Digital valves

FIG. 8 83 Generic loop powered digital valve controller. 

The state variables that provide valve position control are used to diagnose the health of the final control element. In addition, some digital valve controller designs integrate additional sensors into their construction to provide increased diagnostic capability. For example, pressure sensors are provided to detect supply pressure, actuator pressure (upper and lower cylinder pressures in the case of a springless piston actuator), and internal pilot pressure. Also, the position of the pneumatic relay valve is available in some designs to provide quiescent flow data used for leak detection in the actuator. 

Figure 12-4 consists of a single Safety Transmitter, safety PLC, and single shutdown valve without tight shutoff requirements with a digital valve positioner. The digital positioner would be provided with the capability for Partial Stroke Testing (PST). 

In the SIL 3 Case 2 design, three conventional transmitters are connected to the safety PLC and the two valve assemblies use three way solenoids instead of digital valve positioners. No partial valve stroke testing is done. The results of the PFDavg calculations are listed in Table 12-10. 

Predicting health of final control element of safety instrumented system hy digital valve controller, in Presented at ISA Automation West, R. Ali — Fisher Controls Int l., LLC, 2004- www.isa.org. 

Figure 9.8 Modem control valves using digital valve controllers (Courtesy Emerson Process Management).

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