What is the typical leakage rating chosen for steam control valves these days? ?

The sealing grades for control valves specified in the American standard ASME B16.104 are divided into six levels based on the amount of leakage; the highest grade, level 6, implies zero leakage. According to another American API standard, specified test pressures and leakage limits are set for valves undergoing hydrostatic and sealing tests. You can check the relevant standards, and also consider the requirements of the process system regarding leaks

This post was last edited by feng*aosa on 2019-11-19 22:02. Choose between III and V as needed. 　In fact, even for a Class III valve, a leakage of one thousandth is not a problem for the steam system. The selection of the leakage level is actually a matter of choosing the experimental requirements for that leakage level, as well as the corresponding design and manufacturing requirements. 　Grade III leakage: The leakage test is required to be conducted under a pressure difference of 0.35 MPa. In other words, if the operating pressure difference is greater than 0.35 MPa, the actuator simply cannot close the valve due to the unbalanced force. Therefore, when the operating pressure or pipeline pressure is greater than 0.35 MPa, a Class III valve should not be selected. And valves with a pressure lower than this can be used (in fact, grade II valves can also be employed). Of course, it’s best to leave some margin ； 　The calculation methods for leakage rates of Class IV valves and Class V valves are different. Class IV is measured as a percentage of the rated flow rate, while Class V is measured based on the valve seat area. So the leakage of a Class V valve is not necessarily smaller than that of a Class IV valve ； 　Both Class IV and Class V valves are subjected to leakage tests using the maximum operating pressure as the test pressure difference; therefore, there is no need to consider the issue of failure to close properly that occurs with Class III valves ； 　The main difference between Class IV valves and Class V valves is that Class V valves must be able to be used for shut-off purposes, whereas Class IV valves do not have this requirement. So the choice mainly depends on whether the process requires cutting. Put another way, it is about determining whether manual stop valves need to be installed before and after the valve. 　Class VI valves are designed for valves with O-ring seals; such valves are at least not used in steam systems.

This is closely related to the process requirements, such as the usage requirements and quantities in the downstream section under reduced pressure, the pressure difference upstream and downstream of the pressure reducing valve, the fluctuations in steam pressure in the upstream section, and the moisture content of the steam. If steam is used intermittently in the later stages or there is a large pressure difference between the upstream and downstream areas, the higher the leakage rating of the pressure relief valve, the shorter its service life will be

The standard ball valves from general valve manufacturers are of Class IV!