Valves and Fittings

Why a break pressure tank is needed

In a gravity scheme laid on steep ground, the static pressure at the lower part of the pipe becomes very large (it increases by 1 kg/cm² for every 10 m drop). This can burst pipes, joints and taps, and needs expensive high-class pipes. A break pressure tank (BPT) is a small open tank placed along the pipeline that makes the water pressure zero (atmospheric) at that point, so the static head of the next section is reduced to the height below the tank. Thus lower-class pipes can be used, and pressure at taps stays within safe limits (generally below 60–70 m).

It is also used to remove air and reduce the energy of the incoming water.

Construction

   inlet pipe (from above)
        |      float valve
     ___|_____ /___ vent/cover
    |   ~~~~~~~~~   |-- overflow
    |   water       |
    |_______________|-- outlet to next section
        washout

• The tank is built of stone/brick masonry or RCC with cement plaster, usually 1 m × 1 m to 1.5 m × 1.5 m in plan and about 1–1.5 m deep (volume is just enough to hold a few minutes' flow).
• Inlet pipe with a ball (float) valve closes the supply when the tank is full.
• Outlet pipe to the next section, at 100–150 mm above the floor, with a strainer.
• Overflow pipe, washout pipe for cleaning, a vent, and an RCC cover with a manhole for inspection.
• Free board about 0.3 m. A fence is provided around it and the site is drained.

A public stand post (public tap, tap stand) is a water outlet in a community, where several households collect water free or by payment, from a pipe connected to the distribution system. It is the most common mode of water supply in rural Nepal.

        tap(s)   bucket
         |  |   ___
   ______|__|__|   |
  |  masonry     |  platform with
  |  pedestal    |  drain/soak pit
  |______________|_____
   pipe from main (underground)

Construction

• A GI riser pipe (15–25 mm) rises from the main inside a masonry or concrete pedestal, with 1–2 taps (each tap at 0.6–0.9 m height) fixed on it, and a stop valve below.
• A concrete platform around the tap, with a slope and drain to a soak pit prevents mud and stagnation.

Requirements (criteria)

1. Residual head of at least 7–10 m at the tap.
2. One stand post for 100–200 people (one tap for about 50 persons), with a walking distance of not more than 100–200 m (rural, as per the design guideline).
3. Place it in a clean, accessible, safe position, away from drains and latrines, at a high point near the road; the site should be agreed by the users.
4. Strong, durable fittings and a lockable valve.
5. A suitable drain and soak pit, with a platform that the bucket (and the can) fits under.

Importance in rural areas

It gives clean water at low cost to people who cannot afford house connections; reduces the time and labour of fetching water, especially of women and children; reduces water-borne diseases; can be maintained by the users' committee; and wastage is low because the tap is shared and managed by the community.

An air (relief) valve is an automatic valve fitted on a pipeline to release the air that collects in the pipe and to admit air when the pipe is emptied, so that the line works properly.

Function

• Air accumulates at high points (crests) of the pipeline, and reduces the flow area, increases head loss, causes air-lock and may cause the line to vibrate or burst.
• The valve lets the trapped air out automatically while the pipe is being filled or when it is working.
• It also admits air when the pipe is emptied or when negative pressure forms, preventing collapse of the pipe and vacuum.

        outlet (air)
          |  _|_
          | | o |<- float ball
     _____|_|___|__
    |  body        |
  ==|  water ~~~~~ |==  pipe (crest)
    |______________|

Working

1. The body has a small outlet at the top and a float (hollow ball) below it.
2. When air collects, the water level in the valve body falls and the float goes down, opening the outlet and releasing the air.
3. When all the air has gone, water rises and lifts the float, which presses against the seat and closes the outlet.
4. This is repeated automatically whenever air accumulates again.

It is placed at the summit of the pipe, with an isolating sluice valve below it for maintenance.

Reflux valve (non-return valve)

Purpose: to allow flow in one direction only and automatically prevent reverse flow.

• On the delivery pipe of a pump, so that water does not run back to the pump when it stops, which prevents damage and loss of prime.
• On pipes leaving a reservoir or tank, and in the house connection to prevent back-siphoning of dirty water.

  flow ->  _____hinge
  =======|\      |=======
         | \ flap|
  =======|__\____|=======
  Flap opens by forward flow, shuts by reverse flow

Air relief valve

Purpose: to release the air trapped at the high points of the pipeline, and to admit air to prevent a vacuum when the pipe is emptied.

• Air in the line reduces the discharge, causes air-lock, pressure surges and vibration.

       air out
        _|_
       | o | float ball
   ====|___|====  pipe crest

When air collects in the body, the float drops and opens the outlet; once the air has gone, the water lifts the float and closes the outlet.

Why pressure relief valves are necessary

Sudden closure of valves, pump stops or start, or fast change in flow produces water hammer (a high-pressure wave), and pressure can also become too high in low-lying parts or in pumping mains. High pressure may burst the pipes, joints and fittings, or damage pumps. A pressure relief (safety) valve opens automatically when the pressure goes above a set limit, discharges the excess water and protects the pipeline. It is also used on the delivery side of pumps and where water hammer is expected.

Construction and working

          adjusting screw
              |
           [spring]
              |
        ___ [plug] ___
   =====|__ valve seat __|===== pipe
               |
          discharge pipe -> drain

• A spring-loaded (or dead-weight) valve sits on a seat on a branch connected to the pipeline.
• The spring is adjusted to the maximum allowed pressure.
• When the pressure in the pipe exceeds this value, it lifts the valve off the seat and releases water through the discharge pipe, so that the pressure goes down.
• When the pressure falls below the set value, the spring closes the valve again.
• A relief valve is placed near the pump delivery or at the points of high pressure; the discharge is led to a drain.

Requirement of a break pressure tank (BPT)

In a gravity-flow scheme in hilly country, the difference in level between the source and the lower parts of the line is large, and the static pressure becomes very high (1 kg/cm² per 10 m). Such pressures can burst pipes, joints and taps, or require costly high-class pipes. A BPT is a small open tank in which the water is brought to atmospheric pressure. The next section then starts from zero, so the static head is limited to the drop below the tank, and the pipe class and cost can be reduced. A BPT is usually provided when the static head is more than about 60–70 m (the limit of the pipe class), and at each break of this height. It also removes the air from the line and a small storage gives control of the flow.

Why residual head must be maintained at the stand post

Residual head is the pressure remaining at the tap after all losses.

1. It is needed to push water out of the tap at a reasonable flow so that a bucket is filled in a short time (a minimum of about 7–10 m is provided in the design).
2. It overcomes the local losses in the tap fittings and the riser height.
3. It prevents negative pressure and contamination from entering the pipe by back-siphoning at leaks.
4. It lets water reach the taps at the end of the line even when other taps are open together.
5. Too high a pressure, however, damages the taps and wastes water, so it is also limited at the upper side.

Why a reflux valve is necessary

Without a reflux valve, water flows back when pumping stops or the pressure fails. This makes the pump run backwards (damage), empties the rising main and loses prime, creates water hammer, and allows polluted water to flow back into the supply. A reflux valve prevents these by closing automatically on reverse flow.

A reflux valve (also called non-return or check valve) allows water to flow in one direction only and closes automatically when the flow tries to reverse.

   flow ->   _______hinge
   =========|\       |=========
            | \ flap |   (flap lifts for
   =========|__\_____|=========  forward flow,
        valve seat           falls shut on reversal)

• Construction: a cast-iron body with a flap (door or disc) hinged at the top, which rests on a seat. Forward flow pushes the flap open; when flow stops or reverses, the weight of the flap and the back pressure press it onto the seat.
• Uses: on the delivery side of a pump, to prevent back flow when the pump stops and to protect the pump from water hammer; on pipes from a reservoir that must not be drained back; at the foot of a suction pipe (foot valve); on rising mains and at service connections to avoid back siphonage.

Purposes of valves in a pipeline

Valves are fitted in a pipeline to:

1. start, stop or isolate the flow (for repair, cleaning, extension of the line),
2. regulate the discharge and the pressure,
3. prevent back flow (reflux valve),
4. release trapped air (air valve) or relieve excess pressure (safety valve),
5. drain the line (washout or scour valve).

Reflux valve

A reflux valve (also called non-return or check valve) allows water to flow in one direction only and closes automatically when the flow tries to reverse.

   flow ->   _______hinge
   =========|\       |=========
            | \ flap |   (flap lifts for
   =========|__\_____|=========  forward flow,
        valve seat           falls shut on reversal)

• Construction: a cast-iron body with a flap (door or disc) hinged at the top, which rests on a seat. Forward flow pushes the flap open; when flow stops or reverses, the weight of the flap and the back pressure press it onto the seat.
• Uses: on the delivery side of a pump, to prevent back flow when the pump stops and to protect the pump from water hammer; on pipes from a reservoir that must not be drained back; at the foot of a suction pipe (foot valve); on rising mains and at service connections to avoid back siphonage.

Purposes of valves in a pipeline

Valves are fitted in a pipeline to:

1. start, stop or isolate the flow (for repair, cleaning, extension of the line),
2. regulate the discharge and the pressure,
3. prevent back flow (reflux valve),
4. release trapped air (air valve) or relieve excess pressure (safety valve),
5. drain the line (washout or scour valve).

Cutoff valve (sluice valve)

A cutoff valve (sluice or gate valve) is used to stop or allow the flow completely; it is not used for throttling.

      wheel / spindle
          |
       __ | __
      |  [|]  |   <- gate (wedge) moves up and down
  ====|  | |  |====
      |__|_|__|
         body

• Construction: a cast-iron body with a wedge-shaped gate (disc) moving across the flow by a screw spindle turned with a hand wheel.
• Working: when the wheel is turned, the gate rises (valve open: full bore, little head loss) or goes down across the pipe (valve closed: tight shut-off).
• Uses: at the beginning of branches, at intervals on the main lines (sectional valves), before and after pumps and meters.

Cutoff valve

A cutoff valve (sluice or gate valve) is used to stop or allow the flow completely; it is not used for throttling.

      wheel / spindle
          |
       __ | __
      |  [|]  |   <- gate (wedge) moves up and down
  ====|  | |  |====
      |__|_|__|
         body

• Construction: a cast-iron body with a wedge-shaped gate (disc) moving across the flow by a screw spindle turned with a hand wheel.
• Working: when the wheel is turned, the gate rises (valve open: full bore, little head loss) or goes down across the pipe (valve closed: tight shut-off).
• Uses: at the beginning of branches, at intervals on the main lines (sectional valves), before and after pumps and meters.

Reflux valve

A reflux valve (also called non-return or check valve) allows water to flow in one direction only and closes automatically when the flow tries to reverse.

   flow ->   _______hinge
   =========|\       |=========
            | \ flap |   (flap lifts for
   =========|__\_____|=========  forward flow,
        valve seat           falls shut on reversal)

• Construction: a cast-iron body with a flap (door or disc) hinged at the top, which rests on a seat. Forward flow pushes the flap open; when flow stops or reverses, the weight of the flap and the back pressure press it onto the seat.
• Uses: on the delivery side of a pump, to prevent back flow when the pump stops and to protect the pump from water hammer; on pipes from a reservoir that must not be drained back; at the foot of a suction pipe (foot valve); on rising mains and at service connections to avoid back siphonage.

Flanged joint

A flanged joint connects two pipes (or a pipe and a valve, pump or fitting) by flanges, which are circular plates fixed on the pipe ends by casting, welding or screwing. A gasket of rubber, rubber-fabric or lead is placed between the faces, and the flanges are bolted together, tightening the nuts in turn.

   pipe ==|F|gasket|F|== pipe
          |bolts through holes|

• Used for valves, pumps, meters and exposed pipes, and in pump houses and treatment plants.
• Strong, watertight, and easy to dismantle; but costly, and it cannot take bending or misalignment.

Reflux valve

A reflux valve (also called non-return or check valve) allows water to flow in one direction only and closes automatically when the flow tries to reverse.

   flow ->   _______hinge
   =========|\       |=========
            | \ flap |   (flap lifts for
   =========|__\_____|=========  forward flow,
        valve seat           falls shut on reversal)

• Construction: a cast-iron body with a flap (door or disc) hinged at the top, which rests on a seat. Forward flow pushes the flap open; when flow stops or reverses, the weight of the flap and the back pressure press it onto the seat.
• Uses: on the delivery side of a pump, to prevent back flow when the pump stops and to protect the pump from water hammer; on pipes from a reservoir that must not be drained back; at the foot of a suction pipe (foot valve); on rising mains and at service connections to avoid back siphonage.

A sluice valve (gate valve or cutoff valve) is the most common valve on water mains. It is used to completely open or completely shut the flow, and is placed at junctions, branches and at intervals along the line to isolate a section for repair.

        hand wheel
           |
        spindle (screw)
          _|_
         |   |
   ======| | |======  gate (wedge) moves
         |___|        across the pipe
          body

Construction

• Cast iron body and bonnet, bronze seat rings and a wedge-shaped gate (disc), moved by a spindle with a screw thread and operated by a hand wheel or key.
• The spindle may be rising (stem rises as valve opens: indicates the position) or non-rising (for underground valves with a key and valve box).
• Joints at the ends are flanged.

Working

Turning the wheel clockwise lowers the gate across the flow, closing the valve; turning it anticlockwise lifts the gate fully clear of the water way, opening the valve with little loss of head.

Features

• Advantages: low head loss when fully open (the flow path is straight), tight shut-off, flow in either direction, simple and durable.
• Disadvantages: unsuitable for throttling (gate vibrates and seats erode), slow to operate (many turns of the wheel; this is helpful against water hammer), bulky and costly in large sizes.

A globe valve is used to regulate (throttle) and stop the flow of water in small pipes. Its body is globular, and the water has to change direction through an S-shaped passage inside it.

        hand wheel
           |
        spindle
          _|_
   ===   |_ _|   ===  disc rises/falls
      \_ _|  |_ _/    on the seat
        |__ __|

Construction

• A body with an internal partition having an opening (seat), a disc (plug) that moves on to or off the seat along the spindle, and a hand wheel at the top.
• The inlet and outlet are on opposite sides of the partition, and the water normally enters from below the disc.

Working

When the wheel is turned, the disc moves down on to the seat and cuts off the flow, or goes up and opens the valve. A partly open position gives a regulated flow.

Features

• Advantages: good for throttling and for fine regulation of flow; tight shut-off with a short travel of the disc; quick maintenance (disc and seat can be replaced easily).
• Disadvantages: higher head loss than a sluice valve (due to the changes in direction), and flow can pass in one direction only; used in small sizes (up to about 100 mm); for large pipes the sluice valve is preferred.
• Common as taps, stop cocks and house-service valves.