Adv. Chapter 9: Instrumentation and Control Systems

Computer control systems used for process control are classified by two commonly used terms: the Distributed Control system (DCS) and the Supervisory Control And Data Acquisition (SCADA) system.

Computer control systems used for process control are classified by which two commonly used terms?

Cost savings associated with the use of a computer control system frequently include reduced labor costs for operation, maintenance, and monitoring functions that were formerly performed manually. Precise control of chemical feed rates by a computer control system eliminates wasteful overdosing. Preventive maintenance monitoring can save on equipment and repair costs, and, as previously noted, energy savings may result from off-peak (reduced) electrical power rates.

Cost savings associated with the use of a computer control system frequently include which items?

An analog display uses some type of pointer or graphical display against a scale.

Describe an analog display.

ACCURACY refers to how closely an instrument measures the true or actual value of a process variable, while PRECISION refers to how closely the device can reproduce a given reading (or measurement) time after time.

Explain the difference between accuracy and precision.

Flows measured with Venturi meters take advantage of a basic principle of hydraulics, the Bernoulli Effect: When a liquid is forced to go faster in pipe or channel, its internal pressure drops.

Flows measured with Venturi meters take advantage of what hydraulic principle?

Oil is removed from instrument air by filtering the air through special oil-absorbent elements. Water is removed from the air by passing the moisture-laden air through an air-drying system consisting of a moisture separation column and air dryers.

How are moisture and oil removed from instrument air?

Operational checks on instrumentation equipment are performed by always observing each system for its continuing signs of normal operation, and cycling some indicators by certain test methods.

How are operational checks performed on instrumentation equipment?

Electric shock can cause serious burns and even death (by asphyxiation due to paralysis of the muscles used in breathing).

How can electric shock cause death?

Moisture can be controlled in an electrical panel room containing instrumentation by leaving some power components on (such as a power transformer) to provide space heat. In a known moist environment, sealed instrument cases may be protected for a while with a container of desiccant.

How can moisture be controlled in an electrical panel room containing instrumentation or in sealed instrument cases?

Pumps can be prevented from running after a loss of level signal by electrical circuitry designed so the pump will turn OFF on an open signal circuit and ON only with a closed circuit.

How can pumps be prevented from running after a loss of level signal?

To obtain a technical manual for an instrument, write to the manufacturer. Be sure to provide all relevant serial/model numbers in your request to the manufacturer for a manual.

How can the technical manual for an instrument be obtained if the only copy in a plant is lost?

Computer control systems help operation and maintenance personnel in preparing daily, weekly, and monthly maintenance schedules, monitoring the spare parts inventory status, and ordering additional spare parts, when necessary, printing out work orders, and recording completed work assignments.

How do computer control systems help operation and maintenance personnel?

Velocity-sensing meters measure process flows by measuring water speed within a pipeline or channel. One way of doing this is by sensing the rate of rotation of a special impeller placed within the flowing stream; the rate of flow is directly proportional to impeller RPM (within certain limits).

How do velocity-sensing meters measure process flows?

Single-point ball float levels mount in a sump or tank to sense a fixed elevation. When the water level rises, the ball floats and rolls onto its side, actuating an internal mercury switch. The switch provides a discrete ON/OFF signal used to control equipment or send level data to the control system.

How does a single-point ball float level generate a level signal?

Ultrasonic sensors measure the level of a liquid by emitting a pulse that is reflected back to the receiver. The sensor periodically generates a pulse of waves that bounce off the liquid surface and reflect back. The sensor detects the reflected pulse. Based on the speed of the pulse, the travel time between sending and receiving is measured and the distance is calculated and converted into a level measurement.

How does an ultrasonic sensor measure the level of a liquid?

Pressure is measured by the movement of a flexible element or a mechanically deformable device subjected to the force of the pressure being measured.

How is pressure measured?

Different types of liquid level-sensing devices include floats, displacers, electrical probes, direct hydrostatic pressure, pneumatic bubbler tubes, and ultrasonic (sonar) devices.

List the different types of liquid level-sensing devices.

The inferential flow-sensing techniques that are used in flow measurement are: (1) velocity, (2) differential pressure, (3) magnetic, and (4) ultrasonic.

List the inferential flow-sensing techniques that are used in flow measurement.

The key to proper instrument operation and maintenance (O & M) is the operator's practical understanding of the system. Operators must know how to: (1) recognize malfunctioning instruments so as to prevent prolonged damaging operation, (2) shut down and prepare devices for seasonal or other long-term nonoperation, and (3) perform preventive (and minor corrective) maintenance tasks to ensure proper operation in the long term.

List the three areas of operator responsibility that are key to proper instrument operation and maintenance (O & M).

Alarms may produce visual or audible warning signals.

List the two kinds of warning signals that are produced by alarms.

Examples of modulating control systems include: (1) chlorine residual analyzers/controllers; (2) flow-paced (open-loop) chemical feeders; (3) pressure- or flow-regulating valves; (4) continuous level control of process basins; and (5) variable-speed pumping systems for flow/level control.

List three examples of modulating control systems.

Measurement instruments can serve as an extension of and improvement to an operator's senses of vision, touch, hearing, and even smell.

Measurement instruments can serve as an extension of and improvement to which of the operator's physical senses?

Probes are used to measure liquid levels where mechanical systems are impractical, such as within sealed or pressurized tanks or with chemically active liquids.

Under what circumstances are probes used to measure liquid level?

An operator might bypass the operation of a controller in an emergency or when, in the judgment of the operator, the controller is not exercising control within sensible limits. To bypass a controller, switch to the manual mode of operation.

Under what conditions might an operator decide to bypass the operation of a controller? How could this be done?

Pneumatic instrumentation depends on a constant source of clean, dry, pressurized air for reliable operation.

What are the essential qualities of the air supply needed for reliable operation of pneumatic instrumentation?

The five basic components of the computer control system are: (1) the process instrumentation and control devices, (2) the input/output (I/O) interface, (3) the central processing unit (CPU), (4) the communication interface, and (5) the human machine interface (HMI). These components are the means by which the control system gathers and distributes information for the human operator and the process instrumentation and equipment.

What are the five basic components of the computer system?

The general principles for safe performance on the job are to always avoid unsafe acts and correct unsafe conditions immediately.

What are the general principles for safe performance on the job?

The two general systems for transmission of measurement signals are analog (4-20 mA) or digital data.

What are the two general systems for transmission of measurement signals?

The two main types of chart recorders are the strip-chart type and the circular-chart type.

What are the two main types of chart recorders?

The two basic types of flow readings are rate of flow (volume per unit of time) and total flow (volume units).

What are two basic types of flow readings?

Two of the surest indications of a serious electrical problem in instruments or power circuits are smoke or a burning odor.

What are two of the surest indications of a serious electrical problem in instruments or power circuits?

Chart recorders are typically electrical, powered by the instrumentation and control system electrical supply. Battery-backed power for the recorders may be used, if required, as a backup power source.

What are typical power sources for chart recorders?

The basic principle to guide you in setting up the frequency of ON/OFF operation of a piece of equipment is to set the ON/OFF controls to operate or cycle associated equipment on and off no more often than actually necessary for plant operation.

What basic principle should guide you in setting up the frequency of ON/OFF operation of a piece of equipment?

An electrical "explosion" could shower you with molten metal, startle you into a bumped head or elbow, or cause a bad fall.

What could happen to you as a result of an electrical "explosion"?

Computer control systems record and store (archive) information that historically was recorded on paper strip-charts.

What do computer control systems do with information that historically was recorded on paper strip-charts?

UEL stands for upper explosive limit and LEL stands for lower explosive limit. UEL and LEL indicate the range of concentrations at which combustible or explosive gases will ignite when an ignition source is present at ambient temperature.

What do the initials UEL and LEL stand for?

Factors that can disrupt instrument and control systems include temporary power failures, tripped panel circuit breakers, voltage surges resulting in blown fuses, static electricity, and excessive heat.

What factors can disrupt instrument and control systems?

General tasks expected of operators of instrumentation systems can be summed up, from an operations standpoint, as learning, and constant attention to, what constitutes normal function, and, from a maintenance standpoint, as ensuring proper and continuing protection and care of each component.

What general tasks are expected of an operator of instrumentation systems from an operations standpoint and from a maintenance standpoint?

Instrument supplies and spare parts that should always be available include charts, pens, pen cleaners, and ink, and any other parts listed in the technical manual as necessary for instrument operation or service.

What instrument supplies and spare parts should always be available at your plant?

The most useful piece of general electrical test equipment is the V-O-M, that is the Volt-Ohm-Milliammeter, commonly referred to as a multimeter.

What is a V-O-M?

A primary element or sensor senses and quantifies measured variables such as pressure, level, and flow; at times chemical feed rates and some physical or chemical process characteristics are also sensed. The primary element performs the initial conversion of measurement energy to a signal.

What is a primary element or sensor?

In a fluid (liquid or gas) chemical feeder, the indication of quantity per unit of time, or liquid chemical feed rate, may be provided by use of a rotameter or built-in calibrated pump with indicated output settings.

What is one way to measure liquid chemical feed rates?

A measurement is, by definition, the comparison of a quantity with a standard unit of measure.

What is the definition of a measurement?

ANALOG displays show a reading as a pointer against a marked scale and DIGITAL displays provide direct, numerical readouts.

What is the difference between analog and digital displays?

A motor control station or panel and the related circuitry essentially provide ON/OFF operation of an electric motor or other device.

What is the essential purpose of a motor control station or panel?

The greatest challenge for operators using computer control systems is to realize that just because a computer says something, this does not mean that the computer is always correct. Operators will always be needed to question and analyze the results from computer control systems.

What is the greatest challenge for operators using computer control systems?

Our senses provide us with qualitative and relatively short-term quantitative information; instrumentation measurements provide us with exact quantitative data.

What is the main difference between measurements by our senses and measurements by instruments?

The purpose of an electrical lockout device is to physically prevent the operation of an electric circuit, or to de-energize the circuit temporarily.

What is the purpose of an electrical lockout device?

Chart recorders, which by nature also serve as indicators, give a permanent record of how a variable changes with time by way of a moving chart.

What is the purpose of chart recorders?

The purpose of instrumentation indicators is to give a visual representation of a variable's present value, either as an analog or digital display

What is the purpose of instrumentation indicators?

Recorders are indicators designed to show how the value of the variable has changed with time.

What is the purpose of recorders?

Wearing thin rubber or plastic gloves can greatly reduce your chances of electric shock.

What kind of specific protective clothing could be worn to protect you from electric shock?

Control systems are the means by which such process variables as pressure, level, weight, or flow are controlled.

What kinds of wastewater treatment process variables do control systems control?

Process variables commonly monitored or controlled by process instrumentation include DO, pH, chlorine residuals, electrical conductivity, ORP, alkalinity, turbidity, and temperature.

What process variables are commonly monitored or controlled by process instrumentation?

Pump control panels may include automatic or manual alternators (two pumps) or sequencers (more than two pumps). Tis provision allows the total pump operating time required for a particular system to be distributed equally among all the pumps at a pump station.

What provision allows the total pump operating time required for a particular system to be distributed equally among all the pumps at a pump station?

If a recorder trace is altered from its usual pattern during the process of checking an instrument, the operator causing the upset should initial the chart appropriately and note the time.

What should be done if a recorder trace is altered from its usual pattern during the process of checking an instrument?

When testing the atmosphere before entry in any confined space, first test for oxygen deficiency/enrichment, then combustible gases and vapors, and then toxic gases and vapors.

When testing the atmosphere before entry in any confined space, what procedure should be used?

Some pressure sensors are fitted with surge or overrange protection to limit the effect of pressure spikes or water hammer on the device.

Why are some pressure sensors fitted with surge or overrange protection?

A treatment plant operator must have a good working familiarity with instrumentation and control systems to properly monitor and control the treatment process.

Why must a treatment plant operator have a good working familiarity with instrumentation and control systems?

An operator should be especially careful when working with laboratory instruments because some of these instruments are very delicate and replacement parts, such as glass vessels or pH electrodes, are quite expensive. Moreover, the use of some of these instruments requires the regular handling of laboratory glassware and other breakable items.

Why should an operator be especially careful when working with laboratory instruments?

Operators should become familiar with the testing and calibration of plant measuring and control systems in order to discuss needed repairs or adjustments with an instrument technician and perhaps assist with that work. This knowledge also enables operators to analyze the effects of instrument problems on continued plant operation and to handle emergency situations created by instrument failure.

Why should an operator become familiar with the testing and calibration of plant measuring and control systems?

Operators should be especially careful when working around powered mechanical equipment because the equipment could start unexpectedly and cause serious injury.

Why should operators be especially careful when working around powered mechanical equipment?

Pneumatic instrumentation should be purged with dry air before shutdown to rid the individual parts of residual oil and moisture to minimize internal sticking and corrosion while standing idle.

Why should pneumatic instrumentation be purged with dry air before shutdown?

Regular preventive maintenance duties should be printed on forms or cards (or appear on a computer screen) for use by operators as a reminder, guide, and record of preventive maintenance tasks performed.

Why should regular preventive maintenance duties be printed on forms or cards?

If electric current flows through your upper body, electric shock could harm your heart or your head.

Why should you brace yourself when operating power equipment so that electric current cannot flow from arm to arm in case of a faulty tool?