CASE STUDY: Using Clamp-on Ultrasonic Flowmeters for Accurate Helium Measurement

The portable ultrasonic flowmter accurately measures flow at even the slowest rates.
(Photo courtesy of FLEXIM AMERICAS Corp.)

Since 1996 the demand for helium has been ramping up, as supply remains fairly static. Accordingly, prices have risen sharply. For example, in 2014 the price for crude helium is estimated to increase 30 percent.

The culprit? The Federal Government in the guise of the Bureau of Land Management (BLM).

The danger? Helium is much, much more important than party balloons and parades.

The solution? More private production and storage facilities, but those are some years away.

In the meantime? Conservation using accurate flow measurement to assure maximum efficiency and minimum waste. This is critical because there is a finite amount of helium available, and when it leaks into the atmosphere it rises and releases into space.

Properties of Helium
The helium atom is smaller than that of any other element and second only to the hydrogen atom in lightness. As a result, helium is chemically inert and does not form stable compounds with other elements. The attractive forces between helium atoms are also so weak that helium has the lowest liquefaction temperature of all the permanent gases and, unlike all other elements, does not freeze under its own vapor pressure as the temperature is lowered toward absolute zero. Helium turns to liquid at -268.93 C. Absolute zero is -270 C, making helium the very best element for super-conducting applications.

The Federal Reserve
The first large-scale use of helium for nonscientific purposes was its substitution for hydrogen as a lifting gas in lighter-than-air applications (e.g., dirigibles, zeppelins, and blimps). Even though hydrogen provides about 7 percent more lift than helium, it is much more dangerous to use because it is highly flammable. The military importance of helium as a lift gas in the early 20th century led to the development of the Federal Helium Reserve in 1925.

Losing Money
By 1995 the reserve was $1.4 billion in debt as a result of earlier purchases of helium from private producers and their practice of selling helium below market rates. The debt was due to finally be repaid to the U.S. Treasury Department by the fall of 2013 and, under current law, funding to the federal program would then stop, terminating operations. The BLM assumed market forces would move in to fill the gap. That happened, but at a much slower rate than was anticipated. The anticipated closing of the Bush Dome would have a significant impact on numerous industries throughout the world—everything that uses computer chips, MRI machines, aerospace, super cooling, and scientific research. That meant that the cost of many things would rise dramatically.

A Solution—Kind Of
Fortunately, Congress acted and in September 2013 passed the Helium Stewardship Act that keeps the reserve going and provides a formula that gives guidelines for pricing relative to current market prices.

The problem was that helium prices were at an all-time high. Helium refiners had already been raising prices in anticipation of the Bush Dome’s closure. There were other helium plants and storage facilities in the works, but their completion and production were still years away. There was no choice but for businesses to find ways to conserve and learn to work smarter.

Helium Conservation Strategies
Ever since the space shuttle program was cancelled, NASA and the Kennedy Space Center have been rebranding themselves. While they will continue to build and launch spacecraft for exploration and other government projects, the earlier single-minded focus on the shuttle program has broadened. NASA has now taken its sprawling complex and is essentially leasing space and capabilities to tenants. While those tenants consist mostly of subsets of NASA working in different areas of aerospace, two are private companies—SpaceX and United Launch Alliance—and there are likely to be more in the future.

“It used to be that the only customer we had was the shuttle program,” says Dan Tierney, sustaining systems engineer for URS Corporation, a contractor working with NASA at the Kennedy Space Center. “Since they were the sole user of everything that was supplied, there was no need to monitor usage. Now that the shuttle program has ended and NASA has various customers, we need to bill each one separately for everything we supply them. For liquids and gasses that meant that we had to install metering.

“For a variety of reasons, the two most difficult flows to measure are nitrogen and helium gas,” says Tierney. “Both are vital in rocket and spacecraft use. Nitrogen is a propellant and helium has three uses. It is used to remove atmospheric moisture from cleanroom facilities. Because helium is an extremely small molecule and can find its way through the tiniest opening, it is valuable as a leak detector on spacecraft. Third, it is used in high volumes in launch support. Since helium will not freeze upon expansion, as most gasses will, it is used as a purge gas to sweep out combustible vapors from rocket engines and engine compartments. We call it ‘safing’.”

Enhanced metering was seen as a way to optimize helium use. Helium [is] very expensive—10 times the cost of nitrogen, so conservation is key.

URS performed a major search for meters that could not only measure gas, but also gas at low flow rates. “While many of the meters provided acceptable accuracies, operating and maintenance costs played a big factor,” says Tierney. “All but one of the meter types was intrusive and had to be exposed directly to the flow.” That meant an expensive installation, as well as costly maintenance. Because the meters were exposed directly to the flow, they would experience wear. “That meant that down the road, we would have to shut down, remove the meter, clean or replace parts, and start up again,” says Tierney. “That added greatly to the long-term cost.”
The chosen meter in this case was a clamp-on ultrasonic meter by FLEXIM, which didn’t have to be exposed to the fluid flow stream and showed a high accuracy at all flowrates.

Transit-Time Ultrasonic Flowmeters Applied
“The technique most ultrasonic flowmeters use is called transit-time difference. Transit-time ultrasonic flowmeters exploit the fact that the transmission speed of an ultrasonic signal depends on the flow velocity of the carrier medium, kind of like a swimmer swimming against the current,” says Peter Chirivas, an engineer at FLEXIM Americas, the maker of the meter selected. “The signal moves slower against the flow than with it.”

When taking a measurement, the meter sends ultrasonic pulses through the medium, one in the flow direction and one against it. The transducers alternate as emitters and receivers. The transit time of the signal going with the flow is shorter than the one going against. The meter measures transit-time difference and determines the average flow velocity of the medium. Since ultrasonic signals propagate in solids, the meter can be mounted directly on the pipe and measure flow non-invasively, eliminating any need to cut the pipe. We also adapted the technology to measure flows as slow as that of groundwater.

The ultrasonic flowmeter clamps directly onto pipes, providing flow measurement and leak
detection. (Photo courtesy of FLEXIM AMERICAS Corp.)

Semiconductor
Two East Coast semiconductor manufacturers also opted for ultrasonic metering, but for different applications.
“We use helium in the chip making process,” says Anne, a professional engineer at a major Mid-Atlantic chipmaker. “We use it as a shield gas in clean rooms to keep pollutants off the wafers and as a push gas because it is inert.

“With the cost of helium going up for the foreseeable future, we knew we had some leaks in the system,” says Anne. “We compared our tool data to our flowmeters and we had a mismatch. We were missing some gas and we weren’t sure where it was going. We needed a way to detect the leaks so they could be sealed. Helium can get through the tiniest opening.”

The first time Anne heard of ultrasonic metering as a leak detector was at the Chem Show at the Jacob Javits Center in New York City. “There aren’t that many clamp-on flowmeters that will read gas accurately at a low level,” says Anne. “We had looked at ultrasonics before and we did buy a unit some time ago, but it didn’t have leak-detection capability and it was permanently installed elsewhere.

“We needed something we could use on the distribution network that led to the chip-making tools,” says Anne. “If we put flowmeters on every line it would cost a small fortune, not to mention the production loss from the downtime it would take to install the meters.”

The FLEXIM meter we chose was portable and worked on all of the pipe sizes. It also measured slow flows very accurately. “We’ve been going to each lateral and totalizing the flow over a particular period of time,” says Anne. “Then we compared that to the tool’s mass flowmeters. When we first rented the FLEXIM meter, we compared its performance to our building flowmeters. And the FLEXIM meter was quite accurate.”

After going lateral by lateral and comparing the ultrasonic flowmeter’s results to the tools’ mass flowmeters, the manufacturer identified a leak in one of the tools fairly early in the process. “We put the FLEXIM on and the background levels stayed too high for the flow,” says Anne. “We got a window to shut down each tool and went one-by-one until we found it. So we were able to find and repair a leak with little-or-no loss of production.”

www.flexim.com

Jack Sine is a freelance writer specializing in petrochemical, HVAC, flow technology, and environmental issues. He can be reached at [email protected] or 845 831-6578.