Air valves on pumped industrial mains: stopping surge, trapped air and wasted energy

Where does the air come from?

• Filling: when an empty main fills, the air inside gets trapped at high points.
• Pumps: pumps can draw in air through leaking glands, vortices in a sump or a low water level.
• Dissolved air: water carries air that comes out as pressure drops or temperature rises.

What does trapped air cost?

An air pocket narrows the pipe, so your pumps work harder every hour they run. Less water reaches the far end of the main, which can starve cooling, process or fire water supplies. Moving air pockets cause surge that fatigues joints and bursts pipes. And when a main drains fast, after a burst or a pump trip, a vacuum can form and collapse pipework.

Which type of air valve?

There are three types: small orifice, large orifice and double orifice. We explain each in Which type of air valve do I need? For most pumped mains, a double air valve is the safest all-round choice.

Our HiVent large orifice uses a kinetic design. The air flow holds the float open as air discharges, so the valve won’t blow shut mid-discharge and cause a surge. HiVent valves can be sized for differential pressures up to 0.33 bar, against 0.18 bar for conventional designs, so a smaller valve often does the job.

As a quick sizing check, the large orifice should be at least 1/80th of the pipe’s cross-sectional area. A 50 mm HiVent valve suits mains up to 450 mm, an 80 mm valve up to 700 mm, and a 150 mm valve up to 1,200 mm.

Why do pumped mains need extra care?

On boreholes, dewatering pumps and any vertical pump riser, an air valve at the top of the riser, before the check valve, stops air being pushed into the main. But when the pump starts, the water column races up the riser. If it slams the air valve shut, the surge can force the check valve open and slam it shut again, sometimes several times on a single start-up.
An adjustable throttle on the air valve outlet controls how fast the column rises and prevents this. Where pump trips could cause surge, a vented non-return device lets air in quickly but out slowly, so the water returns gently.
Every air valve should be installed truly vertical, with an isolating valve below it. More in Why is an isolating valve supplied with an air valve?

Quick tip: if your pipeline bangs on pump start-up, tell us. It’s often an air valve problem, and usually a simple one to fix.

Before you enquire, have ready: pipe size, working pressure, a rough profile of the main, and pump details if it’s a pumped main.

Related reading: Which type of air valve do I need? · Can air valves be manufactured in specialist materials? · Check valves on dewatering and pumped systems