Earlier post "A refresh to Process Engineer on few phenomenons in restriction orifice" has discussed briefly how restriction orifice (RO) works. Maximum velocity and minimum operating pressure is expected at the vena contracta, just some short distance downstream of restriction orifice.
Careful review on the pressure profile, the upstream pressure is decrease to minimum and recover back to downstream pressure. In the event fluid vapor pressure is just marginal lower than downstream pressure, minimum pressure at vena contracta would probably lower than fluid vapor pressure. Vapor condensation would result "implosion" and generates noise and severe vibration. This phenomena is well known as cavitation.
One way to avoid cavitation in restriction orifice is to reduce pressure drop across RO so that the pressure dip is maintained as high as possible. This results multiple restriction orifices to break the pressure drop in several steps.
How to determine if a restriction orifice will experience cavitation ?
The following simple assessment method base on work done by Tullis & Govindarajan (1973) may be used.
i) Determine Upstream Pressure (Pu), Downstream pressure (Pd) and Vapor pressure (Pv) base on upstream temperature for conservatism
ii) Determine Cavitation level (Cl)
Cl = (Pd-Pv)/(Pu-Pd)
iii) Based on orifice-diameter ratio (beta), determine allowable Cavitation level (Cl,a) from following chart.
Source : Tullis & Govindarajan (1973)
iv) Cavitation will occur if Cl is less Cl,a, multiple ROs may require.
iv) Cavitation will occur if Cl is less Cl,a, multiple ROs may require.
Restriction orifice is widely used in blowdown line to limit excessive flow to flare system. This is the best location to experience cavitation from restriction orifice and can not be eliminated. Severe noise and vibration is expected. Downstream piping may fail on severe vibration. Acoustic induced vibration assessment shall be conducted to ensure BDV/RO downstream piping always provide high integrity to resistance severe vibration cause by RO.
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