Level Measurement Experiment

OBJECTIVE

To evaluate and study the measurement of level using the following.

1. A differential pressure / level transmitter (ABB model 621ED)

2. A sight glass with millimetre scale.

SUMMARY

The purpose of level measurement and calibration experiment was to evaluate and

study the measurement of level using differential pressure or level transmitter and sight glass

with millimetre scale This experiment is conducted under two different conditions which is

the first one is open tank and heater off while the other one is closed tank and heater off.. The

reading of actual level (LI) is slightly different with the actual level (LG) because there is

parallax error while set the level on the suggested level as the position of the sight glass with

millimetre scale is high. There are small different between reading on the sight glass and the

level indicator. This is because sight glass reading was taken manually by human eye contact

while level indicator was taken by sensor indicator. So, the level indicator is more accurate

than the sight glass. The sight glass less accurate because of human errors while read the

readings.

INTRODUCTION

Level measuring instruments are used to measure fluid (liquid or gas) level either

continuously or discrete depending on the type of application. These instruments may be

indicators, which displays liquid level. Transmitters, which carries level information in the

form of ac signals for control purpose. Level switches are used to monitor high or low level of

liquid depending on the set point. Different types of level instruments work on different types

of principles. The selection of a particular level instrument depends on process parameters

like pressure, temperature, medium and environmental conditions etc.

Principles of Level Measurement

The major principles in level measurement and its classification

1. Pressure based or Differential pressure based level measurement

2. Transit time based level measurement

a) Ultrasonic based level measurement

b) Radar based level measurement

3. Attenuation based level measurement / Radiation based level measurement

4. Conductivity based measurement (Electrical measurement)

5. Capacitance based level measurement

This module covers aspects of level measurement as used in process instrumentation

and control. Level measurement addresses essential knowledge and skill elements associated

with measuring level. In this module student will conducted level measurement in two

conditions where the first condition applied on the tank i.e. closed tank and open tank and the

last condition applied on the temperature. The condition of tank either in closed or open is to

demonstrate the pressure in open system or closed system. Meanwhile the heat supplied to the

system used to demonstrate pressure of system when heat supplied.

There is several type of measurement in industry and one of it is level measurement

with seals. Level measurement with seals allows the level measurement to be removed from

direct contact with the process fluid. Remote seals are useful when:

The process temperature is outside of the normal operating limits of the level

measurement and cannot be brought into those limits with impulse piping.

The process is corrosive and requires frequent level measurement replacement.

The process requires unusual materials of construction.

The process contains numerous solids or is viscous; either condition could plug the

impulse piping.

The application requires the use of sanitary connections.

There exists a need for easy cleaning of the process from the connections to avoid

contamination between batches.

Seals function as an extension of the level measurement. The basic measurement of level

follows the same principle as pressure level measurements without seals: pressure is

proportional to level. The head pressure of the liquid corresponds to its height multiplied by

the specific gravity. The prudent selection of remote seals is important in maintaining a

reasonable performance expectation of the pressure level measurement.

One of the level measurement apparatus used in this experiment is bubbler tubes besides

the bourdon gauge, the bubbler tubes provide a simple and inexpensive but less accurate (±1-

2%) level measurement system for corrosive or slurry-type applications. Bubblers use

compressed air or an inert gas (usually nitrogen) introduced through a dip pipe (Figure 1-A).

Gas flow is regulated at a constant rate (usually at about 500 cc/min). A differential pressure

regulator across a rotameter maintains constant flow, while the tank level determines the

back-pressure.

Figure 1. Level measurement device

As the level drops, the back-pressure is proportionally reduced and is read on a pressure

gauge calibrated in percent level or on a manometer or level measurement. The dip pipe

should have a relatively large diameter (about 2 in.) so that the pressure drop is negligible.

The bottom end of the dip pipe should be located far enough above the tank bottom so that

sediment or sludge will not plug it. Also, its tip should be notched with a slot or "V" to ensure

the formation of a uniform and continuous flow of small bubbles. An alternative to locating

the dip pipe in the tank is to place it in an external chamber connected to the tank.

In pressurized tanks, two sets of dip pipes are needed to measure the level (Figure 1-

B). The two back-pressures on the two dip pipes can be connected to the two sides of a u-tube

manometer, a differential pressure gage or a d/p cell/level measurement. The pneumatic

piping or tubing in a bubbler system should be sloped toward required (when the tank is full

and the vapor pressure is at its maximum). An alternative to a the tank so that condensed

process vapors will drain back into the tank if purge pressure is lost. The purge gas supply

should be clean, dry, and available at a pressure at least 10 psi greater than the expected

maximum total pressure continuous bubbler is to use a hand pump (similar to a bicycle tire

pump) providing purge air only when the level is being read.

Bubblers do consume inert gases, which can later accumulate and blanket processing

equipment. They also require maintenance to ensure that the purge supply is always available

and that the system is properly adjusted and calibrated. When all factors are considered, d/p

cells typically are preferred to bubblers in the majority of applications.

RESULTS Table 1 : Level measurement (Tank T1 as OPEN tank, Heater OFF)

RUN 1 SET 1 SET 2 AVERAGE

A Suggested Level (mm) 700 700 700B Actual Level (LG)(mm) 700 700 700C Actual Level (LI)(mm) 703 703 703D Temperature (TG)(oC) 32 32 32E Deviation = | B-C| (mm) 3 3 3

RUN 2 SET 1 SET 2 AVERAGEA Suggested Level (mm) 600 600 600B Actual Level (LG)(mm) 600 600 600C Actual Level (LI)(mm) 604 604 604D Temperature (TG)(oC) 32 32 32E Deviation = | B-C| (mm) 4 4 4





Table 2 : Level measurement (Tank T1 as CLOSED tank, Heater OFF) RUN 1 SET 1 SET 2 AVERAGE


RUN 2 SET 1 SET 2 AVERAGEA Suggested Level (mm) 600 600 600B Actual Level (LG)(mm) 600 600 600C Actual Level (LI)(mm) 604 604 604

D Temperature (TG)(oC) 32 32 32E Deviation = | B-C| (mm) 4 4 4






Summarized results ; Condition Run 1 Run 2 Run 3 Run 4 Run 5 Average

deviationOpen tank Heater off

3 4 7 2 2 3.6

Closed tank

Heater off

4 4 3 2 3 3.2

open tank closed tank

3

3.1

3.2

3.3

3.4

3.5

3.6

3.7

Graph 1 : Standard deviation for heater off method

DISCUSSION

Calibration is the process of evaluating and adjusting the precision and accuracy of

measurement equipment. Proper calibration of an instrument allows people to have a safe

working environment and produce valid data for future reference. Calibration refers to the act

of evaluating and adjusting the precision and accuracy of measurement equipment. Instrument

calibration is intended to eliminate or reduce bias in an instrument's readings over a range for

all continuous values.

This experiment is conducted under two different conditions which is the first one is

open tank and heater off while the other one is closed tank and heater off. Based on data

tabulated on table 1 and table 2, a summarization of data is tabulated in graph 1. From the

graph, we can see that in heater off condition open tank contribute the most to the value of

deviation at 3.6 rather than closed tank at 3.2. Basically, when the temperature inside tank is

changed, the density of water is also changed. Based on theory, as the temperature increased,

the density of liquid is decreased. Because density influences pressure of a liquid, this will

make the accuracy of pressure level indicator to drop. So, the change in the accuracy will

increase the standard deviation which is difference between readings from the sight glass with

the reading from the level indicator. The measurement for heater off, which is sighted glass, is

not affected by changes in density of the liquid as during run the experiment, the level of

water inside the tank should be set at the suggested level by monitored the reading at the

sighted glass.

CONCLUSION

The two type of measurement have been compare between open and closed tank. The

tank as open tank and heater has the highest reading difference of the level indicator and

direct with indirect measurement. When the heater off condition and open tank contribute the

most to the value of deviation at 3.6 rather than closed tank at 3.2. It is because, density

influence pressure of liquid. Hence, the change in the accuracy will increase the standard

deviation which is difference between readings from the sight glass with the reading from the

level indicator. We can conclude that, closed tank system with heater off give less deviation

than open tank system.

RECOMMENDATION

For the recommendation, during take of reading, at sight glass with millimetre scale

(LG) which is the indirect measurement, the eye should sharp to the level. During the closed

tank, the pressure is adjusted to increase the level of pressure rises there of many error. In

addition, we also must check or ensure there are no troubles shooting before run the

experiment. Connection between high operating pressure and low hydrostatic pressure easy to

occur, so we must check the connection before run the experiment. Obstructed sensing lines

and draining sensing line also must be check. Hence, to get the accurate result, we must avoid

the error and double check for the trouble shooting error.

TUTORIAL

1. Level is measured in many of the processes in the industry. List and explain the most

common reasons for measuring level.

The level of measurement help to decide how to explain the data from variable

values and help to decide statistical analysis. Level is measured for the purpose of

process efficiency, inventory, safety, consistent supply of materials and custody

transfer. It is also a very simple systems employ external sight glasses or tubes to

view the height and hence the volume of the fluid.

2. Name the two methods of level measurement used in the experiment.

dip stick

Sight glasses

3. How depth of a liquid does correspond to changes in liquid pressure? Can a

pressure transmitter be used to measure the level?

The pressure became greater when the deeper into water due to the weight of water

pressing down on it. Hence, the higher the depth the higher the change in liquid

pressure. Yes, pressure transmitter can be used to measure level.

4. The differential pressure transmitter is used to measure level in an open tank or

closed tank system. Discuss how this is done.

We can get the measurement by rising the water until it overflows. A differential

pressure transmitter is located somewhere near the bottom of the vessel. These

transmitters have both a low pressure and a high pressure connection in order to

make them a differential pressure transmitter. The high pressure side is connected to

the port near the bottom of the vessel to sense the weight of the material above it in

the vessel. In non-pressurized vessels the low pressure side may simply be vented to

atmosphere but pressure vessels require that the low pressure side be connected to

the vapor space above the level. The difference in pressure between the high and the

low side causes the diaphragm of the pressure cell to move. In order for the weight

to level conversion to be accurate, the density of the material being measured must

remain constant,

5. Discuss briefly the possible errors affecting the accuracy in pressure/ level

measurement as observed in the experiment.

Connections

High pressure and low hydrostatic pressure easy to occur

Actual level high and indicated level is low

Over-pressuring

fail immediately or diaphragm become distorted

low and high reading

Obstructed sensing lines

Small diameter , clogged with particulate inaccurate reading

Sluggish respond to level changes

To overcome periodic draining and flushing

Draining sensing line

To remove debris

Leaking

6. What are the main objectives of a wet lag calibration?

To vaporised liquid which will be condensate by atmosphere level

REFERENCES

i. Schnake. J., (2007). Liquid Level Measurement, Available at:

http://www.controlglobal.com/articles/2009/guidetopressureleveltrans0902/.

[Accessed on 15 April 2015].

ii. Omega Transaction in Measurement & Control Series, (1995). Pressure or Density

Level Instrumentation, Available at:

http://www.omega.com/literature/transactions/volume4/t9904-12-press.html.

[Accessed on 15 April 2015].

iii. David R. Caprette (2000). All about Instrumentation [Online] Available at:

http://www05.abb.com/global/scot/scot206.nsf/veritydisplay/bb949b50a265ef08c1257

bf6004ccab3/$file/TI_266-EN_B-09_2013.pdf [15 April 2015]

iv. Third Edition; Bela G. Liptak Instrument Engineer Handbook –Process Control

v. Idaho State University’s College of Technology, (1992). Level Measurement.

Available at: http://www.isu.edu/estec/ic-ed-modules/Module-9-Level-

Measurement.pdf. [Accessed on 15 April 2015].

vi. Instrumentation & Control Chulalongkorn University, (2014). Level Instrumentation.

Available at: https://canteach.candu.org/Content%20Library/20053318.pdf. [Accessed

on 15 April 2015].

http://www.controlglobal.com/articles/2009/guidetopressureleveltrans0902/

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Level Measurement Experiment