Limit Switches in Valve Actuators

Valve Position Sensor
Courtesy Westlock Controls

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.

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.

Severe Service Valve Applications

Industrial valve for severe service

Industrial process control applications can be associated with some very stringent and challenging performance requirements for the physical equipment and components that are part of the process chain. In fluid based operations, the control and shutoff valves can be a significant impact point of extreme fluid conditions, requiring careful design and selection consideration to assure proper performance and safety levels are predictably maintained.

Industrial valves that are intended for application at the extremes are generally referred to as severe service valves. While there are plenty of published and accepted standards for industrial valves, one does not exist to precisely define a severe service valve. There is, however, some movement toward the development of severe service standards in some industry segments.

So, how do you know when to focus valve selection activities on severe service valves, as opposed to general purpose valves? There are a number of basic criteria that might point you in that direction:

• Very extreme media or environmental temperature
• High pressure drop operation that may cause cavitation
• Rapid and extreme changes to inlet pressure
• Certain types or amounts of solids contained in the fluid
• High number of mechanical operations
• Thermal cycling

Certainly, any of these criteria might be found in an application serviceable by a general purpose valve, but their presence should be an indicator that a closer assessment of the fluid conditions and commensurate valve requirements is in order. The key element for a process stakeholder is to recognize when conditions are in evidence that might overrun the capabilities of a general purpose valve, leading to premature failure in control performance or catastrophic failure that produces an unsafe condition. Once the possibility of a severe service condition is identified, a careful analysis of the possible operating conditions will reveal the performance requirements for the valve.

There are numerous manufacturers of severe service valves, each seeming to concentrate on a particular niche. The oil and gas industry presents numerous applications for specially designed valves, as do other industries.

You can always get more information, or discuss your special requirements, with a product application specialist. They have access to technical resources that can help with selecting the right valve configuration to meet your severe service applications.

Pressure Regulator Valve or Back Pressure Regulator - Appropriate Application

One of many available configurations
for a pressure regulating valve.
Courtesy Cash Valve

Fluids move throughout processes, driven by pressure produced with mechanical or naturally occurring means. In many cases the pressure generated by the driving source is substantially greater than what may be desired at particular process steps. In other cases, the operation may dictate that a minimum pressure be maintained within a portion of the process train. Both cases are handled by the appropriate valve type, designed specifically to regulate pressure.

A pressure regulating valve is a normally open valve that employs mechanical means, positioning itself to maintain the outlet pressure set on the valve. Generally, this type of valve has a spring that provides a countervailing force to the inlet pressure on the valve mechanism. An adjustment bolt regulates the force produced by the spring upon the mechanism, creating an equilibrium point that provides flow through the valve needed to produce the set outlet pressure. A typical application for a pressure regulator is to reduce upstream or inlet pressure to a level appropriate for downstream processing equipment.

Back pressure valves are normally closed, operating in a logically reversed fashion to pressure regulators. Where pressure regulators control outlet pressure, a back pressure valve is intended to maintain inlet pressure. Similar internals are present in the back pressure valve, with the valve action reversed when compared to a pressure regulator. An inlet pressure reduction in the back pressure valve will cause the valve to begin closing, restricting flow and increasing the inlet pressure. A representative application for a back pressure valve is a multi-port spray station. The back pressure valve will work to maintain a constant setpoint pressure to all the spray nozzles, regardless of how many may be open at a particular time.

Both of these valve types are available in an extensive array of sizes, capacities, pressure ranges, and materials of construction to accommodate every process requirement. Share your fluid control challenges with a process control specialist. Combining your process knowledge with their product application expertise will produce effective solutions.

Pressure Regulating Valve - Type G-4 Cash Valve from Classic Controls, Inc.

Flame Scanners For Combustion Operations

Aim flame scanner through the largest cross sectional area of the flame
Courtesy Fireye

Flame detectors or scanners are regularly deployed in combustion applications as a means of confirming the presence of flame in a combustion chamber. The verification that fuel flowing into the utilization equipment is being properly burned and not accumulating unburned in the combustion chamber is the first line of safety in combustion.

Flame scanners use the characteristics of combustion and the electromagnetic emissions from burning fuel to detect flame and distinguish among flames from multiple burners. The instruments rely heavily upon operating principles utilizing visible, infrared, and ultraviolet light measurement and detection.

In single burner applications, simpler sensor and controller combinations can work suitably, but multiple flame applications are candidates for more complex detection devices and controls which can discriminate among multiple flames. Differences in individual flame characteristics, indicated through combustion products, can be utilized to distinguish between flames from different burners. Some photoelectric detectors can distinguish a signature flicker in flames of any type, invisible to the human eye.

Knowledge and understanding of the flame itself, its emissive attributes, and other characteristics are the key to proper flame detection. This may include the temperature of gases within the flame and its specific gas products. Other than temperature, electromagnetic radiation and ionized gas molecules in the flame are commonly used by flame scanners or detectors.

A variety of flame scanners are available for industrial and commercial use, each optimized for particular application sets. Essentially you have a scanner, which acts as a sensor. The signal from the scanner requires amplification and further processing to provide a reliable control signal. Hardware is available as discreet components, allowing a combination of scanner, amplifier, and control units to be combined into a system tailored for specific application requirements. Integrated systems are also available, with all appropriate detection and amplification circuitry built into a single compact unit.

Share your combustion process challenges with application specialists and combine your facility and process knowledge with their product application expertise to develop effective solutions.

Integrated Flame Scanner for Industrial Combustion Operations from Classic Controls, Inc.