 However, sudden closing of a valve in a piping system may lead to water hammer or implosion so in special cases there may be additional items connected to the shutoff valve, such as a pressure relief valve or an aerator valve.For shutdown valves used in safety instrumented systems it is essential to know that the valve is capable of providing the required level of safety performance and that the valve will operate on demand.The required level of performance is dictated by the Safety Requirements Specification, where eventually a Safety Integrity Level (SIL) is indicated.