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water main pressure testing

Before a waterline can be tied into an adjacent existing system, it must be properly isolated by way of in-line valves, blind flanges, or tapped caps, and pressure tested to make sure it is a pressure capable extension of the nearby municipal watermain system that it will become a part of.  Due to this requirement, a watermain is typically installed in it’s proposed location and ‘stubbed’ just short of the area or areas, often 5 feet shy, of where it will be ‘tied-in’ or tapped to the existing watermain system. However, sometimes local authorities will allow the contractor to tie one end into the existing system, while closing the nearest existing in-line gate valve (especially if no active services exist on that portion of line). This is often preferred by the contractor as it makes it much easier to fill the new main with water, and future ‘tie-in’ time can be avoided as well.

Pressure testing may occur before or after bacteria testing, though local authorities will often require pressure testing to be done after to assure that if any joint infiltration/exfiltration occurs, it happens prior to bacteria tests.

It is important to note that pressure testing should be done prior to any service taps being done, as putting testing pressure against corporation stops is not recommended. It’s also important to note that if a run of new watermain is quite long, the engineer may specify the maximum length or section of watermain that must be separately tested, which can often be between valved sections. As introduced previously, a blind flange is simply a flush cap terminating a watermain at a ‘dead-end’.

An in-line valve opens and closes water flow on the main water line and in most cases considered a strong enough barrier to pressure test against (unless it is a very old valve).   At some point on the pipe run to be tested should be an area where water is to fill the pipe and also an area must be established where air from inside the pipe can be released while the pipe fills with water. It’s important to note that testing a watermain with a large amount of air should be avoided as the air pressure can provide unreliable/faulty test readings. Thus, a tapped cap is often utilized which is similar to a blind flange but is equipped with an opening that can either accept piped water or a vertical standpipe (often referred to as a ‘blowoff’ assembly) which blows off pressurized air from inside the new water main. Placing these air relief valves at high points in the pipes grade is ideal (though not always possible) as this is where air will end up during filling.

The water main can be filled at a tapped cap by piping from a nearby hydrant (which is often avoided by contractors if possible due to amount of time required to fill) or by ‘tying-in’ the water main at one of it’s planned permanent locations, where it can be filled with water much more quickly (in this case the nearest valve on the existing watermain can be opened, closed, and tested against. Once the line is filled with water (with all hydrant watch valves closed), it is typically left to stand for some time to allow for the pipe to reach ‘equilibrium’ and to allow for the pipe to move slightly and seat itself permanently in the trench. Then a test pump is utilized which inputs more water by way of slow increased pressure until the line reaches its specified operating pressure (commonly 150psi for standard size public utility water mains). Then the pump is turned off and the pressure is monitored through a gauge for approximately 2 hours. In the pressure waiting period, if the pressure does not drop below the specified tolerances of its operating pressure, it will be considered a successful test. If the pressure test fails, the contractor must take-on the undesirable task of trying to locate a faulty pipe gasket, a faulty pipe/spigot connection, a faulty valve, a faulty fitting or megalug connection, etc.  This can often be a confusing, costly, and frustrating endeavor where blame is often difficult to determine.

Once the pressure test is done (and the bacteria test is done, see disinfection and bacteria testing) the new watermain can be tied-in at any final points (sometimes flushed once more) and opened for permanent use.

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