Instrumentation Questions for Freshers - 2

1. Briefly explain the different methods of level measurement?

There are two ways of measuring level:

1.   Direct

.   Indirect

Direct level measurement:

 (a) Bob and tape:

A bob weight and measuring Tape provide the most simple And direct method of liquid leve 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.

Indirect level measurement: (A) 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.

(b) 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 air pressure to the bubbler pipe is minutely in   Excess of the liquid pressure in the vessel, so that Air pressure indicated is a measure of the level in The tank.

The method above is 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.

(c) Differential pressure meter:

Connections 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.

(d)  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.

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

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.

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

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.

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

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.

SPAN = (X) (Sp. Graf)

ZERO SUPPRESSION = (Y) (Sp.Grav)

5. How is D.P transmitter applied to a close tank & open tank with Dry leg?

Span = (X) (GL)

HW at minimum level = ( Z ) ( GS )  + ( Y ) ( GL )

HW at maximum level = ( Z ) ( GS )  + ( X + Y ) ( GL )

Where:

GL = Specific gravity of tank liquid. GS = Specific gravity of seal liquid.

HW = Equivalent head of water. X, Y & Z are shown in fig (1.1)

Example:

Open tank with X   = 300 inches

Y    = 50 inches Z   = 10 inches GL =  0.8

GS =  0.9

Span   = (300) (0.8) = 240 inches

HW at minimum level =  ( 10 ) ( 0.9 )  + ( 50 ) ( 0.8 )  = 49 inches

HW at maximum level = (10 ) ( 0.9 )  + ( 50 + 300 ) ( 0.8 )  = 289 inches

Calibrated range = 49 to 289 inches head of water

2.3                                                     DEFINITION OF INSTRUMENT RANGE AND SPAN

Legend:

Range                                      Defined in IEC 60902 as follows: The region of values between the lower and upper limits of the quantity under consideration.

It is expressed by stating the lower and upper limits

(e.g. minus 1 to 10 bars (ga)).    Defined in IEC 60902 as follows: The algebraic difference between the upper and lower limit values of a given range.

It is expressed as a figure and unit of measurement

(e.g. 8 bars).

LRL                                        Lower Range Limit; the lowest quantity that a device is designed to measure.

URL                                        Upper Range Limit; the highest quantity that a device is designed to measure.

Instrument range                      the region in which the instrument is designed to operate. It is a physical capability of the device. The region limits are expressed by stating the LRL and URL.

Instrument minimum span         the minimum distance between the URV and LRV for which the instrument is designed. It is a physical limitation of the device.

Instrument maximum span         the maximum distance between the URV and LRV for which the instrument is designed. It is a physical limitation of the device.

LRV                                        Lower Range Value; the lowest quantity that a device is adjusted to measure.

URV                                       Upper Range Value; the highest quantity that a device is adjusted to measure.

Adjusted range                        The measurement region. It is expressed by stating the LRV and URV.

Adjusted span                          The distance between the URV and LRV.

Example:    A differential pressure transmitter is used to measure the level in a vessel, using a wet reference leg. 0% level corresponds with a differential pressure of -800 mbar and 100% level with - 100 mbar. The catalogue of the selected transmitter lists -1800/+1800 mbar for LRL/URL respectively and span limits of 300 to 1800 mbar, so:

Instrument range	=	-1800 to +1800 mbar
Instrument minimum/maximum span LRV / URV Adjusted range	= = =	300 / 1800 mbar respectively; -800 / -100 mbar respectively; -800 to -100 mbar.
Adjusted span	=	700 mbar.

NOTE:         ‘Adjusted’ range and ‘adjusted’ span are frequently referred to as ‘calibrated’ range and

‘calibrated’ span. This term is however only correct, if a calibration facility is used to set the LRV

and URV. For ‘intelligent’ measuring devices, the supplier is usually calibrating the device at the

LRL/URL and the user is setting the required LRV and URV by remote communication.

2.4                                                            SELECTION OF RANGES

The accuracy (2.8) and adjusted range of an instrument should be selected to cover the operating window (2.1), which includes applicable abnormal operation and alternative operating modes. Unless otherwise stated, the normal design value should lie between

50% and 75% of the adjusted range

NOTE:         For certain applications it might not be possible to combine all normal and abnormal operating conditions in one measurement at the required accuracy. In such cases, a case-by-case analysis should disclose whether additional instruments are required or the accuracy requirements and/or operating window may be relaxed. It might be acceptable to present measured values during some of the abnormal operating cases at a lower accuracy or it might be justifiable for the measurement not to produce a sensible signal under some of the abnormal process conditions during start-up, commissioning, regeneration, emergency conditions and the like.

IPF transmitters should have the same instrument range, adjusted range and accuracy as corresponding process transmitters in order to facilitate measurement comparison. For details and exceptions, see DEP 32.80.10.10-Gen. Trip settings should lie between 10% and 90% of the adjusted range.

The LRV should be selected so that the displayed result represents the zero or sub-zero value of the process variable (e.g. 0-150 tons/day, 0-100% level, 0-10 bar (ga), -1/+3 bar (ga), 0-500 °C, -50/+50 °C etc.). Elevated zero’s (100-300 tons/day, 100-200 °C) should be avoided.

6. What is purge level system?

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.

7. Explain the working of a leveltrol.

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

1.  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 :

2.   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.

3.   RELAY: Amplifies pressure variations at the nozzles.

4.   REVERSING ARC: It is used for the following purposes.

•     Motions take of from Torque tube.

•     Means of reverse control action.

•     Adjustment for specific gravity.

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

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

8. Explain the working an electronic level trol.

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.

9. How will you reverse an action of the leveltrol?

The reversing are 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.

10. What is interface level? How do you calculate it?

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.

11. How will you calibrate a leveltrol in the field?

Calculation # 1 If the calibrating liquid is water:  Process Liquid Density / 1* Displacer height = mm of H2O.

Calculation # 2 If the calibrating liquid is other liquid:   Process Liquid Density / Calibrating liquid density * Displacer height = mm of H2O. (Calibrating liquid height in mm).

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.

12. How will you calibrate on interface level control. ?

On an interface leveltrol there are two liquid 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.

Displacer length: L in mm                                                                                                X

Density Low        : dl in Kg / 1

Density High        : DH in Kg / 1

Alarm / Trip point %: X HH Trip Point: Y in mm

Y= L* (DH X + (1-         ) * dl)

100              100

13. How will you apply wt. lest calibration to a leveltrol.

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 = πr2h

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 applies average of the two weights.

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

The output will be maximum.

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

The output will be minimum.

16. What is the used of Suppression and elevation?

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

17. What are the limitations of leveltrol?

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

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

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.

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

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.

20. Explain the working of an Enraf level gauge?

The Enraf precise level gauge are based on servo powered null-balance technique. A

displacer serves as a continuous level sensing element. Principle:

A displacer with a relative density higher than that of the product to be measured is suspended from a stainless steel wire B that is attached to a measuring drum. A two phase servo meter controlled by a capacitive balance system winds or unwinds the measuring wire until the tension in the weighing springs is in balance with the weight of the displacer partly immersed in the liquid. The sensing system in principle measures the two capacitance formed by the moving center sensing rod E provided with two capacitor plates and the side plates. In balance position the capacitances are of equal value. A level variation wills a difference in buoyancy of the displacer. The center sensing rod will move in the direction of one of the side capacitor plates. This causes a difference in value of these capacitances. By an electronic circuit this change is detected and integrated.

During the rotation of the servo motor the cam driven transmitter continuously change the voltage pattern to a remote indicator of which the receiver motor drives a counter indicating level variation.

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