5 replies to this topic

[luuquocdai]

Dear my friends

I have a problem which I want to consult with you.

In the system where I am working, we have 4 PCVs (regulators) with set pressures as detailed in the attached diagram.

I don't understand what the rules are when setting the pressures for these PCVs.

PCVs 1 & 3 are Fail Close.
while PCVs 2 & 4 are Fail Open.

Please help me by feeling free to give your opinion.s
Thanks in advance.

Luu Quoc Dai
luuquocdai@gmail.com

Attached Files

•  PCV.pdf   18.08KB   60 downloads

Edited by Art Montemayor, 01 July 2010 - 09:58 AM.

[djack77494]

Hard to fathom what the designer was considering, but I can offer my opinion. I'd say that control of the downstream pressure is VERY important since four PCV's are devoted to this purpose. Furthermore, with one in each half of the circuit failing closed, it is more important that the downstream equipment not be over pressurized rather than under pressurized. Continuing, I'd say that PCV2 is normally "in control". It is maintaining downstream at 800kpa. PCV3 and PCV4 are closed since they can easily satisfy their requirements and then close. PCV1 is also doing its job by maintaining the inter-valve pressure at 850kpa.

Now let's look at what happens if a valve fails. If PCV1 fails, PCV2 is ineffective. PCV3 and PCV4 would maintain downstream pressure at 750kpa, a drop of 50kpa from normal. Probably not a big problem. If PCV2 fails, then PCV1 will maintain downstream at 850kpa, an increase of 50kpa from normal. Again, probably not a big problem. If PCV3 fails, you wouldn't even know. If PCV4 fails, then PCV3 will be actively operating to maintain the 800kpa pressure in parallel with PCV2. So downstream pressure is maintained between 750 and 850kpa even if a control valve fails. A pretty robust control system.

[luuquocdai]

thanks, jack.

[chemsac2]

luuquocdai,

How can PCV2 and PCV4 have different set pressures?

Assume these to be for making-up pump-out. If pumpout is 100 m3/hr, PCV2 and PCV4 together would make-up 100 m3/hr equivalent.

However, PCV2 would want to maintain pressure at 800 kPa and PCV4 at 750 kPa. If pressure is maintained at 800 kPa, PCV4 would remain closed and you may not get flow equivalent to 100 m3/hr.

If pressure is maintained at 750 kPa by PCV4, PCV2 would sense low pressure downstream and open more giving flow more than 100 m3/hr.

System would be fluctuating in my opinion. Are line sizes different with different capacity PCVs.

Sachin

[fallah]

Assuming the following:

• The simultaneous failing of the PCV1 and PCV2 would be considered as double jeopardy;
• The set pressure of the PCV2 would be 750kpa; and,
• Both PCV1 and PCV2 would be fail open.

Then,

To maintain downstream pressure between 750 and 850kpa we can eliminate the parallel line containing PCV1 and PCV2 and the single line containing PCV1 and PCV2 would be adequate.

Please correct me if I am wrong.

[fallah]

When the pressure would be maintained, making-up for pump out is achieved and there is no need to worry about that.