What you need to know about cavitation when using restriction plates

A brief introduction to the cause and effect of cavitation in your system

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.

Restriction Orifice Plates

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.

Liquid flow

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

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.

1. Example of severe cavitation

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.

How to combat cavitation

Cavitation can be eliminated in different ways depending on how severe the cavitation is.

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)

  • FL = Critical flow factor or cavitation factor
  • P1 = Upstream pressure
  • Pv = Vapor pressure

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

EMCO Controls anticipates a 10-fold increase in sales of float level switches by 2025

Danish instrumentation company Emco Controls has carved a solid niche for itself in supplying the shipbuilding industry with float level switches and anticipates a 10-fold increase in sales of the product over the next 3 years. It is already selling 10 times as many as it did just four years ago and is satisfied with the current level, though declines to specify number of switches sold.

According to Mads Lisberg, CEO of the company based near Copenhagen, the surge in sales is due to a good design. He says: “We believe Denmark is known for good design and when you do a good design we’re following that same tradition that Denmark is known for.”

He adds that the company had enjoyed modest sales of float level switches for about 30 years to shipbuilders and the process industry. Float level switches are used to monitor liquid levels in containers and sound an alarm when a certain level is reached, either high or low. They can also sound when a pump is starting or stopping.

Emco says its sales received a boost four years ago when it was approached by Norwegian company Odim, which expressed an interest in selling the switches to the shipping industry. This led to a new design and construction. Today, it has buoyant sales and is hungry for new markets. Emco was founded in 1966 by Enevoldsen og Mogensen and is owned by Lisberg, who bought the firm in 1984.

Lisberg says the company listed five demands it wanted met for the new design. The switch had to be lightweight, modern, highly anti-corrosive, production costs had to be low, and finally the construction had to be flexible enough to accommodate customer wishes.

The switches are still made at EMCO’s factory in Hillerød near Copenhagen in line with ISO 9001 standards. While shipping is a major segment, with clients such as shipyards and product agents, other sectors for the company include power stations, water treatment plants, as well as the pharmaceutical and chemical industries.

Emco has supplied switches for AP Moller-Maersk’s eight container vessels built at the Odense Steel Shipyard in Denmark. “We think there’s a big market for us and for a product that is well designed and does something specific…We’re going after yards, shipowners and firms that supply marine equipment. It is particularly seeking to tie up with Far Eastern yards that are building Maersk vessels.

It also makes and sells flow, level and temperature instrumentation devices.