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Lesson plan Grade 11 Science

Why the Denholt Pump Cavitates: Vapor Pressure and Lift

A grade 11 lesson using a syringe demonstration and a fictional pump station record to explain cavitation as boiling at reduced pressure, with head math.

Objectives

  1. Students will explain cavitation as water boiling at reduced pressure inside a pump inlet.
  2. Students will calculate net positive suction head available from atmospheric, lift, friction, and vapor pressure terms.
  3. Students will infer relative strength of attraction between molecules from two liquids' vapor pressures.

Vocabulary

Cavitation
Cavitation happens when the water pressure at a pump's inlet drops so low that tiny vapor bubbles form in the water and then collapse with sharp snaps. It sounds like gravel rattling inside the pump, and it slowly chews up the metal.

Also: vapor pressure, suction lift, friction head, net positive suction head, implosion

Teacher overview

Water boils whenever the pressure around it falls to its vapor pressure, and the inlet of a pump that is lifting water is one of the lowest-pressure places in a water system. This lesson uses a syringe demonstration, a vapor pressure chart, and one summer's records from the fictional Denholt Water Works to explain why a pump that ran quietly in April rattled and lost flow in August. The syringe step comes first because it separates boiling from heating, showing warm tap water boiling well below 100 degrees Celsius once the pressure above it is lowered. The lesson closes with the damage that follows, since vapor bubbles that collapse against an impeller remove metal.

Procedure

Hook and demonstration (7 min): Fill a 60 mL syringe about one-third with hot tap water at roughly 50 degrees Celsius, cap the tip, and pull the plunger back hard. The water boils at about 50 degrees Celsius, far below its normal boiling point. Ask students to explain what changed, since the temperature did not. Repeat with a second syringe holding rubbing alcohol at the same temperature, with the room ventilated, no flame or other ignition source nearby, and the alcohol handled by the teacher only, and have students note that the alcohol boils with far less pull on the plunger.

Direct instruction (8 min): Define vapor pressure as the pressure at which a liquid's molecules escape into the gas phase as fast as they return. Establish two ideas from the demonstration. A liquid boils when the surrounding pressure drops to its vapor pressure, whether that happens by heating the liquid or by lowering the pressure. A liquid with a lower vapor pressure at the same temperature holds its molecules together more strongly, which is why water, a small molecule, behaves less like a volatile solvent than its size alone would suggest. Then show the pitted impeller photograph and name what students are looking at.

Worked example (10 min): Model the suction margin calculation on the board with a set of numbers students will not reuse. For a pump at a fictional site where atmospheric pressure supplies 33.0 feet of head, the static suction lift is 8.0 feet, the suction piping loses 2.0 feet to friction, and the chart gives 0.6 feet of vapor pressure head, the net positive suction head available is 33.0 minus 8.0 minus 2.0 minus 0.6, or 22.4 feet. State the rule the operators use: the pump keeps the water at its inlet from boiling only while the available head stays above the value printed on the pump curve.

Guided practice (20 min): Distribute the handout and have students work items 1 through 8 in pairs, then items 9 through 11 alone. Require units on every line. Circulate and watch for the two usual errors, which are converting pounds per square inch to feet in the wrong direction and adding the vapor pressure term instead of subtracting it.

Formative check (5 min): Ask every student to write the answer to item 3 on a slip and hold it up. If more than a quarter of the class is more than one foot away from the correct value, rework item 3 on the board before continuing. Then ask for a show of hands on a single question: does warmer water make cavitation more likely or less likely?

Close (5 min): Collect an exit ticket answering one question in two sentences. Denholt's operators propose three fixes: lower the pump, replace the suction piping with a larger diameter, or run the pump at a lower flow. Name which one your calculations support and say why.

Why the Denholt Pump Cavitates: Vapor Pressure and Lift — Grade 11 Science
Name: ____________________________ Date: ______________

Student Handout

The Denholt Water Works is fictional, and every figure below is invented for this lesson. Denholt's raw water pump sits above a wet well and lifts water up to the pump inlet, so the inlet pressure is below atmospheric whenever the pump runs. Use 2.31 feet of head per pound per square inch. The vapor pressure chart posted in the pump house gives 0.41 feet of head at 50 degrees Fahrenheit and 1.62 feet of head at 90 degrees Fahrenheit. Show units on every line.

Net positive suction head available equals atmospheric head, minus static suction lift, minus friction head in the suction piping, minus vapor pressure head.

  1. The elevation chart in the pump house lists atmospheric pressure at Denholt as 14.2 psi. Convert that pressure to feet of head.
  2. In April the static suction lift is 12.0 feet, the suction friction loss is 3.2 feet, and the water is 50 degrees Fahrenheit. Calculate the net positive suction head available.
  3. In August the well level has dropped, so the static suction lift is 19.0 feet, the suction friction loss at the higher flow is 4.6 feet, and the water is 90 degrees Fahrenheit. Calculate the net positive suction head available.
  4. The pump curve requires 10.0 feet of net positive suction head. Calculate how many feet short the August condition falls.
  5. The static suction lift changed from 12.0 feet in April to 19.0 feet in August. Calculate that change in feet.
  6. New suction piping would cut the August friction loss from 4.6 feet to 1.8 feet, with the lift and temperature unchanged. Calculate the net positive suction head available under that fix.
  7. With the original piping and the August friction loss of 4.6 feet, calculate the static suction lift that would make the net positive suction head available exactly 10.0 feet.
  8. The vapor pressure head is 0.41 feet at 50 degrees Fahrenheit and 1.62 feet at 90 degrees Fahrenheit. Calculate how many feet of suction margin the summer warming consumed.
  9. Explain what is physically happening inside the pump inlet when the available suction head falls below what the pump requires.
  10. Rubbing alcohol boiled in the syringe with much less pull on the plunger than water did. Explain what a liquid's vapor pressure indicates about the strength of the attractions between its molecules.
  11. The pitting damage appears on the impeller blades rather than in the suction pipe. Explain why the damage occurs there.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 32.8 feet. Multiply 14.2 psi by 2.31 feet of head per psi to get 32.802, which rounds to 32.8 feet.
  2. Answer: 17.2 feet. Take 32.8 feet, subtract the 12.0 foot lift, subtract 3.2 feet of friction, and subtract 0.41 feet of vapor pressure head, which leaves 17.19 feet.
  3. Answer: 7.6 feet. Take 32.8 feet, subtract the 19.0 foot lift, subtract 4.6 feet of friction, and subtract 1.62 feet of vapor pressure head, which leaves 7.58 feet.
  4. Answer: 2.4 feet. The pump requires 10.0 feet and only 7.6 feet is available, so the difference is 2.4 feet.
  5. Answer: 7.0 feet. The lift rose from 12.0 feet to 19.0 feet, an increase of 7.0 feet, which is the largest of the three changes between April and August.
  6. Answer: 10.4 feet. With friction reduced to 1.8 feet, the calculation is 32.8 minus 19.0 minus 1.8 minus 1.62, which leaves 10.38 feet, just above the 10.0 feet the pump requires.
  7. Answer: 16.6 feet. Set 32.8 minus the lift minus 4.6 minus 1.62 equal to 10.0, which gives a lift of 16.58 feet. Lowering the pump by about 2.4 feet from its August position achieves this.
  8. Answer: 1.21 feet. The vapor pressure head rose from 0.41 feet to 1.62 feet, and that increase is subtracted directly from the available suction head.
  9. Answer: The water boils inside the pump inlet. As the impeller pulls water in, the pressure at the eye of the impeller falls, and once it reaches the vapor pressure of the water at that temperature, vapor bubbles form even though nothing has been heated. This is cavitation, and it is the same effect the syringe demonstration produced by pulling the plunger.
  10. Answer: A lower vapor pressure indicates stronger attractions between molecules. Molecules escape a liquid into the gas phase only when they overcome the attractions holding them to their neighbors, so a liquid whose molecules attract one another strongly produces fewer escaping molecules and a lower vapor pressure at the same temperature. Water needed a much stronger pull on the plunger than the alcohol, which shows that water molecules hold one another more tightly than alcohol molecules do.
  11. Answer: The bubbles collapse there. Vapor bubbles form at the low-pressure inlet, then travel a short distance into the impeller, where the blades raise the pressure again. The bubbles collapse almost instantly against the blade surface, and each collapse drives a tiny jet of liquid into the metal. Repeated collapses remove metal in small pits, which is why the damage concentrates on the blades rather than in the suction pipe or at the impeller eye where the bubbles first formed.

Differentiation

Support

Give these students a handout with the suction head equation printed as four labeled blanks in subtraction order, and supply the atmospheric head from item 1 already filled in on items 2 and 3. Provide a number line showing which terms are subtracted, and allow a partner check after each of the first three items.

Extension

Ask these students to explain what would happen to the suction margin if Denholt moved the same pump to a site two thousand feet higher in elevation, and to state what they would need to look up to answer it precisely. They can also sketch the pressure along the flow path from the wet well surface through the impeller and label where the lowest pressure occurs.

ELL supports

Pre-teach vapor pressure, lift, friction, and cavitation with a labeled diagram of a wet well, a suction pipe, and a pump. Provide the frames "The pressure falls because ___" and "The bubbles collapse when ___." Let students narrate the syringe demonstration in their home language before writing the explanation in English.

Extensions

Have students find the elevation of their own community and describe how the atmospheric head available there compares with a coastal town, then explain what that means for a pump lifting water from a deep well. Students can also research why a pump is often set below the water surface rather than above it, and write a paragraph tying that choice back to the suction head equation.