Valve Stem Packing: Many sliding-stem control valves have adjustable packing. Over time, wear from the stem movement degrades the sealing effectiveness of the resilient packing material. When that occurs, the stuffing box bolts need to be tightened to squeeze the packing back onto the stem. The timing for this maintenance adjustment is hard to predict, because it’s a function of how often and how much the valve moves. In the past, the maintenance period could only be predicted by prior experience, assuming the operating conditions for the valve remain constant.
Today, using the diagnostics capabilities available with the new generation of electric control-valve actuators, the valve stem packing life can be more precisely related to the total distance that the valve stem travels against the material. Once this travel limit is reached, then maintenance is required, regardless of the time it takes to reach the limit.
Data loggers are capable of recording this total travel and displaying it through a digital communications link or on a PDA (see first screenshot). By simply reviewing the “Distance Traveled” data, maintenance engineers can now easily review the total travel and predict when adjustment will be needed.
Valve Operating Range: Many control valves have an optimum operating range within their travel. Furthermore, there is a danger that damage may occur to valve seats on some types of valves if a valve is held for a long time in the “Just Open” position — that is, the first 5 percent of valve travel.
Using the data logger in the valve actuator, information on the time spent at each position of the valve can be viewed (see second screenshot). “Dwell Time” information can show if a valve has spent too much time in an undesirable position, giving advance information on possible seat damage. It also can give useful information on the way the valve is controlling the process, and if it has been optimally sized.
In recent years there have been advances made in electric control-valve actuator technology that provide a much higher level of diagnostic capabilities than previous methods. These enhancements can help flow control engineers dramatically improve process precision, as well as attain important asset management and predictive maintenance goals.
In the past, many process control engineers were reluctant to consider electric control-valve actuators due to poor speed of response, inferior reliability and the lack of a viable fail-to-position capability. However, new technologies in this area are capable of controlling fast control loops with quick response and high accuracy, while at the same time providing the durability of pneumatic actuators, even under constantly modulating conditions.
Control Valve Diagnostics
Actuators are the de facto instruments for measuring conditions within the valve to determine valve diagnostics. That is, almost all valve diagnostics methods are dependent on the valve actuator to gather information about two main parameters: the position of the valve and the torque or thrust demand of the valve. From these two key pieces of data a considerable amount of information about valve performance and a valve’s operating condition can be extrapolated and used for process improvement.
In almost every case a valve actuator only delivers the amount of thrust or torque demanded by the valve. These forces are generated by the process media acting on the closure element and internal valve component friction from guides, seals, and stem packing. By measuring the forces generated at different positions in a valve’s stroke, an analysis of the valve’s condition can be made that will indicate changes over the life of the valve. Using this information, technicians can determine the condition of stem packing, seats and other components in the valve that may need repair or maintenance to keep the valve operating at optimum performance levels. In addition, some actuators can also diagnose their own condition.
Traditionally, the most frequently used actuator type for control valves — whether linear-type globe control valves or rotary-ball or butterfly-type control valves — has been the spring-diaphragm actuator. Alternatively, on quarter-turn valves, a rack-and-pinion piston actuator may be used to provide rotary motion. The control of most traditional pneumatic control-valve actuators, however, is almost invariably accomplished by the positioner.
Today, the largest number of positioners supplied is the digital (or “smart”) positioner. These positioners are able to monitor the air supply introduced to the actuator to determine the amount of pressure required to overcome the forces in the actuator and valve. From the force derived from this pressure, the internal friction of the actuator and any opposing spring force is subtracted, leaving the net force required to operate the valve. This force measurement, together with position readout, provides the two previously mentioned key parameters that allow valve diagnostics to be monitored. Almost all control-valve manufacturers have a smart positioner available to compliment their products and provide a level of valve diagnostic information.
The development of microelectronics and microprocessors has had a dramatic impact in the world of control valves, where constant movement is part of the valve function. In fact, it has been in the control-valve arena where many of the techniques for asset management and predictive maintenance have been developed.
Therefore, it might surprise some control-valve users that significant improvements in control-valve diagnostic capabilities can be traced directly to years of success in isolating-valve actuator application scenarios.
|An example of the new generation of electric control-valve actuators is the recently introduced Rotork CVA line of actuators, which are available in linear and quarter-turn actions.|
Because isolating valves, such as pipeline ball valves, large municipal sluice gates and so forth, are infrequently used, there had not been a pressing demand for predictive maintenance diagnostics. However, the availability of new electric actuator technology in the mid-1990s coupled with the trend toward outsourcing maintenance personnel in plants led to the introduction of predictive maintenance in this area.
Specifically, the first generation of electronically controlled actuators in the early 1990s incorporated advanced electronic sensors and optional data loggers to allow torque and position data to be recorded within the actuator. Today, many electric valve actuators have data loggers as a standard feature. Maintenance and manufacturing personnel routinely use this data to monitor the valve and determine if there has been unauthorized operation or if adjustments are needed.
Electric motor operators linked by digital communication systems can now download this information in real time or on demand, so central control rooms can collect data on operating conditions and raise alarms related to preventative maintenance issues by predicting when service needs to be done. Such information has greatly enhanced productivity and operating performance in a wide range of isolating-valve situations.
|The high time spent at fully open was accumulated during start ups. This data confirms the correct sizing and operation of the valve by the amount of time spent at 78 percent open. If the distribution showed a high portion of time spent close to the close position, then that could indicate an oversized valve. If, conversely, the graph showed a high proportion of time spent at the open end, then the valve could be undersized. Time-stamped data allows more in-depth analysis of the operating conditions of the associated plant and optimization of the overall plant operating conditions. Also, the analysis of anomalies and pressure spikes can be achieved with time-stamped thrust charts.|
City of Crystal Lake Finds
A Solution and Gains Diagnostic Benefits
In the real world, there are many applications where the new generation of electric control-valve actuators can help solve some of the most demanding problems.
For example, at its wastewater treatment facility, the City of Crystal Lake (Ill.) uses a hydrothermal three-way needle valve to control the flow of steam into its digester. The steam heats the sludge in the digester to ensure optimization of the process. The objective is to maintain a constant temperature of 98.5 F in the anaerobic digester, utilizing a controlled steam temperature at approximately 200 F.
In addition, another important requirement of the application is that should a power failure occur, the control valve is required to shut off the supply of steam to the digester, otherwise the bacteria that treat the sludge will be impaired and may cause circulation problems.
While the hydrothermal valve was found to be the ideal solution for this application, finding an electric fail-safe actuator proved to be a challenge.
The city’s first attempt at solving this control issue used an electric actuator with a spring to provide closure motion on power failure. Because of the constant compression and relaxing of the spring, this was not a suitable solution, as it resulted in constant loading of the motor. For this reason, James Huchel, Sr., wastewater superintendent for the City of Crystal Lake, decided to try a beta test version of Rotork Controls’ (www.rotork.com) new CVA actuator. The CVA utilizes super capacitors to power the actuator to the desired position should main power be lost. Because there is no spring, the actuator can provide higher-duty cycles and more accuracy.
In addition to meeting the primary requirements of the application, Huchel believes the biggest advantage of the CVA electric control valve actuator is its built-in data logger. The data logger enables him to periodically download information regarding the behavior of the valve. Of particular interest in this application is the amount of time the valve spends in any particular position. Should the valve be closed for extended periods of time, then this information would be useful in managing the operation of the steam boilers and could lead to significant cost savings in boiler operations.
Data logger records also provide information about the thrust demand of the valve and help in scheduling planned maintenance. Specifically, the records can sum the total travel traversed by the valve stem, providing an indication on valve-packing wear. In the past, the hydrothermal valve at the Crystal Lake facility required stem-packing replacement after a certain amount of operation. Now, with data log information, the exact time for planned maintenance can be anticipated and conveniently grouped with other maintenance work.
The operation experience so far at Crystal Lake has shown that precision electric control-valve actuators coupled with data logging capability can enhance the overview of maintenance requirements, as well as provide improved process control.
Chris Warnett has over 32 years experience in the valve and actuator industry. A registered mechanical engineer, Mr. Warnett has worked in actuator design and application in both Europe and the United States. He has extensive experience working with valve diagnostics and is the author of a guidebook about using valve actuators as diagnostic instruments. He currently serves as manager of the Rotork CVA product line. Mr. Warnett can be reached at firstname.lastname@example.org or 585 247-2304, ext. 282.