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Air Valves - van xả khí tự động

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Air Valves - van xả khí tự động

Bài gửi  duyminh on Wed Jun 08, 2011 3:32 pm

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Air Valves - van xả khí tự động

Three Basic Types of Air Valves
There are three basic types of air valves which are standardized in American Water Works Association (AWWA) Standard C512: Air Valves for Waterworks Service. They are air release valves, air/vacuum valves and combination air valves.

It is important to understand the functions and limitations of each valve type, so that valves can be located and sized properly for a pipeline.

Air release valves
Air release valves are probably the best known air valve and are typically furnished in sizes 0.5-in (13-mm) through 3-in (76-mm). The valve has a small precision orifice in the range of 0.0625-in (1.6-mm) to 0.5-in (13-mm) to release air under pressure continuously during pipeline operation. The air release valve has a float to sense the presence of air and a linkage mechanism that gives the float mechanical advantage in opening the orifice under full pipeline pressures.

An air release valve can also be used between a vertical turbine pump and a power actuated pump check valve to prevent surges in the piping between the pump and the check valve. In this application, the opening of the check valve is delayed with a timer until the air release valve can discharge the air in the pump column to achieve a controlled 1-ft/sec to 2-ft/sec (0.3-m/sec to 0.6-m/sec) flow velocity in the pump column.

For a 20-ft (6-m) lift, the delay time will be about 10 to 20 seconds. Because the valve has limited vacuum flow capacity, a timer is also needed to delay the pump restart so that the water level in the pump column has time to return to its original level.

Air release valves have a limited capacity for admitting and exhausting air. For this reason, most pipeline locations require both air release and air/vacuum valves for exhausting and admitting large volumes of air.

Air/Vacuum Valves
An air/vacuum valve is installed downstream of pumps and at high points to exhaust large volumes of air during pump startup and pipeline filling. The valve also will admit large volumes of air to prevent a vacuum condition from occurring in the pipeline and to allow for draining.

A float in the valve rises with the water level to shut off the valve when the air has been exhausted. Upon the loss of pressure due to draining, line break, or column separation, the float will drop and allow air to reenter the pipe. It is important to note that under normal operation, the float is held closed by the line pressure and will not relieve accumulated air. An air release valve is needed to relieve air during system operation.

There are two variations of air/vacuum valves that warrant discussion. First, air/vacuum valves can be equipped with an anti-slam device, which controls the flow of water into the valve to reduce surges in the valve. The anti-slam device is useful at highpoints where column separation or rapid changes in velocity occur. Column separation can be predicted by computer transient analysis, but the following general guidelines can be used to help locate anti-slam devices:

1. When the flow velocity is greater than 8-ft/s (2.4-m/s), the surge potential can be as high as 400-psi (2760-kPa). Also, when the fill velocity exceeds 2-ft/s (0.6-m/s), high surges can result.
2. High points where a vacuum forms on shutoff will exhibit rapid flow reversal.
3. Systems where the time for the water column to reverse exceeds the critical time will see high surges even from small changes in velocity.
4. Fast closing pump discharge check valves may prevent slam but still cause line surges.
5. Systems with booster stations can see great fluctuations in line velocities on power failure.
6. If the pipeline discharge creates a siphon on shutdown, rapid flow reversal can be expected.
Second, a well service air valve is an air/vacuum valve equipped with a throttling device or an anti-slam device (4-in and larger valves) for use with vertical turbine pumps. These pumps start against an empty pump column and a closed pump check valve and therefore start rapidly and accelerate the fluid. well service air valves require special consideration during sizing.

The throttling device (3-in and smaller valves) controls the air discharge rate so that the pressure surge caused by the pump water column reaching the closed pump check valve is minimized. The throttling device has a second independent vacuum port to allow air flow back into the line after pump shutdown so that the static suction water level can be restored without allowing a vacuum to form in the pump column.

The dual port throttling device should have an open vacuum port separate from the exhaust port so that the air flow into the device is not restricted by exhaust piping.

Combination Air Valves
The combination air valve combines the functions of both the air/vacuum and air release valves and is an excellent choice for high points.

A combination valve contains both a small air release orifice and a large air/vacuum port in one assembly. On smaller valves, usually less than 8-in (200-mm), the float and lever mechanism are contained in a single body design. On larger sizes, a dual body design consisting of an air release valve piped to an air/vacuum valve is furnished as a factory assembled unit.

Single body units have the advantage of being more compact and typically less costly. Dual body units are advantageous for air release valve sizing and maintenance because the air/vacuum valve is still in operation while the air release valve is isolated and under repair.

By combining various sized air release and air/vacuum valves, a dual body combination valve can be made for almost any application. Some designers use only combination air valves on a pipeline because all air valve functions are included and a mistake in field installation will not leave the pipeline unprotected.

WQA Certified Lead-Free
Certified to NSF/ANSI 61
Meets AWWA C512 requirements
UL Listed for fire pump service
Manufactured under a certified ISO 9001 quality system
Available in sizes 1/2" - 6"
Maintains systems flow efficiency
Releases unwanted air pockets during system operation
Protects system against air related surges

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