VALVES IN PIPE LINES

Types of valves Line valves Gate valves or sluice valves Butterfly valves Globe valves Needle and cone valves Blow off valves or Drain valves or Scour valves Air valves Air release valves Air inlet valves Kinetic air valves Pressure relief valves Check valves or reflux valves Ball valves or ball float valves Automatic shut-off valves Automatic burst control

Line valves Main line valves are provided to stop and regulate the flow of water. The spacing varies with the terrain traversed by the line. Principal considerations in location of the valve accessibility and proximity to special points Function of economics and operating problems Placed at major summits of pressure conduits.

Gate valves or Sluice valves Used to regulate the flow of water through the pipes. Usually placed at the summits of pressure conduits, they can be cheaper and less stronger materials and can be easily operated with less force. For economy, in large dia pipes, valves of smaller dia than the pipe itself are generally used

Butterfly valves Used to regulate and stop the flow especially in large size conduits. Butterfly valves with sliding parts have the advantages of ease of operations, compact size, reduced chamber or valve house and improved closing and retarding characteristics. Involve slightly higher head loss than sluice valves Not suitable for continuous throttling. Sealing is not as effective as sluice valves at high pressures.

Globe valves Used in small bore pipe work and as taps and although a variation is used as a control valve. Have a circular seal connected axially to a vertical spindle and hand wheel.

Needle and cone valves More expensive Well suited for throttling flow. Have a gradual throttling action as they close. Needle valves may be used with counter balance weights, springs or actuators to maintain constant pressure conditions either u/s or d/s of the valve or to maintain a constant flow. Resistant to wear even at high flow velocities Not commonly used in water supply but occasionally used as water hammer release valves when coupled to an electric or hydrualic actuator.

Blow off valves or Drain valves or Scour valves Used to remove the entire water from the pipe. Small gated off-takes are provided at low points. Used for inspection, repairs. When opened, water comes out of these valves quickly under gravity and discharged into some natural drainage channel or sump in which it can be pumped out. There should be no direct connection between the valve and the sewer or drain, to avoid the possibility of pollution travelling into the pipe. For safety, two drain valves are generally placed in series.

Air valves Special kind of valves placed at summits on both sides of sluice valves. When placed on summits which are very near hydraulic gradient line, they ensure the safety of the pipe against collapse. When placed below the ordinary sluice valve, they will protect the pipe against negative pressure which may be developed, when during steady flow, the sluice valve is suddenly closed.

Air inlet valve – will open out automatically as soon as the pressure in the pipe falls below a certain fixed predetermined value, and allows air to enter the pipe. Air Relief valves are required to be provided at all the summits to remove the accumulated air which will obstruct the free flow of water.

Kinetic air valves In case of ordinary air valve, single orifice type, the air or water from the rising main is admitted in the ball chamber of the air valve from one side of the ball. Disadvantage: Once the ball goes up, it does not come down even when air accumulates in the ball chamber. Due to air rushing in, it stirs the ball making it stick to the upper opening which does not fall down unless the pressure in the main drops.

Pressure relief valves Used to reduce water hammer pressures in pressure pipes. This valve is adjusted to open out automatically as soon as pressure in the pipe exceeds a certain fixed predetermined value. Due to the opening of this valve, certain water will get out of the pipe, and thus, reducing the water pressure. As soon as the pressure reduces, the valve will close automatically.

Check valves or Reflux valves Also known as ‘non-return valves’ It prevents the water to flow back in the opposite direction. It may be installed on the delivery side of the pumping set. Check valves are also required at inter-connections between a polluted water system, so as to prevent the entry of pollution into the pure water. Foot valve – check valve installed at the end of suction line Foot valve prevents draining of the suction when the pump stops.

Ball valves or ball float valves Used to maintain a constant level in a service reservoir or elevated tank or stand pipe Equilibrium type of valve – most effective. In most severe operating conditions, needle type of valve will give better service – more expensive. The disadvantage – Valve may operate for long periods in a throttled condition. This can be avoided by arranging for the float to function in a small auxiliary cylinder or a tank. When water reaches the top of the auxiliary tank, the ball will rise quickly from the fully open position to the closed position without shock. The valve will not open again until the water level in the reservoir reaches the base of the auxiliary tank.

Automatic shut off valves Used on the mains to close automatically when the velocity in the main exceeds the predetermined value in case of accident to the line.

Automatic burst control When large steel mains suitably protected against corrosion and laid properly, particularly at the change of direction and the ground is not liable to subsidence, the possibility of a major burst is ruled out. The insertion of a smaller power operated bypass valve alongside the main valve and provision of automatic control arrangements for the main valve to close at a fairly rapid rate, followed by the smaller bypass valve at a much lower speed.