5 replies to this topic

[chemtan]

In a flare purge line, I can see a PCV and an RO.

The datasheets of PCV says:

Inlet = 400KPag
Outlet = 380KPag
IA Failure = retain last state.

RO datasheet:
Max flow = 22kg/hr
Max. Delta = 380 KPag

So the sequence of process is:
400KPag purge gas (Natural gas) to PCV

I can see that RO is here to control purge gas flow so that if PCV malfunctions. But the DP against RO is so high... wasn't it enough to have a RO alone on this line who could drop the pressure from 400KPag to 2KPag (it is already dropping 380KPag to 2KPag).

Is there any other purpose of having PCV + RO control on purge gas that I'm not able to grasp??

Attached Files

•  Purge gas (PCV + RO).pdf   8.7KB   86 downloads

Edited by chemtan, 15 January 2010 - 04:48 AM.

[shan]

In a flare purge line, I can see a PCV and an RO.

The datasheets of PCV says:

Inlet = 400KPag
Outlet = 380KPag
IA Failure = retain last state.

RO datasheet:
Max flow = 22kg/hr
Max. Delta = 380 KPag

So the sequence of process is:
400KPag purge gas (Natural gas) to PCV

I can see that RO is here to control purge gas flow so that if PCV malfunctions. But the DP against RO is so high... wasn't it enough to have a RO alone on this line who could drop the pressure from 400KPag to 2KPag (it is already dropping 380KPag to 2KPag).

Is there any other purpose of having PCV + RO control on purge gas that I'm not able to grasp??

This is not a PCV (Pressure Control Valve). It is just a block valve (on/off) to open or shut down the gas line. 20 Kpa pressure drop is not purposely controlled. I mead it is OK even if the pressure drop is 10 Kpa or 30 Kpa.

If you have no RO, you will lost restriction of gas flow. If you have RO only, you will have no way to shut down the purge line. Therefore, you need the valve and RO both.

Edited by shan, 15 January 2010 - 08:57 AM.

[ponnusj]

A PCV on the purge line u/s of the RO may be to maintain the RO u/s pressure steady, incase of fluctuating Natural Gas pressure. Flow through an RO will vary with varying u/s pressure.

[shan]

A PCV on the purge line u/s of the RO may be to maintain the RO u/s pressure steady, incase of fluctuating Natural Gas pressure. Flow through an RO will vary with varying u/s pressure.

To me, it is simply a gate valve at the upstream RO. How do you figure out the valve is a PCV to maintain RO upstream pressure steady not to maintain RO downstream pressure steady without any pressure element?

[chemtan]

It is a PCV, a self regulating type. I have attached an updated flow chart with more clarity. Please excuse me for its standard, I used MS-word for this purpose.

I cannot understand that;

1. If it is a PCV + RO: PCV is to regulate pressure at RO inlet (provide steady 380KPAg). then why we have RO Delta P of 378KPAg. if you see the diagram, the RO downstream pressure is just 2KPAg.

The PCV providing 20KPA Delta is of any use? I think RO can be sized to drop 400KPAg to 2 KPAg (it is already dropping 380KPA to 2KPA) - why is it not a simple gate valve instead of PCV?

Attached Files

•  PCV + RO (Updated).pdf   9.42KB   48 downloads

Edited by chemtan, 19 January 2010 - 03:03 AM.

[joerd]

The flow through the RO is only dependent on its upstream pressure. So, indirectly, the PCV can regulate the flow, and will ensure constant flow rate. This setup works well if you have unsteady pressure conditions in the upstream system and you still want a fairly constant purge flow rate. You can also easily turn down the flow by lowering the PCV set pressure. This may be (but I'm guessing) why the pressure drop is low for the PCV, because typically you would like to turn down as much as you can (= save fuel gas) while maintaining a good purge rate. Another reason could be wear of the PCV if you give it a high pressure drop; it's cheaper to replace the RO if it wears out.