How to Select the Right Steam Flow Meter for Your Application
Steam is essential to many industrial operations, such as chemical plants and refineries, food processing, district heating systems, and geothermal and nuclear plants using steam turbines to generate electricity. Accurate measurement is critical for improving processes, productivity, and energy efficiency while measuring the cost of steam and reducing energy and maintenance costs.
Choosing the right steam flow meter is critical to supplying the optimal amount of steam, minimizing operating costs, and complying with government regulations—while supporting overall plant profitability. However, selecting the right steam flow meter for your application isn’t always straightforward and often requires balancing accuracy, installation constraints, and long- term costs.
In this blog, you’ll learn:
- Why is steam flow difficult to measure
- Which flow meter technologies work best for steam
- Key application factors that affect accuracy and costs
- When vortex flow meters outperform alternative technologies
The Difficulties of Measuring Steam Flow Rates
Steam flow measurement is one of the most challenging measurements to make. Flow rates for saturated and superheated steam are difficult to measure because the density of saturated steam varies with either steam temperature or steam pressure. Superheated steam varies with both steam temperature and steam pressure.
These physical characteristics mean that volumetric flow meters developed for liquids or gases, such as orifice plate differential pressure meters, may not be the best choice for measuring steam.
Instead, using advanced technology like a multivariable vortex flowmeter is more reliable and beneficial to the end-user. Multivariable vortex meters are the ideal measuring device for saturated and superheated steam, providing key data to improve steam production efficiency and allocation.
Application Considerations for Selecting a Steam Flow Meter
To measure steam flow data repeatably and accurately, steam flow meters must match the intended application conditions. These measuring devices are highly specialized and each one is made to order based on the customer’s specific application, in terms of pipe size, expected flow rates, steam type, environmental conditions, and more.
Pipe Size
The pipe diameter is a critical dimension for sizing the flow meter. Some meters, such as the orifice plate, must be manufactured to fit precisely inside the pipe. Other types, such as ultrasonic meters, do not need to be inserted in the pipe. However, all types of meters must be calibrated to measure flow within the specific pipe size.
Flow Rate Range and Turndown
When specifying a steam flow meter, you must know the required flow range, which is the span of flow rates over which the meter can measure accurately and repeatably. The turndown would then be calculated as the ratio of the highest to lowest flow rate that the meter can measure, for example, 30:1.1
Steam Quality (Dry or Wet)
When specifying a steam flow meter, it's important to know the type of steam you will be measuring, as this determines how the flow meter is configured at the factory. Some types of meters can handle certain conditions that others can’t, and an incorrect setup results in incorrect steam flow readings.
Wet steam is steam that still contains water droplets that have not evaporated or that have
recondensed as the steam flows through the pipe. Steam that contains less than 2% water droplets by weight (mass) is considered dry saturated steam. Finally, steam that contains no water droplets but is heated above the saturation point is called superheated steam.
This means that choosing the wrong flow meter can result in measuring incorrectly, which can be a costly mistake.
Operating Conditions (Pressure & Temperature)
The equipment selected must be rated for the steam pressure and temperature limits expected for the application.
Installation Requirements (Space Availability/Limitations)
As steam flows through straight pipes, the flow profile is naturally smoothed and conditioned. However, the pipe layout, with bends, tees, constrictions, inlets, and outlets, disturbs this smooth flow profile, so it’s difficult for flow meters to get an accurate reading. For this reason, the length of straight run available before and after the meter is an important factor in choosing and installing a flow meter.
The required straight runs are determined by the meter technology as well as the layout. If there is not enough straight run available, flow conditioners placed before and after the meter often can shorten the straight run requirements for that meter.
One of the greatest challenges to measuring steam flow is shutting down an existing process to install a flow meter. Choosing a meter with a hot tap, such as the InnovaMass 241s insertion vortex flow meter with a retractor, allows for installation without process shutdown.
The hot tap meter is welded onto the outside of the pipe with an adapter and ball valve that will allow the sensor to be installed and lowered into the pipe without interrupting the flow of steam. By contrast, a cold tap meter requires the operation to shut down while the meter is installed, which can add greatly to the cost.
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Sierra InnovaMass with Retractor
Vortex Flow Meter Isolation Valve
Types of Steam Flow Meters
Several different types of steam flow meters are available, and each has benefits and drawbacks. The technology for each type is widely different and the accuracy and repeatability, as well as the appropriate applications, vary for each.
Here are some types of flow meters that are used for measuring steam flow:
- Orifice plate flow meters
- Turbine flow meters
- Different versions of variable area flow meters
- Ultrasonic flow meters
- Vortex shedding flow meters
In this article, we will examine three of the most common types – orifice plate, ultrasonic, and vortex.
Orifice Plate Flow Meters
Orifice plate meters are a type of differential pressure device that uses a plate with a small orifice hole to block the fluid flow through the pipe. The fluid pressures before and after the blockage are measured and compared, and the pressure differential (ΔP) is used to calculate the volumetric flow rate.
Differential-pressure devices can be used for extreme pressures and temperatures and are suitable for liquids, gases, and steam. However, their low turndown ratio (4:1) may deliver erroneous readings at low pressures.
In the case of steam, changes in pressure and/or temperature will change the density and therefore DP meters require compensating devices to measure the mass flow rate.
Even a “small” change of 10% in steam pressure will result in a 10% error in non-compensated mass flow. This means that, in a typical orifice plate installation, the volumetric flow rate measured by the device must be compensated by measuring temperature and pressure, and then these three measurements (ΔP, T, and P) are integrated with a flow computer to calculate the mass flow rate.
This will require two additional meters to be installed to measure temperature and pressure, along with the orifice plate and two differential pressure meters. Outputs from all of these must then be routed separately to a computer to calculate the mass flow rate.
Ultrasonic Flow Meters
Ultrasonic flow meters pulse soundwaves through a flowing fluid and measure the reflected wave. The wave moves faster with the flow than against the flow, and this difference in transit time is proportional to fluid velocity.
This type of meter has no moving parts, so maintenance is minimal. They do not obstruct the flow path and tend to be extremely accurate, but they also can be expensive. Ultrasonic flow meters only work ultra dry steam and very low flow rates and pressures. So most typical steam applications do not fit into these limitations.
Vortex Shedding Flow Meters
A vortex meter uses the concept of vortices, which are swirls of fluid caused by an obstruction in the flow. The meter is inserted into the pipe where it places a wedge-shaped vortex shedder bar with two sharp edges directly in the flow. The edges of the bar create an alternating series of vortices flowing from the left and right sides of the bar down a “vortex street.”
Measurement Principle of Vortex Flow Meters
The vortices appear with a frequency that is proportional to the fluid velocity. A sensor bar placed in the flow after the shredder bar measures the vortex frequency over time, which is then converted to a volumetric flow rate by an onboard computer.
Like many devices commonly used for steam flow, vortex shedding meters are inherently volumetric flow measurements. Any change in pressure or temperature will change the steam’s mass flow rate, which can create significant errors in measurement.
This means that in a typical installation, the volumetric flow rate must be compensated by adding sensors to measure temperature and pressure. These three measurements (actual steam velocity, T, and P) then are integrated with a flow computer to calculate mass flow.
A more advanced version of the vortex flow meter, the multivariable vortex mass flow meter, includes a velocity-temperature-pressure “VTP” sensing head built into the body of the meter. The onboard computer uses these three variables to calculate mass flow rate, density, and/or volumetric flow rate, depending on requirements.
Remember that the density of saturated steam varies with either steam temperature or steam pressure, while superheated steam varies with steam temperature and steam pressure.
Insertion multivariable vortex flow meters allow one instrument and one process connection to simultaneously measure mass flow rate, temperature, steam pressure, volumetric flow rate, and fluid density. So, no matter which type of steam is being measured, multivariable vortex flow meters ensure the flow meter’s density calculations and mass steam flow rate are correct.
Available Models
InnovaMass 240S Inline and 241S Insertion Vortex Volumetric and Multivariable Steam Flow Meters
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Benefits
Multivariable vortex flow meters from Sierra Instruments provide steam accuracy of +/-1% of reading, 30:1 turndown plus pressure and temperature compensation.
- Highly reliable with no moving parts
- Ideal for saturated or superheated steam, gas, and liquid
- Flexibility with volumetric or multivariable versions
- Multivariable for five measurements in one device (mass flow, volume, density, pressure, temperature)
- Energy (BTU) Monitoring for real-time monitoring of energy consumption
- No leaks with all welded gasket-free flow body
- Loop power option available
- Hot tap solutions available
Applications
Our InnovaMass vortex flow meter product line includes the 240S inline and 241S insertion models, built for accurate volumetric or mass flow measurement of liquids, gases, and steam.
The 240S is an inline flow meter that is a true workhorse in applications ranging from large pipes and ducts to high-pressure and saturated or superheated steam. The 241S is an insertion probe designed for pipes and ducts up to 72 inches (2 m) in diameter. It is available with an optional hot tap, so no process shutdown is required for installation.
InnovaMass vortex shedding flow meters are commonly used in a wide range of industrial applications, including:
- Chemical plants and refineries
- Food & Beverage
- District Heating Systems
- Geothermal
- Boiler systems
- Gas processing Plants
- Power Plants
- Glass & Ceramics
Steam Flow Meter Cost Considerations
It is important to find the best long-term value for the application when considering the total cost of ownership for a steam flow meter. The initial purchase price of the meter should be balanced with the additional costs of installing, calibrating, and maintaining the meter.
Some meter designs require more maintenance than others, which adds costs for labor and supplies. For example, orifice plate meters are particularly sensitive to the formation of scale and so exhibit frequent plugging and also require yearly maintenance of the plate.
For some types of meters, the cost will increase proportionately with the size of the pipe to be metered, but that is not always the case. For example, ultrasonic meters do not need to be inserted into the flow stream, so pipe size is not a significant factor in the price.
When planning the plant layout and the location for installing a steam flow meter, it’s important to respect the upstream and downstream straight pipe run limits established by the manufacturer. Some meters work better than others in short-run situations, but flow straighteners can be added to the pipe to reduce the length of run required for accurate measurement.
An important factor to also consider is the cost of purchasing, installing, and maintaining any extra equipment required along with the meter. Traditional velocity flow systems require separate temperature and pressure sensors to calculate mass flow rates. These compensating devices allow calculation of the mass flow rate, but each one requires extra invasions of the pipeline, as well as the installation of tubing, valves, and manifolds, all of which add to the cost of installation and maintenance.
With the InnovaMass multivariable vortex steam flow meter, a single device measures five process variables with one process connection. This equates to lower initial cost, less complex installation, and reduced maintenance costs, which can contribute to significant overall savings to maximize steam productivity. Total cost-of-ownership drops, reducing flow meter installation labor and maintenance costs while still providing accurate steam flow meter rates.
Accurate & Reliable Steam Flow Measurement Solutions
At Sierra Instruments, we bring decades of experience in steam flow measurement, delivering proven performance, accuracy, and reliability across industrial applications.
Contact us to find out how we can meet your steam flow measurement needs or click below to explore steam flow meters from Sierra Instruments.
[1] https://www.epa.gov/sites/default/files/2015-07/documents/steam_flow_measurement.pdf