Q&A for Ultrasonic Level Switches

Ultrasonic level switches with single
and multiple level measurement points
Courtesy Magnetrol International

Ultrasonic level switches appear, at first glance, to be a renamed version of a vibrating tuning fork level switch. They have a similar appearance and tank mounting scheme, but a closer look at the technology of the two instruments reveals that they rely on different operating principles to indicate when liquid level reaches their fixed switch point.

A previous article , with an accompanying video, provided some comparison between the two detection methods. Here is the operational difference summarized. A vibrating tuning fork device monitors the resonant frequency of the vibrating fork and responds when the frequency shifts due to immersion of the fork in liquid. An ultrasonic level switch transmits an ultrasonic pulse across a gap, measuring the intensity of the received signal and determining whether the signal passed through liquid (high level received signal) or air (low level received signal). While both technologies are effective, the ultrasonic level switch can be applied over a wider range of liquid density and does not require recalibration or adjustment for a change in media density.

Magnetrol International, globally recognized innovator in level measurement technology and instruments, recently answered a few application questions in their blog about their Echotel ultrasonic switches. The questions, along with answers penned by Magnetrol's product manager Tom Kemme, are shared here.

Question: Can ECHOTEL be used in applications that have entrained air?

Answer: Yes, ECHOTEL ultrasonic switches can be used in applications that have entrained air. All ECHOTEL gap switches feature a tip-sensitive transducer that provides superior performance over side gap transducers that are offered by other companies. Side gap transducers allow gas bubbles to adhere to the upper surface of the gap, which cause false dry gap alarms. Tip sensitive transducers allow these bubbles to pass through the gap. Applications with severe turbulence or entrained air should use the Model 961 switch, which offers a time delay adjustment. Up to 10 seconds of delay can be used to disregard entrained air and reliably detect the true liquid level.

Question: We are considering adding level alarm switches to our process to provide high-high level indication in several tanks. Instead of switches with relay outputs, we are considering the current shift output. What are the advantages of a current shift output?

Answer: Current shift electronics simply shift the current output from 8 mA when the level switch is in the normal operation, to 16 mA to indicate a level alarm. ECHOTEL Model 961 also has a user selectable fault signal of 3.6 or 22 mA. Current shift switches are 2-wire loop powered, which allows them to be offered with intrinsically safe approvals. This allows these switches to be put into hazardous area locations at a lower cost since rigid conduit is not necessary. Since current shift switches provide constant indication of either a normal (8 mA), alarm (16 mA), or a fault (3.6 or 22 mA) condition, they are sometimes referred to as a transmitter for the price of a switch.

Share your level measurement requirements and challenges with process measurement specialists, combining your own process knowledge and experience with their product application expertise to develop effective solutions.

Industrial Control Valves

Cutaway view of industrial control valve
Courtesy Cashco

Control valves are at the heart of almost every fluid based industrial control process. Understanding their basic operation and function is essential for the process engineer, operator, or other stakeholder. A previous blog provided a good information source for those wishing to learn about control valves or review and hone their technical knowledge. The applications for which control valves are used seem uncountable in their variety, but common operating traits of control valves enable their use in every one.

Cashco, globally recognized manufacturer of industrial control products including control valves, regulators, controllers, pressure/vacuum relief vents, and flame and detonation arrestors, will be exhibiting at Interphex in Booth 3464 on March 21 through March 23 at the Jacob Javits Center in New York City. Knowledgeable personnel will be on hand to discuss your industrial control applications and challenges. If your plans include attendance at Interphex this year, take advantage of the opportunity to speak face to face with representatives of an industry leader.

E-Book on Fired Heaters and Combustion Efficiency

Modern high power boiler room

Yokogawa, globally recognized leader in a number of process control fields, has authored an e-book which provides useful insight into how operators of combustion based equipment and systems can improve efficiency and enhance safety by employing modern technology.

[All quoted passages in this article are from the Yokogawa e-book]

The Yokogawa e-book Combustion & Fired Heater Optimization offers “an analytical approach to improving safe & efficient operations” related to the use of combustion & fired heaters in the process industries. Through presenting an overview of combustion sources, such as furnaces and fired heaters, the book states that while “fired heaters pose a series of problems from safety risks to poor energy efficiency,” those problems “represent an opportunity for improved safety, control, energy efficiency and environmental compliance.” Fired heaters “account for 37% of the U.S. manufacturing energy end use.” Tunable Diode Laser Spectrometer (TDLS) technology helps mitigate safety concerns by “measuring average gas concentrations across the high temperature radiant sections.”

The book states that the four main concerns applicable to fired heaters are asset sustainability, inefficient operations, the operator skillset, and safety and compliance. Outdated diagnostics and controls have placed unnecessary stress on operator response, making sustainability of fired heaters difficult. The emissions of fired heaters are generally higher than designed, and can be coupled with control schemes for firing rates little changed over the past 40 years. Operators, generally, lack a clear understanding of design, and even engineering principles of heat transfer are not typically included in education related to fired heaters. Confounding the situation further, “many natural draft heaters do not meet this [safety regulation] guideline with existing instrumentation and control systems.” These complications combine to form a noticeable problem Yokogawa’s technology hopes to address. The company notes how the fired heater relies on natural draft instead of forced air, meaning the heaters “typically lack the degree of automation applied to other process units in the plant.” Offering a full detail of both the control state of most fired heaters and their systems defines the process situation currently considered common in the field, while emphasizing high excess air as providing a “false sense of safety.”

The proposed TDLS system allows for the measurement of “both the upper and lower conditions in a fired heater” by “simultaneously controlling the fuel and air supply based on fast sample intervals.” Safer burner monitoring and heater efficiency results from the TDLS measurements of CO, CH₄, and O₂. The optimization of air flow control reduces “O2 concentration … from 6% to 2%” and increases the furnace’s thermal efficiency. Combustion control is achieved by managing fuel flow and the arch draft. The TDLS integrated system works in tandem with already established logic solver systems in the plant. The TDLS technology works as a non-contacting measurement with “full diagnostic capability” and offers “distinct advantages over single point in situ analyzers” via reduction of false readings. Specific gas measurements, fast response time, optical measurement technology, and “high and variable light obstruction” are featured components of the TDLS system highlighted to show the technology’s durability and flexibility. The longevity and reliability of the system is showcased by how the TDLS combustion management system has been operational in a major refinery since 2010. The percentage of excess O₂ in sample fired heaters has decreased by 1% to 1.5%. Measurements by the TDLS system have been verified by other gas analyzers. The furnace conditions in the plant are more efficiently monitored and controlled. As a result, the furnace in the functional environment is “now near its optimum operating point, using minimum excess air.”

Yokogawa presents a process-related problem, then details the key points of the problem while unpacking the causes. The e-book introduces Yokogawa’s technology, explains the mechanics, and demonstrates how TDLS acts as a solution to the problem, supported by a tangible example. The book offers great insight for both the operational principles of fired heaters and a new technology designed to maximize efficiency in the control process.

The e-book is included below. More detail is available from product applicationspecialists, with whom you should share your combustion and fired heater related challenges. Combining your own facilities and process knowledge and experience with their product application expertise will lead to effective solutions.

Fired Heater and Combustion Optimization From Yokogawa from Miller Energy, Inc.

Consolidated Tool Kit for Sensor Maintenance

Configured repair kit contains everything needed
Courtesy United Electric Cotnrols

Keeping things going, working, no matter your industry, is a continuous challenge. In process measurement and control operations, sensors and transmitters perform an essential function that often calls for immediate repair upon failure. Being properly prepared by having any needed parts on hand empowers technicians to quickly and effectively return sensor assemblies or transmitters to operation.

United Electric offers their "Sensor Box™" a kit that is preloaded with everything needed to effect temperature sensor and transmitter repairs in the field. The kit keeps everything needed in one spot, ready to go.

The video shows the convenience with which a technician can use the kit to make field repairs. Share your temperature measurement and process control challenges with product application experts, combining your own process knowledge with their product expertise to develop effective solutions.

Level Measurement Using Isolating Air Flow

Bubbler method of liquid level
measurement is simple and reliable
Courtesy Yokogawa

Remotely utilized measurements of tank liquid level are common throughout the fluid processing industries. Various means of inferential or direct measurement are available, each with its own set of performance, maintenance, and cost attributes that may make it the preferred choice for a particular application.

Inferring liquid level using a hydrostatic pressure measurement is a simple, easy to implement strategy for delivering a continuous level signal to the process operating and control system. There can be situations where mounting the measuring gear near the bottom of a tank or other vessel may be impractical or undesirable. A pressure transmitter mounted low on a tank may expose it to damage from plant traffic or other physical hazards. It is also possible that the tank may not have a suitable fitting located low enough to provide the needed measuring range. Having a potential leakage point at a fitting low on the tank may also be undesirable. Another, and certainly not final, scenario would be an application involving a corrosive liquid which must not come in contact with the pressure sensor.

The bubbler method of liquid level measurement employs an arrangement that overcomes all of the previously mentioned impediments. It can utilize connections at the top of the tank, above the liquid level. The way in which the method works will keep the pressure sensor out of contact with the process liquid, providing isolation from potential corrosive effects.

The apparatus for level measurement using the bubbler method employs a simple dip tube that extends from the pressure sensor or transmitter to nearly the bottom of the tank or vessel. A small amount of purge air or gas continually flows through the dip tube and will bubble out the bottom of the tube. This dip tube arrangement essentially transfers the hydrostatic pressure at the bottom opening of the tube to the pressure transmitter, while the purge gas keeps the liquid from advancing up into the pipe. The bubbler can be used on atmospheric or pressurized tanks with a properly configured pressure or differential pressure transmitter.

Probably the most significant application point of the bubbler method that will distinguish its use from many other level measurement methods is the importance of maintaining air or gas flow in the dip tube. The flow provides isolation of the sensor, but the flow must also be set to a level that will not impact the pressure measurement in the tube itself. If the flow is excessive, backpressure in the tube can be offset from the level imparted by the tank contents, with the result being an incorrect measurement.

Below is an application note from Yokogawa, showing how their pressure transmitters and rotameters can be used to create the setup. Share your process measurement challenges and requirements with instrumentation experts, combining your own process knowledge with their product application expertise to produce effective solutions.

Liquid Level Measurement Using the Bubbler Method from Miller Energy, Inc.

Yokogawa Differential Pressure Transmitter from Miller Energy, Inc.

Yokogawa Rotameter Flowmeters for Industrial Applications from Miller Energy, Inc.

Case Study From CSB: Industrial Plant Heat Exchanger Explosion

Shell and tube heat exchangers

Industrial accidents, whether minor or catastrophic, can serve as sources of learning when analyzed and studied. Operators, owners, and technicians involved with industrial chemical operations have a degree of moral, ethical, and legal responsibility to conduct work in a reasonably and predictably safe manner without endangering personnel, property, or the environment. Part of  a diligent safety culture should include reviewing industrial accidents at other facilities. There is much to learn from these unfortunate events, even when they happen in an industry that may seem somewhat removed from our own.

The U.S. Chemical Safety Board, or CSB, is an independent federal agency that investigates industrial chemical accidents. Below, find one of their video reenactments and analysis of an explosion that occurred at a Louisiana chemical processing plant in 2013. A portion of the reenactment shows how a few seemingly innocuous oversights can combine with other unrecognized conditions that result in a major conflagration.

Check out the video and sharpen your senses to evaluate potential trouble spots in your own operation.

Accurate Level Measurement Contributes to Heat Rate Reduction

Industrial steam turbine

Steam production is a costly operation in any facility, but is of paramount importance in power generation plants. The bottom line of a combustion based power generation facility is sensitive to the cost of input fuel. Measures that can be taken to reduce fuel input for a unit of power output (called heat rate) can translate directly into profitability. An additional benefit of reducing heat rate is a commensurate reduction in emissions.

A major contributor to heat rate reduction is the recovery of heat from the process and transference of that heat into the boiler feedwater. A sizable feedwater preheater of the shell and tube type is used to recover the heat. Shell and tube heat exchanger efficiency can be maximized with accurate control of liquid level.

Magnetrol, globally recognized leader in level measurement technology, makes the case for using guided wave radar level measurement technology as the most advantageous means for this application. The video below describes the process and how the guided wave radar level transmitter can provide the best performance.

Magnetrol has an information kit devoted to heat rate reduction. Share your steam system and level measurement challenges with a product specialist, and ask how you can get the Heat Rate Reduction Kit. Combining your facility and process knowledge with the product application expertise of a specialist will result in effective solutions.