Liquid Pump Blocked Outlet Sizing Flow Criteria

[Berenger]

Hi,

I am sizing relief valves for liquid system for the first time. I usually work with natural gas. I know the code/standard and the equations and I can calculate orifice sizes.

However, I do have a question regarding blocked outlet on a pump. The facility is a pump station with 18 inch pipelines and 144000 barrels per day. My questions are:

1. For blocked flow at pump discharge, do I size using the entire 144000 bpd?
2. If so, the valve size comes out to a 4 by 6 valve. This seems quite large to me and larger than what I have recalled seeing at other pump stations.

Am I right in using the entire full flow for the calculation? If not, what flow should I use?

I will appreciate if anyone who has experience in this is able to help me out.
Thanks,

Berenger.

 

[LittleInch]

Berenger,

I appreciate you're looking for answers, but the usual way when posting a new question is to provide a link to the previous question, i.e.

http://www.eng-tips.com/viewthread.cfm?qid=394207

so that people can see what has already been discussed. The flowrate seems to have doubled, but the same issues are being asked.

Flowrate used could be anywhere from zero to full flow, but you then need to work out where that flow is going to go and for how long. Pumps can handle no flow for a limited period - enough time to initiate a pump trip on no flow or high pressure without having to handle large volumes of liquid.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Berenger]

Thanks, LittleInch,

Yes, the flowrate has doubled to 144000 barrels per day because the client wants a 100% design factor in the facility (first time I’ve heard of that). Anyway, it seems like there will be more crude passing through the facility BUT they can’t tell, definitively, how much.

Pump trips are usually a matter of seconds. However, what if it fails? Usually, from my experience, relief valves are usually a last resort if all else fails. Considering that, the full flow will pass through the pump.

What is your experience? Have you ever sized a blocked discharge relief valve for a pump? How did you proceed?

Unfortunately, these are not included in the API standard.

 

[LittleInch]

Hmmm,

Sounds like the client doesn't know what he's talking about, but unfortunately that's not uncommon....

Blocked outlet often doesn't result in excess pressure but if you need a higher level of reliability, go for a two out of three pressure transmitters system and a higher reliability control system.

You often see relief system like this where you have a big difference in pressure rating e.g. at entry into a terminal, but not normally at a pump station.

I've only seen a full flow relied for a positive displacement pump. Most pipeline pumps are centrifugal so don't need this sort of pressure relief.

Which API standard?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Have you first checked if the max developed pressure at pump discharge exceeds the MAOP of this pipeline?

To the max normal suction pressure, add the pump shutoff head at max speed and max density of the fluid - what does it tell you ?

 

[Berenger]

Yes, @ georgeverghese, the max developed pressure will exceed the pipeline MAOP. Not by much, but by 60 psig.

Littleinch, the pumps are positive displacement pumps. I think the full flow is required to size the safety valve on the pumps. I use API 521.

Thanks, to both of you. I think I have a clearer understanding now.

 

[LittleInch]

Berenger,

PD pumps at no flow will normally greatly exceed their "normal" pressure rating, but yes, in this instance you need full flow + 10% to make sure you have no accumulation.

This is so basic thought that I wonder if it is not already included somewhere?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Agreed, unless there is something you havent told us yet, the Code RV should be sized for full pump flow, given this is a pd pump. No credit should be taken for any internal non Code designed RV in the pump.