Design pressure of piping downstream of an isolation valve that is capped with a pressure gauge

[johcatrac]

Unless I am wrong, isn't the design pressure of piping downstream of an isolation valve that is capped with a pressure gauge, supposed to be equal to the upstream design pressure? And I believe this to be case also even if there are two isolation valves that are normally closed; since valves cannot be guaranteed to be 100% leak tight. By this virtue, the pressure gauge's design pressure should equal the upstream (of the valves) design pressure.

Any feedback or opinion is welcome, please.

 

[EmmanuelTop]

There is no sketch but I agree with your assessment, from what I pictured in my mind. Anyway, I could not imagine why would someone use piping class with lower pressure rating just for a meter or two of piping "downstream of isolation valve and capped with a pressure gauge". It makes no sense. That is why I think you should post a sketch.

What is the purpose of pressure gauge that is located after a closed isolation valve?


Dejan IVANOVIC
Process Engineer, MSChE

 

[johcatrac]

Thanks - the sketch is attached hereweith

 

[BigInch]

You must have the same design pressure downstream. Otherwise it would be prudent to install a pressure reduction valve & relief valve set to the lower design pressure, or a high integrity shutdown valve system (HIPS) inbetween.

 

[EmmanuelTop]

Branch piping should have the same design pressure - you got that from BigInch.

Also, I don't see the purpose of pressure gauge with a "Normally Closed" valve upstream of it - unless this drawing refers to flushing/blowing procedure or similar.

Dejan IVANOVIC
Process Engineer, MSChE

 

[moltenmetal]

It is permissible to isolate high pressure from lower pressure by means of a valve or valves. How many valves, how they are configured etc., depends on the magnitude of the hazard which may result.

If through leakage of valves is cause for concern, a block and bleed or double block and bleed may be required for isolation. In some other cases, a single check valve may suffice. It depends on the hazard.

If, however, a piece of instrumentation such as a pressure gauge may be damaged, or worse still, potentially rupture with injury, merely as a result of the misoperation of a single hand valve (a highly probable event), most designers would find the resulting risk to be unacceptable and would require a means to mitigate that hazard.

Let's say, for instance, that a line may be at high pressure during some operations, but may require accurate measurement of a lower pressure during a separate operation or step. There are a number of ways to do this safely. One is to specify a pressure transmitter rather than a pressure gauge, which has sufficient overpressure handling capacity to survive misoperation of its block valve. Another would be the removal of the instrument during high pressure operation. Another would be the installation of a protection device such as a relief valve or rupture disc. Yet another might be physical, electrical or pneumatic interlocking which prevents the valve from being misoperated. Which of these measures is best for the particular situation requires a basic hazard identification exercise based on the specifics of the application.

 

[LittleInch]

A bit of an odd situation and I think you need to explain what's going on here rather than snippets of information.

Looks like the PI/PG in question is only used on certain occasion to confirm low or no pressure in a system normally with a much higher pressure. In that case the PI/PG really should have a snubber or pressure limiting valve in front of it to avoid over pressurising the gauge accidentally. Also then needs a vent valve to release this pressure at some point on the PI line.

he key lies in the purpose behind this instrument.

you could always use a PT with a display where the body of the PT is rated to full U/S line pressure, but the element inside is ranged for a much smaller span. If you over pressure the PT, I don't think it likes it very much, but it won't break.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

Looks like the valve and gauge are being used for "pressure requlation" in downstream piping and it is being performed by sight and hand... probably not the safest method one could use.

 

[johcatrac]

The gauge is being isolated only to be used under the scenario where we have to bring in external water supply (lower pressure) to the parent system. Parent system design pressure is 145 psig. After external water supply is brought in, this supply will be turned off and the gauge isolation valves will be opened to get the static head pressure reading of the parent system. The maximum static head pressure that could be read is 58 psig. The gauge proof pressure rating is 87 psig (i.e. beyond this pressure, gauge accuracy is not guaranteed). Gauge burst pressure is 4700 psig.

 

[BigInch]

To be safe it must be impossible that your supply pressure is capable of exceeding the downstream piping design pressure.

 

[johcatrac]

So even if the isolation valves are resilient seated, cannot be guaranteed for 100% seat tightness, correct?

 

[BigInch]

Not if they can be opened.

Double block and bleed with cavity drain might work.

 

[moltenmetal]

Do I understand this correctly: your piping is good for 145 psig, and your gauge has a "proof pressure" of 87 psig but a BURST pressure of 4700 psig? That makes no sense, but in that case the consequence of misoperating the valves or allowing them to leak for long periods without venting would be damage to the gauge but NOT rupture. Unless this is very hazardous material, most people would be totally OK with a single block valve in this case, isolating the gauge, with a bleeder to protect the gauge against leakage.