WATER SYSTEMS PIPING C.

21

of the connected pump, pressure vessels, relief valve settings, et cetera, depending
on the type of system and equipment used. Reasonable margin shall be added to
cover variations in expected maximum performance, transients, and control toler-
ances.
The internal design pressure, including the effect of the static head and allowance
for pressure surges, shall not be less than the maximum sustained uid operating
pressure. Consideration shall also be given to pump shut-off pressure.
Piping subject to external pressure shall be designed for the maximum differential
pressure anticipated during operating, shutdown, or test conditions, excluding pres-
sure tests. Refer to Chap. B2. For buried piping this includes loading due to earth
cover and trafc.
In accordance with ASME B31.1, Paragraph 102.2.4, the piping system shall be
considered safe for occasional short operating periods at higher than design pressure
or temperature, if the calculated stress value is not exceeded by more than 15
percent during less than 10 percent of any 24-hour operating period or by more
than 20 percent during less than 1 percent of any 24-hour operating period.
A piping system is considered safe for operation if the maximum sustained
pressure and temperature which may act at any part or component of the system
does not exceed the maximum pressure and temperature determined in accordance
with Code rules by the Power Piping Code ASME B31.1. Allowable stress values
and pressure-temperature ratings are provided by the piping codes and the standards
referenced therein.

Design Temperature. In the design of water distribution systems, the following

guidance is provided in determining and specifying system-design temperature re-
quirements.
The design temperature shall be determined on the basis of the maximum
expected operating temperature. The effects of pumping, throttling, heating, cooling,
et cetera, must be considered in the determination of the design temperature of
the piping system.

Pipe Sizing Criteria. Typically, total piping system cost is approximately 7 to 8

percent of the total plant investment. These values range upward to 30 percent for
municipal water systems and some ships. Selection of pipe sizes, beside affecting
initial cost, will also affect operating costs due to their sensitivity to changes in
pressure drop, heat losses, and maintenance requirements.
Selection of a pipe-line size involves determination of an optimum size. For
instance, if extra pumping is needed to boost the uid pressure or if the heat rate
will be affected adversely, then the cost of the extra energy required becomes a
signicant factor in the evaluations. The optimum pipe size is obtained when the
sum of installed and operating costs is at the minimum.
Piping optimization is not widely used in preliminary calculations. Where the
pressure drop is dened by other considerations, the minimum pipe size compatible
with good engineering practice will be selected.
Other related considerations which have an important impact on pipe size selec-
tion, include:
1. Noisewhich can result from high velocity ow, cavitation or two-phase ow.
2. Vibrationwhich can result from noise, excessive velocities at changes in the
direction of the uid ow, or the causes of cavitation.
3. Erosion or corrosiondue to chemical action of the uid, excessive velocities,
cavitation, and excessive turbulence at ttings, valves, branch connections, etc.
C.22 PIPING SYSTEMS

4. Flow distributionthe more uniform the cross-sectional velocity prole, the

more likely that the above factors will be reduced. This can be achieved by using
reasonable velocities along with a piping layout that will produce a smooth
ow pattern.
5. Cavitationwhich can result from the collapse of bubbles close to a metallic
surface at a high enough velocity to cause erosion, and two-phase ow uids.

Effects of Velocity. Higher allowable velocities will lead to smaller pipe sizes and
higher pressure drops. Excessively high velocities can cause noise, vibration, and
erosion. Velocities in pump-suction lines shall be kept sufciently low in order to
maintain the pumps required net positive suction head (NPSH).
The pressure drop in a system can be decreased by selecting a larger pipe size
or sometimes by using more than one pipe for the total ow.
For water piping systems a velocity in the range of 4 to 15 ft/sec (1.2 to 4.6
m/sec) is acceptable. Depending upon the material selected, piping design and size
is either in the low or high side of this range, considering the economics of system
installation and operation. For example, for brass pipe a velocity between 4 to 15
ft/sec (1.2 to 4.6 m/sec) would be recommended, while for steel pipe, a velocity of
7 to 10 ft/sec (2.1 to 3 m/sec) is the recommended range, while velocities to 30
ft/sec (9.1 m/sec) may be acceptable. Higher velocities are acceptable if materials
less susceptible to erosion (e.g., stainless steel) are selected. Concurrently reducing
vibration and meeting system hydraulic requirements will reduce the pipings suscep-
tibility to erosion. In all cases, it should be recognized that these ranges are recom-
mended only if system operating requirements are also satised. High velocities
are often conducive to water hammer problems.

Pipe-Wall Thickness Selection. After determining the internal diameter of the

pipe, the designer must select materials, consider their strength, and select a pipe-
wall thickness or schedule, as a function of temperature, pressure, corrosion, erosion,
vibration, and external loads, as required.
Pipe-wall thickness determination begins with the basic hoop stress in the pipe
wall. This stress calculation ignores longitudinal wall stress that exists if the pipe
has closed ends. An example of this is a ask or short header.
Advanced analysis shows that for thin-wall pipe, the outside diameter should
be used in the hoop stress equation:

where P internal design pressure, psig (kPa) [gauge]

Do outside diameter of pipe, in (mm)
tmin minimum required pipe wall thickness, in (mm)
S allowable stress, psi (kPa)
This equation, called the Barlow formula, is the basis for most code stress-
pipewall-thickness calculations such as those provided in ASME B31.1 and B31.3.
The formula also applies to thick-walled pipe.
The Barlow formula allows determination of wall thickness for exible pipe
required to handle internal pressure. Pipe-wall thickness must also be adequate to
handle external loads such as soil cover and vehicle loads, vacuum, and buckling.
For exible pipe such as steel, ductile iron, PVC, and HDPE, determination of
thickness for internal pressure and determination of thickness to handle external
B.60 GENERIC DESIGN CONSIDERATIONS

System Fluid Flow Design

The objective of the uid ow design is to determine the minimum acceptable

inside diameter of each segment of the piping system that will accommodate the
design ow rate while maintaining the pressure drop and ow velocity within
reasonable limits.
Most piping systems use pumps to develop the pressure or head required to
maintain the system design ow rates. Piping system pressure drops must be main-
tained within reasonable values to limit the installed size of the system pumps and
their prime movers. Pump and prime-mover size limitations are necessary to control
initial system construction costs and continuing system operating costs. The optimum
pipe size is based on an economic tradeoff between the installed capital cost of
the piping system and the sum of the capital plus lifetime operating costs of the
pumping system.
System ow velocities are limited by design to avoid a number of potential
operating problems. These problems have already been discussed in previous sec-
tions of this chapter. In the absence of any other formal or more limiting criteria,

TABLE B2.7 Reasonable Design Velocities for Water Flowing

through Pipes

Service condition Reasonable velocity, ft/s (m/s)

Boiler feed 815 (2.54.6)

Pump suction and drain lines 4 7 (1.22.1)
General service 410 (1.23.0)
City water to 7 (to 2.1)
Source: Crane Technical Paper 410, Flow of Fluids through Valves, Fittings,
and Pipe. The Crane Company, New York, 1985, pp. 36.

the ow velocities given for water in Table B2.7 and for steam in Table B2.8 are
considered reasonable for normal industrial applications.
The detailed uid ow design of a piping system requires the consideration of
a number of uid parameters including ow rate, viscosity, density, and pipe wall
frictional drag. Further discussions of this aspect of the pipe sizing process are
provided in Chap. B8.

TABLE B2.8 Reasonable Design Velocities for Steam Flowing through Pipes

Reasonable velocity V
Condition Pressure
of steam P [psig (kPa)] Service ft/min m/s

Saturated 025 (173) Heating (short lines) 4,000 6,000 20 31

Saturated 25 (173) and up Powerhouse equipment, 6,00010,000 31 51
process piping, etc.
Superheated 200 (1380) and up Boiler and turbine leads, etc. 7,00020,000 36100

Source: Crane Technical Paper 410, Flow of Fluids through Valves, Fittings, and Pipe, The Crane
Company, New York, 1985, pp. 316.
C.114 PIPING SYSTEMS

TABLE C3.6 Reasonable Design Velocities for Flow of Fluids in Pipes

Reasonable velocity

Fluid Pressure, psig Use Ft/minute Ft/second

Water 2540 City water 120300 25
Water 50150 General service 300600 510
Water 150 up Boiler feed 6001,200 1020
Saturated steam 015 Heating 4,0006,000 67100
Saturated steam 50 up Miscellaneous 6,00010,000 100167
Superheated 200 up Large turbine 10,00020,000 167334
steam and boiler leads

Source: Courtesy of Stone & Webster.

discharge of the condensate from the trap vessel. Steam traps are discussed in detail
in Chap. A2.

Determining Reasonable Flow Velocity. Before proceeding beyond a preliminary

layout of piping systems, it is necessary to determine pipe sizes which allow reason-
able velocities and friction losses. The maximum allowable velocity of the uid in
a pipeline is that which corresponds to the permissible pressure drop from the point
of supply to the point of consumption or is that which does not result in excessive
pipeline erosion. The economic optimization of line sizes is discussed in Chap. B8
in this handbook. The values of velocity listed in Tables C3.6 and C3.6M (Metric)
are reasonable for use in such cases. The lower velocities should be used for small
pipes, and the upper limits for large ones. These values represent good average
practice and may be used as a guide in many cases where actual pressure drops
are not computed. Additional steam line velocity information is given in Chap. B8
in this handbook.
Erosive action on valve seats and similar exposed parts also affects permissible
velocity. This action is much more pronounced in the case of wet steam than with
superheated steam, and velocities should be correspondingly lower when there is
much moisture in the steam.
High velocities are sometimes used in dry steam lines where excess pressure
exists, to absorb the higher pressure drop. The high velocity is not in itself objection-

TABLE C3.6M (Metric) Reasonable Design Velocities for Flow of Fluids in Pipes

Reasonable velocity

Pressure m per m per

Fluid (kPa gauge) Use minute second
Water 172276 City water 3691 0.611.52
Water 3451034 General use 91183 1.523.05
Water 1034 up Boiler feed 183366 3.056.10
Saturated steam 1103 Heating 12201830 20.430.5
Saturated steam 345 up Miscellaneous 18303050 30.550.9
Superheated steam 1380 up Large turbine 30504570 50.976.2
and boiler leads

Source: Courtesy of Stone & Webster.