Control Valve

Quite a few threads have covered this aspect. In brief

PRV (Pressure relief valve) controls the system pressure. If the pressure crosses its set value it releases a bit of fluid through its relief port to maintain the pressure back to the set value (minus the hysteresis). It is an appendage if the pressure is below the set value. It is used normally to protect the system against the over-pressurisation. In the earlied thread I have given this as the pop weight of the presuure cooker. The actual PRV also is same, there is one conical orifice perpendicular to the flow direction. The orivice is closed by a spring loaded cone. When the pressure increases, the spring retracts and the extra fluid escapes, the orifice closing as soon as the pressure falls below the spring pressure.

PSV (Pressure Safety valve) This acts like an emergency switch and opens up when the pressure crosses the set value and releases the fluid. It usually releases the whole lot of it and not closes back. The example was the blowout valve of the pressure cooker. Once this is opened, the system pressure is undefined and ideally is the atmospheric.

FCV (Flow control valve) and also PCV (Pressure control valve) : This has to work on the feed back.Like the PRV there is the orifice and the cone. the opening is controlled by the down stream pressure and not by the spring. If the down stream pressure reduces, (or if the differential pressure across orifice is measured meaning flow has reduced ) then the cone is opened to allow lesser restriction to the flow and thereby increasing the flow. The reverse is the case if the downstream pressure increases and then the opening reduces to restrict the flow. This can only be effective if there is a significant flow, unlike the other two. Most of the PCVs or FCVs i have used uses a simple elastic diaphragm, pressurised by the two pressures on either side of it or the downstream pressure and a spring and the flexing of the diaphragm is translated into pin(cone) movement.

This is in breif. However you can go through the catalogues of Parker and a few others like masoneilan , ForbesMarshall, Tyco etc

These are what we have used, but a lot others are available too.

I am taking this opprtunity to elaborate PSV as per my understanding is pressure sustaining valve rather than pressure safety valve. Please reply considering pressure sustaining valve.

Guest #1

Yes, indeed. FCV (Flow control valve) and also PCV (Pressure control valve) :--

this elaboration of FCV & PCV is the root cause of confusion and that's why i am trying to conclude that pressure control valve (i.e pressure reducing valve) and flow control valve are similar equipment.

And similarly pressure sustaing valve is also nothing but similar like pressure reducing & flow control valve which maintain upstrem set pressure.

PSV as pressure sustaining valve (or back pressure sustaining) I have never had an opportunity of using. But as per the theory that I know it is the Flow/ pressure control valve connected in reverse (ie the diaphragm is operated by the upstream pressure) and as the pressure builds up it releases the fluid to maintain pressure and if it drops, it closes the orifice to choke the flow.

For PRV, Minus the Hysteresis????? Where or how to take this Value?? Thanks Sb

That can be defined by the manufacturer. purchaser.

In fact we define in our spec three values for pressure

1. Opening pressure (trip) or set pressure.

2. Closing pressure.

3. Pressure drop across valve at the rated full flow (ideally but actually never zero you know )

Guest #1, The title of your thread is "Control Valve" however two parts of your actual question address Relief Valves (PRV) and Safety Valves (PSV). These are not specifically considered "Control Valves". They are considered safety valves. The third part of your question does address a Control Valve. However your understanding of the function of a Flow Control Valve (FCV) is not correct.

Comments from another forum:

(http://www.eng-tips.com/viewthread.cfm?qid=182331)

Per the Instrument Society of America:

Safety Valve (PSV): An automatic pressure relieving device actuated by the static pressure upstream of the valve and characterized by full opening pop action. It is used for gas or vapor service.

Relief Valve (PRV): An automatic pressure relieving device actuated by the static pressure upstream of the valve which opens further with the increase in pressure over the opening pressure. It is used primarily for liquid service.

Safety Relief Valve (PSRV): An automatic pressure relieving device suitable for use either as a safety valve or relief valve, depending on application.

Your final about the difference from a manufacturing point of view can be best understood bu looking at the actual valves via or drawings, pictures or site visits.

Also:

Here is a link to pictures for "pressure safety valves":

http://images.google.com/images?client=firefox-a&rls=org.mozilla:en-US:official&channel=s&hl=en&q=%22Pressure+Safety+Valve%22&um=1&ie=UTF-8&ei=wtBdSs2tF-SEmQesnIh6&sa=X&oi=image_result_group&ct=title&resnum=4

Here is a link to pictures of some "pressure relief valves":

http://images.google.com/images?hl=en&client=firefox-a&channel=s&rls=org.mozilla%3Aen-US%3Aofficial&um=1&sa=1&q=%22Pressure+Relief+Valve%22&btnG=Search+images&aq=f&oq=

Here is a link for pictures for "Control Valves":

http://www2.emersonprocess.com/EN-US/BRANDS/FISHER/CONTROLVALVES/Pages/ControlValves.aspx

I hope this helps.

Gust #1

PSV stand for pressure sustaing valve rather than pressure safety valve taken into consideration while posting this discussion and that's why control valve title is selected.