CAS Newsletter April 2011
IN THIS ISSUE
Floats & Displacers
Floats and Displacers are simple level measurement devices. They are somewhat identical in their look but they work on different operating principles.
Float level switches work upon the buoyancy Principle according to which as liquid level changes a (predominately) sealed container will, providing its density is lower than that of the liquid, move correspondingly. In other words, the buoyancy principle states that the buoyancy force action on an object is equal to the mass of liquid displaced by the object.
Displacers operation is based upon the Archimedes Principle which says that when a body is immersed in a fluid it loses weight equal to that of the fluid displaced. By detection of the apparent weight of the immersed displacer, a level measurement can be inferred.
Displacers and floats are strictly applied for level detection in case of moderately non-viscous and clean process liquids. They present their best operation in switching applications and over for small periods. One can achieve spans of up to 12m also, but in that case their use happens to be extremely costly.
Float Level Switches
Float level switches are mainly employed for level measurement in narrow level differential fields, for example high level alarm or low level alarm applications. One of the significant types of float is a magnetrol float level switch which consists of a plain float and operates via a magnetic coupling action. The switch is designed in such a way that some part of float remains submerged in the liquid as it rides on the liquid surface. The float goes up and down on the surface depending upon the level of fluid in the tank. This causes a magnetic sleeve to travel in or out of the region of a magnetic switch resulting in its activation. A non-magnetic tube is also provided in the design which acts as a barrier and helps in separating the switching arrangement from the controlled fluid. Float level switches exist in diverse shapes such as spherical, cylindrical and many other forms as shown in the figure below.
These float based level switches include: a magnetic piston, a reed switch and a mercury switch. Among different float switch designs, the oldest and most precise one employed for continuous level detection is the tape level gauge. Float level sensors are usually prepared from materials like stainless steel, PFA, Hastelloy, Monel, and several other plastic components. It is always required of floats to have their weights less than the minimum likely specific gravity of the liquid being measured. There are basically three kinds of Float level controls which are listed below:
Figures indicating Top mount and Side mount operating principles are shown below.
Top Mount Operating Principle
Side Mount Operating Principle
An extensive choice of float level switches is accessible in the market which may include mercury, dry contact, hermetically sealed and pneumatic switching devices. The upper temperature and pressure limits of float level switches are +1000° F and 5000 psig respectively. They usually work with low specific gravities which can be around 0.32. They exist in variety of models such as single, dual and three switch models. Besides, for level detection of interfaces created between two fluids, customary float rides are available. Float operated control valves are also available which basically perform combined functions of level detection as well as level control via a single level controller. However, their use is limited to areas involving small flows with negligible pressure drops only.
In a typical displacer switch design, a spring is provided which is burdened with weighted displacers. The displacers having weights greater than the process fluid gets submerged in the liquid resulting in a buoyancy force change. This will cause a variation in the net force operating on the spring. In general, the spring will compress with the raise in buoyancy force. Just like the float level switches, a magnetic sleeve and a non-magnetic barrier tube is also incorporated in displacer switches. The magnetic sleeve is attached to the spring and it moves according to the spring movement resulting in activation of switching mechanism. An in-built limit switch is provided in the design which proves useful in level surge conditions since it keeps a check on the over stroking of the spring. The operating principle of a typical Displacer switch is illustrated in the figure below.
Displacer switches are most commonly employed in oil and petrochemical fields as level transmitters and local level controllers. These switches offer extremely correct and consistent measurement results in applications where clean liquids having stable densities are concerned. They are particularly not appropriate for slurry or sludge type applications since coating of the displacer causes a change in its volume and a resulting change in its buoyancy force. Temperature adjustments should also be done for these switches, specifically in areas where changes in process temperature can significantly affect the density of the process liquid.
The performance of displacers can be influenced by non-stability in process density in view of the fact that the displacement i.e. the weight loss of the material is equivalent to the weight of the liquid dislocated. As soon as the specific gravity of the process varies, the weight of the displaced material also varies accordingly, resulting in a change in the calibration. Due to this, one can specifically face problems in cases of interface level detection between two liquids having different densities, where the relative signal depends upon the difference between two densities. An important requirement while working with displacers is that even after commissioning, the liquid being detected must retain its density for getting good repeatability.
Following are the major advantages associated with the use of floats and displacers:
Floats v/s Displacers
Following are the major points of distinction between floats and displacers:
Ultrasonic Level Measurement
Level measurement can be performed via ultrasonic or sonic technology too. Ultrasonic level measurement devices basically employ sound waves for detection of liquid level. They usually work over the frequency range between 20 kHz to 200 kHz.
The working principle of a typical Ultrasonic level sensor is illustrated in the figure below.
In this design, the level sensor is located at the top of the tank in such a way that it sends out the sound waves in the form of bursts in downward direction to the fluid in the tank under level measurement. As soon as the directed sound waves hits the surface of the fluid, sound echoes gets reflected and returned back to the sensor.
The time taken by the sound wave to return back is directly proportional to the distance between the piezo electric sensor and the material in the tank. This time duration is measured by the sensor which is then further used to calculate the level of liquid in the tank. The speed of the sound waves can sometimes be affected due to variations in temperature for which appropriate compensations need to be provided in the sensor design. In general, the medium over the fluid’s surface is air. However, one can employ a blanket of nitrogen or any other vapor also.
Types of Ultrasonic Level Sensors
Ultrasonic level sensors are available in two basic types which are explained below:
Non-Contact Ultrasonic Sensors
This category of ultrasonic level sensors consists of:
In these devices, the microprocessor generates the gate signal and pulses and directs them to the ultrasonic sensor via the analog signal processor. The sensor then transmits a beam of ultrasonic waves to the surface of the fluid. It also receives the reflected echoes from the fluid surface and sends them back to the microprocessor. The microprocessor keeps on receiving echoes of sound waves and performs calculations to determine distance between the sensor and the fluid surface and hence detects the fluid level.
Contact Ultrasonic Sensors
These types of ultrasonic sensors are primarily used to detect fluid level at a specific point only. These level measurement devices consume less energy and basically include:
They consist of no movable parts and hence do not need any calibration. Typically, they are equipped with terminal blocks for connection of a power source and external control devices. The ultrasonic signal crosses a one-half inch gap in the sensor, controlling relay switches when the gap contains liquid. The sensing level is midway along the gap for horizontally mounted sensors, at the top of the gap for vertically mounted sensors. When the level of the fluid drops below the sensing level, the strength of the ultrasonic signal gets reduced. This eventually brings the relay to its former position.
These ultrasonic sensors find their application in vessels or pipes where they are used for automatic action of pumps, solenoid valves, and high or low level alarms. In these areas, two sensors need to be employed: one for filling and emptying tanks and the other one for measuring fluid volumes. They are suitable for use with mostly all kinds of fluids. Their performance does not get easily influenced by coatings, clinging droplets, foam or vapor etc but sometimes, highly aerated or viscous fluids can create trouble by choking the sensor gap.
Key features of ultrasonic level measurement devices are listed below:
Major advantages offered by ultrasonic level measurement technique are mentioned below:
Ultrasonic level measurement technique can not be suitably applied in all fields since use of ultrasonic level sensors includes few setbacks too. Many factors exist which have the tendency to influence the returned echo signal back to the sensor. Some of them include:
Besides, ultrasonic level measurement devices do not work satisfactorily in areas involving vacuum or high pressure conditions.
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