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BLD-PLUM06-08 · Lesson 2 · 10 clock hours · theory

Hydraulic Principles: Pressure, Flow, and Siphonage

Objective. The learner calculates static pressure, velocity pressure, and the pressure differential needed to cause back-siphonage, and explains how each hydraulic condition can be produced in a building water system.

Why Hydraulics Matter in Backflow Prevention

Backflow — the unwanted reversal of water flow in a plumbing system — is a serious public-health hazard. Contaminated water can enter the potable supply whenever the normal pressure balance in the system is upset. To recognize those conditions and select, install, and test the right protective device, you must understand three hydraulic fundamentals: static pressure, velocity pressure, and the pressure differential that drives back-siphonage.

Static Pressure

Static pressure is the force that water exerts on the walls of a pipe when the water is not moving. In a building system, static pressure comes from two sources: the elevation of the water supply (for example, a water tower or a pressurized main) and the mechanical pressure added by pumps.

The relationship between elevation and pressure is direct and constant:

> 1 psi = 2.31 feet of water head (rounded to 2.31 ft/psi for trade use)

> 1 foot of head = 0.433 psi

A water column 100 feet tall exerts roughly 43.3 psi at its base. This conversion is the starting point for almost every pressure calculation in plumbing.

Static pressure is measured when all fixtures are closed and no flow is occurring. The reading on a gauge at a hose bib with everything shut off is the system static pressure at that point.

Velocity Pressure and the Bernoulli Principle

When water moves through a pipe, some of its static (potential) energy converts to kinetic energy — the energy of motion. This conversion is described by the Bernoulli principle: as velocity increases, static pressure decreases.

In practical terms, wherever water accelerates — through a constriction, around a tight bend, or up a tall riser — the local static pressure drops. If the acceleration is severe enough, static pressure can fall below atmospheric pressure (14.7 psi at sea level), creating a partial vacuum. That vacuum is the engine of back-siphonage.

Velocity pressure itself is rarely calculated directly on the job, but the concept is critical: high-flow events (flushing multiple fixtures at once, firefighting demand) reduce local static pressure throughout the distribution network.

Pressure Differential and Back-Siphonage

A pressure differential is the difference in pressure between two points in a system. Water always flows from higher pressure toward lower pressure.

Back-siphonage occurs when pressure at the supply side of a cross-connection drops below atmospheric pressure, creating a negative pressure differential. The atmosphere (or any higher-pressure source) then pushes contaminated water backward into the potable supply — exactly like liquid being drawn up a straw.

Common causes of negative supply pressure in a building system include:

The minimum gauge pressure required to cause back-siphonage is any reading below 0 psig (gauge pressure). At 0 psig the system pressure equals atmospheric; below 0 psig a partial vacuum exists and back-siphonage is possible at any open cross-connection.

Backpressure vs. Back-Siphonage

Though both are forms of backflow, backpressure and back-siphonage have different hydraulic causes.

Backpressure occurs when a downstream system develops pressure higher than the supply pressure — for example, a booster pump, a steam-heated system, or a pressurized recirculating loop. The downstream pressure physically pushes water backward against the supply.

Back-siphonage does not require elevated downstream pressure. It requires only that supply pressure falls below atmospheric. The atmosphere itself provides the driving force.

Knowing the difference matters because code and manufacturer guidelines specify different backflow prevention devices for each condition. Some assemblies protect against one type only; others protect against both.

Building a Mental Model

Picture a three-story office building. The city main delivers 70 psig at the meter. At the third-floor break room faucet, elevation alone has reduced that pressure by about 13 psi (30 ft × 0.433 psi/ft ≈ 13 psi), leaving roughly 57 psi under static conditions. If a nearby fire crew opens a large hydrant during an emergency, main pressure can collapse to near zero in seconds. At that moment, any submerged hose end, chemical dispenser, or irrigation head connected to the potable piping without proper protection becomes a siphon — pulling contamination in.

Safety Notes

When performing field tests and demonstrations involving vacuum pumps or pressurized acrylic assemblies:

Understanding the hydraulics in this lesson is the foundation for every device selection, installation decision, and field test procedure covered in the rest of this course.

Key terms9

Static pressureThe pressure exerted by water on pipe walls when there is no flow; measured in psi or feet of head.
Velocity pressureThe component of total pressure associated with the kinetic energy of moving water; as flow velocity increases, static pressure decreases.
Head (feet of head)A way to express pressure in terms of the equivalent height of a water column; 1 foot of head equals approximately 0.433 psi.
Pressure differentialThe difference in pressure between two points in a piping system; water flows from the high-pressure point toward the low-pressure point.
Back-siphonageA form of backflow caused when supply pressure drops below atmospheric pressure, creating a partial vacuum that draws contaminated water backward into the potable supply.
BackpressureA form of backflow caused when a downstream system develops pressure greater than the supply pressure, forcing water to flow backward against the supply.
Bernoulli principleThe hydraulic principle stating that as the velocity of a fluid increases, its static pressure decreases, assuming no energy is added or removed.
Gauge pressure (psig)Pressure measured relative to atmospheric pressure; a reading of 0 psig means the system is at atmospheric pressure, and a negative psig reading indicates a partial vacuum.
Cross-connectionAny physical link between the potable water supply and a source of contamination or pollution through which backflow could occur.

Worked examples4

1. A pressure gauge at a basement meter reads 75 psig. A fixture is located 52 feet above the meter. What is the static pressure available at the fixture when no water is flowing?
  1. Identify the elevation head loss: 52 ft × 0.433 psi/ft = 22.5 psi (rounded to one decimal).
  2. Subtract the elevation head loss from the supply pressure: 75 psi − 22.5 psi = 52.5 psi.
  3. No friction loss is present because there is no flow (static condition), so no additional deduction is needed.
Answer: The static pressure at the fixture is 52.5 psig.
2. A technician measures 38 psig at a hose bib on the ground floor. Convert this reading to feet of head.
  1. Use the conversion factor: 1 psi = 2.31 feet of head.
  2. Multiply: 38 psi × 2.31 ft/psi = 87.78 ft.
  3. Round to one decimal place for typical trade use: 87.8 ft.
Answer: 38 psig is equivalent to approximately 87.8 feet of head.
3. During a firefighting event, the municipal main pressure at a building meter drops from 65 psig to 2 psig. A third-floor sink is located 28 feet above the meter. What is the gauge pressure at the sink during the event, and does a back-siphonage condition exist?
  1. Calculate the elevation head loss for 28 feet: 28 ft × 0.433 psi/ft = 12.1 psi.
  2. Subtract from the reduced supply pressure: 2 psi − 12.1 psi = −10.1 psig.
  3. A gauge pressure below 0 psig indicates the pressure at the sink has dropped below atmospheric pressure.
  4. Because the pressure is negative (partial vacuum exists at the fixture), a back-siphonage condition is present.
Answer: The gauge pressure at the sink is −10.1 psig; a back-siphonage condition exists.
4. A closed hot-water heating loop is maintained at 22 psi above the building supply pressure of 55 psig. What is the pressure differential driving backpressure, and which direction will water tend to flow through an unprotected cross-connection?
  1. Determine the heating-loop pressure: 55 psi + 22 psi = 77 psig.
  2. Calculate the pressure differential: 77 psig (loop) − 55 psig (supply) = 22 psi.
  3. Water flows from the higher-pressure region toward the lower-pressure region.
  4. The loop is at higher pressure than the supply, so water will tend to flow from the heating loop backward into the potable supply — a backpressure condition.
Answer: The pressure differential is 22 psi; water will tend to flow from the heating loop into the potable supply (backpressure backflow).
In class: Instructor demonstration using a clear acrylic pipe loop and a vacuum pump to produce visible back-siphonage; learners predict the outcome before each trial and record observations · Math practice set: learners solve eight pressure-conversion and pressure-differential problems using plumber's trade math (psi, feet of head, flow rate) with instructor feedback after each problem · Paired discussion: learners draw a simple schematic of a building system, label where negative pressure can develop, and explain the scenario to a partner
Materials: Acrylic pipe demonstration loop, vacuum pump, pressure gauges, calculator, graph paper, IPC/UPC code books, NCCER Plumbing Level 3 core curriculum textbook
Check: Math practice set: learner solves at least 6 of 8 problems correctly (75%) and accurately describes both backpressure and back-siphonage scenarios during instructor oral check

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Plumbing / ASSE Backflow — Hydraulic principles
1. A water tower has its water surface 115 feet above a building's ground-floor service entrance. Ignoring friction, what is the approximate static pressure at the service entrance?
ASSE Backflow — Backflow causes and definitions
2. Which condition is the primary hydraulic cause of back-siphonage?
NCCER Plumbing L3 / ASSE Backflow — Hydraulic principles
3. According to the Bernoulli principle, what happens to the static pressure in a pipe section where water velocity increases significantly?
NCCER Plumbing — Trade math; State Journeyman exam — Calculations
4. A plumber needs to convert a static pressure reading of 57 psig to feet of head. Which calculation is correct?
ASSE Backflow — Backflow causes and definitions
5. A building supply is at 60 psig. A recirculating chemical injection system downstream operates at 85 psig. What type of backflow condition does this represent?
ASSE Backflow / NCCER L3 — Pressure calculations and backflow conditions
6. During a fire-suppression event, main pressure at a meter drops from 70 psig to 5 psig. A second-floor fixture is 18 feet above the meter. What is the approximate gauge pressure at that fixture during the event?
NCCER Plumbing — Hydraulics fundamentals
7. Which of the following is the best description of gauge pressure (psig)?
ASSE Backflow — System analysis; NCCER L3 — Hydraulic principles
8. A learner is drawing a schematic of a four-story building to identify where negative pressure is most likely to develop. Which location on the schematic should be labeled as the highest risk for back-siphonage under low-supply-pressure conditions?
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