by Bob Donnely
In most process control operations, automated valves have a tendency to experience an increase in torque over time. This results from pressure and a buildup in the media over the life of a process. As a result, a spring-return actuator, which may have been perfectly capable of handling the specified torque when new and initially installed, may no longer be able to completely close the valve as torque increases. Likewise, the valve assembly may experience spring fatigue in the actuator.
The typical solution to this problem is to replace the automated valve assembly, taking on the obvious cost implications of having to buy a new automated valve package, downtime and lost production, as well as labor considerations. Or, perhaps, the user will choose to manually wrench the valve closed each time it fails, which is clearly also a less than ideal solution.
Since about 70 percent of automated valves in process control operations are failsafe — using springs to perform the closing operation when there is a loss of air pressure or when the process requires actuators to normally close the valves — when these spring-return actuators malfunction and cannot completely close the valves they are mounted on, it creates a frustrating dilemma for the plant maintenance engineer. The basic principle of fail-safe pneumatic actuators is to utilize air pressure to compress the springs to open the valve, when the actuator closes, the air used to compress the springs in the center chamber of the actuator is exhausted to atmosphere, allowing the pent up power in the springs to close the valve.
In order to solve the problem of torque buildup in the valve, a simple technology has been developed to utilize the air in the center chamber to increase the closing torque, and, in so doing, overcome the torque increase in the valve. The solution, manufactured by Habonim (www.habonim.com) and called the Impact Air Assist, diverts air from the center chamber of the actuator — air that would normally be exhausted to atmosphere and lost — to the spring chambers, and this additional air pressure increases the spring-end torque. The device has been tested and has been demonstrated to increase the spring-end torque by at least 50 percent up to as much as 200 percent.
Since the device works on the principle of air pressure, the more air there is in the center chamber of the actuator, the greater the boost in spring-end torque. As such, the larger the actuator the more air there is in the center chamber, hence the greater the boost as the size of the actuator increases.
With a one-size-fits-all envelope, the device can be mounted on any spring-return actuator. No electronics or external energy are required for operation. The device features a built-in, nonreturn valve to hold the maximum air pressure in the center chamber of the actuator so that any pressure fluctuations in the system will not trigger alarms. No additional source of air supply is required, and the device does not require a safety backup system. A built-in breather block prevents any suction of external air into the spring chamber.
• Refinery Saves $80,000: An oil refinery recently employed the Impact Air Assist to salvage automated valve packages where the actuators were undersized. The installation saved the refinery an estimated $80,000 — the cost associated with replacing all of the actuators with a larger size and the associated down time and retrofitting labor expenses.
• Paper Mill Ups Plant Production: In another example, a paper mill used the Impact Air Assist to enhance performance in older valve assemblies that were slowing production and causing maintenance alarms. In this case, the automated valve assemblies were still serviceable, but they weren’t able to keep pace with increased plant production requirements. In this scenario, the device enabled a quick and cost-effective alternative to an expensive factory-wide parts replacement. As a result of the greater torque provided by the Impact Air Assist, the life of the existing valves was prolonged and the closing times enhanced. Also, by holding the air constant in the actuator, the device mitigated the previous losses in air pressure that were triggering alarms.
• Valve Closing Time in Gas Application: In a gas application where existing automated valve packages were struggling to meet the maximum specified closing time of 30 seconds, the Impact Air Assist was installed and immediately reduced the closing time to under 10 seconds.
• Guaranteed Valve Closing: Finally, in a case where offshore skids were utilizing hemiwedge-type valves that required high torque closings right at the end of the cycle, the Impact Air Assist was employed to guarantee closing. Since this particular valve design featured a unique torque curve, which rose as the valve mechanism functioned, it was absolutely critical that closing be achieved to avoid any losses or spills. Maximum torque was required in the final movement of the tapered hemi into the seat ring to achieve a unique mechanical seal for low-pressure and low-differential service that the maximum torque is required. By adding the device to the actuator mounted on the valve, this maximum closing torque is assured.
Robert M. Donnelly is the president of Compact (www.compact4.com), a manufacturer of patented pneumatic actuator technology. Compact offers actuators that employ Habonim’s Impact Air Assist for increased spring-end tourque. Mr. Donnelly can be reached at [email protected].