Zero Leakage for Offshore Productions With Rotary Gate Valves

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The graph 1 shows a comparison between the flow and pressure differential for a single and combined orifice. The design of the profiled inlet of the gate offers a significant advantage over more conventional control or choke valves. The profiled design firstly means that an increased flow capacity can be achieved through the valve over a standard square edged orifice. Secondly, and most significantly, this is the first valve on the market to totally eliminate the onset of cavitation within the valve due to the profiled shape and the length of the flow passage.

The Erosion Test Rig Was Designed to...

● measure the erosion and corrosion rate

with and without sand particles

● measure the sand entrainment rate in the

upstream orifice

● bridge of sand particles in the orifice

Flow erosion

One of the key design requirements for the valve was that it should be able to handle a fluid with impurities. The profiled flow orifice was designed with a tungsten carbide insert to resist the velocity impingement of the solid particles. A number of erosion tests were carried out at the StatoilHydro test facility in Porsgrunn, in Norway, to prove the valve’s resistance to erosion. The concentrations of particles on the tests were made equal to that normally expected for fines of 22 ppm weight. The D-50 quartz particle sizes were varied from 215 microns. Tests proved that even at a velocity of 120 m/s, the eroding particles followed the inlet shape of the parabolic insert and had negligible effect on the parallel bore.

Cavitation Control

Cavitation is a phenomenon that occurs in liquid flows. In its most severe form, cavitation can destroy a valve in a matter of hours. Cavitation occurs when local fluctuations near the liquid’s vapour pressure results in the sudden growth and collapse of vapour bubbles (cavities) within the liquid. Cavitation can therefore be avoided by preventing the static pressure from dropping below the vapour pressure of the liquid.

In simplistic terms, every type of control and choke valve, no matter whether single stage or multi stage, demonstrates a pressure profile similar to the graph 2 (red line). In cases where the required outlet pressure is close to the liquid’s vapour pressure, there cavitation can occur when the pressure reduces below and then recovers above the fluid’s vapour pressure. It therefore follows that by preventing the static pressure falling below the vapour pressure – there cavitation can be prevented.

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