What are the advantages and disadvantages of taking pressure readings inside and outside the self-acting pressure control valve, as well as before and after the valve?

This post was last edited by FightingNow on 2015-6-29 09:41. For pressure measurement inside and outside the valve, I believe that measuring pressure inside the valve is generally used to adjust the sensitivity of the valve’s operation, and it is usually used in combination with pressure measurement before and after the valve. Taking pressure readings outside the valve refers to taking readings before and after the valve; there are no advantages or disadvantages to this approach – it simply means that the pressure that can be controlled differs. Taking pressure readings before the valve allows for control of the pressure prior to the valve, while taking readings after the valve enables control of the pressure downstream of the valve. It seems there are no such things as advantages or disadvantages; it’s just that different applications have different requirements! ....

　　The so-called “pressure sampling” is actually pressure measurement sampling. And the purpose of measurement is to control it. Therefore, whether pressure is taken before or after the valve determines whether the self-acting valve is intended to control the pressure before or after the valve. 　　 　　External pressure sampling refers to the process of introducing pipeline pressure into the measuring diaphragm chamber of a self-acting valve by installing sampling tubes outside the valve ； 　　Pressure tapping inside the valve refers to creating a passage within the valve or installing pipes on the valve body to introduce pressure into the measuring diaphragm chamber of the self-acting valve. 　

I’m also looking at it; I’m also concerned about this issue

Someone upstairs has already explained clearly what pressure tapping inside and outside the valve means, as well as what pressure tapping before and after the valve means. Speaking of advantages and disadvantages, taking pressure readings before and after the valve serves different control purposes; there are no real advantages or disadvantages ； There are some differences between pressure sampling inside and outside the valve. With external pressure sampling, since it takes place outside the valve body, it is sufficient to remove the corresponding pipes for maintenance and cleaning. In contrast, with internal pressure sampling, as it occurs inside the valve body, the valve body itself must be disassembled in order to carry out maintenance and cleaning. It’s just that for the maintenance and cleaning of self-acting pressure control valves, it is necessary to stop operation and release the medium before carrying out these tasks; moreover, maintenance and cleaning usually involve disassembling the valve completely. Therefore, the difference isn’t very significant, and there are no real advantages or disadvantages to speak of. As for whether to use pressure tapping inside the valve or outside the valve, it depends on the specific structural design of the self-acting pressure control valve.

There’s something I don’t understand. Pressure sampling is taken before the valve; the pressure may originate from upstream or from the power generated after the moving equipment starts up. Pressure is taken after the valve; then where does that pressure come from? Also, since a self-acting valve needs to have pressure applied to it in order to open, is such a valve fully closed or fully open when there is no pressure in the pipes upstream and downstream of it?

There are no distinct advantages or disadvantages between measuring pressure before or after the valve; it depends on your control requirements – whether you need to control the pressure before or after the valve. In other words, it comes down to the location where pressure is measured: inside or outside the valve. Simply put, it’s a matter of whether pressure guide tubes are used or not. Inside the valve, there is a structure similar to a pressure guide tube built within the valve body; it is not visible from outside the valve. External pressure tapping involves making a hole at a point in the pipeline under your control and using a pressure lead to connect it to the diaphragm head. The advantages and disadvantages of valves inside and outside the valve – something that wasn’t mentioned earlier might be related to heat tracing and insulation. For pressure tapping outside the valve, heat tracing and insulation may need to be considered; sometimes, when the temperature of the medium is high, an isolation tank (such as SAMSON) is also required. For pressure tapping inside the valve, there’s no need to consider these factors (such as Fiher); one just needs to follow the process pipeline. The pressure measurement behind the valve mentioned upstairs refers to the pressure behind the valve body itself. When there is no pressure in the pipeline, it should be fully open.

This post was last edited by cqdfwy on 2017-7-31 at 16:00. When there is no pressure in the pipes before and after the valve, the diaphragm inside the diaphragm-operated pressure control valve is in a free state; therefore, whether the valve core is in the fully closed or fully open position, opening the manual valve upstream will not affect the flow of the medium. Balance is established after just an instant, thereby enabling pressure control.