Wednesday, December 13, 2017

Methods of Continuous Level Measurement in Industrial Process Control

Fuel tanks at refinery
Information about liquid level in a tank is an integral part
of successful process operation and safety.
Many industrial processes require the accurate measurement of fluid or solid (powder, granule, etc.) height within a vessel. Some process vessels hold a stratified combination of fluids, naturally separated into different layers by virtue of differing densities, where the height of the interface point between liquid layers is of interest.

A wide variety of technologies exist to measure the level of substances in a vessel, each exploiting a different principle of physics. This chapter explores the major level-measurement technologies in current use.

Level gauges

Level gauges are perhaps the simplest indicating instrument for liquid level in a vessel. They are often found in industrial level-measurement applications, even when another level-measuring instrument is present, to serve as a direct indicator for an operator to monitor in case there is doubt about the accuracy of the other instrument.

Float

Perhaps the simplest form of solid or liquid level measurement is with a float: a device that rides on the surface of the fluid or solid within the storage vessel. The float itself must be of substantially lesser density than the substance of interest, and it must not corrode or otherwise react with the substance.

Hydrostatic pressure

A vertical column of fluid generates a pressure at the bottom of the column owing to the action of gravity on that fluid. The greater the vertical height of the fluid, the greater the pressure, all other factors being equal. This principle allows us to infer the level (height) of liquid in a vessel by pressure measurement.

Displacement

Displacer level instruments exploit Archimedes’ Principle to detect liquid level by continuously measuring the weight of an object (called the displacer) immersed in the process liquid. As liquid level increases, the displacer experiences a greater buoyant force, making it appear lighter to the sensing instrument, which interprets the loss of weight as an increase in level and transmits a proportional output signal.

Echo

A completely different way of measuring liquid level in vessels is to bounce a traveling wave off the surface of the liquid – typically from a location at the top of the vessel – using the time-of-flight for the waves as an indicator of distance, and therefore an indicator of liquid height inside the vessel. Echo-based level instruments enjoy the distinct advantage of immunity to changes in liquid density, a factor crucial to the accurate calibration of hydrostatic and displacement level instruments. In this regard, they are quite comparable with float-based level measurement systems. Liquid-liquid interfaces may also be measured with some types of echo-based level instruments, most commonly guided-wave radar. The single most important factor to the accuracy of any echo-based level instrument is the speed at which the wave travels en route to the liquid surface and back. This wave propagation speed is as fundamental to the accuracy of an echo instrument as liquid density is to the accuracy of a hydrostatic or displacer instrument.

Weight

Weight-based level instruments sense process level in a vessel by directly measuring the weight of the vessel. If the vessel’s empty weight (tare weight) is known, process weight becomes a simple calculation of total weight minus tare weight. Obviously, weight-based level sensors can measure both liquid and solid materials, and they have the benefit of providing inherently linear mass storage measurement. Load cells (strain gauges bonded to a steel element of precisely known modulus) are typically the primary sensing element of choice for detecting vessel weight. As the vessel’s weight changes, the load cells compress or relax on a microscopic scale, causing the strain gauges inside to change resistance. These small changes in electrical resistance become a direct indication of vessel weight.

Capacitance

Capacitive level instruments measure electrical capacitance of a conductive rod inserted vertically into a process vessel. As process level increases, capacitance increases between the rod and the vessel walls, causing the instrument to output a greater signal. Capacitive level probes come in two basic varieties: one for conductive liquids and one for non-conductive liquids. If the liquid in the vessel is conductive, it cannot be used as the dielectric (insulating) medium of a capacitor. Consequently, capacitive level probes designed for conductive liquids are coated with plastic or some other dielectric substance, so the metal probe forms one plate of the capacitor and the conductive liquid forms the other.

Radiation

Certain types of nuclear radiation easily penetrate the walls of industrial vessels, but are attenuated by traveling through the bulk of material stored within those vessels. By placing a radioactive source on one side of the vessel and measuring the radiation reaching the other side of the vessel, an approximate indication of level within that vessel may be obtained. Other types of radiation are scattered by process material in vessels, which means the level of process material may be sensed by sending radiation into the vessel through one wall and measuring back-scattered radiation returning through the same wall.

Laser

Lasers can be employed essentially as distance measuring instruments, emitting a beam from above the target material and measuring the elapsed time for the emission to return as a reflection from its surface. With no moving parts, this can be an attractive technology for some applications.

The sales and application engineers at Classic Controls are experts in industrial level control. Feel free to contact them with your level measurement and control challenges. Combine your own process knowledge and experience with their product application expertise to develop an effective solution.

Thursday, December 7, 2017

Match Temperature Sensor Configuration to the Application for Best Results

heat tracing temperature sensor
Special construction features can better adapt a temperature
sensor to measuring process conditions.
Image courtesy Pyromation
There are more temperature controlled operations than any of us could count in a lifetime, each with a set of signature performance requirements and design challenges. Matching the means of temperature measurement, the control loop characteristics, and heat delivery method to the application are essential to achieving successful operation.

Step one is to measure the process temperature. This sounds simple until you start researching products and technologies for measuring temperature. Like the temperature controlled operations mentioned previously, they are numerous. To filter the possible candidates for temperature sensing devices, consider these aspects of your application and how well a particular sensor may fulfill your requirement.
  • Response Time - How rapidly the sensor will detect a change in process temperature is a function of how the sensor is constructed and how it is installed. Most temperature sensors are enclosed or encapsulated to provide protection for the somewhat vulnerable sensing element. Greater mass surrounding the sensing element, or a shape that inhibits heat transfer from the process to the sensor, will slow sensor response. Whether the slower response time will adversely impact process operation needs to be considered. More consideration is due to the manner in which the temperature sensor assembly is installed. Not all applications involve a fluid in which the sensor assembly can be conveniently immersed, and even these applications benefit from careful sensor placement.
  • Accuracy - Know what your process needs to be effective. Greater levels of accuracy will generally cost more, possibly require more care and attention to assure the accuracy is maintained. Accuracy is mostly related to the type of sensor, be it RTD, thermocouple, or another type.
  • Sensitivity - Related to the construction, installation, and type of sensor, think of sensitivity as the smallest step change in process temperature that the sensor will reliably report. The needs of the process should dictate the level of sensitivity specified for the temperature sensor assembly.
Take a simple application as an illustration. Heat tracing of piping systems is a common function throughout commercial and industrial settings experiencing periods of cold weather. Electric heat trace installations benefit from having some sort of control over the energy input. This control prevents excessive heating of the piping or applying heat when none is required, a substantial energy saving effort. A temperature sensor can be installed beneath the piping's insulation layer, strapped to the pipe outer surface. A specially designed sensor assembly can improve the performance of the sensor and the entire heat trace control system by enhancing the response time of the temperature sensor. A right angled sheath permits insertion of the sensor beneath the piping insulation while orienting the connection head upright. A surface pad at the tip of the sheath increases the surface contact with the pipe to provide faster sensor response. The surface pad is a metal fixture welded to the sensing end of the temperature sensor assembly. It can be flat, for surface temperature measurements, or angled for installation on a curved surface, like a pipe. The increased surface contact achieved with the surface pad promotes the conduction of heat to the sensor element from the heated pipe in our illustration. This serves to reduce and improve the response time of the sensor. Adding some thermally conductive paste between the pad and the pipe surface can further enhance the performance. While the illustration is simple, the concepts apply across a broad range of potential applications that do not allow immersion of the temperature assembly in a fluid.

A simple modification or addition of an option to a standard sensor assembly can deliver substantially improved measurement results in many cases. Share your temperature measurement requirements and challenges with a process measurement specialist. Leverage your own process knowledge and experience with their product application expertise.

Sunday, December 3, 2017

Controller Reduces Standby Cycling to Conserve Energy

gas fired boilers in boiler room
Improved control can reduce dry firing of boilers,
with substantial energy savings.
Heating of commercial and institutional buildings presents a case where there are energy savings available through the application of an advanced control element able to substantially reduce boiler operation time.

Building owners, boiler engineers, operators and other stakeholders will benefit from this simple and understandable video explanation of some of the inefficiencies associated with boiler operation, and how incorporating an additional control element can minimize boiler dry firing (also called standby cycling). Boiler operation costs can be reduced between 10% and 25%, with a commensurate reduction in carbon footprint, by including the Fireye NXM2G control in the boiler control system.

Watch the video. It's just a few minutes and explains the source of the inefficiency, as well as the solution, in a manner understandable to everyone. More information is available from a combustion product specialist, who can help evaluate the efficiency of your current system or assist with incorporating the latest energy saving features and design into a new installation.

Tuesday, November 14, 2017

Ultrasonic Clamp-On Flowmeter with SIL 2 Rating

SIL 2 capable clamp on ultrasonic flowmeter
FLUXUS F/G70X and F/G80X series meters
Image Courtesy Flexim
Measuring the flow quantity of gases and liquids is a common industrial processing task. There are numerous technologies available for measuring fluid flow, each with its own set of advantages and drawbacks for any particular application. Some of the technologies and methods have been in use for a very long time, with recent enhancements provided by electronics or smart sensor designs.

Ultrasonic flow measurement devices employ a comparatively recent technology to measure gaseous or liquid flow. Whether the transit time differential or Doppler method is utilized, ultrasonic flow meters have a distinctive characteristic in that they can be deployed in a form factor that does not require contact with the the process fluid, nor insertion in the fluid flow path. A common installation method is to clamp the ultrasonic transducer assembly onto the exterior of a process pipe. This makes the technology attractive for applications that involve adding a flow measurement point to an existing piping system.

Flexim, a globally recognized leader in ultrasonic flow measurement, offers a number of permanent and portable units for measuring liquid and gaseous flow rates. Some of their instruments have been certified as SIL 2 capable, along with a host of other third party certifications. The product range includes simple and accurate instruments designed for general industrial use, and extends to multi-beam units intended for applications, such as custody transfer of fluids, that require the highest accuracy and overall performance levels.

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


Thursday, November 9, 2017

Segmented Ball Rotary Control Valve for Natural Gas Regulation

segmented ball rotary control valve for natural gas
Segmented ball valve for natural gas regulation,
shown with Digital Natural Gas Positioner
Image courtesy GE Becker
The manufacturers of valves and other fluid control components for the processing industries have continuously developed new designs and innovated existing designs to deliver better performance for targeted operating conditions. The available basic valve designs, along with their variants, create an immense catalog of potential candidates for each application.

One such design variant is the segmented ball  valve. It is a quarter turn valve, like its cousin the ball valve, but the trim is different. True to its name, the active closure structure is but a portion of what we know of as a common ball valve. Where the closure mechanism of a ball valve essentially is a solid ball with a hole drilled through it, a segmented ball valve more resembles a section of a sphere or hollowed out ball with a shaped opening in the surface. A primary distinction between the two is that a ball valve contains a passageway through the diameter of the ball that rotates around a central axis. A segmented ball valve functions somewhat more like a gate valve and has no machined pathway through the closure, only a formed surface that restricts a portion of the fluid pathway.

The closure in a ball valve can be floating or trunnion mounted. A segmented ball valve will have a trunnion style mounted closure, with rigid support on opposing points of its diameter. Ball valves, with their rotating fluid pathway resembling a short tube, are generally not the best option for flow control, being better suited for isolation applications. The segmented ball valve functions similar to a sliding gate valve, providing an increasing or decreasing shaped opening as the shaft is turned.

GE Oil & Gas, under the Becker brand name, utilizes segmented ball valves for natural gas regulation applications. There are other industrial applications where this valve type can deliver superior performance and overcome a number of otherwise challenging conditions.

The brochure included below provides more detail on the segmented ball valves targeted for natural gas operations. There are some good illustrations that detail the valve construction. Share your fluid control challenges of all types with valve specialists, leveraging your own process knowledge and experience with their product application expertise to develop an effective solution.

Friday, November 3, 2017

Flexim Wins Recognition for Innovative Flow Measurement

ultrasonic flow meter
Flexim F704 Ultrasonic Flow Meter
Image courtesy Flexim Americas
Flow Control Magazine, which targets solutions for fluid movement, measurement and containment, handed out its annual Innovation Awards recently. Among those receiving honorable mention was Flexim Americas Corporation, for the Fluxus Cryo that provides noninvasive measurement of cryogenic fluids. Special design adaptations prevent ice build up on the measurement apparatus that that can plague other technologies.

Ultrasonic flow measurement offers some distinct advantages over other available methods, with high accuracy, no intrusion into the media, and no moving parts. While the award was specifically for a cryogenic application, Flexim ultrasonic flow measurement instruments are available for an extensive array of applications.

For more information, share your flow measurement requirements and challenges with process instrumentation experts, leveraging your own process knowledge and experience with their product application expertise to develop effective solutions.

Thursday, October 19, 2017

Magnetic Flow Meters: Principles and Applications

magnetic flow meter, magmeter, or flowmeter
Magnetic flowmeters are well suited for flow measurement
with conductive fluids.
Image courtesy Yokogawa
Fluid process control operations rely on the operator's ability to accurately determine qualities and quantities of liquid or gaseous materials. In terms of appraising and working with fluids (such as liquids, steam, and gases) the flow meter is a staple tool, with the simple goal of expressing the delivery of a subject fluid in a quantified manner. Measurement of media flow velocity can be used, along with other inputs, to determine volumetric or mass flow. The magnetic flow meter, also called a magmeter, is one of several technologies used to measure fluid flow.

In general, magnetic flow meters are sturdy, reliable devices able to withstand hazardous environments while returning accurate measurements to operators of a wide variety of processes. The magnetic flow meter has no moving parts. The operational principle of the device is powered by Faraday’s Law, a fundamental scientific principle stating that a voltage will be induced across any conductor moving at a right angle through a magnetic field, with the voltage being proportional to the velocity of the conductor. The principle allows for an inherently hard-to-measure quality of a substance to be expressed via the magmeter. In a magmeter application, the meter produces the magnetic field referred to in Faraday’s Law. The conductor is the fluid. The actual measurement of a magnetic flow meter is the induced voltage corresponding to fluid velocity. This can be used to determine volumetric flow and mass flow when combined with other measurements.

The magnetic flow meter technology is not impacted by temperature, pressure, or density of the subject fluid. It is however, necessary to fill the entire cross section of the pipe in order to derive useful volumetric flow measurements. Faraday’s Law relies on conductivity, so the fluid being measured has to be electrically conductive. Many hydrocarbons are not sufficiently conductive for a flow measurement using this method, nor are gases. On the other hand, water and aqueous solutions tend to exhibit sufficient conductivity to apply magmeter technology.

Magmeters apply Faraday’s law by using two charged magnetic coils; fluid passes through the magnetic field produced by the coils. A precise measurement of the voltage generated in the fluid will be proportional to fluid velocity. The relationship between voltage and flow is theoretically a linear expression, yet some outside factors may present barriers and complications in the interaction of the instrument with the subject fluid. These complications include a higher amount of voltage in the liquid being processed, and coupling issues between the signal circuit, power source, and/or connective leads of both an inductive and capacitive nature.

In addition to salient factors such as price, accuracy, ease of use, and the size-scale of the flow meter in relation to the fluid system, there are multiple reasons why magmeters are the unit of choice for certain applications. They are resistant to corrosion, and can provide accurate measurement of dirty fluids – making them suitable for wastewater measurement. As mentioned, there are no moving parts in a magmeter, keeping maintenance to a minimum. Power requirements are also low. Instruments are available in a wide range of configurations, sizes, and construction materials to accommodate various process installation requirements.

As with all process measurement instruments, proper selection, configuration, and installation are the real keys to a successful project. Share your flow measurement challenges of all types with a process measurement specialist, combining your own process knowledge and experience with their product application expertise to develop an effective solution.