During designing an actuated valve, instrument engineer shall concern to valve safety factor which is a ratio between “torque produced by an actuator” to “torque required by a valve to actuate when changing position” (close to open or vice versa). Valve and actuator commonly obtained from different manufacturer, therefore it is a responsibility of instrument engineer to ensure that combination of the selected valve and actuator will operate properly and meet the safety factor specified by project.
To obtain the information simply doing the following step.
After deciding valve size, valve rating, valve material, etc., go to valve catalog and select one valve that meet the specification. Then, obtain valve torque data. Valve torque is affected by maximum differential pressure across valve (Max dP). This value is determined by process department and shall be informed to vendor. Max dP usually occurs when valve is fully close so that one side at maximum pressure while the opposite site is at no process fluid condition.
Afterwards select an actuator that can produce torque higher than the required valve torque. Note that torque produced by an actuator is affected by power source, e.g. instrument air pressure in pneumatic actuator.
Usually vendor will provided safety factor table along with quotation, following with engineering review. Below is one example of safety factor table:
To obtain the information simply doing the following step.
After deciding valve size, valve rating, valve material, etc., go to valve catalog and select one valve that meet the specification. Then, obtain valve torque data. Valve torque is affected by maximum differential pressure across valve (Max dP). This value is determined by process department and shall be informed to vendor. Max dP usually occurs when valve is fully close so that one side at maximum pressure while the opposite site is at no process fluid condition.
Afterwards select an actuator that can produce torque higher than the required valve torque. Note that torque produced by an actuator is affected by power source, e.g. instrument air pressure in pneumatic actuator.
Usually vendor will provided safety factor table along with quotation, following with engineering review. Below is one example of safety factor table:
(Click To Enlarge)What is the recommended safety factor value? Many projects require 1.5 or 2. Note that safety factor which is too high could lead to miss-operation of an actuated valve.
5 comments:
Those were very good information!
Valve actuators are really needed for complex tasks in the high power market.If the load requires accurate positioning, the electric actuators as well as the valve actuators has the advantage among others.That is why,to familiarize yourself in this kind of industrial application , knowing how an automation works is a wisely action and will positively keep us in track.
@fccu: Agreed, also each type of actuators (hydraulic, pneumatic or electric) has its own characteristics and advantages. In process industries e.g. oil and gas, the number of electric actuator is less compared to that of pneumatics. It is applied in big valve such as sea water intake on-off service.
Safety is always an issue. Thats why, years of research must undergo to make this successful.
Thanks.
It's important to ensure that the vendor isn't too conservative to ensure that the actuator works correctly.
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