Picture 1: Torque Calculation for opening a Manual Valve
Torque is a measure of the force required for a valve operator to rotate the valve closure member to an open or closed position. As an example, if a ball valve hand wheel requires 200 N with a single hand and the diameter of hand wheel would be 300mm (0,3m), the required torque for opening and closing the valve by operator is 200x0,30=60N.M. (refer to picture 1).
The gear box of the valve increases the torque / force that is produced by the operator and transfers it to the valve stem and closure member. Picture 2 illustrates the torque measurement on a concentric butterfly valve during a Factory Acceptance Teste (FAT). The measured value refers only to the required torque for opening the valve on the hand wheel. Torque on the stem is greater and can be calculated by multiplying the torque on the hand wheel by the gear box ratio.
Picture 2: Torque Measurement on a Butterfly Valve
Actuators such as electrical actuators supplied with electrical motors produce more torque/force for valve operation than operators for manually operated valves. In addition, pneumatic and hydraulic actuators using 7 bar air or 180 bar hydraulic oil as examples can put more force and torque on the valve compared to when performing this manually.
2. What Valve Design Parameters Affect Torque?
However how do valve design parameters affect torque values? Certainly increasing the size and pressure class of a valve makes the valve closure member larger, and therefore higher torque and force values are required when operating the valve. When comparing ball and butterfly valves, butterfly valves have a lighter closure member (disk) compared to the ball valve, therefore the torque values for butterfly valves are less than heavy ball valves.
What other parameters might affect torque values in complex ball valve designs? One aspect to consider is seat material. Metallic seal material such as 13Cr -4Ni are categorized as martensitic stainless steel with a quenching heat treatment, which gives high hardness and produces more friction to the metallic ball than a soft seat material such as PEEK or PTFE. PEEK seats can be quite hard which creates more ball friction compared to PTFE seats, and in these instances the torque is increased. Designing the seat of a valve for a Double or Single Piston effect influences the torque value of a valve. A Double Piston Effect Seat is pushed stronger towards the seat in a ball valve which requires a larger torque to cover its friction with the ball. The ball design in terms of floating and trunnion designs also affect torque values. Trunnion balls have a support underneath as a plate or flange that reduces the force/torque compared to floating ball valves. In addition, the bore of a reduced bore ball valve reduces the size of the ball and the force/torque for its operation.
The second question is: how do torque values affect the valve and its operator? A higher torque normally requires a larger operator, such as a gear box or actuator.
3. Torque Problem with a Valve (Real Case)
In this case a modular valve with two floating ball valves, a needle valve as a bleed, and a 22Cr material body and internals, was initially selected with a floating ball. The modular valve had a metal seat from 22Cr duplex material, which was causing high friction between the seat and ball, thus increasing its torque. The valve size was small (¾”) and the pressure class was 1500 (see picture 3).
Picture 3: ¾” modular valve class 1500 in 22Cr duplex
The valve was used to double isolate pressure gauges in case of maintenance and/or calibration. We were not able to operate (close) the valve during the test since the lever was short and a high force was needed to cover friction between the 22Cr metallic ball and seat, considering the fact that 22Cr duplex is a very hard material with high friction forces. What was the solution? The valve manufacturer advised we could operate the valve more easily with a trunnion instead of floating ball design. A trunnion support would reduce the required force and torque for the valve significantly. Unfortunately it was too late to change the valve design. The other solution was to replace the valve levers with longer options to facilitate operation, meaning less force would be needed to open and close the valve since the lever length is longer and the torque is constant. An alternative option we considered was to change the lever to a hand wheel and a gear box for ease of operation. However due to the limited space available a longer lever was not an option, nor adding a gear box and hand wheel. The problem was eventually solved by opening the valve, polishing the ball and seat to reduce the roughness and friction between them, and thus facilitating a smoother operation. It is important to note that the force for opening and closing the valve should cover frictions such as stem and packing and ball and seat contact.
4. Why are torque values important?
Required torque values to operate the valves should be estimated as a basis for proper operator type and size selection (e.g. a lever or hand wheel plus gear box operation type, length of lever or diameter of hand wheel, actuator model selection). The valve manufacturer should provide torque values for opening, closing and running the valve closure member between open and closed positions. The actuator supplier should select the proper actuator model to meet the torque required by the valve supplier and facilitate the opening and closing of the valves. In fact, excessive torque estimation by the valve supplier makes the operator size such as an actuator or hand wheel much larger than required. This can be problematic for offshore applications in the offshore industry where space limits on the platform are a main consideration. Conversely, an under estimation of the required torque for operating valves can lead to operators that are smaller than required, and hence making the valve more difficult to operate.