Level Measurement

Troubleshooting, advantages, disadvantages and application of the bubbler type level measurement system

The comprehensive troubleshooting instructions for the bubbler system level and density measuring system are provided in the table below.

Troubleshooting, advantages, disadvantages and application of the bubbler type level measurement system 1

What is a common failure for a dip tube bubbler level sensor?

The level control loop is not stable and level control valve hunting

Troubleshooting, advantages, disadvantages and application of the bubbler type level measurement system2
Possible causeSolutions
Internal blockages in the bubbler dip tube Remove the bubbler dip tube from the tank and blow air through it to look inside for any blockages. (NOTE: Mark the location of the dip tube using a permanent marker before removing it in order to ensure appropriate depth after replacing.)
Air ports of reversing relay blockedIf a reversing relay has been installed in the control valve, check its functionality and condition, and search for any obstructions in the air connections port.
Condensation in the filtersVerify whether condensation has formed in the air filters near the air supply system or in the air pressure regulator of the bubbler system control panel. Drain as necessary.
The dip tube is immersed in the water at the wrong position.  Depth may play a role. In order for the bubbler to function properly, the dip tube should typically be submerged 4 to 6 inches into the water. Sometimes, dipping the dip tube deeper into the water will help performance run more smoothly.
In correct air pressure and flow set value Verify that the air pressure and flow set value are as desired. Refer the instrument data sheet
Troubleshooting, advantages, disadvantages and application of the bubbler type level measurement system

Improper measurement – not matching with actual level

Possible causeSolutions
Probabilities of crystallization of solids in the bubble tubesRemove the cleanout plugs and push a rod through the bubble tubes or flush with a suitable solvent if there is a propensity for particles to crystallize there or if dirt tends to accumulate there. 
Verify the range of both the field transmitter and the PLC/DCS/SCADA.

Pressure transmitter output signal that fluctuates 

Possible causeSolutions
Uneven and insufficient support for the Dip Tube (the tube was able to move).Find the dip tube and examine it to see whether it’s been damaged. In the event that it is broken, it will have to be replaced.
Angle brackets and clamps should be used to fasten the dip tube to the tank’s side.
V-notch missing from dip tube entrance For further information, please refer to the attached figure. 
Please refer to the design document as well as the instructions provided by the manufacturer.
Leaks in the bubble tubesAll leaks must be stopped for the Bubble-Tube Liquid Level System to function effectively.
Turbulence inside the tank as a result of the process sequence Use a longer dip tube: By utilizing a longer dip tube, you may be able to lessen the influence of the turbulence on the dip tube if it is concentrated in the top section of the tank. 
Move the dip tube to a different location in the tank where there is no turbulence.

The output of the transmitter having Zero Shift 

Possible cause
Changes in pressure drop caused by a limitation or a lengthy tube before the dip tube.1. Reduce the length of the tube: If the tube before the dip tube is too lengthy, it can cause a significant pressure drop that affects the accuracy of the measurement. 
2. Increase the air supply pressure: Increasing the air supply pressure can also help to reduce the pressure drop and improve the accuracy of the measurement
Dip tube air flow being more restricted as a result of corrosion.1. Replace the corroded dip tube: If the corrosion is severe, you may need to replace the dip tube altogether. Make sure that the replacement dip tube is made of a material that is more resistant to corrosion, such as stainless steel.
2. Clean the dip tube: If the corrosion is not severe, you may be able to clean the dip tube to restore the air flow. Use a suitable cleaning solution and follow the manufacturer’s instructions carefully.
3. Conduct regular maintenance: Regular maintenance of the bubbler level measurement system can help to prevent corrosion and ensure that the measurement remains accurate.
Out of calibration(zero error)Check the transmitter’s calibration and functionality.

The level signal output displays 0% 

Possible causeSolutions
Problem with power supply1. Verify that the signal terminals are receiving power.
2. Look for reversed polarity in the power supply lines.
3. Check the terminal voltage range of 10.5 to 42.4 Vdc in transmitter terminals.
4. Across the test terminal, look for an open diode in transmitter.
5. Check that the transmitter is not set to operate in the multidrop mode(HART application).

The value from the transmitter mA output is either low or high.

Possible causeSolutions
Issue with the pressure transmitter and possibly an incorrect configuration1. Check the manifold’s valve positions.
2. Validate the calibration range  values for the 4 and 20 mA.
3. Make sure the output isn’t in an alert situation.
4. Check to see if a 4–20 mA output trim is necessary.
5. Confirm that the transmitter has been correctly calibrated.
6. Check that the pressure calculations match the requirements of the application.
Check to see if the high-pressure (HP) and low-pressure (LP) side pressure connections in the transmitter are reversed.Ensure that the tubing connections are done correctly.
If any forcing (bypass activation) is applied to either the PLC/DCS or the field transmitter, the value won’t change.Remove the force after obtaining the required approval or after finishing the interlock rehabilitation procedure.

 Process Variable(level) reading is erratic

Possible causeSolutions
Improperly set damping valueEither the transmitter or the control system should be adjusted for dampening.
Signal wiring has loose connections.Make sure the signal connections are secure.
The pressure transmitter might be defective.Replace or service the pressure transmitter.

The pressure transmitter’s milliamp output is erratic.

Possible causeSolutions
Transmitter’s power supply is  not sufficient (drop)Make that the transmitter’s power supply has sufficient voltage with proper current rating.
External electrical interferenceMake sure there are no outside electrical interferences. 
Installing an appropriate signal isolator.
Improper ground connectionsMake that the transmitter is grounded properly.
Make sure the shield for the twisted pair is grounded at just one end.

What are the disadvantages of bubbler level indicator?

Bubbler level systems require extra attention and maintenance

  • Material from the process can be collected at the flow pipe’s tip, where it hardens and seals the opening.
  • The regulator may malfunction in cold areas and weather due to moisture that freezes in the air supply.
  • Because the accuracy of the bubbler system depends on a steady stream of clean instrument air, dirty air will cause the regulator to clog more frequently.
  • Even though installation costs can be quite low, this necessitates more frequent maintenance, which raises the cost. Additionally, accuracy can vary by up to 10% of full scale because it is constrained by the regulator.
  • Changes in media density have a direct impact on the need for system calibration. 
  • Variations in product density have a direct impact on calibration. Additionally, it is frequently necessary to routinely clean this equipment. 
  • The pipe’s tip has the ability to gather liquid from the process, harden, and fill the hole. 
  • For usage in non-vented containers, bubblers are not appropriate.

What are the advantages of bubbler level measurement?

Comparing the bubbler to other level and differential measurement devices, there are a number of advantages.

  • Temperature, turbulence, pH, conductivity, surface foam, and solids content are all unaffected..
  • The dip tube is the only part of the system that comes into contact with the liquid being measured, making it more reliable than other level measurement techniques.
  • The sensor has a longer lifespan and better calibration stability since it is not in touch with liquid.
  • calibrated with accuracy between 1% and 0.075% 
  • The distance between the instrument panel and the object being measured can be up to several hundred feet.
  • is appropriate for applications involving corrosive, acidic, risky, volatile, molten, cryogenic, or radioactive liquids.
  • A benefit of the bubbler system from the perspective of maintenance is that the labour necessary is not at the tank side, minimizing the amount of time the maintenance worker is exposed to the gasoline kept in the tanks.
  • Complete isolation from the measured liquid is provided by the purge gas (compressed air).
  • When an Auto Blow Down system for the dip tube is acquired and combined with the Level Bubbler, maintenance requirements are kept to a minimum.
  • When discussing the benefits of bubblers, simplicity of design and low initial cost are frequently mentioned. 

What are the applications in which bubbler systems are often used?

  • The level bubbler can be used to start an airburst backwash and check the in-take screens for debris. A dip tube is placed on either side of the screen to do this, and a backwash may be started to clean the screens once the pressure differential between the two reaches a certain level. The name of this system is a Differential Level Bubbler.
  • To regulate the intake pumps, the level bubbler can be used to gauge the wet well’s level.
  • The level bubbler may be used to gauge liquid levels in a variety of applications, including sanitary waste facilities.
  • Ultrasonic level transmitters can be retrofitted with a level bubbler in their place.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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