SpinOffs

Finding the root cause of a failure by narrowing down multiple overlaid symptoms can be a long and tedious process—and very often the true underlying problem is not what one might expect by a casual examination of the symptoms. That was the potential difficulty a customer of ours wanted to avoid when they investigated the catastrophic failure of a super-critical, feed-water pump used for a powerplant boiler.

I was a firefighter (we never call ourselves firemen) for more than ten years.  While this was only a part time, volunteer gig, I was a turbomachinery developer and modeler as my full-time job.  Obviously, firefighters use all kinds of turbomachinery and I found it interesting to experience it from the user’s perspective.

Cavitation occurs in a pump when the bulk liquid pressure starts to approach the vapor pressure of the liquid, and cavities of vapor form as seen in Figure 1.  This figure shows the inlet of a cavitating pump tested in Concepts NREC’s water flow loop.  These cavities can degrade the head rise and efficiency of the pump, create instabilities, and cause significant damage to the pump impeller and other components.  To avoid these effects, a pump must operate with an inlet head that is at least as high as the required net positive suction head (NSPH) that the pump was designed to achieve.  The NPSH is the difference in the inlet head and the liquid vapor head (or vapor pressure converted to liquid head).  Many pumping systems would benefit from the ability to operate at lower NPSH and so reducing the required NPSH of a pump is often of great concern to pump designers.