Chapter 6 Valve Actuators and Position Indicators

DESCRIBE linear variable differential transformers (LVDT) to include basic construction and theory of operation.

An LVDT uses the extension shaft or control rod as a movable core of a transformer. Moving the extension between the primary and secondary windings of a transformer causes the inductance between the two windings to vary, thereby varying the output voltage proportional to the position of the component being monitored.

What does a simple synchro system consist of? A. Synchro motor and control transformer B. Synchro generator (transmitter) and receiver C. Synchro generator (transmitter) and differential transmitter D. Synchro motor and differential receiver

B. Synchro generator (transmitter) and receiver

What is the best way to check the position of a manual valve?

Check the valve towards the closed position

Which of the following valve actuators will always fail "as is". a. Pneumatic valve b. Solenoid valve c. Hydraulic valve d. Motor operated valve

D. Motor operated valve

What type of position indicator does a magnet activate? A. Potentiometer B. Limit switch C. Strain switch D. Reed switch

D. Reed switch

A valve actuator that consists of a handwheel attached directly to the valve stem is a _____________ valve actuator.

Manual

A valve actuator that uses a reversible electric motor connected to the valve stem through a gear train is a ___________________ valve actuator

Motor Operated

DESCRIBE the basic construction and theory of operation of motor operated valves (MOVs)

Motors are used mostly for open-close functions although they are adaptable to positioning the valve to any point in its travel. Electric motor actuators are typically used when a large force is required to produce valve motion or when the valve stroke is very long. The motor is usually a high speed reversible motor connected through a gear train to reduce motor speed and increase torque at the stem.

Hydraulic actuators normally use __________________ as hydraulic fluid.

Oil

DESCRIBE the proper methods or techniques for operating and verifying the position of manual valves.

To physically check a valve position, turn the valve handwheel a short distance in the "shut" direction. If the handwheel turns, the valve is open. If the valve handwheel will not turn, the valve is already shut.

There are five basic types of synchros that are designated according to their function. The basic types are:

Transmitters Differential transmitters Receivers Differential receivers Control transformers.

What type of position indicator uses a variable resistor attached to the valve stem extension? a. Potentiometer b. Limit switch c. Linear variable differential transformer d. Reed switch

a. Potentiometer

STATE the methods that can be used to determine or verify valve position locally or remotely.

-Physically check the valve using the handwheel. -Use system indications such as flow noise and feeling for flow vibration and/or temperature changes. -Check the position of a rising stem valve by observing whether the stem is up or down. -Non-rising stem valves sometimes have small mechanical pointers that are operated by the valve actuator simultaneously with valve operation. -Power actuated valves typically have a mechanical pointer that provides local valve position indication. -For valves with remote position indication, valve position indication is provided using open and closed lights or a meter.

DESCRIBE Pneumatic Diaphragm Actuators

A common type of actuator used in power plants is the diaphragm actuator. These actuators can be forward acting or reverse acting. Looking at the valve actuator, one can tell which side of the diaphragm is pressurized. This air pressure will be used to oppose a spring or a pre-loaded dome pressure. We must also determine if the valve is reverse seated. This information will tell us how the valve operates.

DESCRIBE limit switches and reed switches to include basic construction and theory of operation.

A limit switch is a mechanical device used to determine the physical position of components. An extension on a shaft mechanically trips the switch as it moves from one position to the other. The limit switch gives ON/OFF output that corresponds to the component position.

Double-acting pneumatic operators

Double-acting pneumatic operators have pneumatic pressure applied to both sides of the diaphragm or a piston for positive opening and closing control. In most cases, there is no spring and the differential pressure will determine the position of the valve and the valve will fail as-is (its last position) on loss of pneumatic pressure. A double-acting pneumatic operator can also be designed for use with a spring to provide a fail position of open or closed.

DESCRIBE the basic construction and theory of operation of hydraulically operated valves

Hydraulic actuators use a piston to convert hydraulic pressure into valve stem motion. Hydraulic fluid is fed to one side of the piston while the other side is vented back to the low pressure reservoir. A basic hydraulic actuator system consists of an oil reservoir, an accumulator, a pump, directional control valves, and a cylinder and piston arrangement.

The following diagram represents a ______________________ position indicator.

Linear Variable Differential Transformer (LVDT)

There are four types of pneumatic actuators. They are the diaphragm, cylinder, _______________, and vane.

Piston

DESCRIBE Pneumatic Piston Actuators

Piston actuators are similar to diaphragm actuators. A controller is used to vary the air signal sent to a valve positioner. The positioner will direct air to one side of the piston to open or close the valve. A position indicator is provided for the operator to determine valve position.

What are the four basic types of pneumatic actuators.

Pneumatic Diaphragm Pneumatic Piston Pneumatic Cylinder Pneumatic Vane

DESCRIBE the basic construction and theory of operation of pneumatic operated valves

Pneumatic actuators typically use air, nitrogen, carbon dioxide, or inert gas (to prevent chemical reactions) pressure to position a valve. They can be used to provide automatic or semi-automatic valve operation. Pneumatic actuators open, close, or throttle the valve. Pneumatic actuators translate pneumatic pressure into valve stem motion by allowing the pneumatic pressure to act on a diaphragm or piston connected to the stem.

DESCRIBE Pneumatic Vane Actuators

Pneumatic vane actuators are designed for rotary motion (quarter-turn) valves and can be directly mounted to the stems of these valves without any intermediate gearing. This lightweight unit has very few moving parts and is manufactured in single vane or multiple vane designs. Vane actuators can provide fail-safe operation in the event of pneumatic failure.

DESCRIBE potentiometer position indicators to include basic construction and theory of operation.

Potentiometer position indicators use an extension that is physically attached to a variable resistor. As the extension moves up or down, the resistance of the attached circuit changes, changing the amount of current flow in the circuit.

Single-acting pneumatic operators

Single-acting pneumatic operators can be designed to open or close with pneumatic (air or gas) pressure. Their failed position (open or closed) is determined by the action of the spring.

DESCRIBE the basic construction and theory of operation of solenoid operated valves

Solenoids provide automatic, two-position (open or closed) valve control. Solenoids are only capable of either fully opening or fully closing a valve. That is, solenoids are either ON (energized) or OFF (de-energized). Solenoids actuators use magnetic force to move the stem of a valve.

DESCRIBE the synchro position indicators to include the basic construction and theory of operation.

Synchro equipment consists of units that electrically govern or follow the position of a mechanical indicator or device. An electrical synchro has two distinct advantages over mechanical indicators: (1) greater accuracy, and (2) simpler routing of remote indication.

DESCRIBE Pneumatic Cylinder Actuators

The pneumatic cylinder actuator is a pneumatic power actuator that uses a piston under pneumatic pressure in a cylinder as the driving force. The pneumatic cylinder is designed with a fail-safe position. When pneumatic pressure is lost, spring action dictates the valve's fail position.