Mr. Stanky

For the closed discharge system in safety and relief valve, the forces acting on piping system during steady state flow may be considered as self-equilibrium. This means the forces do not create large moment or stresses on piping system. However, unbalanced transient force will act on piping system by traveling of pressure wave during transient flow. So, your analysis need to include these transient forces applied on each elbow points.
Generally the safety/relief valve opens very short time, within few millisecond and the fluid through the valve orifice will be chocked condition with acoustic velocity(Mach=1). And the flow of downstream will be supersonic or subsonic flow, and sometimes may generate shock wave. These flow conditions will depends on the condition of downstream such as back pressure, pipe size and length, frictional effects,

In my experience, this transient force is not significant for the flare systems in chemical and petrochemical plant. Therefore; I add just some guides and stoppers on the system from safety valve to flare header without detail calculation or analysis.
However, if you want to check it more detail, it will be very difficult and tedious job to get reasonable transient forces without the special software. I had used the SRVA which was developed by Sargent & Lundy to generate time history data for the main steam line safety valve system of nuclear power plant about 6 years ago. This analysis requires many physical properties and valve information. Also, Caesar II can generate time history data for safety and relief valve system and it would be helpful to you, but I have never used this module because I just check this force by adding supports without detail analysis.

The best way, I think, is to check it by conservative methods with simple hand calculation or the practices in Code and Standards as Mr. John Breen mentioned. Also, I recommend one article, Analysis of Power Plant safety and Relief Valve Vent Stacks, Transactions of the ASME, October 1975, G.S.Liao. I remember Caesar II also refer to this article in Safety/Relief Valve Analysis.