Answer / ravi

Flashing:It is formation of vapour bubbles at vena-
contracta.

Cavitation:It is formation & collapse of vapour bubbles at
Vena-contracta.

Noise:Mech Noise,Hydrodynamic Noise,Aerodynamic Noise.

These all effects can be minimised by proper sizing of
Control valve as well as proper type & material of Valve
Body.

Answer / www.islamqa.com

# Flashing: it's formation of vapour bubbles in liquid
stream as valve cause pressure drop to the liquid below its
vaporization pressure.

# Cavitation: it's formation and collapse of vapor bubbles,
collapse occures due to pressure recovery at valve
downstream which cause bubbles to implode, giving microjets
and very high pressure waves (in range 100000 psi).

# Noise: is a result of turbulent fields generated through
the flow stream caused by many parameters:

- pipeline restrictions (orifice, valves, ..)
- pressure fluctuation
- unstable flow and very high flow velocity

and the noise radiation depends on pipeline material and
diameter and thickness.



# Here is the main guidelines to avoid flashing and
cavitation:

# 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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