Monitoring critical valves

Wednesday, 07 July, 2004

Smart field devices and testing software provide a major leap in safety valve reliability.

Of the three major elements in a safety instrumented system (SIS) - sensors, logic solvers and final elements - the latter are usually the ones that give plant engineers the most headaches. Studies have shown that 40-50% of safety-related problems occur in an SIS' final elements, which typically include shutdown valves, venting valves, isolation valves and critical on-off valves.

Similar to the cause of many physical ailments in people, not enough exercise is a leading cause of valve problems. While humans can resolve the problem by spending less time on the couch and more time in the gym, remedies aren't nearly that simple in process manufacturing facilities. In continuous processes such as those one finds in oil refineries, chemical plants and power plants, safety valves generally remain set in one position.

The lack of movement - or valve exercise - combined with harsh plant conditions can cause valves to stick. In a worst-case situation, operators won't discover a valve is malfunctioning until an emergency occurs, and the device fails to move into a safety state.

The only way to ensure that a safety system valve will function when it's needed is to periodically test it. In fact, the length of time between tests has a major effect on the safety system's Probability of Failure on Demand (PFD) and, thus, can affect a system's SIL rating. Lengthening the interval between test has a linear effect on the PFD, so if the length of time between tests is doubled, the PFD is doubled as well.

"Therefore, it is imperative that these valves be tested frequently in order to reduce the PFD and meet the target SIL rating," says Riyaz Ali, development manager for Emerson's Fieldvue instruments.

Ideally, operators would be free to test the full range of the valve (0-100%, or full-open to full-close) frequently. However, in most cases this it not practical because fully stroking a shutdown valve generally requires shutting down the process. Consequently, plants usually schedule full-stroke tests during planned shutdowns, which typically take place once every 2-3 years. However, Ali notes that improved preventive maintenance and system reliability have resulted in less frequent shutdowns and thus fewer full-stroke tests.

Fortunately, engineers have come up with a way around this situation, based on the fact that full stroking of the valve isn't necessary to determine whether the instrument is stuck. By stroking the valve by as little as 10%, plant personnel could improve its reliability in an emergency situation. In this test, a small ramp signal, too small to disrupt the process, is sent to the valve to trigger a small movement, thus confirming that the valve can be moved.

Nevertheless, even this solution presents problems. The most popular method of partially stroking valves involves using mechanical limiting devices, such as a pin, valve stem collar or a valve handjack, that restrict a valve's movement.

If an emergency occurred during testing or a limiting device was inadvertently left affixed to the valve after a test, then the valve would likely not be able to perform its intended function. In addition, this testing method requires the physical presence of personnel - in locations where hazardous conditions can frequently exist - to install and remove the limiting devices and conduct test procedures. In fact, manual partial-stroke testing is inherently unsafe because the safety shutdown function is not available during testing.

However, a new generation of smart field devices, software for testing and analysis, and digital communications protocols is eliminating these problems and increasing safety system reliability while reducing costs. Smart positioners have grown increasingly popular in recent years. These microprocessor-based, current-to-pneumatic digital valve controllers provide internal logic for control of valves while also using the Hart communications protocol to provide plant personnel with details about the instrument's condition.

In a typical installation, a smart positioner is installed in a 4 wire system. A logic solver provides two separate outputs: a 4-20 mA output serves as the primary control signal and is used to control the valve position during partial-stroke testing. The same pair of wires is also able to transmit valve diagnostic information back to the operator via a Hart digital signal superimposed on the 4-20 mA signal.

The second output from the logic solver is a 24 VDC signal, connected to a solenoid valve. This serves as an independent means of sensing an SIS command to the safety valve, should the analog connection to the valve fail. This arrangement is known as a 1-out-of-2 (1oo2) failsafe configuration. Instruments such as Emerson's DVC6000 Series Digital Valve Controllers with AMS ValveLink software for emergency shutdown solutions do not require the presence of personnel in the field during testing because of the technology's inherent communication capabilities. Initiating the automated routine test does not require the technician to remove the instrument's cover or be near the valve.

The new generation of digital valve controllers and diagnostic software delivers a wealth of information about the valve's condition that enables operators and maintenance personnel to spot points of failure that might not otherwise be apparent. AMS ValveLink software can maintain 'signature' files showing the ideal state of the instrument, which are then compared with results acquired during partial-stroke testing. Thus, plant personnel can quickly spot conditions such as packing problems (through friction data), leakage in the pneumatic, pressurised path to the actuator (through the pressure vs travel graph); valve sticking, actuator spring rate and bench set. ValveLink also creates a time-stamped audit trail of testing that can be reviewed, analysed and used for regulatory compliance purposes. So, while final control elements have been the weak link in SIS in the past, companies such as Emerson are using advanced technology to enable users to spot potential problems easily and cost-effectively.

For further information contact Emerson Process Management
471 Mountain Highway, Bayswater 3153

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