Tank to Pump

Hydraulics & Pressure

Net Pump Discharge Pressure

NPDP

Net pump discharge pressure is the pressure the pump itself actually adds — the discharge pressure minus whatever the intake is already supplying. It separates the pump's own work from the help a pressurized hydrant is giving.

A pump schematic with an intake pipe on the left feeding a central pump and a discharge pipe on the right, above a horizontal bar split into a water-blue intake share and a gold net-pump-added share that together make up the held discharge pressure; dragging the intake slider shifts the split between hydrant assist and a drafting vacuum.

Net pump discharge pressure · interactive

HYDRANT ASSIST61 psi net pumpA positive intake is helping: the hydrant already supplies part of the discharge pressure, so the pump only has to add the difference. The net pump pressure sits BELOW the discharge reading.

  • Intake (given)20
  • Net pump added61
Drafting (vacuum)Strong hydrant

The discharge gauge is held at 81psi. NPDP is what the pump itself adds — discharge minus intake — capped at the pump's ceiling of 250psi, exactly as the engine derives net pump pressure. On a hydrant the intake helps, so the pump adds less; drafting, the intake is a vacuum, so the pump adds more than the discharge reads. These are training estimates; follow your SOPs and your apparatus's own gauges and limits.

Drag the intake from a strong hydrant down into a drafting vacuum and watch the split: NPDP is what the pump itself adds (discharge minus intake), so a positive hydrant feed means the pump adds less, while drafting forces it to add more than the discharge gauge reads.

When a hydrant feeds the intake under pressure, the pump does not have to create the whole discharge pressure from scratch — it only adds the difference. Net pump discharge pressure is that difference, and it is what reflects how hard the pump is truly working.

Drafting from a static source is the opposite case: the intake brings nothing (in fact the pump must overcome lift), so the net pump discharge pressure is the full job. Thinking in net terms keeps the operator honest about the pump's real load.

Formula

NPDP = PDP − intake pressure

PDP = pump discharge pressure; intake pressure = what a pressurized hydrant supplies. From draft the intake adds nothing and the pump also fights lift, so NPDP is the whole job.

Common questions

What is net pump discharge pressure?
Net pump discharge pressure is the pressure the pump itself actually adds — the discharge pressure minus whatever a pressurized intake is already supplying. It separates the pump's own work from the help a hydrant is giving.
Why is net pump discharge pressure higher when drafting?
Because a static source brings no intake pressure, and the pump must also overcome the lift to pull water up to itself. With nothing helping on the intake side, the pump does the entire job, so the net pressure equals the full discharge pressure.

Where this shows up

See also