CPV before control valve

[DYV1973]

LS,

We have a system in which 2 fluids (A and B) are blended together. A is the main flow to which B is added with a control valve (pneumatic with positioner). The system runs without pump, but the driving force is gravity (tanks are 50 metres higher). As flow and pressure are fluctuating, someone suggested to install a constant pressure valve (constant pressure on the outlet of the CPV towards the control valve).

Can someone tell me what influence we can expect on the stability of the control valve? Will both valves be influencing eachother?

Thanks,
DYV

 

[BigInch]

We have PV, FCV, CV, PCV, and now ...CPV? What does CPV mean? If I had to guess I'd say "constant pressure valve". Maybe CPV was supposed to tell me these things, but I'm not sure,

Why/How does the pressure variation (from liquid level in the tanks?) adversely affect mixing these two products now?

How do you control the amount of B mixing with A now? In other words, what signal positions CPV's positioner, pressure, flow rate, color, temperature, and where is that being measured? At tank B or downstream of the A & B junction point?

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[DYV1973]

Yes, CPV is a constant pressure valve. It has two sensing lines before and after the CPV, and a spring to adjust the pressure after the CPV.

The situation now is without the CPV, and the pneumatic control valve (equalpercentage, parabolic plug) is used for controling the blended product. This is initially done on ratio (by measuring both flows), and after a defined period the ratio is adjusted by a viscosity control of the blended product.

Problem is that pressure in B has to stay higher than pressure in A. Due to pressure fluctuations in B, we now have problems in keeping the control stable.

So my question was, what will be the influence of a CPV, when it is installed before the control valve.

 

[StoneCold]

Personally I would be hesitant to add more parts to this thing. It sounds like it is too complicated already. Why do you switch from ratio control to viscosity control? Can't you just run on one or the other all the time? If one of the flows is much larger than the other could you put the smaller flow on a metering pump that ran on viscosity control or ratio control with the other flow. I think that would be more robust. But I suppose that if the CPV valve is not a lot of money you could try it out. I am doubtful that it will help. But I have been wrong lots of times.

Regards
StoneCold

 

[BigInch]

I agree with StoneCold, that you'd probably have a better system with a dosing system of some kind. Trying to control mixing flowrates, with varying pressure doesn't work. If you still want to try, I'd go with downstream pressure control valves coming off both tanks, not two valves in the same line.

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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain