ACADs (08-006) Covered

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

Description

Supporting Material

5.4.1.7

Measuring LevelTerminal 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 IndirectDirect: 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 IndirectDipstickFloatSightglassFloat & chainBubbler tubeCapacitancePressure or DP transmitterUltrasonicHJTCWarrick Conductivity LevelMagnetrol

Ways to measure levelAntenna level sensorBubblerCapacitance probeConductivity probeDiaphragm detectorDifferential PressureDipstickDisplacerFloatImpedance probeLevel gaugeMagnetic FollowerOptical levelRadiation level sensorResistance tapesRotating paddlesSightglassSlip tubesTape level devicesThermal level sensorTime domain reflectometryUltrasonic level detectorsVibrating reed switches

Considerations in level measurementAtmospheric vs. pressurized vesselConductive vs. non-conductive fluidTemperature, specific gravity of the fluidTurbulance, foam, fog, dust, or a mixed layer at fluid surfaceOpen flow channelsHead correction, range suppression or elevationMixed fluids, fluid boundaries, slurries, solidsContinuous monitoring or point level detection

DipstickDipsticks

Pneumatic DipstickCombines a dipstick and a bubbler tube

Float & mechanical linkage

Archimides PrincipleA 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 TransducersMeasuring level with pressure differential

Reference to AtmosphereLevel Instruments

Calculating Dry Reference Leg Calibration ValuesCalculate 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 temperaturePressure tap located 2 above the bottom of the tankTransmitter 2 below the bottom of the tank

Dry Leg Level Instrument

Calculating Dry Reference Leg Calibration ValuesCalculate zero and span calibration values for the following example:Tank pressurized with 2 psi NitrogenThe tank is full when it has 30 of water at room temperaturePressure tap located 1 above the bottom of the tankTransmitter 4 below the bottom of the tank

Wet Reference LegLevel Instrument

Specific GravityWater 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 FactorsTank levelReference leg levelSpecific GravityPressureTemperature

The ASME steam tables can be used to determine specific gravity for pure water or steamhttp://www.higgins.ucdavis.edu/webMathematica/MSP/Examples/SteamTable

Pressure = H x Sg x DH = height of fluid column

Sg = Specific Gravity of the fluid

D = Density

Calculating Wet Reference Leg Calibration ValuesCalculate zero and span calibration values for the following example:Tank pressurized at 2250 pounds with 800 degree F water and steam mixtureThe tank is full when it has 40 of water at 800o FThe tank is empty when it has 12 of waterPressure tap located 1 above the bottom of the tankTransmitter 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 secondThe sound waves reflect against the process medium and return to the transducerThe microprocessor based electronics measure the time of flight between sound generation and receiptThis time is translated into the distance between the transducer and the process medium below

Conductivity Probe Method

Capacitance Level Sensor Also called RF level sensorsUses the process material as the dielectric of a capacitorAs the dielectric (level) changes, capacitance changesThis change (picofarads) is conveted to a level signal.We use these on RCP oil levels.

Radar Level InstrumentsSometimes called microwaveSimilar to ultrasonic, but uses 1MHz+ RFCan be used in high temperature steam environments (FW heaters)Mist, steam, dust and foam can be ignoredNon-contact surface level measurementLow power (2 wire)

On to Flow Measurement

**Direct means direct contact with the processFor example, a dipstick, a sightglass, a float or a displacer

Indirect means some other way besides direct contactFor example, a pressure transducer or an ultrasonic device

*Sight glass or gaugeglass

Mention various elaborate arrangements - 2 color sightglasses, backlit

Problems with the specific gravity since the lines can cool considerably from process temperature

-mention oil level in the diesel

*Toilet bowl float*This is inside of a chiller. The float is linked directly to a valve to control the level of freon*Monkey balls or Balls on the walls

Magnetic reed switches along the center column and a magnet in the floating ball

Used in containment for post accident monitoring of containment level*The magnetic bond method was developed to overcome the problems of cages and stuffing boxes. The magnetic bond mechanism consists of a magnetic float which rises and falls with changes in level. The float travels outside of a non-magnetic tube which houses an inner magnet connected to a level indicator. When the float rises and falls, the outer magnet will attract the inner magnet, causing the inner magnet to follow the level within the vessel*Difference between a float and a displacer

*Chain floats used on big tanks, at water rec, Diesel fuel tanks*Magnetrol

*Bubblers used in the spray ponds, circ water canals

Start by explaining what backpressure would be if there were no water (Zero)

Then add an inch of water, then 2 inches, etc

*Theory behind a Rosemount***Mention problem with a bug in the atmospheric reference line

Vent becomes plugged?

*Condensate Storage Tanks, surge tanks EC, EW

Usually tagged as dry leg

What happens if water gets into the dry reference leg? Indicated level goes down?

Talk about condensation into the reference leg

*Pressurizer, steam generators, Equipment drain tank level are wet legs

What happens if the wet reference leg is drained? Indicated level goes up. Mention TMI

What happens if you open equalizer? What happens to level in ref. Leg and what happens to indicated level

Density of the fluid in the dry reference leg - temp compensation*Pressurizer, steam generators, Equipment drain tank level are wet legs.,Share O/ E at Four Corners when the first rosmont xmitters were installed on steam drum level and the heat traced turned on .

What happens if the wet reference leg is drained? Indicated level goes up. Mention TMI

What happens if you open equalizer? What happens to level in ref. Leg and what happens to indicated level

Density of the fluid in the dry reference leg - temp compensation*Milltronics ultrasonic level instrument*Conductivity probes used on RCP bearing oil tanks