3 replies to this topic

[docdan]

I have inherited an unstable pumping arrangement. The flow path:

- supply tank for a medium viscosity liquid

- lobe pump

- shell and tube exchanger

- slow speed, high pressure, multi-cylinder piston pump

- the rest of the process

 

Both pumps are on variable speed drives but the piston pump is usually set to one speed only.

 

Current set-up has PID control on the pressure between the two pumps by manipulating the speed of the lobe pump. The problem is that almost any disturbance sends the PID crazy - rapid, full scale changes in output. The PID tuning is now very slow but it still happens.

 

Solutions under consideration all involve the use of a throwback line around the lobe pump (discharge to suction) on the theory that this will provide some degree of self-regulation: discharge pressure rises => throwback flow rises => discharge pressure drops.

 

Thoughts? Suggestions?

[Art Montemayor]

Only poets can accurately describe a scenario in words.  Engineers need a diagram or drawing to put across the description of a process.

 

I would not arrange the two PD pumps as you describe.

 

I recommend the arrangement that I show in the attached workbook because it is safer and more operable.  It allows for the piston pump to continue to feed the downstream process at the design rate while the lobe pump has the ability to turn off when it oversupplies the piston pump suction drum by use of a level controller turning it off or on.

 

 PD Pumps in Series.xlsx   45.95KB   121 downloads

[xavio]

docdan,

 

Indeed it is a weird way to operate PD pumps, would you mind providing a simple PFD?

How do you determine that the discharge pressure of the lobe pump is too high or too low, so that it is time to adjust the pump speed?

The discharge pressure of the lobe pump will be determined by the backpressure created by the HE and piping, which is fixed, unless flowrate is changed. PD pumps create flow, pressure is the result of flow.

 

Usually, the speed control of the PD pump is intended to adjust capacity (flowrate), because pressure is fixed by downstream equipment.

In your current configuration, it seems that you're trying to control the uncontrollable.

The throwback control should help alleviate the problem, it is indeed a common way to control PD pumps.

However, you will need an FIT (e.g. orifice) at discharge pipe, and take this flow as control variable for the control valve on the throwback line.

Please note that your downstream piston pump will also require similar throwback line, it cannot automatically compensate the change of flow by itself.

 

Mr. Montemayor's suggestion is also meant to control flowrate, albeit indirectly.

The pressure of the suction drum, if not additonally controlled, will be the vapor pressure of the liquid.

If you follow his suggestion, you will get additional benefit for your piston pump, i.e., more stable liquid supply.

The LT output may also be used to control the flow of the piston pump.

 

Hope it helps.

 

xavio

[docdan]

Mr. Montemayor did an excellent job of translating my poetry into a PFD  ....

 

On the one hand, the process requires these pumps in series. OTOH, I understand the need to decouple the action of two PD pumps. It appears that my predecessors attempted to decouple the extremely fast process (pressure in the liquid line between the pumps) by using a slow control element (the VFD). Not very effective, in fact, destabilizing.

 

I agree with Mr. Montemayor that the best decoupling is a surge volume.

 

Thank you both.