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
Perform control-valve preventive maintenance at least once a year and include three key elements: review of the valve specification to account for process condition changes, valve recalibration and physical inspection.
Proper prepurchase valve specification is critical for successful performance and low life cycle cost. Most operators expect at least five years of service from a control valve before major maintenance. When buying a valve, make sure the specifications are correct for the application. Don’t purchase the lowest-priced valve. Having to pull an improperly specified valve for maintenance before a scheduled turnaround will cost far more than any savings achieved at purchase.
Calibration is the most common and important form of preventive maintenance. Operators should perform this routine at least once a year. Accurate calibration requires closing the valve and stroking it to a 100% open position. Control-valve components can be calibrated under full line pressure, but never under full operating conditions. Closing the valve and then opening it fully during production disrupts the process, causes poor product quality and produces a potentially unsafe working condition.
Schedule visual inspections at least once a year during which technicians can check valves for visible signs of wear, corrosion, erosion and packing leakage. During the physical inspection, technicians should inspect the air-filter regulator and bleed off accumulated water.
A successful preventive maintenance program also requires an effective control-valve asset information management system. Proper recordkeeping can be a difficult task, especially for older valves, because finding the original specification information can be a challenge. To assist with recordkeeping, some valve manufacturers offer maintenance- and asset-management database software to help plant personnel maintain accurate data and manage valve total life cycle cost.
Manufacturer’s information and data can be recorded and maintained using maintenance-management software tools. In addition to data storage, these tools can be programmed with flags to alert operators about impending scheduled maintenance.
Digital valve positioners are another data source that help reduce cost. Positioners communicate using Hart and Fieldbus protocols. When manufactured and shipped on a control valve, the digital positioner can provide an initial signature of valve-assembly performance data. This gives the maintenance department an as-manufactured baseline for future comparisons. The operator can extract the scheduled turnaround date for each valve from the database. If a valve is returned to the repair shop within its scheduled turnaround period, the database generates a detailed report that captures the symptoms displayed, the as-received signature, type of problem found, the maintenance performed and a new footprint of the valve (using the digital positioner). The database gives the maintenance department reliable information about premature valve failure and, in some cases, can highlight root-cause errors in valve specification.
Additionally, the valve maintenance database helps the maintenance department reduce inventory. The software identifies the parts that are included in the manufacturer’s recommended spare parts list, verifies which are stored in plant inventory and highlights the parts that are available from the manufacturer within 24 to 48 hours. The team needs to maintain inventories only for long-lead, critical and application-specific parts.
Plants that lack detailed, long-term valve maintenance records need a plant-wide valve survey. This audit should identify every control valve and associated instruments and accessories. The vendor, valve size, tag information, materials of construction and instrumentation mounted on the valve are included. The survey notes the physical condition of the valve, including information about leaking packing, excessive corrosion and broken parts. The survey generates a bad actor list to identify immediate valve maintenance needs as well as a list of critical valves for potential predictive diagnostic testing and further analysis. Valve manufacturers offer this diagnostic service and the associated expertise to ensure critical valves are performing correctly. The cost for the diagnostic testing is insignificant when compared to the cost of a critical valve failing prematurely.
Once these data have been gathered and entered into the database, the plant personnel and the manufacturer should establish a new schedule for preventive maintenance, turnaround frequency, a new spare parts list and an estimate of future manpower resources.
The big cost-saver
Predictive maintenance involves gathering data to predict future valve performance. It’s a more sophisticated approach than preventive maintenance and offers significantly greater opportunities for cost savings. The data typically can be collected in two ways.
The first approach requires an industrial-strength PC and associated hardware for field-testing control valves. The data acquisition tool can compare input signal to valve position, perform frequency-response tests and run diagnostic tests on the positioner, the actuator and the entire valve assembly.
The second method entails gathering data from the digital positioner. Many tests can be run, such as comparing set point to valve position and the response to a step input change. These tests provide valuable data for diagnosing valve performance. For example, the digital positioner can log the total number of strokes the valve has performed and the number of directional changes. Another is the time-near-closed test. A valve that spends an unusual amount of time below a preset limit is oversized and needs to be respecified. Valve oversizing results in higher velocities through the valve, which increases wear and tear, degrades performance and predicts expensive maintenance.
A laptop computer with diagnostic software is recommended, but digital positioner data can be accessed at the valve with a handheld device or it can be collected in the rack room. When in the field, technicians use a HART modem to connect to the input-signal wiring from the control system to download the data and perform tests on the valve. Data collection can be conducted while the valve is offline or online and still in control of the process.
Gathering data is the first step. Analyzing it requires a trained, experienced technician or applications engineer. Like a soothsayer reading tea leaves, a valve diagnostic expert can interpret raw data to predict future valve performance. For example, a diagnostic report can include meaningful operational data, such as valve friction at various valve travel positions. These data can help to diagnose stem, trim or seat wear and can pinpoint root-cause problems or developing conditions. By correctly identifying and pulling only those valves that require maintenance, a plant can save thousands of dollars. Experience shows that only about 50% to 60% of the valves scheduled to be pulled in a turnaround really need to be maintained. Predictive maintenance not only saves repair dollars, it also can shorten the duration of the shutdown, meaning faster plant startup and less lost revenue.
The effective management of control-valve assets represents an attractive cost-reduction opportunity. Installing correctly specified valves can improve plant performance and product quality while extending scheduled maintenance periods. Aggressive preventive maintenance, such as annual calibration and physical inspection, can identify areas of concern before valve assets require costly repair and disrupt operations. Predictive maintenance helps an operator see into the device, pinpoint hidden problems and eliminate the need to pull good valves. With maintenance consuming the lion’s share of a valve’s life cycle cost, an effectively managed maintenance program is essential in a plant operator’s drive for cost leadership.
Mitch Williams is manager of strategic accounts for Dresser Flow Solutions, Avon, Mass. E-mail him at email@example.com.
Figures courtesy of Dresser Flow Solutions.