Each year, underperforming control valves cost the typical process plant thousands of dollars. And that only includes losses from improper maintenance practices. Factor in poor product quality, operating disruptions, related safety compliance and other indirect costs, and the figure can grow considerably larger. The proper care of control-valve assets can generate dramatic savings during the investment’s life cycle.
As its name implies, a control valve modulates the flow of a liquid or gas. It’s the final control element in a loop that includes a measurement device, a control system, a positioner and an actuator to open and close the valve. Control valves are found throughout industrial process plants. A 250,000-bpd refinery might have as many as 2,500 of them installed in its labyrinth of pipes and reactor vessels.
The initial purchase price represents only 20% of the valve’s total cradle-to-grave cost (Figure 1). Almost three-quarters of the total life cycle cost goes for maintenance, which represents a huge opportunity for cost reduction.
Figure 1. It's mostly maintenance
The breakdown of all life-cycle costs for a typical control valve.
Poor control-valve maintenance creeps into a plant’s cost profile like a cancer. In time, it affects a facility’s operations in three major areas:
- Poor product quality: A miscalibrated control valve won’t achieve good process control, allowing pressure or temperature excursions that result in off-spec product.
- High maintenance costs: Poor maintenance practices foster run-to-failure problems that can shut down a major production unit.
- Potential safety hazards: Leaky valves servicing toxic or hazardous materials pose a safety risk, and excessive vibration can damage piping and produce noise levels that violate OSHA regulations.
Unhealthy valves affect the entire production system. If untreated, symptoms grow worse until the process shuts down. Improperly specified and maintained control valves exhibit visible symptoms—you have to know how to recognize them.
Control valves and associated instrumentation must be calibrated before being installed. This ensures the valve will map its range of travel to an input signal and that it optimizes the entire process loop’s control performance. To elaborate, most control schemes include a pressure measurement that’s sent to a control system, which responds by trying to move the valve to a specific position to maintain a certain pressure. The pressure transmitter, the control system and the valve must work together to maintain process stability. As the final control element, the control valve compensates for deficiencies elsewhere in the loop.
Verify valve calibration upon installation and from that point on, reset it at regular intervals. It’s not uncommon to find 25-year-old control valves that haven’t been recalibrated since installation. Such negligence sentences the device to a lifetime of underachievement. Over time, valve calibration continues to deteriorate. Instead of performing at 0.025% accuracy, a control valve can drift to the range of 2% to 5% accuracy, a shift that increases process variability, decreases process quality and fosters final product quality degradation.
Selecting valve type and size properly eliminates cavitation, a damaging condition that destroys valve internals (Figure 2). Also, entrained solids can cause erosion, which can lead to leakage and inadequate valve shutoff.
Figure 2. A real metal eater
Cavitation damage in control valves usually is the result of improper selection.
In compressible flow applications, incorrectly specified valves can produce noise levels of more than 85 db, a condition that violates OSHA noise regulations. When noise levels exceed 105 db, excessive vibration can damage the valve and downstream piping.
Flashing is another fluid flow phenomenon that can lead to valve trim and downstream piping damage. Flashing results when vapor bubbles formed within a liquid process fluid flow through the valve and continue on downstream. The damage flashing produces appears similar to that caused by erosion, however, the wear pattern is much smoother. Proper specification of control valve trim materials and valve geometry is absolutely necessary to avoid or mitigate flashing damage.
Valve packing degradation and leakage is another sign that maintenance is required. Typical valve packing uses Teflon rings or other material around the valve stem to prevent the escape of fluids. Degraded packing won’t provide a tight seal, which allows fluid to escape. A common fix is to tighten the packing flange. However, this increases valve-stem friction and affects the valve’s overall performance. Additional actuator force will be required to overcome the increased friction; the valve’s calibration and performance can be impaired significantly.
Seventy percent of a control valve’s life cycle cost is incurred after installation, much of it during major overhauls and repairs. To reduce costs and maximize uptime, companies extend turnaround intervals from every five years to as much as every eight years. It’s not uncommon for a processing plant to perform maintenance on more than 250 valves during a turnaround. Lengthening turnaround intervals can elevate the risk of unscheduled maintenance as valve assets are pushed beyond their mean-time-between-failures. The most effective way to cut control-valve ownership cost is to use preventive and predictive maintenance to reduce the number of valves that require turnaround maintenance.
Role of PM