Pressure tank sizing for VFD pump system projects is often treated as a minor accessory decision. It is not. A variable-frequency drive can slow the pump as demand falls, but it cannot create stored water volume, absorb every sudden pressure change or distinguish a dripping fixture from meaningful demand without help from the hydraulic system.
That is why a constant-pressure package can have a perfectly reasonable PID setting and still start too often. It is also why installing a much larger tank does not automatically cure unstable pressure. The useful question is not, “How many litres should the tank be?” It is, “What job must the tank perform between the moment demand becomes small and the moment the controller changes state?”
Table of Contents
- The hydraulic job of the tank
- Symptoms that are often misdiagnosed
- Inputs needed before sizing
- A transparent screening calculation
- Turning the number into a commissioning test
- Applying the result to a W713B system
- A field measurement card
Pressure Tank Sizing for VFD Pump System Projects Starts With the Job
In a conventional fixed-speed booster, the tank commonly provides enough drawdown to limit motor starts. In a variable-speed system, the tank may be smaller because the drive follows changing flow. Smaller, however, does not mean unimportant.
In a VFD-controlled booster, the pressure tank normally performs four practical jobs:
- it supplies very small, short-duration demands without requiring an immediate pump start;
- it gives the controller time to recognise that demand has ended before entering sleep mode;
- it absorbs part of the pressure disturbance caused by a fast valve movement or check-valve closure;
- it keeps pressure from collapsing instantly while the pump accelerates from rest.
Grundfos describes constant-pressure boosting as varying pump output to maintain a set pressure while flow and inlet conditions change. The tank does not replace this variable-speed control. It shapes the short interval in which the controller decides whether to run, slow down, sleep or restart.
This distinction matters. A tank selected only by pump motor power can be wrong because a 4 kW pump serving stable process demand behaves differently from the same-size pump serving taps that open for a few seconds at a time.
Four Problems That Can Look Like a Small Tank
Before changing tank volume, record what the system actually does. Several faults produce similar complaints but require different corrections.
| Observed behaviour | First measurement to take | More likely causes to investigate |
|---|---|---|
| Pump wakes every few minutes with all outlets closed | Pressure decay over time | Leakage, failed check valve, leaking toilet or process valve, sensor drift |
| Pressure drops sharply as soon as a tap opens | Tank pre-charge and usable drawdown | Low tank volume, incorrect pre-charge, isolated tank connection, damaged bladder |
| Pressure oscillates while flow is steady | Pressure trend and output frequency trend | PID tuning, sensor location, air in sensing line, oversized pump |
| Drive never enters sleep at low demand | Minimum operating frequency and measured flow | Sleep threshold, minimum-frequency setting, bypass flow, unstable feedback |
| Loud pressure shock occurs when demand stops | Pressure transient and valve-closing time | Fast check valve, poor pipe restraint, high deceleration, inadequate surge control |
The first row is especially important. A larger tank can lengthen the time between starts, but it does not repair a leaking non-return valve. The system would still lose stored pressure; the symptom would simply become slower.
Likewise, a tank does not correct aggressive PID gains. If the pressure signal and drive frequency rise and fall together under stable demand, investigate the control loop and sensor arrangement. The related BEDFORD guide on pump inverter PID settings is a better starting point for that symptom.
Collect These Inputs Before Reaching for a Formula
A useful calculation begins with measured or specified conditions, not a tank catalogue.
- Pressure setpoint: the pressure the system is intended to maintain at the sensor location.
- Wake pressure: the pressure at which the sleeping controller should restart the pump.
- Sleep or upper pressure: the pressure available in the tank immediately before the pump stops.
- Minimum short demand: a realistic low flow, such as one small outlet, not the pump’s rated flow.
- Desired hold time: how long that small demand should be supplied before restart.
- Static elevation: the vertical difference between the tank, sensor and highest outlet.
- Tank pre-charge: measured with the water side depressurised, following the tank manufacturer’s method.
- Tank pressure rating and allowable operating temperature: both must exceed the actual system conditions.
Do not use the pressure shown while the tank is full of water as its pre-charge. Pre-charge is checked with the pump isolated, electrical energy safely controlled and water pressure relieved. Site safety procedures and the tank manufacturer’s instructions take priority.
A Screening Calculation With the Assumptions Visible
Assume a small hotel booster must cover a short demand of 6 L/min for 50 seconds before the VFD-controlled pump wakes.
The required usable drawdown is:
| Input | Value |
|---|---|
Small demand, q |
6 L/min |
Desired hold time, t |
50 s |
Required drawdown, D = q x t / 60 |
5 L |
That 5 L is not the tank’s nominal volume. Only part of a diaphragm tank’s labelled volume becomes usable water between the selected upper and lower pressures.
For a first engineering screen, an ideal-gas relationship can estimate the nominal tank volume:
Vtank = D / [P0(abs) x (1/P1(abs) - 1/P2(abs))]
Where:
Dis required drawdown;P0is tank pre-charge pressure;P1is the pump wake pressure;P2is the pressure immediately before sleep;- every pressure is absolute, uses the same unit and includes atmospheric pressure.
For illustration only, use 2.1 bar(g) pre-charge, 2.7 bar(g) wake pressure and 3.2 bar(g) upper pressure. Converted to absolute pressure, these are approximately 3.1, 3.7 and 4.2 bar(a).
Vtank = 5 / [3.1 x (1/3.7 - 1/4.2)], which is approximately 50 litres.
This does not mean a 50 L tank should be ordered without review. Real diaphragms, manufacturing tolerances, temperature, required reserve, connection losses and the controller’s actual transition pressures change the usable drawdown. Select the next suitable standard size only after checking the tank maker’s drawdown chart.
Some variable-speed tank instructions give a pre-charge relationship based on the pressure setpoint. For example, a Grundfos GT installation document provides manufacturer-specific guidance for its own products. That relationship should not be copied to an unrelated tank as a universal rule. The tank data sheet, controller logic and local pressure-vessel requirements remain authoritative.
The Commissioning Test Matters More Than the Neat Result
Once the tank is installed, verify the assumptions with the real system. A practical test does not require guessing whether the tank “feels right.”
First, bring the system to normal pressure and allow the controller to sleep. Open a known small outlet or use a measured flow point. Record:
- pressure at the beginning of the test;
- flow rate from the outlet;
- elapsed time until the pump restarts;
- pressure at restart;
- time taken to recover to setpoint;
- whether pressure undershoots or overshoots;
- whether the pump returns to sleep after the outlet closes.
The measured drawdown is flow x time. If the outlet delivers 6 L/min and the drive restarts after only 20 seconds, the system supplied about 2 L before waking, not the planned 5 L. Check actual pre-charge, tank isolation valves, upper and lower control thresholds, and whether the pressure sensor is reading the same hydraulic zone as the tank.
Repeat the test at least once after the water temperature and system pressure have stabilised. For multi-pump boosters, also verify that a small demand does not call an unnecessary second pump.
Where W713B Fits Into the System
After the hydraulic behavior is defined, the controller can be selected and configured around it. The BEDFORD W713B water pump inverter supports constant-pressure control, pressure-transducer feedback, built-in PID and multi-pump operation for up to six pumps. The available 220 V and 380 V classes cover a range of small and medium booster duties, but the exact model must be matched to motor nameplate current and supply conditions.
For a W713B project, record the tank-related commissioning values beside the controller parameters. This creates a traceable relationship between hydraulic design and drive behavior:
| Hydraulic record | Controller record |
|---|---|
| Tank nominal volume and model | W713B model and rated current |
| Measured pre-charge | Pressure setpoint |
| Measured usable drawdown | Wake threshold |
| Small-demand hold time | Sleep frequency and delay |
| Sensor and tank locations | PID parameters |
| Closed-system pressure decay | Fault and restart settings |
This is more useful than recording only “system commissioned.” When a contractor returns months later, the table shows whether a changed parameter, lost pre-charge or new leak altered the original behavior.
One-Page Field Measurement Card
Before approving pressure tank sizing for a VFD pump system, the commissioning engineer should be able to answer all of the following:
- What is the smallest meaningful demand the tank is expected to cover?
- How many seconds of hold time are required?
- What are the measured upper and wake pressures at the tank connection?
- Was pre-charge measured with the water side fully depressurised?
- Does the manufacturer’s drawdown table confirm the selected tank size?
- Does pressure decay when every outlet is closed?
- Does the controller sleep and wake at the recorded values?
- Does the highest outlet maintain acceptable pressure during pump acceleration?
- Are the tank rating, relief arrangement and local requirements satisfied?
- Are tank data and controller parameters included in the handover file?
A well-sized tank should make the system calmer and the control decisions more predictable. It should not be used to hide leakage, an unstable sensor signal or a pump selected too far from the real duty point. That is the practical value of treating pressure tank sizing as part of the system design rather than as a final accessory choice.