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Lesson plan Grade 6 Engineering & Design

Bramfell School Backflow Preventer Design Lesson

A 50-minute lesson where students pick a backflow assembly for a fictional school greenhouse, then check the device still leaves enough pressure at the sprinklers.

Objectives

  1. Students will state the criteria and constraints that decide which backflow assembly a connection requires.
  2. Students will subtract stated pressure losses to find the pressure remaining at a sprinkler.
  3. Students will calculate the multi-year cost of an assembly and compare it with a stated set-aside.

Vocabulary

Backflow
Backflow is water flowing backward through a pipe in the wrong direction, which can pull dirty water into the clean drinking water system. Special devices are installed on pipes to stop it.
Cross-connection
A cross-connection is any place where drinking water pipes are linked to something dirty, like a garden hose left sitting in a bucket of soapy water. If the flow reverses, the dirty water can get sucked in.

Also: hazard class, pressure loss, assembly

Teacher overview

Bramfell Middle School is fictional, and every pressure, price, and set-aside figure in this lesson was invented for it. The school is adding a greenhouse whose hose line will run liquid fertilizer through an injector, which creates a cross-connection between the fertilizer and the school's drinking water. If the street main lost pressure, water could be pulled backward out of the greenhouse and into the building, a condition called backflow. Students define the criteria and constraints, choose the assembly the hazard class requires, and then check whether that assembly still leaves enough pressure at the far sprinkler. The misconception this lesson addresses is the idea that any backflow device will serve, when in fact what sits on the other side of the connection decides which assembly is permitted.

Procedure

All figures used in this fictional Bramfell scenario are invented for teaching.

  • Hook (5 min): Draw a hose lying in a bucket of soapy water with the tap open, and ask what would happen to the soapy water if the street main suddenly lost pressure. Take three answers, then name the condition: backflow.
  • Frame the design problem (8 min): Describe the greenhouse fertilizer injector. Have the class list, on the board, what the solution must do (stop fertilizer from entering the building) and what limits it (a spending set-aside, a minimum pressure the sprinklers need, and a legal hazard class). Write these under the headings Criteria and Constraints so students see the two lists as separate.
  • Model the pressure subtraction (10 min): Work one example at the board with numbers that do not appear on the handout. Suppose a different school receives 72 psi at the street, its assembly loses 10 psi, and its pipe and fittings lose 5 psi. Ask the class for the pressure at the sprinkler, then confirm 72 - 10 - 5 = 57 psi. Stress that every loss is subtracted in turn, not averaged.
  • Handout (18 min): Students work items 1 through 10 of the Student Handout, alone or in pairs. Circulate and check that item 1 is answered before item 2 is attempted, since the device chosen there sets the pressure loss used afterward.
  • Formative check (4 min): Ask every student to write the answer to item 3 on a scrap of paper and hold it up, so the whole set can be read at a glance. If more than one third of the class is off, rework item 2 and item 3 together before the debrief.
  • Debrief (5 min): Ask why the cheapest device on the chart is not an option here, and ask what the school would have to change if the street pressure were lower. Close by naming the job of a cross-connection control inspector, who tests these assemblies each year.
Bramfell School Backflow Preventer Design Lesson — Grade 6 Engineering & Design
Name: ____________________________ Date: ______________

Student Handout

Bramfell Middle School and every number below are fictional, invented for this exercise. The street main delivers 68 psi to the school. Three backflow devices are sold for this kind of connection:

Device Cost Pressure it takes away Approved for
Hose bibb vacuum breaker $35 2 psi low hazard only
Double check valve assembly $420 8 psi low hazard only
Reduced pressure zone assembly $980 14 psi high hazard

Liquid fertilizer makes this a high-hazard connection. The greenhouse sprinklers need at least 45 psi to work. Pipe and fittings between the device and the sprinklers take away another 6 psi.

  1. Which device on the chart is approved for a high-hazard connection?
  2. The street main delivers 68 psi. The device you named in item 1 takes away 14 psi. What pressure is left right after the device?
  3. Pipe and fittings take away another 6 psi. What pressure reaches the far sprinkler?
  4. The sprinklers need at least 45 psi. Does the pressure you found in item 3 meet that requirement?
  5. How many psi above the 45 psi requirement is the pressure from item 3?
  6. The reduced pressure zone assembly costs $980 and the double check valve assembly costs $420. How much more does the required device cost?
  7. State rules require a test every year, and the tester charges $85 each time. What do five years of tests cost?
  8. What is the total of the assembly price and the five years of tests?
  9. Bramfell set aside $1,600 for this work. What percent of that set-aside does your answer to item 8 use? Round to the nearest whole percent.
  10. Suppose the street main dropped to 60 psi. Would the far sprinkler still receive 45 psi? Show the subtraction that supports your answer.

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: The reduced pressure zone assembly. It is the only device on the chart approved for a high-hazard connection, and liquid fertilizer makes this connection high hazard.
  2. Answer: 54 psi. The street delivers 68 psi and the assembly takes away 14 psi, so 68 - 14 = 54 psi.
  3. Answer: 48 psi. Pipe and fittings take away 6 more, so 54 - 6 = 48 psi.
  4. Answer: Yes. The sprinklers need at least 45 psi and the design delivers 48 psi.
  5. Answer: 3 psi. 48 - 45 = 3 psi above the requirement.
  6. Answer: $560. 980 - 420 = 560 dollars more.
  7. Answer: $425. Five yearly tests at 85 dollars each give 5 x 85 = 425 dollars.
  8. Answer: $1,405. The assembly costs 980 dollars and the tests cost 425 dollars, so 980 + 425 = 1,405 dollars.
  9. Answer: 88 percent. 1,405 / 1,600 = 0.878, which rounds to 88 percent.
  10. Answer: No. At 60 psi the subtraction gives 60 - 14 - 6 = 40 psi, which is 5 psi short of the 45 psi the sprinklers need.

Differentiation

Support

Give these students the handout with the device chart already reduced to the one approved row, so item 1 is answered for them and the pressure work can begin at item 2. Provide a vertical subtraction ladder with the starting pressure written at the top and two empty boxes beneath it.

Extension

Ask students to determine the lowest street pressure, in whole psi, at which this fictional design would still deliver the sprinklers' minimum, and then to propose one change to the design that would let it work at a street pressure ten psi lower than that.

ELL supports

Provide a labeled diagram showing the street main, the assembly, the pipe run, and the sprinkler in a line, with an arrow and a subtraction sign drawn at each place where pressure is lost. Allow students to answer item 10 with the subtraction sentence alone.

Extensions

Have students walk the school with a staff escort and list every place where a hose could sit in standing water, such as a mop sink, a science lab tap, or an outdoor spigot. Ask each student to mark one location on a simple floor plan and write one sentence about what could be pulled into the pipes there.