8 replies to this topic

[Butterfly]

Hello.
I'm posting in this forum instead of the "Pressure relief" one because i think my question is regarding concept understanding.

My doubt: let's say I have a vessel with 100 psig MAWP, and a PSV set pressure of 100 psig. For single relief device installation allowable overpressure is 10 psia, and relieving pressure is 124.7 psia (API 520).

Supposing I have an operational problem and pressure increases too much, PSV will open when internal vessel pressure reaches set pressure, right??

Once the PSV has opened, can the vessel internal pressure reach the relieving pressure? What first comes into my mind is that if I reach set pressure and the PSV opens, pressure is being relieved, so vessel internal pressure is going down, so I don't see how it could reach 110 % of MAWP (=relieving pressure).

I used 10%, it would be 21% for fire case (maybe there are clearer examples here).

Thanks and sorry if I used the wrong forum.

[kkala]

Supposing I have an operational problem and pressure increases too much, PSV will open when internal vessel pressure reaches set pressure, right??
Once the PSV has opened, can the vessel internal pressure reach the relieving pressure? What first comes into my mind is that if I reach set pressure and the PSV opens, pressure is being relieved, so vessel internal pressure is going down, so I don't see how it could reach 110 % of MAWP (=relieving pressure).....

PSVs have some modulating ability, they do not function as on - off valves. They start opening at set pressure and get fully opened at relieving pressure, allowing more and more flow to pass as pressure increases between these two.
Suppose a fire approaching an LPG bullet. At the beginning its PSV will open at set pressure, to discharge vapor; after a while PSV will open more (as fire gets close) to discharge more vapors. This up to max relieving capacity of this PRV.
Pilot PRVs seem to be better modulated, but there are also pop action pilot valves (practically no modulation).
Fig 2.2 of "Pressure Relief Systems - A quick guide" by C Matthews (Proff. Engineering, 2005) shows a PRV seat, apparently modulating.

Edited by kkala, 11 January 2012 - 01:52 PM.

[S.AHMAD]

Supposing I have an operational problem and pressure increases too much, PSV will open when internal vessel pressure reaches set pressure, right??

Once the PSV has opened, can the vessel internal pressure reach the relieving pressure? What first comes into my mind is that if I reach set pressure and the PSV opens, pressure is being relieved, so vessel internal pressure is going down, so I don't see how it could reach 110 % of MAWP (=relieving pressure).

1. The pressure vill increase further, if the required relieving rate is higher that the actual relieving rate by the PSV.
2. This is the reason for us to determine the required relieving rate for all emergency cases.
3. The PSV will be sized for the maximum required relieving flowrate. This is to ensure that pressure accumulation should not more than 10% or 21% etc.
4. Example: Let we have relieving rate for utilities failure = 10,000 kg/h and blocked discharge = 125,000 kg/h.
5. If we size the PSV for utilities case, what will happen if the the emergency case is for blocked discharge? Pressure will accumulate and may exceed the MAWP.
6. However, if the PSV is sized for block discharge, if utilities failure occur, the pressure will drops and PSV will be closed and may pop again if the pressure increases again (chattering).
7. Under the above scenario, we normally install two PSVs to avoid chattering during utilities failure.

[fallah]

Once the PSV has opened, can the vessel internal pressure reach the relieving pressure? What first comes into my mind is that if I reach set pressure and the PSV opens, pressure is being relieved, so vessel internal pressure is going down, so I don't see how it could reach 110 % of MAWP (=relieving pressure).

Butterfly,

Interesting question!

I exactly got your concern almost reflected to above part of your post; right?

Now my explanation targeted your query:
When the vessel reach the set pressure and the PSV opens, pressure is being relieved, so vessel internal pressure is going to be decreased but at the same time source of the overpressure is pressurizing the vessel! Now if the PSV being sized for pressure relieving equal to set pressure, the PSV (bigger in size) thoretically would be opened, without any overpressure in its upstream, till source of overpressure is removed. But the PSV may practically be operated in unstable manner due to probable flactuations in set pressure which easily affects balancing situation of the PSV in relieving condition.

Thus, the PSV would be sized based on the same set pressure but with 10% (16% for multiple and 21% for fire) overpressure then relieving pressure to be 110% of the set pressure in PSV balancing situation and will lead to the smaller PSV and stable operation (without chattering and ....).

Hope above help you out.

Fallah

Edited by fallah, 12 January 2012 - 01:47 AM.

[Butterfly]

Yes Fallah, you got my doubt.

Thanks a lot to all, you helped me to have clearer understanding

[kkala]

Supposing I have an operational problem and pressure increases too much, PSV will open when internal vessel pressure reaches set pressure, right??
Once the PSV has opened, can the vessel internal pressure reach the relieving pressure? What first comes into my mind is that if I reach set pressure and the PSV opens, pressure is being relieved, so vessel internal pressure is going down, so I don't see how it could reach 110 % of MAWP (=relieving pressure).....

PSVs have some modulating ability, they do not function as on - off valves. They start opening at set pressure and get fully opened at relieving pressure, allowing more and more flow to pass as pressure increases between these two.
Suppose a fire approaching an LPG bullet. At the beginning its PSV will open at set pressure, to discharge vapor; after a while PSV will open more (as fire gets close) to discharge more vapors. This up to max relieving capacity of this PRV.
Pilot PRVs seem to be better modulated, but there are also pop action pilot valves (practically no modulation).
Fig 2.2 of "Pressure Relief Systems - A quick guide" by C Matthews (Proff. Engineering, 2005) shows a PRV seat, apparently modulating.

Kalpesh Unadkat, having placed the negative feedback on above post, is kindly requested to explain the reason. Cold blooded rationale always help in finding the correct answer, which is the truth.

Edited by kkala, 12 January 2012 - 05:05 AM.

[Kalpesh Unadkat]

Nothing special, inadvertantly I pressed the wrong button
Sorry for inconvience caused if any!

[kkala]

I would like to check post No 2 by kkala, reflecting the view that a common PSV has some modulating ability, as formed by filling relevant data sheets. Opinions / advice from other members would be appreciated in order to find the right answer.
1. Let us assume one PSV installed on the gaseous phase of a pressure vessel, with 10% allowable accumulation (covering even fire to some codes). Choked flow will determine the discharge, occurring between 100 and 110% of set pressure (design pressure).
2. If PSV opened suddenly to full opening, discharge flow would vary by only 10% (OK, ~11% by mass), which seems too low. Variation is broader, chattering takes place e.g. at 25% of maximum flow. Consequently PSVs must have some modulating ability, gradually opening (to my understanding) during 100-110% of set pressure (relevant data would be useful). Mentioned Fig. 2.2 of Matthews book shows a conical stem that could support this view.
3. Concerning pilot operated PSVs, there are pop-action (5% blowdown) and modulating action types (Matthews, para 2.9), so let them be out of above discussion.
4. Searching the matter more for spring loaded PSVs, I found "the high lift principle" (Matthews, para 2.5), a feature causing "the valve to pop open when a specific pressure is reached". Fig 2.4 indicates valve opening as accumulation increases. Lift is slow at the beginning, then slope versus overpressure increases steeply before full lift (*). This practically cancels my view of gradual opening (para 2). But how can we explain the (broad) flow variation through a PSV? Or this variation is limited to about 10 % (it must be broader).
I think topic actually concerns the modulating ability / range of PSV, any answers to help?

(*) unfortunately Matthews book in web (evidently advertising issue) is cut 2 pages before para 2.5 of interest http://books.google.... factor&f=false

Edited by kkala, 12 January 2012 - 08:09 AM.

[kkala]

Nothing special, inadvertantly I pressed the wrong button Sorry for inconvience caused if any!

Not at all, but I think one positive feedback should be removed, because you add one for correction and Administrator also did the same.
Rationale of each feedback is important, not numbers of them.
However this was the reason to check PRV function and write post No 8. This may be fruitful, if answers clarify PSV "rangeability".

Edited by kkala, 12 January 2012 - 09:19 AM.