Thursday, March 26, 2015

Thermal Dispersion Flow and Level Technology

thermal dispersion instruments
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.

Sunday, March 15, 2015

Float Operated Level Switch Fundementals

Float Level Switch
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.

Monday, March 9, 2015

Measuring Flow - The Transit-Time Difference Method

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

transit-time difference method
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.