1 Introduction

As an important secondary energy source, steam is widely used in heating, cooling, and production processes as a heat transfer medium. Therefore, increasing steam production, transportation, and use efficiency can save a lot of energy. Accurate measurement is the key to improving energy management. Due to the particularity of steam, there are many difficulties in measurement, and it has been a long-standing problem in flow measurement for a long time. On January 1, 2007, the National Standard GB17167-2006 “Energy Equipment Measurement Equipment Distribution and Management General Principles” has been formally implemented. Under the guidance of the General Regulations, a set of characteristics suitable for China's production enterprises will be gradually established, which is scientific and economical. The availability of practical steam metering equipment is an urgent issue in current energy metering management.

2, the steam flow metering problems

For the sake of discussion, we first list the flow equations for the following standard throttling devices:

(1)

Where: qm - steam mass flow, unit kg / s;
C - Outflow coefficient;
d - the orifice or throat diameter of the throttling element in working conditions, in mm;
ε - coefficient of expansion;
β - diameter ratio (β = d/D );
Δp - differential pressure, unit Pa;
ρ - density of steam under operating conditions in kg/m3

2.1 About steam density

For the flow measurement of steam, people all know that pressure and temperature compensation are needed. However, because of the insufficient understanding of the complexity of steam properties, in the entire set of measurement systems, only the accuracy of differential pressure, temperature, and pressure signals is often emphasized. It is willing to make great efforts to use high-precision transmitters as much as possible, and neglects the importance of density in measurement. In fact, this problem is very clear just by looking at the throttling device flow equation.

From the formula (1), it can be seen that the differential pressure and the density are in the same position in the measurement. However, in actual use, since the influence of the density on the flow measurement is neglected, the careful study and analysis of the density are often neglected. In current flow secondary display instruments (systems), most of the calculations of steam flow density are simple mathematical expressions or look-up tables, and their calculation accuracy is doubtful.

Most of the water vapor applied in the project is just off the liquid or close to the liquid. Its properties are different from those of the ideal gas and should be regarded as actual gases. The physical properties of water vapor are much more complex than those of ideal gases, so they cannot be described by simple mathematical formulas. China has not established a national standard for the “steam thermodynamic properties table” but uses the steam thermodynamic properties table of foreign publications. The International Formulation Committee (IFC), established by the 6th International Water Vapor Nature Conference, formulated the IFC formula for calculating the thermodynamic properties of water and water vapor. Based on this, it formulates new formulas and is familiar to everyone. The "Industrial Formula IFC Formula 1967" (formula IFC-67). China has translated the water vapor thermodynamic properties table of the United States, the former Soviet Union, and West Germany. Regardless of the country's water vapor properties table, any number table that conforms to the IFC 1967 formula is correct. At present, China's relevant regulations have cited this formula.

With the continuous improvement of engineering technology and the level of scientific research, the calculation accuracy and speed requirements for the thermal properties of water and steam have been continuously improved. The IFC-67 formula has drawbacks such as low calculation accuracy, long calculation iteration time, and narrow application scope. It's getting more and more obvious. Therefore, the International Federation of Water and Water Vapor Properties (IAPWS) held in Erlangen, Germany, in 1997 passed and published a brand new water and a joint research team composed of 12 scientists from 7 countries including Germany, Russia, Britain, and Canada. The steam calculation model is the IAPWS-IF97 formula. These two formulae have some differences in the scope of use, regional division, and calculation accuracy, but within the common range of industrial steam flow measurement (temperature 0 °C - 600 °C, pressure 0.1MPa - 5MPa), the deviation of the two formulas is minimal.

2.2 Problems with Wide Range

We all know that the accuracy level of the general instrument is represented by the reference error, while the reference error is a relative error that is simplified and practically convenient, and is the relative error of the instrument at the range. Therefore, the closer the measured value is to the full-scale value, the higher the accuracy. However, in the flow measurement of steam, especially in the case of steam transportation, the actual flow range cannot be accurately determined. Actually, the flow meter often operates at the lower limit of the range and even exceeds the lower limit of the range, which results in a relatively large relative error. Therefore, when designing a steam flow metering system, consider using a wide-range flow meter or a system with wide-range compensation.

The introduction of intelligent wide-range differential pressure transmitters and flow compensation computers with more sophisticated compensation functions makes it possible to have a wide range of intelligent throttle meters. To sum up it should have three conditions: 1. Intelligent wide-range differential pressure transmitter (differential pressure range of 100:1). 2. The digital communication between the differential pressure transmitter and the flow computer (Hart protocol) can not only satisfy the accuracy of full-range differential pressure signal transmission, but also can automatically migrate the measurement range. 3. The flow computer can not only correct the working flow according to working parameters such as temperature, pressure, etc., but also can calculate the outflow coefficient C, flow velocity expandability coefficient ε, etc. in real time. The wide-range intelligent differential pressure flowmeter that meets the above conditions meets the accuracy and the flow measurement range can truly reach 10:1 (or more). This leap of flow meter is a number of technological advancements. As a result, it has changed the traditional understanding of throttling flowmeters.

2.3 About Energy Metering

Thermal energy is a state parameter, and steam of different states and equal masses has great differences in thermal energy. For example, superheated steam at a pressure of 0.8 MPa and a temperature of 200°C contains 2838.6 kilojoules of heat per kilogram of superheated steam at a pressure of 0.8 MPa at a temperature of 220°C per kilogram. Thermal energy was 2884.2 kJ, a difference of 1.6% between the two. Therefore, a settlement unit whose quality is steam cannot truly reflect the value of steam. In view of the above situation, the use of energy as a settlement unit for steam has not only been a consensus in the academic community, but has also been responded to by a large number of steam production and use units. It calls on the state to issue relevant standards to further scientifically regulate steam metering, taking into account People have used the habit of using steam quality as a unit for many years, and as a transition, they should still retain the quality as a steam settlement unit. Changing the trade measurement unit is a more complex issue, but the first implementation of steam energy measurement within the enterprise is feasible on a technical level, and it will be a very beneficial practice at the management level.

2.4 Questions about the transformation of existing systems

At present, the level of automation of production facilities of enterprises is getting higher and higher, most of them use DCS or PLC systems as monitoring and control systems, and the measurement points of enterprises are also integrated into the entire system. In these systems, the compensation calculation model for steam flow is inadequate in software programs. If the procedures for modifying these systems are limited by technical forces and the risk of affecting the real-time, integrity, and security of the entire system, most companies Is not willing to move DCS program. Therefore, in the measurement and transformation, we need to find a solution that not only can meet the requirements of accurate measurement, but also does not destroy the entire instrument control system framework, has good integration with DCS or PLC, and is suitable for data access.

3, steam flow metering equipment plan

3.1 What kind of steam metering system should we have?

To do steam metering, we should first answer the question “What steam metering system should we have?”. We believe that a reasonable and advanced steam metering system should have the following characteristics:

(1) Field instruments meet the accuracy requirements, maintenance-free, low failure rate, and good stability (the verification cycle can be longer).
(2) The compensation is perfect and the compensation algorithm complies with relevant standards.
(3) It has the function of energy metering.
(4) has a wider range.
(5) Has historical data storage, event alarms and other management functions.
(6) Easy to implement networking.

3.2 One-time instrument allocation plan

Our recommended steam flow metering instrument is a YJLB integrated nozzle flowmeter, which is equipped with temperature and pressure transmitters for temperature and pressure compensation.

Throttle flow meter occupies an important position in the steam metering. The throttling flow meter technology is mature, especially the standard throttling device designed and manufactured according to the standard (ISO5167-2003E, GB/T2624-1993) does not require real flow calibration, which is unmatched by other flowmeters. In the study of steam measurement, domestic and foreign scholars used standard throttling devices to conduct a large number of tests, and given a revised mathematical model, so the use of standard throttling devices to measure steam has obvious advantages. In fact, at present, China still uses standard orifice throttling devices for more than 90% of steam meters. Therefore we use standard throttling devices. The reason for choosing nozzles instead of orifices is as follows:

A disadvantage of the standard orifice plate is that the sharpness of the entrance angle is prone to passivation under the fluid scour. It is reported that the relevant domestic authorities have performed tracking verification on the newly installed orifice plate. When the orifice plate is continuously used for 2-3 months, the passivation causes outflow. The coefficient skewness is 1-3%, each serious is above 4%, which has aroused people's great attention. At present, the best way to solve the problem of the passivation of standard orifices is to use standard nozzles. Since the entrance of the nozzles is a smooth curved surface, it is not easy to wear (see Figure 1). Its outflow coefficient is very stable, so the JJG640-94 procedure stipulates that the ISA1932 nozzle verification cycle is 4 years. Furthermore, at the same flow rate and the same β value, the nozzles have a much smaller resistance loss than the orifice plates (only 50-60% of the orifice plate). Long-term operation shows that the nozzles have impact resistance due to their structural advantages. The advantages of deformation, adapt to high temperature, high pressure, high flow media.

Throttle flow meter system composition is more complex, there is a long pressure tube, easy to block, winter operation also need to pressure tube, condensing system for insulation, heat tracing, slightly inadvertently cause failure, and instrumentation Large maintenance workload. The YJLB integrated nozzle (see Figure 2) solves the above problems. The integrated nozzle integrates the throttling element and the differential pressure transmitter and is equipped with an antifreeze isolator (patent number: ZL200520127402.4). It not only shortens the pressure piping, but also eliminates the need for condensation and sewerage systems. This simplifies the system, eliminates the need for insulation and heating, and saves energy while reducing maintenance.

YJLB integrated nozzle features:

â—† The use of standard throttling parts, measurement accuracy is based on;
â—† Patented anti-freezing isolation technology, no need of heating during winter operation, less maintenance;
◆ The nozzle throttling piece has a small resistance loss (about 60% of the same β-value orifice at the same flow rate);
â—† There is no problem of passivation of the orifice, the impact resistance is not easy to be deformed, the coefficient is stable, and the verification period is long (4 years);
â—† Configuration of intelligent differential pressure transmitter, flow measurement range up to 10:1 or more;
â—† Easy to install.

3.3 Secondary Instrument and Data Network Equipment Solution

The recommended secondary flow meter for steam flow measurement is the FC2000-1AEG flow conversion unit (see Figure 2).

FC2000-IAEG flow calculation conversion unit is a new concept of networked flow measurement equipment. It adopts the structure type of instrument cabinet mounting, collects and correlates the flow-related signals at the site, and communicates with the network via RS232/485 serial port, Ethernet port, etc., and also has 4-20mA of temperature, pressure, and compensation flow. Signal output function. The flow calculation and conversion unit can conveniently realize remote supervision and management, establish a distributed measurement management network, and implant high-precision flow calculation links in DCS and PLC systems.

FC2000-IAEG has the following advantages:
â—† Based on relevant international standards and recommendations, national and industry standards, a variety of mathematical models and corresponding calculation software have been established for different media and flow meter types to calculate the steam energy flow.
â—† The parameters such as the outflow coefficient C of the throttling flow meter and the coefficient of flow expansion of the streamer â–¡ are used as dynamic quantities to perform point-by-point operations in real time to achieve a wide range.
â—† It has 4-20mA signal isolation output function such as temperature, pressure, compensation flow, etc. It can send temperature and pressure signal back to DCS.
â—† With the audit record function of historical data storage, alarm recording, power off of the instrument, modification of parameter settings, even if the network system fails, the flow measurement data will be stored in it so as not to be lost.
â—† Installation of instrument cabinet racks eliminates the need to install holes in the instrument panel to save space in the instrument cabinet.
â—† The traffic calculation software used by FC2000-IAEG has passed the certification of the national authoritative department.

The Application of FC2000-IAEG Embedded High Precision Flow Calculation in DCS and PLC System

At present, most control and monitoring systems for petrochemical, chemical, and metallurgical production facilities use DCS or PLC, and the flow measurement points must also be introduced. However, the DCS and PLC functions focus on monitoring and control, and their functions in the operation of high-precision flow are relatively lacking. Both use simplified formulas for flow compensation calculations. If you let DCS or PLC perform complex flow compensation calculations, not only will configuration programming be very complicated, but it will also greatly increase the burden on DCS. Using the FC2000-IAE(G) flow calculation conversion unit, the high-precision flow calculation link can be easily implanted in DCS and PLC systems. The FC2000-IAE(G) flow calculation and conversion unit can transmit operating temperature, pressure, and compensated flow digital signals to the DCS or PLC through the communication interface (RS485, RS232, or Ethernet); it can also pass FC2000-IAE (G The multi-channel isolated analog output port transmits the 4-20mA signal of operating temperature, pressure, and compensation flow to the analog input port of DCS or PLC, as shown in the following figure:

4, steam flow measurement network examples

The primary instrument uses YJLB integrated nozzle flowmeter; the secondary instrument uses FC2000-1A/1AD flow computer or FC2000-1AEG flow conversion unit to form the steam metering system in an independent or combined manner in Beijing Yanshan Petrochemical, Tianjin Petrochemical, Qilu Petrochemical, Jingmen Petrochemical, Shijiazhuang Refinery, Quzhou Refinery, Yizheng Chemical Fiber, Hubei Fertilizer Plant and other enterprises have been applied.

The steam metering scheme has also been successfully applied to the Jiangsu Jiangsu Thorpe Group's steam energy metering network system. The system realizes the networking of steam energy measurement data within the company, and the coexistence of steam quality and energy measurement. The system is structured as follows:

● Flow meter adopts YJLB integrated nozzle flowmeter, equipped with temperature sensor and pressure transmitter.

● The secondary instrument adopts FC2000-1A/1AD flow computer and FC2000-1AEG flow conversion unit. The FC2000-1A/1AD is used at metering points with independent metering panels. FC2000-1AEG is used in DCS control system. When it is needed, it can output temperature and pressure signals to DCS again. Both of them can calculate the steam energy flow.

● The FC2000-1A/1AD traffic computer and the FC2000-1 AEG traffic conversion unit access the enterprise LAN through the Ethernet interface.

● The upper computer system is constructed in the Client-Server mode. The remote server for collecting FcLinker traffic data is installed in the central server to complete the data collection of the interface machine and store the data in the database.

● The data acquired by the acquisition management server is distributed via the Web server on the enterprise LAN through Web pages, including the Instantaneous Data Technology Webpage, Instantaneous Data Sheet Webpage, and Historical Query Webpage. Each mounted terminal can be used. Browser (such as: Internet Explorer comes with the Windows operating system, etc.) to view.

System diagram: