| 摘要 |
A water supply control appliance adapted to be buried in soil beneath local frost level contains a chamber fitted with a piston which is spring-biased in opposition to inlet water pressure. Such pressure forces the piston to fully charged position at one end of the chamber whereupon a piston valve is opened to pass the water to outlet. Closing the supply valve releases pressure on the piston, permitting the piston to move in response to the spring bias. The piston movement pumps water below the piston out to drain and drains water from the outlet lines into the expanded chamber above the piston, safe from frost. Water in the drain line will, at the end of the piston travel, fall back into the chamber above the piston after piston travel is completed, when a piston ring seal spans a port to the drain line.
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