Поплавковый расходомер с металлической трубкой

A metal tube float flowmeter operates on the variable area flow measurement principle, where the flow rate is determined by the equilibrium position of a float inside a vertically mounted, tapered metal measuring tube.

The float reaches a stable position when the following forces are balanced:

• Upward force generated by fluid flow

• Gravity acting on the float

• Buoyancy force of the medium

• Drag force related to fluid density and viscosity

Therefore, the measurement result directly depends on:

• Medium density

• Medium viscosity

• Flow regime (laminar or transitional)

• Float geometry and mass

This is why metal tube float flowmeters are factory-calibrated for a specific medium and operating condition, and recalibration or correction is required if the process conditions change.

Metal tube float flowmeters are technically suitable for stable, continuous flow measurement under harsh process conditions, but they are not universal instruments.

Suitable Conditions:

• High pressure systems: up to 25–40 MPa

• High temperature processes: up to 300–450°C

• Corrosive fluids: with proper material or lining

• Steam, gas, and liquid applications

• Low to medium flow rates with stable flow

Unsuitable or Limited Conditions:

• Strongly pulsating flow

• Highly unstable density or viscosity

• Applications requiring very high accuracy (< ±0.5%)

• Rapidly changing process conditions

• Severe vibration without proper damping

From an engineering standpoint, correct application matching is more critical than the flowmeter model itself.

The typical accuracy of a metal tube float flowmeter is ±1.0% to ±1.5% of full scale (FS) under rated conditions.

Accuracy Influencing Factors:

1. Medium Density Deviation
   Any deviation from the calibrated density will cause proportional flow error.

2. Viscosity Change
   Increased viscosity increases drag force, causing the float to rise higher than expected.

3. Installation Conditions

   • Must be installed vertically

   • Straight pipe lengths upstream improve stability

4. Temperature & Pressure Variation
   Extreme changes may alter medium properties and affect readings.

In practical industrial applications, metal tube float flowmeters are used for process indication, monitoring, and control, not custody transfer.

Material selection directly determines safety, service life, and measurement stability.

Common Material Configurations:

Incorrect material selection may lead to:

• Float sticking

• Chamber corrosion

• Measurement drift

• Safety risks

From a procurement perspective, material compatibility confirmation is mandatory before ordering.

Modern metal tube float flowmeters use magnetic coupling to transmit float position to external devices without process penetration.

Signal Options:

• 4–20 mA two-wire transmitter

• Local mechanical pointer

• Digital LED display

• Limit switches (SPST / SPDT)

Signal Reliability:

• No direct contact with the medium

• Immune to vibration and temperature fluctuations

• Long-term stability for PLC and DCS integration

Typical applications include:

• Flow interlocks

• Low / high flow alarms

• Simple closed-loop control

Metal tube flowmeters are preferred when signal stability and mechanical robustness are more important than ultra-high accuracy.

Based on field feedback, the most frequent procurement errors include:

1. Selecting flow range too close to maximum operating flow
   → Causes float saturation and poor resolution

2. Ignoring medium density and viscosity
   → Results in systematic measurement error

3. Using standard materials for corrosive media
   → Leads to premature failure

4. Assuming metal tube flowmeters are maintenance-free
   → Periodic inspection is still required in dirty media

5. Using them in highly fluctuating flow systems
   → Leads to unstable readings

A properly selected metal tube float flowmeter should operate reliably for 10–15 years with minimal intervention.