Industrial Pressure Switches

Industrial Pressure Switch (Ashcroft)

A pressure switch is a device that detects the presence of fluid pressure. Pressure switches use a variety of sensing elements such as diaphragms, bellows, bourdon tubes, or pistons. The movement of these sensors, caused by pressure fluctuation, is transferred to a set of electrical contacts to open or close a circuit.

Normal status of a switch is the resting state with stimulation. A pressure switch will be in its “normal” status when it senses low or minimum pressure. For a pressure switch, “normal” status is any fluid pressure below the trip threshold of the switch.

One of the earliest and most common designs of pressure switch was the bourdon tube pressure sensor with mercury switch. When pressure is applied, the bourdon tube flex's enough to tilt the glass bulb of the mercury switch so that the mercury flows over the electrical contacts, thus completing the circuit. the glass bulb tilts far enough to cause the mercury to fall against a pair of electrodes, thus completing an electrical circuit. Many of these pressure switches were sold on steam boilers. While they became a de facto standard, they were sensitive to vibration and breakage of the mercury bulb.

Pressure Switch Symbols

Pressure switches using micro type electrical switches and force-balanced pressure sensors is another common design.  The force provided by the pressure-sensing element against a mechanical spring is balanced until one overcomes the other. The tension on the spring may be adjusted to set the tripping point, thus providing an adjustable setpoint.

One of the criteria of any pressure switch is the deadband or (reset pressure differential). This setting determines the amount of pressure change required to re-set the switch to its normal state after it has tripped.  The “differential” pressure of a pressure switch should not to be confused with differential pressure switch, which actually measures the difference in pressure between two separate pressure ports.

When selecting pressure switches you must consider the electrical requirements (volts, amps, AC or DC), the area classification (hazardous, non-hazardous, general purpose, water-tight), pressure sensing range, body materials that will be exposed to ambient contaminants, and wetted materials (parts that are exposed to the process media).

Steam Conservation Guidelines for Condensate Drainage

Any company that is energy conscious is also environmentally conscious. Less energy consumed means less waste, fewer emissions and a healthier environment.

In short, bringing energy and environment together lowers the cost industry must pay for both. By helping companies manage energy, Armstrong and Mead O'Brien products and services are also help protect the environment.

Steam is an invisible gas generated by adding heat energy to water in a boiler. Enough energy must be added to raise the temperature of the water to the boiling point. Then additional energy—without any further increase in temperature—changes the water to steam.

Steam is a very efficient and easily controlled heat transfer medium. It is most often used for transporting energy from a central location (the boiler) to any number of locations in the plant where it is used to heat air, water or process applications.

As noted, additional Btu are required to make boiling water change to steam. These Btu are not lost but stored in the steam ready to be released to heat air, cook tomatoes, press pants or dry a roll of paper.

The heat required to change boiling water into steam is called the heat of vaporization or latent heat. The quantity is different for every pressure/temperature combination, as shown in the steam tables.

Heat flows from a higher temperature level to a lower temperature level in a process known as heat transfer. Starting in the combustion chamber of the boiler, heat flows through the boiler tubes to the water. When the higher pressure in the boiler pushes steam out, it heats the pipes of the distribution system. Heat flows from the steam through the walls of the pipes into the cooler surrounding air. This heat transfer changes some of the steam back into water. That’s why distribution lines are usually insulated to minimize this wasteful and undesirable heat transfer.

When steam reaches the heat exchangers in the system, the story is different. Here the transfer of heat from the steam is desirable. Heat flows to the air in an air heater, to the water in a water heater or to food in a cooking kettle. Nothing should interfere with this heat transfer.

Condensate Drainage - Why It’s Necessary

Condensate is the by-product of heat transfer in a steam system. It forms in the distribution system due to unavoidable radiation. It also forms in heating and process equipment as a result of desirable heat transfer from the steam to the substance heated. Once the steam has condensed and given up its valuable latent heat, the hot condensate must be removed immediately. Although the available heat in a pound of condensate is negligible as compared to a pound of steam, condensate is still valuable hot water and should be returned to the boiler.

The document below provides and excellent reference for understanding the properties of steam and the importance of condensate drainage for an efficient system. You may also download your own copy of the Steam Conservation Guidelines for Condensate Drainage here.

Steam Conservation Guidelines for Condensate Drainage from Mead O'Brien, Inc.

Condensate Drainage ... Why It’s Necessary in Industrial Steam Systems

Condensate drain
(Armstrong).

Abstracted with permission from Armstrong International

Condensate is the by-product of heat transfer in a steam system. It forms in the distribution system due to unavoidable radiation. It also forms in heating and process equipment as a result of desirable heat transfer from the steam to the substance heated. Once the steam has condensed and given up its valuable latent heat, the hot condensate must be removed immediately. Although the available heat in a pound of condensate is negligible as compared to a pound of steam, condensate is still valuable hot water and should be returned to the boiler.

The need to drain the distribution system.

Figure 1: Condensate allowed to collect in pipes or tubes
is blown into waves by steam passing over it until it blocks
steam flow at point A. Condensate in area B causes a pressure
differential that allows steam pressure to push the slug
of condensate along like a battering ram.

Condensate lying in the bottom of steam lines can be the cause of one kind of water hammer. Steam traveling at up to 100 miles per hour makes “waves” as it passes over this condensate (Fig. 1). If enough condensate forms, high-speed steam pushes it along, creating a dangerous slug that grows larger and larger as it picks up liquid in front of it. Anything that changes the direction—pipe fittings, regulating valves, tees, elbows, blind flanges—can be destroyed. In addition to damage from this “battering ram,” high-velocity water may erode fittings by chipping away at metal surfaces.

The need to drain the heat transfer unit. 

Figure 2: Coil half full of condensate can’t
work at full capacity.

When steam comes in contact with condensate cooled below the temperature of steam, it can produce another kind of water hammer known as thermal shock. Steam occupies a much greater volume than condensate, and when it collapses suddenly, it can send shock waves throughout the system. This form of water hammer can damage equipment, and it signals that condensate is not being drained from the system. Obviously, condensate in the heat transfer unit takes up space and reduces the physical size and capacity of the equipment. Removing it quickly keeps the unit full of steam (Fig. 2). As steam condenses, it forms a film of water on the inside of the heat exchanger. Non-condensable gases do not change into liquid and flow away by gravity. Instead, they accumulate as a thin film on the surface of the heat exchanger—along with dirt and scale. All are potential barriers to heat transfer (Fig. 3).

The need to remove air and CO2. 

Air is always present during equipment start-up and in the boiler feedwater. Feedwater may also contain dissolved carbonates, which release carbon dioxide gas. The steam velocity pushes the gases to the walls of the heat exchangers, where they may block heat transfer. This compounds the condensate drainage problem, because these gases must be removed along with the condensate.

Fig 3: Potential barriers to heat transfer: steam heat and temperature
 must penetrate these potential barriers to do their work.

For more information about any industrial steam or hot water system, contact Mead O'Brien by visiting www.meadobrien.com or call (800) 892-2769.

Industrial Valve Automation, Service and Repair

From quarter-turn ball, butterfly, or plug valves, to linear gate and globe valves, Mead O'Brien can handle the most challenging actuation design. Options and accessories such as valve communications, limit switches, fail-safe devices, and solenoid valves are no problem.

With decades of expertise in rack and pinion and scotch-yoke actuators, as well as electric quarter-turn and linear actuators, Mead O'Brien has the experience and facilities to deliver a well engineered automated valve package. Visit www.meadobrien.com.

Understanding Industrial Valve Actuators

Automated Pneumatic
Ball Valve (Jamesbury)

Valves are essential to industries which constitute the backbone of the modern world. The prevalence of valves in engineering, mechanics, and science demands that each individual valve performs to a certain standard. Just as the valve itself is a key component of a larger system, the valve actuator is as important to the valve as the valve is to the industry in which it functions. Actuators are powered mechanisms that position valves between open and closed states; the actuators are controllable either by manual control or as part of an automated control loop, where the actuator responds to a remote control signal. Depending on the valve and actuator combination, valves of different types can be closed, fully open, or somewhere in-between. Current actuation technology allows for remote indication of valve position, as well as other diagnostic and operational information. Regardless of its source of power, be it electric, hydraulic, pneumatic, or another, all actuators produce either linear or rotary motion under the command of a control source.

Thanks to actuators, multiple valves can be controlled in a process system in a coordinated fashion; imagine if, in a large industrial environment, engineers had to physically adjust every valve via a hand wheel or lever! While that manual arrangement may create jobs, it is, unfortunately, completely impractical from a logistical and economic perspective. Actuators enable automation to be applied to valve operation.

Pneumatic actuator
(Jamesbury Quadra-Powr) 

Pneumatic actuators utilize air pressure as the motive force which changes the position of a valve. Pressurized-liquid reliant devices are known as hydraulic actuators. Electric actuators, either motor driven or solenoid operated, rely on electric power to drive the valve trim into position. With controllers constantly monitoring a process, evaluating inputs, changes in valve position can be remotely controlled to provide the needed response to maintain the desired process condition.

Manual operation and regulation of valves is becoming less prevalent as automation continues to gain traction throughout every industry. Valve actuators serve as the interface between the control intelligence and the physical movement of the valve. The timeliness and automation advantages of the valve actuators also serve as an immense help in risk mitigation, where, as long as the system is functioning correctly, critical calamities in either environmental conditions or to a facility can be pre-empted and quickly prevented. Generally speaking, manual actuators rely on hand operation of levers, gears, or wheels, but valves which are frequently changed (or which exist in remote areas) benefit from an automatic actuator with an external power source for a myriad of practical reasons, most pressingly being located in an area mostly impractical for manual operation or complicated by hazardous conditions.

Electric Actuator
(Limitorque)

Thanks to their versatility and stratified uses, actuators serve as industrial keystones to, arguably, one of the most important control elements of industries around the world. Just as industries are the backbones of societies, valves are key building blocks to industrial processes, with actuators as an invaluable device ensuring both safe and precise operation.

Intelligent Transmitters Help Coal Plant Reduce Costs and Improve Performance

Effective, profitable power plant operation requires managing capital-expense turbine, boiler, and combustion equipment, along with many other assets that must be precisely balanced. Reliable readings of pressure, temperature, and other process variables are critical to success.

While analog transmitters are known for accuracy and reliability, maintenance costs increase with age, and flexibility for performance improvement is limited. To reduce long-term operating costs and maintain quality service to more than 300,000 customers, a Michigan power utility launched a program to replace its aging analog transmitters with modern digital models.

The utility uses transmitters for draft indications on the boiler and pulverized mill area. They read pressure on the boiler and the turbine as well as combustion and steam heating equipment. Some of the instruments send data to a centralized distributed control system (DCS), which manages the set points that control the sensitive interactions. Other instruments simply indicate various pressure states to operators and maintenance technicians.

When this power utility implemented its first DCS, all transmitters were analog. At the time, mixing and matching multiple brands of analog sensors was difficult, and in some cases impossible, due to proprietary mounting configurations. Managers at this Michigan power plant wanted to be certain that they selected a digital sensor that would not lock them into a single vendor.

To learn how this power company came up with a the solution and to learn the results, read the complete document below. For any process instrument requirement, visit Mead O'Brien at www.meadobrien.com or call (800) 892-2769.

Intelligent Transmitters Help Coal Plant Reduce Costs and Improve Performance from Mead O'Brien, Inc.

Selecting the Right Valve Automation Partner for the Power Industry

Electrically automated gate valve
in generating facility.

Having knowledgeable, experienced and skilled vendor-partners is crucial in mitigating safety, environmental, and health risks, as well as meeting power generation facility construction and production goals. Partnering with the right valve automation company means that you will have industry experts there to ensure the success of your project, meeting your budgetary and performance goals, and passing critical know-how along to your staff.

But what should you look for in a valve automation partner?

Understanding and Meeting Expectations
Your valve automation partner needs to understand your industry, the application, and the upstream and downstream processes affected by the automated system being installed. Your partner must have a full understanding of all types of valve automation, including pneumatic, hydraulic, electro-hydraulic, and electric actuation. A full understanding of the morass of technical and administrative requirements is critical. These include a knowledge of applicable codes, industry standards, environmental concerns, maintenance requirements, back-up systems, and emergency processes. A strong candidate will consider all of these factors for every power plant valve automation job.

Engineering, Experience, and Precision
Qualified engineering staff and experience are critically important factors in selecting your valve automation partner. A qualified partner should have engineering staff with decades, not just years, of experience in applying, specifying, designing, and fabricating automated valve systems for the power industry. Additional experience in other industry segments is a plus, but a working history and a proven, successful track record in power plant automation is mandatory. All production technicians should be factory trained with valid certification. Whether a 1/2” ball valve with simple electric actuator, or 48” valve gate with extensive controls, your valve automation partner needs to reliably and consistently ensure conformance to specifications.

Involvement
Your valve automation partner needs to remain involved in every step of the process - from specifying, quoting, fabrication, delivery, installation, and training. The best valve automators stand by the customer after the automated valve systems are shipped. They see the project through to completion, paying great attention to detail. They generally obsess over the smallest details. For instance, a critical area is actuator-to-valve adaption design and configuration and a good candidate will pay very close attention to that piece. Good valve automation partners create and provision high quality drawings and wiring diagrams. They carefully ensure all requested settings and configuration meets specification and is completed. Finally, they maniacally Q.C. the completed automated valve though in-depth cycle testing before shipping. A good valve automation partner truly understands that this investment in detail upfront, eliminates costly downstream errors and mistakes.

Documentation and Tagging
Your partner should provide very detailed documentation for each automated package. Documentation packages should be very detailed and include valve Cv, actuator sizing calculations, material selection criterion, ISA data-sheets, dimensional drawings, operational testing data, seat leak test data, packing leakage data, and switch setting verification. Your valve automation partner should standardly provide valve packages tagged with stainless steel stamped serial numbers that provide traceability back to original components, fabrication, and testing.

Facilities
Your valve automation partner should have in-house capabilities for lifting, moving, testing, and storing large valve assemblies. The facility needs adequate pneumatic and electrical service to power any system they build. The space and ability to move large valves with lifts, hoists, and jacks is important. Storage space for sub-assemblies and finished goods, high pressure leak testing stations, seat leak testing stations. In-house CAD systems and on-premise machine shops provide an environment for better quality control and communication. Close proximity between engineers, designers, and technicians supports efficient communication and full understanding of customer needs. An on-site training room, with all required electrical and pneumatic testing rigs should be available. Finally, large docking facilities and easy access to major highways minimizes transportation issues and lowers cost.

Training

Valve automation training facility
(courtesy of Mead O'Brien)

Your plant maintenance crews must have 100% familiarity with the operation, maintenance, and troubleshooting of your new valve systems. Your valve automation partner must have the capability to provide practical, and hands-on, training on all facets of valve automation. Programs must be customizable to customer needs and special situations.

By doing your due diligence, and thoroughly evaluating a prospective valve automation partner, you are establishing a framework of confidence and trust that minimizes risk and provides peace of mind that your critically important power plant valve systems will provide safe, efficient, and reliable performance for years to come.