1. SCOPE …………………………………………………………………………….2. REFERENCE DOCUMENTS 3. DEFINITIONS 3
4. GENERAL ………………………………………………………………………..4.1 Closed Discharge Systems 4.2 Open Discharge Systems 4.3 Flare Relief Systems ……………………………………………………..4.4 High Pressure Relief Systems (Reciprocating Compressors) 4.5 Liquid Relief Systems 4.6 Open Systems ……………………………………………………………..
1. Scope
This standard establishes design requirements for piping in thermal relief systems. ASME B31.1, B31.3,
B31.4, and B31.8 codes shall form the basis for calculating piping loads due to thermal, dead weight,
seismic, wind, impulse, and harmonic conditions where applicable.
2. Reference Documents
Reference is made in this standard to the following documents. The latest issues, amendments, and
supplements to these documents shall apply unless otherwise indicated.
SABIC Engineering Standards (SES)
P01-E01 Design Conditions and Basis for Pressure Piping
P01-E02 Design of Piping Systems for Stress and Pressure Criteria
P01-E03 Flexibility, Support, and Anchoring of Piping Systems
3. Definitions
Back Pressure.
The pressure existing at the outlet of the automatic pressure relieving device due to
pressure in the discharge system.
Closed System.
An installation where the effluent is conveyed to a distant location by a discharge pipe
that is connected directly to the outlet of the safety valve.
Open System.
An installation where the effluent is discharged directly to atmosphere or to a discharge
pipe or stack that is uncoupled from the safety valve.
Owner.
SABIC
Relief Valve.
An automatic pressure relieving device actuated by the static pressure upstream of the valve
which opens further with the increase in pressure over the opening pressure. Primarily used in liquid
service.
Safety Valve.
An automatic pressure relieving device actuated by the static pressure upstream of the
valve and characterized by full opening pop action. Primarily used in vapor or gas service.
Set Pressure.
The pressure setting at which the automatic pressure relieving device is adjusted to open
under the specified operating conditions.
4. Thermal Expansion Relief in Piping
4.1 Closed Discharge Systems
4.1.1 For closed discharge systems in which safety valves connected to relatively long runs of pipe are
discharged, emphasis should be given to anchoring and guiding the long run of pipe to account for
transient flow effects which may result from air being entrained in the discharge piping. Consideration
should be given to increasing the design pressure of the closed discharge pipe by a factor of 2 over the
steady state operating pressure to account for the possible pressure wave steepening into a shock wave.
4.1.2 Steady state reaction forces for closed discharge systems should be considered as self-equilibrating
and occuring at the point of discharge. Its magnitude shall be calculated in accordance with the
methodology set forth for open discharge systems in ASME B31.1 Appendix II Par.2.3.1.
4.1.3 When more than one safety valve discharges into a common header, the interaction loads from their
discharge shall be evaluated and considered in the stress analysis of the piping system.
4.2 Open Discharge Systems
4.2.1 The methodology for evaluating reaction forces associated with safety valve discharges in open
systems shall be in accordance with ASME B31.1 Appendix II Par. 2.3.
4.2.2 Pressure relief piping, including supports, shall be designed to resist reaction thrust loads from valve
discharge, minimise vibration, and allow for valve removal.
4.3 Flare Relief Systems
4.3.1 Flare headers shall have sufficient flexibility to allow for thermal expansion as well as for seismic,
wind, and when applicable, slug loading. Flexibility may be achieved with piping offsets and expansion
loops.
4.3.2 For flare relief systems in wet service and subject to slug flow, provisions shall be made to ensure
anchors are placed at every change in direction on straight run segments. Precautionary steps shall be
taken in the design of directional anchors to ensure rotation of anchors does not cause piping to bind at
supports.
4.3.3 For slug loading in flare relief systems, slug loads at an elbow may be determined through the
following formula:
F= (1.4.4(Rho)A(V)**2)/g
Where:
Rho = fluid density (lbm/ft**3)
A = cross-sectional flow area (ft**2)
V = velocity of fluid (ft/sec)
g = gravitational constant (32.2 (ft-lbm)/(lbf-sec**2)
In the absence of actual velocity data, a velocity of 30 ft/sec shall be used.
4.3.4 Flare headers within the radiant heat circle of flare stacks shall be reviewed for the potential of
thermal bowing.
4.4 High Pressure Relief Systems (Reciprocating Compressors)
For relief systems in high pressure reciprocating compressor applications, provisions shall be made to
minimize relief valve vibration due to harmonic and impulse loading through the use of holddowns, guides,
and directional anchors on valve inlet and
valve discharge piping.
4.5 Liquid Relief Systems
Reaction loads resulting from relief valve discharge in liquid service shall be calculated as follows:
F = (SG*(GPM))/722A
Where:
F = Reaction force, lbs.
SG = Specific gravity of commodity
GPM = Flowrate, in U.S. gallons/minute
A = Area of outlet pipe, inches
4.6 Open Systems
4.6.1 Pressure Safety Valves
a. Discharge piping may be either connected or uncoupled from the pressure safety valve.
Discharge stack piping directly connected to the pressure safety valve shall be guided to minimize
potential vibration during discharge. The tip of discharge piping open to atmosphere should be plain
end. Ends beveled at angles are permissible provided the resulting overturning moment during
discharge does not result in excessive bending stresses in the valve.
b. Fixed stack designs shall conform to ASME B31.1 requirements outlined in Appendix II par. 2.3.1.
4.6.2 Rupture Disks
a. Discharge piping connected to rupture disks shall be designed to consider the thrust load
resulting from the bursting of the disk. Discharge piping should be routed as straight as possible.
b. The thrust load resulting from a rupture disk may be calculated from:
F = 0.378*(Ko + 1)*A*P
Where:
F = Reaction force, lbs.
Ko = Ratio of specific heats, Cp/Cv
A = Orifice area of disk, in*2
P = Inlet pressure at time of opening, (set pressure + 14.7)