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Dall Tube Flow Meter: Low Pressure Loss Differential Pressure Elements
Quick Answer: A Dall tube flow meter is a differential pressure element that creates a measurable DP signal with very low permanent pressure loss. The device handles clean liquid, gas, and steam lines from DN50 to DN1200. Typical accuracy is between ±0.5 percent and ±1.5 percent of rate when matched with a 4-20 mA HART DP transmitter.
Silver Automation Instruments supplies Dall tube flow meters as part of its differential pressure product line. The Dall tube works like a shortened Venturi tube. Fluid enters the inlet cone and passes through a reduced throat section. A high pressure tap sits upstream of the throat. A low pressure tap sits at the throat. The difference between these two pressures gives the flow signal.
Engineers often ask why they should choose a Dall tube over an orifice plate or a Venturi tube. The main reason is energy cost. A Dall tube has a short body and a sharp annular slot in the throat. That design produces a strong differential pressure. At the same time the flow stream recovers most of the pressure downstream. Permanent pressure loss is usually between 2 percent and 5 percent of the differential signal for clean fluids. An orifice plate can lose 50 percent to 70 percent of the differential pressure. This difference matters on large lines with high flow velocity.
Here is the thing. Low pressure loss is not just a lab number. It changes the pump size and the electricity bill. A water utility in Peru replaced a DN300 orifice plate set with a Dall tube in a raw water line. The permanent pressure loss dropped below 0.08 bar at 850 cubic meters per hour. That plant saved enough pump energy to justify the replacement within 14 months.
The Dall tube fits specific process conditions. Use it for clean fluids that do not contain solid particles or sticky deposits. Common applications include cooling water, boiler feed water, compressed air, natural gas, steam, and light hydrocarbons. We supplied a Dall tube element to a chemical plant in Vietnam for a DN150 solvent transfer line. The unit measured toluene flow at 12 bar and 32 °C. That customer wanted a short laying length because the pipe rack was crowded.
When a Dall Tube Makes Sense
Choose a Dall tube when you need low permanent pressure loss and a short installation length. The device is shorter than a classical Venturi tube. For a DN200 line, a Venturi tube may need 1.5 meters of face to face length. A Dall tube of the same beta ratio may need less than 0.8 meters. This saves space in skids and pump stations.
The Dall tube is not covered by ISO 5167 as a standard primary device. That means you need a calibration report or a geometric tolerance report from the manufacturer. Silver Automation Instruments provides a dry calibration certificate for each Dall tube. We calculate the discharge coefficient from the machined dimensions. On request we can also arrange wet calibration at an independent flow laboratory.
Technical Specifications and Selection Data
We build Dall tube flow meters in line sizes from DN50 to DN1200 under the Silver Instruments DLT series. Standard body materials are carbon steel and 316L stainless steel. Other materials are available for corrosive fluids. Process connections follow ASME B16.5 flanges from 150 to 600 psi or EN 1092-1 flanges from PN16 to PN100. Standard temperature range is -40 °C to 350 °C. Higher temperature designs are possibl
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The beta ratio usually sits between 0.4 and 0.75. Accuracy depends on the beta ratio, the upstream straight pipe run, and the DP transmitter. In a typical fully developed flow profile, accuracy is ±0.5 percent to ±1.5 percent of rate. Repeatability is ±0.2 percent of rate. Turndown with a good quality DP transmitter can reach 10 to 1 or better. Standard DP transmitter ranges run from 0-1 kPa up to 0-250 kPa.
Pair the Dall tube with a differential pressure transmitter from Silver Instruments. We recommend a smart DP transmitter with 4-20 mA HART output. For gas and steam service, specify a transmitter with a stainless steel diaphragm and adapters for temperature compensation. For liquid service, a manifold with impulse tubes works well. Provide the flow range, pipe size, fluid, operating pressure, and operating temperature. Our engineers will select the beta ratio and the DP range.
Straight Pipe Run Requirements
The Dall tube needs less straight pipe than an orifice plate in some installations. But you still need enough upstream length to remove swirl. For a single elbow upstream, use at least 12D of straight pipe. For two elbows in the same plane, use at least 15D. For a control valve upstream, use at least 20D. Downstream straight pipe can be as short as 5D. If space is tight, a flow conditioner can reduce these lengths. But adding a flow conditioner adds a small pressure loss. That tradeoff depends on the site.
Frequently Asked Questions
Is a Dall tube the same as a Venturi tube?
No. A Dall tube has a shorter body and an annular throat slot. It produces a similar DP signal but its permanent pressure loss is lower and its physical length is smaller.
What fluids can a Dall tube measure?
Clean liquids, gases, and steam work well. Avoid slurries, fluids with fibers, and sticky fluids. Solid particles can collect at the throat slot and change the discharge coefficient.
Do I need a flow computer?
Not always. For liquid flow, a DP transmitter with square root output can send a linear 4-20 mA signal to a PLC or display. For gas and steam, a flow computer or a PLC with pressure and temperature compensation gives better mass or energy flow data.
Can a Dall tube be used in a short straight pipe section?
Yes, but limits apply. The minimum upstream straight run depends on the upstream fitting and the beta ratio. Provide a site sketch to Silver Automation Instruments and we will confirm the installed accuracy.
What information do I need to send for a quote?
Send your fluid name, flow range, pipe size (DN), pressure (bar), temperature (°C), and flange standard. Tell us if the line is horizontal, vertical, or inclined. That is enough for a first selection.
Request a Quote from Silver Automation Instruments
We supply Dall tube flow meters and matching DP transmitters for export to Southeast Asia, Oceania, Latin America, the Middle East, and Africa. Send us your pressure (bar), temperature (°C), pipe size (DN), and flow range. Call us at +86-25-68650347. Use WhatsApp at +86-25-52155837. WeChat is +86 15365082610. A technical engineer will reply with a model selection and price within one business day. For other process loops we also supply Coriolis mass flow meters, electromagnetic flow meters, ultrasonic flow meters, vortex flow meters, oval gear flow meters, and thermal mass flow meters.
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