11.2 Elevation Head, Minimum Pressure & Hazen-Williams
Key Takeaways
- Elevation head is 0.433 psi per foot of water (inverse about 2.31 ft/psi); a sprinkler 20 ft above the gauge needs +8.66 psi.
- Add elevation when the head is above the reference gauge; subtract only when the head is below that gauge.
- Hazen-Williams friction rises with flow and pipe length and falls as C and internal diameter increase.
- NFPA 13 (2022) Table 28.2.4.8.1: C = 120 wet steel (including deluge), C = 100 dry and preaction steel, C = 150 listed plastic (including CPVC) and copper/brass/stainless.
- A smaller C is rougher pipe and demands more pressure for the same flow, length, and diameter; dry steel at C = 100 is about 1.40 times the friction of wet steel at C = 120.
Elevation is a pressure tax, not a flow formula
Section 11.1 found the pressure at the sprinkler. The water column between that sprinkler and the gauge you are reading still has to be paid. Independent OpenExamPrep material treats this as Level I algebra, not as a Level II branch-line worksheet.
NFPA 13 hydraulic work uses 0.433 psi per foot of elevation (some pocket cards round the twin value 0.434 psi/ft, which is 1 / 2.31). The inverse used on many exams is 2.31 ft of water = 1 psi. Memorize both directions so a stem can give feet or psi.
Worked elevation: a sprinkler is 20 ft above the riser gauge.
20 × 0.433 = 8.66 psi
Check with the inverse: 20 / 2.31 ≈ 8.66 psi. Same tax.
If that head already needs 12.8 psi from the 20 gpm / K-5.6 example in 11.1, the gauge 20 ft below must show at least 12.8 + 8.66 = 21.5 psi before you add pipe friction. Elevation does not change K and does not change Q by itself. It only changes the pressure you must have at the lower reference point.
Sign convention, in one sentence: heads above the gauge add psi to demand; heads below the gauge credit psi. A basement head 12 ft below a first-floor gauge is 12 × 0.433 = 5.20 psi of elevation credit at that head relative to the gauge. Do not credit elevation that the water cannot actually use (a closed valve, a dry pipe still full of air, or a stem that already gave you pressure at the elevation of the heads).
Another original check: a roof manifold 38 ft above the underground flange needs 38 × 0.433 = 16.5 psi of elevation before any fitting is entered on a calculation sheet. 38 / 2.31 ≈ 16.5 psi as well. If a choice says “38 psi because feet and psi are interchangeable,” that choice is padding.
Hazen-Williams: what Level I has to see without grinding a branch line
NFPA 13 pipe friction for water-based systems is Hazen-Williams. In U.S. units the loss rate is:
p (psi/ft) = 4.52 × Q^1.85 / (C^1.85 × d^4.87)
You do not need to hand-calc an entire remote branch on Level I. You do need to read the formula as four knobs:
- Q up → friction up (steeply; the 1.85 exponent means a modest flow increase is a larger pressure increase).
- Length up → total friction up (psi/ft times feet of equivalent pipe).
- C up → friction down (smoother interior).
- Diameter up → friction down (the 4.87 exponent makes diameter the strongest knob).
The exam-ready consequence: a smaller C demands more pressure for the same flow, length, and diameter. That is why dry-system steel and wet-system steel are not interchangeable on a calculation even when the pipe schedule looks identical.
Which C to pick (NFPA 13 2022 Table 28.2.4.8.1)
| Pipe or tube | C-value |
|---|---|
| Black or galvanized steel, wet systems including deluge | 120 |
| Black or galvanized steel, dry systems including preaction | 100 |
| Black or galvanized steel, dry/preaction with listed nitrogen (8.2.6.9) or the table’s vacuum provision | 120 |
| Listed plastic, including CPVC | 150 |
| Copper tube, brass, or stainless steel | 150 |
| Unlined cast or ductile iron | 100 |
| Cement-lined cast or ductile iron | 140 |
The nitrogen path is not “any dry system with a bottle.” Section 8.2.6.9 requires a listed, permanently installed nitrogen generator that can hold at least 98 percent nitrogen throughout the system at a stated leakage rate, plus a means of verifying concentration. If those provisions are not in the stem, dry and preaction steel stays C = 100.
An older field habit that “galvanized is always 120, even dry” is not how the 2022 table is written. Wet galvanized is 120; dry/preaction galvanized is 100 unless the nitrogen or vacuum row applies. Listed CPVC and copper are 150 in wet systems; do not assign them the steel dry-system 100.
How much more pressure does C = 100 really cost?
Hold Q, length, and diameter fixed and change only C. Friction scales as 1 / C^1.85, so the ratio of C-100 friction to C-120 friction is (120/100)^1.85 = 1.20^1.85 ≈ 1.40. The dry steel system needs about 40 percent more friction pressure in that pipe than the same wet steel system. Level I should recognize the direction and the roughly 40 percent figure; Level II can grind the equivalent-length table.
Equivalent-length fitting multipliers in Table 28.2.3.2.1 are a different table. They start from C = 120 fitting lengths. A smoother pipe (C = 150) needs a longer equivalent length of that smooth pipe to represent the same fitting loss; a rougher pipe (C = 100) needs a shorter equivalent length. Do not invert those multipliers into a claim that C = 150 pipe has more friction than C = 100 pipe. Pipe friction still falls as C rises.
Minimum pressure at the supply is the stack, not one cell
At the reference gauge you are stacking:
- Sprinkler pressure from Q = K√P (section 11.1),
- Elevation at 0.433 psi/ft,
- Friction from Hazen-Williams along the pipe and fittings,
- Device losses the calculation sheet actually lists (alarm valve, backflow, etc.).
A sprinkler is 20 ft above the riser gauge. Using 0.433 psi/ft, how much elevation pressure must be added to the sprinkler demand at that gauge?
Per NFPA 13 (2022) Table 28.2.4.8.1, which Hazen-Williams C-value is correct?
Two hydraulically identical steel mains carry the same flow. One is a wet system (C = 120) and one is a dry system without nitrogen (C = 100). Which statement is correct?