Field-style article image prepared for pressure tank sizing for booster pump.
A field technician commissioning a ten-story office building discovers that the booster pump starts every ninety seconds during normal occupancy, far exceeding the manufacturer’s recommended six starts per hour. The pressure tank was undersized during specification, forcing the pump into rapid cycling that will shorten bearing life and waste energy.
Pressure tank sizing for a booster pump determines the buffer volume needed to limit pump starts while maintaining system pressure between defined cut-in and cut-out points. The tank stores pressurized water during periods of low demand and releases it before the pump restarts, protecting mechanical components and reducing operating costs.
Proper sizing depends on four variables: pump flow rate, acceptable cycle rate, system pressure differential, and the tank’s drawdown capacity at operating pressures.
要点
- Tank volume depends on pump flow rate (GPM), acceptable starts per hour, and the pressure differential between cut-in and cut-out settings
- Drawdown capacity—the usable water volume between cut-in and cut-out—determines effective tank size, not total tank volume
- Pre-charge pressure should be set at 60-80% of the cut-in pressure for diaphragm tanks to maximize drawdown
- Undersized tanks cause rapid cycling; oversized tanks add cost without performance benefit beyond reducing starts below the acceptable threshold
- ASME-rated bladder or diaphragm tanks are standard for commercial booster applications above 125 PSI working pressure
Calculating Required Tank Volume
The fundamental sizing equation relates tank volume to pump flow rate and cycle frequency:
**V = (Q × t) / D**
場所：
- V = required tank volume (gallons)
- Q = pump flow rate (GPM)
- t = time between pump starts (minutes)
- D = drawdown capacity (expressed as decimal, typically 0.25-0.35 for properly pre-charged diaphragm tanks)
Time between starts is calculated as 60 minutes divided by the desired number of starts per hour. For a pump rated at six starts per hour maximum, t = 10 minutes.
A booster pump rated at 50 GPM (https://projectcalc.app/blog/booster-pump-sizing-guide) with a six-start-per-hour limit requires: V = (50 × 10) / 0.30 = 1,667 gallons assuming 30% drawdown. This is the minimum tank size to prevent excessive cycling under continuous demand conditions.
Most commercial applications target four to six starts per hour under normal operation. More frequent starts indicate undersizing; fewer than two starts per hour suggests the tank is larger than necessary for cycle protection.
Determining Drawdown Capacity
Drawdown capacity is the percentage of tank volume available between the pump’s cut-in and cut-out pressures. It depends on three factors: tank pre-charge pressure, cut-in pressure, and cut-out pressure.
For a diaphragm tank, drawdown is calculated using the acceptance formula (https://www.amtrol.com/asmesupport/):
**Drawdown (%) = (P1 – P0) / (P1 + 14.7) × (P2 + 14.7) / (P2 – P0)**
場所：
- P0 = pre-charge pressure (PSIG)
- P1 = cut-in pressure (PSIG)
- P2 = cut-out pressure (PSIG)
- 14.7 = atmospheric pressure conversion factor
A system with 40 PSIG cut-in, 60 PSIG cut-out, and 30 PSIG pre-charge yields approximately 33% drawdown. The same pressure differential with 20 PSIG pre-charge drops to 28% drawdown due to inefficient use of tank volume.
Pre-Charge Impact on Usable Volume
Pre-Charge (PSIG) | Cut-In (PSIG) | Cut-Out (PSIG) | Drawdown (%) |
28 | 40 | 60 | 35 |
32 | 40 | 60 | 33 |
36 | 40 | 60 | 30 |
24 | 40 | 60 | 37 |
The table shows a 40/60 PSIG system across different pre-charge settings. Pre-charge set at 70% of cut-in (28 PSIG) maximizes drawdown without risking diaphragm damage from excessive compression.
Pressure Differential Selection
The spread between cut-in and cut-out pressures affects both drawdown and system stability. Wider differentials increase drawdown percentage but cause greater pressure variation at fixtures.
Most commercial booster systems use a 20 PSIG differential. A 40/60 PSIG setting provides adequate buffer for multi-story buildings while keeping fixture pressure reasonably constant.
Narrow differentials below 15 PSIG reduce available drawdown and may cause the pump to cycle more frequently than the tank can accommodate. Variable-speed booster systems (https://engineerfix.com/how-a-booster-pump-with-pressure-tank-works/) with pressure transducers often eliminate the fixed cut-in/cut-out logic entirely, using the tank solely as a surge suppressor rather than a cycle buffer.
Applications requiring tight pressure control—such as laboratory or process water systems—may need variable-speed control rather than relying on tank sizing alone to manage pressure variation.
Application-Specific Sizing Adjustments
Standard sizing calculations assume steady demand patterns. Real installations require adjustments for peak flow conditions and usage profiles.
**Peak Demand Multiplier**: Size the tank for 1.5 to 2.0 times average flow if the system experiences short-duration peak demands. A domestic water booster serving restrooms should accommodate simultaneous fixture use during shift changes.
**Duty Cycle Constraints**: Pump manufacturers specify maximum starts per hour to prevent motor overheating and mechanical wear. The tank must be large enough to keep starts below this limit during the longest continuous demand period, not just average conditions.
**Multiple Pump Systems**: Duplex or triplex booster sets require tank volume based on the lead pump flow rate, not total system capacity. The tank buffers demand variations for a single operating pump; additional pumps stage in only when flow exceeds lead pump capacity.
**Minimum Tank Size**: ASME diaphragm tanks below 20 gallons (https://www.rafsun.com/how-to-size-a-pressure-tank-for-a-booster-pump/) provide minimal cycle protection in commercial systems. Even low-flow boosters should use at least a 40-gallon tank to account for real-world demand variability.
Installation and Pre-Charge Verification
New tank installations arrive with factory pre-charge, typically 38-40 PSIG for standard models. This setting rarely matches the specific cut-in pressure of the booster system.
Pre-charge adjustment requires draining system pressure, accessing the air valve on the tank, and using a low-pressure tire gauge to verify and adjust the charge. Set pre-charge to 70% of cut-in pressure before filling the system.
Bladder and diaphragm tanks lose pre-charge over time through permeation. Annual verification prevents gradual loss of drawdown capacity that leads to increased cycling. A waterlogged tank—indicated by no air pressure at the valve—has lost diaphragm integrity and must be replaced.
Tank location affects performance. Mount the tank on the discharge side of the pump, after the check valve, to prevent backflow from collapsing the bladder during pump-off periods. Vertical orientation with the connection at the bottom ensures complete drainage of the water side.
よくある質問
Can I use a larger tank than the calculation indicates?
Yes, oversizing within reason does not harm performance. A tank up to 50% larger than calculated reduces starts further but adds cost and requires more floor space. Tanks more than double the calculated size provide diminishing returns—the system will already be starting well below the acceptable cycle rate.
How does altitude affect tank sizing?
The atmospheric pressure conversion factor in the drawdown formula assumes sea level (14.7 PSIA). At 5,000 feet elevation, use 12.2 PSIA instead. This reduces effective drawdown slightly, requiring a larger tank to achieve the same cycle protection. Most sizing tools (https://www.mepwork.com/2017/12/booster-pump-calculation-sheets.html) include altitude correction factors for pressures above 3,000 feet.
What happens if pre-charge is set too high?
Pre-charge above 90% of cut-in pressure leaves almost no drawdown capacity. The water side of the diaphragm has minimal room to expand, and the tank functions as a nearly rigid vessel. The pump will cycle on small demand changes, defeating the purpose of the tank.
Do I need separate tanks for each pump in a duplex system?
No. A single properly sized tank serves the entire booster set. The tank volume is based on lead pump flow, and the staging logic brings additional pumps online when demand exceeds what the tank and lead pump can deliver together.
結論
Verify tank size using the pump’s actual flow rate at operating head, not nameplate capacity, since centrifugal pumps deliver less flow at higher pressures. Run the calculation with the flow rate from the pump curve at the system’s required discharge pressure.
After installation, monitor starts per hour during normal demand periods for the first week. If the system exceeds the manufacturer’s recommended cycle rate, the tank is undersized for actual usage patterns, and a larger tank or additional capacity is required before mechanical problems develop.