What are the aperture selection criteria for air valves?

Author: Chen Yifei

Suction aperture D1

About 1 / 5 ~ 1 / 8 of the main pipe diameter D, which is positively related to the slope of the pipe section.

If the suction hole diameter is large, it is recommended to divide it into two valves in parallel.

The response sensitivity of suction orifice to negative pressure shall not be less than 0.2s@-2kpa, and the faster the better. The larger the diameter, the lower the negative pressure sensitivity may be. Therefore, it is recommended that small-diameter multi valve be connected in parallel, rather than single valve and large-diameter; It is suggested to decompose the large-diameter suction valve into 2 ~ 3 or even 4 small-diameter suction valves based on the principle of area equivalence, such as 1 * dn400 ≈ 2 * DN300 ≈ 3 * DN250 ≈ 4 * DN200, at least two valves in parallel; The common specifications with diameter in the range of DN50 ~ 200 have high negative pressure sensitivity, but the larger the orifice is, the greater the suction capacity is. In case of transient negative pressure, it is more favorable to restrain the negative pressure.

Micro exhaust aperture D2

About 1 / 100 ~ 1 / 350 of the main pipe diameter D, the pore size is positively correlated with the flow rate and negatively correlated with the working pressure; Generally, D * (1 / 200 ~ 1 / 250) is taken. For example, about 10mm micro exhaust orifice is required for d2000 pipeline; However, it is recommended to decompose it into 2 7mm or 3 6mm orifices.

Proper amount of empty pipe water filling exhaust aperture D3

About 1 / 19 ~ 1 / 25 of main pipe diameter D, usually 1 / 20; For example, d2000 pipeline requires about 100 mm high-speed exhaust orifice.

However, after the empty pipe is filled with water, it is closed manually or automatically to exit the transient process.

Special requirements for vent valve aperture for water hammer protection

Empty pipe filling period

First fast and then slow. In the early stage, the water can be filled slightly faster, but in the later stage, it must be slow to make the water filling speed reach about v = 0.3m/s. This speed requires that the exhaust aperture D3 be in the range of 1 / 10 ~ 1 / 22 of D, that is, the exhaust aperture should reach about 1 / 20 of D in the later stage of water filling.

Moreover, after the empty pipe is filled with water, the exhaust orifice shall be closed (manual valve closing or automatic valve closing).

Transient water column separation and bridging period

Quick response to transient negative pressure: the response sensitivity of suction orifice to transient negative pressure shall not be less than 0.2s@ – 2KPa, and the faster the better.

The larger the suction aperture is, the better, but it is recommended to divide it into two or even four, and the single aperture is less than 200mm.

The micro row aperture is about 1 / 200 of D; The ratio of inlet and exhaust aperture is ≥ 33:1; That is, the ratio of air inlet and exhaust capacity is ≥ 1000:1. At the moment of water column bridging, it has the ability to lock most of the temporary air bags to act as buffer elastomers; After the transient process, it has the ability to slowly eliminate the stored gas, and the exhaust time is no more than 1 ~ 2 hours (usually, the pipe system will stop running; in this way, exhaust is not required).

Installation distance / interval of vent valve

The standard spacing required by Chinese GB 50013 and CECS 193 standards is about 500 ~ 1000m;

American AWWA M51 recommends 400 ~ 800m;

German dvwg recommends 300 ~ 600m.

We recommend: install vent valves of standard size (composed of 1 / 6 + 1 / 200 + 1 / 20 orifices of D) on the main high convex nodes in a large interval of about 300 ~ 800m (standard interval);

However, small-size economical composite air valves (composed of 1 / 20 middle orifice of D + 1 / 300 micro row orifice) shall be installed / arranged on secondary high convex nodes with small spacing of 100 ~ 300m;

The interval setting method and orifice selection results of the above vent valves also need to meet the special protection requirements under some emergencies, such as pipeline explosion.

Suggestion: set a gas collecting neck at the bottom of the vent valve, and its height and cylinder diameter are about 1 / 4 ~ 1 / 2 of D; Or directly use D600 * h600mm to D300 * h300mm. It is recommended to install 1 ~ 2 small diameter safety valves on the top.

Air valve selection

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