Vortex flow meters are widely used for steam measurement, especially when equipped with temperature and pressure compensation to determine mass flow rates. However, due to varying environments and the nature of steam, distinctions between superheated and saturated steam are crucial for accurate measurements. This article discusses the nuances of measuring steam with vortex flow meters and how to account for these variables.

Understanding Superheated and Saturated Steam

    The density of steam, which can affect measurement results, changes with temperature and pressure. Steam is categorized into superheated steam and saturated steam, each requiring different measurement considerations.

1. Superheated Steam

    In flow computers, the density and thus the mass flow of superheated steam can be derived from temperature and pressure charts. However, during long-distance transport or due to inadequate pipe insulation, superheated steam may lose heat, reducing its temperature and entering a critical saturation state or even condensing into water droplets, turning into wet saturated steam.

Impact on Vortex Flow Meters:

• The output of a vortex flow meter is proportional to the fluid velocity passing through the measurement tube.

• When measuring wet saturated steam, the impact of water droplets on the meter's output is negligible.

• The output is considered to be caused by the dry part of the wet saturated steam, whose density can be accurately determined through pressure or temperature compensation.

Billing Implications:

• If the agreement is to bill only for the dry part of the steam, the impact of condensate on measurement is minimal and can be disregarded.

• If condensate is billed as steam, the measurement result from the vortex flow meter will be lower.

2. Saturated Steam

    When a vortex flow meter is installed after a pressure reducing valve, a significant pressure drop in saturated steam can cause adiabatic expansion, partial evaporation of water droplets, and a decrease in the temperature of both phases as they absorb heat. If the temperature drop is minimal or the humidity is high before evaporation, the steam will quickly reach a new saturation temperature corresponding to the new pressure, maintaining saturated steam status. If the pressure drops significantly or the humidity is low, the steam may become superheated.

Impact on Measurement:

• If the design anticipates steam becoming superheated or uses temperature and pressure compensation, the phase change does not affect the measurement.

• If designed for saturated steam using only pressure compensation, a small error may occur due to the density difference between superheated and saturated steam temperatures.

• If designed for saturated steam using only temperature compensation, significant errors may arise.

Solutions to Measurement Challenges

1. Installing the Flow Meter Before the Pressure Reducing Valve: This ensures that the steam does not undergo phase changes, maintaining measurement accuracy.

2. Adding a Pressure Transmitter: If the flow meter must be installed after the valve, a pressure transmitter can be used for temperature and pressure compensation.

3. Using a Constant Pressure Value: If the pressure reducing valve is stable, the upstream pressure value can be set as a constant in the display instrument for temperature and pressure compensation.

    Accurate measurement of steam using vortex flow meters requires understanding the state of the steam and the environmental conditions it encounters. By considering these factors and applying the appropriate compensation methods, you can ensure reliable and precise measurements.