October 2025
Special Focus: Valves, Pumps, Compressors and Turbomachinery
Using ASME B31.1 Appendix II calculation to identify oversized vent pipe diameter on boiler PSVs
The American Society of Mechanical Engineers (ASME) Power Piping Code B31.1, Appendix II is a non-mandatory appendix and provides the “Rules for the design of safety valve installations.” This Appendix to ASME B31.1 serves to provide designers with design guidelines for the installation of pressure safety valves (PSVs) and vent pipes.
Most PSVs on boiler installations are fitted with a vent pipe to route the PSV discharge steam and release it at a safe location. The vent pipe can be rigidly fixed to the PSV discharge elbow, or it can be the “umbrella” type. In the umbrella-type setup, the vent pipe is larger than the diameter of the PSV discharge elbow and is detached and sits above the discharge elbow, serving as a “hood.” The umbrella-type arrangement is generally used on PSVs fitted on steam boilers, as it allows for thermal movement of the boiler without affecting the position of the vent pipe. However, there is a likelihood of blowback occurring from the space between the PSV discharge elbow and the vent pipe inlet. This could be a hazard for personnel in the vicinity. As such, the ASME B31.1 methodology provides the criteria and the calculations to avoid blowback.
This article provides insight into ASME’s blowback calculation and takes it a step further by using the calculation to identify cases where the vent pipe diameter is oversized.
The nomenclature used in the ASME B31.1 methodology is detailed here:
W = Mass flowrate, pound mass per second (lbm/sec)
ho = Enthalpy at PSV set point, Btu/lbm
gc = Gravitational constant, 32.2 lbm-ft/lbf-sec2
J = 778.16 ft-lbf/Btu
a, b = Constants
P1 = Pressure (psia) at PSV discharge elbow outlet, calculated from ASME equation in Section II.2.2.1
P2 = Pressure (psia) at bottom of the vent pipe, calculated from P3 (P/P*) where (P/P*) is taken from the Fanno Line curve in ASME Fig.II-2.2.1-2
P3 = Pressure (psia) at the vent pipe outlet, calculated as P1*(A1/A3)
P4 or Pa = Atmospheric pressure, psi
A1 = X-sectional area (in.2) of PSV elbow
A2, A3 = X-sectional area (in.2) of PSV vent pipe at inlet/outlet
V1 = Velocity (ft/sec) at PSV elbow outlet calculated from ASME equation in Section II.2.2
V3 = Velocity (ft/sec) at vent pipe outlet calculated as V3 = V1
V2 = Velocity (ft/sec) at vent pipe inlet calculated as V2 = V3* (V/V*) where (V/V*) is taken from the Fanno Line curve in ASME Fig.II-2.2.1-2
k = Specific heat ratio=1.1 for saturated steam
ASME METHODOLOGY FOR PSV VENT PIPE DESIGN
ASME B31.1, Appendix II provides the detailed calculation method for determining the design pressure and velocity in the PSV discharge elbow and the vent pipe. This method is summarized in the following steps:
Step 1. Calculate the pressure (P1) that exists at the PSV discharge elbow outlet (Eq. 1):
Step 2. Determine the velocity (V1) that exists at the discharge elbow outlet (Eq. 2):
Step 3. Determine the pressure (P3) that exists at the vent pipe outlet (Eq. 3):
Step 4. Determine the velocity (V3) that exists at the vent pipe outlet (Eq. 4):
Step 5. Determine the velocity (V2) and pressure (P2) that exist at the vent pipe inlet. Velocity (V2) and pressure (P2) are back-calculated from (V3) and (P3) using the friction losses in the vent pipe f*Σ (L/D) and the Fanno Line curves presented in Appendix II, Fig. II-2.2.1-2.
Final step. Check for blowback at the open space where the PSV discharge elbow is inserted into the vent pipe by the following inequality [momentum check equation (Eq. 5)]:
The left side represents the momentum at Points 1 and 2 (FIG. 1), and the right side represents the forces at Points 1 and 2. If this inequality is satisfied and the momentum is greater than the opposing forces, blowback will not occur.
FIG. 1. Typical boiler steam drum PSV with an umbrella-type vent pipe.
Using the ASME method to identify an oversized vent. At a client’s facility, the steam boiler is fitted with an umbrella-type PSV with a setpoint of 275 psia. The PSV is expected to relieve 48,000 lbs/hr of saturated steam. The size of the PSV elbow is 4-in. nominal pipe size (NPS), and the size of the vent pipe at the elbow outlet needed to be evaluated. The choice was to select a 6-in. vent pipe or an 8-in. vent pipe.
It was suspected that the vent pipe could be oversized at 8-in. NPS. Therefore, a calculation was performed for the 6-in. vent and the 8-in. vent, using the ASME methodology (FIG. 2).
FIG. 2. Example calculation using B31.1 methodology for an oversized 8-in. vent and a normal 6-in. vent.
A calculation summary is shown in FIG. 3.
FIG. 3. Calculation summary shown pictorially.
In FIG. 2, it was observed that the first term (P2-Pa) on the right side of the Momentum Check Equation was less than the second term (P1-Pa)A1 on the right side. This apparent “anomaly” indicated that the 8-in. vent pipe was oversized, and it shows that the forces at the vent pipe inlet [(P1-Pa)A2 are less than the forces at the PSV elbow exit (P1-Pa)A1. This was confirmed when another calculation was done using a smaller 6-in. vent pipe. Results with the 6-in. vent pipe indicated no blowback per the momentum check equation (Eq. 5), and at the same time, the value of (P2-Pa)A2 was greater than (P1-Pa)A1.
Result. For the 8-in. vent pipe, the right side of the momentum check equation (Eq. 5) is negative, indicating that the 8-in. vent pipe is oversized.
Takeaway. The momentum check equation (Eq. 5) is meant to ensure that no blowback occurs. At the same time, the values on the right side of the equation provide an indication of whether the vent pipe is oversized or not. If the first term on the right side of the equation is less than the second term, this is an apparent “anomaly” that is indicative of an oversized vent pipe.
ACKNOWLEDGEMENT
The author would like to thank Mohinder Nayyar, ex-chair of the ASME B31.1 Committee and editor of Piping Handbook for reviewing this paper and providing valuable comments.
The Author
S. Zaheer Akhtar is a Senior Technical Process Engineer with TAI Engineers and has more than 35 yr of experience in the chemical, gas and power generation industries. Akhtar served for 9 yr with Exxon Chemicals at its ammonia-urea manufacturing plant and more than 20 yr with Bechtel Power Corp. and its affiliates, such as Bechtel Oil & Gas, Bechtel Nuclear and Bechtel Egypt. He also worked as a Plant Engineer at the Ghazlan Thermal Power Plant in Dammam, Saudi Arabia with Saudi Electric Co. Akhtar has been a participant member of ASME Performance Test Code Committees 4.3 (Air Heaters), 19.2 (Pressure Measurement) and 19.3 (Temperature Measurement). He has also authored several papers covering various process/mechanical topics related to industry. Akhtar earned an MS degree in chemical engineering from the University of Manchester Institute of Science & Technology (UMIST) – England, and is a Professional Engineer registered in the state of New York.
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