title: "Jockey Pump Sizing: Pressure Maintenance Flow, Head, and Settings"
description: "Learn the formula, units, assumptions, and a worked example, then see how to avoid common pump calculation errors in real industrial systems."
Jockey pump sizing begins with the smallest leakage flow that causes measurable pressure decay in a closed fire protection system, not with the sprinkler demand handled by the main fire pump. A pressurized system loses water through packing glands, threaded joints, pressure switches, and test connections. The jockey pump must deliver enough flow to offset that leakage and restore system pressure before the main fire pump starts, while operating within a head range that keeps it from interfering with emergency response.
Proper sizing requires four linked decisions: estimating the normal leakage flow, calculating the differential head from suction to required discharge pressure, selecting a pump curve and driver that place the duty point near best efficiency, and coordinating pressure switch settings so the jockey pump stops above the fire pump start point. Undersized jockey pumps run continuously and mask real leaks; oversized pumps cycle rapidly, wear mechanical seals, and may fail to maintain pressure during small demand events that should not trigger the fire pump.
Belangrijkste opmerkingen
- Jockey pump flow is sized for system leakage, typically 1% of fire pump capacity for aboveground systems or up to 10% for large underground installations with more joints and potential leak paths.
- Required discharge head equals the fire pump churn pressure plus the margin specified by the system designer or applicable code, minus any available suction head, to ensure the jockey pump stops above the fire pump start setting.
- The selected pump curve must place the leakage duty point within the pump’s allowable operating envelope defined by the manufacturer, avoiding cavitation, overheating, or excessive radial thrust.
- Pressure switch start and stop settings must be staggered according to the control sequence specified in the system design, preventing overlap or hunting between jockey and fire pump operation.
- Verification involves measuring pressure recovery time after a controlled draw-down and confirming that the jockey pump alone restores pressure without starting the fire pump during normal leakage.
Define What the Jockey Pump Must and Must Not Do
The jockey pump maintains system pressure between fire events by compensating for minor water loss. It runs intermittently in response to small pressure drops caused by leakage, temperature changes that contract water volume, or brief test flows. The pump must restore pressure quickly enough that normal system variations do not start the main fire pump, which is reserved for actual fire demand.
What the jockey pump must not do is mask significant leaks, supply fire demand, or operate outside its pressure envelope. If leakage exceeds the jockey pump’s capacity, system pressure continues to fall until the fire pump starts. That event signals a problem requiring investigation, not a call to upsize the jockey pump. The jockey pump also must not deliver pressure high enough to challenge relief valves, overpressure system components, or prevent the fire pump from starting during an actual fire.
The boundary between acceptable leakage and system failure depends on pipe condition, joint quality, and maintenance history. The jockey pump handles the expected background rate, while larger losses trigger alarms and fire pump operation.
Estimate Normal Leakage and Pressure-Maintenance Flow
Normal system leakage depends on piping material, joint type, system age, and total pipe length. A general sizing rule uses 1% of the fire pump rated flow for aboveground-only systems and up to 10% for large underground installations, but measured data from pressure decay tests provides a more accurate starting point.
To measure baseline leakage, isolate the system from external supply, record the initial pressure, and measure the time required for pressure to drop a known amount. Leakage flow can then be estimated from the pressure change rate and system volume:
Q_leak = (ΔP × V) / (Δt × K)
Waar:
- Q_leak = leakage flow rate, GPM
- ΔP = pressure drop, psi
- V = system volume, gallons
- Δt = time interval, minutes
- K = bulk modulus factor for water, approximately 20,000 psi for small pressure changes at ambient temperature
For a hypothetical system holding 2,000 gallons that loses 5 psi in 30 minutes:
Q_leak = (5 × 2,000) / (30 × 20,000) = 0.017 GPM
This calculation captures compression and thermal effects. Actual leakage is typically higher because small drips from packing glands and weeping joints do not register as pressure loss in short tests. Add margin to the calculated value according to system condition and designer judgment to account for unmeasured minor leaks and provide capacity for normal system expansion.
Jockey pump flow is then set at the estimated leakage rate plus margin, not at a fixed percentage of fire pump capacity. Avoid selecting a pump much larger than needed; excess capacity causes rapid cycling, short run times that prevent proper bearing lubrication, and frequent starts that stress the motor and pressure switches.
Determine Required Differential Head
Jockey pump head must overcome the elevation difference between suction and the highest system point, friction losses in the jockey pump’s discharge piping, and the pressure needed to stop the pump above the fire pump start setting. The required discharge pressure equals the fire pump churn pressure plus the margin specified in the system design. Churn pressure is the shut-off head the fire pump develops at zero flow, listed on the certified pump curve.
Required discharge head is:
H_d = (P_stop – P_suction) × 2.31 / SG + h_friction
Waar:
- H_d = total dynamic head, feet
- P_stop = jockey pump stop pressure, psig
- P_suction = suction pressure, psig (negative for lift, positive for flooded suction)
- SG = specific gravity of water, typically 1.0
- 2.31 = conversion factor, feet of head per psi for water
- h_friction = friction head loss in jockey pump discharge piping to the system connection, feet
For a hypothetical jockey pump with flooded suction at 10 psig, stopping at 130 psi, with 3 feet of friction loss in its discharge run:
H_d = (130 – 10) × 2.31 / 1.0 + 3 = 277 + 3 = 280 feet
If suction is taken from atmospheric storage, subtract the static suction lift and add friction on the suction side. Always calculate head from the actual suction condition, not from an assumed pressure.
The pump curve selected must intersect the system curve at the leakage flow and calculated head. That intersection is the duty point. Verify that the duty point falls within the pump’s preferred operating range specified by the manufacturer. Most jockey pumps are multistage centrifugal or regenerative turbine types with relatively flat curves, so small flow variations do not produce large head swings.
Select the Pump Curve and Driver
Once required flow and head are known, select a pump whose published curve places that duty point near best efficiency. Jockey pumps are commonly vertical multistage centrifugals for compact installation, regenerative turbine pumps for high head at low flow, or horizontal end-suction pumps where space permits.
Compare the required duty point against manufacturer curves. The selected pump should place the duty point within the acceptable operating zone shown on the certified curve. Avoid selecting a pump with capacity far exceeding the leakage flow; low-flow operation causes internal recirculation and premature wear.
Driver sizing accounts for the power required at the duty point plus the motor service factor. Calculate brake horsepower:
BHP = (Q × H × SG) / (3,960 × η_pump)
Waar:
- BHP = brake horsepower at the shaft
- Q = flow, GPM
- H = total head, feet
- η_pump = pump efficiency at the duty point, decimal (from manufacturer curve)
For a hypothetical 3 GPM at 280 feet with pump efficiency of 0.45:
BHP = (3 × 280 × 1.0) / (3,960 × 0.45) = 0.47 HP
Select the next standard motor size with adequate service factor to provide thermal margin and accommodate efficiency variation. Do not use a motor rated far above the calculated load; the motor runs below its rated point, operates at poor power factor, and may not develop sufficient cooling airflow.
Specify the motor for continuous duty, with insulation class and enclosure suitable for the installation environment.
Coordinate Start and Stop Settings with the Fire Pump
Pressure switch settings create the operating sequence: jockey pump starts and stops within a band above the fire pump start point. The control sequence specified in the system design ensures the jockey pump runs first, stops cleanly when pressure is restored, and does not interfere with fire pump operation.
Hypothetical example settings for a 100 psi system:
- Fire pump start: 90 psi
- Jockey pump start: 110 psi
- Jockey pump stop: 120 psi
The differential between jockey start and stop provides enough pressure rise to stop the pump without excessive cycling, but not so much that the pump overshoots and challenges system ratings. The gap between fire pump start and jockey stop ensures that normal jockey pump operation does not drop pressure low enough to start the fire pump.
Adjust settings based on actual churn pressure and system ratings. If the fire pump’s churn pressure and system component ratings allow, the jockey pump stop must preserve the safety margin specified in the design. If the jockey pump cannot reach the required stop pressure, check for incorrect pump selection, excessive friction losses, or air in the suction line.
Pressure switches must be calibrated before commissioning using a deadweight tester or calibrated pressure gauge. Include pressure switch calibration in periodic maintenance to detect drift or fouling.
Check Relief, Check Valve, and Pressure Ratings
A relief valve protects the jockey pump from deadhead operation if discharge valves are accidentally closed. Size the relief valve according to the pump’s shutoff flow at churn pressure, with the relief set point above the jockey pump stop setting per the manufacturer’s recommendations. The relief valve discharges to a drain, sump, or storage tank, not back to suction, to prevent a circulating loop that overheats the pump.
A check valve on the jockey pump discharge prevents backflow when the fire pump starts. Use a spring-loaded silent check valve rather than a swing check; silent checks close quickly and do not slam when the jockey pump stops. Position the check valve close to the pump discharge flange, between the pump and the connection to the main system piping.
Verify that all jockey pump components—pump casing, piping, fittings, valves, and gauges—are rated for the maximum pressure the pump can develop at shutoff per the certified curve.
Test Pressure Recovery and Detect Hidden Demand
Commissioning tests verify that the jockey pump restores system pressure within acceptable time and identifies hidden demand that was not apparent during sizing. Conduct a controlled draw-down by opening a test valve at the system’s high point or remote end, allowing pressure to drop to the jockey pump start point, then closing the valve. Measure the time required for the jockey pump to restore pressure to its stop setting.
If the jockey pump runs continuously without reaching its stop pressure, diagnose the cause before adjusting settings:
- Check for open drain valves, hose connections, or test ports
- Inspect mechanical seals, packing glands, and flange gaskets on the system side
- Verify that the fire pump discharge check valve is not leaking
- Measure actual jockey pump discharge pressure; loss of impeller stages, wear, or air binding reduces developed head
- Confirm that no building systems are accidentally cross-connected to the fire system
Continuous jockey pump operation means leakage exceeds the pump’s capacity. Repair leaks rather than upsizing the jockey pump.
Jockey Pump Data Sheet
Record the following information on a project-specific data sheet for installation, commissioning, and maintenance reference:
Parameter | Value | Units | Notes |
|---|---|---|---|
Rated flow | GPM | At duty point | |
Totale dynamische opvoerhoogte | feet | At duty point | |
Uitlaatdruk | psig | At duty point | |
Suction pressure | psig | Static, or negative if lift | |
Pump type | Multistage, regenerative, end-suction | ||
Number of stages | If multistage | ||
Motor horsepower | HP | ||
Motor voltage/phase | V | ||
Jockey start pressure | psig | ||
Jockey stop pressure | psig | ||
Fire pump start pressure | psig | ||
Relief valve setpoint | psig | ||
System design pressure | psig | ||
Fire pump churn pressure | psig | ||
Estimated system leakage | GPM | From test or calculation | |
Observed recovery time | minutes | After controlled draw-down |
Update the data sheet after any pump repair, setting adjustment, or system modification.
FAQs
Can I use a variable-speed drive on a jockey pump to reduce cycling?
Variable-speed control can smooth jockey pump operation by modulating speed to match leakage rate rather than running at fixed speed until the stop pressure is reached. However, most jockey pumps are small fractional-horsepower units where the cost and complexity of a VFD may exceed the benefit. If rapid cycling is a problem, verify that start-stop differential meets design requirements, check for system leaks, and confirm the pump is not oversized.
What happens if the jockey pump fails and pressure drops to the fire pump start point during normal operation?
The fire pump starts and maintains system pressure until the jockey pump is repaired. This is a design feature. The system remains protected, but the event should trigger immediate jockey pump service because extended fire pump operation at near-shutoff flow overheats the pump and wastes energy. Install a run-time alarm on the fire pump to alert operators if it runs outside of test or fire conditions.
How do I size a jockey pump for a system with both aboveground and underground piping?
Use the higher leakage estimate that applies to the underground portion. Alternatively, conduct a pressure decay test on the complete system to measure actual leakage, then size the jockey pump for that measured rate plus margin. Do not split the difference or average the two conditions; the underground section typically dominates total leakage.
Should the jockey pump take suction from the same point as the fire pump?
Yes, in most cases. The jockey pump should take suction from the fire pump suction supply—either a common suction header from the water source or a tee off the fire pump suction piping—so that both pumps see the same suction pressure and water quality. Coordinate with the system designer and follow applicable fire protection standards for pressure maintenance pump connections.
Can I replace a failed jockey pump with a different model that has higher flow and head ratings?
You can replace the pump, but verify that the new curve places the system’s leakage duty point in the acceptable operating range shown on the manufacturer’s curve and that the new stop pressure does not exceed system component ratings or challenge relief valve settings. Match the replacement pump’s curve and pressure limits to the original design, or re-verify all settings and component ratings before commissioning.
Conclusie
Jockey pump sizing balances leakage compensation, pressure coordination, and fire pump protection. Start with measured or estimated system leakage to define required flow, calculate head from suction conditions and the fire pump’s churn pressure, select a pump curve that places the duty point in the manufacturer’s acceptable range, and coordinate start-stop pressure settings per the system design to maintain separation from fire pump operation. Verify system pressure ratings, install relief and check valves per manufacturer instructions, and conduct a recovery test during commissioning to confirm that the jockey pump restores pressure without continuous operation. Document all settings and actual performance on a project data sheet for future maintenance and troubleshooting.