The asymmetry nobody thinks about
A composite FRP vessel rated for 10 bar internal pressure has effectively no rating against vacuum. The physics is one-sided: internal pressure loads the filament winding in tension — its strength; external pressure (which is what an internal vacuum creates, courtesy of the atmosphere) loads the shell in buckling — its weakness. Full atmospheric vacuum is about 1 bar of inward load over the whole shell: roughly ten tonnes per square metre, pressing on a structure with no designed resistance to it. The polyethylene liner distorts and delaminates, the shell can ovalise or crumple, and the damage is frequently invisible from outside while the liner hangs collapsed inside. Storage tanks are even more vulnerable — a sealed plastic tank can be crushed by a few hundredths of a bar of vacuum, which a healthy pump or a draining pipe generates without effort.
Where the vacuum comes from
Nothing exotic — ordinary operations create it:
- Draining. Water leaving a closed vessel must be replaced by something. With no air path, the water column leaving through a low drain pulls vacuum above itself — the faster the drain and the taller the column, the harder the pull.
- Downhill pipework / siphoning. A system feeding a lower elevation keeps siphoning after pumps stop, sucking on everything upstream. Long drops to a lower reservoir are the classic vessel-killer.
- Pump suction. A booster drawing from a line or tank that outruns the supply pulls the deficit as vacuum.
- Thermal collapse. A vessel or tank sealed hot (or steam-sanitised) pulls vacuum as contents cool and contract.
- Backwash-to-drain transitions on tall installations, where the drain line's water column can briefly siphon on the vessel.
What a vacuum breaker does
A vacuum breaker is a normally closed air-admittance valve mounted at the system's high point. In normal operation, system pressure holds it firmly shut and it does nothing. The instant internal pressure dips below atmospheric, it opens and admits air, breaking the vacuum before it can build — then reseals when pressure returns. No power, no maintenance beyond an occasional function check, and sized in small pipe threads. It is among the cheapest components in the catalogue guarding some of the most expensive.
Placement and practice:
- High point, on the vessel or immediately adjacent — on the inlet connection or upper pipework where air admitted can actually reach the space the water vacates.
- Every FRP vessel that can drain or siphon deserves one — which in honest terms means most installations with a drain valve, a low delivery point, or elevated feed.
- Sealed storage tanks need a vacuum relief path sized to the maximum outflow (a breather with insect screen at minimum); a tank that gulps when a valve opens is asking.
- After the event: a vessel that has been vacuumed once is suspect regardless of appearance — liner damage hides. Note it, inspect it, and involve the supplier before returning it to pressure service.
Rule of thumb: if water can leave a closed FRP vessel or tank faster than air can get in, fit a vacuum breaker at the top. It is the classic asymmetric bet — a trivial part against a written-off vessel — and the time to fit it is at installation, because the first drain-down is exactly when it earns its keep.