(P&G). Indeed, the +/-0.1 percent to +/-0.2 percent mass flow rate accuracy typically claimed for Coriolis flowmeters is the primary reason that Cincinnati-based P&G makes wide use of these instruments throughout its 118 plants worldwide, Reizner says.
But there’s “a dirty little secret” about Coriolis flowmeters that your vendor’s local sales rep may not tell you about, Reizner warns. Namely, that Coriolis flowmeters don’t work well for two-phase flows. “If we have a liquid that’s moving through a pipe, and it’s got air bubbles in it, that can cause all kinds of problems for the meter,” he says.
Entrained air in liquids can cause high inaccuracies in Coriolis flowmeters—with errors of 20 percent or more—that can happen insidiously and without warning, according to Reizner. These problems can occur at very low aeration levels, even at void fractions (the volumetric percentage of gas in a liquid) as low as 2 percent, he says. And because the inaccuracies may not be detected immediately, it can cause quality issues. “We’ve scrapped many, many thousands of dollars worth of products because of this problem,” Reizner says.
Reizner’s remarks came during a conference presentation at National Manufacturing Week, Feb. 23-26, in Chicago. And in keeping with the title of his presentation, “Coriolis—the ALMOST perfect flowmeter,” Reizner also had plenty of good things to say about Coriolis flowmeters.
Unlike most other types of flowmeters, which measure volumetric flow rates, Coriolis flowmeters measure mass flow rates. This is significant for a company such as P&G, which produces products ranging from laundry soaps and household cleaners to hair colorings, shampoos, perfumes and deodorants based on precision blending of chemicals. “For people like us who are in the chemical processing industry, mass flow metering is very important vs. volumetric,” says Reizner, “and Coriolis flowmeters are pretty much the mass flowmeter that’s available today.”
Coriolis flowmeters measure mass flow based upon a twisting force induced on a vibrating tube when a fluid passes through the tube. In addition to their inherent high accuracy, Coriolis flowmeters also offer wide rangeability, Reizner says, which is important for measuring the addition of materials at both low and high rates during product blending. A lack of moving parts makes Coriolis flowmeters nonintrusive, he adds. “If you look through the meter, all that’s there is a pipe. There’s nothing inserted into the pipe.” This also reduces maintenance requirements.
Limitations of Coriolis flowmeters include a maximum pipe diameter of six inches. But for P&G, that generally isn’t a problem, Reizner says. The company makes some paper products such as paper towels and toilet paper, which rely on 24-inch pipelines, he notes. For those processes, P&G uses magnetic flowmeters. “But most of the things we make are specialty chemicals like shampoos, deodorants and aftershave lotions, and for things like that, a three-inch pipeline is pretty huge, and two-inch is probably the most typical size.”
Compared to other kinds of flowmeters, Coriolis flowmeters are also more expensive. “They’ve got lots of advantages, so we pay a lot for them relative to the others,” Reizner says. But he notes too that some vendors have recently introduced lower-cost Coriolis units that are “still quite excellent as far as accuracy and rangeability.”
Despite the advantages of Coriolis mass flowmeters for chemical processing applications, there are several areas in which Reizner would like to see improvements. Among other things, Coriolis flowmeters with better dynamic response are needed, he says. Reizner would also like to see vendors do more testing and product development using both viscous and non-Newtonian fluids of the kind that make up most of P&G’s product line. Instead, most vendors currently work primarily with water in their labs.
As for the inaccuracy issues related to aerated liquids, P&G is pushing Coriolis mass flowmeter vendors to develop solutions in that area as well. And there are some encouraging signs. “There are a couple of vendors who claim to have dealt with, or are dealing with this aeration issue. They’re two of the smaller companies in the industry, and they’ve got a lot of marketing going on,” Reizner observes. “So they’re really helping to push everybody to start looking at this issue more seriously.”
Wes Iversen, [email protected]
