1 February 2006
Rotary Control Valve Offers Smart Solution
Valve handles dirty service, controls emissions, and boasts a high temperature range, but proper maintenance remains essential
By Jari Kirmanen and Jean-Guy Lagacé
Due to increasing concern about environmental emissions, the petrochemical industry has paid more attention to emissions coming from control valves. As a result, rotary valves, which typically have lower gland emissions than globe valves, have become more attractive. Eccentric plug valves in particular have established a firm foothold in petrochemical applications. But in addition to low emissions, eccentric plug valves are proving suitable for quite a few applications. For example, in the pulp and paper industry, rotary control valves are necessary because many applications include the control of fibrous flow media.
The high-performance intelligent valve positioner, sometimes defined as valve controller, is an important device when a long-term control solution is required. Even though its role may sometimes be crucial, the performance of a control valve does not result solely from the smart positioner.
The control valve itself must also be working well. Such problems as high friction, shaft wind-up, and backlash have been associated with rotary control valves. But with modern, well-engineered, eccentric rotary plug valves, these can be a minimal distraction.
"A very strong advantage of these valves for petrochemical plants is reduced emission of volatile organic compounds over long periods. The standard packing arrangement meets EPA emission requirements beyond 100,000 cycles in laboratory testing, and in years of service in a plant," says Hank Boger, a voting member of ISA-SP75 Control Valve Standards.
The eccentric plug valve is highly suitable in many applications. The valve's ability to handle dirty service, its low emission control, and its high temperature range are invaluable.
Achieving loop performance starts with correct valve selection and sizing. Analyzing the installed flow characteristic is important for optimizing control performance. Loop performance is at its best only if all the components in the control loop function correctly. To achieve performance throughout the valve life cycle, it is essential to predict when control valves need service. A method based on predictive maintenance and using an intelligent valve positioner and advanced online diagnostics is possible.
Asset and device management based on FDT/DTM technology provides the means of using advanced device diagnostics and even of integrating them into the DCS system.
Friction is one of the most common factors that may cause poor performance in a control valve.
In linear valves, friction comes about mostly because of the packing. In rotary valves, typically the sealing (or seat) causes friction. In addition to friction, problems such as shaft-windup and backlash are commonly associated with rotary control valves.
However, backlash can be reduced by using well-engineered valve-to-actuator couplings, some of which, typically clamped designs, are even completely backlash-free. Eliminating seat friction in eccentric rotary plug valves transpires because the plug detaches from the seat as soon as the plug opens.
Dynamic flow forces, which can cause instability, especially at small valve openings, can be minimal too with a plug that balances flow forces. Typically, diaphragm actuators operate control valves, which reduce the friction in the actuator and ensure smooth operation. Direct mounting between the actuator and the intelligent valve controller with internal pneumatic piping is preferred.
Lastly, an intelligent control valve should have also a high-performance, smart valve positioner. The intelligent, digital positioner is the heart of the intelligent control valve. It should be easy to use, have automatic tuning and calibration, and withstand adverse conditions like dirt, moisture, and rough handling.
The best of intelligent valve controllers can measure the valve's performance, store measured results in its memory, and warn the user if performance decreases. Advanced diagnostics can work online without disturbing the process.
To study the performance of a control valve, the ISA open-loop step sequence test is widely used. The same test can define valve dead band and speed of response.
Fluids containing impurities or dirt constitute very demanding applications for a control valve.
These valves tend to stick easily or become completely clogged if dirt can lodge in the valve trim, bearing, or stem area. In such cases, linear valves have problems because the rising stem tends to draw dirt into the gland packing area. This typically increases valve friction and leakage.
On the contrary, rotary valves do not draw dirt into the gland or bearing area, which makes them more resistant to dirty fluids. One example is the heater pass valves in a refinery. Hot dirty crude oil flows through these valves, which makes the situation very demanding for linear valves. Eccentric plug valves in particular have proved their excellence in dirty fluid service. Eccentric plug valves do not contain any cavities inside the valve where fluid can stick, therefore they can handle even fluids that are prone to crystallize or polymerize. Process pipes may typically contain some debris, especially in start-up. In such conditions, rotary valves have the unique design feature that any debris flushes away when the valve opens.
In petrochemical plants, the most critical environmental impact comes from emissions to atmosphere and to surface water. Now that petrochemical companies are promoting sustainable development, it is clear low fugitive emissions through valve stem packing are an important factor. Due to rotary action, emission control is much easier with rotary valves than with linear valves. In laboratory tests, 10-to-100-times higher emissions have been produced using standard linear valves than with standard rotary valves. It is very likely that under actual process conditions, the differences are even greater because the rising stem tends to "pump" impurities into the gland packing area, and therefore increase the leakage.
"One big advantage that rotary valves have over globe valves is that the guides in rotary valves are less likely to be blocked by sand or other impurities contained in crude oil. They also offer wider rangeability and higher pressure drop capabilities over globe valves for comparable actuator sizes. Finally, rotary valves offer cost, size and weight advantages," concludes expert consultant Hans Baumann.
Baumann is a voting member of ISA-SP75, Control Valve Standards and holds over 120 patents on valve related technologies.
Rotary valve manufacturers have developed live-loaded packing designs, which are almost equal to linear valves with a bellows gland. These packings work for hazardous flow media. Live-loaded packings used in a rotary control valve are very simple designs, which require only minimal maintenance, so rotary valves are an easy solution to improving overall plant emission control. On-site studies have shown significant reductions are possible by switching rising stem valves to rotary valves.
In general, eccentric plug valves are suitable for a very wide range of applications, including steam, condensate, oil, and hydrocarbon. Their streamlined design, whereby fluid flows around the plug, means they can handle flashing as well as erosive services. Temperature and pressure ranges are not as wide as for linear valves but still wide enough to handle most of the applications found in a petrochemical plant. Temperatures from -200 to 400+ degrees Centigrade are possible. Typically, pressure classes up to ANSI 600 are available.
The best of eccentric plug valves can also be equipped with anti-cavitation and noise-reduction trims in order to widen their application range into high-pressure-drop ratio services. However, such valves may not be suitable for very high-pressure applications(>100 bar).
Reaching loop performance
The control valve is only one parameter affecting loop performance. Closed control loop performance is the sum of various factors, which play a part in getting the best performance out of a control loop. However, of all the components in a control loop, the valve does the hardest work and is therefore prone to cause problems. For the valve to work properly, careful valve selection and sizing is imperative, and it should receive regular service to ensure a long-lasting, high-performance control solution.
As to valve sizing and selection, even an advanced control method with the most accurate transmitter cannot compensate for problems caused by poor valve selection. To optimize valve selection, the size to choose should have a valve opening that under normal conditions should be somewhere in the range from 60 percent to 80 percent open. It is also important to aim for an installed flow characteristic that is as linear as possible within the control range. Linearity of the installed flow curve relates directly to the installed gain, which is to say, the slope of the installed curve. A common problem is an oversized valve.
It is crucial to service valves at regular intervals to keep the process sufficiently efficient and to maintain loop performance throughout the whole life cycle. Servicing valves before it is actually required could work, but it would be a rather expensive and time-consuming way of handling maintenance. Waiting until valves fail and cause a possible un-scheduled shutdown can be also very costly.
Ideally, only those valves that really require maintenance should get attention during a shutdown, so valve diagnostics and/or a monitoring program is necessary. During a shutdown, it is possible to monitor and analyze valves to check whether they need servicing or not by using a valve signature test. However, this so-called off-line diagnostic can be very time-consuming. Instead, the better approach is to analyze valve data just before shutdown, while the process is still in operation. In this way, we can pinpoint those valves requiring maintenance beforehand. This requires devices with online diagnostic capabilities.
Online diagnostics make it possible to monitor valve performance, while the process is running, not only during shutdowns. The aim of predictive maintenance is to indicate decreasing valve performance and to warn the user before failure is so bad that it causes excessive process variability or even an unexpected shutdown.
Online diagnostics can continuously monitor valve performance, but analyzing the results can be very time-consuming and labor-intensive. The most efficient way to carry out predictive maintenance and online diagnostics is to utilize valve controllers, which are capable of storing results in their memory and send warnings and alarms based on performance limits stored in their memory. In this way, no additional labor is necessary to analyze and study the results continuously, because the intelligent valve controller, with the help of advanced asset management software, can measure valve performance automatically.
About the Authors
Jari Kirmanen (firstname.lastname@example.org) is an ISA member and part of the Finland section in District 12, Europe. He is a field systems engineer. Jean-Guy Lagacé (JGLagace@tr.kruger.com) is senior instrumentation technician at Kruger Inc., a privately-owned pulp and paper company in Canada.
Standards Prove the Case
The ISA standards group number 75 includes more than 20 documents relating to control valves, on-off valves, and actuators. The open-loop step sequence test measures and defines dead band and speed of response among other performance statistics. ANSI/ISA-75-25.01-2000 Test procedure for control valve response measurement from step inputs defines the testing and the reporting of step response of control valves that are used in throttling closed loop control applications, as well as methods and criteria for performing response tests and evaluating test results for three alternative environments: bench testing, laboratory testing, and in-process testing.
The standard does not define the acceptable control valve performance for process control nor does it restrict the selection of control valves for any application.
This standard defines the testing and reporting of step response of control valves that figure in throttling closed-loop control applications. A control valve consists of the complete, ready-to-use assembly of the control valve body, actuator, and any required accessories.
The most probable accessory is a valve positioner. The technical report most germane to this situation is ISA-TR75.25.02-2000 Control Valve Response Measurement from Step Inputs. The standard defines methods and criteria for performing response tests and evaluating test results for three alternative environments—bench testing, laboratory testing, and in-process testing.
Bench testing is testing without flow such as in a plant instrument shop, laboratory, or control valve-manufacturing site. Laboratory testing is testing with flow in a laboratory. In-process testing is in a plant during normal plant operation with process flow.
Linear valve: Another name for a globe valve. It refers to the linear or straight-line movement of the plug and stem.
Globe valve: Valve with a linear motion, push-pull stem, whose one or more ports and body have a globular shaped cavity around the port region. This type of valve characterizes itself by a torturous flow path, and some call it a low recovery valve because some of the energy in the flow stream is dissipated. The inlet pressure will not recover to the extent that it would in a more streamlined high recovery valve.
Packing: Sealing system that normally consists of a deformable material such as TFE, graphite, and asbestos. It is usually in the form of solid or split rings contained in a packing box that compress tightly so as to provide an effective pressure seal to keep process liquid from leaking.
Packing box: The chamber located in the bonnet, which surrounds the stem and contains the packing and other stem-sealing components.
Backlash: Play or loose motion in an instrument due to the clearance existing between mechanically contacting parts.
Bellows sealed valve: Valve that utilizes a bellows to replace the conventional packing gland. One end of the bellows welds to the rising stem and the other seals against the valve body.
Cavitation: The rapid formation and depletion of air bubbles that can damage the material at the solid/liquid interface under conditions of severe turbulent flow.
Eccentric and Rotary Action Ensures Tight Shut-off
As the eccentric plug rotates 90 degrees from open to closed, it moves into a raised eccentric seat. In the open position, flow is straight through and flow capacity is high.
As the plug closes, it moves toward the seat without scraping the seat or body walls so there is no plug binding or wear. Flow is still straight through making the throttling characteristic of this valve ideal for manual throttling of gases and liquids.
In the closed position, the plug makes contact with the seat. The resilient plug seal presses firmly into the seat for dead-tight shutoff.
Eccentric plug and seat design assure lasting shutoff because the plug continues to move into the seat until firm contact and seal takes place.
Source: Southern Manufacturing Group
Working control value: Calibrate current-to-pressure transducers (I/P), valve positioners, and control valves.
Ultrasonic spots leaky valves: There are significant savings involved in identification and monitoring of critical valves.
Valve ranking and partial stroke
ISA-SP75 Control Valve Standards Committee
Control Valves By Guy Borden, Jr. and Paul G. Friedmann
Control Valve Primer, 3rd Edition By H.D. Baumann
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