ACADs (08-006) Covered

Keywords

Calibrate, direct, inferred, transmitter calibration, gravity correction, density correction, hydrostatic head detector, float.

Description

Supporting Material

5.4.1.7

Measuring Level

Terminal Objective: Given the appropriate equipment and procedures the I&C Technician will calibrate and

maintain level instrumentation. Mastery will be demonstrated by successful completion of Lab

Performance Exercises and written Exam.

• Contrast direct and inferred methods of measuring level, giving an example of each

• Describe the theory of operation of a given level measuring device

• Given an example of a hydrostatic head detector, calculate the transmitter calibration input pressure for a dry reference leg system using specific gravity and density corrections

• Given an example of a hydrostatic head detector, calculate the transmitter calibration input pressure for a dry reference leg with zero suppression

• Given an example of a hydrostatic head detector, calculate the transmitter calibration input pressure for a wet reference leg system

Direct versus Indirect

• Direct: Level is measured by direct contact with the fluid surface. Either the fluid surface is visible to be compared directly to a scale, or direct contact is made with the fluid surface.

• Indirect: Level is inferred by means other than direct contact with the Fluid surface. Fluid level is inferred by its effect on some type of device.

Classify as Direct or Indirect

• Dipstick• Float• Sightglass• Float & chain• Bubbler tube• Capacitance

• Pressure or DP transmitter

• Ultrasonic• HJTC• Warrick Conductivity

Level• Magnetrol

Ways to measure level

• Antenna level sensor• Bubbler• Capacitance probe• Conductivity probe• Diaphragm detector• Differential Pressure• Dipstick• Displacer• Float• Impedance probe• Level gauge• Magnetic Follower

• Optical level• Radiation level sensor• Resistance tapes• Rotating paddles• Sightglass• Slip tubes• Tape level devices• Thermal level sensor• Time domain

reflectometry• Ultrasonic level detectors• Vibrating reed switches

Considerations in level measurement

• Atmospheric vs. pressurized vessel• Conductive vs. non-conductive fluid• Temperature, specific gravity of the fluid• Turbulance, foam, fog, dust, or a mixed layer at fluid

surface• Open flow channels• Head correction, range suppression or elevation• Mixed fluids, fluid boundaries, slurries, solids• Continuous monitoring or point level detection

Dipstick

Dipsticks

Pneumatic Dipstick

• Combines a dipstick and a bubbler tube

Float & mechanical linkage

Archimides Principle

• A body wholly or partly immersed in a fluid is buoyed up with a force equal to the weight of the fluid displace by the body

What is the difference between a float and a displacer?

Chain Float

Bubbler Tube

Capacitive-Type Transducers

Measuring level with pressure differential

Reference to AtmosphereLevel Instruments

Calculating Dry Reference Leg Calibration Values

Calculate zero and span calibration values for the following example:

• Vented tank (at atmospheric pressure)• The tank is full when it has 12’ of water at

room temperature• Pressure tap located 2’ above the bottom

of the tank• Transmitter 2’ below the bottom of the

tank

Dry Leg Level Instrument

Calculating Dry Reference Leg Calibration Values

Calculate zero and span calibration values for the following example:

• Tank pressurized with 2 psi Nitrogen• The tank is full when it has 30’ of water at

room temperature• Pressure tap located 1’ above the bottom

of the tank• Transmitter 4’ below the bottom of the

tank

Wet Reference LegLevel Instrument

Specific Gravity

Water in our pressurizer is xxx degrees but the water in the reference leg is only xxx degrees. How does this effect differential pressure seen at the level transmitter?

Compare indicated to actual level.

What can be done to compensate?

Wet Reference Leg Calibration Factors

• Tank level• Reference leg level• Specific Gravity• Pressure• Temperature

• The ASME steam tables can be used to determine specific gravity for pure water or steam

• http://www.higgins.ucdavis.edu/webMathematica/MSP/Examples/SteamTable

Pressure = H x Sg x D

• H = height of fluid column

• Sg = Specific Gravity of the fluid

• D = Density

Calculating Wet Reference Leg Calibration Values

Calculate zero and span calibration values for the following example:

• Tank pressurized at 2250 pounds with 800 degree F water and steam mixture

• The tank is full when it has 40’ of water at 800o F• The tank is empty when it has 12’ of water• Pressure tap located 1’ above the bottom of the

tank• Transmitter 8’ below the bottom of the tank

Ultrasonic Level Measurement

How does it work?• Ultrasonic sound waves are

pulsed from the transducer about 3 times per second

• The sound waves reflect against the process medium and return to the transducer

• The microprocessor based electronics measure the time of flight between sound generation and receipt

• This time is translated into the distance between the transducer and the process medium below

Conductivity Probe Method

Capacitance Level Sensor

• Also called RF level sensors• Uses the process material as the dielectric of a

capacitor• As the dielectric (level) changes, capacitance

changes• This change (picofarads) is conveted to a level

signal.• We use these on RCP oil levels.

Radar Level Instruments

• Sometimes called microwave• Similar to ultrasonic, but uses 1MHz+ RF• Can be used in high temperature steam

environments (FW heaters)• Mist, steam, dust and foam can be ignored• Non-contact surface level measurement• Low power (2 wire)

On to Flow Measurement