BRANDCO FIELD RESOURCES
How to Measure Fire Pump Suction and Discharge Pressure
Suction pressure describes conditions at the pump inlet. Discharge pressure describes conditions at the outlet. Reading both at the same operating point helps separate the pressure available from the water supply from the pressure added by the pump. Recording discharge pressure alone cannot tell the whole story.
Brandco’s Fire Pump Sender sends both pressure readings and RPM to a Remote Display. That makes it easier to coordinate pump measurements with discharge flow readings, particularly when the test header and pump room are separated.
Understand the three pressure values
Suction pressure may be positive or below atmospheric pressure, depending on the system and operating condition. The instrument must cover the expected range. Discharge pressure generally reflects both the incoming supply and the pump’s contribution. Net pump pressure is used to describe that contribution.
For a basic comparison in the same pressure units, start with discharge pressure minus suction pressure. An illustrative reading of 160 psi discharge and 40 psi suction gives a 120 psi pressure difference. This arithmetic is not automatically the final corrected net pressure for every installation: measurement-point elevations and velocity-head differences can matter.
Do not compare a raw discharge reading with a curve labeled net pump pressure. Confirm that the quantities, units, reference elevations, and operating speed are comparable before drawing a conclusion.
Choose the correct test connections
Identify the designated suction and discharge gauge or test ports from the installation documents. A convenient tap farther along the system may include intervening losses and may not represent the intended pump measurement point. Document where each reading is taken.
The Fire Pump Suction & Discharge Pressure set uses a suction sensor rated from -15 to 215 psi and a discharge sensor rated from 0 to 500 psi. Confirm that both the normal readings and possible test conditions fall within the applicable working limits. A burst-pressure specification is not an operating allowance.
A practical measurement workflow
1. Prepare and identify the instruments
Check calibration status, sensor condition, units, and the identification assigned to each channel. Verify that the suction sensor will be connected to suction and the discharge sensor to discharge. Labeling the cords at setup is useful when several people are handling equipment.
2. Make the connections safely
Use the approved isolation and depressurization procedure before opening a test connection. Select compatible, pressure-rated fittings and follow the sensor installation instructions. Keep cords supported and clear of hot surfaces, rotating equipment, access routes, and places where they can be snagged.
3. Verify the display before flowing water
Confirm communication at the intended operator location and check that the displayed channels correspond to the connected sensors. If re-zeroing is required, follow Brandco’s menu instructions: the pressure sender must be disconnected from any pressure source. Do not zero away an actual system pressure.
4. Capture a matching set of readings
At each stabilized operating condition, record suction pressure, discharge pressure, flow, and RPM together. Note changing supply conditions or unstable readings. A pressure reading taken before a flow adjustment does not describe the same operating point as a flow reading taken afterward.
Interpret changes without jumping to conclusions
A change in discharge pressure can come from a changed inlet supply, a changed flow condition, a speed difference, or a pump problem. Comparing both pressure channels helps narrow the investigation. For example, a lower discharge reading accompanied by a similar reduction in suction pressure has a different meaning from an unchanged suction reading with reduced pressure rise.
Check the measurement setup before diagnosing the pump. Reversed channels, a wrong pressure unit, a poor connection, or measurements at different moments can create misleading results. The Peerless operation manual places pressure readings alongside speed and flow when assessing performance; evaluate the set of measurements together.
Document what the numbers mean
Record the test-point locations, sensor identification, calibration information, operating condition, raw readings, and any corrections applied. Keep the reference curve and test requirements with the report. NFPA 25 provides the applicable ITM framework; the adopted edition and installation determine the required evaluation.
Frequently asked questions
Is discharge minus suction always the complete answer?
It is the basic pressure difference. The qualified evaluator must determine whether elevation and velocity-head corrections are needed for the arrangement.
Can a regular pressure-only sensor measure suction vacuum?
Only if its rated range includes the expected vacuum condition. Select the appropriate compound-range suction instrument; do not infer capability from the connector.
Can the display replace the test report?
No. Saved readings are inputs to the report. The report must still explain the test conditions, evaluation, and findings.
Related resources and references
Fire pump flow-test workflow · Measuring RPM · Brandco display settings · NFPA 25