# How to model Relief Valve (Pressure Safety Valve) thrust force in START-PROF

START-PROF can estimate the support loads and stresses caused by relief valve discharge by static method.

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. There's a several methods to estimate the dynamic equivalent thrust force F:

• ASME B31.1 method
• Teploelectroproject RTM 34 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, N. May be computed by the following equation: where

W - mass flow rate (relieving capacity stamped on the valve by 1.11), kg/sec

gc=1 - gravitational constant,

Pa - atmospheric pressure, N/m²

A1 - exit flow area, m²  A1=p(D-2t)²/4

V1 - exit velocity (node  36), m/sec P1 - static pressure, N/m² h0 - stagnation enthalpy at the safety valve inlet, MJ/kg

J = 999835.0529 m*N/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 Input data:

• Diameter of discharge pipe: 0.219 m

• Wall thickness of discharge pipe: 0.016 m

• 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(0.219-2∙0.016)²/4 = 0.027465 m²

a = 1.9143 MJ/kg

b = 4.33

W = 48∙1.11 = 53.28 kg/sec

stagnation enthalpy for steam at 64 kgf/sm² and 538°C h0 =  3.506 MJ/kg

J = 999835.0529 m*N/MJ

P1=53.28/0.027465*(4.33-1)/4.33*(2*(3.506-1.9143)*999835.0529/(2*4.33-1)/1)^0.5 = 961693 N/m²

V1 = (2*999835.0529*(3.506-1.9143)/(2*4.33-1))^0.5 = 644.6 m/sec

F1 = 53.28*644.77 + (961693-100000)*0.027465 = 58019 N

F = DLF ∙ F1 = 2*58019 = 116039 N 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:   