Which type of flowmeter for my steam application?
In Part 1 of this series of Tutorial articles on different types of flowmeters for steam applications, we covered the advantages, disadvantages and typical applications for Orifice plate flowmeters, Turbine flowmeters and Variable area flowmeters.
Different types of flowmeters have their own advantages, limitations & application benefits. To ensure accurate and consistent performance from a steam flowmeter, it is essential to match the flowmeter to the application.
This Tutorial article will cover:
• Spring loaded variable area flowmeters
• Pitot tube flowmeters
• Vortex shedding flowmeters
Follow the links for more tutorials on flowmeters:
Spring loaded variable area flowmeters
- Turndowns of 25:1 are achievable with normal steam velocities (25 m/s), although high velocities can be tolerated on an intermittent basis, offering turndowns of up to 40:1.
- Accuracy is ±2% of actual value.
- Can be tailored for saturated steam systems with temperature and pressure sensors to provide pressure compensation.
- Relatively low cost.
- Short installation length.
- Size limited to DN100.
- Can be damaged over a long period by poor quality (wet and dirty) steam, at prolonged high velocity (>30 m/s).
- Flow metering of steam to individual plants.
- Small boiler houses.
Pitot tube flowmeters
- Presents little resistance to flow.
- Inexpensive to buy.
- Simple types can be used on different diameter pipes.
- Turndown is limited to approximately 4:1 by the square root relationship between pressure and velocity as discussed in Tutorial 4.2.
- If steam is wet, the bottom holes can become effectively blocked. To counter this, some models can be installed horizontally.
- Sensitive to changes in turbulence and needs careful installation and maintenance.
- The low pressure drop measured by the unit, increases uncertainty, especially on steam.
- Placement inside the pipework is critical.
- Occasional use to provide an indication of flowrate
- Determining the range over which a more appropriate steam flowmeter may be used.
Vortex shedding flowmeters
- Reasonable turndown (providing high velocities and high pressure drops are acceptable)
- No moving parts.
- Little resistance to flow.
- At low flows, pulses are not generated and the flowmeter can read low or even zero.
- Maximum flowrates are often quoted at velocities of 80 or 100 m/s, which would give severe problems in steam systems, especially if the steam is wet and/or dirty. Lower velocities found in steam pipes will reduce the capacity of vortex flowmeters.
- Vibration can cause errors in accuracy.
- Correct installation is critical as a protruding gasket or weld beads can cause vortices to form, leading to inaccuracy.
- Long, clear lengths of upstream pipework must be provided, as for orifice plate flowmeters.
- Direct steam measurements at both boiler and point of use locations.
- Natural gas measurements for boiler fuel flow.
Click here to read part 1 of the flowmeter tutorials.