What causes water hammer in a distribution system?
- A sudden change in the velocity of the water
- Air dissolving in the water at high pressure
- A slow rise in demand during the morning peak
- Corrosion deposits narrowing an old main
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A. A sudden change in the velocity of the water
Moving water has momentum. Stopping or starting it suddenly, by slamming a valve or hydrant shut or by a pump tripping, sends a pressure wave through the pipe that can break mains. Operating valves and hydrants slowly prevents it.
Reference: Hydraulics: water hammer (pressure surge)
A circular clarifier is 40 ft in diameter with a water depth of 16 ft. What is its volume in gallons? Use area = 0.785 × D² and 7.48 gal/ft³.
Answer in gal.
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150320 gal
Surface area = 0.785 × 40² = 0.785 × 1,600 = 1,256 ft². Volume = 1,256 × 16 = 20,096 ft³. Gallons = 20,096 × 7.48 = 150,318 gal, about 150,320 gal.
Reference: Cylinder volume: V = 0.785 × D² × depth; 7.48 gal/ft³
What oxygen range is acceptable for entering a confined space without supplied air?
- 19.5% to 23.5%
- 16.0% to 19.5%
- 20.9% to 25.0%
- 12.0% to 21.0%
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A. 19.5% to 23.5%
Normal air is 20.9% oxygen. OSHA treats an atmosphere below 19.5% as oxygen-deficient and one above 23.5% as oxygen-enriched, which is a fire hazard. Entry outside this range requires ventilation or supplied-air breathing apparatus.
Reference: OSHA 29 CFR 1910.146: permit-required confined spaces (hazardous atmosphere)