BP Process Safety Series - Safe Handling of Light Ends
BP Process Safety Series - Safe Handling of Light Ends
BP Process Safety Series - Safe Handling of Light Ends
Safe Handling
of Light Ends
A collection of booklets
describing hazards and
how to manage them
ii
This booklet is intended as a safety supplement to operator training courses, operating
manuals, and operating procedures. It is provided to help the reader better understand
the why of safe operating practices and procedures in our plants. Important engineering
design features are included. However, technical advances and other changes made
after its publication, while generally not affecting principles, could affect some
suggestions made herein. The reader is encouraged to examine such advances and
changes when selecting and implementing practices and procedures at his/her facility.
While the information in this booklet is intended to increase the store-house of knowledge
in safe operations, it is important for the reader to recognize that this material is generic in
nature, that it is not unit specific, and, accordingly, that its contents may not be subject to
literal application. Instead, as noted above, it is supplemental information for use in
already established training programmes; and it should not be treated as a substitute for
otherwise applicable operator training courses, operating manuals or operating
procedures. The advice in this booklet is a matter of opinion only and should not be
construed as a representation or statement of any kind as to the effect of following such
advice and no responsibility for the use of it can be assumed by BP.
This disclaimer shall have effect only to the extent permitted by any applicable law.
Queries and suggestions regarding the technical content of this booklet should be
addressed to Frdric Gil, BP, Chertsey Road, Sunbury on Thames, TW16 7LN, UK.
E-mail: gilf@bp.com
All rights reserved. No part of this publication may be reproduced, stored in a retrieval
system, or transmitted, in any form or by any means, electronic, mechanical,
photocopying, recording or otherwise, without the prior permission of the publisher.
Published by
Institution of Chemical Engineers (IChemE)
Davis Building
165189 Railway Terrace
Rugby, CV21 3HQ, UK
IChemE is a Registered Charity in England and Wales
Offices in Rugby (UK), London (UK), Melbourne (Australia) and Kuala Lumpur (Malaysia)
2007 BP International Limited
ISBN-13: 978 0 85295 516 1
First edition 1961; Second edition 1964; Third edition 1984; Fourth edition 2005;
Fifth edition 2007
Typeset by Techset Composition Limited, Salisbury, UK
Printed by Henry Ling, Dorchester, UK
v
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 Light ends defined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 ABCs of light ends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 As a gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2 As a liquid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4 Light ends are everywhere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5 Handling light ends on process units . . . . . . . . . . . . . . . . . . . . . 18
5.1 During start-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.2 During shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.3 During processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6 Storage of light ends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
6.1 Activating storage vessels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
6.2 De-activating storage vessels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
6.3 Operations during active service . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
7 Handling LPG at truck transport and
railroad car terminals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
7.1 Operations common to loading and unloading tank
cars and tank trucks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
7.2 Operations concerning truck transports . . . . . . . . . . . . . . . . . . . . . . 43
7.3 Operations concerning railroad tank cars . . . . . . . . . . . . . . . . . . . . . 44
8 Design considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
8.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
8.2 Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
8.3 Fire protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
8.4 Design considerations for loading facilities . . . . . . . . . . . . . . . . . . . . 66
9 Handling of LPG and light ends emergencies . . . . . . . . . . . . . . 67
9.1 Behaviour of light ends spills and fires . . . . . . . . . . . . . . . . . . . . . . . 68
9.2 LPG incident response strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
10 Some points to remember . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
vi
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
Appendix 1: Example of operator fire safety
checklist for LPG storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
Test yourself! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Acronyms and abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
NOTE: All units in this booklet are in US and metric systems.
1
Introduction
Skill and knowledge are required to handle all hydrocarbons whether at the
well head, on a process unit or as finished products. However, one group of
these hydrocarbons, known as light ends, has proven to be particularly
hazardous. Experience has taught that only well-trained personnel, using
properly designed equipment, can handle them safely.
Light ends are more difficult to contain in equipment than heavier hydrocarbons
and are more hazardous if allowed to escape. The low viscosity of light ends
(Figure 1), compared to heavier hydrocarbons, greatly aggravates the problem
of containing them.
1
S A F E H A N D L I N G O F L I G H T E N D S
Most of these hydrocarbons normally are gases at atmospheric temperature
and pressure. To handle them as liquids, they must be confined under pressure
or be held at low temperatures, or both. If the liquid leaks from a container, it
will quickly vaporize, mix with air (oxygen) and probably develop a flammable
mixture. The hazard here is increased because most light ends are heavier
than air and will spread along the ground (Figure 2) where there are many
possible sources of ignition (refer to Chapter 3 for more details).
Figure 1 Light ends are more difficult to contain than heavy hydrocarbons.
S A F E H A N D L I N G O F L I G H T E N D S
2
Propanes ratio of gas volume to liquid volume at standard temperature and
pressure is approximately 270:1, and for butane the figure is 225:1.
Serious fires and explosions have occurred because light ends have not been
contained in refinery units (Figure 3), in storage tanks (Figure 4), at loading or
unloading facilities (Figure 5a), during transport (Figure 5b) or at end-user
locations (Figure 5c).
Figure 2 Most light-ends vapours are heavier than air.
ACCIDENT
Figure 3 This fire at a refinery catalytic crack-
ing unit started with a propane leak. The white
circle on bottom left indicates the position of a
fire truck where seven firefighters were killed.
1 cubic foot (7.5 gallons/28.3
litres) of liquid = 225 cubic feet
(6.3 m
3
) of gas
Figure 4 Fire at refinery storage
area. Eighteen firefighters were
killed during this incident, when
the first sphere BLEVEd.
Figure 5a This loading-rack fire resulted
when the truck driver forgot to disconnect
his fill hose before driving away.
continued
Figure 5b This bulldozer hit a gas
pipeline. The driver was killed by the
resultant fireball.
S A F E H A N D L I N G O F L I G H T E N D S
3
Light ends may also form explosive mixtures within process systems, storage
tanks, and tank cars or truck transports. Special attention is required to remove
air from a system before light ends are introduced and to prevent air from
entering while light ends are present.
Major releases of LPG can be caused by:
tank overfilling, which forces liquid out via pressure relief safety valves.
This booklet will present information on the fundamental characteristics of light
ends with the main focus on LPG (see definition in Chapter 2) and fuel gas as
used on petrochemical plants. It will describe many tried and proven operating
practices, common in the industry today, for handling light ends at refineries.
Also, several important design considerations for equipment are outlined. LNG
will be the subject of a future, more detailed publication.
Figure 5c This factory was destroyed and three employees
killed in what was reported to be a propane explosion.
2
Light ends defined
Light ends
Light ends, as discussed in this booklet, are either a single hydrocarbon or a
hydrocarbon mixture having a Reid Vapour Pressure (RVP) of 18 psia (pounds
per square inch absolute) (1.25 bars a) or more (Figure 6).
4
S A F E H A N D L I N G O F L I G H T E N D S
Light ends will vaporize rapidly at room temperature and pressure. Common
individual hydrocarbons meeting this definition are methane, ethane, propane,
butane and pentane, but also propylene, ethylene, butadiene, etc. Mixtures of
hydrocarbons that qualify include natural gas and fuel gas (though the latter
typically includes hydrogen and other impurities). These contain a large amount
of methane and ethane. LPG (liquefied petroleum gas), which is either propane
or butane or a mixture of the two, is also a light end.
Refinery products (such as commercial motor gasoline and naphtha) having
Reid vapour pressures less than 18 psia (1.25 bars a) are, therefore, not
considered to be light-ends mixtures. However, it must be remembered that
these products may contain light ends. Gasoline, for example, normally
contains butane and pentane. Vapour from a gasoline spill will often contain a
large amount of butane and pentane and will be hazardous.
Figure 6 Hydrocarbons having a Reid vapour pressure of 18 psia (pounds per square
inch absolute) (1.25 bars a) or more are light ends. Those with less are not.
S A F E H A N D L I N G O F L I G H T E N D S
5
TVP: True Vapour Pressure
The true vapour pressure of a liquid is the absolute pressure exerted by the gas
produced by evaporation from a liquid when vapour and liquid are in
equilibrium at the prevailing temperature and the gas/liquid ratio is effectively
zero.
RVP: Reid Vapour Pressure
The vapour pressure of a liquid determined in a standard manner in the Reid
apparatus at a temperature of 37.8C and with a ratio of vapour to liquid volume
of 4:1. Used for comparison purposes only.
3
ABCs of light ends
3.1 As a gas
Light ends (except C5s) are gases at atmospheric temperature and pressure.
Methane and ethane (major components of natural gas) are transported long
distances by pipeline. These gases are used as fuel in various industrial
operations as well as for heating home and office buildings. These particular
hydrocarbons, as well as other light ends, are also handled in refineries as
gases in pipelines and in process equipment. Gas moving through a pipeline is
normally under pressure. If a leak develops, gas will escape, mix with air and
spread over a large area.
Hydrocarbon gas will burn if mixed with the proper amount of air (oxygen).
Since we have no way of knowing the proportions of gas and air in the vapours
around a leak, we must assume that any leak will develop into a flammable
mixture. It also must be assumed that nature will supply a source of ignition.
Experiments have shown that if too little gas is blended with air, the mixture will
not burn. Also, if too much gas is present, the mixture will not burn. These
proportions of fuel are beyond the lower and upper flammable limits. Ordinarily,
we speak of a mixture being too lean or too rich to burn readily. The following
table lists the flammable ranges, at normal atmospheric pressure and
temperature, of several of the light-ends vapour-air mixtures:
6
S A F E H A N D L I N G O F L I G H T E N D S
Volume % Vapour in Air
Lower Limit Upper Limit
Methane 5.3 14.0
Ethane 3.0 12.5
Propane 2.2 9.5
Butane 1.9 8.5
Pentane 1.4 7.8
Natural gas 3.86.5 13.017.0
Many natural and fuel gases which are mostly methane and light hydrocarbons
(frequently mixed with hydrogen and carbon monoxide) are lighter than air.
When they leak out of any container, they rise into the atmosphere. Unless the
S A F E H A N D L I N G O F L I G H T E N D S
7
leak is very large, these gases are diluted in a short time to the point where
they will not burn. However, should the leak occur inside a pump room or other
building, the gases cannot escape readily, and an explosive mixture can easily
develop (Figure 7).
LNG (Liquefied Natural Gasmainly methane) is lighter than air but when
stored at cryogenic temperatures (below 162C/260F), vapours coming
from spilled product are very cold and dense and act like heavier gases until
warming above 107C/160F.
Figure 7 Fuel gas, leaking from a rup-
tured 3/4-inch line, accumulated in
this pump room and ignited. The
resulting explosion caused the dam-
age shown. The line ruptured when
water froze in it.
Ethane and the other light ends are much more dangerous since they are
heavier than air. If these hydrocarbons leak out of a container, they will settle in
a cloud along the ground. LPG vapour is twice as dense as air (Propane 1.5;
Butane 2.0). They are not easily diffused into the atmosphere unless the wind
velocity is at least 10 miles per hour (4.5 m/s) (Figure 2). Also, at ground level
there are usually many more sources of ignition, such as internal combustion
engines, welding equipment and fired heaters (Figure 8).
S A F E H A N D L I N G O F L I G H T E N D S
8
3.2 As a liquid
Light ends are often stored and handled as liquids. This is particularly true of
propane, butane, pentane and LPG under pressure, and methane and ethane
refrigerated.
Pressure
To keep propane as a liquid at 100F (38C), it must be kept at a pressure of at
least 189 psia (13 bars a); normal butane, 52 psia (3.6 bars a); isopentane,
21 psia (1.5 bars a); and LPG, 52 (3.6 bars a) to 189 psia (13 bars a). If these
pressures are not maintained, the liquid will vaporize quickly. Thus, a small
amount of liquid leaking from equipment handling light ends will vaporize and
form a large blanket of vapour (Figure 2). This is why it is so important to keep
light ends confined.
Heavier hydrocarbons at atmospheric temperature do not have to be handled
under pressure and, normally, do not vaporize sufficiently to be a hazard except
in the immediate area of a leak.
Figure 8 Some ignition sources.
S A F E H A N D L I N G O F L I G H T E N D S
9
Viscosity
The relatively low viscosity of light ends compared to heavier hydrocarbons
adds to the problem of containing them within pressurized equipment (Figure 1).
At 100F (38C), kerosene is about fifteen times as viscous (thick) as propane
and six times as viscous as pentane; thus, light ends flow through flanged joints
and packing much more easily than heavier hydrocarbons. Light ends will flow
through openings where even water cannot go.
Boiling point
The low boiling points at sea-level atmospheric pressure of propane
(43.8F/42C) and butane (31.1F/0.5C) create hazards in depressur-
ing equipment in which there may be water or heavy hydrocarbons. As water
freezes, it expands, applying tremendous pressure on the equipment (refer
to BP Process Safety Booklet Hazards of Water for more details). Also,
equipment metallurgy must be adequate to prevent brittle fracture at low
temperatures.
S A F E H A N D L I N G O F L I G H T E N D S
10
A combination cracking unit was severely damaged by a fire
and explosion, and one fatality resulted, because it was not recognized that
vaporizing butane could lower the surrounding temperature below that
required to freeze water. In this accident, a stabilizer tower and reflux drum
were being pumped out, depressured and purged of hydrocarbons in
preparation for a shutdown. During depressuring, residual light hydrocarbon
vaporized, cooling the drum and freezing water in a drain connection. Since
nothing could flow from the drain, the operators assumed all liquid was out of
the drum and left the drain valve open by mistake.
Later, the drain thawed, and a large amount of liquid escaped and vaporized.
The gas flowed along the ground, blanketing a large area before it flashed at
a furnace. Had the operators recognized that the drain could be blocked with
ice, they would have made sure it was open (Figure 9), found that the drum
was not empty, closed the drain valve and proceeded to remove the
hydrocarbons from the drum safely. Another lesson to be learned from this
accident is that equipment should be thoroughly steam purged, if possible, to
be certain that all residual light ends are vaporized.
ACCIDENT
Figure 9 Make sure that drains are not iced.
Water drain procedures are given later in this booklet (refer to Section 8.2).
S A F E H A N D L I N G O F L I G H T E N D S
11
A flash fire followed by a BLEVE (Boiling Liquid Expanding
Vapour Explosion) occurred within the LPG tank farm area of a refinery at
Feyzin, near Lyon, France. Eighteen people were killed and 81 injured. The gas
storage facility, which contained four 1200 m
3
propane and four 2000 m
3
butane
storage spheres, was destroyed. Two spheres suffered BLEVEs, creating a
crater 35 16 metres 2 metres deep (115 52 7 ft). A piece of sphere
weighing 63.5 kg (140 lbs) was found 300 metres (1000 ft) away. The fire spread
to nearby liquid hydrocarbon storage tanks.
The events leading to the accident started with a water draining operation on a
propane storage sphere. Water originating from process treatment units
accumulated in the bottom of the spheres and had to be periodically drained off.
The water draining arrangement consisted of an uninsulated pipe that
discharged to ground underneath the sphere. Both valves had been opened.
When the operation was almost complete, the upper valve was closed and then
cracked open again. Initially nothing came out of the pipe, so the operator
continued to open the upper valve to clear out what he assumed to be ice or
hydrate when the blockage suddenly cleared with a large release of liquid
propane. The operator found it impossible to close the upper valve again as it
had frozen in the open position due to the low temperature created by the liquid
propane flashing to vapour across the valve (44F/42C). He then tried to
close the lower valve, but this too was frozen in the open position. With a clear
path to atmosphere, liquid propane continued to escape forming a vapour cloud
that drifted across the site, through the boundary fence across a motorway, 60
metres (200 ft) away. Although traffic was stopped on the motorway, the gas
cloud was ignited by a passing car about 160 metres (525 ft) away.
The sphere was engulfed by a large fire fed by the escaping propane that
caused the pressure relief valve to lift. Escaping propane vapour ignited. The
local fire brigade arrived quickly, but they were untrained in how to fight an
LPG sphere fire. They concentrated their efforts on cooling the adjacent
spheres. After 1 hour, the burning sphere exploded releasing a wave of liquid
propane that burned in a rising column of fire giving out very high levels of
thermal radiation (BLEVE). The firemen nearby were killed. Fragments of the
sphere cut into the legs of an adjacent sphere, causing it to topple over, with its
relief valve discharging liquid propane into the fire. Half an hour later a second
sphere suffered a BLEVE. Three other storage spheres collapsed and
ruptured, as their supporting legs were not provided with any fire protection.
ACCIDENT
S A F E H A N D L I N G O F L I G H T E N D S
12
The low boiling points of butane and propane create two other operating
hazards. One of these is that operators may suffer from severe frostbite if liquid
butane or propane contacts their skin.
The rapid vaporization of these liquids can cool the skin sufficiently to cause
frostbite. Gloves and goggles should be worn if there is danger of exposure of
hands or eyes to liquid propane or butane. If butane or propane contacts your
skin, remove the liquid immediately and wash the affected area with lukewarm
water.
The other hazard is that liquid butane and propane vaporize when suddenly
released and frequently have the appearance of a white steam cloud. When a
cloud is observed in an area where light ends are processed, it should be
viewed with suspicion. Investigate immediately to be certain that it is steam
and not a dangerous cloud of hydrocarbon.
The white cloud which sometimes appears around a liquid-propane leak results
because the vaporizing liquid absorbs heat from the atmosphere and
condenses particles of moisture in the air. However, this does not always
happen, and you cannot depend on being able to detect a light-ends leak by
looking for a white cloud.
Expansion
Almost all liquids and solids will expand as they become warmer. This is why
the liquid in a thermometer rises as it becomes warmer. Expansion due to an
increase in temperature is called thermal expansion.
If for example, one litre of propane at 0C is heated to 40C its volume
increases to about 1.13 litres. The same volume of butane subjected to the
same temperature rise expands to about 1.08 litres. In general, the lower the
density the greater the rate of expansion or contraction. This coefficient of
expansion is about 10 times that of water, and must obviously be taken into
account when a container is filled with LPG. If a container is filled completely
with LPG and then subjected to a temperature rise, the developed pressure
rapidly rises above the design pressure of the container, because the
expansion is constrained. To avoid the risk of this, care is always taken to
ensure that a vapour space remains in all operating conditions. Limits are
defined in design codes and local regulations.
Thermal expansion of light hydrocarbon liquids can cause serious problems.
Two failures of sample containers in refinery laboratories were due to
overpressure because of expansion of the liquid as it warmed up, and they
could have been avoided if the potential danger had been recognized and
certain precautionary measures taken.
To minimize this hazard, the following general rules for the safe sampling of
light liquid hydrocarbons were developed. These rules apply to sampling
procedures for all liquid hydrocarbons having a Reid vapour pressure of
18 psia (1.25 bars a) or more.
S A F E H A N D L I N G O F L I G H T E N D S
13
The sample containers are to be of 75 and 150-millilitre capacity and are to be
manufactured of monel metal. Under special circumstances, containers as
large as 300-millilitre nominal capacity may be used, provided that special
permission is obtained from an authority designated by the refinery manager.
Sample containers are to be designed for a pressure rating of 5,000 psi
(345 bars) and hydrostatically tested at 8,000 psi (552 bars). A hydrostatic test
of 8,000 psi (552 bars) is to be performed once every five years, and the
containers are to be inspected visually at frequent intervals for corrosion.
Figure 10b compares the Air Force surplus type of container with the 5,000-psi
(345 bars) unit.
The first of these failures was a 1,900 millilitre sample
container that had been filled with field butane at a line pressure of 220 psi
(15 bars) and a temperature of 26F (3C) (Figure 10a). This was an Air
Force surplus oxygen container of two-piece welded steel construction with a
design pressure rating of 500 psi (34.5 bars) and a working pressure rating of
400 psi (27.6 bars). The regular sampling procedure called for venting the
container to assure an adequate vapour space, and this step apparently was
overlooked. The container was brought into the laboratory liquid full, without
any contained air dissolved in the butane.
In the laboratory, the contents of the container approached the room
temperature of 75F (24C). The resulting thermal expansion of the liquid
could have subjected the container to a pressure of more than 1,000 psi
(69 bars), if bursting had not occurred. When it burst, the vaporized contents
flashed at a furnace in which carbon was being burned from a sample of
cracking catalyst.
ACCIDENT
Figure 10a A 1,900-millilitre sample container ruptured, and the vapour
flashed. Thermal expansion of the liquid resulting from a 49F (27C)
temperature rise could have subjected the container to at least 1,000-psi
(69 bars) pressure, if bursting had not occurred.
S A F E H A N D L I N G O F L I G H T E N D S
14
The following are important safety measures.
Make sure that no LPG containers are stored in enclosed spaces, including
laboratories. Use outside well-ventilated areas, in shade and away from
drains, doors and windows for storage.
All rooms where piping is likely to leak (such as from flanges, valves) should
be considered as confined spaces.
Beware of low points (for example, sewers, drains, pits, trenches) where
LPG can accumulate.
For more detail on asphyxiants, refer to BP Process Safety Booklet Hazards of
Nitrogen and Catalyst Handling.
Colour and odour
Light ends are colourless in both the liquid and vapour forms. Any coloration of
the liquid observed, say during sampling, should be investigated immediately,
as it could indicate contamination.
The cloud that appears when LNG or LPG leaks from any point is white, but
this is not the colour of the product itself. It is chilled water vapour condensed
from the air by the evaporating LNG or LPG. It cannot be trusted to show the
limits of the flammable cloud (LEL-UEL) but is a good indicator of the gas
direction.
LPG for commercial use must be given a distinctive odour so that it can be
detected as soon as a leak occurs. The British Standard Specification BS
4250:1987 Parts 1 and 2 requires that the gas be detectable in air at
concentrations of one fifth of its lower limit of flammability. Odorants in use
throughout the industry include ethyl mercaptan, thiophane and amyl
mercaptan.
Unsaturated LPGs (like propylene, butylenes, etc.) do smell strongly.
Other hazards
LNG is stored as a liquid in refrigerated vessels below 162C (260F). Any
contact with a liquid spill can result in severe cold burns (instant frostbite) and
death if the affected surface is significant.
4
Light ends are
everywhere
We must recognize that light ends are found nearly everywhere in refineries.
Process units involved in light-oils operations (such as alkylation, isomerization,
polymerization, reforming, gasoline blending, catalytic cracking, crude topping
and several others) all handle light ends.
LPG products are loaded, unloaded and stored in refineries. Propane storage
drums, butane spheres, pentane spheriods and fuel-gas holders may be found
at many locations throughout refinery process areas and tank fields.
Light ends also are found on many heavy-oil process units. Propane is stored
and mixed directly with heavy oils in propane dewaxing units and in propane
deasphalting units. Some solvent dewaxing units, such as MEK units, also
utilize propane, but only as a refrigerating agent.
Remember that everywhere fuel gas is handled, light ends are present.
17
S A F E H A N D L I N G O F L I G H T E N D S
5
Handling light ends
on process units
Air and light ends must not be brought together in refining equipment except
under rigorously controlled conditions. If air and hydrocarbons are permitted to
mix in flammable proportions, it can be assumed that nature will provide a
source of ignition which will start a fire.
Operators should remember that in many refinery process units, the light ends
may be processed at temperatures high enough (750F/400C and higher) to
self-ignite in air. This temperature is called the autoignition (spontaneous-
ignition) temperature.
Operators must always be on the alert for leaks of any kind. Pump and valve
packing should be watched carefully for leaks and if any occur, the packing
glands should be pulled up. Drain and bleed valves must be kept closed and
plugged and the action logged (Figure 13).
18
S A F E H A N D L I N G O F L I G H T E N D S
Figure 13 When drainage is complete, drains should be plugged or blinded. These
drain points should be checked regularly to confirm plugs/blanks presence.
Solid alloy steel plugs
S A F E H A N D L I N G O F L I G H T E N D S
19
Water must also be eliminated on start-up. The start-up procedure of most units
requires steam purging through vessels, drums and lines to purge air from the
system and, in some cases, requires steam testing of vessels to detect leaks.
During the steaming period, water will collect in pump casings, bottoms of
vessels, and low spots in lines and exchangers. This water must be drained
frequently.
There is little, if any, danger of tank foamovers where light ends are handled
since they are not normally stored at temperatures over 200F (93C), and they
vaporize before water begins to boil.
However, a problem can arise when water is not drained from a light-ends
storage tank that provides charge stock for a processing unit. If such charge
stock is pumped to a fractionating tower, the water may flash to steam in the
tower and cause serious damage to the internals.
Further details about the hazards of water in process units may be found in the
BP Process Safety Booklet Hazards of Water.
5.2 During shutdown
Residual light ends must be removed from refining equipment before air is
admitted. This is accomplished by a steam or other inert-gas purge, followed
whenever it is possible by overflowing the equipment with water.
Detailed techniques on how this should be accomplished are outlined in the BP
Process Safety Booklet Safe Ups and Downs for Process Units. The low boiling
points of propane and butane create hazards in depressuring equipment
containing water or heavy hydrocarbons. When propane and butane vaporize,
5.1 During start-up
Air must be eliminated from process equipment before light ends are
introduced on start-up. The techniques for accomplishing this are described in
the BP Process Safety Booklet Safe Ups and Downs for Process Units. One
important fact to remember is that just one gallon (3.8 litres) of liquid propane
introduced into a 10,000 gallon (38 m
3
) vessel containing air at atmospheric
pressure and at 60F (15.5C) will provide enough fuel for a serious explosion
(Figure 14).
Figure 14 One gallon
of propane vaporized in
a 10,000-gallon (38 m
3
)
air filled drum has the
energy equivalent of 13
pounds of TNT. A source
of ignition can turn that
innocent looking drum
into an exploding bomb.
S A F E H A N D L I N G O F L I G H T E N D S
20
temperatures can be lowered sufficiently to freeze water in drains. At one
refinery, the operators failure to recognize this resulted in a serious fire.
Details of this problem are outlined in Chapter 3.
Blinds must be installed as close as possible to the closed valves used to
exclude light ends. By locating blinds near the valves, the volume of light ends
released from between the blind and valve, when the blind is removed, is
minimized. This principle holds for all oils. It is particularly important with light
ends since they leak past valves readily because of their low viscosity, and
they vaporize quickly.
Maintenance and turnaround items
The (de)commissioning procedures should take account of the following
checks and inspections:
Pressure relief valves
Pressure relief valves should always be removed, tested and inspected and re-
set at frequent intervals, based on local regulations and previous history; and a
written scheme of examination (WSE) drawn up by a competent person.
A register of relief valves should be maintained and each individual valve
identified by a works identification number.
The relief valve tail pipe should be adequately supported and covered with
loose fitting plastic caps to prevent ingress of water.
The tail pipe should have a drainpipe provided which directs away from the
vessel to prevent any flame impingement on to the shell.
Vessels
LPG vessels should be inspected internally at least every 10 years and
hydrotested or in accordance with the WSE (specific inspection techniques
may be applied to replace or complement the hydrotest).
Welds
Radiography or other non-destructive (or non-invasive) inspection method of
piping welds should be carried out regularly as per inspection schedule.
Water accumulation
Nozzles and areas where water may accumulate and cause pitting corrosion
should be subjected to more frequent inspections involving both external
examination and ultrasonic testing.
Ground slope/gradient
Check after vessel overhaul or other major works in the bund to ensure slope
away from under sphere or bullet has not been changed or altered by general
maintenance work, civil works on the bund floor or vegetation growth or other
S A F E H A N D L I N G O F L I G H T E N D S
21
event that may prevent liquid LPG draining down the slope away from under
the vessel.
Filters/strainers
Checks should be carried out for pressure drops every six months.
Stairways/handrails
Stairways and handrails and their supports should be checked for signs of
corrosion and for general wear and tear.
Pipe supports
LPG piping supports should be checked for alignment, function and resting
positions to ensure piping is adequately supported.
Passive Fire Protection (PFP)
Regular checks must be made to ensure that PFP integrity is intact on vessel,
supports, isolation valves, etc.
5.3 During processing
Light ends often come from processes where they are mixed with for example,
H
2
S, cyanides, phenols, ammonia. This may create metallurgical/operational
issues and not dealing with these properly has led to many major accidents.
Operating procedures
LPG facility operating manuals should be readily available at the control room
location. This manual should include emergency instructions, maintenance
procedures, Safety Checklist, flowsheets and process and instrument
diagramsall of which must be up to date.
Facility technicians or operators knowledge and skills for routine and emergency
operations should be checked at annual intervals to ensure competence.
Air hazards
Operators must be constantly alert for the entry of air (oxygen) into process
equipment, especially so when changes in operating methods are made on a
particular process unit or on units supplying feed for it. Air or oxygen may enter
light-ends facilities when (1) dissolved oxygen is present in the feed stream, (2)
equipment is operated under a vacuum, and (3) water is used for process
washing. Also, it is possible that air may be accidentally introduced into
processing equipment.
For example, injection systems used for adding corrosion inhibitors or antifoam
agents must not be pumped empty. In such an event, air could enter the
injection line, or hydrocarbons could back out to the atmosphere through the
same line. Remember, hydrocarbons must not be mixed with air in pressure
vessels or lines except under rigorously controlled conditions.
S A F E H A N D L I N G O F L I G H T E N D S
22
Laboratory research has shown that oxygen may be absorbed by oils and later
released (somewhat like bubbles in soda water). This may cause a
nonflammable vapour space to become flammable by the process of oxygen
concentration or enrichment. Let us see how this is possible.
Assume that the propane stream contains a small amount of oxygen dissolved in
the liquid. When the propane is pumped into storage, some propane vaporizes
into the space above the liquid. Also, along with the propane vapour, oxygen
leaves the liquid, and an equilibrium vapour phase is built up containing a much
higher concentration of oxygen than was present in the liquid (Figure 17).
Now suppose that the liquid level is raised in the vessel and the vapour space is
not vented. Much of the propane vapour under pressure condenses back to a
liquid; but the gaseous oxygen, because it redissolves very slowly in the liquid,
becomes more concentrated in the vapour because of compression (Figure 18).
The percentage of oxygen in the vapour has thus increased many times over
what it was in the liquid entering the storage vessel. Lighter hydrocarbons, such
as propane and butane, are particularly hazardous. They can carry with them
more oxygen than heavier hydrocarbons, and they frequently are stored under
conditions which can lead to the oxygen concentration effect described.
When there is a possibility of air accumulation anywhere in a process unit, the
vapour spaces of the equipment involved should be sampled routinely, and
routine checks for oxygen in the feed should be considered. A value of 0.4%
oxygen is often used as the maximum admissible value before loading LPG.
Figure 17 Equilibrium
vapour phase.
Figure 18 Propane condenses
more readily into liquid under
pressure, leaving the vapour
space more concentrated in
oxygen.
Propane storage drums and an alkylation unit (Figure 15) were
extensively damaged (Figure 16) because of the unrecognized introduction of
oxygen. In this case, oxygen was carried in solution with the propane stream.
ACCIDENT
Figure 15 Arrows indicate location
of three propane storage drums
near an alkylation unit.
Figure 16 Propane enriched with oxygen
detonated. Ignition was probably caused by
iron sulphide. The three propane storage
drums (Figure 15) fragmented. Arrow (Figure
16) indicates remains of one of the propane
storage drums. Other equipment was dam-
aged by fire from the released hydrocarbons.
S A F E H A N D L I N G O F L I G H T E N D S
23
The sampling of liquid light ends requires much care. General rules for the
proper sample container to use and general procedures for obtaining samples
are described in Section 3.2 under Expansion.
Hot work
Hot work, such as welding and cutting on an operating unit, should not
be permitted inside the unit limits except under rigorously controlled
conditionslikewise, vehicles powered by internal combustion engines or other
drives that provide a source of ignition should not be allowed within battery
limits of an operating unit without written authorization of that process unit.
Occasionally, hot work is permitted close to areas where light ends are present.
However, this is done only when operating management has made certain that
the operation will be safe and will continue to be safe until completed. The
determination of the safe operation requires testing of the atmosphere for
combustibles at the hot work location and at the adjacent areas, under strict
adherence to good permit to work rules.
This can be done with combustible-gas analyzers. Several instruments are
available, both manual and automatic (continuous) types.
More information on this subject is presented in the BP Process Safety
Booklets Hazards of Air and Oxygen, Confined Space Entry and Safe Tank
Farm and (Un)loading Operations.
Water hazards
Operators must be particularly careful when draining water to open drains from
vessels containing light ends, and the operator must be present during the
entire operation. Operators must be certain that they have closed the valves
tightly and have replaced the plugs after draining water. Ice in a valve will also
stop flow and prevent complete closing of the valve. When the ice thaws later,
hydrocarbons will escape (see Figure 9 on page 10).
Water drawoff, whether manual or automatic, should be to a closed system. An
open drain or telltale should be installed in the drain line to determine when
the water has been completely drained to the closed drain.
In some instances, it may not be advisable to remove all of the water, with
resultant light ends blow through. In this event, a small water collection pot can
be installed with a gauge glass to permit controlled water drawing with a water
seal at all times. This installation must meet the process system specifications
and must have freeze protection if needed.
Water used for process washing or for flushing can carry air into hydrocarbon
systems. This air may accumulate in a process unit, developing into a
flammable mixture of hydrocarbon and air. Whenever water is used for process
washing or for flushing, vapour spaces of the process equipment involved
should be sampled routinely for oxygen.
Gasoline-blending hazards
The blending of butane into gasoline base stock in atmospheric storage must
be avoided. A closed circulating pipe system (under pressure) should be used.
S A F E H A N D L I N G O F L I G H T E N D S
24
When butane is introduced into a tank containing a low-vapour-pressure stock,
the initial charge of butane may cause vapour agitation of the stock, especially
if the liquid level is low. Such agitation may generate electrostatic sparks on the
oil surface and result in a serious explosion before the oil surface is blanketed
by the butane vapour.
This operation is believed to have caused a refinery tank explosion and fire
involving about a $1,700,000 loss (Figure 19). Only a small amount of butane
had been added to the tank when the explosion occurred. Static electricity was
believed to have been the source of ignition.
ACCIDENT
Figure 19a Tank blending of
butane is thought to have
caused this tank farm fire.
An explosion occurred in the
tank while butane was being
blended with gasoline. The
roof was blown off. It fell on
pipe manifold, broke cast-
iron fittings and started a
ground fire.
The incident above occurred in the 1960s. The one below occurred in the 21
st
Century, showing that some lessons from the past are still valid.
Batch blending was going on in a 7,000m
3
unleaded gasoline
tank when a fire occurred. Fifty-six fire trucks tackled the fire, over more than
30 hours, successfully protecting adjacent tanks. The investigation team found
that the blending calculations were wrong, and three times too much butane
was being sent to the tank. A bubble of light ends probably lifted and tilted the
roof, creating enough static or metal to metal friction to ignite the vapours.
ACCIDENT
Figures 19b & c
Bad control of blending operations or of high RVP products has caused multiple
floating roof sinkings.
S A F E H A N D L I N G O F L I G H T E N D S
25
Relief valves
Relief valves are installed on equipment to prevent internal pressure from
building up to a point which might damage the equipment. Special facilities
operated by trained personnel are available to sites for testing and resetting
relief valves to the proper pressure. No one else is to tighten down or otherwise
alter in any manner the setting of a relief valve.
Flare-stack hazards
A flare-stack system may become a fire and explosion hazard unless given
proper attention. The stack flame, pilot light and lighting device can be sources
of ignition. Also, the stack knockout drum or piping may contain pyrophoric iron
sulphide deposits which catch fire spontaneously when exposed to air. A fire or
explosion could result if air were leaking into a flare-stack system.
Flare systems are usually designed with a continuous purge or water seal to
prevent air from being drawn back into the system.
On catalytic reforming units, the mixture of flared gas is often lighter than air,
largely because it contains appreciable quantities of hydrogen. This lighter than
air gas may mean that a vacuum will be present in the piping and knockout
drum at the base of the flare.
Under these conditions, air will try to get into the system. Operators must be
careful to see that no bleeders or vents are left open.
On starting up, air should be purged from a flare system with steam or other
inert gas before hydrocarbons are vented.
Some ignition systems for flare stacks have air and fuel gas connected to a
common header. Wherever such systems are in use, the air and fuel-gas
connections should be disconnected or blinded after the flare is ignited. This
prevents air from leaking into the fuel gas or into the flare stack.
Drain the flare-stack knockout drums regularly so that
slugs of liquid are not vented out the stack. If
automatic liquid level controllers are used, check their
operation often. Any liquid vented will be ignited by
the flare and fall to the ground where it may cause a
serious fire or injury (Figure 20). A flare was pumped
over at one refinery, and the resulting fire caused
extensive damage.
Figure 20 Burning liquid may be vented from flares if the
knockout drums are overfilled.
Also, flare stacks have frozen on refrigerated storages because steam was
supplied to the flare tip and condensate fell back into the stack.
Refer to Appendix 1 for regular checks to be conducted during routine
operation of LPG facilities.
6
Storage of light ends
Light ends are stored underground or above ground either as liquids or as gases,
depending primarily on the particular hydrocarbon or hydrocarbons involved. In
either case, they may be stored at ambient temperatures or under refrigeration.
Except for a brief description of underground storage, this booklet will be limited
to the discussion of above ground storage at atmospheric temperatures.
The various types of underground storage include:
mechanical spark from the loading arm hitting a metal casing during the
backswing movement (note that enclosed areas such as the one shown
below should be avoided);
electrostatic ignition.
ACCIDENT
Loading arm coupling
Truck coupling
Figure 43a LPG loading station after fire.
continued
S A F E H A N D L I N G O F L I G H T E N D S
40
The connection between the loading arm and the truck detached because of
a weakened connection, despite a previous standard leak test carried out by
the driver and loading operator. Both the thread of the loading arms coupling
nut and the thread of the truck connection showed significant deviations from
the manufacturing standard including:
Provide spark-free and fit-for-purpose wrench tools for LPG thread type
connections and remove unsuitable and non-spark-free tools from service.
Non-sparking tools need to be regularly checked to make sure that they
have not become degraded by bits of grit or iron embedded in the working
surfaces.
Use thread test kit (or equivalent tools) to continuously monitor thread
measures on both refinery and truck sides (see Figure 43b), identify wear
and tear and remove unsuitable connectors from service immediately.
Figure 43b Thread test kit.
S A F E H A N D L I N G O F L I G H T E N D S
41
Unloading
Before unloading, loaders should check the shipping notice to be certain that
the correct quantity and type of product are contained in the carrier.
During cold weather, the temperature in a tank car or truck transport may drop
below 31F (0C). Below this temperature, butanes vapour pressure will be
less than atmospheric, and unloading may be difficult due to the vacuum
present. Air should not be used to pressure the container! Use refinery inert gas
if it is available at the required pressure. If it is not, use bottled nitrogen, being
careful not to overpressure the tank. Another desirable practice is to route
some of the butane from the unloading-pump discharge through a heat
exchanger and back into the vapour space on the tank car or truck.
Propane vapour can also be used, if the propane which dissolves in the butane
does not harm the subsequent use of the butane.
Figure 44 Example of road carloading station using flexible hoses.
Loading and unloading
Before an LPG hose is completely disconnected after use, gas should be
vented from the hose to reduce the pressure in it and to prevent it from
swinging about violently. It is desirable to vent the hose to atmosphere through
an elevated vent stack. Make sure that all hoses are properly disconnected
when the unloading operation is complete and before moving the truck
transport or tank car. Figure 5 on page 3 shows a serious fire that resulted
when a truck driver forgot to disconnect the fill hose.
Relief valves are mandatory on all LPG truck transports and tank cars in most
countries. A loader should never try to adjust or tamper with a safety relief
valve. Vents from relief valves should be directed up and away from the truck
transport or tank car.
S A F E H A N D L I N G O F L I G H T E N D S
42
Regulations covering the transport of hazardous materials have been defined
and published under Title 49, Code of Federal Regulations in the US; the
European Agreement concerning the International Carriage of Dangerous Goods
by Road 20032 (ADR 2003) and Regulations concerning the International
Carriage of Dangerous Goods by Rail 20033 (RID 2003). There are similar rules
in other countries. These regulations have real meaning and purpose to protect
the public and penalties for violation can be severe.
A serious accident occurred in Oklahoma City, Oklahoma,
due to improper safety relief valve venting on an LPG tank truck. A fire of
undetermined origin occurred at the rear compartment of the trailer. The fire
heated the tanks, causing the discharge of fuel from the propane relief valve.
The safety relief valve vent discharged into the enclosed rear compartment,
and an explosion occurred. Five people were killed and 21 injured (Figure
45). The resulting fire destroyed a lumber yard, house, shed and an
automobile.
ACCIDENT
Figure 45 An improperly installed relief valve vent on this truck was the
apparent cause of a serious explosion and resulting fire.
Example of a firefighter training
simulation of a pool fire warming
a LPG truck vessel and the relief
valve lifting (note that the released
product from relief valve has not
yet ignited).
S A F E H A N D L I N G O F L I G H T E N D S
43
7.2 Operations concerning truck transports
No transport should be allowed to approach the rack while another transport is
loading or unloading.
Loading-rack or other knowledgeable personnel should inspect each transport
for leaks and other obvious mechanical defects. Leaks of any kind should be
reported to the supervisor. Loaders should also be sure that the proper
equipment is on the tanks and that it is in good working order.
Some truck transports are equipped with pumps for unloading that are driven
directly by the truck engine. The use of such pumps for unloading at the
refinery LPG rack should be avoided whenever possible. An explosion-proof
electric motor-driven pump, provided at the unloading facilities, is preferred.
Approval to use power take-off pumps should be obtained from management
after a careful study of the accident exposure. Consideration must be given to
such items as area congestion, personnel exposure, ability to limit any difficulty
to the unloading facilities, and available fire protection equipment.
Be sure that the transport tanks are level before loading or unloading. If the
tanks are not level, the relief valve connection may be flooded (Figure 46).
Should the tank be overpressured for any reason under these circumstances,
liquid would be vented. It is also important to keep the tanks level to obtain an
accurate gauge of the contents.
LPG transport trucks are subject to maximum filling density regulations.
Figure 46 Make sure trucks
are level at loading racks.
Avoid flooded relief valves.
The maximum is based on the loading temperature and the specific gravity of
the product being loaded, and can be found in a group of charts or tables
usually located at the loading area.
Three different methods are in use to measure the amount loaded into a truck
transport:
temperature-corrected meters.
Any one of the three may be used.
S A F E H A N D L I N G O F L I G H T E N D S
44
The following are some of the important points for loading the tankers. When
the truck is in position, all electrical systems should be shut off to eliminate
ignition sources. The truck wheels should be chocked and the tanker earthed.
The liquid and vapour hoses should be connected, with careful observation
for leaks. A knowledgeable person must stand by during the entire loading
or unloading operation. Odorant must be added if required, and proper
documents and placards must be provided. Assurance that the loading hoses
are disconnected must be obtained before the truck leaves the rack.
A sign in large visible lettering at the loading area can be helpful. Such a sign
would summarize the loading and unloading procedure, with additional detail in
a procedures manual at the loading rack.
7.3 Operations concerning railroad tank cars
Figure 47 Example of rail car unloading station using flexible hoses.
Loading and unloading procedures must comply with regulations. In addition to
safety problems within the refinery which must be resolved satisfactorily, there
are other safety problems outside the refinery for which the refinery bears great
responsibility. The regulations are designed to assist in this. Special instructions
to the loader-unloader will assure that both of these needs are satisfied. It is to
be expected that inspectors from the local authorities (such as the Department
of Transportation in the US, the DRIRE in France or the HSE in the UK) will
inspect the facilities and review the loading procedures that must comply with
local regulations (such as the ADR-RID in Europe).
S A F E H A N D L I N G O F L I G H T E N D S
45
The following are some of the important points in loading tank cars:
The tank test date and the relief valve test date must be checked to ensure
these dates have not been exceeded.
The suitability of the car for the product to be loaded must be verified.
The handbrake must be set and the wheel chocks applied to prevent car
movement.
When these steps are completed, the liquid and vapour lines can be hooked
up and the gauge rod set for about 30 inches (76 cm).
The permissible loading level must be calculated, after which the gauge rod
(when installed) can be reset to the outage of the particular car being
loaded.
After 15 minutes, the gauge rod should be raised one inch or more, the
valve checked for vapour, and the gauge rod then lowered until white mist
appears. The gauge rod should be read and the gauging operation
repeated. The gauge should be recorded to the nearest 1/4 inch.
The decal should be in place inside the dome cover, giving the After
Unloading instructions.
The dome cover should be closed and sealed, and the Department of
Transportation placards placed in the four holders on sides and ends.
For unloading tank cars, if gallonage is needed for unloading purposes,
measure temperature of contents, raise gauge rod to full extension, and
depress slowly until white mist is expelled. Use temperature reading, gauge-rod
reading, specific gravity of the product and the outage tables of the tank car to
obtain the gallonage. After unloading, all connections should be secured, and
the instructions on the decal After Unloading (inside dome cover) should be
followed.
In many cases, the tank car lessor (Union Tank Car, for instance) provides a
booklet on suggested procedures for loading and unloading tank cars. This is a
valuable reference.
46
S A F E H A N D L I N G O F L I G H T E N D S
8
Design considerations
8.1 Materials
Cast iron should not be used for equipment parts containing light ends under
pressure. This includes pumps, vessels, piping and fittings. Commonly
overlooked are equipment closure items, such as packing glands, mixer seal
housings and compressor valve covers. Preferred material in these cases is
steel. Cast iron is very brittle and is easily broken if it is hit sharply or cooled
rapidly with water during a fire.
At one refinery, the quick closing of a valve on the discharge side of a cast-iron
pump caused a pressure surge which ruptured the pump (Figure 48). Cast
steel pumps are much more suitable for withstanding such pressure pulsations.
ACCIDENT
Figure 48 Cast iron pump casing ruptured by pressure pulsations.
In 1948 in Texas City, two breaks occurred in the 6,800 foot
(2 km) long line used for transferring a propane-propylene mixture (under
pressure of 250 pounds per square inch). A low hanging cloud of gas
covering 68,000 square feet (6,317 m
2
) formed and drifted over the Texas
City-to-Galveston Highway before it flashed, killing seven persons and
injuring fourteen others (Figure 49).
continued
ACCIDENT
S A F E H A N D L I N G O F L I G H T E N D S
47
A small amount of corrosion will start leakage through the threads of a plugged
nipple, especially when light ends are being handled. Asuitable alloy should be
used for screwed plugs used in drains, vents, carbon-steel lines, pumps,
exchangers and other equipment in which light ends are contained.
Materials (such as brass, bronze and aluminum) which will not withstand the
high temperatures reached during a fire should be avoided for equipment
handling light ends.
Small bore screw fittings on pressure vessels and lines should be eliminated
whenever possible. No small bore screw fittings should be permitted to be
directly connected to a pressure source. The concern is that the threads will
strip under pressure or be incorrectly screwed on during maintenance work,
resulting in failure. Small bore connections (including sight glasses) should be
fitted with flow restrictors to limit the rate of release in case of failure.
Flanges should be minimized where possible by welded piping to reduce leak
potential.
One break occurred at a cast iron tee and the other 748 feet (70 m) away in a
cast-iron pipe collar. Both leaks are believed to have been caused by excessive
stresses in the pipeline. Welded steel piping should be used in light ends
service. Large screwed pipe couplings and valves fail very quickly when
exposed to fire (Figure 50). Such failures probably will add more fuel to the fire.
Figure 49 Breaks at cast iron finings in a propane-propylene pipeline caused
this Texas City-to-Galveston Highway flash fire.
Figure 50 Screwed pipe failure due to fire.
S A F E H A N D L I N G O F L I G H T E N D S
48
Material characteristics should be able to withstand all service conditions,
including start-up and shutdown but also colder conditions created by escaping
LPG in case of a leak. Materials in contact with LPG can experience relatively
low temperatures, even at normal pressures and ambient conditions. Rapid
offtake of vapour from a vessel can result in low temperatures within the vessel.
Similar low temperatures can be produced in pressure reducing equipment
and it is essential that both metallic and non metallic components in the system
are made from materials that retain their basic properties over a sufficiently
wide temperature range.
The loss of lean oil flow in a gas treatment plant created
a major reduction in temperature (temperatures in parts of the plant fell
to 48C/ 54F) of an LPG heat exchanger, causing embrittlement of the
steel shell. That heat exchanger ruptured, releasing a cloud of gas and oil. It
is estimated that the cloud travelled 170 metres (560 ft) before reaching fired
heaters where ignition occurred. After flashing back to the point of release,
flames impinged on piping, which started to fail within minutes. Alarge fireball
(Figure 51) was created when a major pressure vessel failed one hour after
the fire had started. It took two days to isolate all hydrocarbon streams and
finally extinguish the fire.
Two employees were killed and eight others injured. The incident caused the
destruction of one plant and shutdown of two others at the site. Gas supplies were
reduced to 5% of normal in the area, resulting in 250,000 workers being sent
home across the state as factories and businesses were forced to shut down.
ACCIDENT
Figure 51a Fireball during the LPG
plant fire.
Figure 51b Ruptured heat
exchanger.
S A F E H A N D L I N G O F L I G H T E N D S
49
8.2 Equipment
Direct-fired heaters should be avoided whenever possible on light ends units.
When it is necessary to use a direct-fired heater, it should be located far
enough away from the nearest equipment containing light ends so that there is
very little possibility of flammable mixtures reaching the heater.
Electrical equipment. A full hazardous area classification should be carried
out and only equipment suitable for use in the zones defined by the
assessment installed. The BP Process Safety Booklet Hazards of Electricity
and Static Electricity covers in detail the proper safeguards for electrical
equipment and area classifications.
Eliminate enclosed areas. Pumps and compressors should not be placed in a
totally enclosed area. If they are, even a small leak can gradually build up the
concentration of hydrocarbon vapours until the room contains a flammable
mixture. Exhaust systems can remove hydrocarbon vapour effectively in such
installations, but it is much better to locate the equipment outside where the
danger of accumulation is virtually eliminated.
Flare stacks should be of sufficient height and be located far enough from
equipment containing light ends that the flare will not provide a source of
ignition. The stack height also should be such that in case of a flameout, gas
issuing from the stack (if heavier than air) will not reach the ground in sufficient
concentration to be flammable. The base of flare supports should be
fireproofed.
Knockout drums designed to drop out liquids before they reach the flare,
should be provided.
There are instruments on the market which can detect the presence of an
explosive mixture and will sound a warning whenever one develops. These
devices can be used to warn vehicles not to enter an area where gas is
present. They also can be used at locations on units where leaks might
otherwise go undetected for a period of time or where railroad tracks and roads
come close to the unit battery limits. Detectors need regular calibration, can be
poisoned, and failure modes may be different from one type of detector to
another.
No instruments leads containing hydrocarbons should be brought into control
rooms.
Sight glasses. New installations should use magnetic followers (or similar
technology) instead of sight glasses.
LPG pumps should be located well away from storage vessels and outside
any vessel bunded area.
Pump seals are a potential area for loss of containment. It should be confirmed
that the degree of seal release protection is appropriate to the location and that
seal performance has, historically, been satisfactory.
S A F E H A N D L I N G O F L I G H T E N D S
50
Mechanical seals are more commonly used than packing on centrifugal pumps
in light ends service because there is very little or no leakage from a
mechanical seal in good operating condition. For this reason, packing should
not be used. Special provisions must be made, however, to prevent excessive
spillage when a seal fails. One of the more common methods used to do this is
to install in the pump case a throttling bushing which has a close clearance
between the bushing and the shaft (Figure 52). In selecting the proper seal for
a pump, consideration must be given to operating temperature and pressure,
and to the characteristics of the stock. Mechanical seals must be installed
correctly. Most trouble with seals is caused by improper installation or poor
operation.
Figure 52 Typical pump mechanical seal details. Throttling bushing reduces
leakage if the mechanical seal fails.
Good industry practice rates the order of protection for pump seals as shown
below:
Other configuration seals are normally tandem seal arrangements where the
seal fluid is at a pressure less than that of the process/system pressure or
where there is no seal fluid in the case of dry running outer seals.
Caution must be exercised when terminology for seals is discussed. For further
information, refer to API 619.
The need for permanently installed condition monitoring instrumentation should
be assessed particularly where pumps are sited in not normally manned or
unmanned locations. Otherwise, a regular program of condition monitoring is
needed.
check if the sampling point is labelled and that samplers are adequately
trained in the correct procedures.
S A F E H A N D L I N G O F L I G H T E N D S
53
Remote isolation valves. Manual only valves may be inaccessible under fire
conditions and therefore remote operated vessel valves should be provided.
Emergency isolation valves should be fitted on all pipework (except piping
for relief valves and level measuring devices below 3 mm (0.125) for liquid and
8 mm (0.333) for vapour) connected to LPG storage vessels containing more
than 10 m
3
. Smaller vessels should be considered on a case by case basis.
The location of the remote operating controls should be outside of any fire area or
high radiant heat area caused by fire. These controls should also be located
upwind of the vessels in case of a gas leak. Additionally, the facility to close the
valves automatically (such as a fusible link) in the event of a fire should be installed.
The valves should be fire-proofed and located as close as possible to the inventory
(if possible inside vessel, if not welded on vessel outlet pipe, or on 1
st
flange after
vessel). When possible, it is good practice to locate the first flange on liquid lines
far away from the storage vessel, so any fire from a flange leak will not impinge
on the vessel (see picture below).
First flange
Note that there is no gasket, bolted
flange or manual and control valve
under the vessel, therefore
minimizing incidents in direct
proximity of large inventory. The
number of liquid line piping
connections to the bottom of the
tank is also minimized compared
to the majority of spherical tanks
(compare with other pictures in this
booklet) that have at least five of these which are one inch or greater, typically:
10-inch discharge;
4-inch recirculation;
slope to drain away from under the vessel for pool fire impact reduction;
flash fire;
earliest identification of potential ignition sources the gas may envelop and
actions including heaters, naked lights extinguished, electrical isolation
(Only if this is possible in a safe time frame before any gas migrates to the
equipment);
pedestrians/passers-by;
actuating any external emergency plans to evacuate people and halt traffic,
etc;
expect gas ignition at any time, even if there does not appear to be any
ignition source.
do not attempt to extinguish the jet fire by using dry powder or heavy water
application;
actions to be taken to deal with the spill once the foam has been
successfully laid.
The applied foam blanket will need regular topping-up to maintain vapour
suppression. Once portable equipment is in position and functioning correctly,
responders should retire to a safe distance from the general hazard area and
observe the foam application from a distance.
Figure 76a Example of foam blanket over an
LNG spill. This foam was applied by fixed foam
pourers but, for condensate spills, portable
foam application will achieve the same results
if the response crews are well trained.
Figure 76b Another example of foam blanket over
an LNG spill. This foam was applied by portable
equipment, but in this case note the vapours
migrating in the centre of the photograph.
Foam application may need to consider environmental impact and therefore
this strategy should be reviewed in advance to ensure that such an impact is
either eliminated or reduced. In some countries, there may be a need to contain
all firewater and foam applied for incident control.
Use of water curtains or spray around a liquid release may, in some cases, be
the best response to contain gas or prevent migration or aid early dispersion.
Use of waterspray/water curtain
Response strategies that may include gas dispersion from a liquid pool using
water spray need to consider several important safety factors:
causing ignition through an electrostatic discharge from the water jet directly
into the LPG liquid;
the outer shell will prevent direct flame impingement on the inner tank;
the insulation between the outer and inner wall will greatly slow heat transfer
to the LNG.
Thermal radiation levels from the resulting rising fireball are sufficiently high
and wide to result in fatalities, serious burn injuries, and damage to plant and
equipment.
In fighting fires where liquid butane and propane are stored in horizontal
cylindrical pressure vessels, operators should be aware that these vessels
sometimes fail at a circumferential seam. When this occurs, the sudden release
of pressure sends the vessel off like a rocket (Figure 82). No one should be
positioned in front of either end of such a vessel during a fire.
S A F E H A N D L I N G O F L I G H T E N D S
80
Figure 82 Light ends
pressure vessels may
become rockets if they fail
at circumferential seams.
However, this does not mean that staying near the sides is safe either as
during a BLEVE, tank debris can fly in any direction, not just from the
ends.
A propane tank fire started after two teenagers driving an
all-terrain vehicle ploughed into unprotected propane piping at a farm. This
above ground piping ran from the propane storage tank to vaporizers, which
fueled heaters located in barns and other farm structures. The 42-foot long,
cigar-shaped storage tank contained propane liquid and vapour under
pressure, and the tank was about half full at the time of the incident. The
collision severed one pipe and damaged another, triggering a significant
propane leak under the 18,000 gallon tank. About five minutes later, propane
vapour leaking from the damaged pipes ignited and burst into flames,
engulfing the tank and beginning to heat the propane inside.
Because of the flames, arriving firefighters could not approach a manual shut-
off valve to stop the propane leak, and they decided to let the tank fire burn
itself out. The fire chief on the scene believed that in the event of an
explosion, fragments would be thrown from the tanks two dome shaped
welded ends. The areas near the sides of the tank, he believed, would be
relatively safe. Shortly after their arrival, firefighters approached the sides of
the flaming tank and began spraying the surrounding buildings to prevent the
spread of fire.
Just seven minutes later, the burning propane tank ruptured completely,
experiencing a Boiling Liquid Expanding Vapour Explosion or BLEVE.
continued
ACCIDENT
Figure 83 A piece of
the 18,000-gallon tank
inside a turkey barn
(picture, U. S. Chemical
Safety and Hazard
Investigation Board).
S A F E H A N D L I N G O F L I G H T E N D S
81
The propane tank was blown into at least 36 pieces, some of which flew 100
feet or more. Some of the shrapnel struck firefighters, killing two of them.
Other pieces smashed into buildings, leaving nearly $250,000 in property
damage.
Figure 84 Firefighters positions at the time of the BLEVE (Illustration NFPA).
Figure 85a Typical fireballs from BLEVE events.
S A F E H A N D L I N G O F L I G H T E N D S
82
Figure 85b LPG vessel failures under fire.
The only strategy for response to an impending BLEVE is to evacuate to a safe
distance. ABLEVE will typically form a fireball although this will be a short-lived
event. Like a flash fire, the duration of the fireball, by itself, is normally
not sufficient to cause steelwork failures. However, any vapours present in the
fireball area will ignite and may lead to secondary vent fires or other fire events.
Note that vessel failure under fire can occur with any size of LPG vessel. Figure
85b shows an example of a 33 kg LPG bottle which ruptured after being
involved in a fire, a depot and a LPG truck during unloading.
S A F E H A N D L I N G O F L I G H T E N D S
83
Personal Protective Equipment (PPE) for responders
Selection of PPE
Responder PPE should always meet an EN, NFPA or equivalent standard.
PPE should be viewed as a system in terms of total wearer protection, rather
than separate items that can be mixed to suit individual budgets.
The level of responder intervention and exposure will determine the levels of
PPE required. For instance, at the lower exposure end, if the responder actions
are simply to actuate a fire system switch and check evacuation status, then the
PPE selection may be for only normal facility coveralls (which will probably be
fire resistant in any case). However, where responder actions require manual
deployment of fire monitors, foam monitors, use of breathing apparatus, etc,
there will be a need for fire turnout gear. This will typically consist of:
fire coat;
fire trousers;
fire boots;
fire gloves.
The use of fire resistant (see below for clarification) materials for such PPE is
critical to responder protection. In addition, responder normal workwear needs
to consider fire resistant coveralls to ensure maximum protection is provided
under fire exposed conditions. This becomes obviously important when
responders may be working nearby to a gas release (knowingly or otherwise),
where there is potential for a flash fire event.
Fire retardant treatments versus inherently fire resistant
Materials that are inherently fire resistant, such as Nomex fibres, offer
continuous fire resistance for responders. This resistance can be provided in
coveralls, two piece protection suit or fire turnout gear. There are materials
such as cotton that have been treated with fire retardant chemicals to offer fire
protection capability but such material, when subjected to heat or flame, will
often result in the mass release of these chemicals thus making the garment
unfit for further service.
Additionally, the regular cleaning of fire retardant treated clothing will result in
washout of the protective chemicals. Therefore, the preference should be for
inherently fire resistant clothing rather than treated clothing.
S A F E H A N D L I N G O F L I G H T E N D S
84
Scenario-specific emergency response plans
Although facilities should have emergency procedures in place for higher level
control of an incident, generic and specific Emergency Response Plans (ERPs)
should be prepared for credible serious or major incidents at facilities.
The requirement to prepare an emergency response plan at major hazard
sites, such as many of those that will be handling light ends, is contained
in regulations such as OSHA PSM in the USA or SEVESO in Europe (COMAH
in UK).
The ERPs should be:
fit-for-purpose;
easy to use;
API
o Std. 2510, Design and Construction of Liquefied Petroleum Gas (LPG)
Installations
o RP 2510AFire Protection Considerations for the Design and Operation of
Liquefied Petroleum Gas (LPG) Storage Facilities.
SIGTTO:
o Liquefied Gas Handling Principles on Ships and in Terminals
o Liquefied Gas Fire Hazard Management
90
S A F E H A N D L I N G O F L I G H T E N D S
S A F E H A N D L I N G O F L I G H T E N D S
91
o A Guide to Contingency Planning for Marine Terminals Handling
Liquefied Bulk Gases
o Cargo Firefighting on Liquefied Gas Carriers
o Safety in Liquefied Gas Marine Transportation and Terminal Operations
(CD)
o Liquefied Gas Carriers: Your Personal Safety Guide.
visual inspection of vent tail pipe drain for corrosion and to ensure it is not
blocked or clogged;
checks for indications of minor leaks at pump seal area, ancillary systems
such as seals, lubeoil, etc.;
visual inspection of supports for small bore piping associated with pumps for
damage, corrosion or general wear and tear.
Support legs:
Visual inspection:
visual inspection of light cover seals for signs of rain or water entry indicating
seal degradation and potential gas/vapour ingress.
Electrical equipment and fittings:
visual inspection of covers, seals and wiring for corrosion, integrity, wear and
tear.
Earthing/bonding:
Visual inspection for corrosion, wear and tear at the cable and fixing points on
bullet or sphere or sphere legs and ground.
Grass, brush, vegetation, trees:
check presence and legibility of warning signs, safety signs and fire systems
signs.
A1.2 Fire protection and fire safety checks
The following checks should be either made or arranged by the fire response, fire
safety team, or fire safety representative, depending on the location of the LPG
facility.
Quarterly
Gas detection:
test for LFL actuation and alarm functions and any executive actions;
re-calibration of detectors.
6 monthly
Water deluge system:
visual inspection of piping and manual valves for damage, corrosion and
general good condition;
visual inspection for piping condition, corrosion and damage or wear and tear;
functional test for flow, stream pattern adjustments and valve operation;
False
False
3. Natural and fuel gases which are mostly methane are lighter than air at
ambient temperature.
True
False
4. Ethane, propane and butane are lighter than air at ambient temperature.
True
False
False
False
7. Pumps with double mechanical seals have the highest degree of protection
against leaks.
True
False
False
False
False
A N S W E R S
1 T / 2 F / 3 T / 4 F / 5 F / 6 F / 7 T / 8 T / 9 F / 1 0 T
S A F E H A N D L I N G O F L I G H T E N D S
96
Acronyms and
abbreviations
CO
2
Carbon Dioxide
ERP Emergency Response Plan
FCCU Fluid Catalytic Cracker Unit
LNG Liquefied Natural Gas (mainly methane)
LPG Liquefied Petroleum Gas (PropaneButane)
O
2
Oxygen
PPE Personal Protective Equipment
PRV Pressure Relief Valve
WSE Written Scheme of Examination