Riyaz Ali, senior director of the Instruments Business Unit at Emerson Automation Solutions, says safety systems with emergency shutdown, venting, isolation and other types of safety valves require each valve to close or open immediately when a process upset or emergency occurs. His article, How Digital Valve Positioners Can Simplify Safety Life Cycle Phases in the December 2018 issue of Processing magazine, explains how digital valve positioners and modern digital techniques simplify testing of safety valves. Here’s a summary of the article.
Emergency valves are not continually opening, closing or throttling like a typical control valve—but instead normally remain in one static position for long periods of time, and are expected to operate reliably when an emergency occurs. Valves remaining in one position for long periods of time are subject to becoming stuck and may not operate when needed. To ensure their required reliability and availability in safety systems, they need to be tested frequently.
Unfortunately, the traditional method of testing the final element of a safety system requires a shutdown, and/or for the safety system to be rendered inoperable during the time the valves are being tested, both very undesirable and expensive options.
Emergency shutdown valves must either close or open immediately when a process upset occurs.
The 20:20:20 Rule
The 20:20:20 rule applies to the entire safety life cycle (SLC) of a safety instrumented system (SIS): the Analysis Phase typically takes 20 weeks, the Design Phase takes 20 months, and the Operation Phase takes 20 years.
In the Analysis Phase, the valve designer first performs a SIS risk-tolerance analysis to determine what level of safety is needed and what hazards can be expected. In the Design Phase, the valve designer must specify a Final Element (valve) that will provide the required SIL suitability. In the Operation Phase, the valve is continuously proof tested to ensure proper operation.
The Operation Phase spans the longest time of the SLC in which all safety instrumented functions (SIFs) must remain in operation to maintain the desired safety integrity levels (SILs).
Avoiding a Shutdown
In the past, plant turnarounds were scheduled every two to three years. However, with increased system reliability and more inclusive predictive maintenance programs, turnarounds at many plants are now being scheduled to occur every five to six years. This means safety valves are tested less frequently, which may prevent them from meeting the target SIL.
Two methods are available for testing valves on line without shutting down the process. The first involves installing a bypass valve around the safety valve. This allows steam, gas or liquid to flow around the safety valve while technicians exercise it to ensure proper operation, but there’s a superior method.
A better solution for on-line testing is partial proof testing by digital device. Safety engineers recognize the most likely failure mode of a discrete shutoff valve is remaining stuck in its normal standby position. Testing for this type of failure requires stroking the valve only a small amount to verify it is not stuck.
Partial proof testing can be accomplished online with a digital valve controller. It receives feedback about valve travel position, along with existing supply and actuator pneumatic pressures. This allows it to diagnose not only itself, but also the valve and actuator to which it is mounted.
Partial proof testing can be accomplished online with a digital valve controller. A logic system (top left) or an asset management system commands the digital positioner (bottom right) to execute the test, and logs all the test data obtained by the positioner.
Typically, the partial-stroke test moves the valve 10% from its original position, but it can be up to 30% if allowed by plant safety guidelines and the particular requirements of the process.
Even though partial-stroke testing does not eliminate the need for full-stroke testing—which is required to check valve seating, etc.—it reduces the required full-stroke testing frequency to the point where it can most likely be tested during the next plant turnaround, even if it is five to seven years away.
Because the digital valve controller communicates via the HART protocol, the partial-stroke test can be initiated from a HART hand-held communicator, from a personal computer running the positioner companion software, from a panel-mounted pushbutton hardwired to the positioner terminals, or from Emerson’s Safety HOST DeltaV SIS.
For safety reasons, an operator must start the partial proof test, either locally or remotely.
Valve Diagnostics During Testing
Emerson’s Fisher ValveLink software runs the test, diagnoses the results and produces a report.
If the pressure is within limits, ValveLink software then sends a command to the valve to open or close 10%, records the pertinent data, and returns the valve to its normal standby position. And, because the digital positioner is equipped with position and pressure sensors, it can also measure valve stiction, pneumatic pressure required to move the valve, the speed at which the valve moves, air leaks and many other parameters.
While the smart positioner provides performance and safety benefits through automated, online partial-stroke testing, many additional benefits can be realized. These include eliminating expensive pneumatic test panels, reducing manpower requirements, lowering base equipment cost and shortening testing time. In addition, remote testing results in fewer maintenance trips to the field, as well as the establishment of an automated test routine.
Considering all benefits, the use of smart positioners in an SIS is a sensible and economical pathway to enhanced SIS reliability.
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