Reaction force for Relief valve in close system.
As mentioned in API 520,
Pressure relief devices that relieve under steady-state flow conditions into closed system
unsually do not create large forces and bending moments on the exhaust system, Only at
points of sudden expansion will there be any significant reaction forces to be calculated.
Closed discharge systems, however do not lend themselves to simplified analytical
techniques. A complex time history analysis of the piping system may be required to obtain
the true values of the reaction forces and associated moments.

By API 520 the reaction force in an open discharge system would be

F=129.W. SQRT[K.T/(K+1).M] + 0.1.A.P

I am having Closed system line with psv of 6" x 4"
set pressure of 38 bar
selected Orifice area 6.38 sq in.
opertating temp 46 deg c,
Molecular weight 22.52
Ratio of specific heat 1.44
Flow of gas 103,700 kg/hr

If my system would have been open system
the force would have been approx 3 tons,

But in my present case, can any body throw light on what would be the loads coming in closed system,
is there some thumb rule or common practice to calculate the reaction force in closed system.