WorldWide Drilling Resource

80 MARCH 2015 WorldWide Drilling Resource ® Cavitation? by Bill Corey Pentair Water Training Institute I got a call from a customer who had a problem with a 1900-gallon per minute (gpm), 150-horsepower pump. It was actually the salesman who was calling and said the customer was com- plaining he wasn’t getting the 1900 gpm as advertised. As a matter of fact, there were bubbles coming out of the well with the water. The salesman said, “I think he’s cav- itating.” Needless to say, I asked some ques- tions which started with: “How deep is it to water?” The answer surprised me. He had an 85-foot well with the water table at 65 feet. This meant the pump was only sitting 10 to 15 feet underwater. So I told the salesman I didn’t think it was cavitation. The customer was complaining he was only getting 1300 gpm; and low flow is not something you normally see with cavitation. I said I thought it was air being pulled in by the lowwater table. The sales- man was very insistent it was cavitation, so I asked him what creates cavitation. The answer to this question is real- ly quite clear: low pressure. Something most don’t take into consideration is the fact submersibles have an NPSHR (net positive suction head required) just like aboveground pumps. They think it’s underwater, so there will always be water going to the pump, but NPSHR and draw- down can see it does not happen. At 1300 gpm, the pressure should be just fine; if anything, maybe a little high. So cavitation was not the problem. But when you draw a well down, there is a possibility if it draws down too far, you will break suction and pull air into the pump. Of course, the air bubbles will then exit the well in the water and you will be able to see them. That is what was happening here. Moral: Just because there are bub- bles, doesn’t mean it is cavitation. The correct question to ask is, “Where did the bubbles come from?” If the flow does not indicate cavita- tion, then it probably is not cavitation. Bill michele@ worldwidedrillingresource.com Can the Carrizo-Wilcox Aquifer Help the Texas Water Shortage? Compiled by Bonnie Love, Editor WorldWide Drilling Resource ® Texas is among the many states in the U.S. dealing with drought conditions. Some folks in Central Texas are consid- ering using a relatively untapped resource, the Carrizo-Wilcox Aquifer to ease the water strain. The Carrizo-Wilcox Aquifer extends from the Rio Grande in South Texas, into parts of Arkansas and Louisiana, and supplies water to 60 Texas counties - nearly 10-12 million people. It is believed this particular aquifer holds hundreds of trillions of gallons of water, which some believe will be enough water to sustain growth for centuries in areas around Austin and San Antonio. This area of Texas is experiencing severe drought conditions. In fact, Lakes Travis and Buchanan, the region’s water supply reservoirs, are currently only 34% filled. Those living above the Carrizo-Wilcox area aren’t convinced the plan to pump billions of gallons of water from the aquifer each year is a good idea. They worry about overpumping and fear the aquifer could be drained within just a few decades. “It’s not a matter of availability,” said James Beach, a hydrologist for the firm LBG-Guyton who studies the Carrizo- Wilcox for a groundwater management district, the Central Texas water provider Aqua, and San Antonio’s water utility. “The volume of water is there. It’s more a question of impact.” One of the biggest impacts would be farms and residents with shallow wells. To prevent the wells from going dry, they would most likely have to drill deeper wells and lower their pumps. Water marketers, companies which collect groundwater rights and sell the water to cities, may agree to place millions of dollars into a fund to assist landown- ers with the expenses associated with lowering the pumps. However, the bigger issue is what impact the additional pump- ing will have on the area overall. It has been suggested, drilling in the deepest portions of the aquifer could help minimize the impact on those with shal- low wells. However, continuous ground- water removal from even the deepest portions of the aquifer will cause water levels to decline throughout the entire area. Another issue with this theory is the quality of the groundwater may de- cline the deeper you go; deeper waters tend to have greater mineralization with higher iron content and other natural chemicals. There are also environmental issues to consider. Studies indicate the Carrizo- Wilcox Aquifer supplies water to the Colorado River, which is at its lowest flow level in decades. Modeling by George Rice, a former Edwards Aquifer Authority hydrologist, suggests pumping water from the aquifer could cause the aquifer to pull water out of the river instead of putting water into it. The repercussions down- streamwould be devastating to fishermen who depend on the river’s freshwater flows for a steady supply of oysters and shrimp in Matagorda Bay. One thing is for sure, with an increas- ing population and current drought con- ditions, this aquifer needs to be carefully monitored, managed, and most impor- tantly protected.