Page 363 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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9.4 Methods and Devices for the Prevention of Backflow and Backsiphonage 323

Figure 9.24 Double Check Valve

“raw” water through outside fire pumper connections (Siamese outlets), and the detection
of any water movement in the fire line water due to fire line leakage or deliberate water
theft. It consists of two spring-loaded check valves, a bypass assembly with water meter
and double check valve, and two tightly closing gate valves (see Fig. 9.25).
The addition of test cocks makes the device testable to ensure proper operation of
both the primary checks and the bypass check valve. In the event of very low fire line
water usage (theft of water), the low pressure drop inherent in the bypass system per-
mits the low flow of water to be metered through the bypass system. In a high flow
demand situation, such as that associated with deluge fire capability, the main check
valves open, permitting high-volume, low restricted flow through the two large spring-
loaded check valves.

9.4.9 Residential Dual Check
The need to furnish reliable and inexpensive backsiphonage and backpressure protection
for TO individual residences resulted in the debut of the residential dual check. Protection
of the main potable supply from household hazards such as home photography chemicals,
toxic insect and garden sprays, termite control pesticides used by exterminators, and so
forth, reinforced a true need for such a device. Figure 9.26a shows a cutaway of the device.

Figure 9.25 Double Check Detector Check