how to protect the PTFE vlave packing and B/B gasket from damage during post weld heat treatment.

[ram21881]

can anyone please advise how to protect the PTFE vlave packing and B/B gasket from damage during post weld heat treatment for both small /large bore piping mainly for bonney forge products.
appreciate your helps.

 

[Danlap]

Conservatively you cannot. For B/B (Bolted Bonnet) valve it is highly recommended to dismantle the valve prior PWHT (and replace the "already" compressed packing/gasket after).
Your question is not detail enough:
- What valve type is this? Quarter turn or rising stem?
- Size and Class?
- Is this Spot or Oven PWHT
- What is the B/B (Body to Bonnet) gasket material

Since PWHT is normally well planned (enough time to buy packing set), will be done during pre-fab or prior full operation, normally there wouldn't be much hustle of dismantling the valve. But maybe you have different reason of not doing it
Nature of parameter you are mentioning in general are (open for detail input from others):
a. PTFE packing design temperature 150 to 200 degree Celcius. Exposing higher temperature will damage it
b. PWHT is approximately 650 deg Celcius for 1 hour per 1 in. wall thickness
c. Unless otherwise specified, controlled cooling after PWHT is required (approximately reduce 100 degree Celcius per hour)

So the question will come back to you. What stands between the PWHT area and packing chamber? and what is the heat transfer rate along? Does 650 degree will reach packing area within 2-3 hours of PWHT?
If yes, The only way to do PWHT without dismantling the valve is costly and have high risk (usually fails);
- Monitor the temperature of the packing and gasket during PWHT and cooling process
- Blow "cooler" air above Bonnet area. calculate whether it will really kept the bonnet as cool as 200 degree Celcius or below.
Risk:
- PTFE structure will still change due to high temperature
- Shall blowing cooler air not done properly, it will disrupt the PWHT process or even worse start crack propagation on other parts.

Better spend 500-3000 EUR of packing set per valve or re-engineered them rather than doing this risky task. Or consult with BFE whether it is possible

Regards,
MR




Greenfield and Brownfield have one thing in common; Valve(s) is deemed to "run to fail" earlier shall compared to other equipments

 

[niteshgargeck]

if you are going to do stress relieving then most probabely it will be low alloy steel valve. in low alloy steel high pressure valve always ask vendor to give graphite packing not ptfe because on your working temperature it will fail for sure.
and yes while doing pwht temperature will go more then 600 so your ptfe packing ill fail u cant save it.
so better to get valve with graphite packing.