Annubar Flow Meters
An Annubar is a proprietary type of insertion flow meter based on the averaging pitot principle, used to measure the flow of liquid, gas or steam that flows through a pipe. Various manufacturers make averaging pitot tubes, however Annubar is the brand name of a range of averaging pitot tubes made by Emerson Rosemount. The name annubar has become synonymous with averaging pitot tubes despite other manufacturers marketing their own version e.g. the Torbar made by ABB.
Annubar Bar Working Principle
The averaging pitot tube has multiple vented ports on its upstream side, i.e. higher pressure side, which all lead to a single pressure tapping point. This means the pressure tapping point receives an average upstream pressure.
The upstream ports are blocked from the downstream port by a barrier in the tube. The downstream port is connected directly to a downstream pressure tapping point.
The averaging pitot tube acts as a partial obstruction to the fluid flowing. Flow around this obstruction causes a differentail pressure (DP) to form between the upstream and downstream side of the tube. This DP is proportional to the square of the velocity of the fluid in the pipe, in accordance with Bernoulli’s theorem.
Why use an Annubar?
There are a few reasons why you may consider using an annubar:
Flow Characteristics
In some situations it may be difficult to find a single point in a flow stream profile that represents the average flow. Using an annubar allows the average flow to be approximated.
Installation Cost
Annubars are inexpensive, they do not require special flanges, spool sections, or piping modifications. Common flange types e.g. ANSI/ASME flanges, or even Pak-Lok assemblies can be used. Depending on line size the differentail pressure transmitter is likely to be the most expensive component in the hook up.
Low Pressure Loss
Permanent pressure loss is a big factor when choosing a primary flow element. Annubar sensors creates less permanent pressure loss than virtually any other DP flow primary element.
Suitable for Large Diameter Lines
The forces exerted on a primary flow element can be large, especially in large diameter lines. Annubars are available in models that extend the entire diameter of the pipe for support at both ends. This makes them robust, yet economical for use in larger pipelines.
Annubar Straight Length Requirements
A minimum number of upstream and downstream straight run pipe runs are recommended when installing an annubar as obstructions in the pipe, e.g. valves, bends, elbows etc will have an adverse impact on the flow profile. Although you should always consult the manufacturer to discuss your own specific application, the following can be taken as an approximate guide:
Pipe Bends; Allow a minimum of 8 upstream straight lengeths and 4 downstream straight lengeths.
Pipe Elbows; Allow a minimum of 23 upstream straight lengeths and 4 downstream straight lengeths.
Pipe Reducer; Allow a minimum of 12 upstream straight lengeths and 4 downstream straight lengeths.
Pipe Expander; Allow a minimum of 18 upstream straight lengeths and 4 downstream straight lengeths.
Upstream Valve; Allow a minimum of 30 upstream straight lengeths and 4 downstream straight lengeths.
A flow straightener or straightening vanes may be used to reduce the length of straight run required.
Disadvantages of Annubars
Dirty Fluids
Dirty fluids can block the the ports of an annubar. There are two methods of cleaning an Annubar to restore performance. Mechanical cleaning is the more certain method, but does require removal of the Annubar primary element. Alternatively purging the Annubar with an external fluid source under a higher pressure can prove effective.
High Viscosity Fluids
In general it is not recommended to use an annubar in flows where the viscosity approaches or exceeds 50 centipoise.
Technical Library
The following pages on Control and Instrumentation.com give more information, and explore in more detail topics associated with flow measurement basics:
