water quality and system

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Figure 4-4.

A weir with a concrete weir structure. Courtesy:

U.S. Department of the Interior, Bureau of Reclamation.

at the main office receives the data from the meters. Computer programs automatically print the bills to be mailed out to the clients.

Flow Meter Installation - Flow meters sometimes fail. Either the device becomes plugged, the taps where the pressures are being read foul and do not read correctly, the case cracks, the gauges stop reading or the gears strip. While these types of events do not happen often, it may be necessary to change the meter, take it apart and repair it, or replace it.

Facility managers should be sensitive to the problems of shutting down the water lines or wastewater lines to service the meter. Small meters, those installed in pressure lines 6 inches in diameter or less, can be installed within a few minutes. A large meter, say in a 48-inch diameter line, could take a couple of days to replace depending upon how long it takes to drain the lines and where the drained water will be sent.

A bypass line is often installed around the meter to allow flows to pass through the system when the meter is taken out of service for repair. This way, the water service continues, the line with the meter in it is shut down, and the meter can be removed and

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Figure 4-5.

Parshall flumes. Courtesy: U.S. Department of the Interior, Bureau of Reclamation.

repaired or replaced. The facility should remember to bill for water supplied while the meter is out of service. Past billing records are used to estimate water supplied while the meter is out of operation.

Pressure Recorders

Instrumentation and recording of pressures is the second most used of the water system measuring tools. (There are usually more pressure gauges but they do not lead to as many arguments.) Pressure gauges measure the pressures of the fluids inside the pipe. Since water will flow from regions of higher to lower pressures if line water is flowing, plumbers and maintenance personnel use pressure gauges to tell them the direction of flow.

The other item of information from pressure gauges is the amount of stress on the inside walls of the pipes. If pressures are too high, risk of pipe rupture is a concern, while if pressures are too low, risk of contamination of the pipe from waste lines is a concern.

Most pressure gauges indicate locally, however, electric signals can be used to send information to a remote location many miles away, like flow meters. Pressure gauges are often installed with a valve so that the valve can be closed, the gauge removed and replaced

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and then opened again. See Figure 4-6 to view a typical pressure gauge.

Figure 4-6.

A typical pressure gauge. Pressure gauges measure the pressure of the fluids inside the pipe. If pressure gets too high, the cause should be determined and corrected immediately.

A pressure gauge is usually tapped into the side of the pipe with a small hole, say ¾-inch to 1/4-inch.

A direct-reading gauge has a fine spring inside, with a small flexible diaphragm. As the pressure increases, the diaphragm moves. The diaphragm is attached to a small mechanical linkage that in turn moves a dial on a clock face. Gauge faces come in all types. A large gauge, perhaps 4 inches in diameter, might be marked every 2 pounds while other gauges will be marked in 5-, 10or 25-pound increments.

Remote-reading gauges are installed similarly to direct-reading gauges. Remote-reading-type gauges use the same diaphragm expansion concept, except that the mechanical link to move the dial on the clock face is replaced with a dial on a rheostat (an electrical device.) The rheostat measures a varying electric current, then computer chips and other electronic components converts the electrical signal into a digital readout similar to what is seen on an electronic calculator. Again, this signal can the sent through wires to a remote station many miles from the meter.

It has been shown that it is more likely that the dial-type meter will be misread than the digital meter, but for most installations this reading may not be that important. The problem with dial-type pressure meters is that personnel will sometimes not write down the correct reading because a mistake is made interpreting the position of the needle.

Like flow meters, pressure instruments and pressure recorders can become expensive. A dial-type pressure gauge might be $25.00 while a digital device will be more expensive. In addition, the electrical devices require power from an electric source. Often, a

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design specifies an electric-type meter or instrument when the meter vault is in the middle of a large field or along the edge of the highway and there is no electrical power available. The cost to run conduit and wire to the metering vault from one of the nearby buildings can exceed the cost of the meter itself. Facility managers should not be put into a position to purchase any more costly pressure gauges than are necessary to accomplish the mission.

In addition, the facility manager should note that electronic instruments require a different staff skill level. Staff to repair and purchase lots of electronic instruments can result in the need for hiring additional staff that may have not been initially planned.

Pegging Pressure Meters - A word of advice for facility managers about pressure gauges: Occasionally, water hammer or mismanagement of the system will result in a sudden extreme pressure increase. If the increase is more than can be read by the pressure gauge, the increase will cause the needle of the gauge to hit the stop at the extreme end of its range. When the needle hits, it will give a little metallic sound, most accurately described as rink. This is called "pegging the gauge," and it is a well known term to plumbers and engineers familiar with this type of equipment and systems. "Pegging the gauge" is not goodin fact, it can be dangerous because it indicates extreme pressures that could be serious. Any craftsman who does not treat "pegging a gauge" seriously should be disciplined. If they continue, they should be replaced.

Pipe supply systems can withstand a small measure of sudden extreme spikes, but these are common to startup. The cause of the spike should be identified and the system modified immediately to prevent it. Most systems will "peg" several times before they fail, but if they do "peg" and the it is not, the pipe system will fail and a potential disaster is about to happen.

Thermometers

Monitoring temperature is important where water is used for heating and cooling. Thermometers are used to measure the temperature of hot water systems.

Temperature readings can also be important at sewage treatment plants were microbes are used as a part of the wastewater treatment process. Other water supplies may need the temperature monitored because of processes downstream where the water temperature is importantcooking or food processing, for example.

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The simplest way to test hot water is to turn on the hot water tap and stick a thermometer in the flow. Otherwise, a hot water tank can have a thermometer on it to tell the operations personnel what the temperature is inside the tank. Most dial thermometers use a diaphragm or an expanding bellows that changes shape with the increase in temperature. The diaphragm is attached to a spring that moves a dial on a clock face. Other thermometers operate on a bulb principle. For these, a recess is attached to the hot water tank and the thermometer bulb inserted into the recess. The recess in the tank or pipe is called a thermowell.

Like the flow meter and pressure gauge, thermometers can be hooked up to digital readouts to give the temperature in direct readings. The advantages are the same as pressure gauges in writing down the readings.

Red-Line/Warning

Gauges can be purchased with red and green backgrounds on the clock face.

Green means, of course, safe, and red means danger.

The advantage of this type of dial face on the gauges is that the operator does not have to understand what pressure or temperature is too highthe gauge's red line notifies him immediately.

Calibration

All instruments should be checked regularly to verify their accuracy. A meter or gauge that does not read accurately is not worth having in the system and should be removed or replaced. Most facilities have a calibration procedure where instruments are taken out and checked on a routine basis.

For complex industrial plants and regulated industries such as hospitals, routine calibration is a requirement and the facility manager is asked to produce the records that calibrations and readings have been performed. This investigation is called an audit. It is not an audit of the books the see where the money went, but instead a check of the procedures used in the facility to confirm that risk is minimized via routine checks.

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Hydraulics

As mentioned above, there is an occasional need for a facility manager to understand the rudiments of hydraulics to enable him or her to have a better understanding of the problems the staff and engineering consultants face in managing a water system. The very basic simple terms used in fluid mechanics are included here.

Water flows to the lowest point, water always runs downhill, and water seeks its own level are all fundamental non-scientific explanations of water flow. The specific term for understanding water flow is called fluid mechanics.

Water has weight but it will not compress. Suppose we have a container full of water. Squeezing water in one place will cause all the water in the container to be squeezed the same amount. If squeezed hard enough, the weak wall of the container will break and a leak will result.

One of the most significant principles in hydraulics is gravity. Water will run downhill until something contains it, then it will fill the space, flowing into and filling the shape of the container from the lowest point upward until one of three conditions are met:

1.The water stops coming in and the fluid stops moving and fills the space.

2.Water keeps coming in until the space is full and then water will flow out into the lowest adjacent space and begin to fill it until it is also full.

3.If an opening is created below the water surface in the space, the water will flow out of it.

If water is flowing in faster than it is flowing out, the volume in that space will increase. If the water flows out faster than it is flowing in, the volume of water in the space will decrease.

Pipe, by providing a uniform round surface follows these principles by filling from the bottom up and filling the space. However, pipe in a upsidedown U shape will have a bubble in it, as flow fills the shape up on one side it will flow over to the other side until each side of the U is filled. ^ bubble of air will be present at the top of the upside-down U.

This bubble can be flushed if the flow is suddenly increased and decreased but it may not be completely eliminated. The bubble of

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air pinches off some of the flow. As a result, many systems will have an air vent to let the air out of the pipes at high points. In the reverse case, when the pipe is full and water is drawn down, there needs to be a way for air to enter through the vent.

For large utility pipes of high diameters and thin steel walls, the pipes have been sucked fiat similar to a paper soda straw. In the early 1950s, in a steel pipe siphon over the Ogden River Canyon in Utah, a pipe was sucked closed by this phenomena. The engineers looked at it and thanks the ductility of steel, a decision was made to fill the pipe again. It worked. Air vents were installed the next day.

Fortunately for facility managers of most systems, the common steel pipes discussed in Chapter 3 will not have this problem because the pipe wall is thick enough that the suction of the water does not have enough force to collapse the pipe.

Venting pipe supply lines at high points to release these ''bubbles" is an important element of any pipe system.

Water Hammer

Water hammer is the sudden pressure spike when a flowing fluid is suddenly closed off. It is called water hammer because of the rumble and shudder of the piping as the moving fluid slams to a sudden stop. What happens here is that a long tube of water is moving steadily through the pipe. The weight of this water can be extensive and while the pipe is not moving, the water flowing inside the pipe is. Essentially a lot of weight is moving fairly fast and suddenly, when the valve closes, the water stops. In theory, the spike of an instant closure is infinite. In reality, the valve does not close in an instant and the water and pipe stretch slightly. The rattle is usually the result of movement in the pipe as it stretches. The sound of water hammer is indicative of pressures great enough to bend metal and continued operation of a pipe system with a water hammer problem can potentially destroy the pipe.

Methods to prevent water hammer include the use of a stem rise of pipe which forms a dead column of air just upstream from the valve. This short piece of pipe becomes a bubble of air that forms a "cushion" for the water when it suddenly stops.

Stem risers used for residential and small pipe systems in the 1/2- to ¾-inch category are usually installed behind the sink or the lavatory. For preventing water hammer in larger pipes, valves are selected which close slowly using a screw mechanism. This gradually slows the water flow down before closing it off completely.

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Case Study: Water Cost Savings Can Pay

Often, supply water is used for process cooling, but it is best to recycle this water and treat it.

One facility used water to cool a compressor that ran seven days a week. The water wasted over a year would have paid for a complete refrigeration recirculation system, had an economic analysis been done.

When an economic analysis finally was done, the recirculation system was installed the following week, saving water bills and preventing a needless waste of fresh, treated water.

For large utility systems, the valves are electrically timed to close at a rate that allows the entire tube of fluid [o slowly come to a halt. Engineering studies and computer programs are used to model the potential of water hammer for large systems. This type is analysis is not expensive, and it is well worth the evaluation trot any pipes larger than about 18 inches.

More Complex Hydraulics

Without going into a lot of mathematics and complex terms, this short discussion of fluid mechanics may prove beneficial to the facility manager when discussing water management problems with engineers or maintenance personnel.

A pipe system of flowing water has three elements of energy: the height of the pipe, the pressure inside the pipe, and the energy of the velocity as it flows (see Figure 4-7). The height of the pipe is relevant to the fluid upstream or downstream. The pipe wall must be strong enough to keep the pressure inside the pipe from bursting through the pipe wall. For the right pipe to be selected, the pressures need to be calculated. For building systems, codes have standardized the pressures and the pipe types to prevent rupture. For utility systems and aqueducts, engineers prepare a detailed analysis to ensure recommended pipe pressures are not exceeded.

To conclude our explanation of hydraulics, we also need to understand friction losses. As the water flows through the pipe, the inside wall of the pipe is not moving and the water right at the wall rubs against the pipe and it slows down. Water out in the center of the pipe moves faster because it does not get affected as much by rubbing against the walls. The faster the water moves, the more

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Figure 4-7.

Engineering drawing illustrating hydraulic relationships. Reprinted from Fluid Mechanics with Engineering Applications with permission of McGraw-Hill Book Co., New York City.

friction builds up at the walls resisting the flow. Over a long stretch of pipe, this friction loss is calculated for different velocities. As the water flows, the friction reduces the pressure. Pumping systems must push the water through the pipe and overcome the drag of the water rubbing along the walls. Most of the engineering calculations for water systems are to account for these friction losses. Some pipes have inside walls that are more slippery than others.

To account for friction loss, engineers calculate the pressures from elevations and flows. The engineers then select the next standard size of pipe that meets these requirements. It is not practical to try to have an odd size fabricated for just one job. Finally, calculations are made at one-third capacity and at two-thirds capacity. Flows are not always at the maximum, and it is necessary to know what the pressures and velocities will be when flow is less than the maximum amounts.

Today, most engineering companies use standard computerized mathematical models that run on personal computers to solve these problems. The software costs $150-$5,000 depending upon the size of the system the computer can handle and the number of flow and pressure variables.

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Smooth Means More Water

In a recent advertisement in a plumbing magazine, a manufacturer boasted that his pipe had a better friction coefficient. What this translated to was that the manufacturer s pipe could carry 10 percent more water than its competitors because the pipe's smoother insides did not offer as much friction loss the flow of fluid through it.

These computer programs allow quick calculation of alternative methods and pipe sizes; modeling has proven effective at optimizing systems.

The theories used by engineers to estimate pressures, flows, and sizes is based upon experimental methods, tests and results obtained early in this century. They are tools for estimating how new systems will behave, but they are not exact. Built into the formulas is a slight "factor of safety," such that flows should be slightly more and pressures slightly less. Depending upon the details of calculations, the attention paid to construction, and the accuracy of assumptions, most large supply pipelines deliver within 2-5 percent of the design flows. Table 4-1 provides the names and vendors of some water system computer modeling software programs. The facility manager should ask the engineer about the various options examined for his system and make sure the consultants have considered most of the significant elements. The facility manager should also request an assessment of both the tangible and intangible assets of the options.

Wastewater Hydraulics

Wastewater pipes carrying sanitary sewer and storm water are almost always larger than supply water pipes. The larger diameter is the result of two factors the facility manager needs to understand if he is to be a successful water manager. The first of the two factors is that the pipes carry solid material. For sanitary sewers, the solids are bits of paper, garbage and human excrement. For stormwater, the solids are paper, trash, stones, sticks and other debris. The second of the rectors that cause wastewater pipes to be larger than supply water pipes is a result of the hydraulic design.

Wastewater systems are designed to flow less than full. That is, the flow of the water combined with solids does not completely fill