Hydrocarbon Processing - December 2020 - 70

Maintenance
and Reliability
M. G. CHOUDHURY, Consulting Engineer,
Thane, Maharashtra, India

Stress qualifications for standard
and nonstandard valves in piping systems
Various types of valves are used in refineries, petrochemical
plants, fertilizer plants and utilities. As a part of a low- or highpressure/temperature piping system, these valves are subjected to fluid pressure/temperature, as well as to external loads
from a connected piping system. A design engineer should
perform a stress analysis of stress-critical piping systems for
qualifying the piping system (including fittings, valves, etc.),
as per applicable design codes.
An approach is defined here to highlight the requirements
for stress qualification of the standard pressure-temperature
(P-T) rating for nonstandard valves, either individually or as
part of piping system in which they are installed.
Importance of valve rating. In a piping system, piping com-

ponents like valves, flanges and other elements are essential for

the safe and reliable operation of the plant, but they are also
much heavier and/or stiffer than the pipe and pipe fittings. A
stress analysis of these critical piping systems is performed with
the use of piping stress analysis software. Such software considers the stiffer components as rigid elements and does not calculate the stress on these elements.
When the piping stress analysis software computes the stiffness matrix and solves for loads, the rigid valve will transfer the
load from one side of the pipe to the other. In this analysis, a rigid-body element does not deform or show induced stress values
or failures. However, if the piping stress analysis software does
not consider the imposed piping load and moments on valves for
stress qualification, and if it does not predict the stress failure of
the rigid elements, then what is the basis for qualifying the total
piping system? The basis must include standard P-T rating valves
for different load cases including sustained, thermal and others.
Standard P-T rating valves must be qualified with the use of
finite element analysis (FEA). The structural strength of valve
bodies and flanges must be analyzed under the composite action of fluid pressure and external piping loads. Also, some piping systems contain nonstandard valves, such as the slide valve
in an FCC unit. What approach should be followed for qualifying these valves?
Case study for stress qualification of standard P-T
valves. A hypothetical high-temperature and high-pressure

FIG. 1. High-temperature/high-pressure piping system with vertical loop.

TABLE 1. Piping details and process design parameters
Size

14 in.

Means of compliance

ASTM A335 P22 (seamless)

Pressure rating

1,500 lb

Operating pressure, P1

42.5 kg/cm2g

Design pressure, P2

47.3 Kg/cm2g

Hydrotest pressure, HP

138.4 Kg/cm2g

Operating temperature, T1

400°C

Design temperature, T2

515°C

DECEMBER 2020 | HydrocarbonProcessing.com

piping system configuration with a vertical loop (FIG. 1) is considered for the case study. The important parameters and process design conditions are given in TABLE 1. This piping configuration, along with the valve assembly, is analyzed by the piping
stress analysis software for different load cases.
Analysis results for this case show that piping system stresses are higher in both the expansion and sustained conditions;
however, these stresses are within the ASME B31 code limits
(TABLE 2 and TABLE 3). The code expansion stress ratio and the
code sustained stress ratio represent the ratios between generated and allowable stresses. W + P + T and W + P represents the
operating and sustained load cases, respectively, where W, P and
T are the operating weight, pressure and temperature of the piping system for the respective operating conditions.
TABLE 5 shows that flange leakage calculations based on
the generated piping moment are performed, and stresses are

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