Pressure, Level and Flow Detectors

Question 1

State the theory of operation for a Bourdon tube pressure element.

Answer 1

A flattened, thin-walled tube, elliptical in shape and having two long, flat sides and two short, round sides is bent into an arc of a circle. One end is sealed and connect to an amplifying indicating gear, the other is exposed to pressure. The pressure expands the flat sides of the tube and the sealed tip moves. Typical max movement is 1/4 to 3/8 of an inch.

Question 2

What is the accuracy of a Bourdon tube?

Answer 2

An ordinary commercial type is within 1% of the maximum graduation at any point above the first 5% of the scale. A precision type is usually guaranteed accurate within 0.5% of the maximum graduation at any point above the first 5% of the scale.

Question 3

State the theory of operation for a Bellow pressure element.

Answer 3

Usually built as a one-piece, collapsible, seamless, metallic unit with deep folds formed from thin-walled tubing. A spring opposes compression of the bellows and the action of the bellows and the spring provide the indicated pressure via linkage.

Question 4

Where does the travel of the bellows occur - compression or expansion side of equilibrium?

Answer 4

The flexibility of a metallic bellows is similar to that of a helical, coiled compression spring. The relationship between increments of load and deflection is linear up to the elastic limit. However, this linear relationship exists only when the travel of the bellows occurs due to a minimum compressive force. All travel of the bellows must be made on the compressive side of the pressure equilibrium point.

Question 5

State the theory of operation for a Diaphragm pressure element.

Answer 5

It consists of a metal disc built into a housing. One side of the disc is exposed to the pressure to be measured, the other to atmospheric pressure. The distortion of the diaphragm under pressure is transmitted to a gauge via linkage connected to the center of the diaphragm.

Question 6

What are the limitations of a Diaphragm pressure element?

Answer 6

Under pressure, a circular metallic diaphragm will exhibit an “S” shaped deflection curve. Starting at the origin and approaching asymptotically the point of bursting pressure. The proportional limit occurs at approximately 0.5% of the bursting load. The deflection of a flat metallic diaphragm is proportional to pressure linearly only for a small range of low pressures and low vacuums. To correct this the disk is corrugated which produces approximately 4 times the deflection of a flat disk.

Question 7

State the operation of a variable capacitance element.

Answer 7

Any change in the high or low-pressure process fluid is transmitted across isolating diaphragms to the dielectric fluid. The change in the dielectric fluid pressure affects the sensing (center) diaphragm, which acts as the center plate of the capacitor. The center diaphragm deflects in proportion to the change in the differential pressure. As the center sensing diaphragm deflects, the distance between the plates change, therefore, the capacitance changes. By maintaining a constant AC voltage across the capacitor plates, the current will be proportional to the change in the differential pressure.

Question 8

State the theory of operation of a variable capacitance element.

Answer 8

Any change in the high or low-pressure process fluid is transmitted across isolating diaphragms to the dielectric fluid. The change in the dielectric fluid pressure affects the sensing (center) diaphragm, which acts as the center plate of the capacitor. The center diaphragm deflects in proportion to the change in the differential pressure. As the center sensing diaphragm deflects, the distance between the plates change, therefore, the capacitance changes. By maintaining a constant AC voltage across the capacitor plates, the current will be proportional to the change in the differential pressure.

Question 9

State the theory of operation for a Force Balance instrument.

Answer 9

Operates on the principle of balancing forces to provide a signal for measuring temperature, pressure, level or flow. With this instrument a force produced by the input (change in pressure, flow, etc.) is balanced by a force produced by the output (output signal representing pressure or flow). In other words, the output of the instrument (to a meter or recorder) balances the force applied by the sensing element. The output is compared to the input, and balanced. All of the elements discussed previously (bellows, bourdon tube, diaphragm) are used in force balance instruments.

Question 10

What are the two main categories of pressure instrument failures?

Answer 10

Overrange and ruptures of the element

Question 11

What happens to indicated pressure after a Bourdon tube has been overranged?

Answer 11

The pressure indication returns to some value greater than the original. Most gauges are designed to handle approximately 35 percent beyond upper range without damage. Above 35 percent overrange, the gauge will be permanently deformed and will not return to its original shape.

Question 12

What happens to indicated pressure after a Bourdon tube has been overranged?

Answer 12

The pressure indication returns to some value greater than the original. Most gauges are designed to handle approximately 35 percent beyond upper range without damage. Above 35 percent overrange, the gauge will be permanently deformed and will not return to its original shape.

Question 13

What happens to indicated pressure after a Bellows pressure element or a diaphragm pressure element has been overranged?

Answer 13

Higher than actual pressure readings following the overpressure transient.

Question 14

What happens to indicated pressure after ruptures or leaks in bourdon tubes, bellows and diaphragms?

Answer 14

Low pressure readings, typical response is for the reading to fall to 0 psig,

Question 15

What happens to indicated pressure after ruptures or leaks on force balance instruments?

Answer 15

Since force balance instruments typically use bellows and diaphragms as the pressure sensing elements, leaks or ruptures will result in low pressure readings as well.

Question 16

What happens to indicated pressure for a leak or break in the sensing line of a D/P Cell?

Answer 16

Result in a lower differential pressure and a lower than normal pressure reading.

Question 17

What happens to indicated pressure during changes in the reference pressure of a D/P Cell?

Answer 17

If the reference pressure increases, the indicated pressure will decrease. Think Containment Pressurization.

Question 18

For a D/P instrument, what is the purpose of an equalizing valve and what happens when it is operated?

Answer 18

It is for removing the instrument from service. The equalizing valve connects both the lowpressure connection and the high-pressure connection across the bellows of the cell. This prevents overloading the measuring element by exposing it to high pressure on one side only when valving in or out.

Question 19

What effect do changes in temperature have on pressure and differential pressure detectors?

Answer 19

Pressure detectors are usually made of materials with a low temperature coefficient of expansion. Therefore, varying temperature conditions do not cause significant variations or errors in pressure measurements. However changes in the temperatures surrounding pressure detectors filled with fluids will affect the density of the fluids and can cause a change in the indicated pressure due to the change in density.

Question 20

What effect do changes in temperature have on pressure and differential pressure detectors?

Answer 20

Pressure detectors are usually made of materials with a low temperature coefficient of expansion. Therefore, varying temperature conditions do not cause significant variations or errors in pressure measurements. However changes in the temperatures surrounding pressure detectors filled with fluids will affect the density of the fluids and can cause a change in the indicated pressure due to the change in density.

Question 21

What effect do changes in pressure have on pressure and differential pressure detectors?

Answer 21

Variations in the atmospheric pressure surrounding pressure detectors can also affect the accuracy of the pressure instrument. Bourdon tubes can be affected by a change in atmospheric pressure, since atmospheric pressure provides a reference force on the exterior of the flatten bourdon tube.

Question 22

What effect do changes in radiation have on pressure and differential pressure detectors?

Answer 22

Radiation levels near pressure or differential pressure detectors affect the detectors’ reliability. Extremely high radiation environments permanently embrittle the metal of the detectors. This changes the characteristics and elasticity of the sensing mechanism, introducing errors. High radiation levels can also affect sensitive electronic circuits housed in the detectors.

Question 23

State the theory of operation for an open vessel level measuring device using a differential pressure cell.

Answer 23

Because the tank is open to atmospheric pressure, it is necessary to use only the high pressure connection of the D/P transmitter. The low pressure connection is vented to atmosphere. Therefore, the differential pressure applied to the transmitter is a function of the hydrostatic head pressure from the level of liquid in the tank. As level increases from a minimum, the D/P increases across the D/P cell. This increasing D/P cell signal can be directly related to an increasing level indication.

Question 24

State the theory of operation for an open vessel level measuring device using a differential pressure cell.

Answer 24

Because the tank is open to atmospheric pressure, it is necessary to use only the high pressure connection of the D/P transmitter. The low pressure connection is vented to atmosphere. Therefore, the differential pressure applied to the transmitter is a function of the hydrostatic head pressure from the level of liquid in the tank. As level increases from a minimum, the D/P increases across the D/P cell. This increasing D/P cell signal can be directly related to an increasing level indication.

Question 25

State the theory of operation for a dry reference leg level measuring device using a differential pressure cell.

Answer 25

The high pressure (HP) connection of the transmitter is connected to the tank at or below the lower range value to be measured. The low pressure (LP) connection of the transmitter is connected to a “reference leg” that is connected to the tank at or above the upper range value to be measured. The reference leg is pressurized with gas or vapor pressure, but no liquid is permitted to remain in the reference leg. The reference leg must remain dry so that there is no liquid head pressure on the low pressure side of the transmitter.

Question 26

State the theory of operation for a dry reference leg level measuring device using a differential pressure cell.

Answer 26

For closed tanks that can be pressurized, The high pressure (HP) connection of the transmitter is connected to the tank at or below the lower range value to be measured. The low pressure (LP) connection of the transmitter is connected to a “reference leg” that is connected to the tank at or above the upper range value to be measured. The reference leg is pressurized with gas or vapor pressure, but no liquid is permitted to remain in the reference leg. The reference leg must remain dry so that there is no liquid head pressure on the low pressure side of the transmitter.

Question 27

State the theory of operation for a dry reference leg level measuring device using a differential pressure cell.

Answer 27

For closed tanks that can be pressurized, The high pressure (HP) connection of the transmitter is connected to the tank at or below the lower range value to be measured. The low pressure (LP) connection of the transmitter is connected to a “reference leg” that is connected to the tank at or above the upper range value to be measured. The reference leg is pressurized with gas or vapor pressure, but no liquid is permitted to remain in the reference leg. The reference leg must remain dry so that there is no liquid head pressure on the low pressure side of the transmitter.

Question 28

State the theory of operation for a wet reference leg level measuring device using a differential pressure cell.

Answer 28

The reference leg level is maintained continuously full, at the level of the piping from the vessel, by the condensing action of a condensing pot or by a fill connection. This column of liquid applies a hydrostatic head pressure to the high pressure side. The value of this hydrostatic head pressure is constant since the reference leg is maintained full. The high pressure side is now the reference leg, and the low pressure side is the tank level.

Question 29

Level instruments that are density compensated and calibrated for cold conditions indicate what when conditions are hot?

Answer 29

Lower than actual. As a heatup progresses, actual water level will swell due to expansion of the water. Since water mass is not changing, indicated level remains constant.

Question 30

If a wet reference leg leaks or breaks for a differential cell used for level indication, how will indicated level be affected?

Answer 30

DP goes down and indicated level goes up.

Question 31

If a wet reference leg leaks or breaks for a differential cell used for level indication, how will indicated level be affected?

Answer 31

Actual level remains the same, DP goes down and indicated level goes up.

Question 32

If a variable leg leaks or breaks for a differential cell used for level indication, how will indicated level be affected?

Answer 32

Actual level goes down, DP goes up and indicated level goes down.

Question 33

For level detectors with a wet reference leg connected to the “high pressure” side of a D/P cell, If a reference leg leaks or breaks, how will indicated level be affected?

Answer 33

Actual level remains the same, DP goes down and indicated level goes up. creates a lower D/P sensed across the D/P cell, resulting in an indicated level higher than actual level.

Question 34

For level detectors with a wet reference leg connected to the “high pressure” side of a D/P cell, if a variable leg leaks or breaks, how will indicated level be affected?

Answer 34

Actual level goes down, DP goes up and indicated level goes down. A break in the variable leg of the D/P cell creates a higher D/P being sensed by the D/P cell, resulting in the level instrument indicating a low level.

Question 35

For level detectors with a wet reference leg connected to the “high pressure” side of a D/P cell, if a diaphragm leaks or breaks, how will indicated level be affected?

Answer 35

This allows the high pressure and low pressure chambers to equalize and produces a minimum D/P signal. The minimum D/P signal will result in a maximum level indication. Same as leakby on an equalizing valve.

Question 36

For level detectors with a wet reference leg connected to the “high pressure” side of a D/P cell, if a diaphragm leaks or breaks, how will indicated level be affected?

Answer 36

This allows the high pressure and low pressure chambers to equalize and produces a minimum D/P signal. The minimum D/P signal will result in a maximum level indication. Same as leakby on an equalizing valve.

Question 37

For level detectors with a wet reference leg connected to the “high pressure” side of a D/P cell, what impact do temperature changes have?

Answer 37

An increase in ambient temperature will cause the density of the wet reference leg to decrease. This will result in a lower D/P sensed by the D/P cell, and indicated level will be greater than actual level. The opposite effect produces a lower indicated level when the ambient temperature decreases.

Question 38

For level detectors with a wet reference leg connected to the “high pressure” side of a D/P cell, what impact do pressure changes have?

Answer 38

Level measuring differential pressure cells are not directly affected by changes in ambient pressure. Any pressure variations in the environment are felt on both sides of the open vessel D/P level instrument and cancel each other. The dry and wet reference leg level detectors are only exposed to system pressure and therefore are not affected.

Question 39

For level detectors with a wet reference leg connected to the “high pressure” side of a D/P cell, if a diaphragm leaks or breaks, how will indicated level be affected?

Answer 39

This allows the high pressure and low pressure chambers to equalize and produces a minimum D/P signal. The minimum D/P signal will result in a maximum level indication. Same as leakby on an equalizing valve.

Question 40

For level detectors with a wet reference leg connected to the “high pressure” side of a D/P cell, if flashing occurs in the reference leg, how will indicated level be affected?

Answer 40

The D/P cell senses a smaller differential pressure due to the reduced height of water in the reference leg. The smaller differential pressure will indicate a vessel water level that is greater than the actual level.

Question 41

What are the two major categories of flow measurement?

Answer 41

(1) flow rate and (2) total flow (total quantity). Flow rate is the amount of fluid that moves past a given point per unit of time. Total flow is the amount of fluid that moves past a given point during a specified time.

Question 42

Describe the theory of operation for an orifice plate flow detection device.

Answer 42

The orifice plate is a thin, circular metal plate with a sharp-edged hole. Flow through a sharp-edged orifice plate is characterized by a change in velocity that reaches a maximum at a point downstream from the orifice. DP across the high side (upstream of the plate) and low side (downstream of the plate) is converted to flow measurement.

Question 43

Describe the theory of operation for an orifice plate flow detection device.

Answer 43

The orifice plate is a thin, circular metal plate with a sharp-edged hole. Flow through a sharp-edged orifice plate is characterized by a change in velocity that reaches a maximum at a point downstream from the orifice. DP across the high side (upstream of the plate) and low side (downstream of the plate) is converted to flow measurement.

Question 44

Describe the theory of operation for a Venturi tube flow detection device.

Answer 44

Consists of a converging conical inlet section, a cylindrical throat, and a diverging recovery cone. The high pressure tap is at a distance one-half the pipe diameter upstream of the inlet cone. The low pressure tap is at the center of the throat regardless of the flow direction.

Question 45

Describe the theory of operation for a Venturi tube flow detection device.

Answer 45

Consists of a converging conical inlet section, a cylindrical throat, and a diverging recovery cone. The high pressure tap is at a distance one-half the pipe diameter upstream of the inlet cone. The low pressure tap is at the center of the throat regardless of the flow direction.

Question 46

Describe the theory of operation for a flow nozzle flow detection device.

Answer 46

Uses changes in It is simpler in design and less expensive than the venturi tube. It is widely used for the measurement of high velocity flow rates. The flow nozzle can measure flow rates up to 60% greater than an orifice plate of the same diameter and at the same DP

Question 47

Describe the theory of operation for a flow nozzle flow detection device.

Answer 47

Uses changes in DP. It is simpler in design and less expensive than the venturi tube. It is widely used for the measurement of high velocity flow rates. The flow nozzle can measure flow rates up to 60% greater than an orifice plate of the same diameter and at the same DP

Question 48

Describe the theory of operation for a flow nozzle flow detection device.

Answer 48

Uses changes in DP. It is simpler in design and less expensive than the venturi tube. It is widely used for the measurement of high velocity flow rates. The flow nozzle can measure flow rates up to 60% greater than an orifice plate of the same diameter and at the same DP

Question 49

Describe the theory of operation for an elbow meter flow detection device.

Answer 49

The elbow meter is another flow measuring device based on differential pressure. As the fluid changes direction in a pipe bend, a low pressure area is created on the inner pipe radius. A high pressure area is created on the outer pipe radius. This pressure difference is proportional to the square of the volumetric flow rate. The tap locations are critical.

Question 50

Describe the theory of operation for an elbow meter flow detection device.

Answer 50

The elbow meter is another flow measuring device based on differential pressure. As the fluid changes direction in a pipe bend, a low pressure area is created on the inner pipe radius. A high pressure area is created on the outer pipe radius. This pressure difference is proportional to the square of the volumetric flow rate. The tap locations are critical.

Question 51

Describe the theory of operation for a differential pressure detector flow detection device.

Answer 51

Flow rate is directly proportional to the square root of the differential pressure.

Question 52

Identify the effects of gas or steam on liquid flow rate indication.

Answer 52

Gases and vapors in liquids tend to collect at the top of the upstream face. These conditions change the density of the upstream fluid and cause inaccurate flow measurement. If gas or steam bubbles pass through the liquid flow detector orifice, the fluid density will fluctuate. This small amount of gas passing through the flow orifice will cause pressure fluctuations. These pressure fluctuations will be picked up by the D/P cell and will ultimately cause fluctuations on the indicating flow meter.

Question 53

State the reason for density compensation in flow detectors, and explain the process used to accomplish density compensation.

Answer 53

Density compensation converts volumetric flow rate to mass flow rate using a pressure signal to boost the D/P signal.

Question 54

State the reason for density compensation in flow detectors, and explain the process used to accomplish density compensation.

Answer 54

Density compensation converts volumetric flow rate to mass flow rate using a pressure signal to boost the D/P signal. Without density compensation, the indicated flow is less than actual flow.

Question 55

Describe the theory of operation for a differential pressure detector flow detection device.

Answer 55

Flow rate is directly proportional to the square root of the differential pressure. if the flow rate increases the D/P increase. And if the flow rate decreases the D/P decreases.

Question 56

Given a potential failure mode for a differential pressure cell used for flow indication, describe how the indicated parameter will be affected for a reference leg leak.

Answer 56

If a leak develops on the high pressure side of the D/P cell, the high pressure side pressure will lower. The net change will be a decrease in the D/P and reduced indicated flow. Similarly, if the low pressure piping leaks, an increase in sensed D/P results, and a higher than actual flowrate is indicated.

Question 57

Given a potential failure mode for a differential pressure cell used for flow indication, describe how the indicated parameter will be affected for a reference leg leak.

Answer 57

If a leak develops on the high pressure side of the D/P cell, the high pressure side pressure will lower. The net change will be a decrease in the D/P and reduced indicated flow. Similarly, if the low pressure piping leaks, an increase in sensed D/P results, and a higher than actual flowrate is indicated.

Question 58

Given a potential failure mode for a differential pressure cell used for flow indication, describe how the indicated parameter will be affected for a diaphragm leak.

Answer 58

Similar to the high pressure piping leak, a ruptured diaphragm or an open or leaking equalizing valve will decrease the D/P across the D/P cell to minimum. The reduced differential pressure will indicate minimum flow.

Question 59

Given a potential failure mode for a differential pressure cell used for flow indication, describe how the indicated parameter will be affected for a diaphragm leak.

Answer 59

Similar to the high pressure piping leak, a ruptured diaphragm or an open or leaking equalizing valve will decrease the D/P across the D/P cell to minimum. The reduced differential pressure will indicate minimum flow.

Question 60

Given a potential failure mode for a differential pressure cell used for flow indication, describe how the indicated parameter will be affected for a eroded or plugged orifice.

Answer 60

If an orifice or flow nozzle erodes to a larger size, then the indicated flow will decrease due to the decrease in D/P across the orifice or nozzle. Debris lodged in an orifice plate will increase D/P and cause indicated flow to be higher than actual.

Question 61

Given a potential failure mode for a differential pressure cell used for flow indication, describe how the indicated parameter will be affected for a eroded or plugged orifice.

Answer 61

If an orifice or flow nozzle erodes to a larger size, then the indicated flow will decrease due to the decrease in D/P across the orifice or nozzle. Debris lodged in an orifice plate will increase D/P and cause indicated flow to be higher than actual.

Question 62

Given a potential failure mode for a differential pressure cell used for flow indication, describe how the indicated parameter will be affected for a failure of density compensation signal.

Answer 62

If the pressure signal for a density compensated flow instrument fails low, indicated flow will decrease to a lower value. Conversely, if the pressure signal for a density compensated flow instrument fails high, indicated flow will increase to a higher value. This is due to the fact that changes in pressure will cause changes in density without a corresponding change in sensed DP.