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Facts worth knowing about pneumatic actuators for control valves

 

Simple design, robust EPDM diaphragm, low maintenance and suitable for low-hysteresis control: The pneumatic actuator can be used universally and offers special safety functions that are particularly in demand in the chemical and petrochemical industries.

But what exactly is a pneumatic diaphragm actuator? For what purposes is it used? And what advantages does it offer? Now read everything you need to know about diaphragm actuators!

What is a pneumatic diaphragm actuator?

The diaphragm actuator belongs to the class of pneumatic actuators and is the most widely used compared to electric, hydraulic and manual actuators. Due to its simple design and reliability, this type of actuator is primarily used in the industrial sector (chemical and petrochemical industry) and in power plants.

 

The mode of operation briefly explained: externally supplied compressed air sets a diaphragm connected to the valve stem in motion. An electric positioner regulates the compressed air supply and thus determines the exact position. The positioning force of a pneumatic actuator depends on the diaphragm area as well as the pressure value of the compressed air. Common areas range from about 80 cm² up to 2,800 cm².

Good to know: In the field of building automation, electric actuators are preferred because the external compressed air supply is often not available.

The advantages of diaphragm actuators:

  • Extremely reliable
  • fast operation possible
  • achieve high actuating forces, even at low control pressures
  • unrestricted use in explosion-proof plant areas
  • good hysteresis and good response sensitivity
  • robust
  • low maintenance
  • best cost-performance ratio

The disadvantages of diaphragm actuators:

  • large size
  • limited travel
  • Requirement of an infrastructure for the external compressed air supply

What is the structure of pneumatic actuator with diaphragm?

Diaphragm actuators can be distinguished according to three types:

Diaphragm actuator with columns  

The column actuator essentially consists of two housing chambers made of steel or stainless steel, a diaphragm plate, spring(s), an actuator rod and the special diaphragm with fabric insert. With the help of a crosshead and the columns, it is possible to adapt this design for different valves.

Diaphragm actuator with yoke (central connection)

Actuators with central connection do not have actuator columns installed. With a cast yoke, this type of actuators can be adapted to fittings.

Diaphragm actuator with lever

Pneumatic lever actuators are used to operate butterfly valves, ventilation dampers and various control devices. The limited stroke of a column actuator is thereby increased by a lever.

How does a diaphragm actuator work? 

Pneumatic diaphragm actuators take advantage of the proportional relationship between control signal and actuator travel. The air pressure on one side of the diaphragm is converted into a force acting on the diaphragm plate and spindle. This force triggers movement of the actuator spindle in one direction until the positioning force is balanced by one or more actuator springs. The proportional relationship between the spring travel and the restoring force results in a linear relationship between the control signal and the actuator travel.

These functionalities in combination with low friction (hysteresis) and the high actuating forces explain the popularity of this actuator type.

 

Operating principles and fail-safe position of a pneumatic actuator

Diaphragm actuators play a key role in plant safety due to their special safety function. The behavior in case of power failure is an essential criterion for the selection of a pneumatic actuator. To be able to move the process to a safe state in the event of a failure, it must already be determined during sizing whether the actuator should open or close the control valve.

The following happens: If compressed air is no longer supplied, the spring return force moves the control valve to a rest position - the fail-safe position. The diaphragm and springs can be arranged in such a way that the valve either closes or opens when the supply air fails.

The so-called single-acting diaphragm actuators are available in two versions. The installation position of the spring assembly and the pressure connection determine the mode of operation: 

Direct acting

Air pushes the diaphragm down and closes the valve.

Spring opens the valve. 

Fail-safe position: Valve open 

Reverse acting 

Rising air pressure opens the valve.

Spring closes the valve.  

Fail-safe position: Valve closed

What are double-acting pneumatic actuators?

Double-acting pneumatic actuators are equipped with one diaphragm or with two diaphragms and are springless. Both chambers are pressurized with the supply compressed air. The position of the spindle is determined by the difference between the two pressures.

These actuators without springs do not have a fail-safe action, which means they are used less frequently in industry. Nevertheless, springless actuators have advantages: Larger strokes and significantly higher positioning forces are possible.

What does the preload mean in a pneumatic actuator?

The following example illustrates why a preload is necessary in a pneumatic actuator:

In a reverse actuator, where compressed air opens the valve, springs with spring strengths in the nominal range 0.8 - 2.2 bar are used.

  • Before installation in the actuator, the springs have the maximum length L0.  
  • If a spring is installed in the pneumatic actuator, the length is reduced to L1. This is called the airless state of the actuator without attachment to the valve.  
  • L2 determines the length of the spring in the nominal range. Here, the actuator is pressurized with 0.8 bar from below. This can mark the starting point (valve closed) at valve stroke 0 mm.  

Preload: If the spring is compressed beyond the nominal range or preloaded to L3, the starting point (valve closed) of the valve stroke is shifted. In the example, the actuator is preloaded to 1.08 bar and the starting point of the valve stroke is shifted by 20 mm.

What do you need the preload for in actuators?

The process conditions for the control valve specify which positioning forces are to be achieved with the valve closed or open. The greatest advantage of preloading is the utilization of the complete spring force range. Instead of using an alternative spring assembly for the actuator, the existing spring assembly with the appropriate preload is usually sufficient for the positioning forces. For direct diaphragm actuators, the preload can be used to ensure better stroke stiffness for stable modulating operation.

What supply air is needed to get the pneumatic actuator going?

From which supply air the actuator spindle raises or lowers depends on the spring forces opposing the air force. In pneumatic actuators in which the springs close the valve (reverse-acting), the spring forces are utilized as much as possible. To obtain high closing forces, spring forces between 2.9 and 3.5 bar are used.

Diaphragm actuators from A. Hock

The pneumatic actuator of the 2000 series from A. Hock offers all the advantages that a diaphragm actuator should have: Reliability, low-hysteresis control and maintenance-free operation. In addition, customers benefit from the wide variety of versions and fast delivery, within one week.

Do you have general questions about the diaphragm actuator or our products? Then please contact us! We will be happy to advise you.

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