Interview Question for Instrument & Control Engineer

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Q:Difference between floater and Displacer ? Role of Specific Gravity & Process density in Selection of displacer ?

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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 weight of the liquid volume 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.

Float Level Switches|979x414

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 gage.

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:

  1. Top mounting
  2. Side mounting
  3. External cage

Figures indicating Top mount and Side mount operating principles are shown below.

Top Mount Operating Principle

Float Level Switches - Top Mount Operating Principle|601x401

Side Mount Operating Principle

Float Level Switches - Side Mount Operating Principle|601x401

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.

Displacer Switches

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 Switch Design|524x455

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:

  • They perform extremely well with clean fluids.
  • Use of these level sensors proves to be very accurate.
  • They are flexible to extensive changes in density of the medium.

Floats v/s Displacers

Following are the major points of distinction between floats and displacers:

  • “Float Switches are available with a glandless design and are capable of fail safe operation in extreme process conditions, unlike displacers, which if the torque tube fails can provide a leak path.”
  • A float generally rides above the surface of liquid whereas a displacer remains either partly or totally immersed in process liquid.
  • Displacer switches are considered to be additionally stable and dependable as compared to standard float level switches in case of turbulent, surging, frothy and foamy services. However in case of refineries, the use of displacers is decreasing owing to their high installation cost and inaccurate performance due to process density changes. In these applications, float level switches have been found to be reliable and useful.
  • Settings of displacers can be changed very easily since they can be shifted at any place along the length of the suspension cable. Moreover, these level devices have the provision of interchangeability between tanks. This is due to the fact that the differences in process density can be endured by varying the tension of the spring attached to the displacers.
  • Testing the appropriate working of a displacer switch is much easier than a customary float level switch since the former requires just lifting of a suspension whereas the latter necessitates filling of liquid in the tank upto the actuation mark.

What are different types of orifice plates? State their uses.

Different orifice plates are: 1. Concentric 2. Segmental 3. Eccentric

  • Concentric: These plates are used for ideal liquid as well as gases and steam service. Concentric holes are present in these plates, thats why it is known as concentric orifice.
  • Segmental: This plate has hole in the form of segment of the circle. This plate is used for colloidal and sherry flow measurement.
  • Eccentric: This plate has the eccentric holes. This plate is used in viscous and sherry flow measurement.

How do you identify an orifice in the pipeline?

An orifice tab is welded on the orifice plate which extends out of the line giving an indication of the orifice plate.

Why is the orifice tab provided?

Following reasons justify for providing orifice tab:

  1. Indication of orifice plate in a line
  2. The orifice diameter is marked on it.
  3. The material of the orifice plate.
  4. The tag number of the orifice plate.
  5. To mark the inlet of an orifice.

Explain Bernoulli’s theorem. State its application.

Bernoulli’s theorem states that the ‘total energy of a liquid flowing from one point to another remains constant’. It is applicable for non-compressible liquids. For different types of liquid flow Bernoulli’s equation changes. There is direct proportion between speed of fluid and its dynamic pressure and its kinetic energy. It can be used in various real life situations like measuring pressure on aircraft wing and calibrating the airspeed indicator. It can also be used to low pressure in the venturi tubes present in carburetor.

How can a D.P. transmitter be calibrated?

D.P. transmitter can be calibrated using following steps:

  1. Adjust zero of Xmtrs.
  2. Perform static pressure test: Give equal pressure on both sides of transmitter. Zero should not shift either side. If the zero shifts then carry out static alignment.
  3. Perform vacuum test: Apply equal vacuum to both the sides. Zero should not shift.
  4. Calibration procedure: Give 20 psi air supply to the transmitter and vent L.P. side to atmosphere. Connect output of the instrument to the standard test gauge. Adjust zero. Apply required pressure to the high pressure side and adjust the span. Adjust zero gain if necessary.

How is flow measured in square root?

Flow varies directly as the square root of pressure. Thus, F=K of square root of applied pressure. Since this flow varies as the square root of differential pressure. The pressure pen does not directly indicate flow. Thus flow can be determined by taking the square root of the pen. Assume the pen reads 50% of the chart. So, flow can be calculated using the pen measure in the chart.

Name different parts of a pressure gauge. Explain the use of hair spring in the pressure gauge.

Pressure gauge includes following components:
a. ‘C’ type bourdon tube.
b. Connecting link
c. Sector gear
d. Pinion Gear
e. Hair spring
f. Pointer
g. Dial

Use of hair spring: Hair spring is responsible for controlling torque. It is also used to eliminate any play into linkages.

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