Relieving Temperature for Pump PRV 1

[raisie]

Hi all,

I am a Junior Mechanical Engineer and have no experience in PRV sizing, apart from all the research I have done during the last few days.
I am sizing a PRV for a PIG Launching System placed in the discharge line of Positive Displacement pump. The liquid is Crude Oil (16 deg API). I have some questions on which I would greatly appreciate any elaboration.
- First of all, all the codes/standards I have come accross (API standards) are for sizing vessel PRV's. Are there specific standards for sizing the kind of PRV mentioned in my case?
- In order to size a PRV, the manufacturer requires information regarding the relieving conditions. I have been doing a lot of research on how to determine the relieving temperature of a PRV located at the discharge of a positive displacement pump. However, I cannot find any clear instructions for doing so. Are there specific guidelines for this?

Thanks in advance!

 

[GHartmann]

You are protecting the downstream piping from blocked in flow.

The sizing basis should be the maximum flow of the pump plus some safety margin (say 15%)

The discharge temperature will be the temperature of the fluid when it is likely to be over pressured. This point is more important for vaporized liquids. You will be more concerned about the density or specific gravity of your fluid at the relieving conditions

You need to have your conclusions reviewed by someone with more relief valve experience

You need to find some continuing education courses to learn about relief valves

http://www.wermac.org/valves/valves_pressure_relief.html

http://www.wermac.org/valves/valves_pressure_relief.html

http://www.leser.com/en/tools/safety-valve-tutorial.html

I will get on my pet peeve for this subject. Sizing a relief valve is easy; just follow the industry formulas. Determining the sizing BASIS is a completely different issue. Too many engineers try to do the sizing first without understanding the sizing basis.

 

[LittleInch]

I agree with GH above. The question I have is what is this PRV protecting? Any positive displacement pump needs to have a full flow PRV set-up between it's discharge and the first isolation valve. Is it this PRV you are getting the design basis for or some other PRV further down the line? You need to look at the system / P & ID to see if there are any other PRVs upstream of your planned one and make sure your valve isn't going to lift before the "main" pump PRV. You need to define the case you are using to protect the piping and equipment from overpressure and for that you need to understand your system and what the potential causes are that are not protected by other devices.

If you can post a sketch or the P & ID, it will be useful to take this discussion any further.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[micalbrch]

In addition to the other answers: Is your PD pump an oscillating type and - if yes - does it have a pulsation dampener on the discharge side to reduce the pulsations? In this case you must consider that the pulsation dampener is a pressure vessel and that the PRV must be sized acc. to the applicabl standards.

 

[raisie]

Thank you all for the replies.

The PRV is meant to protect the piping of the system.
I have attached a PFD to illustrate the system.
I hope it is clear for further discussion.
Thank you in advance.

 

[GHartmann]

Your sketch confirms what we already discussed.

The PSV capacity should match the maximum capacity of your pump.

The relieving temperature should be based on your analysis of your typical operating conditions


Good luck

 

[micalbrch]

Important: Make sure that the PRV supplier is aware that the PRV shall not have a free outlet but shall be connected back to the pump's suction line. That probably affects the sizing of the PRV.

 

[raisie]

Thanks guys!

Indeed, when I mentioned to the supplier that the valve should be connected to the (Positive Displacement) pump outlet and that it shall be connected back to the pump's suction line, they recommended using a Hydraulic Bypass Relief Valve or a Constant‐Pressure Pump Governor Actuator.
I am now starting to wonder which one will be more suitable for this application and why?
Any ideas?

 

[don1980]

Common relief valves are not ideally suited for pump deadhead protection. Other devices, like those suggested by pump vendor, are better for this type service. What you need for a pump application is a highly modulating type device. PSVs in liquid services can be described as modulating devices, but they do so very imprecisely. And they require a significant pressure reduction (blowdown) before they will reclose. This can cause flow disruptions, wasted energy, and high wear & tear on the PSV due to frequent and prolonged openings of the valve. A far better device is one that is designed for this service, such as a hydraulic bypass valve (e.g. Fulflo valves that you see on almost all lube oil skids). These devices are specifically designed to be highly modulating in liquid service.

For protecting a pump from deadhead, there is also a range of reliable instrumented options, such a power monitor.

Sometimes it's necessary to install a PSV to protect a pump from deadhead, but for the reasons explained above, that should not be your first choice.

 

[GHartmann]

I am going to have to respectfully disagree with the last post. The relief in question is not being installed to protect the pump, it is being installed to protect the downstream piping from overpressure according to ASME requirements. Anything other than a relief valve, such as power monitoring will not meet the intent of ASME. They can be, and are used to keep the pump away from relieving conditions.

Since the Constant‐Pressure Pump Governor Actuator is essentially a control valve, I do not believe it would satisfy the requirements of ASME overpressure protection.

The Hydraulic Bypass Relief Valve is just a form of a relief valve. I believe it does satisfy the ASME requirements.

http://www.fulflo.com/catalog.htm

So to conclude, the Fulflo device would be MY recommended choice

 

[e43u8]

@GHartmann,

When a PRV is installed on the discharge line of a PD pump there might be two situations regarding system pressure integrity:

1-If design pressure of the downstream piping is lower than the MAWP of the pump; then the PRV is to be set at mentioned design pressure and would protect downstream piping from pverpressure...

2-If design pressure of the downstream piping is equal to (or in any case higher than) the MAWP of the pump; then the PRV is to be set at MAWP of the pump and would protect both pump and downstream piping from overpressure...

Indeed, a modulating pilot operated PRV might be the best choice to be installed on the discharge line of a PD pump to protect pump/downstream piping from overpressure...