1 June 2002
Smart positioners to the rescue
By Riyaz Ali
Dangerous chemical leaks add to the inherent risks of industrial processes in the chemical or oil and gas industries. And while safety instrumented systems (SISs) are meant to reduce the likelihood of an emergency, they can't always prevent catastrophes.
SISs typically consist of three elements: field sensors to determine an emergency situation, a logic system to determine what action to take, and a final control element to implement the action. While all three elements of the SIS are crucial to safely isolate the process plant in an emergency, they aren't fail safe. There's no such thing as risk-free operation.
Adding a smart positioner to an SIS reduces base equipment cost, testing time, and manpower requirements by eliminating the need for expensive pneumatic test panels and skilled personnel for testing. And it permits remote testing, saving time and reducing maintenance inspection trips to the field.
Final control elements (emergency shutdown, venting, isolation, and critical on/off valves) normally remain static-in one position-and reliably operate only in an emergency. Because they're in one position for a long time, they have a tendency to become stuck in that position (due to process fluid contamination and corrosion) and may not operate when needed. The result: an explosion, fire, or leak of lethal chemicals into the environment.
Frequent testing is imperative with such volatile processes. Estimates indicate final control elements, particularly discrete (on/off) safety valves, cause as much as 40% to 50% of loop operational problems.
If an emergency occurred, the SIS couldn't respond properly. The SIS can fail in two ways: a nuisance or spurious trip (resulting in an unplanned and costly process shutdown) and an undetected (covert or hidden) failure, which allows continued, dangerous process operation.
The only sure way to completely test a final control element is with an in-line test that strokes the valve from 0% to 100% (full open/full close). To close a shutdown valve completely, you would have to totally shut down the process, causing lost production time. Operations managers are reluctant to completely shut down a profitably operating process just to test the safety systems. They usually wait until turnaround time, traditionally every two to three years and now as long as six years.
Companies have devised methods for testing the SIS valves online so they don't have to shut down the process. They might install a bypass valve around each safety valve. The disadvantage is the process is unprotected while the bypass is in operation. Testers might also inadvertently leave the safety valve in the bypass position after testing, leaving the process unprotected until an operator discovers the error.
There's also the mechanical limiting method, which involves a mechanical device such as a pin, valve stem collar, or valve hand jack to limit valve travel to 15% or less of the valve stroke. These tests involve complex and expensive pneumatic test panels.
A major drawback: The safety shutdown function is not available during the test period. Likewise, the safety valve could remain in this mechanically limited condition, and a casual inspection might not always catch it. So the valve could be out of service for a long time, unbeknownst to operators.
You can add a smart positioner to any valve-style configuration, including linear sliding stem, rotary, and quarter turn with spring and diaphragm actuators, spring-return piston actuators, or double-acting piston actuators. Two types of installation are possible: four wire and two wire. Both installations use a solenoid valve and provide a redundant pneumatic path.
The actuator pressure will always be able to exhaust to allow the valve to move to the safe position. If the solenoid valve fails, the actuator pressure exhausts through the pneumatic path in the smart positioner. If the smart positioner fails, the actuator pressure exhausts through the solenoid valve.
In a four-wire system the logic solver provides two separate outputs: a 24-VDC signal for the solenoid valve and a 4-20 mA DC current signal for the smart positioner. Both signals are generated based on sensors and logic internal to the logic solver. It requires an additional pair of wires but permits the smart positioner to continue to communicate even during emergency demand conditions. In a two-wire system installation, the logic solver provides a single 24-VDC source to provide power for both the solenoid valve and the smart position, reducing wiring costs in new installations and requiring no additional wiring in existing installations.
Smart positioners are digital valve controllers-microprocessor-based, current-to-pneumatic instruments with internal logic capability. They traditionally convert a current signal to a pressure signal to operate the valve. They receive feedback of the valve travel position plus supply and actuator pneumatic pressures, allowing them to diagnose not only themselves but also the valve and actuator to which they are mounted. Using smart positioners as part of the final control element permits partial-stroke, online testing of the valve and eliminates the need for special mechanical-limiting devices.
Partial-stroke testing confirms the valve is working without disturbing the process. Because there's no need to shutdown the process, testing can occur more frequently. Because smart positioners hold the programming of the test procedure, partial-stroke testing happens automatically and requires no operator attention.
Typically the partial-stroke test moves the valve 10% from its original position but can move it up to 30% if plant safety guidelines allow. Although partial-stroke testing doesn't eliminate the need for full-stroke testing (required to check valve seating), it does reduce the required full-stroke testing frequency so it most likely undergoes testing during plant turnaround.
Some smart positioners alert the operator if a valve is stuck. As the positioner begins the partial stroke, it continually checks the valve travel to see whether it's responding properly. If it isn't, the positioner will abort the test and alert the operator that the valve is stuck. This will prevent the valve from slamming shut if the valve does eventually break loose. IT
Riyaz Ali is a development manager at Fisher Controls International Inc. in Marshalltown, Iowa. He will present his paper at the ISA Western Regional Conference, Training & Exhibition on 18-19 June 2002 in Las Vegas.