Allowable Leakage

July 26, 2019

 

 

 

If you look closely at the image of Boston’s Quincy Market, built in 1825, you’ll note a fire hydrant in the foreground. That hydrant is attached to the massive Massachusetts Water Resource Authority system. It’s noteworthy to me because in 1975, I attached pumps and hoses to that hydrant in order to pressure test and disinfect the then new distribution pipelines serving this history district. 


Pressure testing meant fill the lines, seal them off again, attach the pumps, raise the hydraulic pressure to 150 psi, and wait.

On a good day, the gauge’s needle would stay pinned to 150 for an hour: the standard test. Some days the needle would hold for 30 or 40 minutes, then precipitously drop. This meant a rupture in the line, and finding it meant looking for the geyser erupting. 

On a challenging day, the pressure gauge would slowly decrement, and, at intervals, we’d pump more water in from calibrated barrels to maintain pressure and then measure the volume of the water loss. 

These were large diameter iron pipes sealed with flexible O rings. Ideal was a flawless seal, but engineering standards permitted a small amount of leakage. 

A hard day was one where the leakage was just above the allowable, but small enough to be really hard to find. 

The tests were validated by the Clerk of the Works, usually a consulting engineer. These were the days I hoped for an experienced one. 

An inexperienced engineer calculated the line length, pipe diameter, and number of joints to establish the allowable; then matched it to the additional volume we’d added. Over or under determined pass or fail.

Experienced engineers would ask for additional tests if the loss was closely above the allowable. We’d extend the time, raise and lower the pressure, and look for patterns. Did the loss accelerate? Was it decreasing? Did the hydraulic patterns appear to be influenced by pneumatic dynamics. In short, they knew stuff that wasn’t in the book and used that to make more discerning judgments.

One time it went the other way. We had several miles of line, a leak that was potentially troublesome, and was simultaneously troublesome to find. 

After a long and fruitless search, and being literally sophomoric, I applied casuistry in my appeal to a junior engineer. If we calculated the allowable over the entire length, I argued, the test was a pass. 

I almost had him, but he called his senior partner over to the site for a consultation. “Shimatta,” I thought. 

The senior engineer heard me out, gave me a grudging, “nice try” look, and told me to find the leak. 

There was by the book, and there was right and wrong. 

And I’m reminded of this on a day I’m testing my own systems for integrity.
 

 

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