PCV or LCV control setup? 1

[bouk]

I'm trying to work out whether I can use a level control setup for my solution. I've attached a schematic showing only the water stream which I am dealing with.

My pump is a positive displacement pump. At the moment my configuration uses a pressure control setup but I am not sure if this is possible and I think it would be better to use a level control setup, but then my LCV is linked to the level in the separator and I am not sure about the control of the pump?

The separator is large and fluctuations are not expeced.

 

[BigInch]

I have no idea what you are trying to indicate on your drawing, but the best way is to read and use THE SAME variable FOR control that you want TO control. Whenever you can develop the control parameter in terms of the same control variable, do it. Ie, don't use pressure to control level, or level to control pressure. Use pressure to control pressure and flow to control flow. Sometimes you might have to use temperatue, or some other variable, to control flow, but don't do that unless you really have no other methods available.

If you want to control the level in the tank, use level sensors. If you want to control suction pressure, use a pressure gage. Don't try to control suction pressure by keeping the level in the tank within some range. Tank level controls should be reserved for things like prohibiting overflow and for not sucking crap off the tank bottom.

 

[JJPellin]

You do not provide enough information to make a meaningful response. I cannot determine what your diagram is supposed to show. In one of the versions, it seems to show that you are controlling the flow from a positive displacement pump by pinching on a control valve on the discharge. This can't be right.

The vessel is described as a three phase separator. In my world, that usually means separation of water, oil and vapor in one vessel. If so, then you must control the interface level between the water and the oil relative to the vessel internals (weir). Depending on what the vapor is, it may also be necessary to control the pressure to get a good separation of vapor from oil. But, all of this is pure speculation.

Please provide more information.


Johnny Pellin

 

[bouk]

Thanks for the responses so far, it is very difficult to explain without speaking face to face, so perhaps someone could provide me with contact details of a control engineering company that I could discuss use of a PCV setup against use of an LCV set up when running produced water from a separator through a hydrocyclone via a positive displacement pump...

 

[BigInch]

Use level control.

 

[ccfowler]

bouk,

Since you have a positve displacement pump, it would make more sense to be controlling a recirculating (relief) flow back to the source tank to control the flow rate (and, consequently, pressure) downstream from the pump. In any case, you need a pressure relief valve and relief line (probably back to the source tank) to avoid excessive pressure in the pump discharge line.

If I was designing such a system, I would have BOTH a pressure relief valve and line to protect against excess pressure from the positive displacement pump AND a control valve separately regulating the relief flow back to the source tank.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[dcasto]

use the PCV diagram and lose the PCV vavle. I am assuming that the level transmitter will run a VFD on the pump. If you need a slightly higher pressure in the clyclone, use an RO down stream

make sure your pump can lubricate itself at low speeds, if not you'll need a recycle line.

 

[Pumpsonly]

Hope I am not too late to this thread. You did not mention what are your trying to control and no one will be able to make the correct recommendation without this piece of vital info.
I would assume that your are trying to control the flow rate (and hence the pressure in side it )to the hydrocyclone which specified a narrow range of flow rate through it for the effective separation desired.
With a PD ( progressing cavity?) pump,the flow rate aspect is very much taken care off if it is sized according to the rated flow of the hydrocyclone. Pressure control will be applied to the outlets of the cyclone if the down stream pressure are not constant to maintain the flow split. RO can be used for this purpose.
Level sensing ( not level control )in the separator vessel is required for safe guarding the pump from dry running.
Your hydrocyclone vendor will be able to recommend the most suitable control system.

 

[ports394]

dcasto, I like your method. But it does mean the level in the hydrocyclone will fluctuate. Will that effect the OP's process?

OP: 99% of the time, never, ever, ever, put a flow control valve at the end of a positive displacement pump. When it pinches, you're creating a lot of back pressure, because that pump is CONSTANTLY running at X RPM and moving X * volume/rev forward. And if that valve ever closes, something will go boom.