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Improving flow measurement accuracy with flow conditioners

One way to reduce plant operating costs is to measure liquid and gas flow more accurately. It goes back to the truism – ‘You can’t control what you don’t measure’.

The first step is to analyse the different media and flow instruments in place, with an eye toward those affecting the costliest processes. Prioritise the search by potential cost impact to achieve the best return on investment.

Some common critical measurement applications are necessary for efficient operations and include pump protection, plant gases distribution, fluid additive/injection monitoring, compressed air consumption, boiler fuel gas and air optimisation, tank blanketing, analyser flow assurance, stack gas monitoring (CEMS, QAL1, MCERTS) and flare gas measurement.

Poor measurement accuracy in these applications can be expensive. The next step is to audit the efficiency of the flow instruments that are performing these critical measurements.

Start by considering which flow measurements are the most critical to efficiently process throughput and the quality of the end product. Are you primarily measuring liquids, gases or steam? Which media is the most expensive to generate and move through the plant?

Once critical processes have been identified and the costs prioritised, look at the flow meters and switches already in place. If the plant moves liquids, gases and steam, several different flow sensing technologies may be installed.

Once the types of flow meters and switches in place are determined, decide whether they need to be replaced due to their age. Also check to see if there are enough devices in place to do the job.

Before making decisions about upgrading flow instruments, operators should think about implementing flow conditioners to enhance existing instruments.

One of the leading causes of inaccurate flow measurement is asymmetrical, or distorted, flow conditions in the upstream and downstream pipe runs from the flow meter or switch. Commonly known as swirl in the pipe this condition ‘confuses’ many flow sensors because their accuracy depends on a steady-state uniform flow regime.

For example, several types of flow meter can be affected by swirl and asymmetrical flow conditions inside the pipe. Depending on the technology, these meter sensing technologies require a minimum of 6-10 upstream and up to 5 downstream straight pipe diameters for accurate measurement.

Unfortunately, most process industry plants are rich with bulky equipment makes installing the required pipe straight runs needed for accurate flow measurement difficult. Often, plant upgrades and retrofits require adding more flow meters in locations that are difficult to accommodate without flow conditioning devices because of their straight-run pipe requirements.

Flow conditioners reduce straight pipe diameter requirements and can correct the flow profile distortions caused by upstream flow disturbances. These unpredictable flow profile variations are neutralised by a flow conditioner, which creates a consistent and predictable outlet flow profile to the flow meter.

The process requirements, especially the viscosity of the process media, will likely shorten the list of potential candidates when selecting a flow conditioner.

Four different flow conditioner designs can accommodate many process requirements:

  • The insertion sleeve-style is the most cost-effective version. It can be installed directly into existing pipe or specially designed to meet unique piping configurations.
  • The meter run-style, tab-flow conditioner offers pipe section replacement for new and existing piping systems. It is a seven-pipe diameter, long-spool piece comprised of three-pipe diameters of flow conditioning internals and a built-in downstream settling chamber.
  • The short run-style, tab-type flow conditioner offers a simple flow conditioning pipe section replacement for new and existing piping systems. It is a three-pipe diameter, long-flow conditioning spool piece with just the tab flow conditioning internals.
  • The elbow-style tab-type conditioner is a 90-degree long radius elbow. It offers flow conditioning elbow replacement for new and existing piping systems. It can isolate irregular velocity profiles and swirl normally like that encountered in a single elbow, and it can isolate irregular velocity profiles entering the flow conditioner.
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