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.

Your search for a rule-of-thumb answer may not be fulfilled.

First of all, if the relief system of yours involves a Category M Design, you'll have to get a FULL fluid transient study, and follow up with a time-history (or response spectrum)analysis of CAESAR II program. Granted, a fluid transient study using either analog or digital computer isn't readily accessible for most stress engineers. Alternatively many do develop his or her methods to approximate the impact loads on a CONSERVATIVE basis. But the truth is, no one in this Forum will likely volunteer his or her calculations for your C-M Design, considering the grave risk that may incur.

On the contrary if it is NOT a C-M Design, all you need is to observe the industry RULE on closed-discharge relief systems, viz: (1) ignore the thrust loading, and (2) reinforce the branch connection to the header or knock-out drum where it connects. The degree of reinforcement depends on circumstances. For a low pressure system, a simple reinforcing pad on branch will usually suffice. But for a higher pressure system relief, like yours at 550 psi, acoustic vibration could come into play. Perhaps you can use a full encirclement pad on header and a thick wall pipe branch extending to a few diameter distance away.

One excellent article on this subject you should search for is "Review Relief Systems for Mechanical Integrity" published in HydroCarbon Processing Magazine/June 1999. All authors, including Mr. Joe Okopinski, an expert stress engineer, were from KBR, Houston, Texas.

So long.