Thermal Dispersion Flow and Level Technology

Thermal dispersion level and flow
instruments (courtesy of Magnetrol)

Thermal dispersion instruments work on the basis of heat transfer. The sensing probe consists of two separate components, both RTDs (temperature detectors). One RTD is used as the reference point and measures the temperature of the fluid right where the probe is immersed. The second RTD is self-heated to a known temperature and maintained. A resulting a temperature differential is created between the two RTDs. By varying the power to the self-heated RTD, the set point can be changed which allows the user to set the instrument for a specific application.

Convective heat is the mechanism of heat transfer for thermal technology level switches based on the principle that a liquid has a thermal conductivity far greater than the thermal conductivity of its corresponding vapor. When the sensor is dry, there is a temperature difference between the two sensors. When fluid comes in contact with both RTDs, there is a cooling effect as the liquid absorbs the heat from the self-heater RTD. The resulting temperature differential drops, and creates a point for high level reference. When the level drops and the sensor goes dry, the temperature difference increases again. The instrument electronics senses the increase in temperature difference and provides a low level reference.

When used for flow applications, the temperature difference under a low flow or no flow condition is controlled by the set point. As the flow rate increases, the sensing RTD is cooled by the fluid moving past the heated sensor - the greater the flow, the greater the cooling. Conversely, the reduction in the temperature differential between the two RTDs indicates that the flow rate is exceeding the set point of the instrument.

Float Operated Level Switch Fundementals

Float Level Switch
(courtesy of Magnetrol)

Float operated level switches are suitable for use on clean liquid applications alarm, pump control and safety shutdown applications.

These float type units are typically designed, fabricated and certified to compliance with ASME B31.3 specifications.

The design of float operated level switches is based upon the principle that a magnetic field will penetrate non-magnetic materials such as 316 stainless steel. In the case of a float type level switch, the float moves a magnetic attraction sleeve within a non-magnetic enclosing tube which in turn trips an electrical switch mechanism. The enclosing tube of housing provides a pressure seal for the chamber as well as the process.

As the liquid level rises in the chamber (refer to Figure 1), the float moves the magnetic attraction sleeve up within the enclosing tube, and into the field of the switch mechanism magnet. Resultingly, the magnet is drawn in tightly to the enclosing tube causing the switch to trip, “making” or “breaking” the electrical circuit.

As the liquid level falls, the float drops and moves the attraction sleeve out of the magnetic field, releasing the switch at a predetermined “low level” (refer to Figure 2). The tension spring ensures the return of the switch in a snap action.

Measuring Flow - The Transit-Time Difference Method

Transit-time difference Method
(courtesy of FLEXIM)

The Transit-Time Difference method exploits the fact that the transmission speed of an ultrasonic signal depends on the flow velocity of the carrier medium.

Similar to a swimmer swimming against the current, an ultrasonic signal moves slower against the flow direction of the medium than when in flow direction.

The Measurement Principle

Diagram of FLEXIM transit-time
difference flow meter design.

For the measurement, two ultrasonic pulses are sent through the medium, one in the flow direction, and a second one against it. The transducers are alternatively working as an emitter and a receiver.

The transit-time of the ultrasonic signal propagating in the flow direction is shorter than the transit-time of the signal propagating against the flow direction. A transit-time difference, Δt, can thus be measured and allows the determination of the average flow velocity based on the propagation path of the ultrasonic signals.

An additional profile correction is performed by proprietary FLEXIM algorithms, to obtain an exceptional accuracy on the average flow velocity on the cross-section of the pipe - which is proportional to the volume flow.

Since ultrasounds propagate in solids, the transducers can be mounted onto the pipe.

The measurement is therefore non-intrusive, and thus no cutting or welding of pipes is required for the installation of the transducers.

Cycle Chargers - GlobalCharge Remote Power Systems by Global Thermoelectric

Cycle Chargers are the most fuel-efficient remote power system available today. They offer the highest efficiency of any prime remote power system and represent some of the toughest equipment on the market. Cycle Chargers are designed for long-term unattended operation, but can also be effective in support of grid supply. 

Global Thermoelectric, a leader in remote power systems, offers GlobalCharge - a fully self-contained remote power system offering continuous prime power from 300 Watts to 6000 Watts.

GlobalCharge is a self-contained remote power system that is ideal for applications where fuel consumption is a priority, or where only liquid fuel is available.

The technology associated with the charging system is proven as it has been used in specialized applications for over 15 years in the military, coast guard and border control. Global Thermoelectric has adapted this technology for use in industrial applications.

The result is a quiet, discreet product that operates reliably in areas where site access is difficult and site visits less frequent.

For more information contact:

M.S. Jacobs
800-348-0089
www.msjacobs.com

Magnetrol Hygienic Level Control Solutions

Here is a short video that illustrates the use of several level control technologies - guided radar level, ultrasonic level and thermal dispersion -  in hygienic applications.

The video shows us the benefits of each technology and where the Eclipse, Echotel and Thermatel controls are typically used.

For more information on level control in Western PA and West Virginia, contact MS Jacobs at 800-348-0089 or www.msjacobs.com

Clamp-on Ultrasonic Flow Measurement Advantages in Pipes and Storage Tanks

Clamp-on ultrasonic flow meter
(courtesy of Flexim)

In order to accurately pump and store fluids, reliable and accurate flow measurement is essential. Intrusive flow meters such as turbine meters and orifice plates are problematic when it comes to maintenance and long-term accuracy.

A better solution is a clamp-on, externally mounted, ultrasonic flow meter. These types of meters simply mount right to the outside of the process piping and do not expose the sensor to the process media. Nor do they require the pipe of vessel to be open for maintenance. Furthermore, they provide high accuracy and repeatability needed for leak detection and minimization of product losses.

Another advantage of using clamp-on flow meters is to use sonic velocity to determine what hydrocarbons are flowing through the pipe. Sonic velocity, when corrected for temperature change, is a good way to distinguish most hydrocarbons. By comparing the actual sonic velocity in a given application, and comparing it to known sonic velocity data, hydrocarbons at given temperatures can be identified.

This comes in particularly helpful in detecting interface changes. Different product batches are easily recognized by the meter. As interface changes are detected with a high degree of sensitivity, product mixups are significantly reduced.

Additional advantages of clamp-on, ultrasonic flow meters:

• No wear and tear - no clogging effects
• Quick and accurate detection of Interface changes - reducing amount of product mixing
• Accurate measurement on thick walled pipes and exotic pipe materials

Happy Holidays from M.S. Jacobs and Associates

On behalf of everyone at M.S. Jacobs and Associates, we wish you a very Happy Holiday Season and a prosperous New Year!