Measure for measure

Using CFD to improve flare measurement accuracy. By Marc Laing

Gas flaring is an important safety critical operation during hydrocarbon production, required by law to be part of the pressure relief system on all platforms. The use of flares is not routine, and the flare system is used to burn the excess hydrocarbons that cannot be recovered or recycled during emergency situations or abnormal operation – so called ‘flaring events’.

The flare must be permanently lit ready for use, and so whilst it largely prevents emission of hydrocarbons to the atmosphere, it does instead lead to the emission of CO2. However, due to imperfect combustion, especially during flaring events, there is still a small, unintended emission of hydrocarbons. Another point of note is that whilst the pilot light of the flare may burn continuously, the vast majority of the emissions occur during the ‘flaring events’ where the flow rates are many orders of magnitude higher, hence any flowmetering system must be able to accurately cover this large range of flow rates, i.e. have a very high turndown ratio.

In recent years, environmental regulations have imposed limits on flaring and failure to comply with these limits can result in highly punitive fines being incurred. Therefore, it is of critical importance to ensure that the quantity of flare gas is metered accurately.

However, gas flaring is not a simple measurement process, primarily due to the large variations in conditions often found in a flare stack. In order to fully appreciate the challenges involved in measuring flare gas, it is important to By Marc Laing first appreciate these varying conditions and the effect they have on measurement.

Firstly, it is worth noting that flares are usually of bespoke design and are entirely based on individual production sites. For example, an installation may have one flare to deal with all operating conditions, while others may have two or more often differentiating between low pressure (LP) & high pressure (HP) applications. Typically, HP flares are more complex when compared with LP flares and require additional equipment to achieve ideal flaring conditions.

Given that flare gas is essentially a waste product, operators prefer to use cheaper metering technologies, and traditionally flare gas systems use single path ultrasonic flow meters (USMs). The flowrate in a flare gas line is highly variable, but in general the flowrate will be low. However, this can instantaneously increase by one, or even two, orders of magnitude during a blowdown situation, which requires a very high turndown ratio in the meter and a USM is typically the best available technology to achieve this requirement. Although blowdown conditions represent only a very small amount of time, they have a dominating effect on the annual quantity of gas emitted, thus accurate metering of these conditions is important.

Single path USMs are by design very sensitive to disturbances in flow profile. The general requirements of such a flow meter are that it is installed with 20 pipe diameters of straight undisturbed flow upstream of the meter, and ten pipe diameters of straight undisturbed flow downstream (data for a GF868 single path USM). Ideally, within this 20D upstream and 5D downstream, there should be no valves, expansions or elbows.

In flare lines, due to space constraints on offshore platforms, it is often the case that a perfect location with ideal upstream and downstream setup does not exist. The disturbances in the flow path upstream of the USM make accurate measurement even more difficult to achieve. To predict the magnitude and nature of any metering error, it is likely an auditor will require Computational Fluid Dynamics (CFD) analysis to be carried out.

To use CFD to model bespoke installations and determine flare gas metering errors, it is necessary to construct a 3D Computer Aided Design (CAD) model to represent the installation. The CAD model is then meshed in order to solve the fluid mechanics. The partial differential equations that govern fluid flow and heat transfer are highly non-linear and must be solved numerically. Therefore, to analyse fluid flows, flow domains are split into smaller sub domains. The governing equations are then numerically discretised and solved inside each of these subdomains. The subdomains are often called finite volumes, elements or cells, and the collection of all elements is called a mesh.

CFD is a very powerful tool when modelling flare gas meters as it is possible to predict the fluid velocity in the direction of the transducer path (or paths). This must be done twice – once for the as-installed case and once for what would be an ideal installation (fully developed flow profile). Once the transducer path velocity data is extracted from CFD, the error can be calculated, based on calculating the volumetric flowrate of the ideal installation and comparing that to the volumetric flowrate predicted for the as-installed case. An example of the as-installed velocity profile is shown top right.

Once the calculations have been carried out using CFD, correction factors can be applied directly to the flow computer and the error correction is done in real time on the meter. This means that no burden is placed on an operator to perform corrections and potential sources of permanent error are removed.

The error correction is carried out in the form of providing velocity distribution factors (VDF’s) to the flow computer. These can be generated easily once the CFD data is available. It is also important that the CFD itself is accurate when modelling flare line installations. Single phase CFD simulations are now very well established with a vast amount of available test data to validate models where required.

As flare legislation becomes more rigorous, and emission trading schemes are developed and rolled-out worldwide, there will be a pressing need for operators to invest in technologies such as CFD, that can assist in overcoming the many challenges of flare gas measurement.

TÜV SÜD National Enginering Laboratory
Modelling at TÜV SÜD National Engineering Laboratory (NEL), a global centre of excellence for flow measurement and fluid flow systems and the National Measurement Institute responsible for the UK’s National Flow Measurement Standards.
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