LEVEL

Que. :  Explain the different methods of level measurement in brief ?

Ans. :  There are two ways of measuring level :

            ( 1 ) Direct & ( 2 ) Indirect.

( 1 ) DIRECT LEVEL MEASUREMENT :

1. BOB AND TOP :

2. SIGHT GLASS : This consists of a graduated glass tube mounted on  the side of  the vessel. As the level of the liquid in the vessel change, so does the level of the liquid in the glass tube.

( 2 ) INDIRECT LIQUID LEVEL MEASUREMENT :

1. PRESSURE GAUGE : This is the simplest method, for pressure gauge 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.

2. PURGE SYSTEM :  In this method a pipe is installed vertically with the open and at zero level. The other end of the pipe is connected  to a regulated air r supply and to a  pressure gauge. To make a level measurement the air supply is adjusted so that pressure is slightly higher than the pressure due to height  of the liquid. This is accomplished by regulating the air pressure until bubbles cab be  seen  slowly  leaving  the open end of the pipe.

The method above are suitable for open tank applications. when a liquid is in a pressure vessel, the liquid column pressure can't be used unless the vessel pressure is balanced out. This is done through the use of different pressure meters.

DIFF.PRESSURE METHOD : Connection are made at the vessel top and bottom, and to the two columns of the D.P. meter. The top connection is made to the L.P. column of the transmitter and the bottom to H.P. column of the transmitter. The difference in pressure in the vessel is balanced out, since it is fed to both the column of the meter. The difference in pressure deducted by the meter will be due only to the changing, level of the liquid.

DISPLACER TYPE LEVEL MEASUREMENT :

The leveltrol is one of the most common instruments used measuring level in closed tanks. This instrument works of Archimedes principle. The displacer in immersed in the liquid due to which there is loss of weight depending on the specified gravity of the liquid. This displacer hangs freely on a knife transmitted to the pneumatic or electronic counterpart at the other end.

Que. :  Explain how you will measure level with a different pressure transmitter.

Ans. :  The bottom connection of the vessel is connected to high pressure side of the transmitter.

Different Pressure  = H  X  D

This difference pressure is applied to H.P. side of the transmitted and calibrated.

Que. :  How is D.P. transmitter applied to a close tank ?

Ans. :  In close tank the bottom of the tank is connected to the high pressure side of the transmitter and top of the tank in connected to L.P. side of the transmitter. In this way the vessel pressure is balanced.

Que. :  How is D.P. transmitter applied to an open tank ?

Ans. :  On an open tank level measurement the L.P. side is vented to atmosphere. Whatever pressure acts is on the H.P. side which is a measure of level.

Que. :  What is purge level system ?

Ans. :  This method is also known as bubbler method of level measurement. A pipe is installed vertically with its open end at the zero level. The other end of the pipe is connected to a regulated air supply and to a pressure gauge or to ^P transmitter. To make a level measurement the air supply is adjusted so that pressure is slightly higher than the pressure due to the 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. The gage then measures the air pressure needed to over come the pressure of the liquid.

/\ P  = H  X  D

USE : On for corrosive liquids where the transmitter cannot be directly connected to process eg... Acids, Some organic liquids.

Que. :  Explain the working of a leveltrol.

Ans. :  The leveltrol is used for measuring level 0 liquids in a closed vessel.

PRINCIPLE. : It works on Archimedes principle "The loss in weight of a body immersed in a liquid is equal to amount of liquid displaced by the body". The leveltrol basically consists of the following :

DISPLACER : It is consists of a 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 buoyancy 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 : Amplifies pressure variations at the nozzles.

REVERSING ARC : It is used for the following purposes.

1. Motion take of from Torque tube.

2. Means of reverse control action.

3. Adjustment for specific gravity.

PROPORTIONAL UNIT. : Converts primary motion to a proportional output air pressure.

CONTROL SETTING UNIT : Provides a motions of varying the set point.

Que. :  Explain the working an electronic leveltrol.

Ans. :  The variation in buoyancy 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 the change in level and specific gravity of the liquid. The resulting torque tube movement varies the angular motion of the rotor in the RVDT (Rotary Variable Differential. Transformer) providing a voltage change proportional to the rotor displacement, which is converted and amplified to a D.C. current.

Que. :  How will you reverse an action of the leveltrol.?

Ans. :  The reversing arc serves as motion take off arm from the torque tube. It is provided with a slot on each side of the center so that link can be connected either for reverse or direct action.

Que. :  What is interface level ? How do you calculate it ?

Ans. :  When a vessel is filled with two liquids of two different specific gravities the level measurement refers to as interface level.

DP = H  ( D - d )

On a level set the difference of two specific gravities.

Que. :  How will you calibrate a leveltrol in the field ?

Ans. :  1. First close both the primary isolation valves and drain the liquid inside the chamber.

2. Adjust the zero to get 0% output.

3. Connect a transparent PVC tube to the drain point as shown in hook up.

4. Fill it to the center of the top flange.

5. Adjust the specific gravity or span adjustment  ( Electronic Level ).

6. Fill it up to 50 %, check linearity.

 Que. :  How will you calibrate on interface level control. ?

Ans. :  liquids of two different specific gravities.

1. The level will be zero when it is full of lighter liquid.

Zero % level  =  H  X  d.

H = Displacer length

d = Specific gravity of lighter liquid.

2. The level will be 100 % when it is full of heavier liquid.

100 % level  =  H  X  D.

D  =  Specific gravity of heavier liquid.

Calibration : ( with water )

1. Fill H X d level with water adjust zero.

2. Fill H X D level with water adjust Sp. gravity or span.

3. Check linearity.

Que. :  How will you apply wt. test calibration to a leveltrol.

Ans. :  Wt. test calibration method :

1. Remove the displacer from the torque arm.

2. Apply equivalent weight on the torque arm that is equal to the wt. of the displacer. Adjust zero % output.

3. For Span : V = TTr2h

Loss in weight = Wt. of float - wt. of the float immersed in liquid

Loss in weight = [ wt. of float - Vol. x d ]

Span wt. = (wt. of float - Loss in wt.)

r = radius of the displacer.

h = ht. of displacer.

4. Apply equivalent wt. equal to the (Wt. of float - Loss in weight). Adjust Span to get 100 % out put.

5. To check linearity apply average of the two weights.

Que. :  What will happen if the displacer has fallen down while in line ?

Ans. :  The output will be minimum. ( wrong )

            The output will be maximum  (100 %) true.

Que. :  What will happen if the displacer has a hole in it while in line ?

Ans. :  The output will be minimum

Que. :  What is the used of  Suppression and elevation ?

Ans. :  Suppression and elevation are used on Level applications where (1) transmitters are not mounted on same level (2) Wet leg. i.e. condensable vapors are present.

Que. :  What are the limitations of leveltrol ?

Ans. :  The limitations of a level control that it cannot be used for lengths more than 72".

Que. :  How will you commission D.P. transmitter in field in pressurized vessel.

Ans. :  1. Close both the isolation valves, Vent the H.P. side.

            2. Fill it with the sealing liquid.

            3. Open the L.P. side vent valve.

            4. Adjust zero with suppression spring.

            5. Close the L.P. side vent valve.

            6. Open both the isolation valves.

Que. : How will you check zero of a level D.P. transmitter while is line ?

Ans. :  1. Close both the isolation valves.

            2. Open the vent valve on L.P. leg and H.P. leg drain.

            3. Check and adjust zero if necessary.

Que. :  Explain the working of an Enraf level gauge ?

Ans. :  The Enraf level precise level gauges are based on servo powered null balance technique. A displacer serves as continuous level sensing element.

Principle :

A displacer A with a relative density higher than that of a product to be measured, is suspended from a stainless steal wire B tat is attached to a measuring drum. A two phase servo motor controlled by a capacitive balance system winds unwinds the measuring wire until the tension on the weight springs is in balance with the wt. of the displace part immersed in the liquid. The sensing system in principle measures the two capacitance formed by the moving central sensing rod E provided with two capacitor plates and the si plates. In balance position the capacitance are of equip value. A level variation will cause a difference in buoyancy of the displacer. The center sensing rod will move in to direction of one of the side capacitor plates. This causes difference in value of this capacitance. By an electrolyte rotation of the servo motors the can driven transmitter continuously change the voltage pattern  to remote indicate of which the receiver motor drives a counter indicating low variation.