What is a wet riser?
A wet riser is a vertical pipe kept permanently charged with water and fed by pumps, providing water at outlets on upper floors. Taller buildings use them where a dry riser cannot deliver enough pressure.
The difference from a dry riser is that the water is already there, so there is no wait while an appliance charges the main. That matters most in buildings tall enough for the delay to be significant.
Being permanently charged means it has permanently live plant: pumps, a water supply and power. Those all need maintaining and testing, which makes a wet riser a considerably larger ongoing commitment than a dry one.
Pump failure is the risk that matters, and it is invisible until the system is needed. Regular running under test conditions is what makes the difference between a system and a pipe.
In practice
A tall building where the riser pump has never been load tested.
It runs when exercised and nobody has established that it delivers what the system needs at the top of the building.
Running and performing are different tests, and a pump that starts is the easier of the two to demonstrate.
The failure would only be discovered at the worst conceivable moment.
What goes wrong in practice
- Pumps not run under test, so failure is discovered at the incident.
- Power supply arrangements to the pumps never proven.
- Outlet valves seized through disuse in the upper floors nobody visits.