Student Learning Outcomes
After reading this chapter, you should be able to:
- Explain the differences between primary and secondary instrumentation
- List the various ways water utilities use SCADA to operate a distribution system
- Define the term telemetry and how it relates to a SCADA system
There are many different processes that need to be monitored by water utility operators. These include, but are not limited to flow rates, meter totalizers, chemical dosages, pressures, levels, and various water quality parameters.
Primary instrumentation is an instrument used to measure process variables. Some of the more common process variables measured in a water distribution system include but are not limited to, flows, pressures, levels, chemical dosages, and temperatures. This sort of process flow measurement provides water utility operators information on the efficiency and overall operation of the system. Pump stations, groundwater wells, storage tanks, and other facilities should be monitored to ensure they are operating correctly and to help maintain the quality and quantity of the drinking water supply.
Measure the flow of water is an important aspect of any distribution system. Flow measurements are used to monitor flows coming into a distribution system (wells, treatment plants, and purchased water sources), flows moving through a distribution system (pump stations), and flows delivered to customers (water service). Measuring flows is important for accounting for the amount of water being purchased, pumped, and sold. Flows can also be used to help track when a piece of equipment needs to be maintained and/or replaced. Meters for measuring flows are typically differential pressure and velocity. They either transmit a read directly to a register (similar to a car’s odometer) or have a pulse or electronic output for monitoring at remote locations.
Pressures on the inlet or suction side of a pump and pressures on the outlet or discharge side of a pump are important parameters to track. Too high or too low of pressure can cause problems with pumping equipment and within a distribution system. Pressure sensors can provide direct-read outputs or provide electronic pulses, which can transmit readings to remote locations. There are four common pressure sensors:
- Strain gauge—This is the most widely used pressure gauge in modern instrumentation. It consists of a section of wire fastened to a diaphragm. The diaphragm moves changing the resistance of the wire. This changing resistance can be measured and transmitted by electrical circuits.
- Bellows sensor—A bellows sensor uses flexible copper that can expand and contract with varying pressures. This is a direct reading pressure gauge.
- Helical sensor—A spiral wound tubular element that coils and uncoils with changes in pressure is a helical sensor. This is a direct reading pressure gauge.
- Bourdon tube—This is a semicircular tube with an elliptical cross-section that tends to assume a circular cross-sectional shape with changes in pressure. This is a direct reading pressure gauge.
Water levels in groundwater wells and water storage tanks are commonly measured. The depth to groundwater is important to ensure the groundwater table is not drawn down too low and it also indicates when the water level drops below the bowls within a well. At this point, a well should be shut off. Water levels in storage tanks are also important to measure. Water storage tanks provide millions of gallons of water to consumers and it is critical that storage tanks do not overflow or run empty.
- Float mechanisms—A simple and inexpensive type of liquid level measuring device is a float that rides on the surface and drives a transducer through an arm or cable.
- Diaphragm element—This type of level sensor operates on the principle that the confined air in a tube compresses in relation to the head of water above the diaphragm. The change in pressure sensed is then related to a change in the head of water.
- Bubble tube—A bubble tube provides a constant flow of air in a tube, which is suspended in the water. The pressure required to discharge air from the tube is proportional to the head of water above the bottom of the tube. Bubbler tubes are not very common and are being replaced with newer electronic equipment.
Direct Electronic Sensors
Probes, variable resistance devices, and ultrasonic sensors are also being used to measure levels. A probe can be suspended in the water and has an electronic circuit that detects a change in capacitance between the probe and water. It then electronically converts this information into water depth. A wound resistor inside a semi-flexible envelope makes up a variable-resistance level sensor. As the water level rises, a portion of the resistor element temporarily shorts out and changes the resistance of the sensor. This resistance is converted to a level output signal. Transducers are common water level measuring devices, which translate the head of water over the unit into a signal (typically 4-20mA), which is then converted to feet of head or pressure.
There are two main types of temperature measuring devices used in water, they are thermocouples and thermistors. A thermocouple uses two wires made of different materials, which are joined at two points. One wire is referred to as the sensing point and the other the reference junction. Temperature changes between the two points causes a voltage to be generated, which can then be read directly or transmitted. A thermistor uses a semiconductive material, such as cobalt oxide, which is compressed into a desired shape from the powder form and then it is heat-treated to form crystals to which the wires are attached. Temperature changes are reflected with a corresponding change in resistance through the wires.
Primary instrumentation provide real-time measurements of the condition of various pieces of equipment. Some of this information can be used for planning and scheduling routine maintenance and also can be used to indicate when something is out of the norm. These instruments are composed of a sensor, which responds to a physical condition being measured and an indicator, which converts the signal into a display on an indicator.
Several parameters are monitored for different pieces of equipment in a distribution system. Electrical sensors are used to monitor voltage (volts), current (amps), resistance (ohms), and power (watts). A D’Arsonval meter is used to measure volts, amperes, and ohms on equipment. It is a current sensing device where an electromagnetic core is suspended between the poles of a permanent magnet.
While these types of sensors have been used over the years, digital sensors, which indicate values directly, are more commonly found in the industry nowadays.
The status of equipment is an important parameter to monitor. Common equipment status monitors include, but are not limited to vibration, position, speed, and torque sensors. Any time equipment turns on, off, or simply just runs, vibration occur. This is normal when the components within the equipment are in good condition and the flow of energy is smooth. However, as components age and begin to wear, vibration can increase. A vibration sensor, especially in locations where daily visual inspections are not possible, can be connected to the power circuit and shutdown the equipment if vibration exceeds a specified value.
Similar sensors measuring speed, torque, position, and various other parameters can also be used to monitor equipment and help protect against excessive damage by shutting down equipment at specified set points.
In addition to sensors used to monitor the status of equipment, various other processes are commonly monitored within the water utility industry. Measuring water quality is important and common. While measuring water quality parameters in drinking water treatment plants is routine, there are several water quality parameters monitored within distribution systems.
One of the most common water quality parameters measured in distribution systems is the disinfectant residual. Chlorine or chloramines are chemicals used to make sure the biological integrity of drinking water is maintained. Chlorine and chloramine residual analyzers are commonly used to measure the water quality on sources of supply such as groundwater wells and purchased water sources. The measurements can trigger alarms or can be automatically adjusted if the measured parameter falls outside predetermined set points.
Secondary instrumentation converts signals from sensors and primary instrumentation. There are several ways instrumentation receives and transmits (indicates) parameters. There are a variety of ways including, but not limited to direct-reading indicators, which express values such as gallons per minute (gpm) for flows, volts from motors, and pressures expressed in pounds per square inch.
Some receivers and indicators collect and record data. Charts are sometimes used to express the values such as the strip chart shown below. Other recorders display total accumulated values or some combination of data collection and expression.
On analog devices, the values will range smoothly from the minimum and maximum values. They are generally easier to read the relative position of the value being displayed throughout the entire range. If values fall between the scaled values on the display they can easily be estimated. On digital devices, the accuracy tends to be better than analog systems and they are very easy to read. The values are typically decimal numbers on mechanical or electronic displays. However, estimating the exact value when the reading falls between divisions on the display is difficult.
When sensors and the indicators are not located in the same area, some type of equipment is needed to send the signal from the sensor to the indicator. In these instances, telemetry is often used. Early telemetry systems used audio tones or electrical pulses. Digital systems are common and se a binary code to transmit signals. A sensor signal feeds into a transmitter, which then generates a series of on-off pulses. The number of on-off pulses represents a number. For example, the pulse sequence of off-on-off-on represents the number five (5). The transmitting device in a digital system is referred to as a remote terminal unit (RTU) and the receiver is called a control terminal unit (CTU).
Whenever multiple signals need to be sent from more than one sensor over the same transmission line is needed, there are several employable methods. Tone-frequency sends signals over one wire or radio signal by having tone-frequency generators in the transmitter. Each parameter is sent at a different frequency. There are filters within the receiver, which sort out the signals and send them to the proper indicator. An example of this is a single voice grade telephone line. As many as twenty-one (21) frequencies can be sent over these types of systems.
Scanning equipment is used to transmit the value of several parameters one at a time in a specified sequence. The receiver decodes the signals and displays each one in a specific turn. Scanning equipment can also be combined with tone-frequency to allow even more signals over a single transmission line. Polling is a system used where each instrument has its own unique address. A system controller sends out a message requesting a specific piece of equipment to transmit its data.
Duplexing is the last process of transmitting signals we will discuss. There are three (3) types of duplexing systems: full-duplex, half-duplex, and simplex.
- In full-duplex systems, the signals can pass in both directions at the same time
- Half-duplex systems only allow signals to pass one direction at a time
- A simplex system only allows signals to pass in one direction
The idea of instrumentation and control systems is to obtain the ability to make changes or corrections in the parameters being measured. Yes, it is very valuable to know what a chlorine residual is at a groundwater well, but if you need to visit the location to make actual changes based on the signals being received from a sensor then valuable time can be lost. Therefore, control systems are extremely useful. A control system allows for adjustments to be made based on the data being transmitted and received. Control systems can be broken down into four (4) main types of systems.
- Direct Manual—A direct manual system, is the simplest and least complicated control system. Components are controlled by an operator that must physically visit each location to make a change. For example, if a signal is transmitted that requires a system to be turned off, in a direct manual control system, an operator must drive out to the location and manually turn the component off. This type of system has a low initial cost and has little complicated equipment to maintain. However, it does require labor and operator expertise and judgment.
- Remote Manual—In a remote manual control system, an operator can make adjustments to systems and components from a remote location. This type of system still requires operator expertise and judgment, but it requires less physical labor. An example of this type of control system could be when a pump needs to be turned on or off. Instead of requiring an operator to physically visit a location to perform this task, it can be controlled remotely from a control room or some remote location.
- Semiautomatic—This type of control system combines manual control from a remote location (control room) with automatic control of specific pieces of equipment. An example of this could be a circuit breaker. A breaker will disconnect automatically in response to an overload, but then it must be reset manually. This “resetting” can be remotely or at the facility.
- Automatic—Full automatic control is when equipment can turn on and off or adjust their operation in response to signals from sensors and analytical instruments. There are two general modes of automatic controls: on-off differential and proportional.
- On-Off Differential control systems turn equipment either full on or off in response to a signal. The rate of the equipment would need to be adjusted manually.
- Proportional control systems adjust variables automatically. Proportional control systems can be broken down into three (3) main types.
- Feedforward proportional control measures a variable such as chlorine dosage. The flow of water is being measured and the faster (or more) water flows through a meter, the chlorine feed system increases the amount of chlorine. This type of system is useful, but it cannot account for varying chlorine demand.
- Feedback proportional control measures the output of a process and will then react to adjust the operation of the piece of equipment. This type of system is also referred to as a “closed-loop” control system because it continuously self corrects. These systems can be troublesome if there are wide variations in the water flow rate.
- Combined control systems adjust in response to changes in the flow rate, but an analyzer monitoring the chlorine dosage makes minor adjustments in the feed rate of the chemical to maintain the selected residual being measured in the finished water.
Supervisory Control and Data Acquisition
The processes discussed in this chapter are wrapped up together into a Supervisory Control and Data Acquisition (SCADA) system. SCADA a system used in a variety of industries including drinking water treatment and distribution systems. There are field devices (primary instrumentation) such as sensors, which read various parameters, sending signals, which are received and transmitted (secondary instrumentation) through telemetry, to a centralized computer system. This allows an operator a complete view of a distribution system to see how things are operating.
A variety of components and processes are monitored in a drinking water distribution system, including but not limited to storage tank levels, pump station flow rates, pressures, groundwater well depths, and chemical feed such as disinfection systems.
Here is a simple example of a how a SCADA system works in a water distribution system.
A groundwater well provides water to the distribution system. So, how does this well turn on and start pumping water? Years ago, an operator would drive out to the well, open the gate to the facility, insert a key to unlock the control panel, and then turn a switch to power it on. Water would enter the distribution system through a network of pipes. If a customer turns on a faucet, water would come out. What is no one was using water when the well was turned on? The water would then flow to a water storage tank. This tank would begin to fill. What would happen if the well was not shut off? Obviously enough, the storage tank would overflow. So, how would an operator know when to shut the well off? Without some sort of computer system, signal, or alarm, the operator would need to drive to the tank and visually look at the level of water. Then, the operator would drive back down to the well site and shut off the well. This is not very complicated, but it is labor-intensive and time-consuming. Fast forward to the age of computers. A sensor could be installed in the water storage tank to monitor the level. With the help of a SCADA system, specific level set points can be programmed to tell another set of sensors when to turn on and off something like a groundwater well. This is a very simplistic example of how a SCADA system works, but it adequately explains the process of a sensor sending a signal, a computer system reading this signal, and responding with a function.
A control room is typically equipped with a human-machine interface (HMI). This interface is a window into the SCADA system. It graphically displays all the facilities within a system. The tanks, pumps, sources of supply are all interconnected to make sure water is continually distributed throughout the system. The system is usually equipped with various alarms to remotely notify operators through pagers, text messaging, or some other type of remote notification. This allows the system to continually function without someone sitting at a computer twenty-four (24) hours a day, seven (7) days a week. An alarm is received by an operator and then the fully trained operator would either verify the alarm was cleared after the process returned to normal operation or inform the operator that additional tasks might be needed to fix the system.
- Which of the following is used to measure pressure?
- Strain gauge
- All of the above
- Which of the following is used to measure low pressure?
- Bellows sensor
- Helical sensor
- Bourdon tube
- All of the above
- Bubble tubes must be installed ___________.
- At the top of the tank
- In the middle of the tank
- At the bottom of the tank
- Anywhere in the tank
- Which of the following temperature measuring devices uses two wires?
- Which of the following temperature measuring devices uses cobalt oxide as a semiconductive material?
- Which type of control would not account for varying chlorine demand?
- Feedback proportional
- Feedforward proportional
- All of the above
- Which of the following is the smallest measurement of power?
- Amperes is a measurement of ___________.
- SCADA stands for ___________.
- Self Contained Analog Digital Assembly
- Superior Computer And Data Acquisition
- Supervisory Computer And Digital Assembly
- Supervisory Control And Data Acquisition
- Which of the following allows signals to only pass in one direction?
- All of the above