Ethan Grossman: Getting dirty in engineering
A series of stories that borrow from daily life experiences to illustrate engineering concepts.
You can learn a lot from experience, even if you don’t realize it at the time. Many times, those experiences are the ones we remember and contribute to “aha” moments later in life.
Growing up, this usually involved getting dirty in one way or another.
The dirtiest job I ever had was was when I worked in a fish market as a teenager. I worked in the crabtank, which was a shack that hung over the harbor. There were live crabs and lobsters people would pick out, and we would boil or steam them. Self-priming pumps would draw seawater out of the harbor and through sand filters into the tanks. The circulating water was critical to keeping the crustaceans alive.
The dirty part was when the pumping system failed and the crabs would start to die. I remember quite vividly showing up to work at 6 a.m., and being instructed to “sniff” every dead crab to determine if it was safe to cook. The bacteria in shellfish builds up within hours and makes the crabs unhealthy to cook. To this day, quite honestly, I don’t think I could forget the smell of a crab gone bad.
Once my career developed from crabmonger to plumbing engineer, I had one of those “aha” moments when I learned about self-priming pumps, pressure differential and net-positive suction head. A self-priming pump is something we should all be familiar with and able to specify. Self-priming pumps have the unique property of being able to perform a suction lift. Designing a system where the pump is installed well above the fluid being transferred requires us to understand net-positive suction head.
Have you ever seen the experiment where someone boils water at 70° F? It involves another property we should all be familiar with: Vapor pressure. Vapor pressure is the pressure at which a fluid starts to vaporize. For water at 70°, the vapor pressure is 0.36 psia. The way the experiment goes is someone puts a glass of water under a bell jar and pulls a vacuum. Miraculously, the water boils as if it had been sitting on the range.
The important lesson here is that when a centrifugal pump impeller gets spun around at high speed, it creates a low-pressure zone that relates to the pump property of NPSHr or “net-positive suction-head required.” If the low-pressure created gets too low, it can cause the fluid to boil. Then, when the fluid gets re-pressurized on the discharge side, the vapor bubbles collapse and ping. I always thought it was called cavitation because the collapsing bubbles created cavities in the impeller. After researching this, it appears the term actually comes from vapor cavities forming in the fluid itself.
The amount of suction head required is something we rarely consider because, quite honestly, there is usually ample amounts of suction head available in commercial plumbing applications! The natural state of ambient conditions supplies us with 14.7 psi of atmospheric pressure to get the fluid into our pump. This translates to about 34 feet of head. Theoretically, a pump should be able to lift water 34 feet out of a pit, or the harbor at high tide as was the case with 17-year-old me. As reality dictates, we need to subtract the NPSHr for the pump from the net-positive suction head available.
NPSH is ultimately what will keep a pump happy, and in the case of the crabtank cantilevered over the harbor, we can use Formula 1 to see how a filtration system design may have stopped pumping.
One of the components that hasn’t been mentioned is the sand filter itself that was part of the system. My 17-year-old self does not remember if it was on the suction or discharge side, but you can see how, either way, the pressure loss through a sand filter can take away that 6.2 feet of head available on the suction side — and friction loss on the discharge side could easily “dead head” a self primer with a flat pump curve.
Directly affecting people
One of the things about being an engineer that fascinates me is we design systems that directly affect people. Whether that person is a dishwasher, a crabmonger or a brain surgeon, the tools of their trade depend upon reliable designs.
Some of us may have held down a dishwashing job in our past, so we can understand what goes on in a commercial kitchen. I’m not sure any of us have gone from practicing brain surgery to designing a medical gas system. It’s not always easy for us to understand how our designs are actually used in the field, so it is important for us to gather information and educate ourselves.
Continuously being curious about how people use the systems we design only makes us better engineers.
Information gathering can be a delicate art: How do we ask intelligent questions in a way that doesn’t make us look incompetent? Or, how do we make determinations without coming across as an egotistical “know it all?” I’ve been painted into a corner more than once when a colleague or client has looked at me to make a determination and I have to let them know that I don’t have all the information I need. More than once I have received a frustrated reaction as if they are saying: “You are the engineer! You are supposed to know!”
While we may have classroom training to understand how things work in theory, we don’t always have the life experiences to meld the two together, as how cooking dead crabs relates to pump theory.
Each of us has different life experiences and can learn from one another. Continuously being curious about how people use the systems we design only makes us better engineers.
Relish the practical experiences that you encounter; they will prime your mind for great designs and problem-solving skills.