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.