Sunday, January 19, 2003

Bubbles 102

In Bubbles 101 I wrote about bubbles in a pot. Now let's see what happens if the liquid that contains bubbles is contained in a piping system.

First you need to know a little about piping systems. If their contents are moving, that's because the pressure is higher where the liquid is coming from than it is where it's going to. In your house, you might have about 40 PSIG of pressure at the faucet when it's closed, but at the end just past the aerator the pressure is 0 PSIG whether the faucet is open or not. The greater the pressure difference the greater the flow.

For a straight level pipe the pressure drops off more or less linearly with length. Throw in direction changes, rough joints, valves, pumps, or changes in elevation, direction, temperature, pipe material or the properties of the contents and things get a lot more complicated in a hurry - in particular, there can be rapid changes of pressure from point to point. But the bottom line is that the pressure drops in the direction the flow is going.

As the pressure drops, it's easier for bubbles to form. That's why your Coke went flat and you change your cake recipe in the mountains. Anyway, it's not a problem if you just have a straight run of pipe - the bubbles just get bigger as you go downstream.

But if you throw in something as simple as an elevation change, the bubble formation rate changes. The bubbles will want to rise, perhaps even opposing the direction of the flow and introducing more flow resistance. More flow resistance means less pressure, and thus more bubbles...

At one power plant I worked at (which has never been in operation), an essential raw cooling water pipe was making incredibly loud noises. The 24" pipe exited a heat exchanger to a high point, turned horizontal and passed through two butterfly valves (one of which was not fully open), and then made a 90° turn straight down to a big discharge header which flowed to the heat sink. I told my supervision and anyone who would listen that the pipe was cavitating.

What's cavitation? Well, when the flow of fluid in a pipe is disturbed, the pressures in it can become unstable. When that is true, bubbles can flow into a volume where the pressure is too much for them, so they collapse. In the body of the fluid that's no problem, but if it happens at a solid surface bounding the fluid the effect is like a tiny chisel, and the pressures reach very high levels. Given enough time, this will chisel away anything while vibrating and making noise at deafening levels. (Conceptually similar mechanical things happen if large drops of water form in the steam in a steam turbine - they impinge on the blades and cause noise, vibration and ultimately failure.)

Anyway, after much bitching a work order was executed to work on the valve that had been throttled (not fully open). It wouldn't seal any more - imagine that. When they took the valve out, the maintenance guys were astounded at how thin the pipe walls had become and how shiny the normally rusty carbon steel surface was. They wound up replacing some pipe and changing operations practices to escape this problem.

(So cavitation produces sound. It turns out that sound can produce cavitation too. Terrific - about now that probably sounds about as useful as an appendix transplant, but in fact that's how ultrasonic cleaners work.)

Given a chance, bubbles will wind up collecting in the high points of the system, making big bubbles. Thus any decently designed piping system will have vents at the high points so the system can be filled fully before operation and perhaps periodically during operation. Fail that and then you're subject to hammering, which is just cavitation writ large. But when such a big "bubble" breaks or changes directions, the force when the water hits the pipe can be great enough to rip pipe right off its hangers and even rupture it. Or just annoy you - it's that loud slamming sound you might hear when you shut off the water suddenly.

Big bubbles can also effectively cut off flow in a system if they're in a pipe. Such a bubble complicated the response to the Three Mile Island accident. They can also prevent positive displacement or metering pumps from working effectively or delivering at all, which can be a real problem at chemical or water treatment plants.

Bubbles are a really big deal with nuclear reactors, and not just because of the TMI event. Boiling water reactors (BWRs) (like Dresden, Quad Cities, Clinton and LaSalle County in IL, Hatch in GA, River Bend in LA, Perry in OH or Grand Gulf in MS) are designed to have a certain amount of bubbling in the reactor core in normal operation, but only so much (the presence of bubbles affects both the rate of heat removal from the core and the rate of power generation - more bubbles -> less power). The much more common pressurized water reactors (PWRs) (like Braidwood and Byron in IL, Callaway in MO, Wolf Creek in KS, Farley and Bellefonte in AL, Indian Point in NY, Sequoyah in TN, Davis-Besse in OH and TMI, Susquehanna, Peach Bottom and Beaver Valley in PA) are not to have noticeable bubbles in them under any circumstances, and when they do you've got a problem.

Stay tuned for the next exciting segment of Bubbles: The Series....

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