Water Hammer - The importance of cross discipline communication.

Imagine opening the lid of a below ground vault to find it full of water. Pump motors, VFDs and a control panel all submerged. It's the worst nightmare of fountain owners and there is a culprit that may put any system at risk.

Water hammer is the resulting force of the momentum or inertia of a stream of water which is abruptly stopped. The force of water hammer may spike to pressures in the hundreds of PSI and for PVC pipe, this is a deal breaker. There are three main variables that equate to the pressure spike of a water hammer. The velocity of the water, the length of pipe flowing the water and the close time of the mechanism that stops the water. Often fountain designers will specify the maximum water pressure entering the vault and that the water is back flow prevented. Typically, a solenoid valve is used to allow make up water into the water feature system. But, a critical piece of information is often unknown; where is the back flow preventer with respect to the vault?

Take a situation where a 1" cold water supply feeds the make up water on a vault. The pressure is coming from the city at 80 PSI but it's reduced to 40 PSI just before the vault. A "slow" closing solenoid valve is used and has a close time of 0.50 seconds. This all sounds like it is per specification but one aspect often outside the control of the fountain designer is the location of the back flow preventer. What wouldn't appear like a problem to a licensed plumber or utility, a back flow preventer 30 or 50 feet away from the vault may result in a catastrophe. Because there can be poor communication between trades in situations like this, it is critical the fountain designer, installer and GC communicate regularly on aspects such as this.

The equation to calculate water hammer is P = 0.07 x (V x L / t). Assume the back flow preventer is 50' away from the vault. When the solenoid valve closes it immediately stops a stream of water 50' long and sends a shock wave back from the closed solenoid valve to the BFP. Using the equation above we know the velocity V of water in a 1" pipe at 80 PSI is near 40 FPS. Length L is the distance from the BFP to the solenoid valve of 50'. The close time of the solenoid valve is 1/2 second. Therefore P = 0.07 x (40 x 50 / 0.5) = 280 PSI. If that same BFP is installed 10' from the vault, the Pressure would be reduced to 56 PSI.

This shows why it is critical for open communication across disciplines and how a simple BFP location which looks fine to one group may be a critical error for another groups equipment.