Valve Magazine - Summer 2008

Figure 5 (left) – MEML as it mounts between the actuator and ESD valve. Figure 6 (right) – Disengaged (top) and engaged (bottom). Rotation of the flag causes engagement of the internal mechanical stop. For remote operation, a small quarter-turn actuator replaces the manual flag.

sure) and that optional sensors are available to provide feedback for event verification and recording. Additionally, integral PSTDs are optimally sized for each actuator and require no special mounting provisions.

Typically, color coding is used to identify the correct key for each MIML (Figures 2, 3 and 4).

Mechanical-External-Manual-Local (MEML): MEMLs are similar in concept to MIMLs except that the MEML is separate from the actuator. Suppliers of these devices point out that MEMLs are actuator independent and are suitable for new ESD valve installations as well as for retrofitting to existing valve/actuator packages. Optional remote engagement is offered as are travel sensors for position feedback.

Both MIML and MEML PSTDs are suitable as add-ons to accessory-based PSTDs as a means to provide hard travel stops (Figures 5 and 6).

Mechanical-Integral-Automated-Remote (MIAR): MIAR-type PSTDs provide a pressure-activated, integral hard stop to prevent spurious shutdown trips. As with accessory-based PSTDs, local manual engagement is optional as are sensors that provide feedback for predictive maintenance. Suppliers point out that full closure response to an emergency is not impeded during PST or actuator seal replacement.

A CASE STUDY

AUTOMATED INDUSTRIAL PLANT NEEDS

EXTENDED LIFE ACTUATION.

EXTENDED CYCLE LIFE ACTUATORS ARE USED FOR BFV APPLICATIONS UP TO30 INCH. THE ACTUATORS HAVE PROVIDED OVER 5,000,000 CYCLES OF CONTINUOUS SERVICE RESULTING IN LOWER OPERATING COSTS AND INCREASED PROFITS. THE ACTUATORS ACHIEVE 90 DEGREE