5 replies to this topic

[flarenuf]

Hello all,

 

i am conducting a poll here to find out the different conceptions that exist as to how a PSV works in respect to pressures especially to when a PSV reaches fully open .

Some engineers see a PSV fully open at 110% of set with the %open vs press curve between 100% and 110% either being linear or specific to a manufacturers valve.
Similarly for the drop in pressure from 110% down to 100% , linear vs pressure
others take fully open at set pressure and start to close at closing pressure so teh valve is in effect still fully open till 95% of set.

 

using simple figures below perhaps some of you could comment as to yes or no in respect to your views.

( i am polling general ideas here not exact)

 

 

                                 121% max accum pressure for fire case

valve fully open        110%  max accum pressure for non fire case

set pressure             100%  ( design pressure say)

start to open               98%

closing pressure         95%

reseating pressure     93%

 

see attached dwg

looking fwd to comments on this

flarenuf

 

 

 

Attached Files

•  API521 PSV pressures_002.bmp   561.24KB   96 downloads

Edited by flarenuf, 04 February 2014 - 11:12 AM.

[Lowflo]

I think you need to more narrowly define this question because the answer will vary depending on liquid vs gas scenario, liquid vs gas trim, and blowdown ring setting.

 

My opinion is that the actual values (answers) will vary widely, even after tightening the phrasing of the question. Manufacturers are probably the best source for general information on this subject. I say "general" because I don't think they have the test data to provide specific answers. 

[Mark-TR]

I do agree with Lowflo, vendor are the best source.

 

please note that opening curve changes with manufacturer and type of valve.

 

-A valve can be pop-up action or proportional

-it can be conventional, balanced or pilot operated.

 

the linear approach is just a rule of thumb in order to model the opening of a PRV.

 

But in fact what PRV vendor guarantees is that the PRV will be fully open at the required overpressure and provide the corresponding flowrate. 

 

I advise you to look in Pentair website for the engineering handbook, you will see the different curves for different types of PRVs

http://valves.pentai...cm106-35825.pdf

 

as Lowflo said these curves are general approach and should be considered as a rule of thumb approach.

 

kind regards

[paulhorth]

Flarenuf,

I will first admit that I don't have any specialist knowledge about the design or testing of PSVs, so I am ready to be corrected on what I say below.

Considering a conventional valve in gas service, my understanding of how the valve opens is as follows:

• The valve disc has a larger diameter than the valve seat, with an overhanging lip.
• At the set pressure, the upstream pressure on the valve disc balances the spring force, and the valve disc starts to lift.
• as soon as the disc lifts off the seat, the shape of the disc edge is such that an additional area is exposed to the upstream pressure.
• This additional area provides additional lift force which is enough to compress the spring and lift the disc to full lift. This is the POP action which I always understood to achieve full opening at the set pressure. The lift required is small relative to the spring length, so the extra force needed is not much.
• Transiently. at initial opening, the back pressure has not yet built up. When flow is established in the tailpipe, the built-up back pressure can be up to 10% of the set pressure, so the upstream pressure also has to rise by up to 10% to keep the valve in balance, or the valve will tend to close. That is the reason for the 10% back pressure rule with a conventional valve.
• But - since the valve is always larger than required for the calculated relief flow, the pressure might then fall a little until the back pressure closes the valve until the reduced capacity matches the calculated relief load. I don't know if this happens.

I know this analysis does not quite line up with your diagram, so it might be incorrect. I also understand that pilot-operated valves can have a modulating rather than a Pop action, and also valves in liquid service are different.

 

The valve recloses when the pressure has fallen below the set pressure, because of the effect of the exposed area of the valve disc, and the flow passage, which can be adjusted with the blowdown ring.

 

Paul

[sukanta87]

I agree with lowflow. Thanks paul for your simple and easy explanation.

Would anyone suggest how to determine the POP-off relief valve's (indicated in yellow zone)  required flowrate? Though it depends on circumstances or scenarios. Informed from the experienced personnel that it's 5% of the stream flowrate i.e; 0.15 MMSCFD in this case for sizing the safety valve. Some literature says it can be also 100 % (3 MMSCFD). And here am considering this for regulating and metering station (RMS) only.

 

 Untitled.jpg   32.97KB   4 downloads

 

 

 

[fallah]

Hi,

 

Appears those valves you mentioned as relief valves to be self regulating PCV...Anyway, in general, with limited available info:

 

If there would be the possibility of blockage at downstream, i.e. at boiler side, because credit cannot be taken for PCVs to be in closed position the required relieving rate from each relief device should be half of the normal flow rate (1.5 MMSCFD if 3 MMSCFD is normal flow) through the station.

 

If there would be the possibility of each PCV failure to be in wide open position the required relieving rate of relevant relief device is the difference between the maximum flow rate (PCV wide open) and normal flow rate passing through that PCV (might be around 10 percent of 1.5 MMSCFD as normal flow rate passing through the each branch).

 

The actual required relieving flow rate for each relief device would be the higher value in two mentioned cases...

Edited by fallah, 23 September 2014 - 02:42 AM.