Inaccurate measurements can cause short term and long term heachaches for oil and gas companies. Such headaches may include financial losses (due to an incorrect amount of product being bought or sold), or operational downtime for installations (in order to diagnose and solve problems). Let’s discuss several key factors that contribute to inaccurate measurements, and identify strategies to solve these problems.
Cause #1: Ugly Piping
Ensuring optimal operation of a flow meter is essential to maximizing measurement performance. Installation effects due to the upstream piping layout can cause severe issues with the meter’s performance. Bends in the pipe (such as elbows or tees), partially open valves, reducers, expanders, probes, filters, or strainers can all cause severe disturbances of the flow. These disturbances can be in the form of a distorted flow profile (where the flow velocity isn’t symmetrically distributed across the pipe) or swirl, (where the flow is rotating around the inside of the pipe).
Two simple ways to avoid these issues include (1) using longer meter run lengths, or (2) simply eliminating these upstream fittings entirely. However, neither of these options are realistic in practice. To fully eliminate swirl, 200 – 300 pipe diameters are required, which can make a meter run hundreds of feet long and impossible to use in the real world. Further, those fittings are the critical components needed to actually construct the meter station in the first place.
Use of a high performance flow conditioner is an easier way to deal with flow disturbance issues. Decades of testing and use have proven that high-performance flow conditioners are an effective and efficient method of both improving the measurement accuracy of fluid flow and isolating the flow meter from upstream disturbances. A flow conditioner works by creating a state of ideal flow. Flow profile distortions are largely eliminated, and fully developed flow is restored while swirl is reduced by up to 90%-95%. When used with modern multipath Ultrasonic Meters, the flow conditioner can reduce path errors by up to 90%!
Cause #2: Wall Roughness
The condition (or wall roughness) of the pipe wall can also be a significant contributing factor to the quality of the flow measurement. This is because the wall roughness is the single most important factor in the generation of a fully developed flow profile. The shape of the flow profile is due to the wall friction, which is in turn due to a particular wall roughness and flow reynolds number. It is also this wall friction that is responsible for eliminating distorted flow profiles and swirl.
For the best measurement accuracy, the pipe wall must remain constant in its roughness. Varying wall roughnesses can create a distorted flow profile itself due to the uneven wall friction pulling on the flow. This can be caused by dirty or rusty pipe. Walls that are too smooth can result in a reduced ability to repair flow profiles, and reduced swirl due to the reduced friction.
To remedy this issue, a consistent roughness spec should always be used. The pipe wall must be rough enough to maintain a fully developed flow, but not so rough that it causes accumulation and excessive buildup of dirt and other contamination.
Using a flow conditioner can also eliminate a significant amount of concerns of the flow profile quality immediately in the meter tube. The flow profile generation of the flow conditioner will quickly take priority over that of the pipe wall itself, allowing the fully developed flow profile to be restored in a much shorter distance.
Cause #3: Valving
While valves are a necessity for any oil and gas installation, their use can be extremely problematic for measurement accuracy. If a valve is partially closed upstream of a flow meter, this can result in significantly distorted flow and a large measurement error. If there is a downstream control valve that is creating excessive noise, this can cause operational issues with an Ultrasonic meter.
To avoid these issues, ensure that all valves are operating properly and can fully close. You can also isolate the valves with enough piping or a flow conditioner. To mitigate noise, a flow nozzle can be utilized to help relocate the valve pressure drop.
Cause #4: Process Conditions
Piping and installation effects aren’t the only issues contributing to a measurement error. Improper measurement of the process conditions can result in further meter errors. If a gas chromatograph isn’t functioning or sampling properly, the flow computer will be using incorrect flow properties for the resulting flow calculations. Inaccurate temperature or pressure readings can result in incorrect density, speed of sound, discharge coefficient, or reynolds number calculations. Therefore, it is extremely important to ensure that instrumentation is calibrated and operating properly before it is installed in the field, and then verify the operation at regular intervals.
Cause #5: Flow Quality
Flow quality is vital to accurate measurement. Single-phase flow is considered the ideal measurement as it is easier to measure and calculate. Flow measurement becomes very complicated when dealing with multiphase flows. These are situations where there are liquids present in gas applications, or multiple insoluble fluids of different densities in liquid applications. These liquid applications can also contain gas, or have a risk of vapour breakout in flows operating near their bubble point. In general, multiphase flows need to be avoided because they are typically outside the realm of current measurement resources and experience; the operational parameters for many of the flow meters that we use today simply have no way to take into account multiphase flow behavior. These are problems above and beyond ‘simple’ installation effects like swirl and flow profile distortion.
Call Us Today
There are several variables that can cause inaccurate flow measurement. It is vital to be aware of these causes, to act proactively, and to apply preventative solutions to your projects. We invite you to check out our website for more information regarding flow measurement. Call us today to speak to one of our experts: (403) 236-4480
