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Hydraulics practice questions

26 questions from the backflow prevention tester practice test.

Hydraulics practice test

10 questions, graded as you go, with the reason for every answer. No sign-up.

What this covers

Sample questions

One psi of pressure will support a column of water about how high?

  1. 0.433 ft
  2. 14.7 ft
  3. 33.9 ft
  4. 2.31 ft
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D. 2.31 ft

One psi equals about 2.31 feet of water (1 divided by 0.433). The value 0.433 is psi per foot, the conversion going the other way.

Reference: Hydraulics constants: 2.31 ft of head per psi

A gauge on a water line at sea level reads 50 psig. What is the absolute pressure in the line?

  1. 64.7 psia
  2. 35.3 psia
  3. 50.0 psia
  4. 14.7 psia
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A. 64.7 psia

Absolute pressure is gauge pressure plus atmospheric pressure: 50 + 14.7 = 64.7 psia. Subtracting 14.7 instead gives 35.3, which is the common slip.

Reference: Hydraulics: psia = psig + atmospheric pressure

Which event is most likely to cause backsiphonage in a building's water service?

  1. A booster pump starting up on the building's domestic water piping system
  2. A water heater firing while no fixtures are being used
  3. A break in the water main at a lower elevation down the street
  4. A pressure-reducing valve failing in the wide-open position
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C. A break in the water main at a lower elevation down the street

A main break at a lower elevation drains the main and can pull pressure at higher services below atmospheric, siphoning water backward. A booster pump raises downstream pressure, which is a backpressure cause rather than a backsiphonage cause.

Reference: Backflow hydraulics fundamentals; EPA Cross-Connection Control Manual (EPA 816-R-03-002), ch. 2

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