Level Measurement Techniques: Minimizing Guided Wave Radar Probe Buildup

Level measurement applications in natural gas, condensate and crude processing have some special requirements that are not always evident from instrument data sheets. The potential of solids or other materials building up on a guided wave radar probe is one example. The experience of Magnetrol® field engineers has led to the development of some simple but effective level measurement techniques to address field issues related to buildup that may not be evident in data sheets.

Level Measurement Techniques Radar Probe Buildup

A sampling of probes for the Eclipse® GWR Transmitter

Natural gas, condensate and crude processing applications can experience paraffin, asphaltenes, grit and grime. The degree to which any of these can accumulate on guided wave radar (GWR) probe buildup varies by application. Even in applications where buildup isn’t typically prevalent, it can happen over time, during cold weather periods, or when bringing units up or down due to temperature, pressure and process material fluctuations. Like distillation columns, chambers/cages/bridles may require cleaning from time to time. Below are some good practices that can minimize buildup and reduce maintenance time.

  • The use of enlarged coaxial GWR probes with more clearance reduces the chance for buildup to occur.
  • Consider using a chamber probe whenever possible. Magnetrol® offers a unique family of chamber probes, which combines the sensitivity and performance associated with coaxial probes with the viscosity immunity of a single rod.
  • Insulate the probe necks of overfill probes to reduce any cooling at the top of the probe inside the vessel, chamber, cage or bridle.
  • Chambers should be insulated even in warm weather locations. The temperature differential between a warm/hot vessel (like a separator) and uninsulated chamber/cages can be significant, resulting in paraffin deposition and/or viscosity increases.
  • Insulate chamber flanges to reduce any cooling at the top of the probe.
  • Use probes with integral flushing connection to simplify flushing/dissolving paraffin or grit. Flushing connections are an option available on all MAGNETROL coaxial GWR probes.

Eclipse Microsite

IMPROVE POWER PLANT EFFICIENCY WITH EFFECTIVE LIQUID LEVEL CONTROL

The global power generation industry is rapidly changing. Increases in power consumption, economic growth and environmental pressures are creating significant opportunities for safe and reliable plant operation.

Power Plant Efficiency Opportunities
Level control applications in power plants lend themselves to performance improvements that can enhance a plant’s overall safety, efficiency or profitability. Technologies offering more precise level indication that are not affected by process variables provide operators with the ability to better manage overall power plant performance. For example, feedwater heaters in coal-fired plants historically suffer from inefficiencies due to poor level controls, which increase heat rate, thus reducing efficiency. The illustration below indicates some of the most common level control applications in the power generation industry.

Liquid Level Applications

Power Plant Efficiency Opportunities

  1. Fuel Oil Storage: Crude oils with lower flash points represent a greater fire hazard and require safety-certified liquid level switches and transmitters.
  2. Open Atmosphere Sumps: Level control in collection and processing basins must often tolerate corrosive media, punishing weather conditions and liquids with high solids content.
  3. Condensate Storage: Accurate, reliable liquid level monitoring in the condensate storage tank ensures the proper supply of make-up water.
  4. Deaerator: Pressure fluctuations are extensive in the deaerator and result in flashing, thereby requiring level controls that can withstand varying temperatures and pressures.
  5. Condensate Drip Legs & Drains: Level instrumentation must contend with high temperatures and pressures associated with drip legs, to ensure proper functioning of the condensate collection system and prevent damage to the turbine.
  6. Steam Drums: Precise level in the steam drum is important to optimize steam/water separation and steam quality.
  7. Condenser Hotwell: Level control in the hotwell can prevent make-up water loss in the turbine cycle due to leakage, steam venting or other usage.
  8. Feedwater Heaters: Feedwater heater level is controlled to prevent damage to expensive hardware, while at the same time optimizing level control to improve efficiency (heat rate) during base load, as well as load following operations.
  9. Boiler Blowdown Tank: Good boiler blowdown practices reduce water treatment needs and operation costs, as well as the chance of catastrophic explosion.
  10. Lubrication Oil Tanks: Reliable level monitoring of lube oil reservoirs ensures proper functioning of turbines, electrical generators and other equipment with integral lubrication systems.
  11. Ammonia/Caustic/Acid Storage: Managing hazardous and non-hazardous chemical storage inventory and replenishment safely and reliably is critical to ensure availability during normal operation.
  12. Cooling Tower Basin: Proper level control in the cooling tower basin eliminates low-level damage to pumps, while preventing costly overflow conditions. Vulnerability to foam from chemical injection and modest buildup considerations are fundamental to selecting the correct level technology.

Liquid Level Control Solutions
Magnetrol offers the power generation industry one of the most complete lines of liquid level control technology solutions, as well as extensive power plant efficiency applications experience for challenging process control environments.

  • Series 75 Sealed External Cage Liquid Level Switch: Self-contained units designed for side mounting to a tank or vessel with threaded, socket weld or flanged pipe connections.
  • Series B40 High-Pressure/High-Temperature Liquid Level Switch: Specifically designed for HPHT service conditions such as boilers, available in rugged industrial or ASME B31.1 construction.
  • Tuffy® II Side-Mounted Float Switch: Compact-sized, float-actuated device for horizontal mounting in a tank or vessel through threaded or flanged pipe connections.
  • Digital E3 Modulevel® Displacer Transmitter: Advanced, intrinsically safe two-wire instrument utilized buoyancy principle to detect and convert liquid level changes into a stable output signal.
  • Displacer Type Liquid Level Switch: Offering a wide choice of alarm and control configurations, these units are well suited to simple or complex applications, including foaming or surging liquids and agitated fluids, and typically cost less than other level switch technologies.
  • Eclipse® Model 706 Guided Wave Radar Transmitter: Loop-powered, 24 VDC device utilizes diode-switching technology for outstanding signal strength, and offered with a comprehensive probe offering for a wide variety of applications.
  • Pulsar® Model RX5 Pulse Burst Radar Transmitter: The latest generation of non-contact radar transmitters offers lower power consumption, faster response time and ease of use, compared to most loop-powered radar transmitters.
  • Model R82 Pulse Burst Radar Transmitter: An economical, loop-powered, non-contact radar transmitter that brings radar performance to everyday applications.
  • Magnetic Level Indicators: AtlasTM and Aurora® magnetic level indicators offer reliable visual indicator solutions, with or without accompanying continuous level transmitters.
  • Jupiter® Magnetostrictive Transmitter: High accuracy and high linearity at a reasonable price.

Power Plant Efficiency

 

CHEMICAL STORAGE MONITORING

On a recent visit to evaluate several level transmitters on chemical storage applications at a new combined cycle power plant, I had the opportunity to work with the EPC firm, plant personnel and the chemical supplier. This provided an interesting look at the level instrumentation package for the chemical storage side of things from an engineering perspective, as well as from a daily operations point of view.

AMMONIA, ACID AND CAUSTIC CHEMICAL STORAGE
Although important measurements, the ammonia, acid and caustic storage tanks are not difficult level applications for Magnetrol® and Orion Instruments® by any stretch. However, I found that small nuances on how the applications are monitored relative to technology can have a dramatic effect on the day-to-day practicality and reliability of the type of instrument(s) used. Additionally, there are safety considerations when replenishing these chemicals, which can be addressed simultaneously with inventory monitoring by implementing a few simple, cost-effective modifications to the instrumentation package.

Chemical Storage MonitoringThese chemical storage tanks can be horizontal or vertical vessels six to ten feet in diameter/height, with the ammonia storage tank usually the largest of the three. It is not uncommon to see some type of non-contact level transmitter (Ultrasonic being the most prevalent) installed to provide level indication to the control room with a local display at the base of the tank, either in series with the 4-20 mA transmitter output or repeated from the control room. The signal to the control room tracks inventory, acts as a high alarm for overfill protection and establishes the resupply interval. The local display facilitates monitoring the offload of chemicals from the supplier’s truck.

There are a number of level technologies you could throw at these applications. My preference is Through-Air (Non-Contact) Radar or Guided Wave (Contact) Radar on the acid and caustic tanks and the latter for the ammonia storage. This doesn’t imply non-contact Ultrasonic Level transmitters or other technologies are not up to the task. Simply put, radar is indifferent to the changes in the contents of the vapor space of these vessels occurring throughout the course of the day. Oftentimes, these changes affect the Ultrasonic burst causing what I refer to as a nuisance alarm, e.g., it loses the signal intermittently or the level indication becomes erratic only to recover about the time a technician arrives on the scene. These types of issues are difficult to isolate since they are intermittent and not linked to an installation or configuration anomaly or hard fault in the instrument. If a competitively priced technology that does not require calibration and is unaffected by changing process conditions is available, I take that path. Radar meets these three criteria.

PERFORMANCE PERSPECTIVES FROM THE I&C TEAM
Discussions with the I&C technician and chemical supplier during the evaluation process were confined to reliability, remote indication and performance verification of the level transmitter. Both preferred an independent visual indication along the lines of a Magnetic Level Indicator (MLI) on all three tanks. From the technician’s perspective it allows for easy verification of the level transmitter’s performance and adds a layer of redundancy in case the transmitter was out of commission for whatever reason. I have a lot of confidence in our instrumentation hitting the mark right out of the gate, but I have to admit it is nice to have a second opinion on the reading following initial commissioning. Sticking these tanks is usually not an option during normal operation.

LEVEL GAUGE READABILITY DURING PRODUCT TRANSFER
The chemical supplier’s insight focused on readability during the transfer of product from the truck to the tank. Even though he normally offloads a fixed quantity of material which the vessel should accommodate based on level indications prior to dispatch, knowing the level during the transfer process is a safety measure to prevent overfilling. His comment was that MLIs can be read easily from a distance with the occasional glance while managing other tasks, whereas, he had to be on top of an LCD-type display to monitor progress. This was particularly important when working with the ammonia storage tank.

CONSIDERATIONS WHEN TOP FILLING
One item worth pointing out that would simplify the commissioning of the instrument is the close proximity of the top-fill piping to the instrument mounting nozzle on the smaller acid and caustic tanks. Since non-contact technologies are ubiquitous on these applications and rely on projecting a circular or elliptical signal footprint perpendicular to the surface of the material being measured, locating the instrument nozzle as far away from the turbulence generated while top filling is the ideal. Furthermore, such close proximity allows the spray pattern created as the chemical enters the vessel to interact with the transmitted beam, which could cause a loss of signal during the fill process leaving the supplier blind as to the remaining space in the vessel. Tweaking the instrument’s configuration to overcome such obstacles is possible with time and patience. On the flipside, separating these two entry points during the design phase of the vessels would eliminate any potential problems without adding cost or extending the commissioning time.   The present nozzle/fill line configuration is another point to argue the case for an MLI or opting for Guided Wave Radar technology to eliminate any potential interference and excessive turbulence near the transmitter.

After surveying each application and visiting with the I&C department and chemical supplier, I reviewed the various options with the engineering team. Our collaboration yielded three options to improve the reliability and enhance the day-to-day functionality of the instrumentation package.

THREE OPTIONS TO IMPROVE RELIABILITY AND FUNCTIONALITY
Chemical Storage Monitoring 1
The easiest modification was to replace the originally specified Ultrasonic technology with an entry level Through-Air Radar (MAGNETROL Model R82), an easy fix to improve reliability by eliminating the vapor space issues noted above without adding to the overall cost. The balance of the inventory management scheme remained the same, i.e., remote LCD indication and so forth.

Taking things a step further, we considered incorporating a Guided Wave Radar (MAGNETROL Eclipse® Model 706) in place of the non-contact technologies. The Guided Wave Radar would add cost to the instrument itself as compared to an Ultrasonic or entry level Through-Air Radar. However, if we leverage its remote transmitter option in lieu of purchasing an independent remote indicator, we can offset most of the additional cost by eliminating the secondary display and its associated costs: wiring, mounting, configuring, etc. Another benefit to this approach is more flexibility in the instrument nozzle mounting location relative to the vessel top fill piping arrangement. The Guided Wave Radar is a contact measurement whose sensing element can ignore turbulence and the chemical spray pattern previously mentioned. This added benefit not only simplifies commissioning of the instrument, but separates the measurement from the tank dynamics for improved reliability.

Lastly, we looked at including some of the “wish list” items the I&C technician and chemical supplier noted and added a visual indicator (ORION INSTRUMENTS Atlas™ or Aurora® models) for level verification, redundant and diverse measurement technologies and improved readability during normal plant operation. This approach does add cost to the instrumentation package even when taking into account the elimination of peripheral items included in a minimal installation. Aside from the ammonia storage tanks, which traditionally have the process connections in place to accommodate an externally mounted device, adding similar process connections to the acid and caustic tanks would bump up the cost of the vessel as well. In the grand scheme of things, the additional costs are minor compared to the long-term benefit, but it is something that needs to be taken into consideration for chemical storage monitoring.

Improving Solar Power Efficiency Through Level and Flow Control

Solar technologies use the sun’s energy to provide electricity, hot water, process heat and cooling. According to the U.S. Energy Information Administration, solar power presently provides less than 1% of U.S. energy needs, but this is expected to increase with the development of more efficient solar technologies. One way to enhance solar power efficiency is through the use of level and flow instrumentation to drive process improvement.

TYPES OF SOLAR COLLECTORS

Solar Power EfficiencyDifferent solar collectors meet different energy needs. Passive solar designs capture the sun’s heat to provide space heating and light. Photovoltaic cells convert sunlight directly to electricity. Concentrating solar power systems focus sunlight with mirrors to create a high-intensity heat source, which then produces steam or mechanical power to run a generator that creates electricity. Flat-plate collectors absorb the sun’s heat directly into water or other fluids to provide hot water or space heating.

SOLAR LEVEL AND FLOW APPLICATIONS

Heat Transfer Fluid Storage: Large-scale solar collectors for electric power generation require a heat transfer fluid (water, thermal oils, or ionic liquids) to absorb the sun’s heat for generating steam. Arrays of mirrored panels convert the sun’s energy into +750° F (+399° C) thermal energy that’s hot enough to create steam for turbines. The mirrors focus sunlight onto pipes of heat transfer fluid that run along the mirror’s centerline. The fluid then boils water to produce steam. Thermal fluids also help provide hot water and heat. Thermal fluids are typically stored in pressurized tanks that require level monitoring.
Recommended Continuous Level Technologies: Displacer Controller, Guided Wave Radar
Recommended Point Level Technologies: External Cage Float

Hot Water Storage: High-temperature solar water heaters provide energy-efficient hot water and heat for large industrial facilities. Thermal storage in buffer tanks provides interfaces between collector subsystems and energy-using systems. The preferred solar storage vessel is a vertical cylindrical tank designed for the maximum pressure of the supply water source, which may be as high as 150 psi.
Recommended Continuous Level Technologies: Displacer Controller, Guided Wave Radar
Recommended Point Level Technologies: External Cage Float

Pump Protection: Flow switches protect pumps from damage due to leaks or if a valve is accidentally closed downstream. A flow switch will actuate an alarm and shut down the pump when flow drops below the minimum rate.
Flow Alarm: Thermal Dispersion Flow Switch for High/Low Alarm, or Flow Switch

LEVEL AND FLOW INSTRUMENTATION FOR SOLAR POWER EFFICIENCY

Instrumentation_for_Solar_power_efficiency

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Level Control’s Impact on the Efficiency of Wind Turbines

Wind energy is one of the fastest-growing forms of electricity generation in the world. U.S. wind power market share is expected to reach 3.35% by 2013 and 8% by 2018. More optimistic industry experts predict that wind energy will meet 20% of the nation’s energy needs by 2030. As supply and demand for wind power grows, so will the need to increase the efficiency of wind turbines, to drive costs out of the process and deliver cost-effective renewable energy solutions.

Wind Energy Systems

Large wind conversion systems are most commonly deployed for power grid electricity generation. Smaller systems are used for water pumping. A system of blades mounted on a tower is turned by the wind to either produce mechanical work directly (via a water pump), or to employ a generator to transform mechanical work into electrical energy (wind turbines). Utility-scale wind turbines for land-based wind farms have rotor diameters ranging from 165 to 325 feet (50 to 100 meters).

Wind Turbine Level Applications

efficiency_of_wind_turbines_image2WIND TURBINE OIL RESERVOIR: As wind energy technology advances, higher demands are placed on turbine lubrication systems. Lubricant reservoirs of up to 550 gallons (2,000+ liters) serve as oil storage in centralized systems to provide lubrication for the blade bearings, blade tilt, main bearing, azimuth bearing, meshing gears, generator bearings, cylindrical gears, bevel gears, rolling and sliding bearings, worm gear units, and gear couplings. The oil reservoir is monitored using continuous or point level.
Recommended Continuous Level Technologies: Guided Wave Radar, Ultrasoni
Recommended Point Level Technologies: Float Actuated, Ultrasonic

WIND TURBINE GEARBOX: Gearbox and bearing lubrication are of particular importance due to the complexity of the gearbox and the high mechanical loads. Gearbox and bearing problems are a common cause of downtime, and loss of oil through a small leak has led to catastrophic wind turbine failures. Along with vibration, temperature, and flow sensors, a low level gearbox oil alarm is a critical safety control.
Recommended Point Level Technologies: Float Actuated, Ultrasonic

Water Pumping Level Application

WATER PUMPING STORAGE: For industrial and agricultural use, a water pumping windmill is typically placed above a well or near a river. Next to the mill a storage tank is placed to provide a buffer supply of water for when the mill is not operational. Ferro-cement and steel tanks are typically used.
Recommended Continuous Level Technologies: Guided Wave Radar, Displacer Controller, Pulse Burst Radar (Through Air), Ultrasoni
Recommended Point Level Technologies: Float Actuated, Ultrasonic

Instrumentation for Wind Energy Applications

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Improving Biopharmaceutical Process: Level Control of Caustic Soda Solution

A Magnetrol Application Study:
A biopharmaceuticals company needed an accurate and repeatable level measurement of caustic soda solution, which was used to control the biopharmaceutical process of continuous fermentation.

The application had been originally equipped with a competitor’s float system, and due to the typical “lift-off” issues that accompany those systems, it only had a measurable level of 5″ (13 cm) above the bottom of the storage tank.

Because the storage tank was only 27.5″ (70 cm) high, and any leftover solution had to be discarded, it resulted in both wasted process media and loss of yield during fermentation.

The product temperature was about +77° F (+25° C), with a pressure of 18.9 psi (1.3 bar). The container was periodically sterilized at +249.8° F (+ 121° C) for 60 min.

Biopharmaceutical_Process_TankProblem:
Since the continuous fermentation was controlled by the addition of caustic soda solution, fermentation stopped when the storage tank was almost empty and the measurable level was near 5″ (13 cm).

Since the tank was not completely emptied after completion of a cycle, every additional liter could be seen as an increase in the overall yield.

The installation position of the level measurement was fixed; and to further complicate things, a non-standard flange was required.

Until recently, the customer was unsuccessful in finding a suitable measurement technology that would offer an increase in the yield or measurement range while utilizing the required installation location.

Solution:
By utilizing an Eclipse® Hygienic Model 705 guided wave radar transmitter (GWR) with a single rod X7MF probe, the customer now has a solution for accurate biopharmaceutical process control.

The GWR probe was manufactured with a special flange to meet the installation requirements. This allowed for simple replacement of the Eclipse GWR transmitter on the vessel without modifications.

In order to ensure an increase in yield, the probe was bent by the customer at approximately 50% of the rod length at 45°. This allowed measurement down to the lowest point of the tank, which led to a significant increase in yield.

Furthermore, using the 20-point strapping table in the GWR transmitter, the vessel was accurately calibrated for tank volume.   This brought an additional advantage because the customer was then able to operate more efficiently, resulting in better control of the fermentation process.

Result:
By using ECLIPSE GWR technology, the customer has increased yield by about 20%.

Considering the cost of fermentation and production cycle of about one batch per week, the customer realized a payback period of less than two months.

Visit the Eclipse Model 706 microsite now!

Key Questions Answered About Thermal Dispersion Flow Meter Technology

For technical information about thermal dispersion flow meter technology, Flow Control magazine’s thermal mass flow measurement technology portal provides a comprehensive resource. Tom Kemme, our thermal dispersion product manager, answers questions about the technology in the digital portal’s Ask the Expert column. This week’s blog shares some recent Q&As.

magnetrol_ta2_thermal_dispersionQuestion: What kind of pressure drop can I expect across a thermal dispersion flow meter?

Answer: Most thermal flow meters you’ll find are insertion probes, and the outside diameters are typically less than one inch. Very little pressure drop can be expected in comparison to other technologies that may block more of the flow path or need to impose a pressure drop in order to make a flow measurement.

Question: What type of configuration is needed in the field when a thermal flow meter is being installed?

Answer: This will vary by manufacturer, but typically the only thing needed in the field is to wire the unit up and install it into the pipe. Thermal dispersion flow meters are calibrated by the manufacturer for the individual application. This way the 4-20 mA output, pipe size and type of gas is already taken into account. There are always ways to communicate with the instrument if needed, but it will vary what configurations can be changed. Some suppliers have a display that the user can operate; sometimes, external hardware/software is needed. There are many settings that can be accessed and modified in our unit, although typically this isn’t needed.

Question: The rotatable housing—why is it needed, and how do I do it?

Answer: Thermal mass flow meters measure flow in one direction. When the flow meter is being installed, the flow arrow on the probe should be in the direction of the flow in the pipe and parallel to the piping. At Magnetrol®, we calibrate with flow going from left to right. Depending on the application, the display may not be facing the user or they would be inclined to install with the flow arrow in the wrong direction in order to see the display. The NPT threads connecting the probe to the housing did not allow for rotation. With the straight threads we use, now the user can loosen a set screw at the bottom-front of the housing and rotate the head to the desired location, and then tighten it back down after rotation. A standard 2.5mm Allen key can be used. There is a second set screw in place to prevent over-rotation of the housing. The rotatable head allows the user to place an order without having to specify direction of flow, simplifies installation and adds ease-of-use when working from the transmitter display.

Flow Measurement and Instrumentation

Guided Wave Radar Versus Load Cells For Cell Culture Process Measurement

A Magnetrol® Applications Study:

Fermentation, widely used to process food, biochemicals, beer, wine and spirits, or ethanol, is also employed to produce a wide range of biopharmaceutical products. Biotechnology’s unique signature in making medicines is to utilize proteins for making drugs rather than the chemicals of traditional pharmaceuticals. Living cells—either bacteria or more complex mammalian cells—serve as micro-factories that can manufacture the appropriate proteins. Bioreactors cultivate animal cells. Fermentors cultivate microbes. Cell culture processing is a specific kind of fermentation that applies similar techniques for growing cells from living microbes.

Our customer, a U.S. biotech company, relies on cell culture fermentation to make leading-edge medicines. Their cell culture process largely consists of transferring 98% of a tank’s contents to another tank and topping off the 2% with new media. Level transmitters ensure that this process is accurate.

Because biotechnology companies produce complex molecules for human use, there is need to monitor and control all aspects of this complicated process to ensure safe, viable and consistent products. Any means that can create new economies in large-scale production without diminishing established levels of quality are highly desirable, which is what happened when hygienic Eclipse® guided wave radar (GWR) devices were implemented during a recent company expansion.

cell_culture_processIn planning the new cell culture fermentation units for the expansion, the cell culture processing system was given a stringent review.

The previous system utilized load cells. However, the load cells and the yearly calibration verification turned out to be unreasonably expensive. That’s when MAGNETROL proposed replacing the load cells with hygienic ECLIPSE GWR transmitters in 304 stainless steel housings and Model 7EF-E hygienic probes. How can GWR technology replace load cells?

Load cells measure mass. In addition to level, ECLIPSE GWR technology measures volume. For ECLIPSE transmitters to measure volume, our customer’s tanks were first strapped, which is a procedure of precise physical measurement of the tank geometry and calculation of capacity tables. Since the density was precisely known, our customer could then multiply the volume by the specific gravity to obtain an inferred mass. This is how volume measurement obtained through the ECLIPSE device was able to replace a load cell’s mass measurement.

Our customer was impressed that the replacement resulted in benefits to the bottom line, because the upfront cost for the ECLIPSE units was 75 percent less compared to load cells. In addition, yearly dry calibration verification costs were 10 percent lower than the cost required to calibrate load cells.

However, based on the fact there was not an installed base and history of GWR technology at this facility, our ECLIPSE solution was first installed on a trial basis. Our customer’s requirement was to accurately measure down to the last 15.8 gallons (60 liters) of the 990-gallon (3,750-liter) vessel in order to obtain the ROI needed to fund the project. To meet this goal, a hand-fabricated, curvilinear ECLIPSE GWR probe, which extended down the vessel’s sidewall and inclined toward the very bottom outlet valve, was used.

Six ECLIPSE GWR probes were cut and shaped on-site so that the tip of the probe came as close as possible to the outlet on the dished vessel. After strapping and calibration, the probes accurately measured down to the last 8 gallons (30 Liters), resulting in control performance that was twice the goal outlined by the biotech company.

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Direct Measurement of Mass Flow Rate in Industrial Process Operations

Increasingly, industrial process operators are recognizing the advantages of the direct measurement of mass flow rate for monitoring gases. The following article discusses the difference between volumetric flow and mass flow measurement for gas control applications, and is excerpted from the Magnetrol® Thermal Dispersion Mass Flow Measurement Handbook.

An Introduction to and Benefits of Thermal Dispersion Mass Flow Measurement
Accurate mass flow measurement of gas is difficult to obtain. The main reason is that gas is a compressible fluid. This means that the volume of a fixed mass of gas depends upon the pressure and temperature it is subject to

Consider a balloon containing one actual cubic foot of gas at room temperature (70° F) and atmospheric pressure. An increase in the room temperature causes the balloon to expand. An increase in the pressure surrounding the balloon results in a decrease in volume. Although the volume of the balloon changes with variations in pressure and temperature, the mass of the gas inside the balloon has remained the same. This illustrates how pressure and temperature affect the actual volume

There are many well-established methods of measuring the actual volumetric flow rate. However, the measured flow rate will vary with changes in temperature and pressure. For virtually all industrial process operations, the user wants to measure the mass flow rate instead of the actual flow rate. Chemical reactions work on the basis of mass relationships of ingredients. Combustion is based upon the mass flow rate of the air and the fuel. Gas consumption in a facility is based upon mass flow rate. To accurately measure mass flow, the actual flow rate must be adjusted to correct for any change in temperature and pressure.

Thermal mass flow technology is a method of gas flow measurement that does not require correction for changes in process temperature or pressure. Thermal mass flow technology also has a benefit of measurement at low velocities and greater turndown capabilities than those obtainable with other flow measurement devices. Turndown is the flow range for which the device is accurate (maximum flow / minimum flow).

What is Mass Flow Rate?
Mass Flow is the measurement of the flow rate without consideration of the process conditions. Mass flow is equivalent to the actual flow rate multiplied by the density. M = Q x ρ, where Q is the actual flow and ρ is the density. As the pressure and temperature change, the volume and density change, however the mass remains the same.

To obtain standardization of gas flow measurement, Standard conditions of Temperature and Pressure (STP conditions) are utilized. Gas flow measured at STP conditions is corrected from the actual process conditions to standard conditions (more information about standard versus actual process conditions can be found in our Thermal Dispersion Mass Flow Measurement Handbook).

The simplest way of measuring mass flow of gas is in units of cubic feet per minute or cubic meters per hour, corrected to STP conditions. This is referred to as SCFM (standard cubic feet per minute) or the metric equivalent of Nm3/h (normal cubic meters per hour). The density of a gas at standard conditions is known, thus providing a relationship between SCFM and pounds per hour or between Nm3/h and kg/h.

The conversion between the volume at actual conditions and the volume at standard conditions is based on the ideal gas law — actual volume increases in direct proportion to an increase in absolute temperature, and decreases in direct portion to an increase in absolute pressure. Consider the balloon example — as the temperature increases, the volume expands; as the pressure increases, the volume shrinks.

Absolute pressure of zero psia (pounds per square inch at absolute conditions) is a perfect vacuum. One atmosphere of pressure is defined as 14.69 psia or zero psig. The conversion between psia and psig is easy: PSIA = PSIG + 14.69. If you have a pressure gauge calibrated for psig, it will read zero at sea level and only measure gauge pressure above atmospheric pressure. The following chart will help clarify this.

Measuring Mass Flow Rate

Absolute zero is defined as the temperature where molecular motion stops. It is defined as 0 K (Kelvin) which is -273.16° C or 0° R (Rankine) which is -459.67° F. To convert between actual temperature and absolute temperature, simply add 460 to the temperature in degrees Fahrenheit or 273 to the temperature in Celsius.

Once we establish a set of conditions as a standard temperature and pressure (STP conditions), we can convert between the flow rate at actual conditions and the flow rate at standard conditions.

Mass Flow Rate Measurement

The subscript (a) refers to actual conditions; the subscript (s) refers to standard conditions.

Unfortunately, not all STP conditions are universal. Many users consider one atmosphere and 70° F as STP. Some industries use one atmosphere and 60° F as standard; others use one atmosphere and 32° F as standard. The metric equivalent is Normal conditions which are based on a pressure of one bar (14.5 psia) and 0° C.

The important issue is that Standard Conditions are not Standard and a mass flow meter needs to be able to permit the user to select the desired STP condition. An error of approximately 8% will occur if there is a difference in STP conditions between 70° F and 32° F.

Once a set of standard conditions is identified, the density of that gas at these conditions is known. Therefore, it is a simple matter to convert from SCFM to mass in pounds per hour:

Mass Flow Measurement

In this formula, the density in pounds per cubic foot is the density at the specified STP conditions.



Mass Flow Rate eBook

It’s Official! Magnetrol® Marks New Headquarters Opening

Despite the gray weather, the outlook for Magnetrol® International was 100% sunny during its official ribbon-cutting ceremony and open house, celebrating the grand opening of the company’s new corporate headquarters and manufacturing facilities in Aurora, Illinois, USA.

On July 12, MAGNETROL employees, families, friends and special guests gathered for the ribbon-cutting ceremony and open house. In all, 661 were on hand to watch the ribbon cutting, including Illinois Congressman Bill Foster, Illinois State Senator Linda Holmes and Aurora’s Assistant Chief of Staff Chuck Nelson. Fourteen MAGNETROL officers and directors cut the ribbon, which was held by two veteran MAGNETROL associates, Crys Fiedler (39 years) and Pepe Silva (37 years).

Asked about their role in the ribbon-cutting, Mr. Silva said, “I was very proud to be chosen to participate in the ceremony,” with Ms. Fiedler adding, “It was an honor to be asked.”

MAGNETROL President and CEO Jeff Swallow emphasized that creating the new corporate headquarters demonstrated a significant team effort. “Who would have thought that this company, which was started 83 years ago in a garage in St. Louis, Missouri, would be celebrating another start in a state-of-the-art, 231,000 square-foot, new corporate headquarters?” said Mr. Swallow. “Certainly not me. But here we are.”

“That’s what makes this company great,” added Vice President and Chief Operating Officer John Heiser. “It’s the perseverance, it’s the passion and it’s the drive that each of our associates, each one of our members of this company, each one of our partners has, that makes us what we are today.”

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