Pump return flow overheats supply tank

[deanquest]

Greetings,

We are pumping a liquid refrigerant from an outside supply tank to inside the plant. The pumping system is located outside the plant next to the tank. We are using a centrifugal pump and a bypass pressure regulating valve to maintain constant supply pressure to the plant. The bypass flow is being returned to the supply tank.

We have sized the pump to provide the required flow at a constant pressure to the plant on a cold winter day when the differential pressure across the pump is at its maximum. However, the flow through the same pump on a hot summer day is nearly twice the cold day flow rate. This is due to the warmer, increased vapor pressure of the refrigerant in the supply tank. We need to maintain the same discharge pressure to the plant as in the winter.

I am concerned the increased flowrate during the summer months may cause the supply tank to overheat due to increased recirculation of bypass flow. With the flowrates we're talking about, it is possible the heat being generated through the pump may not be dissipated to atmoshpere through the shell of the supply tank.

I've heard of using an orifice to limit pump flow, but I would think that would also reduce pump flow during the colder months when almost all the flow the pump is capable of generating at the required pressure is needed inside the plant. Short of installing a variable speed controller for the pump, can anyone suggest a method of reducing pump flow during summer using mechanical components that will still allow us to have full pump flow during winter?

Thanks to all who took the time to read and consider this post.

Dean Quest

 

[vpl]

a temperature control valve and a bypass line back to the tank?

Patricia Lougheed

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[cvg]

a throttling valve may be better than an orifice

 

[impeller1]

use a valve to adjust your flow and pressure requirements..you can throttle the valve on the warm summer days and open th vale fully on the winter days to allow max flow

 

[CH5OH]

didge the bypass pressure regulating valve and install VFD on the pump motor.
control logic:
if the refrigerant flow demand increases, the pump RPM increases.
if the refrigerant flow demand decreases, the pump RPM decreases.
I assume the installation has somewhere a compressor to
(to liquefy the refrigerant)?

 

[deanquest]

Thanks, Everyone.

I appreciate all the posts. All the suggestions offered to help are thankfully received.

In the end, I believe we will have to install a VFD as CH50H has recommended. All the mechanical resolutions to this problem require periodic maintenance efforts which are difficult to rely on when maintenance workers are typically putting out fires in order to keep production up.

I just wanted to check with the experts to see if a simpler and less costly solution was available.

I wish everyone a swell weekend!

Thanks again,

Dean Quest

 

[BigInch]

I hope you don't think that a VFD will help your maintenance situation.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[JJPellin]

I think a VFD might work well. But the spill-back arrangement is definitely not the right choice. The additional flow you get in the summer is because the higher vapor pressure means higher suction pressure. The pressure requirement of your system is constant. So, with higher suction pressure and a constant downstream pressure, the flow would have to go up. What you don't mention is where this places the pump on the curve. If the flow in winter was at 100% of BEP (Best Efficiency Point) and the flow in summer was 150% of BEP, then the spill-back to the tank would be running the pump too far out on the curve for good reliability. A simple pressure regulating valve in the outgoing flow could maintain the same downstream pressure by maintaining the same flow. You would waste some energy by building up the extra pressure, just to bleed it back down across the control valve.

However, if the flow in winter was 50% of BEP and the flow in summer was 100% of BEP, then the in-line pressure control valve would pinch the pump back to a less efficient (and less reliable) operating point in summer.

A VFD would solve this problem. Rather than just pinching the pump back to a lower flow rate (as a percent of BEP), it would lower the BEP. An orifice would be a bad choice since it cannot adjust for changing suction pressure.

Johnny Pellin