API-521 Addendum May-2008 Clause 5.19.5 provides some tools to take credit of low pressure system piping capacity to reduce the relief rate due to tube rupture.
However, these are recommendations & the interpretation is left to engineer to take the suggested credit.
In order to get experts opinion / confirmation, I have put my interpretation for respective clause (marked in red). Request you to confirm / correct my understanding for the same.
API-521 Addendum May-2008 Clause 5.19.5 states following;
- To determine the influence of piping, either in eliminating the need for a relieving device or in reducing relieving requirements, the configuration of the discharge piping and the contents (liquid or vapor) of the low-pressure side should be considered.
- Interpretation of clause: In case of compressible fluids on high pressure side, piping volume can be considered as expansion vessel that may drop pressure below design pressure of low pressure side. In that case PSV on low pressure side can be eliminated.
- If the low-pressure side is in the vapor phase, full credit can be taken for the vapor handling capacity of the outlet and inlet lines, provided that the inlet lines do not contain check valves or other equipment that could prevent backflow.
- Interpretation of clause: In case of compressible fluid on low pressure side, PSV capacity shall be equal to volumetric flow rate that low pressure side piping can handle.(Considering limitation of Mach no / available pressure drop margin?)
- If the low-pressure side is liquid-full, the effective relieving capacity for which the piping system may be credited shall be based on the volumetric flow rate of the low-pressure side liquid that existed prior to the tube rupture. However, if a detailed analysis is performed, a capacity credit may be taken for acceleration of the low-pressure side liquid.
- Interpretation of clause: In case of incompressible fluid on low pressure side, then PSV capacity = mass flow calculated for tube rupture (as guided by API) – equivalent mass flow considering same volumetric flow of low pressure side.
- If the piping system to the low-pressure side of heat transfer equipment contains valves, their effect on the capacity of the system when overpressure occurs should be taken into account. Valves provided only for isolation may be assumed to be fully opened. In calculating relieving-capacity credit for the piping system, one should consider the valves used for control purposes to be in a position equivalent to the minimum normal flow requirements of the specific process. However, this assumption cannot be made if the valve could automatically close because of the emergency situation.
- Interpretation of clause: Limitation on capacity due to control valve opening can be considered as PSV capacity, provided the valve is not provided with any interlock to open / close considering emergency.
- Thanks & Regards,