Flow measurement of liquid, gas and steam according to the differential pressure principle has been recognised principle for many years using orifice plates, venturi tubes and flow nozzles. A restriction in the pipeline creates a pressure drop when the fluid flows. The pressure drop is determined by the velocity of the fluid.
The method is thoroughly described in many standards, practices and books. Orifice plates can also be used for pressure control and flow limitation. This special use is not covered by international standards and is only scarcely mentioned in literature.
For a restriction orifice plate the flow quantity through the orifice will vary with the pressure before and after the orifice plate.
The restriction orifice plate is mostly applied in non-critical flow limitation. The use of the expression “non-critical” must not be compared to the word “critical” as in critical flow devices. Non-critical is here to be understood as non-complicated with less requirements for the accuracy.
This could for instance be after a control valve in order to divide the required pressure loss into two elements. This is done to reduce the noise in liquids and to avoid cavitation. It is advisable to avoid cavitation in order to protect the process elements before the restriction orifice plate in blow-down systems.
If the pressure in Vena Contracta (because of high velocity) decreases to the vaporisation pressure of the liquid, a cavitation zone is created. This cavitation zone will act as a flow limitation.
Cavitation occurs frequently in liquid flow restrictions where large pressure drops exist. According to the Equation of Bernoulli the liquid flow velocity increases when the bore area is reduced causing a pressure drop to occur. The lowest static pressure is reached at highest velocity in Vena Contracta. If the lowest static pressure is lower than the vapour pressure, steam bubbles will form.
After the liquid has passed through the restriction, the velocity decreases, and the static pressure increases. This leads to a collapse of the formed vapour bubbles. The collapse of the bubbles generates noise and will cause the plate as well as the pipe to erode. It is therefore important to avoid a full cavitation through a liquid flow restriction. Consequently, to avoid incipient or full cavitation it is important that the differential pressure does not create a static pressure in Vena Contracta that is lower than the vapour pressure.
When the steam bubbles collapse caused by the increased pressure, chock waives are formed resulting in extremely high static pressures lasting for an ultra-short period of time. The internal part of the pipe downstream will be damaged due to this in combination with the erosion caused by the local heating of the inner pipe. Cavitation generates loud noise which sounds like pebbles running in the pipeline.
Cavitation can be eliminated in different ways depending on the severity of the cavitation. A multistage single hole or multi hole depressurizing unit is reducing the total required pressure drop in stages. The number of stages depends on the pressure drop, the plate cavitation factor FL and the selected hole design (single or multi). The distance between each plate must not be less than D, preferably 1,5 – 2 X D
A differential pressure of ΔP as indicated below, should be used:
ΔP < FL(P1 – Pv)
The FL factor depends on hole design – single hole, multi hole, cylindrical or conical holes. The FL value is between 0,5 and 0,7