Estimation of reaction forces

This can be evaluated using the Omega method, if applicable (see Annex 8) or other HEM model (see Annex 4). The Omega method can be used to obtain G and the exit choke pressure, PE for the upstream pipe. An isenthalpic flash calculation can then be performed from the stagnation pressure at the start of the pipe to the choke pressure, PE, in order to evaluate the mass fraction of vapour, x, at the pipe exit. If the flow is not choked, then the term (PE - Pa) becomes zero. 

DIERS[1) presented a series of design charts, based on the Omega method, which can be used to evaluate the thrust force. These charts do not include the dynamic load factor, FD. If a load is suddenly applied, as will be the case following operation of a relief system, the piping will. experience a dynamic load of approximately twice the applied load. It is therefore usual to use a dynamic load factor of 2 in equation (12.1). Leung121 also discusses the use of the Omega method to calculate reaction forces. 

It is difficult to use the Omega method to obtain the pressure at a bend, but some estimate can be made knowing the upstream pressure and downstream choke pressure. 

DIERS recommended that reaction forces for both the two-phase flow and gas/ vapour-only flow be evaluated because the forces for gas/ vapour flow may be the larger. For the frictionless flow of ideal gas in a constant diameter bursting disc system, the thrust force is given byt3]  

DIERS[1] gives the following equation for the thrust force due to flow of an ideal gas in a safety valve system  

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