Location of Pressure Relief Valve?

[TomosSmith]

I am involved with the design of a pumphouse which consists of four (4) fixed speed centrifugal pumps (duty/assist/assist/assist), each pump being large, 4500m3/hr at 50m total head.

As the characteristics of the system demand are largely unknown and the demand is likely to be highly variable, a relief valve is required to dump any excess flow.

My questions are;

1. It is better to have a relief valve in the discharge line of each individual pump (4 relief valves, one for each of the 4 pumps), or to have a single, larger, relief valve in the common discharge manifold?

2. Can the relief valve discharge be piped back to the suction manifold or is it preferred to discharge the relief valve elsewhere.

Thanks for any assistance. I would also appreciate any suggestions for good pump house design literature.

Best regards,
Tom

 

[Glenfiddich]

what are you pumping and what temperature is it at normally? are the pumps in series or parallel?

 

[SeanB]

I am not sure if a relief valve is what you need. You talk about dumping excess flow - but I relief valve is to protect against over-pressure. You may want to clarify a little bit.

 

[MJCronin]

Tom..

Why don't you have a combination of fixed-speed pumps and VFD driven pumps ?

I agree with SeanB... the typical usage of a relief device on a pump discharge is to protect the pump against heat buildup during dead-head flow conditions. And, it is mostly multi-stage pumps ( boiler fw pumps) that require this protection.

-MJC

 

[BigInch]

Big pump typical relief is for waterhammer.

Don't run it into pump suction. Get rid of it into a relief tank or holding pond, I don't know what liquid you have.

If you need relief (waterhammer situation), you will probably need a lot of relief and piping it back to suction will overpressure suction lines, if the flowback can go anywhere at all. Your pumps will probably need to be shutting down anyway, or an inadvertant trip caused the hammner in the first place, so dumping it back into suction will make hammer worse and maybe on lower allowed pressure capacity pipe as well. There you will tend to reverse flow and create another much worse hammer possibility there too.

You can probably get by with one relief station on a common discharge, but it is highly dependent on the piping configuration and how the pumps react. Do a real simulation and get the real answers.

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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[BigInch]

BTW one relief station, but probably quite a number of relief valves. Also depends on what valves attempt to close when hammer or pump trip occurs and where they are located in relation to the pumps and the relief station. That may force choosing between RV on individual discharge lines and a common station.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[TomosSmith]

Thank you for all your replies and advice.

In response to your various questions:

Glenfiddich - The medium is sea water, at a temperature in the range 15-35 degC (location-persian gulf). The pumps are in parallel.

SeanB - I agree with you. To clarify, the four pumps are in parallel and serve a shipyard site distribution system, where the water is used for cooling processes on vessels which are docked. As the requirements of these vessels are unknown and the range of demands is likely to be large, it was decided to control the pumps from a discharge manifold pressure transmitter. I agree with you that a relief valve is not required to dump any flow as I orinally wrote. However, I think that due to the fact that the demand may drop below the Qmin of a single pump, as required for pump cooling, a relief valve will be required for this. Do you agree? Perhaps a relief valve is also required to prevent water hammer as others have suggested?

MJCronin - Thanks, I agree with you and SeanB on the purpose of the relief valve. You seem to suggest that relief valve for cooling purposes may not be required for a large single stage centrifugal pump, is this the case? I will ask the pump manufacturer about this. I agree that a combination of VSD and fixed speed may be an option.

BigInch - Yes I agree that the discharge line should not be piped back to the suction side. I also agree that relief may be required for waterhammer. A situation may arise where a vessel is ballasting at a flowrate of about 2500m3/hr and then the operator closes a valve to shut it off. Therefore, the waterhammer may be significant in this system. I haven't done any waterhammer calculations, do you have a feel for the required size of the relief valve for waterhammer? What is the benefit of having multiple relief valves over a single one in a sinlge station? Do you know of any good guidance or literature on waterhammer calculations?

Thanks again,
Tom

 

[BigInch]

With ship operations you can be ABSOLUTELY SURE that you will get inadvertant valve closures! I operated a petorleum marine terminal and I can tell you without doubt IT HAPPENS and true to Mr Murphy's Laws, at the worst possible time. I would prepare to have shutdowns during maximum flow to any single, if not all ships on line.

If you can't determine 4 or less optimum flow points for this system, it is possible that you will do well to consider VSD. VSDs should be suited to your hydraulics, so its just a matter of determining how much variation you will expect to have in flowrate to decide if they are worthwhile. BUT, with 4 pumps, you should also have enough flexibility in the variation of flowrate such that no flowrate will be very far off one of those 4 possible optimum flowrates resulting in any control valve position losses being minor. You can't tell without making at least a cursory study if VSDs or constant speed w/ control valves would be better. I would think VSD are NOT necessary with 3 or 4 pumps, especially if their curves are relatively flat.

In most large systems you will not need recirculation lines if minimum flows of 20% BEP to each pump when operating individually can be maintained. Less than 20%, I would tend to install recirculation lines, if not for heat control, for pump start/stop considerations. Are the drives diesels? You may need low flows during diesel warm-ups periods, yes, even in the Gulf area ... during winter, if you cannot wait long enough for warming up to bring on full flows. I would guess, with your ship service, you probably can wait 15 minutes or so, therefore not required.

I'm not going to guess about your water hammer situation. I would think nothing short of full flow relief would be appropriate. You may be able to have several different relief settings when using several different valves, for example, one valve set for hammer pressures with one pump operating/starting/stopping/valve closure, two valves when two pumps are operating/start/stop etc. Also both maintenance and reliability can be improved, not to mention costs. I don't think you'll find a reliev valve for 4 pump full flows anyway, so perhaps no choice in that matter.

If you're unsure of detailed water hammer hydraulics in terms of pump starts/stops, bringing large diesel driven pumps on line in proper sequence, reverse flows back into pumps (any check valves in there?) valve closure times and Cv vs valve positions, I would obviously suggest you find a good transient pipe flow consultant. Can't imagine who that might include(?)

My webspace has a little information about waterhammer, but just google it and you'll get lots of hits. If you need more help in that, just call or write me directly.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[Artisi]

TomosSmith (Mechanical)
For pumps of this size - upward of 750Kw, correct trouble free operation is a little more complex than a hit or miss approach of should I or shouldn't I fit by-pass lines/ relief valves etc. IMO you should be consulting with / employing an engineer well versed in this type of operation to undertake a full hydraulic study of all aspects of the operation.

 

[TomosSmith]

Thanks again for your advice BigInch and Artisi.

With regard to your post and questions BigInch; yes, I will consider that maximum flow shutdown will occur. I will also consider VSD, but as you have suggested will look at different flow scenarios and also look at the pump curves. The pump curves are relatively flat, so you may be right that VSD is not required with 4 pumps, I will investigate here.

As the minimum flow is unknown, I think that recirculation lines should be considered.

The drives are electric.

With regard to water hammer, I will investigate further. I am thinking that full flow relief should be provided at this stage. Perhaps with some detailed calculations it can be shown that a lesser relief capacity is acceptable. I will also look into having relief valves with different settings. I also agree that it may not be possible to find full flow relief vales and several smaller ones is the only solution.

Yes, check valves have been included.

Thanks for the offer of your services. I will keep it in mind but for the short term, I will try and struggle on by myself! I'll also keep you in mind for future projects.

With regard to your post Artisi; thanks, advice noted. I’ll be sure to run my designs and calculations by a specialist at some stage.

Thanks again for your assistance.
Tom

 

[BigInch]

I just offered help. Service is another department.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain