Pump Problems

Pump B is bigger than pump A. VSDs are available, and something is controlling them, be it system flowrate or pressure, whatever.

All that will happen is that the VSD will run pump B slower than it is running pump A, to achieve the same requirement.

Instead of running slower the impeller of the bigger pump can be reduced to achieve the same effect. At least check it out.

Also check the efficiency at the slow downed speed vs smaller impeller.

Pump A will have a range of impellers ask for details from pump A manufacturer along with performance curves check your new requirements and select you may have to reduce the pump impeller based on your requirement of horsepower,RPM,Flow rates and head and system curve NPSH of the pump you have to Dynamically balance the impeller in case you have to machine it to reduce the diameter never operate the pump if the disgened head is more than operating head you will end up with pump motor over heating and burnouts further cross check the pipe sizing and friction losses with pipe flow charts you could solve your problem yourself. good luck

crm

If both your pumps are VFD driven; check at which speed the rated point (flow @ developed head) is given.

If pump B is rated 6750GPM@330 ft it might well be operated at a lower speed producing 5835GPM@290ft .

Regards,
Richard

ChemP,

Remember that the pump is not an isolated component. If pump B is used and produces a higher flow, the rest of your system will have a greater pressure loss, so the actual flow and pressure will be somewhere in between the two. Since you would be using it on a VFD, all you have to do is make the speed control for the VFD be from a pressure transducer on the discharge line. Then, you can set it to operate at 290ft of pressure and the VFD will set the pump speed up or down to maintain this discharge pressure. Or, if you do not need to have precise control, manually set the speed so the observed discharge pressure is the desired 290ft.

If you are in doubt, ask the pump vendor to give you the speed and horsepower the pump would be operating at, to achieve a discharge pressure of 290ft. Since you have not changed the rest of the system, the flow rate will be the same as what you had previously. You'll be OK.

---JMM

Hi Chemp

adding to what is mentioned by crm, i would suggest using throttling process.The Q-H curve of a centrifugal pump can be altered to reach the required discharge and head.

Sayed Sarhan

throttling process

practically throttling can be used as short term solution as it will cause reduced flow rates ie droop in GPM down stream of the pump.

crm

Your avatar doesn't indicate your location, but, since you use US flow and head values, I am assuming you are in the US. And, I will further assume that the speed of the two pumps is 3450 RPM since you are using a horizontal split case pump, and you need that speed to achieve the heads you list. If I am wrong, get back to me and I will re-figure the values I am about to present. Further, at 3450 RPM, the Pump B delivers 6750 GPM @ 330 feet TDH. This is on a VFD and the power is 60 Hz power.

Now you don't indicate which is most important to you, the 5835 GPM of Pump A or the 290' TDH. However, pump B operating at 2982 RPM on your VFD (that would be at 51.86 Hz) would deliver 5834.3 GPM but the head would only be 246.5'. That same pump operating at 3235 RPM would deliver 6329.3 GPM and the head would be 290.2' operating on the VFD at 52.26 Hz.

As has been stated by others, another option is trimming the impeller, but the results would be very similar since the only way to increase or decreae the capacity or head of a centrifugal pump is to increase or decrease the tip speed of the impeller. That can be done by managing the actual rotative speed of the spinning impeller or to manage the diameter of the impeller. Since you have VFD's there now, why not use them. Whatever controls the VFD now, either flow or head, can be set to constantly deliver either the flow you need or the head, whichever is important to you. If you trim the impeller you will have made a permanent alteration to the pump. Go with the VFD.

Hi

thanks for the reply

Pump A is 1450 rpm @ 50Hz and Pump B is 1493 rpm @ 50 HZ

My requirement is both flow and head that is 5835 GPM and 290 TDH.

As u mentioned to operate Pump B on VFD at a particular RPM such that i get near to 5835 RPM and 290 TDH, am i correct?

Also wht would be the implication on the pump performance if my pump will operate at a reduced RPM always.

As u mentioned that impeller trimming is not a good option, however i think using a VFD and trimming the impeller both might work in favour of the pump in terms of power and also meachanical performance.

Can u also highlight the cal., of achieving a particular flowrate and head at diff RPM and Hz

Bye

Pump B operating at 1291 RPM would deliver 5836.7 GPM at a head of 246.7 feet. The VFD would be reading 43.24 Hz. The pump would also deliver 6329.5 GPM @ 290.2' of head and operating at 46.89 HZ.

The pump efficiency would be about the same as it would be at full speed. Operating the pump at a reduced speed continuously has no detrimental effects on the pump. Quite the contrary. The pump operating at a slower speed means less friction on the bearings, less heat generation, less operating current and all the other benefits of operating slower. It is no different than using a belt drive for the pump and operating at lower speed.

The relationship netween the pump characteristics and the pump speed are as follows:

The pump delivery (flow) is directly proportional to the pump speed

The pump head (pressure) varies as the square of the speed

The Horsepower varies as the cube of the speed and

The efficiency is nearly unchanged.

Not having benefit of the pump curve, I can not determine the horsepower required for your particular application, but you can figure that by multiplying the flow (GPM) X head (feet) and then dividing by the hydraulic efficiency X 3960.

As for trimming the impeller, there would be not advantage to doing that. The result would be that you would have a new constant speed curve from which you would have to calculate the information I have shown you above. I wouldn't do it.

The calculations that I show were derived from the relationships I showed you above. I use a computer program to calculate them. The program is available from many pump manufacturers. The one I use was provided by Cornell Pump Co. located in Oregon. Hope this helps.

Pump manafacturers M/s Grundfoss and Bell and gosset also provides pump sizing software free of cost you need to get in tuch with them

crm

You can accept this change and can operate pump B with VFD to meet any varied demand. If you operate Pump A you pressure will drop when the flow exceeds 5835GPM. Also pump A is also having VFD which can be used in any other plant and will get the advantages of VFD.