How accumulators meet short peaks and when boosted or pressurised systems are needed.
When the mains is not enough
Sometimes the incoming main has adequate pressure but cannot deliver enough flow at peak times, or a tall building needs pressure at high level. Accumulators and boosted systems address these in different ways.
Accumulators
In a mains-charged, unpumped arrangement, an accumulator stores water under pressure behind a diaphragm with a gas charge. A pumped charging arrangement has separate pump and control requirements. During quiet periods the main slowly refills, or recharges, it. At a peak the stored water is released to add to the mains flow, so the system can meet a short high demand that the main alone could not. The key design point is recharge time: if peaks come too close together, the accumulator does not refill between them and the benefit is lost. Size it to the peak demand and the available recharge period.
Boosted and pressurised systems
Where pressure or flow is genuinely insufficient, a pump set is used. There are two common arrangements. Boosting from a stored break tank protects the main and gives a buffer. Boosting more directly from the supply needs the water undertaker’s permission and stays within any flow-rate limit they set. Variable-speed control aims to maintain the selected outlet pressure within the set’s operating range as demand changes.
Always check what the water undertaker allows before boosting from the main, and confirm sizing against BS 8558, BS EN 806 and the pump manufacturer data.
Booster set details that decide performance
- A correctly selected and maintained inlet air gap separates the boosted side from the main. The pump draws from the tank; the main still supplies water to refill it, and that demand can affect running pressure. Suction performance depends on the stored level, inlet design, replenishment and low-level protection.
- Restricted suction pipework or a low tank level can reduce available inlet head. Cavitation occurs if local pressure falls low enough for vapour to form; it can bring noise, erosion and loss of duty. Compare inlet conditions with the pump data before calling a noise cavitation. A higher discharge pressure does not mean the inlet is satisfactory.
- A suitably sized and charged pressure vessel can supply small draws before the pump starts, reducing cycling. Check usable draw-down and control settings as well as pre-charge against the instructions; the vessel cannot provide its intended buffer when depleted or incorrectly charged.
- Low-water-level protection stops the pumps running dry when the break tank can no longer support them.
- Boosting can push pressure above what downstream fittings and appliances are rated for, so the set needs controls that keep pressure within their working range.
- Select a pressure-reducing valve for the required flow as well as its pressure range and setting. An undersized valve can add excessive loss at peak demand; the set pressure does not tell you its capacity.
- In England, installing a pump or booster that draws more than 12 litres a minute, connected directly or indirectly to a supply pipe, is notifiable. The water undertaker must be told, and must consent, before the work starts.