An actuator that meets a torque requirement at 5.5 bar may fall short at 4 bar. If the plant can only guarantee the lower pressure while the valve moves, the higher catalogue number is the wrong basis for selection.
Start with the lowest available supply during a demanded movement. Then compare output with the valve load through opening and closing, check the mechanical load limits, and define what the assembly must do when air, power or signal is lost.
A catalogue example makes the supply issue visible
Rotork’s K-TORK brochure publishes these selected double-acting torque outputs. They are external manufacturer data used here to explain a sizing check.
| Published double-acting model | Output at 4 bar Nm | Output at 5.5 bar Nm |
|---|---|---|
| KT-10 | 65 | 94 |
| KT-20 | 138 | 199 |
Source: Rotork K-TORK brochure, Performance Data, page 16. Values apply to the models and table conditions stated by Rotork. They do not establish a ZGV actuator selection, a supply relationship or mechanical compatibility.
Now assume a hypothetical valve requirement of 68 Nm at a critical point and an illustrative project factor of 1.25. The resulting screening requirement is:
68 Nm × 1.25 = 85 Nm
The 1.25 factor is an assumption in this example, not a universal sizing recommendation. The project and valve supplier need to define the actual allowances.
At 5.5 bar, the KT-10 table value of 94 Nm exceeds the illustrative 85 Nm requirement. At 4 bar, its 65 Nm value falls below it. Reading only the higher-pressure column would therefore miss the lower-supply shortfall in this example.
The KT-20 value at 4 bar clears that single numerical screen. It still does not approve the complete selection: the remaining travel, valve load limits, connections, speed and failure requirements must also be satisfied.
Define the pressure at the actuator while it is moving
The compressor outlet pressure is not necessarily the pressure available at the actuator inlet during a demanded stroke. Filters, regulators, solenoid valves, tubing and other users on the supply can affect the pressure and flow delivered to the package.
Give the supplier the minimum available dynamic supply pressure at the relevant connection, together with the maximum supply and required action time. That lets the supplier identify the correct sizing condition and select the associated air components.
A static gauge reading taken with nothing moving is useful site information, but it is not a complete demonstration of the demanded stroke. For a package with a specified fast action, the supply flow and the restrictions in the accessory circuit deserve attention as well as pressure.
Keep supply-pressure units clear. The process line pressure, the actuator air supply and the differential pressure across the valve are three separate inputs. A repeated number in different fields does not make them interchangeable.
Compare the whole movement
The valve may need different torque to break away from its seat, continue moving and complete closure. Required loads can also differ between opening and closing. Obtain the manufacturer’s load information for the actual valve trim and conditions.
An actuator likewise has an output characteristic. A single peak torque is insufficient when the critical valve demand occurs elsewhere in the stroke. The review should compare available output and required load at the relevant positions in both directions, including the agreed allowances.
Extend the worked example to three load points
Continuing the same hypothetical example, assume 68 Nm at breakaway, 42 Nm while running and 60 Nm at final seating. Apply the same illustrative factor of 1.25 to each point.
| Assumed load point | Valve requirement Nm | Illustrative screened requirement Nm |
|---|---|---|
| Breakaway | 68 | 85 |
| Running | 42 | 52.5 |
| Final seating | 60 | 75 |
The three valve-load values are assumed for this worked example; they are not ZGV torque data. The published KT-10 value of 65 Nm at 4 bar falls below the assumed breakaway and seating requirements after the factor is applied. It is also below the bare 68 Nm breakaway assumption, so the shortfall in that example is not solely caused by the allowance.
Write the failure action for each event
“Fail safe” needs a defined event and a required final state. Loss of air, electrical power and control signal may affect different components. The system design must establish what the valve is expected to do in each case.
| Event to specify | Confirmation required from the package supplier |
|---|---|
| Loss of pneumatic supply | Required valve position, available stored energy and ability to complete the movement |
| Loss of electrical power | Solenoid or electric-drive response and resulting valve action |
| Loss of control signal | Controller or positioner response and the agreed valve state |
| Return of supply or signal | Defined restart behaviour and any required reset |
Treat ISO 5211 as interface information
ISO 5211 addresses part-turn actuator attachment dimensions, driving components and reference interface torque values. Its public scope also identifies a boundary: attachment of an intermediate support to the valve is outside its scope. The ISO page lists a 2026 edition. ISO 5211 official scope
A matching attachment designation helps define the interface. It does not, by itself, verify the complete bracket, coupling engagement, shaft shape, assembly alignment or load capacity. The supplier should provide the relevant drawings and confirm the assembled load path.
Part-turn and multi-turn duties also need to stay separate. A ball or butterfly valve may require part-turn movement, while a rising-stem gate assembly may require a different combination of torque, thrust, travel and turns. A part-turn torque table cannot size that linear movement by itself.
ZGV’s pneumatic butterfly valve entry, electric butterfly valve entry and electric flat gate entry identify different offered product categories. Their public presence does not establish a common actuator interface or a complete sizing curve for every configuration.
Compare double-acting and spring-return as complete circuits
A double-acting actuator uses pneumatic energy in both directions. Its loss-of-air position is not guaranteed by that description alone: the accessory circuit, trapped energy and process forces matter. A spring-return assembly uses stored spring energy for its designated movement, but its required final state still depends on direction, available load and the complete package.
For spring return, compare air-stroke start and end and spring-stroke start and end with the corresponding valve demands, then review the intervening movement where required. Endpoint values are a useful screen, not proof that the curves never cross. Rack-and-pinion and scotch-yoke constructions can have different output characteristics.
Confirm which spring configuration each catalogue column represents. Columns for different matched configurations should not automatically be treated as pressure-decay performance of one unchanged assembly.
Use minimum dynamic supply for available operating output and the relevant maximum-supply case for mechanical load limits. The permissible limit may be set by the stem, internal drive connection, coupling, bracket or another component. A pressure-control proposal needs its own engineered assessment; a regulator label alone does not approve the complete overtorque protection arrangement.
Finally, define air, power and signal failures separately, including restoration behaviour. A position indication should be reviewed as part of its actual sensing arrangement; it should not be assumed to prove every internal isolation condition.
Ask for a package calculation with identifiable inputs
The calculation package should identify the valve size and trim, service pressure and temperature, operating differential pressure, required loads, allowances, available actuator output and mechanical limits. Supply pressure, spring configuration and accessory circuit should be identifiable wherever they affect the result.
Action time and operating frequency belong beside that calculation. An actuator suited to occasional isolation may need a different assessment for frequent modulation. Electrical duty, pneumatic flow capacity, ambient conditions and environmental protection should match the specified use.
Confirm the assembled valve and operator, including every supplied gearbox, bracket, coupling, solenoid, positioner and indicator. For an automated ZGV enquiry, send the minimum dynamic air supply, maximum supply, required failure states and process duty. Request a package calculation and assembly drawing using those same inputs.
ZGV VALVE · Request a quotation · WhatsApp +86 13732021078
Download the ZGV valve RFQ template and attach your service conditions and required inspection documents.
