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Emerson Annubar Flow Meter Design: Averaging Pitot Tube Technology for Low Pressure Drop
Quick Answer: The Emerson Annubar flow meter uses a multi port averaging pitot tube to measure differential pressure. It delivers low permanent pressure loss and works in large liquid, gas, and steam lines. For many plant applications, Silver Instruments supplies alternative flow meter technologies with similar low pressure drop results and easier field installation.
How the Emerson Annubar Flow Meter Works
The sensor bar sits across the pipe cross section. Multiple sensing holes face upstream. A downstream port reads the low pressure side. The transmitter converts the differential pressure signal to flow rate. This is not a single point pitot tube. It averages several velocity points. That design reduces the effect of flow profile distortion.
Here is the thing. Most engineers skip this part. The sensor body is a T shaped or diamond shaped bar. The holes are positioned according to a logarithmic distribution. That placement produces a better average velocity signal. The output can be 4-20 mA HART or Foundation Fieldbus depending on the transmitter.
Low Pressure Drop Benefits in Real Plants
In a 500 mm cooling water line, an orifice plate can lose 0.2 to 0.8 bar. An Annubar design can reduce permanent pressure loss to less than 5 percent of the measured differential pressure. In practice, this saves pump energy.
We have seen this on customer sites many times. A water treatment plant in Vietnam replaced an orifice plate with an averaging pitot tube on a 600 mm raw water pipe. Pump power demand dropped by about 8 percent. That number came from their control room logs. The payback was less than 14 months.
Typical Applications and Installation Limits
The Emerson Annubar works on clean liquids, saturated steam, superheated steam, natural gas, compressed air, and water. It is not a good fit for slurries, heavy solids, or sticky fluids. The sensing holes can plug. For wastewater sludge or chemical slurries, a Silver Instruments electromagnetic flow meter works better. For clean water from DN50 to DN2400, the Annubar design performs well.
Pipe orientation matters. The sensor needs a straight run upstream and downstream. For a single elbow, allow 10D to 20D upstream. For two elbows in different planes, allow 30D or more. In tight pipe racks, this is hard. An electromagnetic or ultrasonic clamp on meter may be easier.
Specific Parameters and Recommended Pairings
For saturated steam from DN150 to DN600, use the Silver Instruments vortex flow meter with integrated PT100. It handles temperature to 350 °C and pressure to 25 bar. Output is 4-20 mA HART. For low pressure gas, our thermal mass flow meter reads mass flow directly without separate pressure and temperature transmitters. For liquid chemicals, our electromagnetic flow meter with PTFE liner and Hastelloy electrodes handles conductivity down to 5 µS/cm. Each option includes local display and pulse output.
Does Silver Instrument
Silver Instruments is a process instrument supplier, not an Emerson representative. We service many plants that use Rosemount Annubar models. We supply replacements, calibration support, pressure transmitters, and alternative flow technologies. Tell us your pipe size and fluid. We can recommend a direct replacement or a different technology that reduces maintenance.
For example, a customer in Malaysia asked about an Annubar replacement on a 400 mm chilled water line. We supplied a Silver Instruments electromagnetic flow meter with DN400 PN16 flanges. It installed in one shift. No impulse lines. No DP transmitter. Meter accuracy was plus or minus 0.5 percent of reading.
Hidden Costs of Averaging Pitot Tube Installations
The sensor itself is simple. The total installed cost includes impulse tubing, valve manifolds, DP transmitter, welding, and flow computer. In remote sites, impulse line freezing or condensation is a common issue. We have seen this in the Middle East during winter nights and in offshore wind. The DP signal can drift if the impulse lines are not sealed correctly. Direct insertion meters remove this problem.
But if your application has high temperature steam or large gas lines, an Annubar design is still hard to beat on cost. The sensor costs less than a full bore flow meter. It works in pipes where inline meters become too heavy. A DN1200 inline Coriolis meter is rarely practical. A DN1200 averaging pitot tube is common.
FAQ: Emerson Annubar Flow Meter Design
What is the typical accuracy of an Emerson Annubar flow meter
Accuracy is usually plus or minus 0.5 to 1.0 percent of flow rate when installed with a calibrated DP transmitter and sufficient straight run. Repeatability is better, around plus or minus 0.1 percent.
Which fluids work best with an averaging pitot tube
Clean liquids, steam, air, natural gas, and water work well. Avoid slurries, sludge, sticky hydrocarbons, and fluids with fibers. The sensing holes can plug.
What pipe sizes can an Annubar flow meter handle
Typical sizes range from DN50 to DN2400. The actual limit depends on the model and sensor length. For small lines below DN50, use an inline meter such as an oval gear flow meter for oils or a Coriolis flow meter for mass flow.
How much straight pipe does an Annubar need
Allow 10D to 30D upstream depending on the fitting. Allow 5D downstream. If the pipe has a control valve upstream, add more straight run or use a flow conditioner.
Can Silver Instruments supply spare parts for Emerson Annubar meters
We supply compatible pressure transmitters, valve manifolds, thermowells, and flow computers. Send your model code and tag number. We respond with a quotation and lead time.
Send us your pressure in bar, temperature in °C, pipe size in DN, and flow range. Contact Silver Instruments at Tel: +86-25-68650347. WhatsApp: +86-25-52155837. WeChat: +86 15365082610. You can also visit flow-meter.com.au for our full product range.
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