Buried in the bowels of modern infrastructure caught between the placid and pumped, turbulence can be a ruling lord in this domain if not controlled.
Pumps think they are in control, but they do need help. Pumps need uninterrupted and unthrottled flow into the inlet nozzle. Pipe bends and crammed intake configurations can vex performance. You can forget about using a numerical model. It’s impotent here. A numerical model’s no mans land. It’s simply too small and too chaotic of a flow area to model this way.
What can you do? You need to man up and build a scaled down model of the flow area and test it. There are only a few labs in the world that can do this type of testing and one is five miles from my house.
Although it’s been on my must do list for a number of years, I finally toured Clemson Engineering Hydraulics (CEH), a hydraulic consulting firm and physical modeling lab with David Werth, PhD, P.E. who is a founding partner and principal engineer of the lab. David is one of those guys that is full of passion when he is describing the lab and this is exactly the type of person you would want on your team.
He has conducted over 140 physical hydraulic model studies for an extensive variety of pump intakes including those for water/wastewater, cooling water, flood control, and sea-water intakes. He has been involved in the conceptual design and hydraulic modeling of intakes ranging from 100 gpm to nearly 500,000 gpm. When he started this lab over five years ago he literally bet the house by maxing out his credit cards and getting an instrumental loan to start the lab. Thankfully the lab business has grown every year and now is spread over two large industrial buildings. The whole complex seemed to me more like a series of movie back lots where lots of different disciplines converge.
In any given week, they are working on three to six studies which means they are constructing, testing, modifying, or demolishing scaled down models. As we were walking through the facility I felt like I was on a mini world tour. Currently they have projects from Japan, Ireland, Korea, Germany, Argentina, Mexico, and Spain.
Many of these studies are for new installations, but more than half are for facilities that are having trouble or being modified beyond what they were designed to do.
He was quick to discount some general myths of physical modeling such as:
- Physical Modeling is Expensive. Many times they can build a scaled down physical model for less than $30,000.
- Physical Modeling Takes Forever. In as little as a couple weeks his team of four carpenters can build a scaled down model.
- Physical Models are Not Accurate. Flow separation is a significant problem because it can result in large pressure velocity variations around the impeller which leads to imbalanced loading decreased performance vibration. With carefully placed a red dye it is quite apparent how the flow is moving, good or bad.
Some of their model testing simply confirms that under full flow conditions the proposed design would perform satisfactorily. Other testing requires iterative modeling to determine any modifications needed to improve the flow conditions. Transforming basic raw materials like PVC pipe, plywood and acrylic glass into scaled down models requires a bit of ingenuity combined with years of experience. There are no “How-To” books on this. For example, shaping acrylic into different piping configurations requires a little art mixed with hard core engineering.
It is reassuring that in the age of all things digital, analog physical models remain a powerful design tool.