5 replies to this topic

[Jiten_process]

hello,

sorry for restarting this same topic, actually i ws surfing through archive posts in order to know the basics of 'minimum flow recirculation line'. and i have a doubt in the continuous of above topic. Because we are not providing minimum flow recirculation line in many application.

One of the reason of giving 'minimum flow recirculation line' is cavitation. Caviation is a phenomena of occuring vapor bubbles of operating fluid which inturn damage the pump. It mainly occurs when difference betwenn NPSHa and NPSHr is not maintained sufficiently as per required value.

Second when we reduce the operating flow generally by throtteling valve at constant speed, the NPSHa will increase because of the reduction in friction losses and NPSHr will decrease which subsequently widens the the difference between NPSHa and NPSHr which is infact good for the pump (only in a view of caviation).

Now, if my above understanding is true then why caviation occurs when we do not provide the minimum flow. Infact reducing the flow is favorable for the pump (just in a view of caviation). It is true then at very low flow, the temp. of fluid starts getting increase but is it so severe as to boiling the fluid too. (coz take an example of cooling water at atm temp, do i really need minimum flow line for even a turn down of 70-80%, coz anyway temp of water may not go at a boiling point at pressurised condition, plz correct me i m wrong..)

waiting for reply...

[JoeWong]

The NPSHr of centrifugal pump is decreased with flow...

Factors possibily determining centrifugal pump MINIMUM RECYCLE flow are :

i) Fluid temperature rise
ii) Minimum stable flow
iii) Internal recirculation
vi) Thrust capacity

All four factors are affected by pump design.

[Qalander (Chem)]

Dear Jiten_process hello/Good Evening,

Although what is mentioned by Joe is appropriate and accurate & you may find substantial details/ arguments on the issue from his (Joe's) Web blog.
But I suggest you to consider

the situations when there is very little or almost no flow due to throttling and

fluid gains heat and at times reaches elevated temperature

as there is insufficient heat sink(flow-out)

this in-turn may initiate bubbles/vapour generation and disturbs a centrifugal pump operation

here minimum flow line is necessitated; however minimum flow re-circulation is not imperative as I understand.

Moreover the minimum flow recirculation line(depending upon the pump design NPSHa & NPSHr values)ensures the availability of sufficient fluid in the centrifugal pump's suction eye to stop starvation/capitation possibilities whenever the system's NPSHa drops below the needed NPSHr at any moment of pump operation.
This is from operating angle observed on many pumps.
Best regards
Qalander

[ARAZA]

Hi There,

If a centrifugal pump with a high horse power motor have the possibility of deadheading OR there is a possible case of operating case the pump at a much reduced flow. Than there is a case for min. flow, based on the simple fact: The heat produced by the pump motor is dissipated by the fluid leaving the pump. If the pump is deadheaded OR operated at a reduced flow (less than the pump vendor's recommended min. flow) than the liquid may start heating up and there is a rise in vapor pressure, pump may start vibrating and possible damage to the pump.

I would suggest, look at the pump vendor's data-sheet and look for thermal flow & stable flow and compare it with your flow at reduced conditions. If your flow at turndown is less than the vendor's recommended flow than most probably you will require a min. flow line.

Hope this helps.

ARAZA

[Andrei]

Jiten,

I would like to make a correction at your cavitation definition. No, cavitation occurs not when NPSH available is lower than NPSH required. Cavitation occurs when the actual pump suction pressure is lower than the liquid vapor pressure at the suction conditions. NPSH is just a length conversion of the pressure. NPSH available and required are calculated values. The actual NPSH is the indication if cavitation occurs or not.
Yes, personally I've seen a lot of pumps without minimum flow lines. If you install one or not is a design decision. There are cases when you have to because the pump vendor doesn't guarantee the pump performance otherwise. There are cases when you have to install one because the process conditions are such. And there are cases when is a waste of materials to install one. It is the job of the design engineer to asses these requirements. Of course there are standards, and guidelines but ultimately it is the judgment of the engineer.


Regards.

[Cheetha]

Dear Friend,

Coming Straight to the Question of Why provide Minimum Flow protection line.

This is required due to following reasons :

1. Pump protection.
2. Pump operating point flow is less than or equal to pump minimum continuous stable flow. This is due to fact that at pump minimum flow pump suffers from max vibrations and there is every chance of damage to pump. You can refer API 610, 10th edition, which has graph showing the speed and the vibration limits which makes it easy to understand.

So NPSH or the cavitation is not the criteria for providing minimum flow protection to pump. Yes in industry across the world many pumps are running safely and smoothly without this protection line because their operating points are much far away from the minimum flow region of that particular pump.

Hope I have made the point clear.

Regards,

Cheetha