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Distribution Sytem Wide Hammer and Transient Analysis of RPZ

I am attempting to set up a model to demonstrate how a RPZ (Reduced Pressure Principle Backflow Preventer) will respond to Hammer.

For those unfamiliar with the operation of an RPZ, I have attached a document which outlines this very well.

It is basicly a double check with an atmosperic vent between the two checks. This releases water to atmosphere to keep the pressure between the two checks lower than the pressure upstream of the RPZ (upstream is the municipal distribution system).

I will describe below what is occuring in the distribution system, which should explain why I am setting up this model:

A municipal distribution system is experiencing water hammer throughout one of its zones. This zone is served by a pump and a couple tanks. The utlimate source of the hammer is uknown. The pump and tank operations have been thoroughly analyzed and it is generally believed they are not the cause. Its most likely a hydrant being opened or closed too quickly. During the Hammer, the ground shakes around hydrants, 4" service connections dance all over the place , people all over the pressure zone call asking if there's an earthquake, PRV's on the system have blown discs. It would be expected that this would damp out after a few minutes. This lasts for 45 minutues to 2.5 hours. It happens every few weeks or so (highly variable), but it tends to be in the morning on weekdays (pump on the system not running, tanks full).

Five buildings in a development have RPZ's. When these RPZ's are isolated from the distribution system the hammer stops. During the hammer events water shoots out of the RPZ's (from the atmospheric vent between the two checks) and the valves sound like a machine gun (constant opening and closing). It is theorzied that the action of the RPZ's propagates the hammer.

What I would like to know is what is the best way to model this to either prove or disprove that the RPZ's are propagating the hammer. I do have an idea:

Run a model with a fast hydrant closing to start a hammer event to get the graph of the surge (I have done this and the graph of pressure shows the pressure flucuating very quickly above and below HGL (maybe 100 times a second). Add two checks and a TCV in between wherever I want an RPZ. Have the TCV vent to a resevoir at an elevation to match atmospheric pressure (or a tank). Setup the throttling of the TCV to match the graph of the surge (with the TCV opening when pressure is below HGL in the distribution system, which is when the RPZ discharges to atmosphere). I will have to get the closing times of the checks from the RPZ manufacterer or make some assumptions.

Any idea? And if anyone has any other ideas as to what could be causing the hammer to continue for so long, I would appreciate it as well (I know I said the hammer stops when the RPZ's are isolated, but those who say that have some stake in it being the RPZ's fault, so I take it with a grain of salt)

 

Thank you