Temperature Rise in a Centrifugal Pump 1

[Pavan Kumar]

Hi All,

I would like to calculate the temperature rise in a Centrifugal pump under normal operation. I know it based on the pump efficiency. I would like to get the formula with the units for all the parameters involved so that I can perform a calculation. I am trying to calculate the temperature rise in Boiler Pump which takes suction at 95 Deg C. I would like to provide a minimum recirculation back to the Deaerator tank without flashing the hot water. Please reply asap.

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

Hi LittleInch,

When the VFD is at 50% of the oroginal frequency it cannot produce enough to pump to the steamm drum. Now if this water is not sent somewhere else or re-criculated back to the Deaerator then it is as good as Pump Shut-off confition and with quick temperature rise the pump will cavitate. Now at 50% frequency the pump will deliver 30 gpm at a certain head which might be sufficient( have to check hydraulically) to re-circulate back to the Deaerator provided it does not flash.

Now the question at 50% frequency for the temperature increase calculation the BHP at shut-off to be taken at the original RPM or at the reduced RPM?. When I take the BHP at Shut-off at reduced RPM the temperature increase is only 1 DEg F which means there is no flashing????.

Thanks and Regards,

Pavan

 

[EdStainless]

IF you run 50% and don't get circulation then all of the power (whatever comes off of the pump curve for shut in at that speed) goes into heat. That has to be more than 1F.
I am with LI on this, BFW is a poor application for a VFD on a centrifugal pump.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[Pavan Kumar]

Hi EdStainless ,

Yeah what you said is correct if the pump dead heads. But what we are interested is when there is a minimum re-circulation back to the Deaerator, in which case what is the temperature increase. Per my calculation it is only 1 Deg F. Here is the calculation attached, please refer to the tab "Temp Rise Min Flow" for the calculation.

https://files.engineering.com/getfi...ifugal_Pump_Temperature_Rise_Calculation.xlsx

Thanks and Regards,

Pavan

 

[LittleInch]

Why don't you just turn the pump off??

What you also need to advise is how the pressure / head is controlled when in recirc mode to prevent the pump going off the end of the curve.

But you're missing my point which is yes it might rise 1F, but depending on what the volume of water is that you are recirculating this could take a few minutes to rise in total by more than 1F or it might never happen.

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

 

[williamhakespeare123321]

In terms of heat input into the water all you need to do is find the shaft power at whatever flow rate you want to study, find the pump's efficiency at that flow rate and then assume conservatively that all of the inefficiency goes into heating up the water.

 

[Pavan Kumar]

Hi Little Inch,


The BFW sent to the steam drum is sent from the Steam drum to Process Gas coolers and hence there is continuous consumption of BFW. We cannot turn off the pump for this reason. The demand for BFW may increase all of sudden. When the water level in the Steam drum is high the VFD ramps down to almost 30HZ( which is 50% of its original frequency) and so it does not develop the head required to pump up to the steam drum. In this event we want to recirculate the water back to the Deaerator for which the head developed by the pump at 30 HZ is sufficient. Per my calculation at 30HZ the RPM is 1775 ( at 60HZ full frequency it was 3550 RPM). Using pump affinity laws I modified the pump curve and for the head required to pump to the Deaerator I got the flow rate which is 30 gpm and the power consumed is 6 hP. Plugging these values into the formula below, I calculated the temperature rise as only 1 Deg F. With BFW at pump suction at 95 Deg C, the temperature at pump discharge would be 95.6 DegC. With Deaerator operating at atmospheric pressure the recirculated water will not flash. Please let me know if there is anything wrong in my analysis.

∆𝑇=(5𝑥𝐵𝐻𝑃𝑜𝑥𝐶ℎ𝑝)/(𝑄𝑥𝑆𝑝.ℎ𝑡) = (5*6*1)/ (30*1.005) = 0.995 Deg F.

where BHPo is the Brake Horse Power at Shut-off
Chp is the Viscous horsepower correction factor
Q is the minimum circulation flow rate in gpm
Sp. ht is the specific heat of the fluid.

Thanks and Regards,
Pavan


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LittleInch (Petroleum)

Why don't you just turn the pump off??

What you also need to advise is how the pressure / head is controlled when in recirc mode to prevent the pump going off the end of the curve.

But you're missing my point which is yes it might rise 1F, but depending on what the volume of water is that you are recirculating this could take a few minutes to rise in total by more than 1F or it might never happen.

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