Decreasing Pumping Flow Rate

[RJB32482]

Hello,
I am working on the application of an air operated Pacer centrifugal pump for a pumping application. I believe the flow rates we will get using it in our system will be too high. Would the best way to decrease flow rate is to add a Pressure Reducing Valve on the discharge side? Any other methods? Is the PRV is correct sizing would be:
1. Calculate discharge head without PRV.
2. Calculate difference of discharge head needed for the correct flow rate on the curve minus the rate you will get in your system.
3. Find K needed to get pressure drop as high as on the pump curve.
3. Calculate Cv for the pressure reducing valve using Crane manual.

Any other thoughts? Orfice in pipe? Throatting discharge gate valve?

Thanks

 

[JJPellin]

I would definatly not recommend pinching down on a gate valve on a long term basis. The seat may wash out and then you will be unable to get the valve to seal. I am not familiar with the PRV you refer to. I would suggest either sizing an orifice and installing it downstream of the pump or installing a globe valve in addition to the gate valve. A globe valve can be throttled without damaging the seats. The globe valve is also nice in case the conditions change and you need to adjust the flow. With an orifice you will not be able to adjust for process changes on the fly.

 

[MikeHalloran]

The pneumatic motors come with a pressure gage 'for adjusting rpm'.

It would therefore make some sense to run the pneumatic motor from a pressure regulator to slow it down and move the pump curve in toward the origin. They don't give curves for speeds other than 3450 and 2900 rpm, so you'll have to use affinity laws to estimate what the liquid discharge pressure will be at your desired operating point.

I suppose you could get closed- loop control by running the air motor from a pneumatic relay controlled by the liquid discharge pressure.




Mike Halloran
Pembroke Pines, FL, USA

 

[Artisi]

You don't say by what amount you are likely to be over in flow so as pointed out by MikeHalloran slow the pump down, the other alterantive is to reduce the impeller diameter or a combination of both, impeller trim and reduce speed.

Naresuan University
Phitsanulok
Thailand

 

[RJB32482]

Thanks for the insight from everyone.

From my initial calculations (don't have piping layout yet). I would say that the flow would have to be reduced from 50 GPM to around 10 GPM. Curve speed is at 3450 RPM so I would decrease the speed to around 690 RPM (using Affinity Laws. Don't know if that is possible just by turning down the PSI of air I deliver to it. Looks like it might be possible. Any thoughts?

Looks like the motor curve is horsepower vs. speed with different lines for different PSI of air. Is this horsepower BHP or WHP??

Thanks.

 

[mucour]

BHP is Brake Horse Power, the power delivered by the motor shaft to the pump, while WHP, is the liquid horsepower delivered by the pump. So the motor curve should be giving you the BHP.

 

[MikeHalloran]

The fine print says the air motors are intended to run at 2900 rpm, so start with that curve.





Mike Halloran
Pembroke Pines, FL, USA

 

[quark]

Actually, you may not be able to reduce the flowrate to 10gpm, unless your discharge is totally dynamic. Depending upon your head requirement, you may end up between 10 and 50gpm. Further, you should check what is the safe minimum flowrate for the pump before reducing the flowrates.

Throttling the discharge valve may be the simplest option, but this can be expensive with pneumatic prime movers.

The procedure you mentioned above using a pessure reducing valve works fine but speed control is better.

 

[kenvlach]

If the motor only runs properly at a certain speed, let it.
Rather than throttling flow or altering impellor, I suggest adding a recycle loop. Makes it easy to fine-tune desired flow, and can easily get more if conditions change.
Ken