...Plug, ball, gate, or butterfly valves can be used for closed shutdown with varying degrees of success. As a very broad statement, their effectiveness is a function of the abrasive or wearing qualities of the effluent controlled. Of course, the most effective safety valving is either full closed or open, protecting the sealing elements from being compromised by the effluent (except in the case of butterfly valves which have sealing surfaces fully exposed when partially or fully open). All parameters of the intended use must be examined in detail to provide a definitive assessment of the intended application.
Michael Karr, Executive Vice President, COO
Vulcan J.M. Intl., www.vulcanjm.com
Analog Valve Cant Do It
Two key safety principles come into play here: One, the safety system must be independent of the control system and, two, for process control the accepted safe state is the de-energized state. I suppose one could use an analog control valve for the first principle but don't see how it could be used for the second principle. I would say that a control valve cannot be used as a safety shutdown valve.
...Being in the intrinsic safety isolator business, I also must note that the leading companies in our business do not rate analog I.S. isolators for Safety Integrity Levels.
...I must add that the mechanism for the shutdown must allow for quick action when the power is shut off to the valve and I do not think that a control valve is designed for this response.
Mike McElroy, Business Development Manager
NC Solenoid Valves Suffice
Generally speaking, two-way and three-way normally closed solenoid valves with synthetic rubber seals are considered to be "safety shut-off" valves, and are usually listed with an agency like UL as such. This means that in the event of loss of electrical power to the coil, the valve fails in the closed position, which is most desirable. These solenoid valves can be used anywhere that it is required to shut off gases or liquids in the event of power loss.
Mark Emond, Technical Service Representative
Parker Fluid Control, www.parker.com
More Trouble Than Its Worth
Sure it can be done. The reason it hasn't may be due to one or more of the following:
- Pneumatic valve positioners are much more expensive than conventional solenoid-operated valves.
- Not too many safety PLCs on the market offer a certified 4-20 mA output (some do; for example the Quadlog system offers a TUV-certified 4-20 mA analog output module).
- Control valves often aren't designed for tight shutoff, they are typically designed for better control (hence the added cost).
- Typically, the vendors that manufacture and sell control valves and valve positioners have not sought the necessary failure rate data to meet the IEC 61508 standard.
- Due to their operational nature, control valves require more maintenance than shutoff valves and have a higher demand on compressed air.
...Most of todays safety shutoff valves are solenoid-operated shutoff valves. The solenoid valve sits on top of a valve actuator and responds to a discrete (on/off) signal from the safety system. The solenoid will either allow compressed air to drive the valve closed or release air while a spring closes the valve.
...An electrical/pneumatic (I/P) positioner works on top of a control valve actuator. The positioner receives a 4-20 mA signal from the safety system and allows the valve to go to any position between full open and full closed. Some of the possible benefits of using a control valve are:
- Using the positioner, users can partially close the valve to validate system performance. Today, users would have to shut down the process to test shutoff valves.
- The positioners intelligence would allow users to obtain valuable diagnostics for preventive maintenance and safety records.
- Bus technology like Profibus, HART, or Foundation fieldbus are often available with today's positioners, adding additional advantages.
Charles Fialkowski, Product Manager, Safety Systems
Siemens Energy and Automation, www.sea.siemens.com
Its Your Call
A control valve can be used in lieu of a separate safety shutdown valve for SIS applications. One technique is to install a solenoid in series with the valve positioner, downstream of the positioner output. The solenoid valve is operated by a SIS signal when a process incident occurs, and its output supersedes that of the positioner to drive the valve to the shutdown position. In many cases this operating setup eliminates the need for an additional safety valve, piping, and installation space.
...While a control valve can be used for control and as a safety shutdown valve for SIS applications, there are tradeoffs to consider. Upside:
- Using the control valve will eliminate the cost of an additional valve and associated piping.
- The DCS will check for the availability of the modulating valve and will record valve travel as a way to document operation. This is not the case with discrete on/off solenoid-operated valves.
- When the control valve is equipped with a digital valve controller (DVC), the DVC can record valve travel, actuator pressure, setpoint, and other parameters for valve diagnostic purposes.
- A control valve uses a better designed/matched actuator and instrument package than does the typical on/off valve, with the result being tighter control.
Then theres the downside: