Many critical applications must be prepared in the event of a catastrophe, such as an explosion, fire, flooding, earthquake, ect. In the past, process engineers often specified remotely operated on-off valves for standby applications. However, today’s emergency shutdown (ESD) standards have been increased so that valves must now be applied using a strict code of testing, identification, auditing, and performance under adverse conditions associated with catastrophic event.
Under common ESD codes, before an ESD valve is applied in a critical application, it must be designed and tested so that the valve goes to the required fail safe position ( fail open, closed or fail in place) and then holds that position for a particular length of time under given parameters of a possible catastrophic event.
Of the three fail safe positions, fail closed is the easiest to achieve since the actuator spring and fluid pressure are both used to move or assist the valve to the required closed safe function.
However, although rare in emergency shutdown applications, the fail in place position often requires an independent power or air supply source to hold the required position. With the use of digital positioners such as DVC 6200 and controllers, throttling control valves used in ESD applications are expected to perform at a higher standard as a critical part of the safety instrumented function (SIF), which calls for a safety loop between distributive control system (DCS) and the ESD final control element.
When a catastrophic event occurs, sensors send a signal to the (SIF) which calls for a safety loop between the DCS and the ESD final control element. When a catastrophic event occurs, sensors send a signal to the SIF, which disconnects the power to the ESD control valve, moving the valve to the correct failure position.
For self-sustaining control valves employing smart technology with an internal safety integrity level (SIL) function, the digital positioners and controller are linked to self-contained sensors, which after sensing the event can employ the required failure mode. For this reason, the digital positioner or controller must be designed with the necessary Intrinsic temperature, and exterior conditional requirements of the ESD valve.
The likelihood of catastrophic event is rare, it’s critical that the ESD valve must be tested and designed for a particular mean time between failure MTBF parameter which is an important part of SIL requirements. The problem with most valves is when they stay in the one position for a very long period of time, they are progressively more prone to stick or require a greater force to move to the fail safe position upon failure. Knowing the MTBF for a particular valves allows maintenance or a failure test to be performed at certain time intervals to ensure proper performance.
ESD valve standards
The most common standards applied to ESD are IEC 61508, 61511. IEC (International Elecrotechnical Commision) is commonly recognised worldwide as the safety standard for ESD valves, and is a requirement for many global insurance companies. This standard provides stringent guidelines for testing and auditing of the functional safety requirements for safety integrity loop, which includes all aspects of the process loop, sensors, positioners or controller, control valve or any exterior fail safe equipment. IEC demands a zero tolerance level of hardware failure (valve, actuator yoke or cylinder, spring ect.) and a minimal tolerance failure rate, diagnostics associated with ESD valve and SIL software requirements.
Partial Stroking Testing
In the past the only way when the testing of the ESD valve was required, was to carry this out during a system shutdown for routine maintenance. This can be an expensive and time consuming process.
However now with the development of smart positioners on ESD valves. Not only does this save time and expense but now gives diagnostic information of the systems performance and strokes the valve partially 10 to 30 percent of stroke, to ensure their is no stiction on the valve stem or shaft. Each test records the ESD valve signature so you can see from one test to the next if any changes occur and look for obvious signs of high friction levels, this would suggest a possible stuck stem or shaft or stiction taking place. Self diagnostic tests can also be conducted to detect leaks to the actuator, or determine spring rate or bench set.
Fisher FIELDVUE™ DVC6200 Digital Valve Controller
FIELDVUE™ DVC6200 digital valve controllers are HART® communicating, microprocessor-based current-to-pneumatic instruments with linkage-less, non-contact travel feedback. Important functions performed: 1 Traditional valve positioning 2 Automatic calibration and configuration 3 Provide instrument and valve diagnostic information.
Linkage-Less Non-Contact Position
Feedback—There are no wearing parts so cycle life is maximised
Encapsulated Electronics–Resist the effects of vibration, temperature, and corrosive atmospheres