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    Copyright 2009 American Water Works Association

    AWWA eLearningWater System Mechanical Equipment

    Module 3: Instrumentation and Control

    An operators main responsibilities are supervision and control. Supervision means examining system performance information and deciding if it is acceptable. If, in the operators opinion, performance is unacceptable, then an element of the system operation must be changed to bring performance back to an acceptable condition. A setup for which a human operator evaluates the performance continually is said to have open-loop control. Control equipment allows the operator to change valve settings, turn pumps on and off, and otherwise adjust the system for efficient operation.

    When instruments are provided to make the necessary change or correction automatically without the intervention of the operator, the system is said to have closed-loop control. However, regardless of the extent to which automatic control is used, the operator still may need to intervene manually during abnormal or emergency situations.

    Most large utilities now feed instrument readings to one or more computers both to gather information and to assist in providing control. Most small systems, though, continue to maintain simpler, on-site instru-mentation and controls. As computers and electronic control equipment become less expensive, less complicated, and more reliable, they will increasingly be used by medium-size and smaller utilities.

    The main categories of instrumentation and control discussed in this module are

    primary instrumentation: sensors that measure process variables such as flow, pressure, level, and temperature

    secondary instrumentation: instruments that respond to and display information from primary instrumentation

    control systems: manual, automatic, and digital systems that operate final control elements such as pumps and valves

    This module will also discuss maintenance and operation safety for this instrumentation.

  • AWWA eLearning INSTRUMENTATION AND CONTROL 2

    Copyright 2009 American Water Works Association

    Primary InstrumentationSensors that measure quantity of flow, pressure, and other parameters

    are essential to water system operations. The measurements allow the operator to maintain the quality and quantity of drinking water efficiently.

    Basic Instrument ComponentsVarious instruments are used to measure, display, and record the

    conditions and changes in a water treatment plant and distribution system. Some instruments indicate what is happening at a given instant. Others guard against equipment overload and failure. Still others provide permanent records that are used to determine operating efficiency and the need for regularly scheduled maintenance.

    These functions are similar to the functions of an automobile instru-ment panel. The speedometer indicates the current vehicle speed; the oil-pressure light warns of impending engine failure; and the mileage indicated on the odometer can be used to calculate gas mileage and deter-mine the need for regular maintenance. In both the water system and the automobile, the instruments do not replace the operator. They simplify the operators work and help to improve the performance, safety, and reliability of the equipment.

    The simplest instruments have only two parts: a sensor and an indicator. The sensor responds to the physical condition (parameter) being measured, converting it to a signal that can activate the indicator. The signal may be a simple physical motion, or it may be an electrical current or a change in pneumatic (air) pressure.

    The indicator may display the result immediately, or it may be replaced or supplemented by recorders or totalizers to monitor condi-tions over a period of time. The main categories of indicators are direct, remote, and distant. Indicators will be discussed in more detail later in this module.

    The following descriptions introduce the most common types of sensors.

    Flow SensorsThe most significant measurement in water treatment and distribution

    facilities is the flow of water. Day-to-day operational decisions and long-term planning are based on the measurements from flowmeters. Knowledgeof the quantity of flow is needed to control plant inflow, pace chemical feeds, bill customers, check the efficiency of pumps, monitor for leaks, and help control or limit the delivery of water. Analyses of the water

  • AWWA eLearning INSTRUMENTATION AND CONTROL 3

    Copyright 2009 American Water Works Association

    system operations are also derived from treatment plant flow and distri-bution system information.

    The flow passing through treatment plant and distribution system meters might simply be totalized on the meter register. It can also be recorded locally on a chart recorder or transmitted to a central location for recording. It is also usually important to know the flow rate at various times of the day, as well as the flow in various parts of the distribution system. Figure 3-1 shows a chart recording of a water plants instanta-neous flow rate for a period of one week.

    The following paragraphs review various meter technologies for mainline measurements.

    FIGURE 3-1 Typical circular chart recording

    Courtesy of Process Instruments, Inc.

  • AWWA eLearning INSTRUMENTATION AND CONTROL 4

    Copyright 2009 American Water Works Association

    Differential-Pressure FlowmetersThe measurement devices most commonly used for measuring large

    quantities of water are differential-pressure flowmeters (also called head meters). Their popularity is largely due to a combination of flexibility, simplicity, ease of installation, and reliability. The flow velocity reading of a differential-pressure meter is calculated from the difference between two pressures measured in the meter. Differential-pressure flowmeters are available in many forms, such as the venturi, modified venturi, flow tube, or orifice plate. The measurement of flow is a function of detecting two pressure heads, usually one in the normal pipe size and one in a constricted region, called the throat, within the meter. The flow is propor-tional to the square root of the difference between the two pressure readings.

    Velocity-Type FlowmetersVelocity-type, or current-type, flowmeters include magnetic, turbine,

    propeller, multijet, proportional, and sonic flowmeters. In each case, the flow velocity is measured, and the quantity is calculated from the product of velocity times the cross-sectional area of the pipe. The Pitot meter is also a velocity-type measuring meter, but it determines the velocity based on the difference between the flows dynamic pressure and the static pressure.

    Other FlowmetersOther meters are available for mainline flow sensing, such as the

    vortex- shedding flowmeter and the variable-area flowmeter. The open-channel flow detectors (weirs and flumes) represent another type of differential-pressure flowmeter. In both cases, the flow is calculated from the fluid depth or the head that drives the flow.

    Pressure SensorsPressure sensors are used to determine suction and discharge pres-

    sures at pumps, pressure regulators, and selected points in a distribution grid. They also determine pressures of plant waters, eductors, storage tanks, and air compressors. Pressure instrumentation may range from simple, direct-reading pressure gauges to complex pressure-sensing equipment that transmits readings to remote locations. The four most common types of pressure sensors are

    strain gauges

    bellows (low pressure)

    helical elements (medium pressure)

    Bourdon tubes (high pressure)

  • AWWA eLearning INSTRUMENTATION AND CONTROL 5

    Copyright 2009 American Water Works Association

    Strain GaugesThe diaphragm strain gauge sensor is the type used most widely in

    modern instrumentation. As shown in Figure 3-2A, it consists of a section of wire fastened to a diaphragm. As the variable being measured (such as pressure) changes, the diaphragm moves, changing the length of the wire, thus increasing or decreasing its resistance. This changing resistance can be measured and transmitted by electrical circuits. Similar electronic sensors include the variable-capacitance pressure cell, variable-reluctance pressure cell, and vibrating wires.

    Direct-Reading Pressure GaugesThe remaining three types of sensors are direct-reading pressure

    gauges, which were once widely used but are seldom installed today. The bellows sensor (Figure 3-2B) is a flexible copper can. The sensor expands and contracts with changes in pressure. The helical sensor (Figure 3-2C) is a spiral-wound tubular element that coils and uncoils with changes in pressure. The Bourdon tube (Figure 3-2D) is a semicircular tube with an elliptical cross section that tends to assume a circular cross-sectional shape with changes in pressure, thereby causing the C-shape to open up.

    Level SensorsLevel sensors are commonly used to measure the elevation of water

    in wells, as well as the depth of water in storage reservoirs and tanks. They are also used to measure the levels of stored chemicals in tanks. A pressure sensor can readily be adapted to level measurement if it is installed at the base of a tank. As level increases in the tank, the pressure reading increases. The reading can be calibrated in feet (or meters) of liquid. In elevated tanks, the level measurement needed is the level in the elevated portion of the tank, rather than in the tank and riser. Transmittingmechanisms can be calibrated so that zero represents the bottom or minimum level in the elevated portion of the storage tank.

    The common types of level sensors are

    float mechanisms

    diaphragm elements

    bubbler tubes

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