Level and Pressure Measurement Christopher Foot Patrick Lawlor

Level Measurement Why is it important? Three Mile Island

Level Measurement Assess the Situation  Can the level sensor be inserted into the tank or should it be completely external?  Should the sensor detect the level continuously or will a point sensor be adequate?  Can the sensor come in contact with the process fluid or must it be located in the vapor space?  Is direct measurement of the level needed or is indirect detection of hydrostatic head (which responds to changes in both level and density) acceptable?  Is tank depressurization or process shut-down acceptable when sensor removal or maintenance is required?

Level Measurement 5 Main Types  Radar  Differential Pressure  Admittance-probe  Ultrasonic  Nuclear

Radar Level Measurement Consists of Transmitter, Antenna, receiver/signal processor, operator interface. Electromagnetic wave sent downward toward surface of process fluid, reflected back and collected Level of tank based on difference of time from emission to collection to EM wave.

Radar Level Measurement Advantages  Can detect the liquid level a layer of light dust or airy foam.  Least expensive of 5 common types  Automated calibration. Disadvantages  If foam layer is too thick, level of foam will be transmitted, not process fluid  Interference from rat-holing, angled process material surfaces, solid deposition on antenna

Differential Pressure Level Measurement Also know as Hydrostatic Tank Gauging Based on density of fluid and pressure changes  P = h (SG)

Differential Pressure Level Measurement Advantages  Can be used in either open top or pressurized vessels  Simple theory – easy to apply Disadvantages  Bubbles in system (affecting SG) can cause errors in measurement  Error associated with pressure measurement passed along to level measurement

Admittance-probe Level Measurement Operate in the low MHz range measuring admittance of an AC current circuit that varies with level. Based on Dielectric Constant of the material Often used as level switches

Admittance-probe Level Measurement Advantages  Good for level measurement of solids and other non- reflective materials  Sensitivity easily adjusted by varying probe parameters Disadvantages  Materials with high conductivity can cause a short within the probe  Temperature has a large effect on the system  Likelihood of false indications high with fluids

Ultrasonic Level Measurement Also known as SONAR Similar to radar – issues an electronic pulse (sound wave) and measures time until echo received

Ultrasonic Level Measurement Advantages  Instruments include temperature compensation, data filters and self-calibration options  Can be used to detect the interface level between 2 liquids Disadvantages  Cannot be used with agitated liquids  Signal will be absorbed by foam, dust, mist, humidity

Nuclear Level Measurements Uses Radiation absorption to directly measure level using point source radiation and strip detection Highest absorption indicates lowest level in vessel

Nuclear Level Measurements Advantages  Can be used to measure either level or density of a process material  Radiation source can be very long-lasting, often outliving the process  Can be installed outside the process avoiding process interruptions as radioactivity can “see” through walls Disadvantages  Radiation safety is very involved and can inhibit the process  Measurements can be skewed by density

Pressure Measurement Why is it important?

Pressure Measurement Asses the situation  What is the range of pressures to be measured?  Is pressure dynamic or static?  What is more important: absolute accuracy or good repeatability?  How much overpressure protection is required?

Types of Measurement Mechanical  U-tube manometer, Bourdon tube, Diaphragm and Bellows Electrical  Strain Gauge, Capacitive sensor, Potentiometric, Resonant Wire, Piezoelectric, Magnetic, Optical

Mechanical Mechanical pressure measurement devices are large and cumbersome. Not suited for automated control loops typical in industry. Mechanical devices:  U-tube Manometer  Bourdon tube  Diaphragm and Bellows element

U-tube Manometer Measures difference in pressure between two points in a pipe. Typical in laboratories.

Bourdon Type Flexible element used as sensor. Pressure changes cause change in element position. Element connected to pointer to reference pressure.

Diaphragm and Bellows Element Similar concept to Bourdon type. Widely used because they require less space and can be made from materials that resist corrosion.

Electrical Have become more common with increased reliance on computerized control systems.

Strain Gauge Measures deflection of elastic diaphragm due to pressure difference across diaphragm. Widely used in industry. Used for small pressure ranges. Measurements tend to drift.

Capacitive Sensor Measures changes in capacitance of electrically charged electrodes from movement of metal diaphragm due to pressure difference across diaphragm.

Capacitive Sensor, cont. Can be operated in balanced or unbalanced mode.  Balanced always has capacitance of zero. Measures pressure indirectly by measuring drift in capacitor arms.  Unbalanced measures ratio between output voltage and excitation voltage. Widely used in industry. Large rangeability.

Resonant Wire Wire is oscillated at resonant frequency by oscillator circuit. Pressure changes cause change in wire tension which changes oscillatory frequency. Generates digital signal. Very precise, used for low differential pressure measurements. Sensitive to temperature variation and has non-linear output

Piezoelectric Measures the charge developed across quartz crystal due to change in pressure. Charge decays rapidly making unsuitable for static pressure measurements. Sensors are very rugged. Pressure can be applied longitudinally or transversally. Used to measure dynamic pressure changes associated with explosions and pulsations.

Magnetic Measures induced current caused by movement of magnetic components from pressure changes. Used in applications where high resolution in small range is desired due to very high output signals. Sensitive to stray magnetic fields and temperature changes.

Optical Detects effects of minute motions due to process pressure changes through partial blocking of an LED. Immune to temperature effects. Excellent stability and long-duration capability.

Questions?

References ns/volume3/pressure.html ns/volume3/pressure.html ns/volume4/ ns/volume4/ Exp16/U-Tube/utube_schematic.htm Exp16/U-Tube/utube_schematic.htm