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Psv Discharge Line Sizing

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#1 Xavi098


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Posted 03 July 2012 - 01:17 AM

I have a relief valve that was sized based on a blocked-in case.
When I calculate its rated capacity under a fire scenario, it is much higher than the required relief load since the blocked-in case was the controlling case. However, the rated capacity under a fire scenario is the controlling case for pipe sizing downstream of the PSV.

Please correct me if I'm wrong, all I could find on APIs 521 & 520 was that the PSV discharge line should be sized based on the relief valve's rated capacity.

The PSV relieves to atmosphere. Since this is a vapor at a very low density, I calculated that the flow will choke at the end of the discharge pipe. If I add a reducer and expand the line to twice the PSVs outlet diameter, I get a choke flow at the reducer. I moved the reducer all the way back to the PSV's discharge flange but does not seem like a good engineering practice since:
- I think the PSV will chatter.
- The discharge pipe is twice the size of the PSV outlet and looks ridiculously big.
- After a fire the vessel will probably be taken our of service along with the PSV.
- PSV is oversized for an external fire scenario.

I cannot trim the upstream pump's impeller nor decrease the upstream pressure for the blocked-in case.
Vessel needs a PSV for fire case.

Should I size the discharge line for the blocked-in case only?
What would be my basis for excluding the fire case as the controlling case for line sizing?


#2 fallah


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Posted 03 July 2012 - 03:06 AM


Your query isn't so clear and you didn't specify type of the PSV which is needed to specify the design basis of PSV discharge line sizing. Indeed, in general you should size the PSV for governing case needs highest orifice area for relieving the overpressure.


#3 katmar


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Posted 03 July 2012 - 03:49 AM

It is not at all unusual for a PSV discharge line to be significantly larger then the discharge nozzle. The PSV is more likely to chatter with a high pressure drop down a small discharge line than it is with the low pressure drop you will get with the large discharge line. Design for the worst case scenario. You will leave yourself open to trouble if you knowingly disregard a potentially hazardous situation.

#4 Mark-TR


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Posted 03 July 2012 - 09:50 AM

PSV discharge line shall take in to account both the blocked scenario and the fire scenario.

Both at rated conditions, as a standard practice Mach velocity shall not be higher than 0.7.

This is to avoid noise and too much force in the line while the PSV opens.

Also note that it is important not to exceed PSV maximum backpressure. Usually this value is 10% of set pressure of your PSV for conventional type and 30% for balanced.

If this rule is not applied, the capacity of PSV is compromised. Generally, valves routed to atmosphere are conventional.

Please note some minor comments to be considered:

As you talk of an upstream pump I suppose you are considering to include an overfilling scenario.

If this is the case the set pressure of PSV shall be also reviewed. You shall consider hydrostatic height between your safety valve and your vessel. This will make a pressure difference at relieving conditions. Therefore, you should consider decrease the hydrostatic pressure if a significant difference in hydrostatic pressure is observed.

Also note that inlet line of a PSV must be also reviewed when a new scenario is considered and evaluated at rated conditions.

I suppose you are considering a clean service, as you talk about discharging to atmosphere. On this case, you should also review if the isometric of discharge line is also good for relieving a liquid fluid.

Generally, when the line relieves vapor, it is routed to safe location 3 meter height in order to relieve the fluid to atmosphere. But if liquid is to be relieved that can make a beautiful fountain of water…which is no good for operators.

Therefore, it is currently a good practice to design a drip leg in the outlet to direct vapor to atmosphere and the liquid to the proper drainage system. A hydraulic seal shall be considered in the outlet drainage of the drip leg.

Kind regard

Edited by Mark-TR, 03 July 2012 - 09:50 AM.

#5 Xavi098


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Posted 03 July 2012 - 12:13 PM

thanks you all for the prompt responses.

Hopefully this makes it a bit more clear:

This is a conventional relief valve.
This is a clean service and the discharge is going to a safe place. Believe me, I am not going to dump HF (aq) or sour water into the atmosphere where the operators can bathe in it.
If I use a discharge line size, diameter, of 3 inches since the PSV is a 2"x3", I get chocked flow through the 3 inch line and my pressure drop through the line is higher than 10% of the PSV's set pressure.
If increase the line size to 4 inches, I will get chocked flow at the reducer and my back pressure will slightly above 10% of the set pressure.
When I increase the line size to 6 inches, the flow becomes chocked at the reducer but the pressure drop through the discharge line will be much less than 10% of the PSV's set pressure: however, I am still chocking the flow at the reducer which is downstream of the PSV.
Since spring loaded relief valves don't really pop open but rather open at the set pressure and reach their rated capacity at a higher accumulated pressure, I'm afraid that chocking the flow with the reducer will not make the discharge line any better than using a 4 inch line.
The problem is that when you try to expalin this the project manager or operations, they laugh.
First, they don't believe that the flow will get chocked (some people don't believe that if you try to squeeze a compressible fluid into a small orifice, etc your velocity will increase drastically hence pressure will drop)
Second, the line looks ridiculously big.

I'm going to take Katmar's advice and size for the worse case scenario until some one gives me a good reason why not to use such a big discharge line.

#6 katmar


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Posted 04 July 2012 - 01:08 AM

If you are relieving to atmosphere it is hard to imagine how you can get choked flow in an expanding reducer on the valve's discharge. It is common for there to be choked flow in the PSV's orifice, and there may be sonic flow at the outlet of the valve itself, but how does the reducer cause choking?

#7 latexman


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Posted 04 July 2012 - 10:05 AM


I suspect you may have a 2J3 PSV, right? The largest flow nozzle choice in the standard PSV sizes (2J3, 3L4, 4P6, etc.) sometimes needs to have inlet and outlet sizes more consistent with the next size up to perform satisfactorily. That is not uncommon, even for installations that do not have long runs. The problem gets worse as you progress up in size, i.e. 4P6 is worse than 3L4. Just having a 2J3 could bump your outlet line up to 4" by itself. Then, if you have a relatively long outlet line or some other issues, that can easily bump it up another notch, to 6". I have seen this and worse in several installations over the years.

If you are confident of your sizing, let the Project Manager laugh, but don't let them intimidate you.

The following is not exactly what you are describing, but it may be somewhat related, especially for larger size PSVs, and you may find it interesting and enlightening. I reference and quote Guidelines For Pressure Relief and Effluent Handling Systems, Center For Chemical Process Safety of the American Institute Of Chemical Engineers, 1998, pages 38-39. Your company may have access to this, possibly in Knovel.

“On certain occasions, the designer can encounter a dilemma when sizing a tail pipe to meet the built-up back pressure limitation for conventional pressure relief valves. Calculations for a tail pipe of the same size as the outlet flange can say that the built-up back pressure exceeds the allowable limit even if the length is reduced to zero.

For gas flow, this problem usually arises only for valves with small ratios of outlet-to- nozzle area and rather high set points (Van Boskirk 1982). For example, the condition is encountered with air flow in a typical 8T10 valve with set pressures above about 150 psig. For flashing two-phase flow, the problem can arise at higher area ratios and lower set pressures. The computational problem can appear to go away if a larger tail pipe size is specified without proper modeling of the flow in the pipe expansion. The dilemma remains nevertheless: if the back pressure is too high even with zero length of outlet size pipe, the back pressure will not be reduced by addition of pipe of larger size.

The computational problem does not arise unless the calculated flow from the valve nozzle is predicted to be high enough to attain choked-flow conditions in the pipe size of the outlet flange. However, choking per se is not the problem. The computational problem arises only if the choked-flow pressure exceeds the allowable back pressure limit for the particular valve.

For the case of gas flow, the problem may well only reflect the conservatism of the calculation models for zerolength pipe and of the criterion for allowable built-up back pressure. There is no test data nor evidence in industrial experience for valves with no tail pipe to show that a real operational problem is indicated. For the case of the flow of flashing two-phase fluids, however, test data on saturated water obtained in the DIERS program show that a serious loss of capacity and stability can occur as the ratio of outlet to nozzle area is reduced below that of a standard 2J3 valve (Sallet and Somer 1985). The set pressure for these tests was about 90 psig. These results are in general agreement with predictions of conditions under which built-up back pressure will exceed the 10% limit (Huff 1983).

The dilemma then is whether or not to change the selection of the device to obtain calculated results in accord with the non-mandatory Code requirement (select a valve or valves with higher outlet flow area for the given nozzle flow area). Present experience does not show the need for such a change in gas flow service, though the designer may choose to do so in extreme cases (if calculated built-up back pressure is considerably above the 10% limit). For flashing two-phase flow service, however, the decision to change would seem wise in light of the DIERS test results unless direct experience shows otherwise.”

Huff, J. E. (1982), “Intrinsic Backpressure in Safety Relief Valves”, 1983 Proceedings- Refining Department, 62 (48th Midyear Meeting), American Petroleum Institute, Washington, DC, pp. 105-111.

Sallet, D. W. and Somers, G. W. (1985), “Flow Capacity and Response of Safety Relief Valves to Saturated Water Flow”, Plant/Operations Progress, 4(4), pp. 207-216.

Van Boskirk, B. A. (1982), “Sensitivity of Relief Valves to Inlet and Outlet Pipe Lengths”, Chemical Engineering, 89, August 23, pp. 77-82.

Edited by latexman, 04 July 2012 - 10:08 AM.

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