Wednesday, June 29, 2016

On-Off Temperature Control Using PLC Ladder Logic

 Diagram of on / off control.
In control theory, an on–off controller is a feedback controller that switches abruptly between two states. It is often used as a control method for a process which can tolerate an ongoing, changing band of change, referred to as the hysteresis. A very common example for temperature are residential thermostats. They control the temperature of your home, turning off at your comfort setting, then after some significant change occurs, and they turn on again to eliminate that difference. The process cycles continually.

A common method of temperature control is an on/off control system using comparison instructions in a PLC program where outputs are energized until the set point is reached.

The video below provides a temperature control example where the heater turns on when the temperature falls to or below 597 degrees, and turns off when the temperature reaches 603 degrees or more.

To control the circuit, S1 is programmed in the heater output circuit. Addressed to the move instruction is a thermocouple that provides an analog value to the temperature. The temperature is moved from the source to the destination when S1 is activated and is displayed on the LED panel.

Using the less than or equal to, and greater than or equal to, instructions addressed to the same integer file the source values have A and B are compared to control the heater. With source a less than source be at the less than equal to instruction, the low temp and heater outputs are enabled. The heater remains on as long as the low temp output is true and the high temp output is false.

As the temperature rises above source B at the less than or equal to instruction, low temp turns off and heating continues. Reaching 603 degrees or more, the high temp output is enabled, since source A is equal to source B of the greater than equal to instruction.

When the high temp output is true, the heater turns off and remains of until the temperature reaches 597 or lower.  The cycle is repeated to maintain the average set point temperature at the other at 600 Fahrenheit.

Thursday, June 23, 2016

Comparing Displacer Transmitter Technologies for Liquid Level Measurement

Reprinted with permission from the white paper
of the same name courtesy of Magnetrol.

Process level measurement has greatly evolved over the years with new technologies. Instrumentation engineers have more demanding requirements that make it essential to have reliable and accurate liquid level measurements. Although based on a more traditional level measurement technology, one of the most trusted devices for continuous liquid level measurement remains the displacer level transmitter. This transmitter operates on the Archimedes principle of buoyancy, which holds that any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object. As liquid level moves upward on the displacer, the buoyancy force increases and there is a vertical motion that can be converted to liquid level using sophisticated software. The two main technologies that are used as displacer level transmitters in the industry are torque tubes and range spring/LVDT (Linear Variable Differential Transformer) technologies.

A torque tube uses a torsion bar that rotates relative to the weight of the displacer in fluid to correspond to a level reading. The range spring and LVDT combination uses an LVDT core that moves as the spring is unloaded by having fluid on the displacer, causing voltages to be induced across the secondary windings and be converted to a level reading.

There are substantial technology advantages that can make using range spring/LVDT based displacer transmitters preferable to a torque tube based instrument. These include improved accuracy, structural integrity, footprint, and maintenance capability.

Measurement Accuracy

The main technology advantage is a more accurate level output given by a range spring/LVDT displacer transmitter. Demonstrated through extensive testing1, the output of a range spring/LVDT displacer transmitter is 4 times more resistant to vibration, 6 times more linear, and 20 times more accurate than a torque tube. When monitoring the process level, it is important to have the most accurate level measurement, because inaccuracies can cause inefficiencies and cost end users money. The range spring helps dampen any vibration that may occur to the level instrument, maintaining a more stable output than a torque tube. The linearity and repeatability contribute to a consistent level output, ensuring the transmitter reads consistently throughout its operation. If there is any sticking in the transmitter, it could cause for the level reading to be different from the actual level.
 Vibration testing completed between Fisher 2500 Leveltrol and Magnetrol Modulevel. Linearity and Repeatability testing done between Fisher DLC3010 with 249BF torque tube and Magnetrol E3 Modulevel.
Structural Integrity

The structural integrity of the displacer transmitter is important to ensure minimal maintenance costs across the life of the instrument. There are two structural components that separate range spring/LVDT technology from torque tubes. The torque tube arm that holds the displacer is mounted on what is called a knife edge. This edge creates a stress concentration on the part, which can create wear over time to cause a failure. The range spring and stem attached to the LVDT core move vertically, completely eliminating the occurrence of friction or wear. The LVDT core is coated with a polymer material to provide smooth vertical motion, and the range spring is protected by a spring cup.

The other component that affects structural integrity is the thickness of the pressure boundary components. In a torque tube displacer transmitter, the torque tube itself serves as the pressure boundary component and has a thickness of 0.01”. In contrast, the enclosing tube for the range spring design is 0.035” thick. The increased thickness helps contain pressure and has more tolerance against any type of corrosion that would erode material away, since these components are exposed to the process media. Also, because the torque tube rotates, there is shear stress induced into the material, to which the enclosing tube for the range spring/LVDT design is not susceptible.

Installation Footprint

Space, while maybe not top of mind when picking a level sensing technology, can play an important role in the selection of the type of level transmitter. Some applications into which new level transmitters are being installed have space constraints and do not have the room to put a device with a large footprint. Torque tube and range spring/LVDT models have very similar sensing elements that use a displacer, but the difference is in the size and area of the transmitter heads. Torque tubes have a very large footprint to complete their level transmitter, which is also confined to a left-hand or right-hand display design. The left hand or right hand design can cause limitations in how it is wiring is configured, as the location of the electronics conduit is fixed. For range spring/ LVDT designs, a vertical and fully rotatable housing head is available to cover a smaller amount of area, and allow for easy wiring depending on where the wiring is coming from.

Maintenance

Lastly, most level transmitters that have moving mechanical parts will typically require maintenance to keep them operating at peak performance. Displacer transmitters are electro-mechanical devices that require some level of maintenance during their operating life.

It is critical, therefore, to consider long-term maintenance efficiencies in device selection. One key consideration is the minimization of wear points in the device design. As discussed above, torque tubes consist of a knife edge that the sensing element rotates on and can wear over time. Another key maintenance factor is downtime. Since the torque tube itself is part of the pressure boundary, if the transmitter requires maintenance, the pressure vessel will have to be de-pressurized. This creates downtime that negatively impacts end users’ bottom line costs. For range spring/LVDT technology, the pressure boundary component is not part of the sensing element. Therefore, if troubleshooting is required on the range spring/LVDT transmitter, it can be removed without having to de-pressurize the entire system. This equates to less downtime and maintenance cost savings.

Summary

In summary, there are benefits to choosing range spring/LVDT technology over torque tube technology. The chart below outlines the major differences:

Wednesday, June 22, 2016

Determining Accuracy in MFCs (Mass Flow Controllers)

Mass flow controller accuracy is often difficult to pin down because there is a variety of ways MFC accuracy and performance is stated.

Since manufacturers differ on how they state accuracy, having a basic understanding of the following is helpful when specifying a Mass Flow Controller. If you are in doubt, its always best to contact an applications expert for a consultation.

To properly understand MFC accuracy there are three variables to consider:
1. CMC (which stands for Calibration and Measurement Capability)
2. Repeatability
3. Linearity
CMC has a dependency on the equipment and processes used to test MFCs. Repeatability and linearity are variables inherent to the MFC.

CMC

In basic terms, CMC is a measure of how closely the calibration method represents absolute accuracy. Since no calibration equipment or method provides absolute accuracy, there is always an uncertainty greater than zero associated with CMC. CMC represents both the inaccuracy of calibration system components and any statistical variation during its use.

Repeatability

Repeatability is the MFC’s ability to repeat a flow measurement under the same conditions in a short period of time. If an MFC was used to create a specific flow rate repeatedly and in rapid succession without changing conditions, the resulting distribution of the flow measurement data points (beyond the variation in the CMC) represents the repeatability of the MFC.

Linearity

A linearity specification is required because all Mass Flow Controllers are nonlinear to some degree. To compensate, a curve-fit correction is applied to the MFC by collecting multiple data points during a calibration process and then calculating the curve fit equation. The linearity specification is a statement to the degree the curve-fit correction compensated for the MFC’s inherent non-linearity.

Each of these three variables contribute uncertainty to the accuracy of an MFC. The sum of these uncertainties represent the MFC accuracy.

Accuracy = CMC + Linearity + Repeatability

It’s important to understand that these three variables relate to the accuracy of the MFC itself. Other variables, namely long-term stability, temperature/pressure coefficients, process conditions vs. calibration conditions, and conversion factors, relate to, and affect, the actual process accuracy.

BCE
21060 Corsair Blvd
Hayward, CA 94545
Phone: (510) 274-1990
Fax: (510) 274-1999
www.belilove.com

Wednesday, June 15, 2016

Wastewater Treatment and Ammonia Concentration Sensing

 Selection of the proper Ammoniaand Nitrate sensor for clean water.
Ammonia is found naturally in water, but when found at higher than natural levels (even in very low concentrations) it is toxic to fish and other aquatic organisms. The controlled discharge of ammonia from wastewater treatment plants (WWTPs), large farms, and landfills is an important topic across the United States. Many states adopted new rules addressing total ammonia discharge. Selecting the proper sensor for accurate and repeatable ammonia and nitrate concentration is critical.

The HYDRA Ammonium Sensor is designed to monitor the nutrient load (NH4+) directly in the aeration basin of a Waste Water Treatment Plant. The HYDRA uses ISE technology to measure the ammonium, potassium and pH. Compensation for the pH dependent concentration equilibrium and potassium ion interference on the ammonium electrode are preformed automatically in the HYDRA C22 analyzer.