Booster Pump Creating Cross-connection Incident (Case 2)

The water service line from the public water system main to this facility supplies water to the domestic water system and the fire standpipe. The fire standpipe is equipped with a fire pump. The domestic water system is on the upstream side of the fire pump.  On the top floor of the building, there is a mixing tank with no backflow protection. The public water system does not provide water to this fecality in sufficient quantity for the domestic water system and for the fire protection standpipe system, if the pump was activated.  If the fire pump was activated a pressure drop would occur in the domestic water system. The water supply for the mixing tank is at the bottom of the tank and it is located on the upper level of the building.  A backpressure condition would occur if the water pressure in the chemical mixing tank was greater than the domestic water system pressure.   A backflow prevention device installed the water line to the chemical tank would prevent non-potable water from entering the potable water system.

Water hammer is a sudden increase in the pressure and is caused when flowing water is immediately stopped due to a valve or hydrant being closed too quickly, or when a pump is started up too quickly, which sends a sudden pressure wave down the water line, shocking the pipes.

Typically the pressure wave of water hammer travels at a velocity of 1440 meters per second. That is similar to saying that if you change the pressure at the end of a pipe, it can be read one mile away one second later. The shock wave travels up and down the pipe through the liquid, banging against each end of the pipe.

The pressure spike (acoustic wave) created can be as high as five (5) times the system working pressure.

Valves closing quickly such as solenoid valves will create a sudden increase in pressure or shock wave in the piping system known a “water hammer.” As previously stated this pressure can increase to as much as five time the system pressure. In this case, if the incoming pressure is 80 psi the water hammer pressure surge would be 400 psi.  If a backflow prevention device is installed on the plumbing system, the second check valve is taking the force of the water hammer pressure. Typically the backflow prevention device is only rated to withstand pressures of 175 psi. This constant pressure surge or shock wave against the second check will over a short period of time cause the second check valve to fail and/or create major problems to the plumbing system.

Temperature and pressure are directly proportional to each other. This means that as the temperature decreases, the pressure also decreases, and as the temperature increases, the pressure increases. As water is heated the volume must increase or the pressure must increase.

As the temperature increase in a closed vessel such as a boiler, the pressure in the boiler increases since the volume of water in the boiler cannot increase. The increased pressure will create a backpressure situation if the pressure in the boiler is greater than the pressure in the potable water system.