Modeling Relief Valve (Pressure Safety Valve) Thrust force
The relief valve discharge thrust load acts on elbow 28. In Start-Prof it should be applied at the end of the elbow in 29 node.
Download this piping model file. See how to open the piping model file
There’s a two methods to estimate the dynamic equivalent thrust force F:
- ASME B31.1 method
- Direct calculation of V1 и P1 by special software like Hydrosystem
ASME B31.1 Method
Equivalent dynamic thrust force can be estimated by equation:
F = DLF ∙ F1
where
DLF – dynamic load factor, depend on first natural period of piping. If period is unknown the DLF=2.0.
F1 – static reaction force, kgf. May be computed by the following equation:
where
W – mass flow rate (relieving capacity stamped on the valve by 1.11), kg/sec
gc=9.81 m/sec² – gravitational constant,
Pa – atmospheric pressure, kgf/sm²
A1 – exit flow area, sm² A1=p∙(D-2t)²/4
V1 – exit velocity (node 36), m/sec
P1 – static pressure, kgf/sm²
h0 – stagnation enthalpy at the safety valve inlet, MJ/kg
J = 101970.408 m*kgf/MJ
a, b – constants according to table below
| Steam Condition | a, MJ/kg | b |
| Wet steam
<90% quality |
0.6769 | 11 |
| Saturated steam
≥90% quality 1.05 kgf/sm² ≤ P1 ≤ 70.31 kgf/sm² |
1.9143 | 4.33 |
| Superheated steam
≥90% quality 0.07 kgf/sm² ≤ P1 ≤ 140.61 kgf/sm² |
1.93291 | 4.33 |
Example
Example project file: ReliefValve.ctp. See how to open the piping model file
Input data:
-
Diameter of discharge pipe: 21.9 sm
-
Wall thickness of discharge pipe: 1.6 sm
-
product: Saturated steam
-
relief valve relieving capacity: 48 kg/sec
-
Steam pressure: 64 kgf/sm²
-
Steam temperature: 538°C
Calculation:
A1=p∙(D-2t)²/4 = 3.14159∙(21.9-2∙1.6)²/4 = 274.65 sm²
a = 1,9143 MJ/kg
b = 4.33
W = 48∙1.11 = 53.28 kg/sec
stagnation enthalpy for steam at 70 kgf/sm² and 538°C h0 = 3,506 MJ/kg
P1=53.28/274.65*(4.33-1)/4.33*(2*(3.506-1.9143)*101970.408/(2*4.33-1)/9.81)^0.5 = 9.8 kgf/sm²
V1 = (2*9.81*101970.408*(3.506-1.9143)/(2*4.33-1))^0.5 = 644.77 m/sec
F1 = 53.28*644.77/9.81 + (9.8-1)*274.65 = 5919 kgf
F = DLF ∙ F1 = 2*5919 = 11838 kgf
Create additional occasional force loading 1.1 in operation mode editor:
If we have more relief valves, then we should add more additional modes (1.2, 1.3, etc.) if it’s open not simultaneously.
Apply dynamic thrust force at node 29 for mode 1.1:
Support loads due to relief valve thrust load:
Stresses due to relief valve thrust load:
