Intoduction to Super Thrust Actuators

The M.E.A. Incorporated series of Super Thrust Actuators was designed to fill an industrial need for high speed, maximum thrust, and a self-contained unit in its own temperature controlled environment.

This Actuator series can be broken down into two main divisions:

Modulating - Also referred to as proportioning type. The actuators assume a position proportional to the command control signal from a controller.
ON/OFF - Also referred to as two-position. The actuator will be driven to one extreme position or another, depending on the control signal which can be a momentary contact or a maintained contact. The units can be stopped in mid position by a stop signal command.

These two divisions can further be broken down into two other categories:

Linear - This actuator has a linear motion.
Rotary - This actuator has a 90^o angular rotation. Its primary use is on plug valves, butterfly valves, dampers, etc.

All Actuators can be supplied to meet any electrical codes but specifically two codes:

NEMA 4 (National Electrical Manufacturers Association)
- NEMA Type 4 Description -- Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt, rain, sleet, snow, windblown dust, splashing water, and hose-directed water; and that will be undamaged by the external formation of ice on the enclosure.
NEMA 7 - Explosion Proof - Class 1 - Group D - Division 1
- NEMA Type 7 Description -- Enclosures are for indoor use in locations classified as Class I, Groups A, B, C, or D and shall be capable of withstanding the pressures resulting from an internal explosion of specified gases, and contain such an explosion sufficiently that an explosive gas-air mixture existing in the atmosphere surrounding the enclosure will not be ignited. Enclosed heat generating devices shall not cause external surfaces to reach temperatures capable of igniting explosive gas-air mixtures in the surrounding atmosphere. Enclosures shall meet explosion, hydro-static, and temperature design tests.

All actuators can also be supplied for nuclear service, underwater service and as digital actuators. A series of actuators has been built for special service such as lifting a hood on a BOF (Basic Oxygen Furnace) furnace for a vacuum control. There are other applications too numerous to outline. However, if a large load is to be moved quickly and accurately, the M.E.A. Incorporated Actuator was designed specifically for this purpose.

The modulating type M.E.A. Super Thrust is basically used where large thrust, stroke and speed are required together with good frequency response. A dissertation on frequency response is attached which explains the reason that M.E.A. Incorporated factory is equipped to engineer and manufacture the necessary brackets and linkage to mount on any type of valve.

Technical Information -- Super Thrust Actuators Modulating

Frequency Response

Many control processes today have a problem of stability due to the increase in size of the system, valving and the necessary high speed of operation to control as well as high speed transient changes.

Valve actuators have been a forgotten link in the average engineer's decision on the necessary hardware to design a complete control system. An engineer spends hours selecting a high precision, high performance transducer, high performance controller and then asks the valve manufacturer/supplier to furnish a high performance valve with an actuator. More often than not, the specification calls for an actuator "nearest vendor standard speed" and often not even that.

There are several important factors that an engineer must know about so as to not properly specify a valve actuator for a given control application.

Frequency response is an expression of how well an actuator output will follow an input signal.
If a 4-20 mA input signal is varied sinesoidally 10% (11.2-12.8 mA), at a frequency of 10 cycles per second, the position output should follow.
- Assume 9" stroke in magnitude better than 0.707 of 10% stroke (10% x 9" x 0.070 - 0.636") and the output signal must not lag the input by more than 45^o. This is 10 cycle response.
- Frequency response is typically shown on a Bode plot with frequency as a base and both phase lag and magnitude plotted on the ordinate. For convenience in loop analysis, the amplitude ratio is shown in DB notation rather than direct algebraic notation. DB - 20 lag Vo/Vi where Vo is the input magnitude and Vi is the input magnitude (both are unity at 0 frequency input). A Magnitude ratio of 0.707 is equal to a minus 3 db.
- In order to rate all instruments equally and aid the control system designer, frequency response is uniformly shown as the frequency at which the magnitude ratio is down 3 db and the phase lag is 45^o.
The total control loop response is limited by the lowest response component in the loop. Invariably, this component is the valve actuator. By increasing the actuator frequency response, the entire control loop will operate at a higher gain, higher speed, more accurately and be more stable.
A speed specification without a frequency response specification is COMPLETELY MEANINGLESS. A good example is where a specification is written for a valve to go from open to close in 3 seconds. Any actuator can do tis but it will be unstable UNLESS the frequency response is correct. Typical example is for a diaphragm operator with a pneumatic positioner that has a frequency response of 0.14 cycles per second. The M.E.A. actuator has a frequency response of 30 cycles per second. This means the M.E.A. operator can move up to 12 inches per second linear stroke without becoming unstable. The pneumatic system could only go 1/3 inches per second before becoming unstable.

Load Sensitivity

Many engineers use the term actuator "stiffness" in place of load sensitivity. What it really means in both cases is the ability to hold valve position regardless of the pressure drop variation across the valve. A good engineering example is a globe valve operation. A valve stem normally flutters with a pneumatic actuator but with the M.E.A. Super Thrust actuator it will not flutter. The load on the globe valve stem can vary for example from 100 pounds to 6000 pounds and the valve stem would only vary position 0.1% with the M.E.A. actuator. A pneumatic actuator is compressible and would flutter 1% and 2%.

Repeatability

Many engineers use the term hysteresis and some even use the term resolution. What it means is the ability for the valve actuator to position the valve within a close tolerance, time after time, for a given input control signal. The M.E.A. Incorporated Super Thrust will repeat within 0.1% whereas the pneumatic operator would be 1% to 2%.

Linearity

This, for some applications, is very important. What it means is the ability to follow a linear curve of input signal to valve position within close tolerance. The M.E.A. Super Thrust actuator will follow within 0.5% whereas a pneumatic operator will be 1% to 3%.

Summarizing, there are other considerations but the above factors are the main basis for specifying valve actuators. Not specifying the correct actuator could result in an unstable control system with all of its costly penalties.

Technical Information -- Super Thrust Actuators Two-Position

Energy

The M.E.A. two-position Super Thrust can furnish up to 100 hp of energy to open or close a valve with only a 1½ hp motor. The M.E.A. unit accomplishes this by storing energy during an "off" period in accumulator. The savings over an electric gear actuator are substantial.

Fail-Safe

The M.E.A. two-position Super Thrust will operate at least once after power failure thus meeting the requirements of EPA (Environmental Protection Agency), OSHA (Occupational Safety & Health Administration) and other agencies. An electric gear actuator is not capable of operating after power failure.

Speed

The M.E.A. two-position Super Thrust will operate at a selected speed which can vary in either direction. Speeds up to 7.5 inches per second are available as standard. Speeds up to 24 inches per second are available as special orders.

Thrust

The M.E.A. two-position Super Thrust can provide thrusts up to 200,000 lbs. Special orders up to 800,000 lbs. are available. The large thrust with high speed (because of the stored energy) can solve many problems with a relatively small actuator compared to an electric gear actuator.

Safety

The M.E.A. Incorporated two-position Super Thrust can fail safe when actuator is engulfed in a fire. A Thermal switch will energize the actuator to a fail safe position. A fire-safe blanket can also be provided.