What you need to know about pneumatic actuators and control valves onboard vessels

Onboard vessels the most used types of valve actuators are the pneumatic ones and they are use for a numerous numbers of applications like: feed water valves, steam valves for tank heating, steam valves for different heaters, purifier’s 3-way valve etc.

Example of steam valve pneumatic actuator. Source: chiefengineerlog.com

These are actuators are fairly cheap compared with other types, easy to adjust and easy to maintain, troubleshoot and repair. Compressed air is quite abundant, cheap and easy to obtain onboard vessel compared with hydraulic oil. Moreover, experience shown that in case of the steam valves in particular, the valve itself tend to fail before the actuator. Often, steam valve seizure due scale deposits lead to actuator failure due overload.

The working principle of the pneumatic actuator is quite simple and is adapted to the system and/or application that is used for. The below explanatory video is a very useful tool for actuator’s working principle understanding (Source: RealPars).

As you have seen above, the working principle is very simple and pneumatic force required to overcome the spring tension can enter either from above or below membrane, depending on the system fail safe requirement.  Actuated or automatic valves that revert to a pre-determined position after the actuating force is removed are referred to as “fail-safe” valves. The fail-safe mode of a pneumatic/spring valve is a function of both the actuator’s action and the valve body’s action. If the fail-safe actuator is set to fail closed then when pneumatic power is removed the actuator’s spring will push the valve to the closed position. If an actuator is set to fail open then when pneumatic power is removed the actuator’s spring will push the valve to the open position.

For example, in case boiler’s feed water system, where the actuator’s fail safe mode should be valve in open position, the pneumatic pressure should come from top of the membrane to close the valve and spring tension will open it. Thus in case of pneumatic failure, the spring will keep the valve open and there will be no starvation in the drum water pipes, which may cause serious damage to drum internal.

Another type of pneumatic actuator often used onboard vessels are the pneumatic torque actuators designed to handle most quarter turn valves.

Exploded view of an air torque actuator. Source: sbakelas.gr

The units are designed from aluminum and can be fitted either as a spring return or double acting and are used for butterfly valves.

These type of pneumatic actuators are mainly used for remote control of the heeling and ballast valves as these actuators are on/off types.

One more type of pneumatic actuator used onboard vessel is the piston type actuator. A piston actuator, like all actuators, is a device that transforms raw energy into motion. In general, the actuator is connected to a piston which is contained inside an enclosure and is mainly used onboard for safety features like ventilation fire flaps, funnel flaps, galley fire flaps etc. These actuators are on/off types and are not usually used for process control.

These are also typically mounted to the upper master and wing valve for sequenced closure during shutdown operations.

Because the pneumatic actuators are operating valves used to control flow of different fluids, these valves are known as Control Valves. So, a control valve is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a PID controller. If you want to read and learn more about valve’s PID controller follow this link.

The below explanatory video is a very useful tool for control valve’s working principle understanding (Source: RealPars).

The control valve adjusts the flow of a fluid, which might be gas, steam, water, or chemical compounds, in order to compensate for the load disturbance and maintain the regulated process variable at a position that is as close as feasible to the point that was wanted. The control valves are perhaps the most essential component of a control loop; nonetheless, they are frequently the element that receives the least amount of attention. The control valve serves as the “muscle” of the system that regulates the process. If the eyes represent the sensors of the process variables and the brain represents the controller, then the hands represent the final element of control in the control loop. Because of this, it is the most crucial component of an automatic control system, despite the fact that it is sometimes also the least understood.

Every control valve has something called an inherent flow characteristic, which describes the connection between the “valve opening” and the flowrate under conditions of constant pressure. Please take into consideration that the term “valve opening” used in this scenario refers to the location of the valve plug in comparison to its closed position against the valve seat. It is not talking about the orifice pass region at all. The orifice pass region is also referred to as the ‘valve throat,’ and it is the point at which the passage of fluid through the valve is at its most restricted and is located between the valve stopper and the seat. There is never an exception to the rule that the relationship between flowrate and orifice pass area is always directly proportional, regardless of the characteristics of the valve. If you want to learn more about control valves characteristics, please follow this link.

Control is typically accomplished by the use of globe valves. The movement of a valve plug in respect to the port(s) placed within the valve body is how the valve controls the amount of flow that passes through it. The valve plug is connected to a valve stem, and the valve stem is linked to the actuator.
The flow rate in a line can be controlled with the use of a control valve, as demonstrated in the following image.

Example of control valve arrangement. Source: http://www.steamline.com

The “controller” is responsible for receiving the pressure signals, comparing them with the pressure drop for the desired flow, and adjusting the control valve to either increase or reduce the flow based on whether or not the actual flow matches the desired flow.
Controlling any one of the multiple process variables can be accomplished through the use of comparable arrangements and the four most typical types of regulated variables are temperature, pressure, level, and flow rate.

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Source and Bibliography:

  • YouTube video training source and credit – RealPars; Engineering Concepts; TecknoMechanics

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