Downstream pipe design conditions of a drain valve that is capped

[johcatrac]

Shouldn't the pressure/temperature rating in the downstream piping stub of a normally closed isolation valve be the same as upstream of the valve?

 

[MickMc]

I would say yes unless there is positive isolation e.g. spectacle blind/spade after valve, assumption is that valve is rated for upstream. Typically seen for closed drain system which is Class 150 but fed from Class 150/300/600 etc.

 

[LittleInch]

For a non flowing stub, you can sometimes argue that the temperature is ambient or lower / higher than the process flow, but pressure should be the same.

Begs the question why this stub is there in the first place??

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

 

[bimr]

The pressure may vary if there is an elevation difference.

 

[KevinNZ]

If the drain piping downstream of the isolation/drain valve is open to atmosphere and no other valves in the line then maximum pressure the drain line can see is dependent on the flow you will get when the upstream system is a full pressure.

Certainly the start of a drain line may see full pressure. But the discharge end may never get this high.

Generally we design drain lines for upstream pressure. We may reduce this for EQ design.

 

[curtis2004]

Little Inch,

I have seen capped valves used for vent and drains in steam power plants. These are required for venting and draining during hydro test. Once hydro test is complete those valves are cap welded. My only guess this is done just make sure that nobody should open those valves after hydro test, because of high pressure high temperature steam inside lines.

If in the future you need to conduct another hydro test (for instance, maintenance or replacement of the lines), they only need to cut one fillet weld to have vents and drains.

Regards,
Curtis