9 replies to this topic

[thorium90]

Dear all,

I have come across a difficult control problem and would appreciate your advice on a suitable method of control.

I have 2 VSD Pumps that pump into a common header. The common header has a flow transmitter. This header provides flow into 5 parallel filters. Each of the 5 branches contain 1 control valve, one flow and one pressure transmitter. All 5 branches exit into a product tank with a level transmitter. It would be desirable to maintain the product tank level. The pressure to each filter would vary over time as the filters gradually foul up and thus the pressure would increase to maintain production.

I'm finding it difficult to find a suitable control scheme that would control flow and pressure in multiple variable speed pumps along parallel lines with parallel control valves.

 

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[Bobby Strain]

Well, two variable speed pumps operated in parallel should always have flow control on each pump discharge. Unless the head curve is very steep and you know that both pumps running together will always have sufficient flow. Without flow controllers on each pump the operator is the controller. So, the first order of business is to put flow controllers on each pump. Then you can use a controller to vary pump speed to maintain tank level. And set the two flow controllers to balance the load. Something like a smart controller with all the functionality to vary speed and adjust to maintain balanced flow.

 

Bobby

[thorium90]

How about the 5 control valves? Would it be better to be flow controlled then?

[curious_cat]

How about the 5 control valves? Would it be better to be flow controlled then?

 

Naive question but what's the actual purpose of those 5 branch valves? What if you let the pressures there float? 

 

i.e. Say a filter starts to foul, the delta-P across it increases so it automatically acts like a throttled valve so naturally the relative flow increases to the other filters in parallel & things even out?

 

So you'd be controlling the level by manipulating the VFD signal for pump rpm & letting the filter inlet pressures float. Will this not work?

 

For faster response you could have the tank LC feed a set point to an FC controlling flow in the main header in cascade mode by rpm control of both pumps?

Edited by curious_cat, 06 September 2015 - 04:23 AM.

[PingPong]

I agree with Bobby: when operating two centrifugal pumps in parallel you need to have a control valve in the discharge of each pump.

 

I agree with curious_cat: why have a control valve upstream each filter? Just let the flow divide over the filters as it likes, but keep a flow indicator at each filter.

[thorium90]

The filters are expensive membranes and control valves are installed on each of their inlets to control the flux going through each one. Over time, those membranes that have fouled more would have their control valve opened bigger to force the same amount of fluid thorough and those that are still clean can be opened less so that they are not over fluxed. Otherwise, the fluid will just follow the path of least resistance and the cleanest membrane gets all the flow and get fouled up immediately (and become impossible to clean or get damaged from too high flow going through the pores) while the dirtier ones get very little flow.

Edited by thorium90, 06 September 2015 - 07:08 AM.

[curious_cat]

About the pumps in parallel on a VFD, do they have identical characteristics?

 

Let's assume they have identical characteristics: Then, one design decision might be whether you want them to be both VFD controlled in parallel or only one gets VFD controlled at a time? i.e. Are they both operating at identical operating points or is one operated at the BEP and other at reduced RPM. 

 

Not sure if I'm conveying the point clearly, but it looks like the same overall operating point could be reached by two quite different operating strategies on the pumps in parallel having a VFD. 

[thorium90]

The VFD pumps are identical, therefore they would each have to operate at the same point as they go into the same common header. The pump discharge only has on/off valves.

Edited by thorium90, 06 September 2015 - 07:41 AM.

[PingPong]

The filters are expensive membranes and control valves are installed on each of their inlets to control the flux going through each one. Over time, those membranes that have fouled more would have their control valve opened bigger to force the same amount of fluid thorough and those that are still clean can be opened less so that they are not over fluxed. Otherwise, the fluid will just follow the path of least resistance and the cleanest membrane gets all the flow and get fouled up immediately (and become impossible to clean or get damaged from too high flow going through the pores) while the dirtier ones get very little flow.

I am sorry, but that operating philosophy makes no sense to me.

 

A filter or membrane feels pressure drop, not flux. If one gets more fouled than the others, I don't see why you would want to keep pushing the same flow through it, thereby running the risk of damaging it due to high pressure drop.

[thorium90]

It can withstand some amount of pressure drop. But each one will have a different pressure drop and require a different pressure to maintain the same amount of flux, and hence a control valve is used. Cleaning will occur when pressure drop has reached a threshold.

Edited by thorium90, 06 September 2015 - 08:17 AM.