Design documentation requirements for spring-loaded pressure relief systems are very detailed, based on the definition in API Standard 521, 6th edition. The Installation Documentation section of API Standard 521 recommends provision of inlet and outlet piping pressure drop calculations as part of a complete analysis.
Inlet pressure drop is an important factor, since it is considered that excessive inlet pressure drop will affect pressure relief valve performance, and potentially result in chatter which may damage the valve and piping. In situations where inlet pressure drop exceeds 3% of set pressure for spring loaded pressure relief valves, previous editions of API Standard 520 Part 2 have allowed for an "engineering analysis" to be conducted for these systems - however the content of the engineering analysis has never been well defined.
The latest version of API Standard 520 Part II provides a more detailed description of an engineering analysis and allows a Force Balance Assessment to be conducted for spring loaded pressure relief valves exceeding 3% inlet pressure drop. This method can predict valve stability during relief flow, and identify whether chatter, flutter or stable flow is likely to occur. Valve characteristics such as valve opening and closing times are critical input parameters for this analysis. If stable operation can be demonstrated, then expensive mitigation efforts, such as modifications to piping or the valve itself, can be avoided.
This paper presents an overview of the force balance assessment in a step-by-step manner. Additionally, a sample of spring-loaded pressure relief systems which exceed the 3% inlet pressure drop criteria is reviewed. The aim of this review is to determine the effectiveness of conducting a force balance assessment, and to identify which systems show stable operation, and which systems were identified as likely to chatter.