All,

I am currently tasked with sizing a few PSVs for the discharge lines of some centrifugal pumps. I am going to describe one that I currently don't understand, although they are all similar.

The pump is for a heavy oil (API 20) which is pumping from an atmospheric tank into another atmospheric tank. The pumps are basically sized to overcome tank head which is very small. Looking over the pump curves the max that they can operate at is 60 psig discharge.

The PSV is on the discharge line of the pump and recycles back to the suction of the pump. In case of a blocked flow scenario down stream, the pump will overpressure the line until it reaches it's shutdown head. The line is rated for 150# which has a design pressure of 285 psig, which could never be reached by the pump. So I don't understand what I'm protecting here.

My guess is that with no motion through the pump, the fluid will heat up and eventually you will have either thermal expansion which could over pressure the line or vaporization which could damage the pump via cavitation.

However, since the PSV just recycles to the pump suction, how is the PSV preventing anything. It just prolongs the situation for a little longer because the fluid still isn't going anywhere it's just cycling through the pump repeatedly presumably still gaining heat.

For a similar pump that is for liquid butane, the PSV recycles back to the source tank which makes sense to me since it's a volatile substance that could easily flash if heated.

IDK, I guess I'm confused about the recycle PSV set up and what it's protecting against when the Pump discharge cannot exceed the design pressure for the line.

Any guidance would be appreciated. I've actually never sized PSVs for pumps before and I'm a little bit confused about their purpose. btw, I'm a Junior

Pauwl,

Good analysis.

If the pump shut-off pressure is lower than piping design pressure, PSV would not be required. The only event that i would think a PSV is required would be the thermal expansion which already captured in your analysis.

Theoretically i only can see PSV is only for thermal expansion in your case. Somehow i have seen people use PSV for pump protection (recirculation) as well as thermal expansion. With PSV, it just buy time for operator to take action (could be short or long...). A proper low flow protection is required for pump protection.

Pawl:

From what you describe, I get the impression that the pump installation is just the product of inexperience (at best) or incompetence (at worse). I would challenge the installation because:

1. I don’t identify any credibility in the need for a PSV on the discharge of this centrifugal pump. I almost suspect that some one saw a discharge PSV on a positive displacement pump somewhere else and thought this might be a good idea – Wrong! If you can’t justify the PSV, then take it out. You are just begging for more maintenance and operational problems. Use the K.I.S.S. principle here: “Keep it Simple, Stupid”. You are correct in using your common sense.
2. The best and most operable type of PSV discharge on a pump (if one is needed) is back to its original source – never back to the pump’s own suction line. A discharge into the suction line is either a cheap attempt to save piping costs or the lack of accessible space or distance. I’ve never come across the last two reasons. A pump suffers from a discharge injected into its suction line – in NPSH, turbulence, and heat-up. I believe it is foolish economy to discharge into a pump’s suction line.
3. Never rely on a safety device to “control” a process. This is naïve and ignorant thinking. A PSV can never be relied upon to furnish the pump’s minimum flow requirements. As Joe infers, you need a minimum flow control valve for that – a totally different animal and for a totally different reason.
4. Like government, the best PSV protection is when you can justify NO PSV installation. That is really fail-safe, and the way I would design this simple oil transfer operation. You are probably out in a remote Storage Farm somewhere and you don’t want to add more equipment or devices than you need. Again, use the K.I.S.S. principle.

I hope this experience helps.

Thanks Guys.

Sorry to beat a dead horse, but you're hitting on topics that are near and dear to my heart, and I feel compelled to add my 2 cents worth.

I want to wholeheartedly endorse all 4 points made by Art. There is seldom (seldom but definitely not never) a reason to install a PSV in the discharge piping of a well designed centrifugal pump system. (Actually, Joe covers the thermal relief case which is one of the few good reasons to install such a PSV.) The described installation sounds as if a PSV is being substituted for a pump minimum flow valve. In my mind, you should not make such a substitution. Either a controlled minimum flow loop, measuring the total flow and using it to control a return flow that goes back to the source, or a self contained modulating valve for the same purpose should be used. I abhor the thought of sending this recirculating flow back to the pump's suction-it's false economy. Likewise, I would endorse Art's points 3 & 4. Perhaps if you cannot maintain a minimum flow through the pump and it is very inconvenient to send the excess flow back to its source, it may be best to just switch off the pump.
Doug

In an economical viewpoint ,for lowering the design pressure of a pump discharge line in order to avoiding higher material class, using PSV with set point lower than shut-off head on discharge line could be logical/acceptable?

Regards

Fallah:

Is your query related to this thread? It is difficult to interpret what you mean in your description, but I think what you are asking is:

In order to lower the working pressure on a pump’s discharge line and allow the use of a lower cost pipe, it is proposed to install a Process Safety Valve that has a set pressure no higher than the pipe’s maximum allowable pressure - but lower than the pump’s maximum developed head (deadhead pressure)?

My comment to the above is that under no explanation would I allow another engineer to try to install a pressure relief safety device with the intention of having such a device act as a pressure controlling instrument. Safety device should NEVER be applied as control devices. I believe I have stated this many, many times in all these Forums. I consider this type of application as an accident waiting to happen. There is no justification to use a safety device on an application for which it was not designed. If you want to reduce the pressure of a liquid line, there are designed and engineered instruments to do this very thing. However, I would not do what you describe. Common sense dictates that if one requires a lower discharge pressure from a centrifugal pump, then the proper and safe thing to do is to simply reduce the impeller size or its speed.

Dear art
Thanks a lot for your comment,but because we have no problem with normal discharge pressure, we are not going to reduce the impeller size.Our problem is limited to coping with shut-off head which increase the design pressure of the line.If we install PSV on discharge line (along with instrumented pressure control system that trips the pump on set point lower than shut-off head ), when shutt-off happens, why could'nt we consider piping under shut-off as pressure vessel that is protected by PSV against overpressure in the case of instrument failure?

fallah,
I understand that the higher pressure possible at shut-off conditions may suggest the need for an increase in piping class that you'd rather avoid. Despite my stated preference for a minimum flow controlled system for your situation, a PSV is certainly an alternative. I would not use an "off the shelf" or pop action safety valve since I think a chattering situation could easily result. But a modulating PSV (which is essentially a control valve) could do the job.
Good luck,
Doug

Doug & Fallah,

My 2-cents opinion...

Minimum flow controlled system using proper control valve with necessary control loop to limit the pump discharge pressure could be an alternative to achieve CAPEX saving as per your presented case. Nevertheless, a modulating type PSV on pump discharge certainly a safety and overpressure protection requirement.

Dear Joe Wong

Would you please describe briefly on modulating type PSV?Any proper reference?

Regards

Please read

Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries
Part I—Sizing and Selection
API RECOMMENDED PRACTICE 520
SEVENTH EDITION, JANUARY 2000

section 2.2.2.6 and refer to fig. 24 & 25 for pop action and modulating type PSV.