Sequence of Gate and Check 1

[PipingAbacus]

What are the reasons, if any, why anyone would prefer to locate the check upstream from the gate va? Or vice-versa. I am a designer and I am not sure if this matters at all.

Please chime in.

BTW, this is my first of hopefully many posts.

 

[pennpiper]

Piping Abacus,
I assume you are speaking specifically about the sequence of the valves in the discharge line of the typical (paired) centrifugal pump.
Correct?
If so then it is simple.

If there is a failure of the pump or any part of the piping components (i.e.: the check valve) you isolate the system by shutting the block valve (Gate) drain the piping and remove or repair the broken item.

If the check valve is outside the block valve and it is broken you cannot isolate it without shutting down the whole unit.

 

[vpl]

To expand on Pennpiper's remarks:

The check valve is there to prevent (or reduce) backflow while the gate valve is there to isolate. You need to look at why the valves are there in the first place and what function they're performing.

It could be that the gate valve is there to isolate a standby system while the check valve is there to prevent backflow when the standby system isn't isolated, but isn't operating either.

Patricia Lougheed

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[zdas04]

Another way of saying what PenPiper said is that if the check valve (on any system from a pump to a meter run, to a wellhead) fails it is handy to be able to shut the block valve to fix it. If the block valve fails, the check valve is not a safe isolation so it isn't much help.

Hydraulically there is no difference.

David

 

[PipingAbacus]

So ... if check valves rarely fail ... then there is a very slight advantage to have it protected by a gate valve. Other than a maintance issue, there is no other reason.

Thanks to all who replied and to those who reply after this email.

 

[waterpipe]

Well, I see it not as a "very slight advantage" but the whole reason for having a block valve. You should see the faces of bosses when the whole process is forced to be shut because of a misplaced check valve ...
I am also thinking that your centrifugal pump always starts and stops against a closed block valve. if the correct order the check valve is "out of the line" while if it would be installed without being protected by the block valve, it will be under static pressure of you delivery line when the pump is closed. I am not sure if this could "shorten" the check valve's life (depending of the CV type and sealing, etc) but I prefer to see it in the correct location.

 

[zdas04]

PipingAbacus,
I would say that differntly. How about "since check valves occasionally fail and since the sequence doesn't make any hydraulic difference why not put it behind the block valve?".

David

 

[rmw]

I read back and couldn't find it. Who said "check valves rarely fail?" I'd go a little past zasd04's generosity and use the term "check valves often fail."

Maybe I just didn't get the good ones, or I have just have had to dig one too many clappers out of downstream equipment but I always design assuming that the check valve can and will fail. Then what?

rmw

 

[SNORGY]

When in doubt, follow convention, unless it's fundamentally wrong.

Convention would put the check valve before the gate valve in the direction of flow.

If nothing else, a reduced port gate valve might impede the travel of a rogue broken flapper.

That said, I have been fortunate in that I have not seen "total failure" in check valves; it's usually been just seat damage, etc..

Regards,

SNORGY.