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# Pressure Control Valve Parallel Operation

12/24/2009 2:23 AM

Hello guys,

i wana know why pressure control valves are connected in parallel for the inlet of veritcal vessel,during the HAZOP of this section there were a debate on the issue but due to lack of control phlosophy it was a little difficult to get clear answer.

jsut i wana any one explain what might be the possible reason,i blieve we are going to alocate the document for the cause,but i need to have some knowldge on the subject.

thank you

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#1

### Re: Pressure Control Valve Parallel Operation

12/24/2009 11:05 PM

Not quite sure if I understand your operation.

If you are using a 1/3 - 2/3 valve arrangement in parallel, the reason is to keep the valve seats from wire drawing when the capacity requirement closes the valve to near 0%. The theory requires the 1/3 valve to open fully and then have the 2/3 valve open for additional capacity. Some programmers open the 1/3 valve fully and when additional capacity is required, they close the 1/3 valve and modulate the 2/3 valve. The point is to keep whichever valve is operating at mid-span or higher to prevent wire drawing.

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Anonymous Poster
#4

### Re: Pressure Control Valve Parallel Operation

12/26/2009 4:53 AM

can you plz define the term wire drawing

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#5

### Re: Pressure Control Valve Parallel Operation

12/26/2009 6:42 AM

Wire drawing = when the pressure and velocity of the fluid or gas you are controlling to literally erode away the valve seat during partial openning of the valve. Leakage will occur after the seat is wire drawn requiring the valve to be rebuilt for proper service.

Wire drawing occurs when the valve disc and seat position operate for extended periods of time close to the shut-off point of the valve, and the water flow erodes or scores a pathway in the seating material that remains when the valve closes tight to the shut-off position. This allows small flows and pressure "creep" from the inlet to the outlet side of the valve at the closed position. Pressure creep into the distribution system is relieved with the operation of plumbing fixtures; however, in systems where fixtures may be lightly used over a weekend, as in a commercial building, the pressure creep may exceed the temperature and pressure relief valve settings on local water heaters throughout the building. The resulting unattended discharge of water from T&P relief valves may cause extensive water damage and bring litigation against the designer's firm.

Wire drawing of valve seat materials can be mitigated by the correct sizing of the valve (smaller is better here, as a smaller valve opens more widely) or the specification of stainless steel seats that resist the scoring action of the water at low flows.

Parallel and Staged Series Arrangements

 Figure 1

The limitations of each valve type can be successfully overcome by their use together, in parallel, with each valve overcoming the limitations of the other. The direct-acting valve is operational for low flows, and the diaphragm valve is operational for high flows above a preset maximum pressure drop of the direct-acting valve. Selecting valve sizes that allow the direct-acting valve to provide for 1/4 to 1/3 of the maximum probable flow demand, and the diaphragm valve to provide for the remaining maximum probable demand, will result in a smoothly operating pressure reduction across the full range of required flow. Arrangements may have the direct-acting valve included in the pilot piping of the diaphragm valve body to reduce the space required for overall installation, or it may be piped separately from the main valve (Figure 1).

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Anonymous Poster
#2

### Re: Pressure Control Valve Parallel Operation

12/25/2009 1:16 AM

I am sorry Ihad to use this thread.well I am with another problem. Our client is asking to include for Online Test cost in the bid and we are at risk not to include that. I need to know what is this Online Test for pressue / safety relief valve. Who does it and what does it cost.

requesting, yours

Akram

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#7

### Re: Pressure Control Valve Parallel Operation

12/29/2009 10:49 PM

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#3

### Re: Pressure Control Valve Parallel Operation

12/25/2009 3:33 PM

Somewhere in the commissioning of this system, the regulators will need to be set to the desired operating pressure, which will probably differ from the factory settings. You want each regulator to open, feed fluid into the vessel, and then close when the desired pressure is reached. The process should be run at a variety of flows to check properly that the regulators can keep up and close off under the range of conditions. Depending on how accessible the regulators are, and how easy/hard it is to manipulate the process flow, this may take from an hour to several hours. Usually, but not necessarily always, the installing contractor would be responsible for this.

[After answering your companion thread on this, I was going to add some remarks, but jcchiefeng's GAs have already said what I was thinking of.] BTW, another reason to have parallel regulators is that one can be serviced while the other remains in operation.

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#6

### Re: Pressure Control Valve Parallel Operation

12/26/2009 7:27 AM

Hello Guest,

You have already received detailed advice on the recommended practice for selecting valves in in a parallel configuration. It would be helpful if you added some info about the control system. Do the valves operate simultaneously or sequential? The sophistication of the control is what provides stability and reliability to cope with varying load conditions. Is your system part of a PLC based process controlled environment, or a simple direct acting local control ?

Remember when talking about valve capacity (its kv rating) that you are specifying the pressure drop in bars in order to determine the actual flow capacity.

Good luck,

Massey.

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