PRV question 4

[Lan123]

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.

 

[bimr]

Yes, that is one approach that may be used.

 

[georgeverghese]

When you say side by side, do you mean the valves are arranged in parallel or in series?

 

[LittleInch]

Could it be fine, yes. Is it a good idea, no.

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.

 

[Lan123]

Thanks again for all the replies.


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.

 

[bimr]

When designing a network, it is important to incorporate fixtures that can be maintained with minimal disruption to normal flow regimes, using hygienic operating and maintenance practices. Bypasses for devices such as flow meters and pressure reducing valves should be installed that will allow the devices to be taken out of service for maintenance.

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.

 

[Gator]

It seems we are talking about a control station and globe valves are typically used for the bypass line (like bimr says). See B. below.

 

[LittleInch]

I appreciate option b turns up, but my normal action it's to redline the bypass valve and delete it. Doesn't matter what notes you put on a p&id, or locked closed, if it's there someone can open it with no instructed system to protect the downstream system. IMHO that's a bad move.

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.

 

[Gator]

What is the fluid, anyway?

In large facilities like refineries there are literally dozens (if not hundreds) of control stations with (usually) globe valve bypasses.

 

[georgeverghese]

When you say this is a struggling country, it may be that the nearest reliable maintenance centre for PRV1 could be far way and it could be some time (days to weeks) before you get this valve repaired and sent back. And to expect operators to manage with a standby manual valve on a parallel line during this time may too optimistic, especially if there are fluctuations in downstream demand flow and / or upstream pressure. It may be okay for short term operation.

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.

 

[Gator]

Actually, the OP wrote "county" and that led me to believe that this was a rural water or NG distribution network and replacement would be in the order of days (if it's in the "western" world). What's the fluid?

 

[georgeverghese]

Gator, Missed that, but I doubt the term "struggling county" makes sense, which would be somewhere in the western world - there may be some remote parts of the US that fit that bill that I dont know.

 

[Lan123]

Thanks all. This is a for a water distribution system in rural US. Having read all the comments I now feel that it is worthwhile to use a PRV for the backup valve as well.

 

[Gator]

Don't forget block and drain valves!

 

[SNORGY]

LittleInch,

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.

 

[LittleInch]

Snorgy,

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.

 

[SNORGY]

LittleInch,

Fair enough. The answer, then, I suppose, is reliability and / or redundancy in the control valve(s). Different approach, nothing wrong with it.

 

[77JQX]

In pressure regulating stations in our municipal water system we generally use a normally closed gate valve in a bypass around the regulating station. The regulating stations themselves generally have two regulating valves - one for normal flow, the other for fire flows. If the low flow valve fails, the large valve can do an "OK" job of regulating low flows while the small valve is out for service. If the large valve fails, we notify the fire department while its out of service, and if there's a fire, the gate valve bypass can be manually opened.