A steam flow measurement using an orifice plate, flow nozzle or Venturi tube shows a constant offset after commissioning. Or the indicated flow rate changes with the outdoor temperature even though pressure and plant load remain almost constant. In such cases, the differential pressure transmitter is often checked first. However, the actual cause is frequently found in the impulse lines between the primary element and the transmitter.
In a conventional differential pressure measurement for steam, the two impulse lines are normally filled with condensate in a controlled manner. This prevents hot steam from reaching the measuring cell of the differential pressure transmitter directly. At the same time, however, two liquid columns are created whose hydrostatic effects should be as identical as possible.
This is precisely where one of the most important sources of error arises: If the condensate pots are not installed at the same height, if the lines have different filling levels or if the high- and low-pressure sides are heated differently, an additional hydrostatic differential pressure is created in addition to the actual differential pressure generated by the primary element. The transmitter measures this error in exactly the same way as the differential pressure generated by the flow.
For steam applications, it is therefore not sufficient to design only the primary element correctly. Condensate pots, impulse-line routing, slope, filling condition, temperature distribution, valve manifold and commissioning are equally important. A highly accurate measuring cell cannot compensate for an asymmetrically designed impulse-line system.
Suitable primary elements can be found at ICS Schneider under primary flow elements. Suitable instruments for measuring the generated differential pressure can be found under pressure sensors and differential pressure sensors.
Table of Contents
- How does differential pressure flow measurement for steam work?
- Distinguishing between orifice plates, flow nozzles and Venturi tubes
- Why flow depends on the square root of differential pressure
- Why condensate is required in steam impulse lines
- What is the purpose of condensate pots?
- Why both sides must be designed symmetrically
- How height differences create zero-point errors
- Routing impulse lines correctly
- Gas bubbles and incompletely filled lines
- Temperature differences between impulse lines
- Frost protection and heat tracing
- Using 3- and 5-valve manifolds correctly
- Checking the zero point only after correct filling
- Density compensation for steam flow
- Systematic commissioning
- Typical fault patterns
- Troubleshooting implausible steam flow readings
- Practical example: constant flow error despite a correctly calibrated transmitter
- Suitable primary elements and differential pressure transmitters
- Conclusion
- FAQ
How does differential pressure flow measurement for steam work?
In differential pressure flow measurement, a defined primary element is installed in the pipeline. This can be, for example, an orifice plate, a flow nozzle or a Venturi tube.
The primary element reduces the free flow cross-section. This locally increases the flow velocity and reduces the static pressure.
Two pressures are measured upstream and downstream of the primary element or at defined tapping points:
- pH = higher pressure on the high-pressure side,
- pL = lower pressure on the low-pressure side.
The resulting differential pressure is:
Δp = pH − pL
This differential pressure is transmitted to the differential pressure transmitter through the two impulse lines.
The higher the flow through the primary element, the greater the generated differential pressure normally becomes.
The quality of the flow measurement therefore depends on three areas:
- Primary element: correct geometry and installation conditions.
- Pressure transmission: symmetrical, correctly filled impulse lines.
- Differential pressure measurement and calculation: correct measuring span, square-root extraction and, where required, pressure/temperature compensation.
Distinguishing between orifice plates, flow nozzles and Venturi tubes
Different primary elements are available for differential pressure flow measurement. The basic measuring principle is similar, but their design characteristics differ.
Orifice plate
The orifice plate is one of the most commonly used primary elements. A defined bore reduces the pipe cross-section and thereby generates the differential pressure required for flow determination.
Its advantages include:
- comparatively simple design,
- proven measuring principle,
- suitable for liquids, gases and steam,
- extensive standardized design options.
One disadvantage is the comparatively high permanent pressure loss.
Flow nozzle
A flow nozzle has a more streamlined inlet than a sharp-edged orifice plate.
It is particularly often used for:
- steam,
- high flow velocities,
- high temperatures,
- demanding energy applications
.
For steam applications, a flow nozzle can be particularly suitable because of its robust geometry.
Venturi tube
In a Venturi tube, the pipe cross-section is first reduced in a controlled manner and then expanded again through a diffuser.
This allows a larger proportion of the static pressure to be recovered.
The main advantage is therefore a comparatively low permanent pressure loss.
The suitable primary element depends, among other things, on pressure, temperature, pipe diameter, required measurement accuracy, available pressure loss and required measuring range.
Why flow depends on the square root of differential pressure
With a conventional differential pressure primary element, the relationship between differential pressure and flow is not linear.
In simplified form, the volumetric flow rate is:
Q ∝ √(Δp / ρ)
and the mass flow rate is approximately:
ṁ ∝ √(ρ · Δp)
Where:
- Q = volumetric flow rate,
- ṁ = mass flow rate,
- Δp = measured differential pressure,
- ρ = density of the medium.
The actual calculation additionally takes into account factors such as geometry, discharge coefficient, expansion factor, pipe diameter and the β ratio of the primary element.
For troubleshooting, however, the square-root relationship is particularly important:
A constant differential pressure error has a particularly strong effect on the calculated flow in the lower flow range.
A zero-point error of only a few millibar can therefore be significantly more problematic at low flow than near the upper end of the measuring range.
Why condensate is required in steam impulse lines
In a conventional differential pressure measurement, hot steam should not reach the measuring cell of the differential pressure transmitter directly.
A condensate filling is therefore established in the impulse lines. The process pressure is hydraulically transmitted to the transmitter through this liquid column.
The impulse lines therefore perform two functions simultaneously:
- transmission of the process pressure,
- thermal separation between the hot steam and the measuring instrument.
During stable operation, both sides should be filled with condensate as completely and reproducibly as possible.
The decisive point is:
The liquid column on the high-pressure side and the liquid column on the low-pressure side must create hydrostatic conditions that are as equal as possible.
If this is the case, the two hydrostatic pressure components act almost equally on the measuring cell and largely cancel each other out when the differential pressure is formed.
What is the purpose of condensate pots?
In conventional steam measuring points, condensate pots or condensate chambers are frequently installed between the pressure tapping point and the impulse line.
They help establish a defined transition between the steam phase and the condensate filling.
This allows reproducible liquid levels to form on the high- and low-pressure sides.
One point is particularly important:
The reference heights of both condensate pots must be identical in accordance with the design of the measuring point.
Otherwise, the two liquid columns start at different geodetic heights and generate a hydrostatic differential pressure even when the system is at rest.
Separate condensate pots are not mandatory in every modern steam measuring point. Depending on the primary element, connection geometry and manufacturer’s design concept, other defined condensate arrangements may be used.
The decisive factor is not the component named “condensate pot”, but a permanently symmetrical and defined condensate column on both sides.
Why both sides must be designed symmetrically
In an ideal steam measuring point, the high- and low-pressure sides are designed to be as hydraulically identical as possible.
This applies particularly to:
- height of the condensate pots,
- diameter of the impulse lines,
- line lengths,
- slope,
- valves and fittings,
- insulation,
- heat tracing,
- ambient influences.
Perfect geometric mirroring is not possible in every installation. However, it is essential that both lines have hydrostatic and thermal conditions that are as comparable as possible.
Examples of problematic arrangements include:
- H side 1 m long, L side 5 m long,
- one line insulated, the other uninsulated,
- one line routed next to a hot steam pipe, the other exposed to ambient conditions,
- heat tracing on only one side,
- different condensate levels.
Such differences can lead to temperature-dependent zero-point shifts even though the differential pressure transmitter itself is operating stably.
How height differences create zero-point errors
A liquid column generates hydrostatic pressure:
phyd = ρ · g · h
If the effective condensate columns on both sides differ by the height Δh, an additional differential pressure is created:
Δperror = ρ · g · Δh
Where:
- ρ = density of the condensate,
- g = gravitational acceleration,
- Δh = height difference.
Example
For a liquid with approximately the density of water, a height difference of 100 mm gives approximately:
Δp ≈ 1000 kg/m³ · 9.81 m/s² · 0.1 m ≈ 981 Pa ≈ 9.8 mbar
The actual density of the condensate depends on its temperature.
An error of almost 10 mbar can already be significant in a differential pressure measurement with, for example, a span of 100 or 200 mbar.
Condensate pots installed at different heights should therefore not simply be “corrected” using a zero trim on the transmitter.
The mechanical and hydrostatic error remains present and may additionally change with temperature.
Routing impulse lines correctly
Impulse lines should be routed so that a defined filling condition is maintained permanently.
In a conventional steam measurement, the differential pressure transmitter is often installed below the pressure tapping points or below the defined condensate level. This allows the impulse lines to operate as liquid columns.
The specific arrangement must, however, always correspond to the respective plant design and manufacturer’s recommendations.
Important factors include:
- continuous slope or rise without unnecessary pockets,
- avoiding local high points where gas can accumulate,
- avoiding unwanted low points,
- sufficient pipe diameter,
- mechanically stable support,
- impulse-line routing that is as symmetrical as possible.
An apparently horizontal impulse line is often more problematic in practice than a line deliberately installed with a defined slope.
The design must also ensure that the lines remain accessible for maintenance, filling, venting and, where required, draining.
Gas bubbles and incompletely filled lines
In steam measurements, the sections of impulse line between the condensate level and the differential pressure transmitter should normally be filled with condensate.
A trapped steam or gas bubble changes the dynamic behavior of the pressure transmission.
Unlike a liquid, gas is compressible.
This can result in:
- delayed measured-value response,
- different dynamics on the H and L sides,
- fluctuating zero point,
- unstable measured values after commissioning.
Local high points in the impulse lines are particularly problematic because gas pockets can accumulate there.
During commissioning, the measuring point should therefore be completely filled and vented in accordance with the intended procedure.
Temperature differences between impulse lines
Not only the height but also the temperature of the condensate influences the hydrostatic pressure.
The density of the liquid changes with temperature.
If both condensate columns have the same height but significantly different temperatures, slightly different hydrostatic pressures can result.
This effect can become relevant with long impulse lines and small differential pressure ranges.
A typical practical problem is:
- the H line runs close to a hot steam pipe,
- the L line runs along a cold outside wall,
- the two liquid columns therefore have different temperatures.
The resulting error can change with ambient temperature.
A typical indication is a zero-point shift that varies with the time of day or season.
Impulse-line routing that is as symmetrical as possible and comparable insulation reduce this risk.
Frost protection and heat tracing
In outdoor installations, the condensate in the impulse lines can freeze.
A frozen line no longer transmits process pressure correctly and can also be mechanically damaged by the volumetric expansion of freezing water.
Depending on the location and plant design, the following may therefore be required:
- thermal insulation,
- electrical heat tracing,
- steam tracing,
- heated instrument enclosures.
The principle of symmetry also applies here.
If the H and L sides are heated differently, different condensate temperatures and therefore different liquid densities result.
Heat tracing should therefore not only provide frost protection, but also create thermal conditions that are as similar as possible on both measuring sides.
In addition, the permissible temperatures of the transmitter, valve manifold, seals and, where applicable, hazardous-area components must be observed.
Using 3- and 5-valve manifolds correctly
A 3- or 5-valve manifold is frequently used with differential pressure transmitters.
A typical 3-valve manifold has:
- an isolation valve on the high-pressure side,
- an isolation valve on the low-pressure side,
- an equalizing valve between the two sides.
A 5-valve manifold additionally provides options for venting, draining or connecting a test instrument.
The valve manifold is particularly important for:
- commissioning,
- zero-point checks,
- calibration,
- maintenance,
- troubleshooting.
The equalizing valve allows both sides of the differential pressure transmitter to be exposed to the same pressure under defined conditions.
However, the exact operating sequence must always comply with the manufacturer’s instructions and the plant operating procedure.
At high static steam pressures, an incorrect valve sequence can expose the differential pressure sensor to severe one-sided loading.
Checking the zero point only after correct filling
A common commissioning error is to correct the zero point of the differential pressure transmitter before the impulse lines have reached a thermally and hydraulically stable condition.
A meaningful zero adjustment requires:
- correctly filled impulse lines,
- vented liquid columns,
- defined condensate levels,
- stable temperatures,
- correct valve positions,
- final installation position of the transmitter.
Only then should it be assessed whether the measuring instrument actually has a zero-point error.
A mechanically or hydrostatically induced offset must not simply be electrically trimmed out.
Otherwise, the indication will only be correct for the current filling condition. As soon as the condensate level or temperature changes, an error will occur again.
Density compensation for steam flow
For steam flow measurements, differential pressure alone is not always sufficient when accurate mass flow is required.
Steam density changes with pressure and temperature.
For mass flow, the simplified relationship is:
ṁ ∝ √(ρ · Δp)
If the plant operates under different process conditions from those used for the original design, a flow calculation error can occur even if the differential pressure itself is measured correctly.
Saturated steam
With dry saturated steam, there is a defined relationship between saturation pressure and saturation temperature. Depending on the measurement concept, a suitable pressure measurement may therefore be sufficient for compensation.
Superheated steam
With superheated steam, pressure and temperature are normally required to determine the current density or the thermodynamic properties required for the flow calculation.
High-quality steam flow measurements may therefore record the following together:
- differential pressure,
- static pressure,
- temperature.
The flow calculation can then be performed in the transmitter, a flow computer or the control system.
An important distinction is:
Density compensation corrects for changing process conditions – it does not correct incorrectly filled impulse lines.
Systematic commissioning of a steam flow measurement
Careful commissioning is essential for reproducible differential pressure flow measurement.
- Check the primary element: Verify type, installation direction, pipe diameter and pressure tapping points.
- Identify the H and L sides: Clearly assign the high-pressure side upstream and the low-pressure side downstream of the primary element.
- Check the condensate pots: Compare reference heights and connection positions.
- Check the impulse lines: Verify diameter, slope and symmetrical routing.
- Flush the lines: Remove welding residues, dirt and deposits before they can reach the measuring cell.
- Fill the lines: Establish the specified condensate level.
- Remove gas pockets: Vent the lines in accordance with the plant procedure.
- Commission the valve manifold: Follow the manufacturer’s valve operating sequence.
- Allow thermal stabilization: Allow the condensate columns and lines to reach stable conditions.
- Check the zero point: Equalize both transmitter sides under defined conditions.
- Check the measuring range: Verify scaling and square-root extraction.
- Check density compensation: Plausibility-check pressure and temperature values.
- Compare the first process value: Compare the indicated flow with steam generation or plant load.
Typical fault patterns in steam flow measurements
| Observation | Possible cause | Recommended check |
|---|---|---|
| Transmitter shows a constant differential pressure in an apparent zero condition | Unequal condensate columns or different condensate-pot heights | Compare geometry and filling levels on both sides |
| Flow indication drifts with outdoor temperature | Different temperatures of the two impulse lines | Compare line routing, insulation and heat tracing |
| Measured value responds unusually slowly | Gas bubble or partially blocked impulse line | Vent the lines and check them for free passage |
| Flow indication fluctuates strongly after commissioning | Lines not yet completely filled with condensate | Check filling and thermal stabilization |
| Measured value suddenly becomes implausible in winter | Frozen condensate | Check frost protection and heat tracing |
| Negative differential pressure signal after maintenance | H and L lines reversed | Check the connection assignment |
| Flow indication is correct at only one operating point | Incorrect square-root extraction, density compensation or primary-element data | Check calculation parameters and process data |
| Measured value is initially correct after zero adjustment but drifts again later | Hydrostatic error was electrically compensated | Check condensate levels and line symmetry |
| One side responds faster than the other | Different line filling or partial blockage | Check the H and L sides separately |
| Transmitter is calibrated but flow remains incorrect | Error in the primary element, impulse lines or flow calculation | Check the entire measuring chain |
Troubleshooting implausible steam flow readings
If a steam flow measurement produces implausible values, the differential pressure transmitter should not immediately be removed.
The following sequence is useful:
- Check process plausibility: Does the indicated flow match the current boiler or plant load?
- Check static pressure: Does the steam pressure correspond to the expected operating condition?
- Check temperature: Saturated steam or superheated steam?
- Check the primary element: Correct installation direction and open pressure tapping points?
- Check H/L assignment: Are the lines connected correctly?
- Compare the condensate pots: Same reference height?
- Check the impulse lines: Same filling condition, no gas bubbles and no blockages?
- Check temperature distribution: Are both lines similarly insulated or heated?
- Check the valve manifold: Are all valves in the correct operating position?
- Equalize the transmitter: Bring both sides to the same pressure under defined conditions.
- Check the zero point: Only now assess sensor offset.
- Check signal and square-root extraction: Verify 4–20 mA scaling or digital flow calculation.
- Check density compensation: Are pressure and temperature correctly measured and configured?
Only after these points have been ruled out should the differential pressure transmitter itself be calibrated or considered as the source of the error.
Practical example: constant flow error despite a correctly calibrated transmitter
In a steam network, the mass flow is measured using an orifice plate and a differential pressure transmitter.
After a plant overhaul, the measuring point consistently indicates approximately 8% more flow than before the maintenance work at a comparable boiler load.
The differential pressure transmitter is checked using a pressure calibrator. Its measurement deviation is within the expected specification.
Step 1: Check the primary element
The orifice plate, installation direction and pressure tapping points are correct.
Step 2: Check the impulse lines
During the inspection, it is discovered that one of the two condensate pots was reinstalled approximately 80 mm higher than the other after the overhaul.
The two condensate columns therefore have different effective heights.
Step 3: Evaluate the hydrostatic error
The height difference generates an additional static differential pressure which the transmitter interprets as part of the actual differential pressure generated by the flow.
Because the flow calculation is based on the square root of the differential pressure, this results in a systematic flow error.
Step 4: Correct the measuring point
The two condensate pots are reinstalled at the intended common reference height. Both impulse lines are then completely filled and vented.
Step 5: Check the zero point again
After thermal stabilization, the transmitter is equalized using the valve manifold. The previously observed offset has practically disappeared.
In operation, the flow indication is subsequently back within the expected range.
Result: The differential pressure transmitter was not faulty. An asymmetrical condensate column had generated an additional hydrostatic differential pressure.
The example demonstrates an important principle:
In steam flow measurement, the geometric and thermal symmetry of the impulse lines is part of the overall measurement accuracy.
Suitable primary elements and differential pressure transmitters for steam measurements
For differential pressure flow measurement with steam, the primary element and differential pressure transmitter must be designed together for the pipeline, steam condition, flow range, pressure and temperature.
WIKA FLC-FN – flow nozzle for steam applications
The WIKA FLC-FN-PIP and FLC-FN-FLN flow nozzles as well as the FLC-FN-VN Venturi nozzle are designed for flow measurement of liquids, gases and steam.
Flow nozzles are particularly suitable for applications involving high temperatures and high flow velocities and are therefore very well suited to industrial steam measurements.
Depending on the application, different designs are available for installation in pipelines or for flange mounting.
Further information can be found under WIKA FLC-FN flow nozzles at ICS Schneider.
WIKA FLC-OP / FLC-FL / FLC-AC – conventional orifice plates
For conventional differential pressure measuring points, WIKA FLC-OP orifice plates, FLC-FL orifice flanges and FLC-AC annular chamber orifice plates are also available.
These designs are suitable for flow measurement of:
- liquids,
- gases,
- steam.
Orifice plates are particularly useful where a simple, established primary element that can be designed according to standard methods is required.
Further information can be found under WIKA FLC-OP / FLC-FL / FLC-AC at ICS Schneider.
WIKA FLC-VT – Venturi tube for low permanent pressure loss
If the permanent pressure loss of the measuring point should be kept as low as possible, a Venturi tube from the FLC-VT series can be a suitable solution.
The downstream diffuser geometry recovers a large proportion of the static pressure.
Venturi tubes are also suitable for liquids, gases and steam.
Further information can be found under WIKA FLC-VT Venturi tubes at ICS Schneider.
Siemens SITRANS P320 – differential pressure transmitter for differential pressure flow measurement
The Siemens SITRANS P320, for example, is suitable for measuring the differential pressure generated by the primary element.
The process transmitter can be used, among other applications, for:
- differential pressure,
- differential pressure flow measurement,
- volumetric flow,
- mass flow
.
Depending on the instrument version, HART communication and extensive diagnostic functions are available.
For a steam measuring point, in addition to the appropriate differential pressure range, the permissible static pressure rating must be considered. The actual differential pressure may be only a few tens or hundreds of millibar, while a much higher steam pressure is simultaneously present on both measuring sides.
Further information can be found under Siemens SITRANS P320 at ICS Schneider.
Always design the primary element and transmitter together
| Requirement | Suitable solution |
|---|---|
| Conventional steam flow measurement | Orifice plate or flow nozzle + differential pressure transmitter |
| High steam temperatures and flow velocities | Flow nozzle |
| Lowest possible permanent pressure loss | Venturi tube or suitable Venturi nozzle |
| Continuous process and trend monitoring | Differential pressure transmitter with analog or digital signal |
| Accurate mass flow under variable steam conditions | Differential pressure measurement with suitable pressure and temperature compensation |
An overview of suitable primary elements can be found under primary flow elements at ICS Schneider.
Conclusion
In differential pressure flow measurement of steam, the accuracy of the differential pressure transmitter alone does not determine the quality of the measurement result.
The impulse lines form two hydrostatic measuring columns. Different condensate levels, heights, temperatures or line routings generate an additional differential pressure that the transmitter cannot distinguish from the actual differential pressure generated by the flow.
Condensate pots or defined condensate levels should therefore be located at a common reference height. The high- and low-pressure lines should be designed as symmetrically as possible in terms of geometry, insulation and temperature.
Complete condensate filling, avoidance of gas bubbles, suitable frost protection and correct operation of the valve manifold are equally important.
A zero adjustment should only be performed once the measuring point is hydraulically and thermally stable. A hydrostatic installation error should never simply be electrically trimmed out.
For accurate mass flow measurement, changes in steam properties must also be taken into account. Pressure compensation and, where required, temperature compensation are therefore part of the complete measuring chain in demanding steam applications.
The following troubleshooting principle therefore applies: First check the primary element, condensate pots and impulse lines, then the valve manifold and zero point – and only after that consider the differential pressure transmitter itself as the source of the error.
FAQ: Differential pressure flow measurement and steam flow
Why are condensate pots used in steam flow measurements?
Condensate pots or defined condensate sections help establish reproducible liquid columns between the steam process and the differential pressure transmitter. This thermally separates the hot steam from the measuring instrument while transmitting the process pressure hydraulically.
Do both condensate pots have to be installed at the same height?
For a symmetrical measuring point, the specified reference heights should be identical. A height difference creates different hydrostatic liquid columns and therefore an additional differential pressure.
How does a difference in condensate-pot height affect the measurement?
The additional hydrostatic pressure is determined by density, gravitational acceleration and the height difference. Even a few centimeters can create a relevant zero-point error in small differential pressure ranges.
How much pressure does a 10 cm water column generate?
At a density of approximately 1,000 kg/m³, a 100 mm water column corresponds to approximately 981 Pa or 9.8 mbar. The actual condensate density depends on temperature.
Why should steam impulse lines be filled with condensate?
The condensate filling transmits the process pressure to the differential pressure transmitter and at the same time prevents hot steam from reaching the measuring cell directly.
What happens if a steam impulse line is only partially filled?
Gas or steam pockets can make the pressure transmission compressible. This can result in delayed, unstable or differently timed responses on the high- and low-pressure sides.
Where should the differential pressure transmitter be installed for a steam measurement?
With conventional condensate-filled impulse lines, the transmitter is often installed below the defined condensate level. The exact position and line routing must, however, comply with the plant design and manufacturer’s specifications.
Why do impulse lines require a slope?
A defined slope prevents unwanted gas or liquid pockets and supports a reproducible filling condition. Local high and low points can interfere with pressure transmission.
Why should the H and L lines be designed as similarly as possible?
Different line lengths, heights, temperatures or filling levels can generate different hydrostatic pressures. Since the transmitter detects only the resulting pressure difference, such differences directly affect the measured value.
Can different insulation cause a measurement error?
Yes. Different temperatures change the density of the condensate columns. With small differential pressure ranges and long impulse lines, asymmetrical thermal conditions can therefore cause a measurable zero-point shift.
Why can the indicated steam flow fluctuate with outdoor temperature?
One possible cause is different heating or cooling of the two impulse lines. This changes the condensate densities and therefore the hydrostatic pressure components.
When is heat tracing required?
Heat tracing may be required if condensate in outdoor installations could freeze. It should be designed so that both sides have comparable thermal conditions while the permissible instrument temperatures are maintained.
What is the purpose of the equalizing valve on a 3-valve manifold?
The equalizing valve connects the high- and low-pressure sides of the differential pressure transmitter. This allows the transmitter to be checked under defined conditions to determine whether it indicates a differential pressure close to zero when the same pressure is applied to both sides.
Can unequal condensate columns be compensated by zero adjustment?
This should be avoided. If the temperature or filling level changes later, the hydrostatic error will also change. The mechanical or hydraulic cause must therefore be eliminated first.
Why is the measurement error particularly critical at low steam flow?
Flow is determined from the square root of differential pressure. A constant differential pressure offset therefore has a particularly large relative effect at low differential pressure values.
Why must steam density be compensated?
Steam density depends on process conditions. If pressure or temperature changes from the original design conditions, the relationship between differential pressure and mass flow also changes.
Is temperature measurement required for saturated steam?
With dry saturated steam, there is a defined relationship between saturation pressure and saturation temperature. Depending on the measurement concept, a suitable pressure measurement may therefore be sufficient for density determination. The specific design depends on the required measurement accuracy and steam condition.
What is different with superheated steam?
With superheated steam, pressure and temperature are no longer uniquely linked by the saturation curve. Both quantities are therefore normally taken into account for accurate density and mass-flow calculation.
Which primary elements are suitable for steam flow measurement?
Typical primary elements include orifice plates, flow nozzles and Venturi tubes. The selection depends, among other things, on steam pressure, temperature, pipe diameter, flow velocity, required accuracy and permissible pressure loss.
Why is a flow nozzle particularly suitable for steam?
Flow nozzles have a robust, streamlined geometry and are frequently used at high temperatures and high flow velocities. This makes them particularly suitable for demanding steam applications.
When is a Venturi tube useful?
A Venturi tube is particularly useful when the permanent pressure loss of the flow measurement should be reduced. The diffuser recovers a larger proportion of the static pressure.
What should be checked first if a steam flow measurement indicates an incorrect value?
First check the process condition, primary element, H/L assignment, condensate pots, impulse lines, filling levels and valve positions. Then check the zero point, electrical scaling and density compensation. Only after these checks should the differential pressure transmitter itself be calibrated or replaced.