Minimising the installation effect of flowmeters
14 January 2014
Craig Marshall, project engineer at NEL, discusses how and why installation effects cause errors in flow measurement, and looks at ways of reducing this problem.
Figure 1: Asymmetrical flow profile caused by an installation effect.
Flow meters are generally designed to work best under ‘ideal’ conditions. The International Standards or the meter manufacturer will advise on the best installation and will usually specify a length of straight pipe upstream and downstream of the meter.
However, one of the biggest problems with industrial metering systems is a failure at the design stage to appreciate the importance of providing the meter with as near to an ideal flow condition as possible. This is particularly relevant in the offshore oil and gas industry, where space is often limited and significant straight lengths of pipe are not, generally, feasible.
In a typical industrial application it may not be possible to achieve fully developed flow, due to pipe work configuration or the placement of in-line equipment such as isolation or control valves. The term ‘installation effects’ is used to describe any source, that can disturb or skew the velocity profile in the pipe away from ideal conditions such as bends, valves or reducers. These installation effects can cause an error in flow measurement that is dependent on the meter type and measurement principle.
The Reynolds number
It is important to understand how fluid actually flows in a pipe and its dependence on a non-dimensional parameter called Reynolds number. Reynolds number is defined as the ratio of inertial forces to viscous forces and, depending on which force dominates, this will determine the nature of the fluid flow within the pipe.
If the Reynolds number is less than 2,000, then the viscous forces are dominant and this means that the flow is laminar - fluid moving along in thin layers with very little mixing between the layers. If the Reynolds number is greater than 5,000, then the inertial forces generally dominate, and this means flow is turbulent - the bulk motion is parallel to the pipe’s axis, but with mixing between the layers. Transitional flow lies between the ranges of the laminar and turbulent regions, switching unpredictably back and forth between the two, or sometimes a mixture of both.
In the majority of cases, inertial forces dominate as Reynolds numbers are typically above 100,000 in liquids and in the millions for gas applications. Laminar flow can be encountered, but usually only in very low flows or with high viscosity fluids. The majority of flow meter technologies are Reynolds number dependent, so it is always important to know the Reynolds number occurring in a pipe line during operation to ensure the most appropriate correction factors are used.
When fluid flows through a pipe, the velocity is not constant across the entire cross-section of the pipe. Due to frictional losses at the pipe wall, a velocity gradient forms in the cross-section and this is the velocity profile.
While a velocity profile takes time to become fully developed, when it does stabilise, it is commonly referred to as an ‘ideal’ profile. An ideal profile develops within a length of pipe which, theoretically, is of infinite length. In reality though, an engineer is looking for the length of pipe which will allow the flow to develop a profile that is very close to ideal. For example, a fluid travelling around a bend will move more quickly on one side of the bend, giving a skewed or asymmetric profile. Figure 1 shows an asymmetrical profile caused by an installation effect.
Asymmetry is not the only issue associated with bends: two or more bends out-of-plane can cause a single or multiple vortex rotation of the fluid in a non-axial direction. This phenomenon is known as swirl and, in some cases, can take over 100 pipe diameters (D) to decay. There have even been cases where swirl has persisted for over 500D before the profile resembles a fully developed one.
Reducing installation effects
Installation effects can cause serious mis-measurement if not dealt with properly. There are a variety of methods to reduce or remove installation effects on meter performance which may or may not be applicable to all applications.
A meter can be relocated to a more suitable location with adequate upstream and downstream straight lengths of pipe. This will allow a natural correction of the velocity profile and fully developed flow to be achieved. However, depending on the profile distortion and amount of swirl present, this may not be feasible with some poor profiles requiring over 100D of straight lengths to achieve fully developed flow. Again, this is particularly true on offshore platforms where space is at a premium.
Methods of correcting the flow profile are also in common use. Flow conditioners or flow straighteners will reduce the amount of straight pipe needed but will not completely eliminate it. Neither are they the complete answer to reducing the effect of flow disturbances due to the pressure drop caused.
Another reason conditioners and straighteners are sometimes avoided is their increased likelihood of partially blocking. This has the opposite effect than intended and will act to disturb the flow profile even more.
Another method of dealing with installation effects is the use of secondary diagnostics. Some meters have the ability to detect when non-ideal conditions are present and can alert users to when this occurs. Moreover, the use of diagnostics is not limited to the detection of installation effects. If confidence can be built in the relationship between distorted flow profiles and meter performance for various meter types then it may be possible to correct for them. By using the diagnostics to estimate the levels of asymmetry or swirl in the flow, a model could potentially produce a correction factor to remove the effect on meter performance. In the near future, the use of diagnostics could eliminate the errors associated with installation effects and other issues that affect meter performance.
NEL is a provider of specialist technical consultancy, research, development, testing, measurement and programme management services, to the energy, and oil & gas industries, as well as government. Part of the TÜV SÜD Group, it is a global centre of excellence for flow measurement and fluid flow systems and is the custodian of the UK’s National Flow Measurement Standards.
Contact Details and Archive...
Most Viewed Articles...