High Pressure Drop Service – Multistage Continuous Resistance (CR) Disk Stack
The CR trim is used in gas and vapour applications where the pressure drop is high enough to cause high noise levels and the resultant energy being dissipated is significant enough to cause substantial damage to standard or drilled hole trims as well as associated downstream pipe work and instruments.
Industry, through the International Standardisation Association (ISA), has recognised that control valves with severe service trims can fail quietly through fatigue and erosion. As a result, the ISA has incorporated Clause 5.3.2 Vibration and Erosion limits into its specification. This clause is of particular importance when specifying critical service control valves as it limits the trim exit Velocity Head to below 480 kPa through each pressure reduction stage of the trim.
Velocity Head (Vh) is a measure of Kinetic Energy such that:
Vh = Kinetic Energy = V2/(2g).
Limiting the trim exit Velocity Head to below 480 kPa ensures that amount of energy being dissipated through each stage of the trim is considered while designing in order to avoid fatigue and failure of the valve and pipe work due to control valve induced vibration.
The guidelines set out in the ISA Guideline Compliant Specification, Control Valves, are the fundamental principles to which the CR Multistage trim is designed. In particular, the design of the CR disk stack ensures that the required energy can be dissipated through each pressure reduction stage of the trim without exceeding the wear properties of the trim material, whilst providing maximum reduction of control valve induced noise and vibration.
The CR Trim provides continuous resistance while controlling the Velocity Head through each stage of the trim at levels that does not create high noise levels.
In liquid applications where the pressure drop is high enough to cause cavitation, the multi stage design of the CR trim eliminates cavitation by ensuring that the pressure drop across each stage is low enough so that the pressure does not fall below the vapour pressure at any stage, thus preventing cavitation from occurring. The design of the CR Trim provides sufficient number of continuous resistance stages which provide staged pressure reduction in order to maintain the trim exit velocity in liquid applications to below 30 m/sec for continuous service single phase fluids and below 23 m/sec for cavitating and multi-phase fluids.
Furthermore, our CR Disk Stack makes for a technically superior and cost-effective alternative to conventional noise abating devices (i.e., silencers) in gas and vapour atmospheric vent applications.
The CR Vent Stack effectively performs the duty of both the pressure let down valve and the atmospheric silencer, eliminating the need for the atmospheric vent silencer all together.
As the CR Vent Stack is installed downstream of the valve, the size of the valve body is reduced and as a silencer is not required the total weight of the “system” is greatly reduced. This provides significant savings to otherwise expensive construction and formwork required to install large and heavy vent silencers.
All CR trims are designed by utilizing latest design & CFD analysis tools / software to derive accurate number of stages and resultant fluid exit velocities through them.