Valves, Actuators & Pump Control


Blowdown applications using multi-stage trim

July 2018 Valves, Actuators & Pump Control

The Arca angle style control valves series 350 and 380 are proven under critical process conditions. If used in steam or other process gas applications at high pressure and temperature, in cavitation-free relaxation of liquids at high pressure differential or in handling of fluids with high solid content, the modular system of valve housings, bonnet and trim styles, in combination with a large range of materials, always offer an ideal solution.

The combination of high pressure ratio and solid-containing fluids is one of the most critical challenges for sizing, selection and design of a control valve. Due to the high pressure ratio, multi-stage valves are indispensable to avoid severe cavitation damages. However, as soon as the fluid contains solid particles, besides the normal fluid-dynamic sizing (determination of the number of throttling stages and their individual flow coefficient), the grain size of the particles has to be taken into account. The cross-section area of each flow channel in the valve must be the size and shape to allow all particles in the fluid (assumed to be of spherical shape) to pass.

Flow channel optimisation

Since the cross sectional area of each throttling stage is explicitly defined by the flow coefficient calculated from the fluid-dynamic valve sizing, only the shape of the flow channel remains to be optimised.

It is obvious that a circular shape is the optimum with regard to the permeability of spherical particles. The newly developed multistage trim MS4, which has only one flow channel at each stage, milled as a spherical calotte into a rigid, multiple guided control rod, approximates this ideal shape and is superior to any multistage parabolic plug in this regard.

Another special feature of this trim style is the protection of the seating surface (which is the most sensitive area in terms of erosion) against over-stroke of the valve. This ensures that the control stage of the valve only opens under conditions of low pressure differential and with a small flow velocity at the seating surface.

The rigid multiple guidance of the control rod is an effective prevention of the valve against mechanical vibrations and the resulting wear, even at extremely high pressure differentials.

Series 350 and 380 are available in sizes up to DN 250 and ratings up to Class 2500 (PN 400). Depending on the process conditions, MS4 trims can be delivered in valve sizes up to DN 100.

For more information contact Desmond Delport, Valve & Automation, +27 (0)11 397 2833, desmond.delport@valve.co.za, www.valve.co.za



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