PRESSURE RELIEF DEVICES
A couple of weeks ago, during an API 570 inspection on a pipe that discharges to the flare system, a PSV was found and showed that the Set Pressure and the Cold Test Differential Pressure (CDTP) were exactly the same, even though it had a non-zero back pressure, and the questions immediately arose, why? Is it possible ?, someone might ask what is the reason. If we recall, the Cold Test Differential Pressure (CDTP) is a set pressure that is configured to be used for PRV (bench test) because under current conditions of use, PRV can be affected by back pressure and the operating temperature, therefore, when testing the valve at room temperature and without back pressure (bench test), it is necessary to adjust the set pressure to compensate for these factors. In other words, the shop test pressure = CDTP. Well the answer is YES, it is possible and for only one reason, there are valves equipped with balanced bellows and the Set Pressures and CDTP will be the same.
Keep these concepts in mind in your future inspections
Metallurgical Engineer/API Inspector
4 年NATIONAL BOARD tells how to calculate the CDTP for a PRD taking into account a temperature differential and a back pressure, CDTP is not more than the difference of the SET PRESSURE minus the superpositions exerted by the effect of the temperature and the back pressure of a valve of conventional design (without bellows). By the way, the temperature correction is given by a multiplying factor that will depend on the quality of the internals of each valve and this is supplied by the manufacturer's brand. For example DRESSER has its own tables of multiplying factors