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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.
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