what happens if we are having very very high cavity and
flashing in a control valve..??? is it stops
working...immediately... or eroded...slowly
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Answer Posted / www.islamqa.com
Actually the valve will not stop working, but as a result
of continous errosion it will be useless and you will need
to change the valve completely.
talking about solution need more details about the
situation but here is the basic guide lines:
# To avoid flashing:
====================
1 - valve selection with high Fl.
Fl:valve pressure recovery coefficient, and this introduced
by MASONILIAN as: Fl=(P1-P2/P1-Pvc)^.5 - and Fl max is 1.
where FISHER introduced Km =(P1-P2/P1-Pv) - and this is a
valve parameter presented by vendor and it's calculated by
lab tests, and usin Km we can get allowable(P1-P2) which
will not cause flashing.
2 - using anti cavitation trim, which generate multi vena
contracta points in the presuure profile, and this prevent
pressure drop below Pv.
3 - To cool down the process material to make Pv very low.
4 - To change the valve position to another place with high
pressure, to make pressure drop through the valve away from
PV.
And anyway if there is no escape from flashing, it's better
to sustain flashing within the valve to assure that we will
not have a cavitation when pressure recover, and liquid can
be recovered and bubbles can implode any other place away
from the valve.
and this can be achieved by heating up the process liquid
so we raise Pv, and a good indication for us to be sure
that flash will be sustained is p2, as we can know P2 at
downstream pipeline and make sure that it's below Pvc of
the process liquid.
Regards,
Ahmed Sabry
Control and Instrumentation Engineer
GPC - General Petroleum Company
ahmedsabry.online@gmail.com
CELL.: 00 2 0121367310
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