Process Engineering Division – Instrumentation and Automation Technology Exchange Forum – Daily Question – Question dated 2020-01-14

The orifice plate is the simplest and cheapest, and under the same conditions, it has the greatest pressure loss. The venturi tube has low pressure loss, so it is often used for flow measurement in large-diameter pipes with an inner diameter ranging from 100 to 800 mm. Because the fluid pressure in large-diameter pipes is low, a lower pressure loss is required. Under the same conditions, the β of the nozzle is smaller. Therefore, if the required pressure difference and flow rate are below the lower limit for orifice plates, nozzles can be used. When measuring corrosive and corrosive fluids, the flow coefficient changes due to deformation at the inlet edge, which has the greatest impact on orifice plates, while having a lesser impact on nozzles and Venturi tubes.

The orifice plate is the simplest and cheapest, and it has the highest pressure loss at the same pressure. The venturi tube has low pressure loss, which is why it is often used for flow measurement in pipes with an inner diameter ranging from 100 to 800 mm. In pipes with larger diameters, the fluid pressure is lower, so a lower pressure loss is required. Under the same conditions, the β value of a nozzle is smaller; therefore, when the required differential pressure and flow rate are lower than those at the lower limit for an orifice plate, a nozzle can be used.

1. Orifice plates, nozzles, and Venturi nozzles are generally used in pipes with a diameter D ≥ 50 mm. 2. To measure the flow rate of high-temperature, high-pressure media, orifice plates and nozzles are used; orifice plates or nozzles are selected for measuring steam flow rate ； Venturi tubes are only suitable for measuring the flow rate of low-pressure fluid media. 3. When a low pressure loss generated by the throttling device is required, nozzles or venturi tubes can be used. 4. When measuring the flow rate of certain dirty or corrosive media, nozzles are a better choice. 5. Among orifice plates, nozzles, and venturi tubes, nozzles offer the highest measurement accuracy, and they also require a shorter straight pipe section. 6. In terms of processing, manufacturing, and installation, orifice plates require the simplest procedures, followed by nozzles, with venturi tubes being the most complex to work with. 7. Subject to meeting technical requirements, orifice plates, nozzles, and venturi tubes should be selected based on principles of cost-effectiveness, suitability, and feasibility

Orifice plates are simple to manufacture and inexpensive, but they have high pressure loss; strict requirements are placed on the straight pipe sections before and after them. Venturi tubes use a fluid throttling mechanism, resulting in the lowest pressure loss; they are more expensive but can be used in a wide range of applications. The nozzle manufacturing process is relatively simple, and it is often used for steam measurement.

The following principles should be followed when selecting throttling devices such as orifice plates, nozzles, and Venturi tubes: 1. Orifice plates, nozzles, and Venturi nozzles are generally used in pipes with a diameter D ≥ 50 mm. 2. To measure the flow rate of high-temperature, high-pressure media, orifice plates and nozzles are used; orifice plates or nozzles are selected for measuring steam flow rate ； Venturi tubes are only suitable for measuring the flow rate of low-pressure fluid media. 3. When a low pressure loss generated by the throttling device is required, nozzles or venturi tubes can be used. 4. When measuring the flow rate of certain dirty or corrosive media, nozzles are a better choice. 5. Among orifice plates, nozzles, and venturi tubes, nozzles offer the highest measurement accuracy, and they also require a shorter straight pipe section. 6. In terms of processing, manufacturing, and installation, orifice plates require the simplest procedures, followed by nozzles, with venturi tubes being the most complex to work with. 7. Subject to meeting technical requirements, orifice plates, nozzles, and venturi tubes should be selected based on principles of cost-effectiveness, suitability, and feasibility

Orifice plates are simple to manufacture and inexpensive, but they have high pressure loss; strict requirements are placed on the straight pipe sections before and after them. Venturi tubes use a fluid throttling mechanism, resulting in the lowest pressure loss; they are more expensive but can be used in a wide range of applications. The nozzle manufacturing process is relatively simple, and it is often used for steam measurement.

Orifice plates are simple to manufacture and inexpensive, but they have high pressure loss; strict requirements are placed on the straight pipe sections before and after them. Venturi tubes use a fluid throttling mechanism, resulting in the lowest pressure loss; they are more expensive but can be used in a wide range of applications. The nozzle manufacturing process is relatively simple, and it is often used for steam measurement.

Orifice plates are simple to manufacture and inexpensive, but they have high pressure loss; strict requirements are placed on the straight pipe sections before and after them. Venturi tubes use a fluid throttling mechanism, resulting in the lowest pressure loss; they are more expensive but can be used in a wide range of applications. The nozzle manufacturing process is relatively simple, and it is often used for steam measurement.

The structure of the orifice plate throttling device is easy to replicate; it is simple and robust, offers stable and reliable performance, has a long service life, and is inexpensive. Nozzle adjustment is achieved by using several control valves to regulate the corresponding sets of nozzles in the adjustment stages, in order to control the amount of steam entering the turbine; the nozzles in these adjustment stages are not arranged in a continuous circle. The pressure loss of a Venturi is low. For the same differential pressure, the pressure loss of an orifice plate is 3 to 5 times greater than that of a Venturi tube

What are the characteristics of orifice plates, nozzles, and venturi tubes? Orifice plate: An orifice plate is a differential pressure device used for measuring flow rate; in combination with various differential pressure gauges or transmitters, it can be used to measure the flow rate of various fluids in pipes. Its features are: 1) The structure of the throttling device is easy to replicate; it is simple and robust, offers stable and reliable performance, has a long service life, and is inexpensive. 2) Orifice plate calculations follow international standards and manufacturing practices ； 3) It has a wide range of applications; all single-phase flows can be measured, and some mixed-phase flows can also be measured. 4) The standard throttle device can be put into use without the need for actual flow calibration. 5) The integrated orifice plate is easier to install; no pressure guiding pipes are required, and it can be connected directly to differential pressure transmitters and pressure transmitters. Nozzles: They feature resistance to high temperatures and pressures, impact resistance, a long service life, a wide measurement range, and high measurement accuracy. Suitable for measuring the flow rate of high-temperature and high-pressure steam in power plants, as well as in heating network pipelines, for fluids with high flow velocities. Nozzles come in two forms: A: ISA1932 nozzle (standard nozzle) ； B: Long-diameter nozzle, Venturi tube features: 1) It generates low resistance to the fluid, around 150 Pa, thus resulting in low energy consumption. 2) High pressure difference, high precision, and wide measurement range. 3) Good stability with smooth pressure difference characteristics. 4) It has a wide range of applications; it can be used with various gases, flue gases, and blast furnace gas containing many impurities, without experiencing blockages even with long-term use. 5) Easy to install and convenient for long-term maintenance. 6) The straight sections before and after are shorter than those of a standard throttling device, approximately 1.5D in front and 1D behind. (7) It features an integrated structure for online self-correction of temperature and pressure.