Operation and maintenance accounts for 79% of the total cost of ownership of a typical control valve, according to Neles, a specialist in flow control solutions for the process industries. With such a hefty associated cost, monitoring valve health and performance is clearly a prudent strategy. “You can have some understanding of what’s happening with a valve just by paying attention to process behaviour,” says Cristian Moraes, head of performance solutions at Neles. “Furthermore, using advanced devices such as smart positioners can provide even more valve health data. Valves ‘talk’; it’s just a case of listening.”
The OREDA Offshore Reliability Data Handbook (www.is.gd/upuhuv) lists 17 potential failure modes for valves, the most common being valve leakage in closed position, failure to close on demand, abnormal instrument reading, spurious operation and failure to open on demand.
“We know as a valve manufacturer and maintainer that around 30% of valves have mechanical issues or limitations,” says Moraes. “These problems can impact lifespan, reliability and the process. Typical indicators might include poor or no control, as well as compromises to safety, the environment or noise levels.”
How can engineers really understand that a valve needs removal from the process? Asset management tools can help. Each PLC or DCS will have its own asset management tool that collects data from smart devices, supporting users to analyse the information and achieve better diagnostics.
CONTROL LOOPS “Another option is to monitor process variables using control loop monitoring systems, which will indicate if the valve is responding efficiently and effectively to commands,” says Moraes.
Today, around 33% of control loops operate in manual mode, in effect relying on operator experience. However, sometimes operators can cause process disturbances, often by opening and closing valves too frequently.
“On average, 20-40% of control loops globally operate with some form of oscillation,” says Moraes, outlining another issue. “If the final element of your control loop is a valve and you have oscillation in the process, it can cause stress, with the valve trying to operate beyond its normal load limits. That said, 80% of valves removed for maintenance have no problems and have therefore been replaced without reason.
“For this reason, we recommend predictive/prescriptive maintenance analysis with multi-variable time series data, as it has a minimal cost impact,” he continues. “Some people think there is high investment involved with acquiring the digital tools needed for predictive maintenance, but it’s not true,” Moraes claims.
Among Neles’s solutions is Expertune PlantTriage, PID tuning software that continually monitors plant to identify issues. Information is prioritised based on technical and economic factors, helping to find the root cause of issues using a complete set of analysis tools.
Implementing an effective valve performance management strategy will offer benefits including: identifying valves that are high risk for production shutdown or slowdown; identifying and prioritising corrective actions for valves that urgently need attention; identifying corrective actions to resolve operating issues such as excessive valve travel; the resolution of stiction issues (see also pp24-5); and identifying valves that are beyond their normal range of operation.
For those with a large installed base of control valves, valve positioners come highly recommended. Jo Kirkbride, actuators, positioners and force measurement product manager at ABB Measurement & Analytics UK and Ireland, offers tips for ensuring optimum valve positioner performance.
“Valve positioners can alter the valve actuator’s position based on a control signal to achieve the desired flow of process medium,” she says. “They are typically used where rapid and accurate control is required without any error or hysteresis.” Whether sliding-stem or rotary control valves, both have a mechanical linkage that provides feedback to the positioner control unit, allowing comparison of the valve’s position with the position requested by the controller. Upon receipt of a signal that differs from the valve actuator’s position, the positioner sends a control signal to the actuator. The feedback linkage confirms the change has taken place and reports on its magnitude.
“Valve positioners allow fine control with better resolution by using the full signal, which compensates for inherent variability in the accuracy of the actuator system,” says Kirkbride. “Further advantages include their ability to cope with large variations in forces and maintain consistent valve position under fluctuating process pressures.”
Accuracy is important in processes where the correct volume or flow of materials is required. Plants might measure the correct volume using a flowmeter, or through level measurement in the vessel with feedback data delivered to the process controller. In such applications, valve positioners offer a fast and accurate response, says Kirkbride.
“For example, upon detecting a flow rate change in a piping system, the measurement would trigger an adjustment to the valve position - where the continuous feedback loop delivers the correct valve position for the desired flow rate. Increased speed is possible because the positioner uses higher pressure and air flow to adjust the valve.”
GOING DIGITAL
Digital positioners are quicker to commission and have a maintenance advantage over mechanical positioners because they have fewer moving parts, so reduce the risk of unplanned downtime and associated disruption, time and cost. Another attraction of using digital positioners is the opportunity to reduce air consumption. With around 90% of the energy used by a compressor ending up as waste heat, compressed air is effectively 10 times more expensive than electricity.
Concludes Kirkbride: “As an example of the savings available, consider an application where positioners spend 50% of their operational time in a steady-state position. If compressed air costs 1p per standard cubic feet per minute, the cost of this wasted energy could be as much as £2,106.78 per year. Replacing these positioners with a digital positioner such as an ABB TZIDC could save between £500 and £1,200 per year, per positioner.”
BOX: VALVE POSITIONER SELECTION
According to ABB Measurement & Analytics. there are four principal types of valve positioners, all of which use either compressed air or electricity to move the valve actuator. Powered by compressed air, pneumatic positioners (1) receive pneumatic signals before supplying the valve actuator with the correct air pressure to move the valve to its required position. Electric valve positioners (2) perform the same function but use electricity as the input signal instead. Electro-pneumatic valve positioners (3; pictured) are a combination of the two, converting current control signals to the equivalent pneumatic signals. A more modern and increasingly popular alternative is the digital or ‘smart’ positioner (4). These devices use microprocessors to position the actuator, while monitoring and recording data. Digital positioners are said to be highly accurate, use less air than traditional non-digital versions and offer online diagnostic features.