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King Air Flow Meter Sizing: Heavy Industrial Metal Tube Variable Area Transmitters for Plant Air Systems
Quick Answer: For plant air and instrument air, size a metal tube variable area transmitter based on real operating pressure, temperature, and normal flow. A DN25 or DN50 all metal rotameter with 4-20 mA HART covers most compressed air headers from 2 to 50 bar. Send your pressure, temperature, pipe size, and flow range to Silver Instruments and we will return a model, range, and price.
What King Air Flow Meter Sizing Means in a Plant Air System
Many plants still run legacy King air flow meters on air headers, instrument air lines, or nitrogen purge panels. The sizing logic is simple. You need a metal tube variable area flow meter that does not crack under pressure. You need a transmitter with 4-20 mA output for the control room. You need a scale in Nm3/h, SCFH, or kg/h that matches the real gas density.
Silver Automation Instruments supplies heavy industrial metal tube variable area transmitters for plant air systems. We size replacements for old King air flow meter installations. We also quote new lines where a glass tube rotameter is not safe.
Plant Air Sizing Data You Must Collect
Do not size a variable area flow meter from pipe diameter alone. Gas flow changes with pressure and temperature. Collect these four numbers before you send a request.
Operating pressure in bar or psi. Operating temperature in degrees C. Normal flow rate in Nm3/h or SCFH. Minimum and maximum flow range. Pipe size in DN. Process connection type. Flange or threaded. Gas type. Air, nitrogen, or mixed gas. Ambient temperature if the meter is outdoors.
Here is the thing. Most engineers skip the minimum flow value. Then the meter is too large at startup. The float sits at the bottom. The 4-20 mA signal stays at 4.0 mA. The plant sees zero flow. Sizing must cover the lowest expected flow at night or during low production.
For plant air systems, we often see operating pressure from 4 to 10 bar. Pipe sizes from DN15 to DN100. Flow ranges from 0.5 Nm3/h up to 500 Nm3/h. Tell us if you have wet air or oil carryover. That changes drain port and material choices.
Why Heavy Industrial Metal Tube VA Transmitters Work for Plant Air
Glass tube rotameters are common in labs. They do not belong in heavy industrial plant air service. Metal tube variable area transmitters use a SS316 or stainless steel body. The float is SS316 or PTFE. The indicator is magnetic. No glass breaks. No direct reading tube leaks.
Silver Instruments builds these units with 4-20 mA HART output. Optional ATEX Zone 1 certification covers gas group IIC. We also add local indication. The operator can read the scale through the metal housing. Maintenance does not need to open the pipe.
In practice, a metal tube VA transmitter handles pressure to 40 bar standard. High pressure options go to 100 bar. Temperature ratings go from minus 20 degrees C to 200 degrees C. This covers compressed air, nitrogen, argon, and fuel gas headers.
All SAV series metal tube VA transmitters have a 10:1 turndown ratio. That means a DN25 with 20 Nm3/h upper range reads down to 2 Nm3/h with useful accuracy. Pressure drop at full scale air flow is normally 20 to 60 mbar. This matters in low pressure plant air networks.
Recommended Silver Instruments Models for Plant Air
We recommend three starting poin
SAV-10 DN15 metal tube VA transmitter. Range 0.5 to 5 Nm3/h air at 6 bar. Output 4-20 mA HART. Wetted parts SS316. Accuracy plus or minus 1.6 percent full scale. Good for small instrument air panels.
SAV-25 DN25 metal tube VA transmitter. Range 2 to 20 Nm3/h air at 6 bar. Output 4-20 mA HART. Process connection 1 inch NPT or DN25 PN16 flange. Good for branch air lines and seal gas.
SAV-50 DN50 metal tube VA transmitter. Range 10 to 120 Nm3/h air at 6 bar. Optional ATEX Zone 1. Process connection DN50 PN16 or ANSI 150 2 inch flange. Good for main plant air headers.
For larger lines, we build DN80 and DN100 units. Flow goes up to 500 Nm3/h. We add local totalizer if the plant needs batch or consumption tracking.
We have seen these units on customer sites many times. A paint manufacturer in Southeast Asia replaced a cracked King air flow meter with a SAV-25. A water treatment plant in the Middle East uses SAV-50 units on instrument air. A food packaging line in Vietnam uses SAV-10 units for nitrogen blanketing. Each one sends 4-20 mA to a PLC.
How to Request a Sizing from Silver Instruments
Send us these values. Pressure in bar. Temperature in degrees C. Pipe size DN. Flow range in Nm3/h or kg/h. Gas type. Process connection. Hazardous area class if any.
We will return a model, a flow range, an accuracy statement, and a price. No vague brochure. No waiting two weeks. Our team replies within one working day.
Contact Silver Automation Instruments. Tel: +86-25-68650347. WhatsApp: +86-25-52155837. WeChat: +86 15365082610.
Installation Notes for Plant Air and Instrument Air
Keep the meter vertical. Use straight run of 5 pipe diameters upstream and 3 downstream if you have a valve or elbow. This is standard practice. For metal tube VA transmitters, the manufacturer tag shows flow direction. Install per the arrow.
If wet air is present, add a filter or water separator upstream. Moisture makes the float stick. That causes a noisy 4-20 mA signal. In cold locations, use a heat traced line for outdoor air meters. Frozen condensate blocks the float.
Do not use flexible hoses on heavy industrial air headers. Hard pipe the meter. Support the pipe on both sides. A DN50 all metal rotameter is not light. It needs proper bracing.
FAQ
Can a metal tube variable area transmitter replace a King air flow meter?
Yes. We match the process connection, flow range, and electrical output. Send the old model number if you have it. We will cross the range to a Silver Instruments SAV series unit.
What flow range do I need for plant air?
It depends on pressure, pipe size, and consumer load. For DN25 branch lines at 6 bar, a 2 to 20 Nm3/h range is common. For DN50 main headers, 10 to 120 Nm3/h is a typical start.
Do you offer ATEX Zone 1 certification?
Yes. We offer optional ATEX Zone 1 for gas group IIC. Specify the hazardous area class when you send the sizing request.
What output does the meter provide?
Standard output is 4-20 mA HART. We can add local indication, pulse output, or a totalizer. Tell us your PLC input type.
How fast can Silver Instruments quote a plant air flow meter?
We reply within one working day if you send pressure, temperature, pipe size, and flow range. Many standard SAV series units ship within 7 to 15 days.