Hydrocarbon Processing - December 2020 - 71

Maintenance and Reliability
within the code limit. TABLE 2 and TABLE 3 results also show that
calculated stresses for valves are within the code limit. From
TABLE 4, it is evident that piping stress analysis software does
not calculate stresses at rigid components, such as valves. The
expansion stress at valve nodes 40, 43 and 46 are zero.
Since the values of stresses at the valve nodes are zero, how
can the generated stress on the standard P-T rating for the valve
in the case study be determined to fall within the code limits?
Must the same valve be qualified independently by another
method? From the outlined analysis, it is evident that the piping
configuration-with the exception of the valve-has stresses
within the code limit. An analytical approach for the independent qualification of the valve is detailed in the next section.
Standard P-T rating valve stress qualification. For the

individual qualification of a valve, FEA of the valve body
structure must be performed to analyze the valve under the
conditions of pressure and temperature rating envelope, along
TABLE 2. Maximum expansion stress in piping system
Maximum expansion stress load case (W + P2 + T2 ) - (W + P1 )
Piping code: ASME B31.3-January 31, 2017
Highest stresses, N/mm2
Code expansion stress ratio, %

81.7

At node 58

Code stresses

196.8

Allowable stresses: 240.8

Axial stresses

5.1

At node 40

Bending stresses

191.7

At node 58

Torsion stresses

At node 59

Hoop stresses

At node 18

203.2

At node 58

Max. stress intensity

Result: Code compliance evaluation passed

TABLE 3. Maximum sustained stress in piping system
Sustained load case at operating pressure (W + P1 )
Sustained load case at design pressure (W + P2 )
Piping code: ASME B31.3-January 31, 2017

with the maximum possible imposed external loads from the
attached piping system.
When performing FEA analysis for the valve body structure, the judgement to be made of external loading from the
piping system for optimizing the valve design can be obtained
using ASME Boiler and Pressure Vessel Code (BPVC) Section
3: NC 3658.3 for dynamic loading, with some safety margins.
An example of such a judgment is the external moment of the
piping system = yield strength (YP) × the maximum possible
section modulus of the connected piping (Z) or the value of
the external moment for a flanged valve. The external loading
value derivation must be performed judiciously, as too much
leeway can result in an uneconomical valve design.
Valve manufacturers perform FEA analysis of these valves
based on type, size, material and P-T rating for design qualification prior to valve production, considering the maximum load
scenario as per valve P-T rating conditions. As allowable stresses of valve material (casting or forging) and section modulus are
usually relatively higher than the connected piping, the same
design valve that has already been qualified by the manufacturer
does not need to be analyzed or qualified for each new piping
system installation.
Manufacturers can also use an experimental method for
the qualification of standard P-T rating valves in place of FEA
analysis. In this method, the manufacturer applies actual internal pressure and external forces and moments in a properly designed test bench. The test bench loading can be applied repeatedly if fatigue effect must also be evaluated.
Nonstandard design (special valve) stress qualification.
Critical/special piping systems in which nonstandard valves
must be installed (e.g., valves used in FCC plant piping, such
as catalyst slide valves, fuel gas slide valves, etc.) require the
manufacturer to perform FEA for qualification, considering the
defined pressure and temperature envelope and the maximum
possible imposed external loads from the attached piping. This
analysis is required for several reasons:
1. In piping systems for such applications, the actual piping
load may be higher than the manufacturer's standard
TABLE 5. Flange leakage calculation at valve ﬂange joint

Highest stresses, N/mm2

Flange ASME BPVC Section 3 NC 3658.3 report

Code sustained stress ratio, %

74.9

W + P2 at node 80

Code stresses

53.2

Allowable stresses: 71

Axial stresses

8.9

W + P2 at node 20

Bending stresses

44.6

W + P1 at node 80

Torsion stresses

W + P1 at node 70

Operating load case design condition: W + T2 + P2
Node

Torsion Bending
moment, moment,
Nm
Nm
0

327,037

G/C,
mm

Flange
stress,
N/mm2

Allowable
pressure/
stress, N/mm2

Ratio,
%

635

190.28

213.25

89.23

Hoop stresses

19.5

W + P2 at node 18

326,643

635

190.05

213.25

89.12

Max. stress intensity

53.5

W + P2 at node 80

351,777

635

204.68

213.25

95.98

364,897

635

211.96

213.25

99.4

Result: Code compliance evaluation passed

TABLE 4. Expansion stresses at valves
Load case
Expansion

From node Code stress, N/mm2 Allowable stress, N/mm2 To node Code stress, N/mm2 Allowable stress, N/mm2 Piping code
40

ASME B31.3

Expansion

ASME B31.3

Expansion

ASME B31.3

Hydrocarbon Processing | DECEMBER 2020

Hydrocarbon Processing - December 2020

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