Limit Switches Applied to Industrial Process Control Valves

Example of switchbox containing limit switches and a
valve position indicator.
Courtesy Flowserve/Automax

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.

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.

Automax Rotary Switches and Positioners from CTi Controltech

Rotork Type K Damper Drive Case Study

Type K Pedestal Mount Damper Drive
Courtesy Rotork

Here is a useful narrative, a case study, describing how a large combustion based plant performed a retrofit of some decades old damper drives with Rotork Type K Type K drives to provide all around better performance and safety.

The Type K Range encompasses rotary and linear pneumatic damper drives that can be applied where precise combustion air and flue gas handling is desired. Proper application of the drives has the potential to increase boiler efficiency, reduce maintenance, lower fuel consumption and reduce harmful emissions. A few key aspects of the Type K drives include:

• Drop-In-Place models for retrofit requiring no field engineering or fabrication
• Damper drives engineered for reliability in high temperature environments
• Products are rated for continuous modulating service
• Highly accurate resolution of 0.25%
• Discrete contacts, along with pneumatic, analog or bus network (MOD) communications

The case study is worth browsing. See how the new drives were effectively incorporated as a retrofit to get an upgrade in performance. Share your combustion application challenges with the experts at CTI Controltech. Combine your knowledge of your own plant and equipment with their product and application knowledge to generate positive solutions.

Rotork Type K Damper Drive Case Study from CTi Controltech

CTI Controltech Offers New Damper Drive Line From Rotork

Pedestal mounted Rotork PM Series pneumatic rotary damper drive
Courtesy Rotork

CTI-Controltech Combustion division has been named as the authorized exclusive representative for all Rotork Type K products in Northern California and Western Nevada. CTI-Controltech has been an authorized representative for other portions of the Rotork line of actuators for some time, and the new Type K line expands the universe of applications for which CTI provides well considered solutions.

The Type K Damper Drives encompass several styles that are particularly well suited for differing applications. Several models are designed to function as drop-in replacements for matching drives, while allowing for the reuse of existing linkage. Numerous arrangements are available in linear and rotary configurations, with pneumatic or electric actuation.

Explore or reacquaint yourself with the Rotork line of rotary and linear damper drives. A document is included below with illustrations of the various models, along with technical features and intended application examples. Take a browse through it and you will find something useful. You can also contact a product specialist with your questions and application challenges.

Type K Linear and Rotary Damper Actuators - Rotork from CTi Controltech

Process Control - Getting Your PLC Project Done

Programmable logic controllers combine inputs, outputs,
and processing in a compact unit

Programmable logic controllers have been around for decades, but continue to supplant component oriented control schemes everywhere. The basic PLC consists of a variety of inputs from the process and its equipment, outputs to the process equipment, and internal processing (logic) that interrelates the two. PLCs have distinct advantages making them a first choice for many process control applications.

• Compactness: A PLC packs a comparatively huge amount of inputs, outputs, control loops, and relay contacts in a small package. Wiring is simplified and severely reduced, since the bulk of the switch logic is accomplished without relays and wiring in the PLC software.
• Functionality: In addition to duplicating the function of countless relays and discrete controllers, the logic functions of the PLC and the integration of multiple controls, inputs, and outputs into a single device empower the designer to provide functionality that is difficult, impractical, or impossible with discrete control component designs.

A programmable logic controller is not purpose specific. In actuality, it does not really do much of anything right out of the box. Every controller must be programmed to perform the tasks necessary to effectively and safely regulate the process. Programming a PLC is not intrinsically difficult. There are various programming tools available for almost every brand that provide options for textual, symbolic, and even ladder logic programming. These tools adroitly accommodate the manner in which the programmer envisions the logic. My personal preference is for ladder logic because it resembles a wiring diagram.

If planning on incorporating a PLC into your control scheme, here are some things you should consider early on to make the project successful.

• Schedule out all the inputs and outputs that will be needed. Incorporate excess capacity. It is likely you will want to add functionality in the future.
• Evaluate the control scheme for potential added features and functions that have not been previously considered. This is especially true when replacing an existing discrete component control system with a PLC based design. There are often relationships among the various inputs and outputs that can be exploited to provide better monitoring or control.
• If there are multiple machines, valves, or other major components in the control scheme, make sure your program design properly coordinates or interlocks their operation.
• Develop operating schemes for responding to faults or malfunction detected in the PLC itself or the connected devices or machines. There may be other safety devices that operate independent of the PLC. Consider them as last resort devices and design your program to handle faults before they become real problems.
• Structure the program thoughtfully. Make it as easy to read and understand as you possibly can because, at some future time, somebody is going to need to decipher what has been done. Include notes and descriptions for every logical block of code. Use labels for program elements that provide useful information about the function of the element. Make allowances for future modification.
• In some cases, it may be useful to produce a text document that describes the function of each logical program block, the reason for its existing structure, how it relates to other functions. This can be a substantial amount of work, but instills discipline in the programming process and provides any operator with a reference for how the control system is supposed to work.
• The function of a PLC is to bring the knowledge of the process operator to bear on the control of the process. The programmer should have substantial knowledge of the equipment being controlled, as well as a good understanding of process operation and goals.

The hardware part of the project will likely be the easy part. Developing a good operating program that serves the long term needs of the operation is the real challenge. It can be difficult to find individuals with combinations of good programming skills, writing skills, and technical knowledge of the process and numerous equipment and control elements. A good solution may be to outsource the programming to a firm experienced in PLC applications involving processes and equipment similar to yours.