Instrumentation Sensors Book

Thermal probes consist of a heating element adjacent to a temperature sensor. When the liquid rises above the probe, the heat is dissipated and the temperature at the sensor drops. The probe is a simple, low-cost, and reliable device for single point sensing.

Beam breaking methods are sometimes used for pressurized containers. For single point measurement, as shown in Figure 8.12(a), only one transmitter and one detector are required. The beams can be light, sonic or ultrasonic waves, or radiation. The devices are low-cost and of simple construction, but can be affected by deposits. If several single point levels are required, a detector will be required for each level measurement, as shown in Figure 8.12(b). The disadvantages of this radiation system are the cost, the need for special engineering, and the need to handle radioactive material. However, this system can be used with corrosive or very hot liquids. High-pressure containers are used where conditions would be detrimental to the installation of other types of level sensors.

8.2.4Level Sensing of Free-Flowing Solids

Paddle wheels driven by electric motors can be used for sensing the level of solids that are in the form of power, grains, or granules. When the material reaches and covers the paddle wheel, the torque needed to turn the paddles greatly increases. The torque can be an indication of the depth of the material. Such a setup is shown in Figure 8.13(a). Some agitation may be required to level the solid particles. This is an inexpensive device and is good for most free-flowing materials, but is susceptible to vibration and shock. If the density of the material is greater than 0.9 lb/ft3 (12.8 kg/m3), then a vibration device can be used, as shown in Figure 8.13(b). The probe vibrates at the natural frequency of a tuning fork, and the frequency changes when in contact with a material. The change in resonant frequency is detected. The probe, which has no moving parts, is rugged, reliable, and only requires low maintenance, but its operation can be affected by other vibration sources. These devices may need protection from falling materials, and the proper location of the probe is essential for correct measurement [10].

Figure 8.12 Liquid level measurements, made using (a) single point beam breaking, or (b) multipoint beam breaking.

Paddle wheel

Vibration sensor

Motor

Vibration unit

Figure 8.13 Sensors for measuring free-flowing solids: (a) paddle wheel, and (b) vibration device.

8.3Application Considerations

A number of factors affect the choice of sensor for level measurement, such as pressure on the liquid, temperature of the liquid, turbulence, volatility, corrosiveness, level of accuracy required, single point or continuous measurement, direct or indirect, particulates in a liquid, free-flowing solids, and so forth [11]. Table 8.2 gives a comparison of the characteristics of level sensors.

When considering the choice of level sensor, temperature effects are a major consideration. Density and dielectric constants are affected by temperature, making indirect level measurements temperature-sensitive (see Chapter 10). This makes it necessary to monitor temperature as well as level, so that the level reading can be compensated. The types of sensors needing temperature compensation are: capacitive, bubblers, pressure, displacers, ultrasonic distance-measuring devices, and load cells.

Floats are often used to sense fluid levels, since they are unaffected by particulates, can be used for slurries, can be used with a wide range of liquid specific

Table 8.2 Level Measuring Devices

weights. Due to their shape, flat floats are less susceptible to turbulence on the surface of the liquid. When the float is used to measure more than 50 cm of liquid depth, any change in float depth will have minimal effect on the measured liquid depth.

Displacers must never be completely submerged when measuring liquid depth, and must have a specific weight greater than that of the liquid. Care must also be taken to ensure that the liquid does not corrode the displacer, and the specific weight of the liquid is constant over time. The temperature of the liquid also may have to be monitored to make corrections for density changes. Displacers can be used to measure depths up to approximately 3m with an accuracy of ±0.5 cm.

Capacitive device accuracy can be affected by the placement of the device. The dielectric constant of the liquid also should be regularly monitored. Capacitive devices can be used in containers that are pressurized up to 30 MPa, can be used in temperatures up to 1,000°C, and can measure depths up to 6m with an accuracy of ±1%

Pressure gauge choice for measuring liquid levels can depend on the following considerations:

1.The presence of particulates, which can block the line to the gauge;

2.Damage caused by excessive temperatures in the liquid;

3.Damage due to peak pressure surges;

4.Corrosion of the gauge by the liquid;

5.If the liquid is under pressure a differential pressure gauge is needed;

6.Distance between the tank and the gauge;

7.Use of manual valves for gauge repair.

Differential pressure gauges can be used in containers with pressures up to 30 MPa and temperatures up to 600°C, with an accuracy of ±1%. The liquid depth depends on its density and the pressure gauge used, and temperature correction is required.

Bubbler devices require certain precautions to ensure a continuous supply of clean dry air or inert gas (i.e., the gas used must not react with the liquid). It may be necessary to install a one way valve to prevent the liquid being sucked back into the gas supply lines if the gas pressure is lost. The bubbler tube must be chosen so that the liquid does not corrode it. Bubbler devices are typically used at atmospheric pressure. An accuracy of approximately 2% can be obtained, and the depth of the liquid depends on gas pressure available.

Ultrasonic devices can be used in containers with pressures up to 2 MPa, temperatures up to 100°C, and depths up to 30m, with an accuracy of approximately 2%.

Radiation devices are used for point measurement of hazardous materials. Due to the hazardous nature of the materials, personnel should be trained in its use, transportation, storage, identification, and disposal, observing rules set by OSHA.

Other considerations are that liquid level measurements can be affected by turbulence, readings may have to be averaged, and/or baffles may have to be used to reduce the turbulence. Frothing in the liquid also can be a source of error, particularly with resistive or capacitive probes.

8.4Summary

This chapter introduced the concepts of level measurement. Level measurements can be direct or indirect continuous monitoring, or single point detection. Direct reading of liquid levels using ultrasonic devices is noncontact, and can be used for corrosive and volatile liquids and slurries.

Indirect measurements involve the use of pressure sensors, bubblers, capacitance, or load cells, which are all temperature-sensitive and will require temperature data for level correction. Of these sensors, load cells do not come into contact with the liquid, and are therefore well suited for the measurement of corrosive, volatile, and pressurized liquids and slurries.

Single point monitoring can use conductive probes, thermal probes, or ultrasonic or radioactive devices. Of these devices, the ultrasonic and radioactive devices are noncontact, and can be used with corrosive and volatile liquids, and in pressurized containers. Care has to be taken in handling radioactive materials.

The measurement of the level of free-flowing solids can be made with capacitive probes, a paddle wheel, or with a vibration-type of device.

References

[1]Harrelson, D., and J. Rowe, “Multivariable Transmitters, A New Approach for Liquid Level,” ISA Expo. 2004.

[2]Vass, G., “The Principles of Level Measurement,” Sensors Magazine, Vol. 17, No. 10, October 2000.

[3]Battikha, N. E., The Condensed Handbook of Measurement and Control, 2nd ed., ISA, 2004, pp. 97–116.

[4]Totten, A., “Magnetostrictive Level Sensors,” Sensors Magazine, Vol. 19, No. 10, October 2002.

[5]Hambrice, K., and H. Hooper, “A Dozen Ways to Measure Fluid Level and How They Work,” Sensors Magazine, Vol. 21, No. 12, December 2004.

[6]Considine, D. M., “Fluid Level Systems,” Process/Industrial Instruments and Control Handbook, 4th ed., McGraw-Hill , 1993, pp. 4.130–4.136.

[7]Gillum, D. R., “Industrial Pressure, Level, and Density Measurement,” ISA, 1995.

[8]Omega Engineering, Inc., “Level Measurement Systems,” Omega Complete Flow and Level Measurement Handbook and Encyclopedia, Vol. 29, 1995.

[9]Liptak, B. E., “Level Measurement,” Instrument Engineer’s Handbook Process Measurement and Analysis, 3rd ed., Vol. 2, Chilton Book Co., pp. 269–397.

[10]Koeneman, D. W., “Evaluate the Options for Measuring Process Levels,” Chemical Engineering, July 2000.

[11]Paul, B. O., “Seventeen Level Sensing Methods,” Chemical Processing, February 1999.

Figure 9.1 Flow velocity variations across a pipe with (a) laminar flow, and (b) turbulent flow.

approximately 5,000, the flow is always turbulent. The Reynolds number is a derived dimensionless relationship, combining the density and viscosity of a liquid with its velocity of flow and the cross-sectional dimensions of the flow, and takes the form:

= VDρ

R (9.1)

µ

where V is the average fluid velocity, D is the diameter of the pipe, is the density of the liquid, and is the absolute viscosity.

Dynamic or absolute viscosity is used in the Reynolds flow equation. Table 9.1 gives a list of viscosity conversions. Typically, the viscosity of a liquid decreases as temperature increases.

Example 9.1

What is the Reynolds number for glycerin flowing at 7.5 ft/s in a 17-in diameter pipe? The viscosity of glycerin is 18 × 10−3 lb s/ft2 and the density is 2.44 lb/ft3.

= 7.5 × 17 × 2.44 =

R 1,440 12 × 18 × 10−3

Flow rate is the volume of fluid passing a given point in a given amount of time, and is typically measured in gallons per minute (gal/min), cubic feet per minute (ft3/min), liters per minute (L/min), and so forth. Table 9.2 gives the flow rate conversion factors.

In a liquid flow, the pressures can be divided into the following: (1) static pressure, which is the pressure of fluids or gases that are stationary (see point A in

Table 9.1 Conversion Factors for Dynamic and Kinematic Viscosities

Figure 9.3 Flow diagram for use in the continuity equation with (a) constant area, and (b) differential areas.

Example 9.3

If a pipe changes from a diameter of 17 to 11 cm, and the velocity in the 17cm section is 5.4 m/s, what is the average velocity in the 11cm section?

Mass flow rate (F) is related to volume flow rate (Q) by:

where F is the mass of liquid flowing, and is the density of the liquid. Since a gas is compressible, (9.3) must be modified for gas flow to:

where 1 and 2 are specific weights of the gas in the two sections of pipe. Equation 9.3 is the rate of mass flow in the case of a gas. However, this could

also apply to liquid flow, by multiplying both sides of the (9.3) by the specific weight (), to give the following:

9.2.3Bernoulli Equation

The Bernoulli equation (1738) gives the relation between pressure, fluid velocity, and elevation in a flow system. When applied to Figure 9.4(a), the following is obtained:

Figure 9.4 Container diagrams: (a) the pressures at points A and B are related by the Bernoulli equation, and (b) application of the Bernoulli equation to determine flow.

where PA and PB are absolute static pressures at points A and B,A and γB are specific weights, VA and VB are average fluid velocities, g is the acceleration of gravity, and ZA and ZB are elevations above a given reference level (e.g., ZA − ZB is the head of fluid).

The units in (9.6) are consistent, and reduce to units of length as follows:

Potential Energy = Z = ft(m)

This equation is a conservation of energy equation, and assumes no loss of energy between points A and B. The first term represents energy stored due to pressure; the second term represents kinetic energy, or energy due to motion; and the third term represents potential energy, or energy due to height. This energy relationship can be seen if each term is multiplied by mass per unit volume, which cancels, since the mass per unit volume is the same at points A and B. The equation can be used between any two positions in a flow system. The pressures used in the Bernoulli equation must be absolute pressures.

In the fluid system shown in Figure 9.4(b), the flow velocity V at point 3 can be derived from (9.7), and is as follows, using point 2 as the reference line:

Point 3 at the exit has dynamic pressure, but no static pressure above 1 atm. Hence, P3 = P1 = 1 atm, and1 = 3. This shows that the velocity of the liquid flowing out of the system is directly proportional to the square root of the height of the liquid above the reference point.