Can We Use Control Valves for Safety Shutdown?

Readers Help a Reader Solve This Control Problem

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  • The SIL level of an SIF loop will dictate whether or not valve redundancy can be eliminated.
  • If a process is critical and no chances are to be taken in averting an incident, then a redundant valve is a must. The second valve could be an on/off unit that is operated via solenoid, or it could be another control valve operated by an independent SIS signal.
  • The existing control valve may not be capable of meeting stroking speed or closure time requirements.
  • Control valves typically are not designed to meet fire-safe standards.

Riyaz Ali, FieldVue Business Development Manager
Fisher Controls Intl., www.fisher.com

Help for Electric Actuators

Each application is different and one must first list what instances would initiate safety shutdown procedures. Then decide under each of these conditions how your valve will respond or be controlled.

...One of the main issues with electric actuators when power is lost is the actuator generally must be manually manipulated to place the valve in a fail-safe state. M-Systems new PSN3 electric actuator will automatically go to the user's specified failsafe state. This is accomplished by the PSN3's internal backup NiCad battery, which provides power to move the valve to the user-specified failsafe state if the main actuator power should fail.

...The main benefit: Safety personnel are not required to go to each valve and manually place them in a failsafe state. This is a major time-saver and eliminates potential human error of placing valves into a non-failsafe state under fault conditions. Additionally, it is possible that this may remove the requirement for separate manual safety valves.

Gary Labadie, Sales/Marketing Manager
M-System, www.m-system.com

Mays Problem

How Much Cooling Is Too Much?

We specify coolers for enclosures that end up in a wide range of ambient temperatures and humidities. My boss says we should simplify operations by using the same refrigeration unit, but this means in some cases the unit will have much more than the calculated minimum capacity. What problems could this cause? How could we avoid them?

Please send us your comments, suggestions, or solutions to this problem. Solutions will be published in our May issue, but we need your responses by April 10 to meet our editorial production deadlines. If you have a solution to this problem, or a problem of your own that you would like to pose to other colleagues, e-mail, fax, or mail it to pstudebaker@putman.net, Fax 630/467-1124, CONTROL Problem Solving Ideas, 555 W. Pierce Rd., Suite 301, Itasca, IL 60143.

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