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

Read
about START-PROF pipe stress analysis software

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

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: