In order to make the distribution of water easy and effective various appurtenances are required to be used in a distribution system.
Some of the appurtenances which are commonly used in a distribution system are as follows:
Appurtenance # 1. Valves:
Valves are provided in the pipelines to control the flow of water, to isolate and drain pipeline sections for test, inspection, cleaning and repairs, to regulate pressures and to release or admit air.
The various types of valves commonly used in a distribution system are described below:
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i. Sluice Valves or Gate Valves:
Sluice valves or gate valves are the most common type of valves used to regulate the flow of water through the pipelines. A sluice valve consists of a disc or circular gate parallel sided or wedge shaped in cross-section and having a nut which engage with the thread of an operating spindle.
The disc or gate is raised or lowered in grooves with gunmetal or stainless steel sealing faces, through the spindle by turning a hand wheel or by turning the cast iron cap with a wrench, thereby opening or closing the passage of water through the pipe on which it is fixed.
There are two types of spindles for raising the gate, a rising spindle which is attached to the gate and does not rotate with the hand wheel, and a non-rising spindle which is rotated in a screwed attachment in the gate. The direction of rotation for opening the valve is usually anti-clockwise. In the fully open position, the gate is withdrawn clear of the water-way and hence this valve is also designated as full-way valve. Sluice valves are available with threaded, flanged or bell-and-spigot ends.
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Sluice valves are sometimes troublesome to operate because they need a big force to open or close against a high unbalanced pressure. Further these valves are not suited for operation in partly open positions as the gates and seating would erode rapidly.
A butterfly valve consists of a circular disc of elliptical shape in cross-section, housed in a cylindrical valve body which forms virtually a continuation of the pipe itself. Rotation of the valve spindle brings the disc either into closed position or into fully open position in which it is parallel with the pipe axis.
Butterfly valves are used to regulate and stop the flow especially in large size conduits. They are sometimes cheaper than sluice valves for larger sizes and occupy less space. The advantage of butterfly valves being ease of operation due to absence of sliding parts, compact size, reduced chamber or valve house and improved closing and retarding characteristics.
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However, butterfly valves involve slightly higher head loss than sluice valves and even in fully open position they offer a fairly high resistance to flow because the thickness of the disc obstructs the flow. Further alike sluice valves butterfly valves are also not suited for operation in partly open positions as the gate would erode rapidly, and require high torques to open them against high pressure.
A globe valve consists of a disc connected axially to a vertical spindle and hand is lifted up by the pressure of the incoming water, thus opening the valve. In globe valves the flow changes direction through 90° twice which results in high head losses. These valves are normally used in pipes of small diameter (less than 100 mm) and as water taps.
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These are also known as reflux valves or non-return valves. A check valve allows water to flow in one direction only and the flow in the reverse direction is automatically stopped by it. There are three types of check valves out of which the swing type check valve is widely used. The swing type check valve consists of a gunmetal door (or disc) hinged at its top edge. The door fits tightly against an annular gunmetal seat under its self weight.
When water flows in the direction of arrow the door is lifted up by the pressure of the flowing water and it is held in open position. In open position the door does not cause any obstruction to the flow of water. When the flow of water in this direction stops, the door swings down and again fits over its seat thus valve is closed and the flow of water in the reverse direction is prevented.
The reflux valve is invariably placed in a pumping main so that if the pump fails or stops, water is prevented from flowing back to the pump and thus pumping equipment is saved from possible damage.
v. Air Valves or Air Relief Valves:
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When a pipeline is filled with water the air present in the pipeline may be trapped. Similarly the water flowing through a pipeline may have entrained air which may also be trapped. The trapped air tends to accumulate at summits or high points in a pipeline, which usually exist in a pipeline because a pipeline is seldom laid parallel to the hydraulic grade line and it has invariably some rise or fall. The accumulation of air results in a serious blockade to the flow of water that may diminish the area of the pipe available for the flow of water thereby increasing head losses and reducing the discharge through the pipeline.
It is therefore necessary to remove the accumulated air from the pipeline and for this purpose air valves or air-relief valves are provided in the pipeline. The provision of air valves in a pipeline also permits air to enter the pipeline when it is being emptied, or when vacuum occurs in the pipeline due to sudden breakdown of pipeline at low points. If air valves are not provided in a pipeline vacuum may occur at summits or high points and the pipe may collapse, or it may not be possible to drain the pipeline completely.
An air valve consists of a cast iron chamber (circular or rectangular), float, lever and poppet valve. The chamber is connected to the pipeline at its top through a short pipe length. In the normal condition, the chamber is full of water entering from the pipeline. The float therefore touches the roof of the chamber and the poppet valve is held in closed position. When air from the pipeline enters the chamber, it starts accumulating at the top of the chamber.
With more and more air accumulating in the chamber air pressure is built up due to which the water level in the chamber gets depressed, consequently the float is brought down and the valve is opened, accumulated air is thus allowed to escape through the valve. When air escapes, water rises again in the chamber and the float is raised which results in the closing of the valve so that water does not escape through it.
The air valve also helps to admit air into the pipe when the pipe is being emptied or when negative or vacuum pressure is created in the pipe. In this case the water level in the chamber is lowered with the result the float drops and the valve is opened thereby air is admitted into the pipe which helps to drain the pipe completely and also to break the negative or vacuum pressure created at any stage in the pipe.
It is thus observed that an air valve operates automatically while allowing air to escape from or to enter a pipe. The air valves are usually located on both sides of sluice valves at summits, and also on the downstream side of all other sluice valves. Further air valves are also provided at changes in grade to steeper slopes in sections of pipeline not otherwise protected by air valves.
The above described air valve is usually designated as a single orifice air valve in order to distinguish it from another type of air valve termed as double orifice air valve. A double orifice air valve is made up of two cast iron chambers interconnected and provided with separate buoyant balls of different compositions and orifices designed to operate for different conditions of high pressure and low pressure of flow in pipeline.
The balls are vulcanite or rubber-covered and are seated on rubber or metal rings. Each ball is slated against an opening at the top of the chamber when the chamber is full and seals the opening.
When air accumulates at the top of the chamber the water level in the chamber gets depressed due to which the ball drops and the accumulated air escapes through the opening. With the release of the accumulated air the chamber is again filled with water and the ball is raised to seal the opening.
Further when the pipe is being emptied or when negative or vacuum pressure is created in the pipe, the water level in the chamber is lowered, and the ball drops thereby permitting air to be drawn into the pipe, which helps to drain the pipe completely and also to break the negative or vacuum pressure created at any stage in the pipe. For high pressures stainless steel floats are used instead of vulcanite or rubber-covered balls.
vi. Pressure-Relief Valves:
The pressure-relief valves also called overflow towers are provided to keep the pressure in a pipeline below a predetermined value, and thus protect it against the possible danger of bursting due to excessive pressure. When the pressure in a pipe exceeds a predetermined value the valve opens automatically and allows certain amount of water to flow out from the pipe to waste, thereby reducing the pressure in the pipe.
A pressure-relief valve consists of a spring loaded disc. The load on the disc can be adjusted by releasing or compressing the spring with the help of a handle. The valve is fitted on an opening provided at the top of the pipeline. As long as the pressure in the pipe is less than the design value the disc is held tightly fitting against the opening and the valve remains closed.
When the pressure in the pipe exceeds the design value, the disc is forced to be lifted up and certain amount of water is allowed to flow out from the pipe to waste, thereby pressure in the pipe is reduced. With the reduction in pressure in the pipe the disc is forced back to fit against the opening and the valve is again closed.
The pressure-relief valves are located at every point along the pipeline where pressure is likely to be maximum. Thus these valves are often placed along the pipeline at suitable intervals at low points where the pressures are high. Further a pressure relief valve is usually provided on the upstream side of a sluice valve so that the pipe lying on the upstream side of the valve is relieved of water hammer pressure resulting from the sudden closure of the sluice valve.
The pressure-relief valves are generally more useful on small pipelines for which the escape of a relatively small amount of water will alleviate water hammer pressures. However, the pressure-relief valves are not sufficiently responsive to rapid fluctuations of water hammer pressure. As such in order to relieve large pipelines of water hammer pressures various other devices such as air vessels, surge tanks, etc., are used.
vii. Scour Valves or Blow-off Valves or Drain Valves:
The scour valves or blow-off valves or drain valves are provided for completely emptying or draining of the pipe for removing sand or silt deposited in the pipe and for inspection, repair, etc. These are ordinary sluice valves which are located at dead ends and depressions or low points in the pipeline. They discharge into natural drainage or sewer, or empty into a sump from which water can be pumped to waste.
In order to avoid the possibility of water in the pipeline becoming polluted, there should be no direct connection between the valve and the sewer or drain, and also two scour valves are placed in series. Further the outlet into the channel should be above the high water level. However, if the outlet must be below high water, a check valve must be placed to prevent back flow.
Appurtenance # 2. Manholes:
Manholes are provided at suitable intervals along the pipeline. They are helpful during construction and later on serve for inspection and repairs. These are usually spaced 300 to 600 m apart on large pipelines. Their most useful positions are at summits and downstream of main valves. They are commonly provided in the case of steel and concrete pipelines and are less common in the case of cast iron and asbestos cement pipelines.
Appurtenance # 3. Fire Hydrants:
A fire hydrant is an outlet provided in a pipeline for tapping water mainly for the purpose of fire fighting (or fire extinguishing). However, sometimes these may also be used for withdrawing water for certain other purposes such as sprinkling on roads, flushing streets, etc.
When a fire breaks out, water is obtained for firefighting from a nearby fire hydrant through a fire hose. For firefighting usually large quantity of water at high pressure is required in order to make it to reach to the place of occurrence of fire. Thus if water at required pressure is available from a fire hydrant, it can be directly used for firefighting through a fire hose connected to the outlet of the fire hydrant.
However, if water at much higher pressure is required the same is developed by attaching a fire engine or a pump to the fire, hydrant outlet. The fire engine or the pump draws water from the fire hydrant boosts its pressure and the high pressure water coming out from the outlet of the fire engine or the pump is used for firefighting through a fire hose connected to the outlet of the fire engine or the pump. At the end of the fire hose a nozzle is provided to develop a powerful jet of water.
The number of fire hydrants in a distribution system and their location depends on various factors such as chances of fire occurrence, requirement of water for fire fighting, utility of buildings, population of area, etc. Generally fire hydrants are placed at all important road junctions and at intervals not exceeding about 300 m.
Appurtenance # 4. Water Meters:
Water meters are the devices which are installed in pipelines to measure the quantity of water flowing through them. The water flowing through pipelines is supplied to various consumers for domestic, industrial and commercial uses and its measurement is necessary to charge the consumers according to the quantity of water supplied to them.
The water meters may be classified into the following two categories:
(i) Inferential type meters or velocity meters
(ii) Displacement type meters
(i) Inferential Type Meters or Velocity Meters:
The inferential type meters or velocity meters work on the principle that the discharge or rate of flow of water through a pipe of given cross-sectional area is proportional to the velocity of flow, thus higher the velocity of flow more will be the discharge.
The commonly used inferential type meters or velocity meters are:
(a) Rotary or turbine meters
(b) Venturi meters
(a) Rotary or Turbine Meters:
A rotary meter used for measuring small domestic flow consists of a small wheel having a series of radial blades or vanes and mounted on a shaft. It is enclosed in a casing which is provided with inlet and outlet. The wheel is caused to rotate by the water flowing through the meter. The number of revolutions per unit time made by the wheel will depend on the velocity of flow of water, i.e., greater is the velocity of flow higher will be the speed of rotation and vice versa.
The rotation of the wheel actuates a clock-like mechanism through a system of gears and the quantity of water flowing through the meter is registered by means of a system of dials. The meter is calibrated to read directly the total quantity of water flowing through it over a certain period of time.
These meters can be used for measuring small domestic supplies as well as large supplies for commercial and industrial use. Further these meters can be used even if water contains suspended particles or sediment. However, at low flows the accuracy of these meters is comparatively less.
(b) Venturi Meters:
The basic principle on which a venturi meter works is that by reducing the cross-sectional area of the pipe, a pressure difference is created and the measurement of the pressure difference enables the determination of the discharge through the pipe.
A venturi meter consists of (a) an inlet section followed by a convergent cone, (b) a cylindrical throat, and (c) a gradually divergent cone. The inlet section of a venture meter is of the same diameter as that of the pipe, which is followed by a convergent cone.
The convergent cone is a short pipe which tapers from the original size of the pipe to that of the throat of the venturi meter. The throat of the venturi meter is a short parallel sided tube having its cross-sectional area less than that of the pipe.
The divergent cone of the venturi meter is a gradually diverging pipe with its cross-sectional area increasing from that of the throat to the original size of the pipe. At the inlet section and the throat of the venturi meter, pressure taps are provided to facilitate the measurement of pressure difference between these sections. The discharge or the rate of flow of water through a venturi meter is given by –
In which,
Q = discharge through venturi meter;
K = constant of venturi meter;
C = coefficient of discharge of venturi meter; and
h = difference of pressure head between the inlet and the throat sections of venturi meter.
A venturi meter is preferably used for measuring the high flows in large pipes, especially for the pipes carrying raw water from the source to the water treatment plant. It is, however, not suitable for measuring small flows.
(ii) Displacement Type Meters:
A displacement type meter measures the rate of flow of water by recording the number of times a container of known volume is filled and emptied. Depending on the type of motion of the moving part in the meter various displacement type meters are available which include reciprocating type, rotary type, oscillating type and nutating-disc meters.
Out of these the nutating-disc meter is commonly used as domestic water meter in United States and the same is briefly described below:
Nutating-Disc Meter:
A nutating-disc meter consists of a disc of hard rubber placed inside a chamber which is provided with inlet and outlet. The water entering the chamber causes the disc to oscillate about its centre with a spiral motion. The oscillations imparted by one complete filling and emptying are recorded by the meter through a gear system in terms of the volume of water passing through it.