LEVEL MEASUREMENT

1.Write the formula for calculating a static head?

Static Head    ρ = pgh

p= Density of the liquid – mass/volume-(kg/m³)

g= free fall Gravity (9.8 m/sec sq.)

h= height of the liquid column (meters)

2.What are the simple methods for measuring level?

Gauge glass, Pressure Gauge – scale graduated in % of level, Rope and weight , Dip Tape…etc

3.How to convert  a pressure gauge into a level gauge?

Calculate the static head using the formula “pgh”. Select a pressure gauge and calibrate it for the calculated static head. Graduate the pressure gauge scale in terms of %of level.

4.What is the density of crude oil?

0.8.

5.What is an “interface dip tape”? Where it used?

Interface dip tape is an instrument used for measuring the total and the interface liquid level in vessel.

For example: It is used in oil storage tanks to measure the total liquid level and water level.

6.Explain how a Leveltrol works? Name the parts of a pneumatic Leveltrol.

A Leveltrol in an instrument used for measuring the liquid level between two known points. The Leveltrol works on the buoyancy principal. Leveltrol has a float, which submerses proportionately with liquid level raise in the float chamber. The amount of submersion of the displacer depends on the liquid density, which produces a torque. The amount of torque produced in measured in terms of % of level.

Parts of Leveltrol:

Float chamber, Float, Torque lever, Knife edge, Feedback Bellows, Air Relay, Restriction, Flapper, Nozzle, Feedback link, Density range, Action change lever, HP and LP flange,

7.What is a static head level transmitter?

A static head level transmitter is used for measuring the total liquid level in the tank.

8.What are the application limits of a Leveltrol and a static head level transmitter?

A Leveltrol can measure only for a short and fixed height of level. A Leveltrol has a high gain output. In can be used for liquid level as well for the interface level measurement. Static head transmitter is used for measuring the total liquid level in the tank. The transmitter output is linear.

9.What is the operating principle of a Magnetrol level switch?

The Magnetrol level switch works on the principle of Buoyancy force.

10.What are the points to consider while selecting a level switch for a particular process?

While selecting a level switch, the following points are to be considered:

#The characteristic of process (corrosive or non – corrosive)

#The process pressure

#The liquid density

#The flange ratings

#Proof pressure of the switch

#The micro switch contact rating

11.What type of level measuring instrument is suitable for closed tank?

If the level measurement is required for a fixed and small height, a Leveltrol is more suitable. If it is for a total height, then either a static head or a differential pressure transmitter with its HP leg connected to the bottom of the liquid level and the LP leg connected to the top of the tank to gas phase.

12.What is Zero suppression and Zero elevation in level measurement?

Zero suppression: when a static head transmitter is installed below the zero liquid level, the transmitter gets a +ve error in the level measurement. This error is corrected  by a zero suppression kit.

Zero elevation: when a static head transmitter is installed above the zero liquid level, the transmitter gets a –ve error in the level measurement. The error is corrected by a zero elevation kit.

13.What is an interface in level measurement?

An interface is the separation point between the two de-missible liquids levels In a vessel. This condition arises when the liquid does not mix due to its chemical composition and difference in their density.

14.How to calibrate a leveltrol for an interface level measurement?

Fill the leveltrol chamber 100% with the lower density liquid and adjust its zero for 4.00mA output.

Drain the liquid and fill the leveltrol chamber 100% with the higher density liquid and adjust its span for 20.00 mA output.

The transmitter on line measures the percentage of higher density liquid in the lower density liquid at a known height.

15.How to calibrate a static head level transmitter for an interface measurement?

Static head level transmitter is not commonly used for measuring the interface level. But the following procedure may be used for calibrating it to measure the interface level in a tank.

Fill the vessel 100% with the lower density liquid and adjusts its zero for 4.00 mA output. Drain the liquid and fill with the vessel 100% with the higher density liquid and adjust its span for 20.00mA output. 100% transmitter level is to be continuously maintained.

The transmitter on line measures the percentage of higher density liquid in the lower density liquid in a know height.

16.Why is a leveltrol more suitable than a static head level transmitter on a separator?

Leveltrol is more accurate in measuring the small height of liquid level. The process pressure change does not affect the leveltrols performance. Leveltrol offers a high gain output verses the change in the liquid level.

17.Why is static head level transmitter more suitable than a leveltrol on a surge tank?

A leveltrol has a limitation in measuring the maximum height of liquid. Considering huge of the surge tank, a static head level transmitter is more suitable.

18. Why is a gap controller often used in controlling a level in a high pressure vessel?

Gap controllers output changes from minimum to maximum and visa versa when the process measurement deviates the set point by the set gap. Hence the final control element i.e. control valve acts like an on-off valve. This helps in minimizing the trim erosion due to a high DP across the control valve while partially open.

19. How to use a DP (differential pressure) transmitter for level measurement?

A differential pressure transmitter gives a linear output for the differential pressure measured across its HP and LP chambers.

Connect the transmitter’s HP leg to the bottom of the tank and its LP leg to the top of the tank. The transmitter will read the liquid level accurately irrespective of the change in process pressure above the liquid surface.

20. Briefly explain the different methods of level measurements.

There are two ways of measuring level.

Direct

indirect

Direct level measurement:

Bob and tape:

Sight glass: this consists of a graduated glass tube mounted on the side of the vessel. As the level of the liquid in the glass tube.

Indirect level measurement:

Pressure gauge: this is the simplest method. A pressure is located at the zero level of the liquid in the vessel. Any rise in level causes an increase of pressure which can be measured by a gauge.

Purge system: in this method a pipe is installed vertically with the open end at zero level. The other end of the pipe is connected to regulated air supply and to pressure gauge to make a level of measurement the air supply is adjusted so that pressure is slightly higher tan the pressure due to height of the liquid. This is accomplished by regulating the air pressure until bubbles can be seen slowly leaving the open end of the pipe.

21. Explain the working leveltrol.

The leveltrol is used for measuring level of liquids in a close vessel.

Principle: it works on archimedec principles “the loss in weight of body immersed in a liquid is equal to amount of liquid displaced by the body”. The level trol basically consists of the following:

displacer: it is consists of cylindrical shape pipe sealed and filled inside

with sand or some weight. The purpose of this is to convert change in level to primary motion. The variation in bouyancy resulting from a change in liquid level varies the net weight of the displacer increasing or decreasing the load on the torque arm. This change is ‘directly proportional to change in level and specific gravity of the liquid.

Relay: aplifies pressure variations at the nozzle.

Reversing arc: it is used for the following purposes.

Motion take-off from torque tube.

Means of reversing control action.

Adjustment for specific gravity.

Proportional unit: converts primary motion to proportional output pressure.

Control setting unit: provides a means of varying the set point.