Using A By-Pass Line To Gain Pumping Volume 4

[Dascav]

Help Appreciated,
This is an extraction pump on a separator of a hydro-extraction vacuum system. The situation is such: Hydraulic extraction volumetric pumping limitation has been found (the motor power will not permit further volume to be pumped before kicking out to high load). The current pump scheme does not employ a by-pass line around this low NPSH centrifugal pump. In order to gain more pumping capacity, the idea was presented to install a sufficiently sized by-pass line that would permit the Total Dynamic Head to be manipulated as currently there is excess suction side head available. Question: Can we expect to gain pumping capacity by doing this? What do you think the pitfalls may be?
Thanks

 

[BigInch]

I'm not totally clear on your system diagram and your description of how you want to change things this is a bit confusing too. Are you trying to transfer excess suction head to the discharge side of the pump by moving some of it to the pump discharge side through a bypass line? I hope you're not trying to do that.

The only ways to gain pumping capacity (more volumetric flow) while using the same pump and motor, is to change the system curve by one or more of the following,

Increase suction pressure,
increase pipe diameter
reduce injection pressure downstream

Or you might be able to change the pump characteristics by,

an increase in pump speed
an increase in pump impeller diameter.

If you're operating near the pump's BEP now, any change to flow or head via the system curve adjustment methods may have adverse consequences on energy consumption.

**********************
"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 am also confused about the configuration of your system. But, it sounds as if you are proposing a bypass to spill pump discharge back to suction. This cannot provide you with any benefit in pumping capacity. The pump will run further out on its curve and demand higher input power from your motor. You can't use the spill-back to raise the suction head in order to drive more flow. The only example I have ever seen where this could be of some energy benefit was a vertical turbine pump that had such a steep curve, that increasing the flow rate actually reduced the horsepower because the drop-off in head was so drastic for a relatively small flow increase. The Break Horsepower on the motor is determined (in US units) by the equation:

BHP = ((Flow in gpm) x (Head in Ft) x (Specific Gravity))/ (Efficiency x 3960)

The only way to reduce the horsepower would be to reduce the flow, reduce the head, reduce the specific gravity or increase the efficiency.


Johnny Pellin

 

[itdepends]

Unless your "excess suction head" means that the pump inlet head is higher than the discharge head- then as noted by others above a bypass line will not help you- it'll result in a higher energy consumption by the pump as the discharge recycles back to the suction

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[rzrbk]

Dascav,

If I understand your system correctly, (additional pump head available, bled thru a restriction valve; motor trips as valve is opened more) the bypass line will not give you extra capacity; it will just send some of the capacity you have going forward now, back to the suction.

Two ways to get extra capacity would be to:
-check with the pump vendor to see if an alternate impeller is available(lower head higher flow impeller)
-Install a larger motor to let the pump run farther to the right of its curve.

It is also possible that trimming the impeller some small amount might allow some extra flow by reducing the amount of head the pump makes. The window for this to work would be very small.