Torque Calculation for Electric Actuated Valves

When selecting electric actuated valves, “torque” is one of the most critical technical topics. Torque is the rotational force an actuator can produce to turn the valve shaft or to move the valve toward the closed position. Belimo’s product documentation clearly shows that rotary actuators are offered in specific torque classes — for example 1 Nm, 2 Nm, 5 Nm, 10 Nm, 20 Nm, 40 Nm and 160 Nm.

The goal of a torque calculation is not to say “whatever the valve diameter is, just fit an actuator to match.” The correct approach is to understand how much rotational force the valve actually requires under real operating conditions. Spirax Sarco specifically states that actuator sizing must be based on the maximum differential pressure and the valve/actuator combination data given in the manufacturer’s technical datasheets.

What Is Torque?

Torque is the moment required to produce rotational motion about a shaft. In electric actuated valves, this moment is needed to move the valve’s internal mechanism from closed to open, from open to closed, or to intermediate positions. Siemens’ basic actuator documentation clearly states that “positioning force” and “torque” values must be taken into account when selecting a valve and when replacing an actuator.

In practice this means: if the torque produced by the actuator is lower than the torque the valve requires, the valve either does not open at all, does not close fully, or operates under strain. Over time this shortens the actuator’s service life, compromises valve tightness and leads to system failures. For butterfly valve applications, Belimo clearly states that the actuator must be strong enough to safely drive the valve from closed to fully open.

Why Is Torque Calculation So Important?

If torque is calculated incorrectly, very typical field problems occur: the valve stalls halfway, leaks on closing, cannot move against high differential pressure, struggles during frequent open/close cycles, or the actuator is overloaded. In its motorized valve data, Siemens gives the “close-off pressure” — the maximum differential pressure against which the valve can safely close — as a separate selection criterion.

The issue becomes even more critical with butterfly valves. According to Belimo’s technical guide, when a butterfly valve is partially open, dynamic flow forces act on the disc and generate a torque in the opening direction; this effect is stated to peak between 60° and 85°. The same source notes that in valves with a resilient seat, the torque required near the closed position can be higher.

The Main Factors That Determine Torque in Electric Actuated Valves

Torque calculation does not depend on a single variable. At minimum, the following factors must be evaluated together:

1. Valve type

Ball valves, butterfly valves, rotary valves and linear control valves do not have the same torque behavior. In butterfly valves especially, the disc geometry, seat design and angular position significantly affect the torque requirement. Belimo states that in high-performance butterfly valve designs, the “double offset” construction reduces seat wear and lowers the required torque.

2. Valve diameter

As the diameter increases, the actuator’s torque requirement generally rises as well. The fact that Siemens offers actuator options with different torque classes within the same valve family in its butterfly valve ordering and combination documents is the direct field counterpart of this.

3. Differential pressure

One of the most important loads the actuator must overcome is the pressure difference across the valve. Siemens and Spirax Sarco sources note that the maximum differential pressure against which the motorized valve safely closes must be given as a separate technical figure. In other words, the same valve may move easily at low pressure but demand far more torque at high pressure.

4. Seat and internal mechanism design

Belimo’s butterfly valve guide states that valves with a resilient seat require higher torque near the closed position, whereas “undercut disc” designs can reduce the close-off torque. For this reason, even two butterfly valves of the same diameter can require different torque due to different seat designs.

5. Flow velocity

According to Belimo’s velocity chart document, at very high flow velocities the liner and disc can be damaged; torque can also increase and even exceed the actuator’s capacity. This shows that the calculation must consider not only static closing but also the flow conditions.

6. Operating mode

Will the valve operate only on/off, or will it modulate? Spirax Sarco notes that modulating electric actuators can have a higher number of starts and different duty requirements. AUMA likewise defines open-close duty and modulating duty classes separately. This shows that torque selection must be considered together with the operating regime, not just the instantaneous moment.

How Is Torque Calculated?

The ideal field method is not to try to devise a theoretical formula, but to base the selection on the valve manufacturer’s required torque / close-off pressure data and then choose a suitable actuator. Spirax Sarco specifically recommends relying on the manufacturer’s datasheets for actuator sizing.

The practical sequence is as follows:

1. The valve type is clarified: ball, butterfly, 2-way or 3-way.
2. The valve diameter is determined.
3. The maximum differential pressure in the line is determined.
4. The required torque or close-off pressure value given by the valve manufacturer for that valve is obtained.
5. An actuator with a safety margin above this value is selected.
6. If the valve will modulate, the duty class and cycling frequency are evaluated separately.

Why You Should Work With Manufacturer Data, Not a Theory

Users sometimes look for a general approach such as “torque = pressure × area × lever length.” This logic can give a rough idea; however, real valve behavior depends on many variables such as seat friction, internal geometry, shaft resistance, angular position, flow force and body design. Belimo’s butterfly valve technical guide clearly shows that torque changes with angular position and that flow forces increase especially at certain angles. For this reason, the manufacturer’s datasheet must be used in practical selection.

How Should Torque Calculation Be Approached for Butterfly Valves?

In butterfly valves, torque calculation is one of the most sensitive issues. This is because the disc works both against the seat and under the dynamic forces of the fluid. According to Belimo, the dynamic forces acting on a partially open butterfly valve produce a torque in the opening direction, and this effect peaks between 60° and 85°. The same source states that a high torque is required near the closed position because of the resilient seat.

Therefore, when selecting an actuator for a butterfly valve, the following logic should be followed:

• the nominal diameter alone is not enough,
• you cannot simply say “this valve is DN100, so 20 Nm is enough,”
• the pressure-dependent torque or close-off class given by the manufacturer must be taken into account. In Siemens’ butterfly valve documents too, actuator torque classes are included in the product code as a separate parameter; this shows that torque is an independent selection criterion.

How Should Torque Calculation Be Approached for Ball Valves?

In ball valves the geometry is generally different from butterfly valves; nevertheless, the torque requirement still depends on pressure and internal seat design. In Siemens’ control ball valve documents, the maximum differential pressure against which the motorized valve safely closes is separately defined. This shows that close-off capacity is directly critical in ball valve selection as well.

The practical approach for ball valves is as follows:

• the close-off pressure given in the valve datasheet and the appropriate actuator combination are examined,
• the control signal and operating mode are determined,
• a safety margin is allowed,
• if necessary, it is taken into account that spring-return versions may produce a lower net force. Spirax Sarco clearly states that in spring reserve versions the electric actuator force may be limited and the manufacturer’s charts must be consulted.

How Should the Safety Margin Be Considered?

In practice, selecting an actuator with only the minimum torque listed in the valve catalog is not always correct. This is because in the field:

• water hammer can occur,
• line pressure can change,
• the seat can age,
• fouling and deposits can form,
• temperature changes can increase friction.

For this reason, a safety margin is allowed in practice. The exact percentage of this margin varies by manufacturer and application; it is not correct to give a single universal figure. The correct method is to base the selection on the manufacturer’s valve-actuator combination table and not to select at the limit. This is exactly why Spirax Sarco’s emphasis that “manufacturer’s charts should always be consulted” is so important.

Why Is Torque Even More Critical in Modulating Applications?

In on/off applications, the valve mainly operates at the open and closed end positions. In modulating applications, however, the valve moves continuously through many intermediate positions. Spirax Sarco notes that modulating electric actuators move continuously to the desired position via a position sensor and feedback; AUMA, in turn, defines separate duty classes for modulating duty. This shows that, along with the torque calculation, cycling frequency and duty class must also be taken into account.

The 7 Most Common Mistakes

The most common mistakes in torque calculation for electric actuated valves are:

1. Selecting the actuator based on valve diameter alone.
2. Not taking differential pressure into account.
3. Not reading the close-off pressure value.
4. Assuming that butterfly and ball valves have the same torque behavior.
5. Neglecting the duty class in modulating operation.
6. Not accounting for the net force/torque difference in spring-return actuators.
7. Selecting by estimation instead of using the manufacturer’s combination tables.

All of these points are directly or indirectly emphasized by Siemens, Belimo and Spirax Sarco in the logic of valve-actuator selection.

Frequently Asked Questions

What is torque?It is the rotational moment an actuator produces to turn the valve shaft. In Belimo and Siemens product groups, this value is given in Nm or lb-in.

Is torque calculated with a formula?A rough engineering approach can be set up; however, for the actual selection, the required torque, actuator torque and close-off pressure data given by the manufacturer for the valve must be used. Spirax Sarco clearly recommends this.

Why do you need to be more careful with butterfly valves?Because flow forces act on the disc and the torque requirement changes with angular position. Belimo’s butterfly valve technical guide explains this in detail.

What happens if the torque is insufficient?The valve does not open fully, does not close fully, may leak, the actuator may be strained and its service life may be shortened. This outcome is consistent with the close-off and sufficient torque requirement emphasized by manufacturer sources.

Conclusion

Torque calculation in electric actuated valves is not a side topic of valve selection but its very core. For correct torque selection, DN size alone is not considered; the following must be evaluated together:

• valve type,
• differential pressure,
• close-off pressure,
• seat design,
• flow velocity,
• operating mode,
• manufacturer’s combination table

Siemens, Belimo and Spirax Sarco sources also point to the same thing: actuator selection must be based on the manufacturer’s technical data, and the maximum differential pressure and required torque must always be taken into account.