Pneumatic Actuator With Air Fail Safe Operation

Pneumatic actuator on industrial valve
Air Fail Safe Actuator
installed on industrial valve
Courtesy Hytork, EPM
Pneumatic actuators are used throughout the industrial process control field to provide remote or automated control of valve operation. This type of industrial valve actuator employs pressurized air to move a piston or diaphragm and apply force to the valve stem, opening or closing the valve. Since, by the general nature of their scale and content, industrial fluid processes tend to carry substantial risk related to safety and financial loss due to failure, the engineer’s task is to plan and design for safe operation, should a wide range of possible failure modes occur. Hytork, an Emerson Process Management brand, provides a solution for fail safe operation of pneumatic actuators.
The Hytork Air Fail Safe Acutator is intended for use on larger valves with high torque requirements. The actuator has a built-in air reservoir that provides reserve air capacity to safely close the valve when the input air pressure fails. The accumulator stores compressed air at the highest pressure seen at the device. The design of the Air Fail Safe actuator delivers a high torque level in a double rack and pinion arrangement that is inaccessible using spring based designs. Learn about the Hytork Air Fail Safe Actuator and discuss your potential applications with a product expert.

Selecting the Right Valve Type - Plug Valves

Valves are the primary fluid flow control device employed in the industrial process control arena. The widely varied applications and requirements have led to an almost daunting array of vendors. valves and options from which to choose. For your particular project or application, the selection candidate pool can be shrunk down to a manageable size if you first select the type of valve that will best suit your needs.

industrial plug valve internal part
The "Plug" in a Plug Valve
Courtesy Fluoroseal Inc.
Valves are generally used to start, stop, redirect, or throttle (control at some intermediate level) the flow of a fluid. They may also be tasked with preventing fluid flow from going in an undesirable direction or regulating pressure. Industrial flow control valve types are generally classified according to the structure or arrangement within the valve body that provides the obstruction to fluid flow. Some of the common types are ball, butterfly, gate, globe, and plug. Surely there are more valve types, and this article is not intended to list them all. Some of our previous blogs have discussed selection considerations for ball, butterfly, and gate valves. Let’s look at one of the oldest valve types, the plug valve.

Plug valves are similar to ball and butterfly valves, with their rotational movement of 90 degrees from the fully open to the closed positions. The “plug” in a plug valve is installed in the flow path within the valve body and rotated by means of a stem extending to the exterior of the body. Plugs are often tapered toward the bottom and are fitted to a seating surface in the valve body cavity. An opening through the plug, the port, can be shaped to provide particular flow characteristics. There are numerous variants of the basic plug valve which may make it suitable for particular applications. One common variant is the lined plug valve, with an interior lining of material suited to provide a measure of corrosion resistance for special media applications.

industrial plug valve
Industrial Plug Valve With Manual Operation Handle
Courtesy DHV Industires
What are the general positives for plug valve application?

  • 90 degree rotation from open to closed provides fast operation.
  • With proper configuration, can be well suited for frequent operation.
  • Availability of corrosion resistant liner may provide comparative cost savings because valve body can be constructed of less expensive material.
  • Design is simple and employs a low parts count.
  • Valve can be serviced in place.
  • Generally, low resistance to flow when fully open.
  • Reliable leak-tight service due to tapered plug wedging action, replaceable sleeve, and injection of lubricant in some variants.

What are some potential negatives for plug valves?

  • Higher friction in the closure mechanism may require comparatively higher operating torque than other valve types.
  • Generally not well suited for throttling applications without special design modifications.
  • Rapid shutoff may not be suitable for some applications where hammering may occur.

As part of your own assessment and selection process, share your application requirements and concerns with an experienced sales engineer. Their product knowledge and application experience can provide the additional input needed to make the best choice for your project.

Downpipe Liquid Level Sensor Purge Control

Downpipe bubbler type tank level indicator
Arrangement for bubbler type tank level indicator
with purge control
Courtesy King Gage
Industrial process control often requires the measurement of liquid inventory volume or mass. If the material is contained within a tank of known shape and dimensions, the volume can be determined accurately by measuring the level of the liquid. Various means are used to determine the liquid level. One of the simplest is the downpipe sensor, sometimes referred to as a "bubbler", where liquid level is inferentially derived using differential pressure. The downpipe is a length of pipe that is open at the end extending down into the liquid contained within the tank. The top end is connected to a purging device that provides a controlled flow of pressurized air or gas into the pipe. The pressure maintained within the downpipe will reach an equilibrium with the pressure produced by the hydrostatic force related to liquid level in the tank or vessel. Accurate measurement of this pressure, along with a thorough knowledge of the liquid's properties, can be used to determine the depth of the liquid within the tank and the corresponding volume.

King-Gage manufactures several industrial process level measurement devices incorporating downpipe purge technology and differential pressure measurement. They have applications in inventory monitoring, process control, hazardous and explosion zones, ballast monitoring, and other areas that benefit from their simple operation, low maintenance, and ruggedness. The company, in its own words describes the unit as an...
...extremely rugged unit designed specifically for hazardous areas requiring flameproof or ATEX (Ex d) rating. A proprietary wet check assembly ensures positive seal of fluids to ensure containment integrity. Loop powered transmitter provides 4-20mAdc output while components are isolated from the process media by a continuous air purge. Total consumption rate is less than 0.083 scfm for energy saving operation with external air/gas supply.The only internal element is a simple length of pipe extending into the tank.
Rugged design requires no setup or air flow adjustment due to its differential pressure regulation that avoids dynamic pressure drops common to other bubblers. This ensures highly accurate level measurement and repeatability within ± 0.2% while preventing turbulence or foaming of tank liquid. Applications include water/wastewater, sulfur pits, most free flowing liquids (including slurries) and temperatures in excess of 160 °C (320 °F) as the transmitter is effectively isolated from the process by the air purge.
The slides below illustrate various installation configurations, including one for explosion hazard areas. Contact an application engineer for more detailed information.

Is a Butterfly Valve Right for Your Application?

There are many manufacturers of industrial control valves, with each producing an array of sizes, types, construction materials, and other distinguishing characteristics that make it necessary to quickly narrow the field of contenders to consider for your valve application in order to be productive. A useful filter that can be applied to the broad offering of valves is selecting a valve “type”. Valve types are generally classified according to their means of closure, with some common types being ball, plug, globe, butterfly and gate, among others. Each type has particular application strengths, some that may prove advantageous enough for your application to exclude all others. Let’s take a look at butterfly valves.
industrial butterfly valve
Butterfly Valve
Courtesy Crane CPE

The closure means for a simple butterfly valve is a disc, positioned in the fluid flow path, that rotates around a central axis through a 90 degree arc. The valve stem serves as the axis and extends through a seal to the exterior of the valve body where it serves as an actuation point. The closed position for the valve has the disc positioned perpendicular to the direction of flow and blocking the flow path. When open, the disc is parallel to the flow path, but still positioned in the center of the flow. Butterfly valves are available in a variety of construction materials, making them suitable for an array of media.

Butterfly valve aspects that might be advantageous:
industrial butterfly valve
Butterfly Valve
Courtesy Crane CPE

  • Closure mechanism allows for comparatively low torque operation.
  • Small size and weight for reduced installation space and valve body support.
  • Low pressure drop in the fully open position. The disc, when parallel to the flow path, presents a thin edge of its profile to the fluid flow.
  • Low maintenance requirements and small parts count.
  • Transition from fully open to fully closed with a 90 degree turn provides for fast operation.
  • Suitable for limited range of throttling applications.

Aspects warranting closer scrutiny:

  • Throttling capability is generally limited to low pressure drop applications.
  • The orientation of the closure may cause cavitation under some conditions.
  • Choked flow can be a concern under certain conditions.

Performance, low maintenance requirements, and cost are always primary considerations. There are many more detailed sources of application information. Consult with a professional sales engineer that specializes in industrial process valves and get the information you need to make a good choice.

Reduce Process Downtime With Device Protection

The list of possible events or conditions that can crash your process is immense. Fortunately, most of the possible occurrences are tagged with extremely low probabilities that allow us to sleep at night without worrying about impending disaster. However, we are engineers and industrial process operators. We are supposed to make sure everything keeps flowing smoothly. Here is a small part of your world that can be made more secure with some practical consideration.

Oil and Gas Industry Installation
Oil and Gas Industry Installation
Industrial processes require two very basic elements, measurement and control. The instruments used to measure process variables and the control devices employed to change them must be kept in operation, in many cases, all the time. You invest time to research available products, searching for the right materials of construction, signal output, accuracy, and a whole range of other attributes that will make your selection perfectly suited to the process requirements. This technical selection process is an essential part of the process design and implementation. What other aspects of your measurement and control hardware may play an important part in maintaining process uptime?

Give some consideration to the conditions or events that might take a particular instrument or control device out of action. With the application of imagination and good judgement, combined with some help from an experienced application engineer, you can develop additional items to incorporate into the design and installation of your process measurement and control elements. Here are some basics to get you started.

What is the security exposure for your device?

Not considered so much in the past, thinking about how someone without the best interests of your organization in mind might create havoc should be part of your general assessment of each installation. Fully understanding the access pathways, physical or otherwise, to your instruments and controls is necessary to thwart unauthorized tinkering or malicious mischief.

What are the real extremes of weather conditions occurring at the device’s physical location?

Data on the weather for every industrialized country, and some that are not so industrialized, is freely available. Check it out. Forget about the average range of conditions. Look for conditions that occur once in a hundred years or more. A small hardening of your installation might provide the needed capacity or strength to withstand the worst weather in a hundred years. Challenge your device vendors with these weather conditions. It might leave your process the only one standing after that freak storm.

What extreme conditions may be generated by the process that could lead to device damage or deterioration?

The normal process conditions are generally known. Consider, though, the instantaneous or short duration occurrences that, while associated with normal operation, may have an adverse impact on installed measurement and control devices. What happens when elements start or stop, open or close? Are there malfunctions of one device that could snowball into a string of failures of other devices?

process measurement instruments
Process Measurement Instruments
What are the physical contact hazards where the device is installed?

Measurement and control equipment needs to be provided with personnel access for maintenance, repair, even real-time observation in some cases. Along with human access comes the wide array of misbehaviors of which we are all capable. There is not one of us that has not tripped and fallen, dropped something, or bumped into something with undesirable results. Plan for contact with people and the things they may carry around your plant or installation. Locate sensitive gear away from major traffic areas and provide protection from unplanned contact with people, dollies, carts, tools, boxes and anything else that may pass through the area.

Are your devices and equipment protected from moisture?

Electronics are an integral part of modern industrial measurement and control equipment. Electronic devices will be damaged or destroyed by exposure to excessive levels of moisture. Moisture exists everywhere in our environment, primarily as vapor or liquid. The ability of moisture to find its way into unprotected enclosures is well known, and you should take precautions to prevent its entry and accumulation in your devices.
Protecting your installed process measurement and control equipment is an investment that pays dividends in process uptime and reduced repair effort and cost. It is challenging to think of all possible hazards, so consult an experienced engineering sales team and tap into their field experience to make your project as good as it can be.