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Quick Answer: An Annubar flow meter is a type of averaging pitot tube that measures the flow rate of liquids, gases, and steam by sensing the difference between impact pressure and static pressure across a pipe cross-section. It creates a permanent pressure drop much lower than an orifice plate, saving thousands of dollars in pumping costs each year on large pipe sizes like DN300 and above. You can use it for steam feed lines in a textile factory, cooling water in a petrochemical plant, or compressed air in an automotive assembly line, and get a stable 4-20 mA HART signal without cutting the pipe.
Most engineers learn about pitot tubes in school, then forget about them. A standard pitot tube measures velocity at a single point. An Annubar is different. Its sensing bar goes across the entire pipe diameter, and multiple sensing ports face upstream. Each port sees a local velocity pressure. Inside the bar, an averaging chamber blends these values, so the output represents the true average flow profile, not just one spot near the wall or centerline. That is the simple but powerful reason why an Annubar can hold ±0.5% to ±1% of rate accuracy in fully turbulent flow.
Silver Automation Instruments supplies insertion-type averaging pitot tubes with direct-mounted differential pressure transmitters. The sensor body is often made of 316L stainless steel or Hastelloy C for corrosive gases. You will see models with a T-shape or bullet-shaped profile that sheds vortices at a fixed point to isolate the low-pressure port from turbulent wake mess. In practice, this design keeps the DP signal steady even when the pipe Reynolds number changes a lot.
The fundamental math is the same as any differential pressure device. Flow rate is proportional to the square root of the differential pressure. If you read 10 mbar at 50 m³/h, you will see 40 mbar at 100 m³/h. A multivariable transmitter adds PT100 temperature input and an absolute pressure sensor, and it can output mass flow of steam in kg/h directly via 4-20 mA HART or Modbus RTU. We have seen a biomass boiler plant in Thailand install six DN400 Annubars with integral temperature compensation and cut their saturated steam measurement cost by 60% compared to a full-bore vortex meter solution.
Because the sensor insertion depth can be supplied to match your exact pipe schedule, even large pipe ID deviations do not cause big errors. A 42-inch cooling water pipe at a desalination plant in Saudi Arabia does not need a huge spool piece. You simply weld a 1.5-inch or 2-inch mounting fitting, insert the probe through a ball valve hot-tap assembly, and lock it in place. That is a full weekend of downtime saved.
Orifice plates are cheap to buy and expensive to own. Their permanent pressure loss is around 60% to 80% of the measured DP. An Annubar produces only 5% to 15% of the measured DP as permanent loss. Take a DN500 natural gas pipeline running at 20 bar. Switching from an orifice to an averaging pitot tube can save an operator over 15 000 USD in compressor energy over five years. That number is not theory. A gas distribution company in Argentina verified it across four metering stations.
There is another pain point. Orifice plates collect dirt upstream and the sharp edge wears out in wet steam or slurry. An Annubar with a stepped-shape sensor does not trap solids and can handle some entrained liquid droplets. We recommend it for saturated steam with dryness fraction above 0.95, or for air with trace oil mist, but not for raw sewage with rags and stones.
On the spec sheet you will see accuracy of ±0.5% of rate if the flow profile is fully developed. Real plant piping rarely gives perfect straight runs. A rule of thumb from the field: keep 15D straight upstream and 5D downstream for a single 90-degree elbow. If you have two elbows in different planes or a control valve close by, install a flow straightener or accept ±1.5% to ±2% uncertainty. Most process engineers are fine with that for balance of plant steam or plant compressed air. They are not doing custody transfer. They just want to know which shift uses more compressed air and whether the boiler efficiency is dropping.
You can get a flanged version, a compression fitting version, or a hot-tap retractable version for dirty gas that needs periodic cleaning. For a DN80 pipe carrying heavy fuel oil at 150 °C, a welded thread-o-let with a 316L probe and PTFE ferrule works well. For a large duct, say 2-meter flue gas line, multiple probes can be manifolded together to overcome stratification. Silver Automation Instruments builds duplex or duplex stainless probes for seawater applications where crevice corrosion kills 304 steel in months.
To get a specific model recommendation and a price with delivery time to your port, just send these four items to [email protected]: pipe inner diameter (DN or inches), fluid name and state (liquid, gas, steam), operating pressure in bar or psi, operating temperature in °C, and the maximum and minimum flow rate. We will reply with a data sheet showing the calculated DP span, permanent pressure loss, and transmitter setup within one working day. You can also reach us by Tel: +86-25-68650347, Whatsapp: +86-25-52155837, or WeChat: +86 15365082610.
Q: Can I use an Annubar for dirty, sticky fluids like crude oil with wax?
A: Normally no. Wax can block the small sensing ports. A thermal mass meter or a Coriolis meter usually fits better here. In some light slurry applications, a purged system helps, but it adds maintenance.
Q: What is the typical turndown of an averaging pitot tube?
A: With a modern DP transmitter, you can get 10:1 turndown on flow. The square root relationship forces the DP signal to drop fast at low flow, so below 9% of max flow the uncertainty rises sharply.
Q: Do I need straight pipe runs upstream and downstream?
A: Yes. Minimum 15D upstream and 5D downstream for a single elbow. Tight piping needs a flow conditioner. We can advise after seeing your piping isometric sketch.
Q: Can an Annubar measure bidirectional flow?
A: A standard model measures in one direction only. Some special symmetrical probes with a bi-directional DP transmitter can handle forward and reverse flow, often used in underground gas storage wells.
Q: Which transmitter works best: DP only or multivariable?
A: For saturated steam or gases where density changes a lot, a multivariable transmitter with PT100 input gives direct mass flow reading. For water and air with stable density, a simple DP transmitter is enough.
