Actuators are the devices which drive the valve stems. There are many different actuators. They range from the simple hand wheel to the latest microprocessor controlled electrical/hydraulic actuators. The following notes introduce the common types of actuator . You will learn about specialized actuators used on a particular site during advanced training.
The Pneumatic Actuator (Diaphragm Type)
Figure 1 Air to Close Pneumatic Actuator
Figure 1 shows a typical sectional view of an air to close, pneumatic actuator. With a minimum air signal (3 psi or 0.2 bar) applied to the loading pressure connection, the spring forces the stem to its maximum upwards position. With a maximum air signal (15 psi or 1 bar) the force on the diaphragm compresses the spring and the stem moves down to the closed position. Any signal between the two will hold the stem at an intermediate position to control the flow.
The actuator can be designed to work in the opposite direction, i.e. air to open. Figure 4-17 shows a typical air to open actuator. The air signal input is applied to the underside of the diaphragm (loading pressure connection).
Air to Open Pneumatic Actuator
The Pneumatic Actuator (Piston Type)
Figure 3 A Piston Pneumatic Actuator
Figure 3 shows a typical pneumatic piston actuator. It is used to operate a ball or butterfly valve. In the position shown the valve is about half way. An increase in the pressure (P1), forces the piston further down and rotates the ball or butterfly. This closes the valve more. The system is balanced by the feed back signal (P2). P2 is provided by the Actuator Control Unit (positioner). The operation of pneumatic positioners will be explained later in the course. If P1 falls the opposite occurs, P2 increases and the valve opens more.
The actuator rotates the V ball through 900. When the valve is closed the ball stops the fluid flow in the same way as an ordinary ball valve. As the valve opens the “V” cut in the ball allows fluid to pass at an increasing rate to the fully open position. So, the “V” ball is an efficient control valve similar to the globe type.
The Electrical Actuator
There are two common types of electrical actuator; the solenoid type used in emergency shut down systems and the motor operated type used when loading tankers from a marine terminal.
THE SOLENOID OPERATED VALVE (SOV)
Figure 4 The Solenoid Valve
This on/off valve is used to remotely open or shut a flow line. These valves normally come in small sizes (e.g. 2″ diameter). It is often used for the control of air supply lines etc. in Emergency Shut-Down (ESD) systems.
THE MOTOR OPERATED VALVE (MOV)
Figure 5 shows one of the latest types of motor operated valve. The valve is either open or closed. An electrical signal goes to an electric pump. This drives the hydraulic fluid to open the valve. If the signal stops, the hydraulic pressure falls and a spring returns the valve to the closed position.
Figure 5 The Electro Hydraulic Actuator
These valves are used in large diameter pipelines (e.g. 20″ diameter). They control the flow of petroleum products being loaded onto a tanker, etc.
The plug or cage of a valve can have different shapes. The different shapes can control the flow in different ways. There are three main types of control characteristics; linear, equal percentage, and quick opening.
The graph below shows how these control characteristics change the flow as the valve is opened.
A quick opening characteristic gives nearly maximum flow for a small % opening distance of the plug, (plug travel).
A linear characteristic provides the same change of flow for the same change in plug travel (e.g. 50% open, 50% flow; 20% open, 20% flow; etc.)
An equal percentage characteristic means the plug travel provides a constant percentage change in the flow rate. This is shown on the graph as a flow which gets faster as the valve opens. At a 20% flow rate a 10% increase means the valve opens to allow the flow rate to increase to 22%. However, a 10% increase in the flow rate at 80% means the valve opens to allow the flow rate to increase to 88%.
Which characteristic is used depends on the property being controlled. A few examples are given below.
Linear – Liquid level and flow control
Equal Percentage – Pressure control
Quick Opening – Pressure relief valves.
The cages and plugs for the different characteristics are easy to tell apart as the following examples show:
Flashing and Cavitation
Flashing and cavitation are two problems with valves which control liquid flow. When you service a control valve you must check the plug and cage for signs of damage caused by these problems. Report any damage as the valve may need to be redesigned to stop more damage which could cause a shutdown.
Figure 6 Pressure Curve through a Valve
Figure 6 shows the effect of a valve throttling a flow steam to control the flow. The valve acts like an orifice plate. The pressure will fall across the valve as the velocity increases through the restriction.
If the pressure of the liquid falls below the bubble point the gases in the liquid under pressure will be released as bubbles. This is called the flashing point. As these bubbles keep hitting the valve plug and seat they can wear away the metal.
As the fluid leaves the valve the pressure increases again. This makes the bubbles implode. This is called the cavitation point. The sudden implosion of the bubbles can wear away the metal on the plug, seat and valve body.
The two pictures below (see Figure 7) are examples of flashing and cavitation damage.
Figure 7 Flashing and Cavitation Damage
Most manufacturers make special cages to stop flashing and cavitation. These cages split the flow into small flow streams. This reduces the possibility of damage and noise as the flow rushes through the valve. Typical examples are shown in Figure 8.
Figure 8 Anti Cavitation/Noise Cages
EMERGENCY SHUTDOWN OPERATIONS
Various valve/ actuator assemblies on a plant are made to “fail open” or “fail closed” in an emergency shutdown. The valve and actuator can be made in the following ways to produce the “fail open” or “fail closed” conditions.
1. Air to close actuator with direct operating valve body
2. Air to close actuator with reverse operating valve body.
1. Air to open actuator with direct operating valve body
2. Air to close actuator with reverse operating valve body.
Most plants use direct operating valve bodies. They get “fail open” or “fail closed” by using air to close or air to open actuators. However, the other combinations are sometimes used.
A piston type actuator can be made either “fail open” or “fail closed” by the positioner. It is also possible for the positioner to hold the valve at it’s last control position. That is usually called “fail intermediate”.
The previous pages have provide a general introduction to valves and actuators. However, the subject can be very detailed and an engineer/technician can spend all his working life on valve operations only. A table is added at the end to summaries the facts you have learned. It gives a summary of the uses of valves.
Summary of Valves and its uses (Table)