April 2011 Vol. 238 No. 4

Features

Accurate Brine Measurement Guides Gas Storage Cavern Development

Dave Thompson’s search for the best non-contact flow meter for really challenging liquids began at Freeport LNG’s natural gas storage facility on Oyster Creek on the Texas gulf shore.

“Three years ago I was with Sub Surface Group as its manager of field operations,” he said. “We were on board to create a salt dome storage facility for natural gas using a solution mining process. The operation was already under way and we were using a combination of meters to measure flow. Of course, wet meters are a problem because anything that is touching brine is subject to corrosion and because of the scaling nature of the salt that collects on surface, plugs orifices, and degrades other signals.

“I was aware there was some new non-contact metering technology that worked well with common liquids, but didn’t know if any of the technologies available could accurately measure slurries. Because the current metering was not as effective as I desired, I decided to see if I could identify a metering technology that could work effectively with brine,” Thompson said.

“I started asking other engineers and other measurement professionals if they had any positive experience with non-contact flow meters. One of them, Walter Cook, once had his own engineering company and had dealings with a firm called Allesco. He suggested I talk to a fellow named Cliff Lane who was an expert on flow meters,” Thompson said.

Allesco is a value-added distributor to the process industry. In addition to providing components and instrumentation, it also offers technical services.

An Ultrasonic Solution
Said Lane, “When I got together with Dave (Thompson), we looked at several possible solutions to his brine problem. There are several providers of non-contact flow meters, mostly using either transit time or Doppler technology. The problem is the brine. To create an underground storage dome, engineers pump water into a salt dome to dissolve the salt and pump the brine out, creating vast amounts of (natural gas) storage area. They have to measure the amount of water and the brine content because it lets them know how much salt has been removed and how much storage space has been created. So they need to know not only the flow rate, but also the amount of salt concentrations in the flow. The concentrations are usually pretty consistent, but they can vary enough to cause problems.

“Dave and I discussed a variety of products, but eventually settled on one that looked promising – a combination of low frequency transit time and Doppler,” Lane recalled.

Ultrasonic Technology
“One of the major benefits of ultrasonic flow meters is that, unlike traditional meters, they contain no moving parts and do not need frequent calibration and maintenance,” said Lane. “Measurements are made using the transit-time difference method. It exploits the fact that the transmission speed of an ultrasonic signal depends on the flow velocity of the carrier medium. An ultrasonic signal moves slower against the flow direction of the medium and faster when it is in the flow direction.

“For the measurement, two ultrasonic pulses are sent through the medium, one in the flow direction and the second against it. The meter’s transducers work alternately as emitter and receiver. The transit time of the signal sent in the flow direction is shorter than that of the signal sent against the flow. The meter measures the transit time difference and calculates the average flow velocity. Since the ultrasound signals propagate in solids, the meter can be mounted directly onto the pipe non-invasively.

“Ultrasonic flow meters are not affected by density which makes them ideal for brine measurements. They automatically compensate for variations in viscosity. Also, because the salt tends to plate, the meter can detect any buildup on the interior of the pipe, compensate for decreased diameter, and send an alert that the pipe needs cleaning,” Lane explained.

Peter Chirivas, senior engineering staff manager at Flexim Americas, manufacturer of the chosen meter, said, “Because we use a low frequency input module with a special low frequency transducer, we are better able to measure through the particulates in a brine slurry. It infers the concentration by measuring what the speed of sound of the liquid is. Since we know the geometry of the pipe, we can directly measure the speed of sound of the moving liquid.

“We’re always eager to work with our customers so they can get the most out of our meters. At Freeport, they gave us data about their brine and our physical chemist in Germany developed an equation that the meter uses to measure that particular brine. Since the meter measures the speed of sound of the liquid, and normalizes for the effect of temperature (the meter has an input for a resistance temperature device or a 4-20 milliamp temperature transmitter), the meter puts these two variables into the equation and the resulting change of sound speed is correlated to the salt concentration in the brine.

So, they don’t have to be in contact with the brine to know what the concentration is. Speed of sound is a very active variable when it comes to brine concentrations. With this technique you get quite good resolution and repeatability. The technique also works with many other applications such as sulfuric acid and nitric acid,” Chirivas said.

Doppler Backup
Chirivas continued, “Then there’s the Doppler aspect of the meter. If the slurry becomes too heavy, the ultrasound signal can get lost. But we’ve built in Doppler technology as well, and if the slurry becomes too heavy, the meter automatically switches to Doppler and then back to transit time when the slurry thins. Doppler isn’t quite as precise as transit time, but it makes for a good backup and keeps important data from being lost.”

Testing For Accuracy
Thompson said, “The folks at Freeport, Dow Stratton Ridge, are very careful about changing how they do things. While they recognized the innate problems with contact meters – clogging, plating, maintenance – they wanted to be absolutely sure before changing to ultrasonic. So they developed a battery of tests that compared the ultrasonic results directly to their wet and alternative meter results, and they weren’t in a hurry about it.

“After six months they decided they’d seen enough and the meters were approved for our contract compliance use as well as for their system measurement worldwide. We installed the ultrasound meters throughout the operation and had great success,” Thompson said.

Mississippi Hub
Thompson was so convinced about the efficacy of low frequency ultrasonic meters for brine solutions that he took them to his next salt dome challenge – Mississippi Hub (MS Hub).

“The project for MS Hub was for development of two salt storage caverns to hold up to 12 Bcf of working gas. The location is in close proximity to major pipelines and pipeline expansion projects. I was brought in by Energy South, the original owner, to set up its operations group. They had some initial problems with their design and were having trouble getting the project off the ground.

“My role grew into that of being in charge of project development, specifically in design review and implementation. In that capacity, I was able to persuade the engineers to give low frequency ultrasonic meters a try. With the success at Freeport, it wasn’t a hard sell and they have been more than pleased with the results – not only the accuracy – but also the greatly reduced maintenance time. They eventually replaced all of their orifice meters with ultrasonics,” Thompson said.

Gas Flow Measurement
When the caverns are completed, the ultrasonic meter work won’t be finished. Said Chirivas, “The meters are capable of measuring almost any kind of flow when properly equipped. While the brine meters can easily be upgraded to measure gas flow, the engineers at MS Hub decided to install dedicated clamp-on gas meters to complete the full measuring cycle.”

Thompson added, “Both locations are happy with the ultrasonic technology and I understand they plan to continue to use the meters to measure gas flow. They realize that when they combine the decreased maintenance cost and the continued use of the meters in their gas distribution operation, this technology is exceptionally cost-effective.”

Author
Jack Sine is a freelance writer specializing in industrial technology. He can be reached at jack.sine@verizon.net.