The Importance of Reliable Valve Torque Measurement for Predictive Maintenance
Rotork IQ intelligent electric valve actuators are designed with the benefit of fifty years’ worldwide operating experience in industries including oil, gas, power generation and water treatment. The compact and rugged mechanical design is combined with innovative solid state control and instrumentation technologies, providing a very high degree of functionality and reliability in the harshest of operating environments, combined with a minimum requirement for maintenance.
Experience has shown that a single-stage worm and wheel gear design in a sealed oil bath provides the greatest mechanical reliability and durability for the actuation of valves in all ambient temperatures - from the deserts of the Middle East to the northern slopes of Alaska. Oil does not exhibit the problems of ‘tunnelling’ and decomposition associated with grease, which can cause a lack of lubrication and premature wearing on the gears. The use of an aluminium bronze material for the worm wheel provides a tough, wear resistant performance and, when required, special oils are available to suit extreme temperature conditions. This combination of material of construction and oil lubrication delivers a proven long service life in the most demanding and arduous applications.
The accurate measurement of the actuator output torque is also a function provided by the worm and wheel. The axial force of the worm shaft is always proportional to the torque produced by the worm wheel and this is not affected by changes in the efficiency of the gearing caused by long-term operation.
In Rotork’s case, the torque is measured by a force transducer rather than disc springs or any other mechanical devices that can wear and change their characteristics over time. The force transducer is entirely unaffected by usage and temperature,
therefore the accuracy of the indicated actuator torque can be relied upon throughout its service life. This is very important, as the torque profile of the valve operating stroke is recorded by the actuator data logger as a ‘footprint’ during commissioning and subsequently recorded during every valve operation. As described below this data forms a crucial part of the information that can be downloaded from the actuator by means of Rotork IQ-Insight software for maintenance purposes. Using this data, maintenance can be planned for only when it is really necessary, removing the requirement for over-cautious, regularly scheduled shutdowns.
Communication with the actuator is performed ‘non-intrusively’ by means of a hand-held setting tool. This begins with commissioning, when a multi-language enabled menu on the actuator’s display window guides the operator through the programming of end of travel limits, torque setting, the number of turns, remote indication requirements, alarms, interlocks and all the other commissioning data demanded by the valve duty. This data is recorded on the actuator data logger, together with the footprint of the valve operating torque profile in both the opening and closing directions. Subsequently, torque output on every valve operation is recorded in historical order, giving the actuator the ability to monitor the condition of the valve.
Using the ‘non-intrusive’ link, this data can be periodically downloaded and then uploaded into a PC or laptop computer running Rotork IQ-Insight software. The software enables the historical record of valve operating torque profiles to be analysed and in this way the trend of valve wear can be established. With this information it is possible to accurately predict when the valve will require maintenance and plan accordingly, ruling out the need for laborious and inefficient maintenance schedules, which are often rigidly planned at frequent intervals "just in case", resulting in a lot of unnecessary work, expense and disruption to operations.
Published in Valve User Magazine Issue 5
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