Torque Testing

The Importance of Torque Testing and Actuator Selection Due to demand from our customers and the desire to supply a ‘Best in class’ service to the oil and gas industry, Geographe Energy’s ongoing investment program recently extended to toque testing equipment. These torque testing units were designed by Geographe personnel and are capable of measuring up to 100,000NM of torque, which, in practical terms, incorporates up to and including 40” valves at all pressure ratings.  The Need for Torque Testing The selection of the correct actuator goes hand in hand with the with the resulting life times of valves. Torque testing of valves and the correct sizing and specification of actuators is a very real way of prolonging valve life and reducing unexpected failures, thus increasing productivity.  Valves and actuators are complex pieces of equipment, with numerous manufacturers, designs, and materials of construction available (design torques differ through different materials). They are often used in conditions of high stress and in contact with high velocity corrosive/erosive fluids. An inappropriate valve and actuator selection can lead to premature failure, resulting in shutdowns and large economic losses. There are also safety issues associated with the premature failure of safety critical machinery. An accurate description of process conditions and valve and actuator purpose is essential when specifying valves and actuators. Particular attention must be paid to the mechanical linkages of the valve and actuator drive chain and mechanical design of the valve and actuator. The various designs available all have their limitations, and accuracy in initial design can prevent many subsequent operational problems. The Use of Safety Factors When Specifying Actuators The use of safety figures when specifying actuators is wide spread within the industry. This is a recommended and approved method of sizing the correct actuator for the valve. The safety factors in ‘safety critical’ machinery is to ensure operation on demand. This safety factor is designed to take into account variances within the manufacturing process resulting in differences in the measured torque figures recorded during ‘in house’ torque testing at the manufacturer’s premises. These variances in manufacture could be due to any one of a number of reasons including tolerance build up within the valve ball to seat interface, in the case of ball valves, and variances in bush diameters and contact points. In addition to this the safety factors take into account changes in torque due to the operational environment including the time between cycles of the valve. This can result in the phenomenon known in the industry as ‘sticktion’ or ‘stickation’, which is the jerky movement of the valve operation after long periods of inactivity. The jerky movement is attributed to the variance in friction coefficients at the interface between the seat and the closing member.  In some instances the valves remain in the closed or open position for long periods of time, which can lead to, in the case of PTFE seat material, cold flow of the seat material around the valve seat to closing member interface thus vastly increasing the valve break torque. (Particularly prevalent in ball valves.) In general valves that perform safety related shutdown operations, centrifugal compressor inlet and discharge valves and divert or blowdown valves are fitted with actuators. It is conventional to use spring return actuators to provide “fail safe” operation of an emergency shutdown valve. In the case of large high-pressure ball and gate valves, the resulting actuators can be prohibitively large and expensive. Double-acting actuators, with suitably protected local pressure reservoir supply, can be used as an alternative. Care should be taken when specifying the speed of operation of actuated valves. The requirement for rapid shut off can lead to potential surge problems (e.g. water hammer) on valves associated with liquid pipelines, unloading lines, and the like. The Equipment The units designed by Geographe utilise tried, tested and proven torquing equipment incorporated in a user friendly / valve friendly configuration culminating in a portable, accurate and fit for purpose piece of equipment. The units come in various sizes, depending on torque characteristics, and are therefore as compact a design as possible. This ensures that onsite testing is achievable without the need for modification to surrounding enclosures or pipework.   Additional In addition to the points noted above it is worth mentioning at this stage the importance of correctly aligning actuator stops on completion of the testing and specification. In all instances it is important that the actuator be transported with the valve and that the actuator stops be set during testing. The reason for this is; if the actuator stops are not set correctly during testing the valve may leak when put into line. The transposing of actuators onto valves that are already in line means that the actuator cannot be set until the valve is tested. This is usually a very costly exercise to undertake and often leads to damage to the softer seat material due to erosion caused by the process fluid as it bypasses the seat, often under high velocities and pressures, due to excessive travel of the closure member. Another point worth mentioning is that care must be taken when reviewing quotations for valve and actuator assemblies to make sure the correct safety factors are being used by the manufacturer or the person putting together the valve and actuator package. In addition to this it is worth clarifying at this stage if the valve and actuator are being tested as a unit, which is not always the case. Conclusion In conclusion suppliers of the actuation packages will only be able to match the process requirements by considering long-term experiences of the users. This demands not only proper sizing including safety factors to receive continuously fast and repeatable stroke durations, but also internal precautions for special bearings, materials, coatings and secondary seals for the actuators and accessories. A major consideration is the proper alignment between valve bodies and actuators, starting with close tolerances in the bracket or mounting plates (as per DIN ISO 5211). Including selecting proper strengths for materials of construction and dimensions for those brackets, and a positive design for the actuator alignment to the bracket itself. The later is not always met in the industrial standard. In conjunction with the above, the actuators weight and center of gravity must be considered when designing the support, especially if the valve is positioned in a vertical line. If the brackets are not designed for the service, but are a generic design then there is a possibility of side loads on the bracket and stem culminating in damage to the stem or stem seals or bracket.

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