Effect of air in the networks

When designing a pipe for a water distribution by gravity system, it is generally supposed that the water flow will fill the pipe. In that case, the flow in the pipe is controlled by the available load, length, diameter, the coefficient of friction of the pipe and singular, continuous and localized losses. If for some reason the pipe is not completely filled with water, the ratio between the available load and the flow will be very different.

This occurs in the following cases:

• When water enters for the first time in a new installation with empty or partially filled pipes.
• If there is cavitation (too much suction involving vaporization of the water) somewhere in the circuit.
• If the pipe is supplied by a source through a coupling box and its production is less than that for which the system was designed.

It is therefore necessary to remove the air as its presence increases the load required to achieve a given flow rate. In fact, the air may act as an obstruction and no water will come out through the pipe outlet.

The air can be removed by using suction pads, which are elements designed for removing and introducing air in ducts. They prevent the pipe from bursting and allow the proper functioning of the network. They consist of hydro-mechanical elements that connected to the pipes at the high points of the layout make the following functions:

• Expulsion of air accumulated in the pipe during the filling process (suction pad for expelling air)
• Continuous expelling of the air which comes from the process of removing gas from the water (air bleed).
• Entrance of water in the pipe line during the emptying process (suction pad of air admission)

By removing air from the pipes, the vents cause the fluid to pass through the entire section of the pipe.

Thus, the flow speed is lower, reducing water hammers on the network and improving the supply of the most disadvantaged zones due to lower head loss.

In the case of pumping systems, the effect of the vents is essential because they allow the entry of a large mass of air when the pumps stop (if the wave of the water hammer is greater than the static pressure), the sockets eliminate air under control, largely decreasing the water hammer.

Suction pads guarantee the maintenance and efficiency of the networks, facilitating its filling and emptying and reducing adverse effects as vibrations or reductions of the transport capacity.

Location of vents

The vents must be placed on:

• High points of each section of the pipe line to expel air while the installation is being filled and during its proper functioning, as well as to admit air during the emptying.
• Reduction down-gradients. Place a vent a maximum of every 600 meters or less in long down-gradients.
• Increased up-gradient. Place a vent a maximum of every 400-800 meters maximum long up-gradients.
• In horizontal sections, place a vent every 500 meters.
• For siphon traps, place them upstream of the retention valve.
• Unloading a pump.

A distinction will be made between monofunctional and trifunctional vents.

Monofunctional vents: work only when the pipe is under pressure, eliminating air bubbles on the network. This type of vent is very important to avoid the accumulation of air, and hence the head loss resulting from a decrease in the working diameter of the valve. It is also important because in the event of an increase in pressure or flow, it prevents air pockets that move and generate bursts caused by the movement thereof. This type of vent works thanks to a small hole blocked by a float. When air is accumulated, the weight of the float equals the pressure generated by the small hole (1 to 5 mm) and falls, letting air out. This makes monofunctional vents with a flange connection greater than DN50 unnecessary.

Trifunctional vents: Apart from having the functionality of a monofunctional vent, they let air in and out in large quantities when the pipe is emptying or filling, respectively. This type of vent is important to prevent the pipe from collapsing under vacuum and because it reduces water hammer produced by pump stops. It also facilitates the pipe filling and emptying operations. The working design incorporates a monofunctional vent and a float that blocks a section according to the diameter of the vent.

Positioning of the monofunctional and trifunctional vents

Monofunctional vents:

1. On each geometric high point for proper pipe bleed and to reduce head loss.
2. Regularly along the pipe, at least every 500 m, to prevent water hammers resulting from uncontrollable movements of pressurized air pockets.
3. Each abrupt change in the down-gradient to avoid the phenomenon of hydraulic high point. Owing to the change in gradient, greater pressures are formed and act as geometric high points capable of immobilizing an air pocket.

Trifunctional vents:

1. At each main high point to allow rapid filling of pipes.
2. Regularly along the pipe, at least every kilometer to prevent air pockets from being trapped during the filling process.
3. Before or after each regulation apparatus, following the gradient of the land to prevent depression of the pipe after closing or opening the equipment.
4. After a pumping system, when the value of the water hammer exceeds the static pressure to pump outlet to prevent the depression caused by the pump stopping from spreading and therefore reduce the resulting water hammer.

Dimensioning of the vents

The vents must be dimensioned following the recommendations applied in the calculation and design programs of specialized companies, including a working maintenance protocol, according to the flow rate and depression to be exerted on the pipe. As we know, fluids are incompressible, so the volume of air to be evacuated equals the volume displaced by the water. Therefore, the air flow rate is equal to the flow rate in the pipe.

Therefore, the dimensioning can be done based on a total burst of the pipe, the drainage speed or simply using the manufacturer’s recommendations regarding pipe diameter versus the diameter of the vent.