Reduce the Costs of Pumping and the Risk of Damages

Background

Air trapped in completely filled pipelines is basically undesirable. Even if air does not get into the pipeline during filling, small gas bubbles (air, carbon dioxide etc.) are also formed during operation. Little by little, these can then merge to form larger bubbles and rise to the top. As a result, they then collect at high points in pipelines and form air cushions there. The consequences can vary considerably:

• The interpretation of tightness tests before the commissioning of pipelines is made more difficult. 
• The reduction of the free crosssection available for flow and the higher pressure losses in the pipeline associated with this can mean that the output of pumps is restricted; pumping costs increase. 
• It can cause vibrations in pumps and valves. 
• Pressure surges are intensified and this can cause damage to pipelines.

For drinking water pipelines there are descriptions e.g. in DVGW data sheet W 334 [1] of how accumulations of air can cause considerable dynamic pressure changes as a result of the different densities of the two media. Therefore pipelines must be air-free and must be kept air-free. Air can get into pipelines in different ways, for example:

• dissolved in water 
• already present in empty or drained pipelines 
• sucked in at high points 
• sucked into the sump of pumps 
• drawn in via an air vessel

The commissioning of a pipeline represents a special case. Directly after construction, the whole system is filled with air and must first be filled up with a liquid. Attention must be paid to the following here: if, when filling pipelines, air is expelled via vent valves, the filling speed must be kept as low as possible. The dreaded pressure surge (Joukowsky surge), which occurs if the float of the vent valve closes the valve seat abruptly at the end of the filling process, must remain below the admissible pressure load (PMA = maximum pressure occurring temporarily, including pressure surges, which a part of a pipeline in operation can withstand [2]) of the pipeline. As a rule, the admissible pressure surge is limited to 3 bars for safety reasons. The filling speed is limited to 0.25 m/s in accordance with DVGW data sheet W 334 [1].

In order to secure the pipeline against inadmissible pressure fluctuations for problem-free operation then, depending on the operating state, ventilation or air release is necessary for the pipeline components. The gas bubbles (air, carbon dioxide etc.) trapped in pipelines reduce the free cross-section available for flow, increase the pressure loss in the pipeline and in some cases cause undesirable pressure surges.

The size and number of vent valves is to be determined according to the nominal width size of the pipeline, the filling volume, the topography and the maximum permissible air speed in the narrowest cross-section of the vent valve (main venting).

As regards the level of ventilation, as a rule it is assumed that the pressure should not fall below the absolute pressure of 0.8 bar (0.2 bar negative pressure) in the pipeline. According to experience, the limit is respected with sufficient certainty if the air entry speed in a correctly dimensioned vent valve is not above 80 m/s this speed of 80 m/s should also not be exceeded for reasons of noise pollution.

Ventilation and air release valves are generally installed in shafts or buildings. They can also be arranged on pipelines which run above ground. However, there are also designs of ventilation and aeration fittings which are suitable for installation underground.

Ventilation and air release

Ventilation by automatic vent valves is necessary in the following cases: 

• draining sections of the pipeline 
• in case of underpressure occurring, to protect the pipeline (for example behind pipe break protection devices)

Air release is not necessary in normal network operation as air release automatically occurs through branches, hydrantes and above all house connections. Even with long-distance pipelines, no forced air release is necessary if the speed of flow is sufficient, even when the pipeline is on a downward gradient, to carry the air bubbles with it. In cases where disruptive accumulations of air can form, automatically acting air release valves are to be provided. Air in water pipelines is mainly to be expected in places where certain conditions arise such as pressure reductions and temperature increases.

So air bubbles form at

• geodetic high points (L 1, L 3, L 6, L 7) and 
• hydraulic high points (L 2, L 4).

Hydraulic high points can possibly form in certain operating situations and are temporary in nature.

Air release by automatically acting air release valves is necessary in the following cases: 

• draining sections of the pipeline 
• in case of underpressure occurring, to protect the pipeline (for example behind pipe break protection devices)

Choice of different aeration and air release devices

Most designs of aeration and air release devices are based on the float principle, with and without lever reinforcement.

Float principle

Ventilation and air release valves using the float principle are designed with a main vent section with a large aeration and air release cross-section and an operational vent section with a smaller aeration and air release cross-section. The main aeration and air release is there to remove or drive large volumes of air from the pipeline. This is the case if pipelines are being filled or emptied.

As soon as the operating medium reaches the large float of the main vent section during filling, it is raised and always remains closed under pressure. Smaller volumes of air which can occur during normal operation are taken away via the operational vent section. The smaller float is then raised by the operating medium and closes the nozzle of the operational vent section. It always opens if air bubbles collect in the housing during operation.

Valve lever function

With aeration and air release valves with a lever function, valves are opened and closed by a lever in the components; depending on the application, different construction forms are used.

Particular construction forms

In addition to the construction forms described above, particular types of design have been developed for operational reasons. Dynamic pressure brakes are used in order to protect aeration and air release valves from pressure surges. There is a shut-off body mounted in the valve housing which can be moved by the flow. When a certain flow speed is exceeded the medium pushes the shut-off body into the valve seating. Only a reduced cross-section remains free.

So that the aeration and air release valve can be isolated from the pipeline for overhaul purposes, a gate valve is often installed before the aeration and air release valve. This means that the aeration and air release valve can be dismantled or cleaned while the main pipeline is still operational. A soft-seated valve is best suited to this function as it allows a free passage. In order to prevent aeration with small aeration and air release valves and only ensure an air release function, aeration and air release valves with suppressed air inflow are often used. These valves have their main application in suction pipelines for mechanically purified water or in the area drinking water.

If air is emitted during the filling of pipelines via vent valves, the filling speed must be kept as low as possible. The dreaded pressure surge (Joukowsky surge), which occurs if the float of the vent valve closes the valve seat abruptly at the end of the filling process, must remain below the admissible pressure load (PMA = maximum pressure occurring temporarily, including pressure surges, which a part of a pipeline in operation can withstand [2]) of the pipeline. As a rule, the admissible pressure surge is limited to 3 bars for safety reasons. The filling speed is limited to 0.25 m/s in accordance with DVGW data sheet W 334 [1].

The size and number of vent valves is to be determined according to the nominal size of the pipeline, the filling volume, the topography and the maximum permissible air speed in the narrowest cross-section of the vent valve (main venting).

Basically, aeration and air release valves are installed in shafts. Their construction is described in DVGW worksheet W 358 [3]. In order to economise on the structure of the shaft, special ventilation and aeration fittings for installation underground are used. On the left on the next page an aeration and air release valve is illustrated which ventilates in the underground area via a surface box. Installation beneath surface boxes ensures accessibility. The illustration on the right shows an above-ground design.

There are aeration and air release valves for supplying and discharging small volumes of air. In this case the valve has a female thread and can be mounted directly on the pipeline. Valves of this kind are predominantly used in building installations.

 

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