3 replies to this topic

[gimenz]

Dear Sirs,

Good morning.

I would like to ask for guidance/advice on this specific matter. I tried to use google, but I couldn't find the specific topic in the results.

I have this manual control valve upstream of a fractionator (vapor effluent from coking will pass thru this valve).
The conditions through the valve are: mixed phase, 430 degC, and 1.05 kg/cm2G.

I would like to ask if there's a need to put 2 PI's or a PDI across the control valve.

If this is not applicable to system I mentioned above, I would like to understand when there is a need to put PI's or a PDI across a control valve.

General information is highly appreciated.

Thank you.

-Ray

[Dacs]

I assume that this valve is in the line as a feed going to a fractionator.

What is your intent with this manual valve? Because if you're thinking on controlling the pressure of the fractionation column, then it's difficult (if not impossible) to do so.

Anyway, looks like (this is highly my opinion) that valve (why manual why why why? ) serves to control the flow. In that case, you need a flow indicator in sight of that valve to regulate the flow. You'd also wanna place a PI upstream of the valve to get the upstream pressure but you don't normally place a PDI on a valve because it will serve little purpose.

If you want to get the fractionating colum pressure, it's usually being done through a PI on the overhead line. PDI are useful for columns to check internals pressure drop.

I learned this from a senior engineer years back that the ultimate purpose of instrumentation is to guide the operator to make sound decisions when running the plant. So ask yourself if placing PI/PDI will serve any purpose for the operator.

Hope this helps

[kkala]

Having seen PDIs across strainers or bag filters, I understand they aimed at monitoring scale formation. Ideally this would be under constant flow rate, though there were cases where flow rate was not measured. Pressure gauges upstream and downstream heat exchangers or hydrocyclones can serve that purpose in a less accurate manner.
PDI across a valve has not been seen. Agreeing with Dacs, flow control is good; but flow measurement on mixed phase flow can have problems * (including scales). Probably there is flow measurement upstream. Then increasing ΔP across the valve will warn about scale accumulation. But PI downstream is not necessary, if fractionator pressure in is (nearly) constant.

* E.g. see http://www.cheresour...-flow-readings/ '> http://www.cheresour...-flow-readings/ , concerning gas/liquid. Opinion here is vaguely worse on similar measurements.

Edited by kkala, 17 August 2012 - 10:10 AM.

[ankur2061]

gimenz,

Differential Pressure Control Valves (DPCVs) which you are calling as manual control valves are commonly used in many applications. They are also mentioned as presure independent control valves (PICV). The main purpose of such control valves is to maintain a constant differential pressure irrespective of the variation in the upstream pressure. Normally DPCVs have in-built differential pressure measurement.

You are calling them as manual valves since they are not modulating in the true sense like a conventional control valve. The modulation is limited to the extent of maintaining the differential pressure across the control valve which is either factory configured or can be configured locally on the valve.

I am providing some links for more information on DPCVs:

http://www.frese.eu/...Valves/Frese-PV

http://www.cranefs.c...FS-DPCV-IOM.pdf

http://kb.eng-softwa...rol Valves Work

Hope this helps.

Regards,
Ankur