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A pressure seal valve needs to have its body/bonnet bolts torqued
down to the specified torque when the valve is at operating temperature
and pressure. This is necessary for the pressure seal gasket to be
properly compressed. The pressure seal concept is designed to seal
with operating pressure. The bonnet is forced up into the gasket with
increasing pressure, which in turn forces the gasket to seal against
the valve body. This will only result in greater sealing capability
when the bolts are re-tightened to resist the added force.
The gasket bolt tightness must
be rechecked after operating test pressure and temperature has been
achieved. Without a live-loaded bonnet, bolt tightness will be lost
when the system loses pressure. A live-loaded bonnet may also lose
some bolt load, due to vibration in transit or bolt creep. It is essential
to check live-loaded bonnet bolt torque at operating pressure. Once
torqued at operating pressure, no future tightening of gasket bolting
will be necessary, unless the valve has been dismantled.
Check the gasket torque at operating temperature and pressure and
adjust to the required value.
Knock-out holes are a feature incorporated in some valves to remove
the segmental packing rings. A loss of pressure-seal contact between
the body/bonnet/gasket can cause steam or other fluids to leak out
of a knock-out hole.
Typically, when a system sees startup or fluctuating pressures, a
wisp of steam can be released through these holes. Flow through the
knock-out hole usually subsides quickly. Reapplying torque to the
body/bonnet bolts at operating temperature and pressure is necessary
to fully seal a pressure seal valve.
Installing a graphite gasket in a pressure seal valve is a delicate
process, please review the recommended
procedure prior to doing this work.
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