Flow measurement using differential pressure
- Verified standardized way of measuring
- Exact measurements even for fast flowing steam and gas
- Absence of moving parts
- Robust and versatile design
- Reasonable price
measuring orifice plates, nozzles and Venturi tubes are used for measuring the flow of superheated or saturated steam, liquids, gases or gaseous homogeneous mixture. The used measuring method is based on the pressure difference - ie. differential pressure that occurs when the passage of the medium reduced cross section. Instantaneous flow rate of the fluid is up to a small non-linearity proportional to the square root of the pressure difference. The transfer of the differential pressure into an electrical signal occurs at a differential pressure transmitter. This electrical signal is processed in the control or in the parent system together with the temperature and pressure in real time. The result of this measurement method is the information on the quantity or the energy of the flowing medium. Measurement computationally based on common solutions Bernoulli equation and the equation of continuity and flow equations established on the basis of experimental measurements.
Orifice plates are the most commonly used primary element for flow measurement based on the principle of differential pressure. It is due to their simple design, high accuracy and, last but not least, favorable price. Produced orifice plates are in accordance with the standard EN ISO 5167 – 2. Diameter range is defined by standard DN50 to DN1000.
Orifice plate can be supplied as a separate disc designed for the operating conditions. In this case, the user must ensure correct mounting and the extraction process, which is often problematic and therefore is often supplied orifice plates along with tappings (pressure or chamber) performed between flanges or welded-in. A welded embodiment is particularly suitable where there are high temperatures and pressures. Another option is to supply the test track, where is part of the pipeline.
Majority of orifice plate is centric. Inner hole must have a sharp measuring edge.
Concentric aperture is not suitable for dirty liquid. Eccentric and aperture orifice plate are designed so that particles can pass through the hole.
There are three types of subscription differential pressure - fillet, distance D before D/2 behind the orifice plate and flange offtekes where the distance from the orifice plate disk to the middle of offteke is defined as 25,4 mm.
Tappings may be terminated thread, pipe weld or flanged. It is also possible to equip condensation, cooling vessels and valves.
Orifice plates are normally manufactured from stainless steel class 17 in order to prevent corrosion, thereby changing the dimensions of the measuring part. The material may be modified based on specific application requirements. The standard allows the production of any material while maintaining the shape and dimensions in accordance with the values in it.
Calculation is performed according to ISO 5167 – 1. Typical uncertainty of uncalibrated orifice plates is + / -0.5%.
Orifice plates typically have a pressure drop of between 40 to 95% of the generated differential pressure. The actual permanent pressure loss depends on the ratio of the pipe diameter and the inner aperture.
Orifice plates can be best used for clean and non-corrosive liquids where there are sufficient stabilization piping and allowed higher pressure loss.
Orifice plate measuring track
For applications requiring high accuracy of flow measurement, it is recommended to include before and behing the measuring element stabilization pipings as a part of the measuring lines.
They can be delivered in between flanges or prepared for weld. Measuring track can be calibrated for maximum accuracy in accredited calibration laboratories.
Input section has a standard length 10D in which the pipes are machined to meet the parameters requirements of ISO 5167. Subsequent section has a standard length 4D. On request it is possible to make a track of any length.
Typical precision calibrated measuring line is in the range of 0.15 to 0.25%.
Nozzle ISA 1932 is used for high speed, low viscous and where the risk of deterioration or destruction of normal orifice plate. Another nozzle advantage compared to the orifice plate is smaller necessity of the stabilization piping, less pressure loss, and higher throughput for the same beta ratio (d/D) - nozzle allows to measure approximately 50% higher flow rate than a orifice plate with similar computational beta and differential pressure. Nozzles are manufactured in accordance with the standard EN ISO 5167 – 3. Diameter range is defined by standard DN50 to DN500.
Nozzle consists of confusor (input section) with a rounded profile and cylindrical neck profile with a sharp finish. Its length depends on the beta ratio. The lower the ratio, the nozzle is shorter.
Fillet consumption uses either chamber or pressure tappings.
Connectivity to differential pressure tapings are the same as in the case of a orifice plate - can be terminated thread, pipe weld or flanged. It is also possible to equip condensation, seal pots and valves.
Nozzles are made of different materials, mostly of stainless steel class. 17, or from the pipe material, all depending on the specific measurement requirements, the nature of the media and so on.
Typical uncertainty of uncalibrated nozzles ISA 1932 is ± 0.8%.
Pressure loss depends on the ratio of beta and ranges between 40-95% generated by differential pressure.
The nozzle can be supplied complete with tappings and straight lengths for a welded or flanged connection.
In most cases nozzle for a custom fit uses welded joint, it is possible to fit between the flanges.
Evaluation of the measurement
For evaluating the pressure differential and conversion to the required data are used differential pressure transducers. We use transducers of the world's leading manufacturers that allow you to transfer to an accuracy of a fraction of a %. The output is a standard 4-20 mA, on customer request can be added digital communication (HART, Fieldbus, Modbus ....), local display and so on.
Before ordering, check the offer process. We will need the following information:
- type of device
- flow rate to be measured during the operation (max, min)
- pipe material, material of the primary element
- flange design
- requirements at the transmitter output
- term of supply
- accessories: condensing vessel
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