I received a question a few days ago about flow measurement and wet gas. Here is a paraphrased version of the question:
The meter would be used to measure gas in and out of storage. The gas may be wet–depending on pressure and temperature. I do not know yet how wet it can be. Ultrasonic meters have been tried in the past and sometime have issues if there is too much water in the gas. How much is too much and is there a general guideline for gas wetness that applies to these meters?
I turned to Emerson’s Karl Stappert who has experience with a number of flow measurement technologies ranging from Ultrasonic to Coriolis effect. I thought his answer was the makings of a great blog post so I thought I’d share it with you.
The “wet” is of course liquids of varying levels that tend to collect on the walls of the piping at any velocity. The problem is that regardless of liquid loading levels, they will build up in low-level areas of the piping at low flow velocities and they get swept out and whipped-up back into the flow stream when high velocity flows are reintroduced. Liquids can collect in the USM [ultrasonic meter] transducer ports bridging the space between the transducer and USM meter body causing the ultrasonic transducer to fail. USMs such as the Daniel JuniorSonic, which has its transducers located at the top of the meter pointing down, will mostly eliminate the liquid bridging on the transducers.
This design mostly eliminates the liquid bridging concern, but there still exists the potential for intermittent failures when the flow transitions from low velocity to high and sweeps liquids up that have collected during the low velocity timeframe. Liquids that are entrained into the gas stream will cause refraction of the ultrasonic sound wave and may degrade signal quality causing some ultrasonic sound shots to fail in a batch of shots. However, failure of all shots in a batch is a rare occurrence and likely only intermittent if ever. It is not possible to totally alleviate these intermittent conditions regardless of liquid load in the flow stream. This is due to the liquid collection issue at low velocities and potential piping configuration issues.
A meter and meter tube inspection and cleaning program should be employed to ensure the accuracy of the meter over time. Also, with the increasing flow capacity of Coriolis flow measurement, this technology can be considered for these slightly wet gas conditions.
Changing wetness conditions is a challenge in gas flow measurement. Both Ultrasonic and Coriolis measurement can be successfully applied. It’s important that any piping issues be addressed to avoid liquid collection areas upstream of the flow meter and that that the maintenance program monitors for wet conditions.
