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- Figure 1. Erratic response due to vibration.
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- Figure 2. Trace after modification.
SEVERE SERVICE
VALVES
SEVERE SERVICE
VALVES
Anti-Surge - Compressor Recycle
(Compressible Fluids)
Although all control valve applications are sized according to operating conditions and to ensure adequate valve openings for control, the anti-surge does require particular attention. Usually, the valve specification states the exact requirements, i.e. the maximum range of the valve design Cv. These are specified to ensure that when the valve is brought into service it can control the flow so that the compressor does not move towards a surge condition. An additional requirement for this application is that the valve responds quickly under either a trip or modulating condition. This requires a sophisticated instrumentation configuration, using volume booster and trip valves. These must be set up to ensure a stable control in operation.
Another problem with this application is vibration. One of the main contributors to vibration is the piping configurations that are often created in an attempt to minimise the footprint of the plant. This, together with adverse operating conditions such as high pressure drops, can result in vibration problems. To counter this, KKI applies a specific set of rules in selecting valves for this application.
The graphs shown in figures 1 to 2 present traces of the actual valve opening conditions during a recycle operation. Under most of the operational range the valve control remained stable. The green and red lines are the input (instrument signal) and output (valve position) of the device. However, in certain instances, the response of the valve was erratic as shown in Figure 1.
This shows that although the input signal (green) is requesting the valve to close, the valve actually remains open, as shown by the red trace. Corresponding with this occurrence was a sudden increase in SPL. This was not aerodynamic noise generation, but the result of high vibration levels at a discrete frequency due to fluid-induced resonance.
In this example, numerous attempts at resolving the problem only partly solved it. A complete review of the process was undertaken, and various modifications to the trim design and guiding were incorporated into the replacement trim. The valve was put back into service and retested. To verify the performance of the modification a slight sinusoidal signal was incorporated into the input signal.
