其功能在于防止污染水回流到饮用水供应系统中，这些装置不需要对改变管线，只需要直接螺纹旋在洒水龙头上即可。
Designed to prevent the flow of contaminated water back into the potable water supply, these devices require no plumbing changes, and screw directly onto a sill cock.
不幸的是，供应给美国的来自世界各地的有着良好教育的人已经开始慢慢减少了，实际上他们其中很多人已经计划回流了。
Unfortunately, supply of talented individuals and good families from rest of the world to America is down to trickle now. In fact many are planning to return.
用户要由安全饮用水供应商在水表处安装法定的双止回防回流阀。
The user installs a dual check back flow preventer at the water meter as prescribed by the supplier of safe drinking water.
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