14 - Section-9 Vents Flaress Drains
14 - Section-9 Vents Flaress Drains
14 - Section-9 Vents Flaress Drains
SECTION 9
1 Introduction
3 Drains System
3.1 System Function
3.2 System Equipment
Figures
9.1 INTRODUCTION
Such a provision is required both during normal operations and during upset
or emergency conditions.
Refer to Figure 9. 1
The vents and flares installed on offshore installations can comprise three
separate systems operating at high pressure (HP), low pressure (LP) and
low-low pressure (LLP). The purpose of these systems is to:
Recover separated liquids and transfer them to the platform drains for
recovery
Gases from the three systems are burnt at HP, LP and LLP flare tips located
adjacent to each other at the end of the flare boom.
The flare system is designed such that in the event of maximum flaring
conditions there is a minimal risk to personnel and equipment due to heat
radiated from the flare tip.
By referring to the system shown in Figure 9.1 it can be seen that three
collection and disposal systems are provided:
LP system comprising the flare drum and the LP) flare tip
LLP system comprising the flare drum and the LLP flare tip
A combined high pressure and low pressure flare tip is shown in Figure 9.2.
Each flare tip is equipped with three pilots which are fed by fuel gas.
Each pilot has a dedicated ignition riser in which a mixture of instrument air
and fuel gas is lit by a flame front generator located on the flare ignition
panel.
The ignited air/gas mixture or flame front is then diverted through the
appropriate ignition riser to its particular pilot, which in turn ignites the
associated flare tip.
Failure of any of the flare pilots, low purge gas flow or high fuel gas filter
differential pressure is indicated on the flare control panel.
Normally cross-ignition takes place between the flare tips, once the first tip
has been lit. However, the system must be checked to ensure that all the
flares are burning.
Hydrocarbon gases, vapours and liquids are released to HP, LP and LLP flare
systems from blowdown valves (BDVs), pressure safety valves (PSVs),
bursting discs (pressure safety elements (PSEs)), pressure control valves
(PCVs) and vents as follows:
Vents pass fuel gas to the LLP flare after use by process and utility
equipment for blanketing purposes, eg on drains, vessels and so on
Hydrocarbons that are vented to the HP flare system are routed to either
one of two HP flare headers, the wet or dry header. The purpose of the two
headers is to prevent the HP flare system becoming inoperative through
possible blockage by hydrates. As hydrate formation is dependent upon
temperature, pressure, free water and hydrocarbon gases being present
under certain conditions, the system is designed to prevent this occurring by
eliminating one or all of these conditions.
Hydrocarbons vented to the wet header contain free water but, because of
process conditions ie temperature and pressure, will not create low enough
temperatures to form hydrates.
Hydrocarbons vented to the dry header do not contain free water and may
be disposed of safely from plant areas that could create low temperatures
when vented, ie if the gas process were vented from a cold gas separator
hydrates will form should water be present.
For this reason the source and nature of the hydrocarbon is taken into
account and routed accordingly.
Purge gas (refer to Figure 9.1) is continually supplied to the HP dry flare
header to blow any liquids that may be present through to the flare drum;
prevent air ingress into the system and ensure a minimum flow at the flare
tip. Likewise a minimum flow is introduced to the HP wet header.
Collections from the HP wet and dry headers enter the HP flare drum as
separate streams. On entering the drum each stream hits a deflector
causing liquids and gases to separate. Gas exits the drum from both ends,
flows to a common header and then to the HP flare tip where the gas is
disposed of by burning. The common header is manifolded to receive gas,
from the gas export line, whenever the line is required to be depressurised.
The liquid level in the drum is automatically controlled between upper and
lower limits by the level indicating controller (LIC). At the upper limit the
HP flare condensate pump A is signalled to start and transfer separated
liquids to the pressure drain flash drum. If the level increases to a high
level, the second pump is signalled to start. When the level drops to the
lower limit both pumps are signalled to stop.
As with the HP system separate wet and dry LP flare headers segregate
collections within the system. Hydrocarbons that are vented to the LP flare
system are routed to either the wet or dry header. The purpose of the two
headers is to prevent the LP flare system becoming inoperative through
possible blockage by hydrates.
Hydrocarbons vented to the wet header contain free water, but because
of process conditions, ie temperature and pressure are not likely to form
hydrates. Hydrocarbons vented to the dry header do not contain free water
and may be disposed of safely from plant areas that could create low
temperatures when vented.
Certain process equipment can be permanently open to the LP flare wet and
dry headers.
Liquid accumulating in the LP drum flows through a level valve (LV) before
passing to the slops oil tank. Liquid level in the drum is automatically
controlled by the LIC acting on the LV.
The single collection header routes released hydrocarbons to the LLP flare
drum which separates out liquids by gravity. Gases leave the top of the
drum and flow to the LLP flare tip for combustion. Separated liquids leaving
the base of the drum flow through a loop seal to the slops oil tank.
Elevation differences between the normal liquid level and the Slops oil tank
allow liquids to be transferred by gravity.
The transfer line maintains a minimum level by means of the liquid seal
contained in one arm of the loop. The loop is continuously vented to the LLP
flare header to prevent liquids being inadvertently syphoned out of the
drum followed by gas breakthrough. Piping downstream of the loop is self-
draining and is normally free of liquid.
Hydrocarbon gases from each of the three flare drums are discharged to
corresponding flare tips through dedicated flare headers. The flare tips are
located at the extremity of a flare boom. The quantity of gas flared is
measured and recorded by means of flowmeters installed in each header.
Released hydrocarbon gases are burnt on discharge from the individual HP,
LP and LLP flare tips. The continuous fuel gas purge to each HP, LP and LLP
system maintains a continuous flame at the flare tips so that any additional
release to the flare systems automatically ignites at the flare tip.
Typical flare tips have tulip type profiles (refer to Figure 9.3). Gas burns on
the outside of the tulip and a protective layer of unburned gas between the
flame and tulip bowl prevents the flame impinging on the bowl. The tips are
designed for smokeless combustion and are capable of burning up to 25%
weight/weight of crude oil or condensate without liquid fall-out and little
smoke production.
The LLP flare tip is normally a conventional open pipe flame with the flame
burning just outside the gas exit point. Combustion characteristics are
dependent on gas flowrate and composition.
The LLP flare is not designed to deal with appreciable quantities of liquid
carryover, which if sufficient will cause burning liquids to fall away from the
flare (flaming rain) presenting an extremely hazardous situation.
In order to prevent the ingress of air into the pipework the system header is
continually purged with fuel gas or nitrogen during startup and shutdown
situations.
Low points of the system headers are fitted with liquid knockout drain pots
in order to remove any liquids formed in the system.
Analyser vent
Closed drains generally collect only hazardous liquids from process vessels
or pipe manifolds and are hard piped from the drain point, ie the drain
line is flanged or welded directly to the vessel or pipe. Once collected the
fluids are directed to the pressure drains flash drum.
(1) The open drains system (atmosphere skid draining, washdown, deluge
and rain water draining). This is further subdivided into:
Closed drain manifolds are sometimes segregated into warm and cold
headers to reduce the potential for hydrate formation within the
system.
Any flash gas from the liquids in the closed drains vessel is vented
directly to the LP flare whilst recovered oil, including that recovered
in the hazardous drains caisson, is pumped back to the separators for
reprocessing.
Figures