Restrictive Orifice The FL The FLOW OW CO CONS NSUL ULTA TANT NT un uniq ique uely ly co cove vers rs re rest stri rict ctiv ive e an and d crit cr itic ical al fl flow ow me mete ters rs Wh Whil ile e re rest stri rict ctiv ive e or orif ific ices es an and d liquid Cavitatin! limitin! flow"rate devices are not flow meters they are widely used to limit or #rovide a #ressure loss $restriction% $restriction% in a flow system
Ans. The difference between limiting restricting flow are often misunderstood.
Crit Critic ical al
(gas (gas/v /vap apor or) )
or
Cavi Cavita tati ting ng
Liqu Liquid id
flow
and
Flow Flow: :
Gas Flow Flow-r -rat ate e is limi limite ted d to a fixe fixed d flow flow-r -rat ate e for for a give given n upstream pressure when a shock front is created at the minimum flow area of the device, this is established when the “ritical! pressure ratio is reached as the downstream pressure is reduced. For air this occurs when the the rati ratio o of upst upstre ream am pres pressu sure re to thro throat at pres pressu sure re is ".#$. %i&u %i&uid id flow flow is limi limite ted d when when the the diff differ eren enti tial al pres pressu sure re e&uals the difference between upstream and throat 'or for an orifice( vena contracta pressure is the vapor pressure of the fluid. )n both both cases cases *ernoulli+s *ernoulli+s no longer longer applies. applies. For a thick orifice the overall pressure loss the difference between the upstream pressure and the downstream pressure. For a stan standa dard rdi ied ed crit critic ical al flow flowme mete ter r the the pres pressu sure re loss loss is appr approx oxim imat atel el " / of the the chok chokin ing g diff differ eren ence ce betw betwee een n upstream and downstream choking pressure.
The
Flow
Consultant includes
critical flow orifices, Torodial Throat and lindrical 0enturi.
thick
Restrictive Orifices:
1estrictive orifices do not limit the flow but fix overall pressure loss for a given flow rate rate. . The The F%23 F%23 245 2456% 6%TA TA4T 4T comp comput utes es the the pres pressu sure re loss loss usin using g the stan standa dard rd orif orifi ice e&u e&uati ation with with the the 5tol tol derived discharge coefficient for pipe taps. 7ipe tap are located 8.# pipe diameters upstream of the plate and 9 pipe diameters downstream '8 :8 and 9 ;
taps(. This prediction e&uation well fits recent overall pressure loss data '(. )t should be noted that while the discharge coefficient for pipe tap orifices are used for restrictive orifices, no taps exist. 7ressure taps are re&uired onl for pipe tap metering orifices which use the same selection option button. The selection of either up or downstream tap is onl for pipe tap metering orifice flowmeters and not necessar for restrictive devices. The option for tap location is provided to accommodate pipe tap meters but is not used for restrictive orifice.
