JPH0519941B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for measuring the dryness of wet steam fed through a pipe. [Prior Art] The dryness of wet steam is an important control target when operating, for example, a steam turbine. In a steam turbine, the steam used in the turbine becomes wet steam and is exhausted, but the saturated steam in this wet steam causes condensed water generation and causes corrosion of the turbine's metal parts. It was also familiar. Furthermore, in devices such as steam ejectors that generate vacuum using steam pressure, it is clear that the degree of vacuum generated is greatly influenced by the wetness of the steam. For this reason, the dryness of wet steam in the exhaust of a steam turbine is measured. Conventionally, for example, the amount of heat exchanged between steam and water in an insulated container was measured, and the ratio of the steam was measured. Find enthalpy,
A method of determining dryness based on this is known. [Problems to be Solved by the Invention] However, in the conventional wet steam dryness measuring method as described above, it is necessary to take out steam from the steam piping during actual measurement. It is difficult to measure the dryness of steam. In addition, the weight of the steam in the measurement state, that is, the weight of the steam whose heat exchange amount with water in the insulated container was measured, is required as data. Since it is wet steam with an unknown temperature, although its volume can be easily measured, it is very difficult to measure its weight, making it difficult to put it into practical use. In other words, in reality, if the volume and weight of wet steam and its temperature are known, the degree of dryness of that steam is also known, and the problem with the prior art is that the weight of wet steam cannot be determined easily and accurately. exists. Therefore, in the day-to-day management of the performance of conventional equipment such as steam turbines and steam ejectors that use steam as a working fluid, priority has been given to equipment management, and the quality of the steam has not been given much consideration. Specifically, for example, if a device does not perform as expected, the cause is first determined in the mechanical structure, and then, if no mechanical problem is found, the steam It is often concluded that the cause is the quality (dryness) of steam, and the reality is that the performance of devices that use steam as a working fluid has not been sufficiently improved. [Means for Solving the Problems] The present invention has been made in view of the above-mentioned problems, and it heats wet steam under a constant temperature condition during pipe delivery (supplying latent heat). ) dry saturated steam, the weight of the dry saturated steam determined from the easily measurable volumetric flow rate of this dry saturated steam, the amount of heat required for heating, and the dry saturated steam and saturated water at the same temperature. By determining the dryness of wet steam from the enthalpy, it is possible to calculate the dryness of wet steam at any position in the steam piping without taking out steam from the steam piping.
The object of the present invention is to provide a method for measuring the dryness of wet steam that can easily and accurately measure the amount of steam and its dryness. The method for measuring the dryness of wet steam according to the present invention is a method for measuring the dryness of wet steam fed through a pipe, in which the steam is measured on the upstream side of the pipe using a heating device placed at an appropriate position in the pipe. Dry saturated steam is obtained by heating humid steam whose steam temperature is slightly higher than the temperature measured downstream, and the weight flow rate G of dry saturated steam is detected based on the flow rate of this dry saturated steam and the steam pressure in the pipe. , the dryness x of the wet steam from the weight flow rate G and its enthalpy h'' of this dry saturated steam, the amount of heat Q required for heating by the heating device, and the enthalpy h' of saturated water at the same temperature as the dry saturated steam. is determined by the following formula: x=1-Q/G・(h″-h′). [Principle of the Invention] First, before describing specific embodiments of the invention, the principle of the invention will be described. FIG. 1 is a Pv diagram showing changes in the state of wet steam, with pressure P on the vertical axis and volume v on the horizontal axis. this
Isothermal lines a~b~e~ with constant temperature in the Pv diagram
Among f, the period between a and b is a liquid section where the volume v gradually increases as the pressure P decreases, and the section between e and f is a section where superheated steam expands adiabatically as the pressure P decreases. It is. And between b and e is wet steam,
That is, it is a zone of a mixture of saturated liquid and saturated vapor,
In this section, for example, if xKg of saturated steam exists in 1Kg of wet steam (1Kg - xKg is saturated liquid), this x is called dryness (1 - x is wetness). Incidentally, the section from b to e in this Pv diagram is on the isothermal line but is not in an adiabatic state. That is, latent heat flows in and out as the dryness x changes.In other words, when the dryness x increases, the saturated liquid vaporizes and converts into saturated steam, but at that time, the heat of vaporization is absorbed, In the opposite case, the saturated steam releases the heat of condensation and is converted into a saturated liquid. That is, in this section, there is no temperature change in the wet steam as a whole, but latent heat is flowing in and out, and if the dryness x is different (if latent heat is flowing in and out), there is an enthalpy difference. Now, the enthalpy h of this wet steam is expressed by the following equation (1), where the enthalpy of the saturated liquid is h', the enthalpy of the saturated steam is h'', and the dryness of the wet steam is x as mentioned above. h= h′+x・(h″−h′) …(1) On the other hand, since the “difference in enthalpy” corresponds to the amount of latent heat, wet steam with weight G and dryness x becomes dry saturated steam (dryness = In the case of conversion to 1), the following equation (2) holds true, where Q is the amount of latent heat of combustion absorbed at that time. G・(h′−h)=Q …(2) Therefore, by substituting equation (1) into equation (2) and rearranging, we get the following
Equation (3) is derived, and the dryness x is determined from this. x=1-Q/G・Ph″-h′) …(3) Now, to determine the weight G of the steam, for example, if wet steam is being fed through a pipe, the weight flow rate needs to be measured. The above-mentioned [prior art]
In this case, there was a problem in that it was difficult to measure the weight of wet steam whose dryness x was unknown. However, in the case of dry saturated steam, if the temperature is known, the specific weight (the reciprocal of the specific volume, specifically the weight per unit volume) is known, so it can be reduced to simply measuring the flow rate of gas. and this is easily doable. In addition, when putting into practical use the principle of the present invention as described above, it is necessary to confirm whether or not the wet steam with a dryness of less than 1 has become dry saturated steam with a dryness of 1 without excess or deficiency. This is because even if the temperature difference between before and after heating the wet steam is 0°C,
If the heating is insufficient, the wet steam will not become completely dry and saturated steam, and on the other hand, if the heating is excessive, the dry saturated steam with a dryness level of 1 will immediately become superheated steam and the temperature will rise. Because it will come. Therefore, in the present invention, the temperature of the steam after heating is very small compared to the temperature of wet steam before heating, and more specifically, it is better if it is smaller, but in practice, a thermocouple used as a temperature detector is used. In consideration of the detection accuracy, etc., in the embodiment described later, the conversion of the wet steam to dry saturated steam is confirmed by heating the wet steam until the temperature rises by 0.1°C. In this way, in the present invention, it is confirmed that the wet steam has completely dried to become saturated steam by making the dry saturated steam become 0.1°C higher than the wet steam. The error may be completely ignored in practice, as explained below. As an example, Table 1 shows 10, 20,
It shows the saturated temperature of dry saturated steam in the range of 30 at., the enthalpy at that temperature, the temperature and enthalpy of slightly higher superheated steam, and the enthalpy difference per unit temperature between the two.

【table】

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on drawings showing embodiments thereof. FIG. 2 is a block diagram of an arithmetic and control system along with the configuration of an apparatus used to implement the method for measuring dryness of wet steam according to the present invention. 1 in the figure is a steam pipe, and wet steam WV is fed from the right side to the left side in the figure. A heating device 3 is attached to an appropriate portion of the steam pipe 1. This heating device 3 includes a heater 31 made of a resistance wire wound around the steam pipe 1;
The heat insulating material 32 prevents the heat generated by the heater 31 from dissipating and heats only the steam inside the pipe through the pipe wall of the steam pipe 1. The wet steam WV flowing in the steam pipe 1 is given latent heat by the heating device 3 and becomes dry saturated steam DV. The heater 31 is supplied with power from the power supply 12, and the power supply line has a current A applied to the heater 31.
and a variable resistor 7 (resistance value: r) that controls the amount of heat generated by the heater 31 by changing the voltage applied to the heater 31. equipped. Note that if the power source 12 is a constant voltage power source, the voltmeter 6 may be omitted. On the other hand, a pressure gauge 13 for measuring the pressure P of the wet steam WV and a pressure gauge 13 for measuring the pressure P of the wet steam WV are located slightly upstream (wet steam WV side) of the installation position of the heating device 3 in the steam piping 1.
A temperature detector 21, such as a thermocouple, is provided to measure the temperature T1 of the WV. On the other hand, there is dry saturated steam slightly downstream (dry saturated steam DV side) of the heating device 3 in the steam pipe 1.
A flow meter 9 for measuring the flow rate (volume flow rate) V of DV and a temperature detector 22 having the same configuration as the temperature detector 21 described above and for measuring the temperature T2 of the dry saturated steam DV are provided. There is. The detection results of each of these detectors, i.e., wet steam
Temperatures T1 and T2 of WV and dry saturated steam DV, pressure P of wet steam W, volumetric flow rate V of dry saturated steam DV, current value A and voltage E applied to the heater 31
is given to the arithmetic and control unit 8. The arithmetic and control unit 8 determines that the difference between the detection signals T1 and T2 of both temperature detectors 21 and 22 is substantially 0, specifically, the wet steam WV is heated by the heating device 3 and given latent heat, and the wet steam WV In order to confirm that all of the saturated water inside has become saturated steam, the temperature rises very slightly (about 0.1°C) and both temperature detectors 21,
The difference ΔT (=T2−T1) between the detected values of 22 is 0<ΔT≦
The resistance value r of the variable resistor 7 is adjusted to the extent that it is maintained within the range of 0.1. In addition, the detected value A (ampere) of the above-mentioned ammeter 5, the detected value E (volt) of the voltmeter 6, and the volumetric flow rate V of dry saturated steam DV measured by the flow meter 9.
(m ³ /H), and then calculate the dry saturated steam DV at each temperature given in advance in the form of a numerical table or approximate polynomial and stored in an appropriate storage device.
The enthalpy h'', specific temperature product v'', and enthalpy h' of saturated water are read out, and the calculations explained in the above [Principle of the Invention] section are performed to calculate the dryness x of the wet steam WV. . Next, the actual procedure for carrying out the method of the present invention using the apparatus shown in FIG. 2 will be explained with reference to the flow chart of FIG. 3 showing the arithmetic processing procedure of the arithmetic and control unit 8. First, the arithmetic and control device 8 detects both temperature detectors 21 and 2.
2 and the detected values T1, T2, and P of the pressure gauge 13 are read. Then, the difference ΔT (=T2−T1) between the two is determined. If the temperature difference ΔT is 0°C (ΔT = 0), there is a possibility that the wet steam WV will not completely dry and become saturated steam DV because the heat generated from the heating device 3 is insufficient as described above. , the arithmetic and control unit 8 reduces the resistance value r of the variable resistor. As a result, the amount of heat generated from the heating device 3 becomes larger than before, so more heat is given to the wet steam WV than before, and the saturated vaporization of saturated water in the wet steam WV is promoted. Ru. On the other hand, the temperature difference ΔT is 0.1℃ or more (ΔT＞
0.1), wet steam WV is dry saturated steam
After reaching DV, the steam is further heated excessively and becomes superheated steam, so the arithmetic and control unit 8 increases the resistance value r of the variable resistor. As a result, the amount of heat generated from the heating device 3 becomes smaller than before, so that less heat is given to the wet steam WV, and heating of the dry saturated steam DV is suppressed. Now, the temperature difference ΔT is 0.1℃ or less (0<ΔT<0.1)
In the case of , wet steam WV becomes dry saturated steam DV and then is further heated to become dry saturated steam DV, but the temperature difference ΔT is only within 0.1°C, so it is not dry saturated steam. The enthalpy difference between DV and superheated steam heated to 0.1℃ is extremely small, and as mentioned above, the enthalpy of superheated steam heated to 0.1℃ can be regarded as the enthalpy h'' of dry saturated steam DV. Under this condition of 0<ΔT<0.1, the arithmetic and control device 8 performs a calculation to obtain the degree of dryness x.That is, the arithmetic and control device 8 first stores the information in the form of a numerical table or approximate polynomial. Enthalpy h'' and specific volume of dry saturated steam DV at pressure P and temperature T1
v″, also the enthalpy of saturated water at temperature T1
Read h′. Next, the arithmetic and control device 8 reads the current value A from the ammeter 5 and the voltage E from the voltage system 6, and reads the amount Q of heating of the wet steam WV by the heating device 3.
For example, when the power supply 12 is a DC power supply, it is determined by the following equation (4). Q=0.24×A×E...(4) Furthermore, the arithmetic and control device 8 reads the steam flow rate V from the flow meter 9, and uses the previously read pressure P and temperature T1.
From the specific volume v″ of dry saturated steam DV at
Calculate the weight flow rate G of dry saturated steam DV after heating using equation (5). G = V Calculate the dryness x. [Effect] As described above, according to the present invention, wet steam is heated at a predetermined position of the steam piping by a heating device until the steam temperature measured on the downstream side is slightly higher than the steam temperature measured on the upstream side. Heating to obtain dry saturated steam, calculate the weight flow rate G of the dry saturated steam based on the flow rate of this dry saturated steam and the pressure inside the pipe, and calculate the weight flow rate G, the amount of heating heat Q by the heating device, and the enthalpy h of the dry saturated steam Since the dryness x of dry saturated steam is determined from ``, there is no wastage of thermal energy for heat treatment of wet steam, and the process is simple, and the weight flow rate can be measured using known measuring equipment such as an orifice type. It is easy to obtain, the equipment cost is low, and the various constants required for calculations such as the density of dry steam can be used by using the widely known constants for dry saturated steam, which eliminates the hassle of setting new constants. The present invention has excellent effects.

[Brief explanation of the drawing]

The drawings show embodiments of the present invention.
Figure 2 is a state diagram of wet steam, Figure 2 is a block diagram of the arithmetic control system along with the configuration of the equipment used to carry out the method of the present invention, and Figure 3 is a flowchart showing the details of the arithmetic control of the arithmetic control device. . 1... Steam piping, 3... Heating device, 5... Ammeter, 6
... Voltmeter, 7... Variable resistance, 8... Arithmetic control device, 9
...flow meter, 12...power supply, 13...pressure gauge, 21,2
2...Temperature detector, WV...wet steam, DV...dry saturated steam.

Claims (1)

[Scope of Claims] 1. A method for measuring the dryness of wet steam fed through a pipe, using a heating device placed at an appropriate position in the pipe to measure the temperature of the steam at a temperature downstream of the steam temperature measured at the upstream side. Dry saturated steam is obtained by heating the wet steam until the measured steam temperature becomes slightly higher, and the weight flow rate G of the dry saturated steam is detected based on the flow rate of this dry saturated steam and the steam pressure in the pipe. The dryness x of the wet steam can be calculated from the weight flow rate G and its enthalpy h'', the amount of heat Q required for heating by the heating device, and the enthalpy h' of saturated water at the same temperature as the dry saturated steam using the following formula: x=1 -Q/G・(h″-h′) A method for measuring the dryness of wet steam.