2.0 Sensors

 

 

 

 

Sensors are used for process monitoring and for process control.  These are essential elements of safe and profitable plant operation that can be achieved only if the proper sensors are selected and installed in the correct locations.  While sensors differ greatly in their physical principles, their selection can be guided by the analysis of a small set of issues, which are presented in this section.  Each issue is introduced here with process examples, and details on the issues are provided in the remainder of this site for the most common sensors in the process industries.

 

 

Exercise 2.01 You have just started your first job as an engineer.  You supervisor presents you with the process drawing in Figure 2.01.  She asks you to provide sensors for this process.  “Please have your proposal ready tomorrow for the design review meeting.”

 

You have two challenges.

1. What variables should be measured?
2.What sensor should be specified for each measurement?

 

For guidance on selecting the process variables, see Chapter 2 and Chapter 24 in Marlin (2000).  Guidance on selecting sensors is provided in this site, with an introduction to the key issues in this section.

 

 

Figure 2.01 Reactor with feed-effluent heat exchanger.

 

When defining sensor requirements and principles, the engineer should use terminology that has a unique meaning, which is not easily achieved.  Therefore, the engineer should refer to accepted standards and use the terminology provided in the standards.  For instrumentation, standards published by the ISA (formerly, Instrument Society of America) are the most relevant.  This section uses terms from the ISA wherever possible.

 

2.0.1 Major issues for selecting sensors

The major issues in sensor selection are summarized in the following.  The relative importance of each issue depends upon the specific application; for example, one application might require excellent accuracy, while another might require only moderate accuracy, but high reliability.  Generally, we find that the greater the requirements for good performance, the higher the cost for purchase and maintenance. Therefore, we must find the proper balance of performance and cost, rather than always specify the best performing sensor.


 

ISSUE

 

COMMENTS

·         Accuracy - Accuracy is the degree of conformity of the measured value with the accepted standard or ideal value, which we can take as the true physical variable.  Accuracy is usually reported as a range of maximum inaccuracy.  These ranges should have a significance level, such as 95% of the measurements will be within the inaccuracy range.

 

Accuracy is needed for some variables, such as product quality, but it is not required for others such as level in a large storage tank.  See Section 24.3 in Marlin (2000) for a discussion on the needs of sensor accuracy.

 

Accuracy is usually expressed in engineering units or as a percentage of the sensor range, for example:

 

Thermocouple temperature sensor with accuracy of ± 1.5 K.

 

Orifice flow meters with accuracy of ±3% of maximum flow range.

·         Repeatability – The closeness of agreement among a number of consecutive measurements of the same variable (value) under the same operating conditions, approaching in the same direction.

 

The term “approaching in the same direction” means that the variable is increasing (decreasing) to the value for all replications of the experiment.

·         Reproducibility – The closeness of agreement among a number of consecutive measurements of the same variable (value) under the same operating conditions over a period of time, approaching from both directions.  This is usually expressed as non-reproducibility as a percentage of range (span).

 

Often, an important balance is between accuracy and reproducibility, with the proper choice depending on each process application.

 

The period of time is “long”, so that changes occurring over longer times of plant operation are included.

 

Reproducibility includes hysteresis, dead band, drift and repeatability.

·         Range/Span - Most sensors have a limited range over which a process variable can be measured, defined by the lower and upper range values.  Usually, the larger the range, the poorer the accuracy, and reproducibility.  Therefore, engineers select the smallest range that satisfies the process requirements.

 

We select ranges that are easily interpreted by operating personnel, such as 100-200 °C, but not 100-183 °C.

 

If a chemical reactor typically operates at 300 °C, the engineer might select a range of 250-350 °C.

 

Since the reactor will be started up from ambient temperature occasionally, an additional sensor should be provided with a range of -50 to 400 °C.

·         Reliability – Reliability is the probability that a device will adequately perform (as specified) for a period of time under specified operating conditions.  Some sensors are required for safety or product quality, and therefore, they should be very reliable.  Reliability is affected by maintenance and consistency with process environment.  Also, some sensors are protected from contact with corrosive process environment by a cover or sheath (e.g., a thermowell for a thermocouple), and some sensors require a sample to be extracted from the process (e.g., a chromatograph).

 

If sensor reliability is very important, the engineer can provide duplicate sensors, so that a single failure does not require a process shutdown.  See Chapter 22 in Marlin (2000) for the use of duplicate sensors in process control.

·         Linearity - This is the closeness to a straight line of the relationship between the true process variable and the measurement.  Lack of linearity does not necessarily degrade sensor performance.  If the nonlinearity can be modeled and an appropriate correction applied to the measurement before it is used for monitoring and control, the effect of the non-linearity can be eliminated.  Typical examples of compensating calculations are the square root applied to the orifice flow sensor and the polynomial compensation for a thermocouple temperature sensor.  The engineer should not assume that a compensation for non-linearity has been applied, especially when taking values from a history database, which does not contain details of the measurement technology.

 

 

Linearity is usually reported as non-linearity, which is the maximum of the deviation between the calibration curve and a straight line positioned so that the maximum deviation is minimized.

 

See ISA (1979) for further details and several alternative definitions of linearity.

·         Maintenance - Sensors require occasional testing and replacement of selected components that can wear.  Engineers must know the maintenance requirements so that they can provide adequate spare parts and personnel time.  Naturally, the maintenance costs must be included in the economic analysis of a design.

 

On-stream analyzers usually require the greatest amount of maintenance.  The cost associated with maintenance can be substantial and should not be overlooked in the economic analysis.

·         Consistency with process environment - Most sensors will function properly for specific process conditions.  For example, many flow sensors function for a single phase, but not for multi-phase fluid flow, whether vapor-liquid or slurry.  The engineer must observe the limitations for each sensor.

 

Some sensors can have direct contact with the process materials, while others must be protected.  Three general categories are given in the following.

  • Direct contact - Sensors such as orifice plates and level floats have direct contact with process fluids.
  • Sheath protection - Sensors such as thermocouples and pressure diaphragms have a sheath between the process fluid and the sensor element.
  • Sample extraction - When the process environment is very hostile or the sensor is delicate and performs a complex physiochemical transformation on the process material, a sample can be extracted.

 

Naturally, the parts of the sensor that contact the process must be selected appropriately to resist corrosion or other deleterious effects.

 

A float can indicate the interface for a liquid level.  However, a float is not reliable for a “sticky” liquid.

 

Also, a turbine flow meter can be damaged by a rapid change in flow rate or liquid entrained in a vapor stream.

 

 

 

 

Sensors in direct contact must not be degraded by the process material.

 

 

The sheath usually slows the sensor response.

 

Samples must represent the fluid in the process.

·         Dynamics - The use of the sensor dictates the allowable delay in the sensor response.  When the measured value is used for control, sensor delays should be minimized, while sensors used for monitoring longer-term trends can have some delay.

 

A greater delay is associated with sensors that require a sample to be extracted from the process.

 

On-stream analyzers usually have the longest delays, which are caused by the time for analysis.

·         Safety - The sensor and transmitter often require electrical power.  Since the sensor is located at the process equipment, the environment could contain flammable gases, which could explode when a spark occurs.

 

Standards for safety have been developed to prevent explosions.  These standards prevent a significant power source, oxidizing agent and flammable gas from being in contact.

·         Cost - Engineers must always consider cost when making design and operations decisions.  Sensors involve costs and when selected properly, provide benefits.  These must be quantified and a profitability analysis performed.

 

In some cases, a sensor can affect the operating costs of the process.  An example is a flow sensor.  In some situations, the pumping (or compression) costs can be high, and the pressure drop occurring because of the sensor can significantly increase the pumping costs.  In such situations, a flow sensor with a low (non-recoverable) pressure drop is selected.

 

Remember that the total cost includes costs of transmission (wiring around the plant), installation, documentation, plant operations, and maintenance over the life of the sensor.

 

See a reference on engineering economics to learn how to consider costs over time, using principles of the time value of money and profitability measures.

 

Links to WWW Resources

The engineer should understand the common sources of measurements errors and how these errors are evaluated experimentally, characterized and considered when several elements are used in series. The following WWW sites provide useful material for further study.

 

The US National Institute of Science and Technology (NIST) has replaced the old National Bureau of Standards and provides information on measurement.

 

The following sample chapters are available from the book by Wheeler and Ganji (2004). Chapter 2 gives a nice survey of measurement issues

 

 

Exercise 2.02 The drawing in Figure 2.02 shows sensors that could be used for monitoring and controlling the process. 

 

How would you select the proper physical principle for each sensor?  For example, what principle should be used by flow sensor FC-1; orifice, venture meter, pitot tube, positive displacement, turbine, or other?  The resources on this site provide information for making these decisions.

 

 

 

Figure 2.02 Reactor design from Figure 2.1 with sensors added.

 

 

 2.0.2 Location of measurement displays

The measurement is displayed for observation by plant personnel.  Typically, the display uses analog principles, which means that the display presents the measurement as a position in a graphical format, which could, for example, be the height of a slide bar or the position of a pointer.  Often, the value is displayed as a line on a trend plot that provides the values for some time in the past.  In addition, the measurement can be displayed as a digital number to provide more accuracy for calibration.  Finally, measurements that are transmitted to a digital control system can be stored in a historical database for later recall and for use in calculating important parameters useful in monitoring process behavior, for example, reactor yields or heat transfer coefficients.

The engineer must ensure that the measurements are displayed where needed by personnel.  Several common approaches are briefly summarized in the following.

 

·                     Local display - A sensor can display the measurement at the point where the sensor is located.  This information can be used by the people when monitoring or working on the equipment.  A measurement that has only local display involves the lowest cost, because the cost of transmission and interfacing to a digital system are not required.  Note that no history of these measurements is available unless people record the values periodically.

·                     Local panel display - Some equipment is operated from a local panel, where sensors associated with a unit are collected.  This enables a person to startup, shutdown and maintain the unit locally.  This must be provided for units that require manual actions at the process during normal operation (loading feed materials, cleaning filters, etc.) or during startup and shutdown.  Usually, the values displayed at a local panel are also displayed at a centralized control room.

·                     Centralized control room - Many processes are operated from a centralized control room that can be located a significant distance (e.g., hundreds of meters) from the process.  The measurement must be converted to a signal (usually electronic) for transmission and be converted to a digital number when interfaced with the control system.  A centralized control system facilitates the analysis and control of the integrated plant.

·                     Remote monitoring - In a few cases, processes can be operated without a human operator at the location.  In these situations, the measurements are transmitted by radio frequency signals to a centralized location where a person can monitor the behavior of many plants.  Typical examples are remote oil production sites and small, safe chemical plants, such as air separation units.

 

Exercise 2.03 For each of the sensors used in the following list, determine the proper sensor location.

 

·         Orifice sensor used for flow control of the plant feed rate.

 

Centralized control room

·         Float sensor used for a high level alarm in a reflux drum.

 

Centralized control room

·         Thermocouple sensor used to monitor the temperatures in a fired heater to prevent damage to the equipment.

 

Centralized control room

·         Pressure sensor used for the startup of a compressor.

 

Local control panel and centralized control room

·         Pressure sensor used to monitor piping for possible plugging.

 

Local display (unless the plugging could occur rapidly, in which case the display should be in the centralized control room)

 

 

2.0.3 The “Smart sensor” revolution

Currently, sensor technology is experiencing a dramatic change.  While the basic physics and chemistry of sensors are not changing, sensors are being enhanced by the addition of microprocessors at the location of the sensor.  This change makes the following features possible that were not available with older technologies.

·                     Digital conversion and transmission - The “signal” from the sensor is no longer simply a single value representing the measured value.  The sensor can transmit additional information, including diagnostics and corrected estimates of a variable based on multiple sensors, e.g., orifice pressures and density.  All values can be transmitted digitally, which allows many sensor values to be sent by the same cabling, which reduces the cost of an individual cable for each measurement, as required with analog transmission.

·                     Diagnostics - The sensor can provide sophisticated diagnostics of its performance and warn when a measurement might be unreliable.

·                     Signal conditioning - The sensor can identify unusual signal characteristics and eliminate noise or “spikes” according to methods defined by the engineer.

·                     Configuration - The range of a sensor can be changed quickly to accommodate changes in process operating conditions.