Limitorque Fluid Power Systems (LFPS)

Limitorque Fluid Power Systems is a group of modular scotch yoke fluid power actuators designed to deliver maximum torque with the lowest possible displacement and overall size. These heavy-duty, fluid-powered valve actuators and control systems are design primarily for the oil and gas industry. The group is categorized into three major sub-groups:

• Gas Powered Actuators - The Limitorque LDG direct gas actuator is designed to operate on high pressure pneumatic supply, including pipeline gases, nitrogen and any other equivalent high pressure source.
• Hydraulic Actuators - LHS and LHH are Limitorque’s range of hydraulic, quarter-turn, scotch yoke actuators. Designed to meet or exceed the most current and stringent safety and reliability standards for application in the oil and gas industry LHS and LHH are suitable for on/off and modulating control of all quarter-turn valves. 
• Pneumatic Actuators - Limitorque’s LPS and LPC are pneumatic quarter turn scotch yoke actuators, featuring a robust design suitable for heavy duty services, and among the longest design lifespans and maintenance intervals in the industry.

Download the Limitorque Fluid Power Systems PDF here.

Mead O'Brien
www.meadobrien.com
(800) 892-2769

The Application of Limit Switches on Automated Industrial Valves

Limit switch with position indicator.

Limit switches are devices which respond to the occurrence of a process condition by changing their contact state. In the industrial control field, their applications and product variations are almost countless. Essentially, the purpose of a limit switch is to serve as a trigger, indicating that some design condition has been achieved. The device provides only an indication of the transition from one condition to another, with no additional information. For example, a limit switch triggered by the opening of a window can only deliver an indication that the window is open, not the degree to which it is open. Most often, the device will have an actuator that is positively activated only by the design condition and mechanically linked to a set of electrical contacts. It is uncommon, but not unknown, for limit switches to be electronic. Some are magnetically actuated, though most are electromechanical. This article will focus on limit switch designs and variants used in the control and actuation of industrial process valves.

Employed in a wide range of industrial applications and operating conditions, limit switches

are known for their ease of installation, simple design, ruggedness, and reliability. 

Automated valve assembly
including actuator and
limit switch.

Valves, devices used for controlling flow, are motion based. The movable portions of valve trim create some degree of obstruction to media flow, providing regulation of the passage of the media through the valve. It is the movement of critical valve trim elements that limit switches are used to indicate or control. The movable valve trim elements commonly connect to a shaft or other linkage extending to the exterior of the valve body. Mounting electric, hydraulic, or pneumatic actuators to the shaft or linkage provides the operator a means to drive the mechanical connection, changing the orientation or position of the valve trim and regulating the media flow. Because of its positive connection to the valve trim, the position of the shaft or linkage is analogous to the trim position and can be used to indicate what is commonly referred to as “valve position”. Limit switches are easily applied to the valve shaft or linkage in a manner that can provide information or direct functional response to certain changes in valve position.

In industrial valve terms, a limit switch is a device containing one or more magnetic or electrical switches, operated by the rotational or linear movement of the valve.

What are basic informational elements that can be relayed to the control system by limit switches? Operators of an industrial process, for reasons of efficiency, safety, or coordination with other process steps, may need answers to the following basic questions about a process control valve:

• Is the valve open? 
• Is the valve closed? 
• Is the valve opening position greater than “X”? 
• Has the valve actuator properly positioned the valve at or beyond a certain position? 
• Has the valve actuator driven the valve mechanism beyond its normal travel limits? 
• Is the actuator functioning or failing? 

Partial or complete answers to these and other questions, in the form of electrical signals relayed by the limit switch, can serve as confirmation that a control system command has been executed. Such a confirmation signal can be used to trigger the start of the next action in a sequence of process steps or any of countless other useful monitoring and control operations.

Applying limit switches to industrial valve applications should include consideration of:

• Information Points – Determine what indications are necessary or useful for the effective control and monitoring of valve operation. What, as an actual or virtual operator, do you want to know about the real time operational status of a valve that is remotely located. Schedule the information points in operational terms, not electrical switch terms. 
• Contacts – Plan and layout a schedule of logical switches that will provide the information the operator needs. You may not need a separate switch for each information point. In some cases, it may be possible to derive needed information by using logical combinations of switches utilized for other discrete functions. 
• Environment – Accommodate the local conditions and hazards where the switch is installed with a properly rated enclosure. 
• Signal – The switch rating for current and voltage must meet or exceed those of the signal being transmitted. 
• Duty Cycle – The cycling frequency must be considered when specifying the type of switch employed. Every switch design has a limited cycle life. Make sure your selection matches the intended operating frequency for the process. 
• Auxiliary Outputs – These are additional contact sets that share the actuation of the primary switch. They are used to transmit additional signals with specifications differing from the primary signal. 
• Other Actuator Accessories – Limit switches are often integrated into an accessory unit with other actuator accessories, most of which are related to valve position. A visual local indication of valve position is a common example. 

Switches and indicators of valve position can usually be provided as part of a complete valve actuation package, provided by the valve manufacturer or a third party. It is recommended that spare contacts be put in place for future use, as incorporating additional contacts as part of the original actuation package incurs comparatively little additional cost. 

Employing a properly configured valve automation package, with limit switches delivering valve status or position information to your control system, can yield operational and safety benefits for the life of the unit. Good advice is to consult with a valve automation specialist for effective recommendations on configuring your valve automation accessories to maximize the level of information and control.

The Rack and Pinion Style Pneumatic Valve Actuator

Rack & Pinion Actuator
(courtesy of Flowserve Automax)

Three primary kinds of valve actuators are commonly used: pneumatic, hydraulic, and electric.

Pneumatic actuators can be further categorized as scotch yoke design, vane design, and the subject of this post - rack and pinion actuators.

Rack and pinion actuators provide a rotational movement designed to open and close quarter-turn valves such as ball, butterfly, or plug valves and also for operating industrial or commercial dampers.

The rotational movement of a rack and pinion actuator is accomplished via linear motion and two gears. A circular gear, referred to a “pinion” engages the teeth of a linear gear “bar” referred to as the “rack”.

Pneumatic actuators use pistons that are attached to the rack. As air or spring power is applied the to pistons, the rack is “pushed” inward or “pulled” outward. This linear movement is transferred to the rotary pinion gear (in both directions) providing bi-directional rotation.

Visual of rack and pinion
(courtesy of Wikipedia)

Rack and pinion actuators pistons can be pressurized with air, gas, or oil to provide the linear the movement that spins the pinion gear. To rotate the pinion gear in the opposite direction, the air, gas, or oil must be redirected to the other sides of the piston, or use coil springs as the energy source for rotation. Rack and pinion actuators using springs are referred to as "spring-return actuators". Actuators that rely on opposite side pressurization of the rack are referred to as "direct acting".

Most actuators are designed for 100-degree travel with clockwise and counterclockwise travel adjustment for open and closed positions. World standard ISO mounting pad are commonly available to provide ease and flexibility in direct valve installation.

NAMUR mounting dimensions on actuator pneumatic port connections and on actuator accessory holes and drive shaft are also common design features to make adding pilot valves and accessories more convenient.

Fully automated valve with rack
and pinion actuator, solenoid, and
limit switch.

Pneumatic pneumatic rack and pinion actuators are compact and save space. They are reliable, durable and provide a good life cycle. There are many brands of rack and pinion actuators on the market, all with subtle differences in piston seals, shaft seals, spring design and body designs.

For more information on any pneumatic or electric valve automation project, contact:

Mead O’Brien, Inc.
www.meadobrien.com
10800 Midwest Industrial Blvd
St. Louis, Missouri 63132
Phone (314) 423-5161
Toll Free (800) 874-9655
Fax (314) 423-5707
Email: meadstl@meadobrien.com