4 replies to this topic

[Chem32]

Dear members,

 

Chilled water flow of 1500 m³/hr is required for the sustenance of the present system at our Plant . An insulated closed tank of capacity 150 m³ with supply pumps (5 nos., 3 running, 2 stand-by) was designed for the mentioned purpose, having a total discharge head of 55 m and capacity of 555 m³/hr each. With recent modifications in the plant, a separate cold well with supply pumps (55 m head, 5 nos., 3 running, 2 stand-by) to process units were established in the plant premises. These changes reduce the required head on the hot well supply pump to around 30 m, as it traverses only up to the cold well via the ammonia chiller system. Now instead of two pumps we are operating only two pumps, because of head reduction the flow got increased. The present power consumption is 220 KW. I don't have the pump performance curve, it is a old pump with the serial number i asked the vendor to provide, but they couldn't find it. My question is whether my pumps are running with lower efficiency? if i replace the pumps with required head will i have any power saving? awaiting for your replies.thanks in advance.

 

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[breizh]

Chem 32 ,

 

You need to perform an hydraulic calculation of your system and then you will know the options . Vendors should support your query and define the best pumps . Now you may consider to use a variable speed drive to reduce the power consumption.

 

hope this helps

 

Breizh

[Chem32]

Dear Breizh

 

If i am not wrong Using VFD will reduce the flow. Our system is having 30m head for 1500 m3/hr flow and the valves are fully opened. What variable i should consider for VFD?

[breizh]

Consider this resource about VFD .

 

Hope this helps

 

Breizh

[PingPong]

we are operating only two pumps, because of head reduction the flow got increased. The present power consumption is 220 KW.

Is that 220 kW for both pumps together?

Is so then that corresponds to a pump efficiency of 60 % (in combination with a motor efficiency of 95 %) for 2 times 750 m³/h and 30 m head..

It is likely that, due to the fact that both pumps are now running at 150 % of design, they now operate well beyond their Best Efficiency Point (BEP).

You could operate three pumps, while throttling the discharge(s) to a combined flow of 1500 m³/h, just to mimic the old situation and see what their power consumption, and efficiency, then was.

 

A new pump of 750 m³/h at a head of 30 meters is likely to have an efficiency of about 75 - 80 % at BEP.

I assume you have vendor curves for the new pumps, so you can easily see what their efficiency at BEP, as well as at their actual operating point, is, so that you have an idea of how a modern pump performs in similar service.

 

According to your "After" sketch you now have a total of 5 pumps running, producing a combined head of 85 meters. Note that adding the new cold well increases the power consumption of the system due to the higher head required, even if the efficiency of the old pumps would not have dropped due to their new operating point beyond BEP.