I&C Level Sensors and Detectors
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A direct method used to determine the height of a liquid in a tank, the design usually consists of a ball or sphere attached to a rotating shaft to provide level indication on a calibrated scale
A direct method used to determine the height of a liquid in a tank, similar in design to the ball float but the range of level measured is limited only by the size of the tank
A diaphragm or bellows pressure gauge, connected between two points and is used to measure differential pressure
Differential pressure (D/P) gauge
A pressure detector calibrated to read out tank level and vertical pipe with a constant airflow connected through the side of a tank to displace water in the pipe
Dip tube (Bubbler) level detector
A visual method used to determine the height of a liquid in a tank, usually made from tubular glass or plastic attached directly to the tank
Instrument used to measure the height of a liquid above a reference line or the hydrostatic head developed by the liquid.
A decrease in the reference leg density of a wet reference leg level detector due to an increase in temperature or a rapid decrease in pressure
Reference leg flashing
IDENTIFY the principle of operation of the following types of level instrumentation
Dip Tube (Bubbler) Level Detector
In the gauge glass method, a transparent tube is attached to the bottom and top (top connection not needed in a tank open to atmosphere) of the tank that is monitored. The height of the liquid in the tube will be equal to the height of water in the tank.
The operation of the ball float is simple. The ball floats on top of the liquid in the tank. If the liquid level changes, the float will follow and change the position of the pointer attached to the rotating shaft.
The operation of the chain float is similar to the ball float except in its method of positioning the pointer and its connection to the position indication. The float is connected to a rotating element by a chain with a weight attached to the other end to provide a means of keeping the chain taut during changes in level.
The dip tube is a vertical pipe connected through the side of a tank at the bottom. An air pressure supply is connected to the pipe, and a constant airflow displaces the water in the pipe. The pressure required to displace the water in the vertical pipe is equal to the hydrostatic pressure due to the liquid level in the tank.
IDENTIFY a common failure for Dip Tube (Bubbler) Level Detector
- Problems that can develop with the use of Dip Tube Level Detector
- The bubbler lines are subject to blockage and need to be cleaned periodically in some systems.
- A blockage could result in higher than actual readings.
IDENTIFY a common failure for Gauge glass
- The problems that can develop with the use of gauge glasses are:
- Gasket material failure resulting in leaks.
- Dirt obscuring the glass so that readings cannot be taken.
IDENTIFY a common failure for Ball float
- Problems that can develop with the use of ball floats are:
- Floats may sink causing low readings.
- Mechanism may jam causing no response to level changes
IDENTIFY a common failure for Chain float
- Problems that can develop with the use of chain floats are:
- Floats may sink causing low readings.
- Mechanism may jam causing no response to level changes.
- Mechanism may jam or the chain may slip off the gears causing no response to level changes.
EXPLAIN the theory of operation of the Open vessel level measuring devices using a differential pressure cell
As level increases from a minimum, the D/P increases across the D/P cell. This increasing D/P cell signal is directly related to an increasing level indication. Similarly, as level lowers, the corresponding D/P signal lowers
EXPLAIN the theory of operation of the Dry reference leg level measuring devices using a differential pressure cell
Measuring the liquid level in a pressurized tank requires that both the high pressure and the low pressure sides of a D/P transmitter be connected to the tank. Otherwise, changes in tank pressure will be sensed by the D/P cell and displayed as a level change
- DP = 0 Tank is Empty
- DP = High Tank is Full
EXPLAIN the theory of operation of the Wet reference leg level measuring devices using a differential pressure cell
- Where the content of the tank is a condensable fluid, such as steam, the dry reference leg design cannot be used. The steam condenses in the reference leg. To prevent the condensation from introducing an error, the reference leg is filled with the same liquid as the tank. The reference leg level is maintained constant at the level in the vessel by the condensing action of a condensing pot or a fill connection.
- This column of liquid applies a hydrostatic head pressure to the high pressure side.
- The value of this pressure is constant because the reference leg is maintained full. Since the pressure on the high pressure side remains constant, any change in the differential pressure is due to a pressure change on the low
- pressure side only
- DP = 0 Tank is Full
- DP = HIGH Tank is Empty
Open Tank Open to Atmosphere
Closed Tank with Dry Reference Leg
Closed Tank with a wet reference leg
DESCRIBE the environmental effects on the operation of differential pressure (D/P) level detectors
- The three factors that can affect the accuracy and reliability of level detection instrumentation are:
- density of the fluid
- ambient temperature changes
Given a potential failure mode for a differential pressure cell used for level indication, DESCRIBE how indicated level will be affected:
Reference Leg leak or break
Variable Leg Leak or Break
Reference Leg Flashing
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