Role of Pump Inertia and Stopping time in Surge mitigation

[Sawsan311]

Dear All,

I am seeking your views on the below:

a) For dynamic surge analysis capturing the scenario of sudden pump trip (of multiple pumps) in case of motor failure, have you come across any design credits given for the pump's inertia which is related to the time the pump will keep on spinning before it eventually stops completely.


b) Additionally, is there any design guideline for advising on the pump trip time with regards to the motor rpm? Typically, I have seen consultants considering 3 seconds which is in my opinion is too conservative specially for large pumps?

c) Secondly, for an ESD initiated trip by a common SIF function such as high pressure, can we prevent the scenario of surge (unlike power failure which cant be controlled) by staggering the motor tripping with a time delay gap between multiple pumps? I think this is acceptable design measure..

d) Additionally, for the design of the lube oil and mechanical seal system, during pump trip, is there a minimum time (similar to settle out in compressor)
which is required to be allocated as part of the lube oil and seal design to protect the pump under standstill conditions? For example in compressors, when the compressor stops, backup lean gas / nitrogen is used for the primary/secondary seals to allow safe shutdown. Is there any similar design requirement for the pump's design?

thanks Regards,

 

[LittleInch]

1) Sudden single pump trip of many? Not normally a surge event.

single pump trip then yes you can add it, but the time is often low and the head decreases as a square of the speed, so often no flow, but if the issue is a vacuum collapse immediately D/S the pump then it can help.

Not sure what you mean by "design credit"? You stick the inertia into the transient analysis and let it do its thing to see if it makes a difference and then design accordingly.

2) - None that I've seen, but I wouldn't trust anything more than 10-15 seconds max. It really is pretty fast so even 5 seconds it's probably only producing 25% of the head or less by then so you need to think in terms of "effectively stopped" rather than 0rpm. Everyone normally starts getting exited about reverse flow if you leave it open too long.

3) Never seen this, but then I tend to deal with single pumps. high pressure usually implies low / no flow so surge isn't so much of an issue.

4) Yes, - ask the vendor but I've seen timers at 15 or 20 mins to continue circulation of lube oil post trip or until the bearing temp goes down to some pre set level.



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[Sawsan311]

Thank you Mr.LittleInch for your swift response,

In cases where you have downstream pipeline network, the tripping of multiple pumps due to common mode failure like power failure, can despite creating low pressure immediately close to the pump's system YET, there will be pressure wave propagation (line packing) through the pipeline network and peaking at locations where hydrostatic pressure build up by pipeline elevation is maximum (can be perceived from steady state runs). The pressure wave can propagate quickly enough after the pump has stopped leading to greater surge effects. Hence, this doesn't necessarily mean low flow with no surge issues.

On the other hand, if the scenario is sudden SDV closure, if the SDV closure time is long enough such that it exceeds the time the pressure wave would propagate back through the line, this will contribute in reducing the surge pressure.

Accordingly, I think pump vendors to be contacted to identify the effective proportional flow cessation speed such that surge impact on the downstream can be identified.

Regards,

 

[LittleInch]

Ok, so you mean multiple pumps in series? I thought you were talking about parallel.

Surge analysis can get interesting in those situations.

You might be surprised at how fast it all happens when you turn the power off...

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

 

[Sawsan311]

No I was referring to pumps in parallel feeding a long pipeline network.

 

[bimr]

One additional function of the pump control valve should be considered: prevent the pump from backspinning after power failure or overload trip. Since pumps today are no longer equipped with flywheels, as with old diesel units, they have a low rotating inertia and come to rest in a just a few seconds. Therefore, after a power outage or pump trip, the pump control valve must close more rapidly to prevent backspinning.

The valve hydraulic controls are equipped with a bypass line equipped with a 2-way solenoid valve (SV) to send the controlled cylinder flow around the normal flow control valve and through a large flow control valve (FCV), thereby closing the pump control valve automatically in 5 to 10 seconds after power failure. This is essential to prevent excess pump backspin and to prevent depletion of the hydro-pneumatic surge tank water back through the pump if one is utilized.

Link

 

[Sawsan311]

Hi Bimr, you are referring to the revser rotation of the pump after a shutdown which can be easily managed by having two non slam dissimilar check valves at its discharge closing fast when the pump trips. I did not understand the part you are referring to the hydraulic controls and the controller cylinder. Any schematic available?

 

[bimr]

Look at the link

 

[georgeverghese]

From an operational perspective, aux lube oil to compressors, once auto started when compressor trips (and main oil pump stops also), is only stopped on manual from DCS - there is no timer stop for aux lube oil pump. The same would apply to pumps also I suspect.