Sizing factors to determine an actuator size for a soft seated ball valve

When sizing pneumatic actuators to suit soft seated floating & trunnion ball valves the torque must first be established for the valve which will vary depending on what seat the valve is fitted with. For example TFM 1600 would be the lowest torque, then PTFE, then reinforced PTFE, then Delrin/ Devlon/ Nylon and PEEK in order of the effect on torque. All these seat types will have a different torque. The valve manufacturer will only publish the theoretical torque and the actuator manufacturer will only publish the theoretical torque. Most of our manufacturers such as APV-Torqturn, Air Torque and Actreg are conservative and publish the lowest torque. Most valve manufacturers such as Australian Pipeline Valve, Starline, JC, Sferova, etc., will publish a valve torque that is in the higher range. Global Supply Line Engineering can do an actual torque test on the valve and actuator which can be significantly different.

Other factors that effect the torque of the valve include the stem packing system. For example a Belleville washer energised packing system fitted to the stem of floating ball valves will provide a superior low maintenance stem seal but may increase the torque slightly and so it goes on. Many of the factors below vary greatly according to the size of the valve. Also, if any of the below special service conditions exist in larger size valves, the user may even consider using a different style of valve than a soft seated ball valve due to the cost and size of conventional pneumatic scotch yoke or rack & pinion actuators. Also soft seated ball valves are more suited to clean service in a standard temperature range.

Soft seated ball valves need to be regularly ‘exercised’, especially when fitted with an actuator for ESDV service. When analysing safety factors, the torque of the actuator at it’s weakest point needs to be considered. But also the critical 'break to open' and 'break to close' strokes need to be evaluated to ensure there is sufficient ‘safety’ as many seat types can become hard to turn if left in one position for too long causing problems in open and fully closing. See the below APV Ball valve and Torqturn Actuator IOM's, which discuss the pro’s and con’s of using the seat sealant injection points in emergency situations to flush out entrained debris and lower the torque of the valve temporarily until it is again cycling freely. (See Ball Valve IOM link below).

GSL Engineering and APV can provide a safety factor sizing guide calculation chart. Click here to view an example. We can also torque test the valve and/or actuator. GSL are the official automation & test facility approved for F.A.T. testing APV, JC, Sferova, Starline, Torqturn, Air Torque, Actreg & Biffi. We also stroke test the actuator while the valve is under full differential pressure. We can also supply and fit Bettis & Rotork.

The real torque value can be high depending upon the following listed factors as these factors can drastically increase (or decrease) the torque. (Many factors such as points 1 and 2 apply more to smaller sizes industrial valves.) Sizing also needs to take into account the maximum stem torque. Larger actuators may require stronger stem materials like 14-4PH, 410, F6A, 4140, 4130, S31803, Nitronic 50, F51 XM19, etc.

Approximate examples (depending on size/ class) of valve torque increases/ decreases (apply to safety factor accordingly) are: -

1. Fluid carrying dust, powder, abrasive particles (+50% to 125%). In extreme cases can be higher. i.e. Up to 200%
2. Slurry, pulp, resins, paste (+50%) can be even higher in some cases (this generally rules out all but very small ball valves)
3. Dry (clean) service (dry gas) (+10-30%)
4. High or low temperature (consult us) again this will effect torque
5. Infrequent use (+20-40%) *1 - *2
6. If a higher minimum air supply than shown above is available more torque will be produced in the air operation phase and vice versa
7. Valve torque is based on a maximum Delta P for class. If you actual working pressure is lower please advise as a smaller actuator may be possible
8. Lubricated service (clean oil or other high lubricity fluid) (- 15%)
9. For very high frequency usage avoid glass filled PTFE (RPTFE) (floating ball valves) as glass fibre can delaminate.
10. Base actuator torque on minimum available air supply.

1* PTFE/RPTFE seats can require more torque to unseat after a period of time due to it's fluidity & ‘memory’. For Devlon/ Nylon seat the valve torque is higher and PEEK higher still (which needs to be factored in to valve torque shown where applicable), however, as these materials are harder, infrequent use has a lesser effect on the increase in torque requirements.

2* Where set ‘fail spring to close’, an additional infrequent use safety factor primarily needs to be applied to spring BTC stroke (as once the valve is 'cracked' the other torques are less affected) Nonetheless, the spring ‘ETC’ if often the weakest point of small to medium scotch yoke actuators. Infrequent use: - Once per month at least an extra 10-20%, once every 4 months 15-25%, over 4 months an extra 20-30%.

For more information and a sample of one of our GSL torque analysis forms click here.

Also stocking Actreg and Biffi (Emerson) Pneumatic Actuators up to 30,000NM. See our website stock list click here.

Global Supply Line - With $60 million inventory, we keep the world's largest valve and actuator stock range. Click here to view and bookmark our full stock list. Click here to view and bookmark our catalogues. Click here to join the GSL Group. Click here to follow our GSL company page for the latest updates.

Australian Pipeline Valve - short lead time, high quality oil and gas valve manufacturer. Specialising in short runs and supplying high specification valves. Click here to view and bookmark our catalogues. Click here to join the APV Group. Click here to follow our APV company page for frequent updates.

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