Measurement of steam flow rate

A few days ago, my company asked me to estimate the steam consumption, but we don’t have any flow meters. . . . All I can do is this: in the case of reaction vessels or heating equipment, convert the heat energy required due to temperature changes in the material into steam enthalpy values, thereby determining the steam demand. (But steam cooling produces water and vapor, which is a big problem. I can’t determine exactly how much of the steam turns into water; I have to assume it all becomes water.) ) Another problematic issue is that for some heating coils, the flow rate can actually be determined once the steam flow velocity is known; however, it’s not possible to measure this velocity. The design manual states that the optimal flow velocity is 20–35 m/s (I think so; check the details carefully). But the pipes were installed initially without using the design-defined flow velocity to determine their diameter. So I guess it can’t be taken from the manual either. . . . . . Hehe, in any case, I think it’s more accurate to calculate using enthalpy and similar values. After all, when the conditions aren’t favorable and there isn’t much data available, the on-site data required for calculating enthalpy are still easier to obtain. . . Just my opinion. . . . I don’t have much experience either; I’m just sharing what I’ve done, so you can use it as a reference.

Is it possible to estimate the steam consumption under current operating conditions, based on the steam usage and feed rate before the flow meter broke down?

Can’t we just buy a target-type or vortex flow meter and replace it?

Back to Floor 4: Our factory used orifice flow meters in the past, with the readings displayed via secondary meters; however, there were issues with those displays. I’m thinking of using a U-tube manometer to show the pressure difference generated by the orifice meters – is that possible?

It should be possible to calculate the instantaneous flow rate, but the cumulative flow rate can only be estimated. If the flow rate remains stable, the estimate can be more accurate. If there are issues with the display, I think (I’m not sure if this is correct) that it should be sufficient for the meter technician to fix it; there’s no need for such complications. Last edited by Pseudo Xiao Bao on 2009-3-1 19:54.]

Regarding the issue of steam flow measurement, I have gathered some relevant information, which I share with the original poster below; I hope it can provide you with some insights. On the measurement of steam flow rate 1. Introduction In metering work, inaccurate measurement of steam flow rate is a common problem, and the main reasons for this are as follows. 1.1 Superheated steam: Steam is a rather special medium; in general, what is referred to as steam is superheated steam. Superheated steam is obtained by heating saturated steam. It contains absolutely no liquid droplets or mist, and is a real gas. The temperature and pressure parameters of superheated steam are two independent parameters, and its density should be determined by these two parameters. After being transported over long distances, superheated steam, as operating conditions such as temperature and pressure change – especially when the degree of superheating is not high – loses heat and its temperature drops, causing it to transition from a superheated state to a saturated or supersaturated state, thereby becoming saturated steam or supersaturated steam containing water droplets. When saturated steam is suddenly subjected to a significant pressure drop and the liquid undergoes adiabatic expansion, it also turns into superheated steam, thus forming a two-phase flow of vapor and liquid. 1.2 Saturated steam Steam that has not undergone heat treatment is called saturated steam. It is a colorless, tasteless gas that is non-flammable and non-corrosive. The content of liquid droplets or mist in saturated steam reflects the quality of the steam, which is generally expressed by the parameter known as dryness. The dryness degree of steam refers to the percentage of dry steam in a unit volume of saturated steam, denoted by “x”. It is difficult to accurately measure the flow rate of saturated steam, as it is hard to maintain a consistent degree of dryness in the steam. Generally, no flow meter can accurately detect the flow rate of two-phase fluids; fluctuations in steam pressure cause changes in the density of the steam, which results in additional errors in the readings provided by the flow meters. Therefore, in steam metering, it is necessary to ensure that the dryness of the steam at the measurement point meets the required standards; compensation measures should also be taken when needed to achieve accurate measurements. 2. Analysis of measurements: At present, flow meters are used to measure steam flow, and the medium being measured is single-phase superheated steam or saturated steam. For steam with frequently changing phase flow, there is certainly a problem of inaccurate measurement. The solution to this problem is to maintain the superheat of the steam and minimize its moisture content; for example, by improving the insulation of the steam pipes and reducing pressure losses in the steam, thereby enhancing the accuracy of the measurements. However, these methods cannot completely resolve the issue of inaccurate steam flow measurement; the fundamental solution to this problem is to develop a flow meter capable of measuring two-phase flowing media. There are many types of flow meters used to measure gas flow rates, among which velocity-type and volume flow meters are the most commonly used. Their common feature is that they can only measure the volume flow rate under operating conditions; since the volume flow rate is a function of the state, it cannot accurately represent the actual flow rate. In engineering practice, the volume flow rate or mass flow rate under standard conditions is generally used for representation. The so-called standard state volume is the volume of a gas at 0°C and 1 standard atmosphere, or the volume at 20°C and 1 standard atmosphere. The use of mass flow rate as a measurement unit is not common at present. When using a calibrated gas flow meter, the normal temperature and pressure of the gas are taken as the design conditions; the volumetric flow rate under these design conditions is converted into a standard volumetric flow rate or mass flow rate. The conversion factor includes a factor related to the gas density, and when the operating conditions of the gas medium deviate from the design conditions, errors will occur in the flow rate readings. Furthermore, changes in the composition, concentration, or temperature of the gas medium also affect flow measurement; therefore, compensation measures are necessary when measuring steam flow, and the compensation factors become more complex due to the changes in the state of the steam. The density of superheated steam is determined by two parameters: temperature and pressure. Moreover, within different ranges of these parameters, the expression for density varies, and it cannot be represented by a single formula; therefore, there is no universal formula for calculating density, and compensation formulas for temperature and pressure must be derived individually. In situations where temperature and pressure fluctuate significantly, in addition to performing temperature and pressure compensation, it is also necessary to consider compensation for the gas expansion coefficient ε. Regardless of the type of flow meter used to measure the flow rate of saturated steam, pressure compensation measures must be employed when operating under conditions of fluctuating steam pressure. This is because the flow equation includes a factor related to the density of the steam; when the operating conditions differ from the design conditions, errors occur in the readings. The magnitude of these errors is related to the difference between the actual operating pressure and the design pressure – a value of P_actual > P_design results in negative errors, while otherwise positive errors occur. The dryness condition of steam is an important factor determining the accuracy of steam flow measurement. Currently, online instruments for measuring steam dryness are under development; once such instruments are incorporated into steam flow measurement and compensation systems, they will undoubtedly further improve the accuracy of measurements. The following three measures should be taken at present: (1) The pipelines for transporting steam must have adequate insulation to prevent heat loss. (2) Drainage should be provided for each section of the steam pipeline; traps should be installed at the lowest points of the pipeline as well as on the pipelines in front of the instruments, to remove condensate water promptly. (3) During boiler operation, it is necessary to avoid excessive drum liquid levels and minimize large fluctuations in load. 3. Selection of flow meters: When selecting flow meters for steam measurement, five main factors should be taken into consideration: the properties of the fluid being measured, the production process, installation conditions, maintenance requirements, and the characteristics of the flow meter itself. Here, the characteristics of flow meters, installation conditions, maintenance requirements, and several points to consider when selecting flow meters are discussed in detail. Currently, the main instruments used for measuring steam flow include vortex flow meters, differential pressure flow meters (orifice plates, venturi tubes, elbow flow meters), split-vane flow meters, Anemometer-type flow meters, and float-type flow meters. The following illustrates these with examples of vortex flow meters, orifice plate flow meters, and elbow flow meters. 3.1 Vortex Flow Meter The vortex flow meter is a new type of flow meter developed based on the Karman vortex street principle. Thanks to advantages that other flow meters do not possess, it has seen rapid development since the 1970s. It is reported that in developed countries such as Japan, Europe, and the United States, the use of vortex flowmeters has increased significantly; they are now widely used in various fields and are set to play a dominant role in flow meters in the future. They represent an ideal alternative to orifice flowmeters. It has the following features: ① It has a simple and robust structure with no moving parts, ensuring high reliability over long-term operation ； ② It is very easy to maintain, and the installation cost is low ； ③ The sensor does not come into direct contact with the medium, offering stable performance and a long service life ； ④ It outputs pulse signals proportional to the flow rate, with no zero drift, high precision, and is easy to connect to a computer ； ⑤ Wide measurement range, with a range ratio of up to 1:10 ； ⑥ Low pressure loss and low operating costs, making it more energy-efficient ； ⑦ Within a certain range of Reynolds numbers, the frequency of the output signal is not affected by the physical properties or composition of the fluid; the instrument coefficient depends only on the shape and size of the vortex generator. No compensation is required to measure the volumetric flow rate of the fluid, and there is no need to recalibrate the instrument’s coefficients after replacing the components ； ⑧ Wide range of applications; capable of measuring flow rates for both gases and liquids ； ⑨ The calibration cycle is 2 to 4 years. However, this flow meter also has certain limitations: ① The vortex street flow meter is a type of velocity-based flow meter, and the stability of vortex separation is affected by the flow velocity; therefore, it requires a certain length of straight pipe section, typically 10D in front and 5D behind it ； ② When measuring liquids, the upper flow velocity is limited by pressure loss and cavitation effects, and is generally between 0.5 and 8 m/s ； ③ When measuring gases, the upper flow velocity is limited by changes in the compressibility of the medium, while the lower flow velocity is restricted by the Reynolds number and the sensitivity of the sensor; for steam, it is 8–25 m/s ； ④ Stress-type vortex flowmeters are sensitive to vibrations; therefore, when installing such flowmeters in pipes with high vibration levels, it is necessary to implement certain vibration-damping measures in the pipes ； ⑤ Stress-type vortex flowmeters use piezoelectric crystals as detection sensors, so they are limited by temperature, typically ranging from -40 to +300°C. 3.2 Differential pressure flowmeters: Differential pressure flowmeters, represented by orifice plate flowmeters, have a long history of use, come with international standards, offer high theoretical accuracy, and are widely employed. However, after decades of use, it has been found that orifice plate flowmeters also have their shortcomings: ① Many factors in operation (mismatches between design parameters and operating conditions, insufficient straight pipe section upstream, misalignment of the orifice plate with the pipeline, contamination of the A surface of the orifice plate, sharp-angle wear, etc.) have a significant impact on their measurement accuracy, thereby increasing the measurement errors ； ② Installation is rather complicated, and maintenance as well as cleaning require a lot of effort ； ③ It requires use with a differential pressure transmitter, which increases the maintenance workload; in addition, pressure guiding pipes must be installed, and these pipes need to be insulated in winter. It cannot be installed outdoors ； ④ The flow range ratio is 1:3, resulting in significant limitations ； ⑤ If not installed correctly, steam leakage is likely to occur ； ⑥ The pressure loss is high, resulting in high operating costs. 3.3 Elbow Flow Meter The elbow flow meter is essentially a 90-degree standard elbow; there is no flow sensor with a simpler structure than this. With the development of the machining industry and the continuous improvement of industry standardization and standardized management, the cost-performance ratio of standard elbow mechanisms used as bend sensors is becoming increasingly favorable. Its features are: ① Simple structure and low cost. ② The sensor of the elbow flow meter is wear-resistant and not sensitive to minor wear. ③ It is easy to install; the direct welding method can be used, which completely eliminates the problem of leaks at the installation site. ④ It has strong adaptability, a wide measurement range, and relaxed requirements regarding straight pipe sections. Any fluid flow in a pipeline that can be measured using orifice plates, vortex flow meters, or average velocity tubes can also be measured with a elbow flow meter. Moreover, in terms of resistance to high temperatures, high pressures, impacts, vibrations, humidity, and dust, elbow flow meters far outperform other types of flow meters. ⑤ The range ratio of the elbow flow meter can reach 1:10. For steam, its applicable range is 0–70 m/s, which can satisfactorily meet the requirements for measuring steam flow. ⑥ Due to its unique measurement principle, the elbow flow meter does not require strict straight pipe sections for practical use; generally, only 5D in front and 2D behind are required, which is far less than the demands of other flow measurement devices. ⑦ Elbow flow meters feature high accuracy and good repeatability; their measurement accuracy can reach 1.14%, while the repeatability accuracy is 0.2%. Once installed, there is no need to disassemble or reassemble them, which ensures optimal accuracy in installation as well. ⑧ The most notable feature of the elbow flow meter is that it contains no additional throttling elements or inserts, which allows it to **reduce the energy required for transporting fluid through the pipes and thus save energy. This advantage is particularly evident in applications involving large systems, large pipe diameters, and low head pressures. For example, to keep an orifice plate flow meter installed in a heating pipeline with a flow rate of thousands of tons per hour operating properly, tens of thousands of kWh of electricity are required over one heating season, which amounts to tens of thousands of yuan in cost. Here, only a pressure loss of a few thousand pascals for orifice plate flow meters is considered; in actual operation, this value is far exceeded. Even with a pressure loss of just a few thousand pascals, the additional operating costs it generates are not negligible. The additional pressure loss associated with measurement using a elbow flow meter is much lower. By replacing orifice plate flow meters with elbow flow meters, operating costs can be significantly reduced, resulting in substantial economic benefits. In summary, the selection of a steam flow meter is extremely important; accurate measurement of steam flow is a prerequisite.

With steam pressure, pipe diameter, and steam temperature, how do I calculate the steam flow rate? We don’t have a steam flow meter, so we have to estimate the steam flow rate – it’s such a headache! ! ! I’m asking the fellow sailors here: is there anyone who knows how to calculate this? For example, what is the steam flow rate at a steam pressure of 7.3 kg, a temperature of 166°C, and a pipe diameter of DN50?

The pressure difference parameter is still missing, I guess