PRV question
PRV question
(OP)
This could be a silly question. Could there be a substitute for a Pressure Reducing valve (PRV). We have to installed a PRV between two pressure zones in our distribution system. To maintain a redundancy in case of the PRV malfunctioning we want to install two PRV units side by side. I am wondering whether a ball valve could be a solution for the second valve? The hydraulic grade line could be broken by throttling the valve until the PRV (Valve 1) is repaired and brought back to service. This will save some money for the struggling County.
Thanks in advance.
Thanks in advance.





RE: PRV question
RE: PRV question
RE: PRV question
Ball valves are terrible at controlling flow or pressure to any level of accuracy or longevity.
You provide no information to make any judgement here.
Is flow constant or varies a lot?
What is preassure drop range?
What is consequence of pressure control failure?
How will you monitor and control your manual valve?
Control valves like pressure regulating valves are normally there for a reason. Changing it for a manual arrangement needs proper thought and design.
For steady flow and as a manual back up I've seen multiple small tubes and orifices which allow someone to go up in steps of flow monitoring the downstream pressure out flow continuously.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: PRV question
Georgeverghese
You are correct. It is parallel...PRV always on duty and the second valve (Ball...) only when the PRV is out of service for maintenance. Will be a patc work for a short time until the PRV is back in service.
Little Inch. I will share more information (Flow range ... Upstream and DS pressure) after the hydraulic model runs are complete.
RE: PRV question
Pressure reducing valves are fairly reliable with minimal downtime. A better choice for the bypass valve will be the globe valve, which is the same style of valve used on the pressure reducing valve.
RE: PRV question
RE: PRV question
If a control valve is there it's there for a reason. Putting a manual bypass valve there allows some idiot operator to open it and all your carefully considered protection schemes go out of the window.
No valve, can't bypass a pressure control device.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: PRV question
In large facilities like refineries there are literally dozens (if not hundreds) of control stations with (usually) globe valve bypasses.
RE: PRV question
And a regular soft seated ball valve is not a good idea for throttling service, unless pressure drop is low and the fluid is non erosive.
As mentioned, the money spent on a fully automated second PRV may be recovered in a short while if and when PRV1 breaks down when you consider the continued business revenue at no loss in process safety / reliability.
Else you may explain why you think a manual ball valve can be a suitable option here.
RE: PRV question
RE: PRV question
RE: PRV question
RE: PRV question
RE: PRV question
Right or wrong, people I have worked with over the years very commonly put double block and bypass valving around pressure reducing valves per Gator's sketch (b). When I worked at a major Canadian EPC which had a huge American parent company, this was even their standard detail.
If there is a specification break across the valve station, we would typically safeguard the downstream piping with a PSV sized for control valve failure. You can also put a restriction orifice on the downstream side of the bypass valve if you are concerned about an overpressure scenario equal to or greater than what you would have to consider for the control valve failing wide open.
But, unless Lan123 has provisions like that, I agree with you; and I most certainly agree that a ball valve wouldn't be my choice in the bypass. I would do a globe.
RE: PRV question
I accept that there are many of these arrangements, but I've always argued against them in design and hazops. You can guard against failure as you indicate, but it is failure of something. Open a manual bypass and you've lost control, whether the change is fast or slow, manual control is never as good as process control loops.
It's much worse at pressure breaks, but I've yet to lose a reasoned argument on one of these arrangements. Where possible I get to delete it before it even gets to a hazop....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: PRV question
Fair enough. The answer, then, I suppose, is reliability and / or redundancy in the control valve(s). Different approach, nothing wrong with it.
RE: PRV question