Pump Discharge Pressure Calculator

PDP = NP + TPL (Formula 6.4)

TPL = FL + AL + EP (friction loss + appliance loss + elevation pressure)

FL = C x Q² x L (Formula 6.1). Q = flow in hundreds of gpm (or L/min). L = length in hundreds of ft (or m).

Use it for a single hoseline, hose that changes size (add a section), siamesed lines, a standpipe, a master stream or an aerial waterway. The same flow runs through every section.

Siamesed lines: pick a siamese set from the list and enter the TOTAL flow and the length of ONE line. For a set that is not in the table (such as three 5 in lines), pick the single hose and set "Lines side by side". The flow is split evenly, as the book describes on p. 239.

Lines of unequal length going to one device: type each length separated by commas (example: 900, 900, 1200). The calculator uses the average, rounded to the nearest whole foot or meter, as in Example 6.21.

Without a supply line, each hoseline has its own discharge. The PDP is set for the line that needs the highest pressure and the others are gated down.

With a supply line (wye, manifold or water thief), the supply line carries the total flow of all lines. Add the supply FL, the appliance loss and the line that needs the most pressure.

If a line is more than one hose size, run that line as its own "one path" calculation and compare.

"Highest total pressure needed" adds NP + FL + EP (+ master stream loss) for each line and uses the largest. "Book method" picks the line with the highest friction loss and adds the master stream loss on top, which is how Example 6.26 (p. 251) is worked. The two give the same answer unless the master stream is on a line with less friction loss.

Solid stream handline: 50 psi (350 kPa)

Solid stream master stream: 80 psi (560 kPa)

Fog nozzle: 100 psi (700 kPa)

Low-pressure fog: 50 or 75 psi (350 or 525 kPa)

Master stream device (includes an aerial waterway on the same stream): 25 psi (175 kPa) regardless of flow. It is added once for each master stream.

Other appliances (wye, siamese, water thief, manifold): 10 psi (70 kPa) each when the flow through them is 350 gpm (1 400 L/min) or more. Below that, the book uses 0. The book text on p. 209 says "350 gpm or greater" while some examples say "greater than 350 gpm"; at exactly 350 gpm this calculator applies the loss.

Book Examples 6.8 and 6.10 charge only the 25 psi when a siamese feeds a master stream, so leave "other appliances" at 0 in that case unless your department counts the siamese separately.

EP = 0.5 x H psi (H in feet) or 10 x H kPa (H in meters). The true value is 0.434 psi per foot; the book uses 0.5 for ease.

Buildings: EP = 5 psi x (stories - 1), or 35 kPa x (stories - 1). The ground floor counts as story 1.

EP is a loss when the nozzle is above the pump and a gain (subtract) when it is below.

GPM = 29.7 x d² x √NP (d in inches). L/min = 0.067 x d² x √NP (d in mm). As in the book's examples, the flow is rounded to two decimals and then to a whole number before friction loss is figured (Example 6.25 rounds 452.50 L/min up to 453).

The book states the coefficients are approximations that usually run slightly high, giving a worst-case estimate. Flowmeters and your department's tested values take priority. Aerial and ladder pipe losses vary by manufacturer; the 25 psi figure is the book's standard value.

That example applies a siamese coefficient to two wyed attack lines. This calculator does not, because siamese coefficients are for lines that are siamesed (their combined flow goes through the set). Each wyed line is figured with its own flow and the single-hose coefficient, which agrees with Appendix C: 250 gpm through 200 ft of 2 1/2 in hose is 25 psi, not 6.25 psi.