Tank to Pump

Hydraulics & Pressure

Pump Discharge Pressure

PDP

Pump discharge pressure is the total pressure the pump must produce so the nozzle gets what it needs after the hose, appliances, and elevation take their share. It is the sum of nozzle pressure plus every loss between the pump and the tip.

A pump-to-nozzle schematic with a stacked bar of four pressure components — nozzle design pressure (gold), friction loss (amber), appliance loss (orange), and elevation (blue). Sliders for flow, hose length, and elevation rebuild the bar, and a live GOOD/LOW/HIGH verdict bands the total against the nozzle's design pressure.

Pump discharge pressure · interactive

WORKING152 psi PDPA normal lay — the pump produces more than the nozzle pressure to overcome friction loss, the appliance, and any climb. That extra is exactly what the components stack up, and it is correct, not over-pumping.

Nozzle type
  • Nozzle pressure100
  • Friction loss52
  • Appliance loss0
  • Elevation0

Friction loss and elevation are computed by the same engine functions the PDP Builder calculator uses — flow and hose length drive friction loss, and elevation is 0.5psi per foot. PDP is meant to sit above the nozzle's own pressure — that extra is what overcomes the line; the verdict flags how hard the lay works the pump, not whether it is “over” the nozzle. These are training estimates; follow your SOPs and the nozzle's own data.

Pump discharge pressure builds from four components — nozzle pressure, friction loss in the hose, appliance loss, and elevation. Pick a nozzle and drag flow, hose length, and elevation to watch each component stack and the total PDP change.

PDP is the number the operator is really chasing at the panel. Start from the pressure the nozzle is built around, then add back everything the water gives up on the way there: friction loss in the hose, any appliance in the lay, and the pressure gained or lost from elevation. Set the throttle so the discharge gauge reads that total and the tip ends up where it should be.

Because friction loss grows with the square of the flow, PDP is not a single fixed number for a given line — it climbs steeply as you move more water. That is why a longer stretch or a bigger nozzle can demand a much higher pump pressure even though the tip pressure target never changed.

Formula

PDP = NP + FL + AL + EP

NP = nozzle pressure; FL = friction loss; AL = appliance loss; EP = elevation pressure (added uphill, subtracted downhill).

Common questions

What is pump discharge pressure?
Pump discharge pressure is the total pressure the pump produces at the discharge so the nozzle gets its design pressure after the hose, appliances, and elevation take their share. You build it up from the tip backward: nozzle pressure plus friction loss plus appliance loss plus or minus elevation.
How do you calculate pump discharge pressure?
Add the four parts: start with the nozzle pressure the tip is built around, add the friction loss of the hose at the planned flow, add any appliance loss in the lay, then add elevation pressure for an uphill stretch or subtract it downhill. The sum is the pressure you set on the discharge gauge. It is a training estimate — your department's SOPs and the apparatus manual govern real settings.
Why does pump discharge pressure change when I open another line?
Opening a second line moves more total water, and the pump trades pressure for flow as it moves down its pump curve — so the pressure sags unless you add throttle. The pressure governor adjusts engine speed to hold your setting, but only while the supply can deliver the added flow.

Where this shows up

See also