Ethan Grossman: The devil is in the details
A series of stories that borrow from daily life experiences to illustrate engineering concepts.
Construction design documents almost always include details. I’d venture to guess that many of us began our careers by putting detail sheets together. This may have involved copying the relevant details from a previous job or utilizing standard details from a CAD library. Details are an important part of our job that can help make our designs as good as they can be.
Once, I worked on the design for a building that was going to utilize an existing mechanical room. Our team had to survey the existing equipment and utilities to make sure they could support the building addition. As I walked into the mechanical room, my first impression was one of awe, not because it was particularly clean or that the equipment was well-maintained; it was the sheer size of the room itself.
It was the most cavernous mechanical room I had seen. As I took in all the available space and oversized equipment, there was one detail that caught my eye: A 2-inch domestic cold water pipe that was hung way up high, down 6 feet from the structure. The reason it caught my eye was be-cause it was swinging back and forth along its axis.
This is the kind of thing plumbing engineers get excited about, like witnessing a bald eagle catch a fish out in nature or finding a rare collectible at a flea market. “This must be that hydraulic shock I’m always reading about!” I thought to myself. Hydraulic shock, also known as water hammer, is the result of high-velocity flow, followed by an abrupt stop in flow.
Seeing that domestic cold water pipe swinging in the air made me wonder where in the contract documents the contractor would have known to install it differently. Maybe they interpreted the use of a trapeze hanger in the literal sense: As a trapeze artist would swing from up high. This type of observation makes me curious about the details of my documents.
A BIM model with a high level of design (LOD) may have had the pipe modelled closer to the steel. In the days before BIM, the specifications may have inherently prescribed how to hang piping based on MSS standards. Another place where the information would be located is on a detail sheet — some sort of detail that shows the maximum hanging distance of pipe below steel. As I think back on what I witnessed, the action was more like the jarring stop of a zip line than the swinging of a trapeze.
The ASPE design guide gives us a formula to calculate hydraulic shock:
I can understand the value of this formula as it tells us what the “pressure spike” will be. If we have water flowing along happily at 60 psi, how high will that pressure jump when a valve is suddenly closed? In the case of a pipe hung 6 feet down from structure, this pressure spike could even result in a moment force that causes the pipe to bend and swing the hanger rod.
Most of the variables in the formula should be familiar to us. Gamma, or “g,” is the specific weight of water, which is around 62.4 pounds per cubic foot. “dV” is what the formula uses to define the change in velocity and “g” is the acceleration due to gravity, I saved the variable “a” for last since it has that sneaky characteristic of being a formula within a formula. On top of that, it is defined by a bunch of words that don’t roll of the tongue so easily: “Velocity of propagation of elastic vibration in a pipe.”
The variable “a” has such a special definition for itself that it deserves its own formula:
The important takeaways from the formula that defines velocity of propagation are the two ratio’s K & B. “K” is the ratio of the modulus of elasticities of water to pipe. The modulus of elasticity is the measure of how “squishy” something is, and for the purposes of our scenario, water is about 80 times less “squishy” than copper. “B” is the ratio of the pipe diameter to thickness of the piping wall. We can see how the formula has many of the ingredients necessary to describe the phenomena of hydraulic shock.
Once I feel that I know enough about an equation to be dangerous, I like to test it out with some real numbers. I’m pretty sure the pipe I saw that day in Boiler Room Cavern was a 2-inch main. As I described, it was hung down unrestrained about 6 feet from the structure. It ran for about 60 feet before turning into the other building. To give the original designer the benefit of the doubt, I’d guess the water was flowing at 6 feet per second (8 feet per second being our recommended maximum design velocity).
Plugging the numbers into our equation gives us a pressure spike of:
A pressure spike of 300 psi in a pipe may be within the limits of a reasonable design, and if it were above a ceiling on a short hanger with a water hammer arrestor installed somewhere, it would likely go unnoticed. The fact that the pipe was swinging in the air brings to light how design issues can become compounded when we don’t pay attention to the details. Details help communicate the finer intentions of a design, and we should take the time to improve upon them whenever we can.
Even with our best intentions, design flaws find their way into being constructed sometimes. They can be like gremlins or annoying red imps with horns, laughing at us, swinging on a trapeze.