Sunday, November 24, 2013

Actuators Uses, Types and Failure Modes

A brief discussion is provided to assist the user in understanding the considerations affecting the selection of the type of actuators required for an application. Manually actuated valves do not change position due to a change in the mode of system operation or an accident. As such, a manually operated valve remains in the last position it was placed in. Manually operated valves are usually furnished with a hand wheel attached to the valve stem or yoke nut which is rotated clockwise or counter-clockwise to close or open a valve such as a gate or globe valve. Manually operated quarter-turn valves, such as a ball, plug, or butterfly valve, is provided with a lever to actuate the valve. There are applications in which the force required to actuate the valve is more than the force manually imparted through a hand-wheel or a lever. These applications include:

● Large valves and valves required to be operated against high-fluid pressure

● When the time required to open, close, throttle, or regulate the valve manually is longer than that required by system-design criteria

● When the valve is required to be operated from a remote location

● When the valve must attain a position (open or close) in the event of an accident or in a particular mode of system operation

The valves to be furnished with an actuator utilizing external source(s) of energy, such as electricity, pneumatics, hydraulics, mechanical springs, or a combination of one or more of these energies, are called actuated valves. Upon failure of the external source of energy, a valve may not be in the required position for accomplishing the design function. One must be aware of failure modes prior to selecting a valve actuator.

Failure Modes

Fail-As-Is (FAI): The valve remains in its last position following upon an external power failure.

Fail-Closed: The valve is provided with external source(s) of energy to place it in the closed position, regardless of valve position before power failure.

Fail-Open: The valve is provided with external source(s) of energy to place it in the open position, regardless of valve position before power failure.

Fail-Locked: Air or inert gas actuators may be provided with a device or devices to seal in actuator pressure upon the loss of normal pneumatic source control.

Types of Actuators

The following are some of the commonly used valve actuators:

Gear Actuators

Spur, bevel, or worm-gear actuators are used to reduce the manual force required to operate a valve. Spur gears are used with globe, angle, and nonreturn valves. Bevel-gear actuators are used on gate valves. Worm-gear actuators are usually used on quarter-turn valves. The use of a gear actuator is warranted when the rim pull force required to manually actuate a valve exceeds a given value, which varies from 50 lb (22 kgf) to 250 lb (113 kgf).

Electric Motor Actuators

An electric motor provides the actuating energy to place the valve in the desired position. Upon loss of power, the failure mode is fail-as-is. The stem speed may vary from 12 in/min (30 cm/min), known as manufacturer’s standard, to 60 in/min (150 cm/min). Special features are required to accomplish stem speeds over 45 in/min (112 cm/min).

Pneumatic Actuators

Pneumatic actuators utilize the motive force provided by a compressed gas such as air, nitrogen, or other inert gas. There are many different types of pneumatic actuators. These include linear, rotary, and linear-to-rotary. Linear-type actuators are used with valves having translating stems. Rotary and linear-to-rotary pneumatic actuators are used on valves having rotating stems. Pneumatic actuators can generate very high thrust and an extremely high torque.

The length and speed of stroke may vary considerably, depending upon the type of pneumatic actuator. These actuators are capable of providing either fail-open or fail-close failure mode upon loss of air. Piston-type air actuators are furnished with springs to open or close the valve upon air failure. Diaphragm air actuators have limited thrust and torque generation capabilities due to limits on air pressure because of diaphragm strength. In addition, the diaphragm actuators have limited stroke ranging from 1 to 4 in (25 to 100 mm).

Air-vane pneumatic actuators are used with quarter-turn valves, and they can be directly mounted on the valve stem. 

Hydraulic Actuators

Hydraulic actuators utilize pressurized liquids, usually oils but sometimes water, or the process liquid is used to provide the motive force for actuating the valve. Like pneumatic actuators, these actuators can help achieve fail open or fail-close failure modes.

Solenoid Actuators

Solenoid actuators have short-stroke and low-thrust capabilities.Two types of actuating methods are used in solenoid valves: direct acting and pilot operated. In the direct acting solenoid valve, the disc is lifted off the seat to a fully open position by magnetic flux generated by energizing the coil, and the disc is returned to its seat by De-energizing the coil. The pilot-operated solenoid valve utilizes the system pressure to provide the actuating force.

Solenoid valves can accomplish all failure modes.
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