Tank volume versus drawdown
Nominal tank volume includes the air and water spaces. Manufacturer drawdown tables state usable water between the specified cut-out and cut-in pressures.
Precharge
Precharge is checked with electrical power isolated and the water side fully drained. Conventional manufacturer instructions commonly specify a value just below cut-in; follow the exact product instructions.
Cut-in and cut-out
Cut-in starts the pump at the lower pressure; cut-out stops it at the upper pressure. Changing either affects drawdown, maximum pressure and system behavior.
Minimum run time
Required drawdown can be screened as pump flow multiplied by desired minimum run time. Actual allowable starts and run requirements come from the pump and motor manufacturer.
Common short-cycle symptoms
Frequent starts, brief on-time, rapidly swinging pressure or cycling with no intentional demand can point to low drawdown, lost air charge, bladder failure, leaks, check-valve leakage or control faults.
Measure before adjusting
Record starts over a defined interval, on/off durations, dynamic demand, cut-in/out pressure and tank drawdown. One symptom does not establish the cause.
Variable-speed systems
Small pressure tanks in variable-speed systems serve different control functions. Use the drive and pump manufacturer’s sizing and precharge instructions rather than a conventional rule.
Safety boundary
Pressure vessels and electrical controls store hazardous energy. Isolate, depressurize and use qualified service where required.
Worked example
A 40 L/min pump with a 1-minute run-time target needs at least 40 L rated drawdown. If an existing tank supplies only 20 L, the ideal cycle target cannot be met even if the tank shell is much larger than 40 L.