introduction
During the operation of winter thermal power plants in the northern part of the country, the ambient temperature of some equipment may be lower than 0 °C, such as open-air boilers (or equipment); during the boiler shutdown, the low temperature area near the doors, windows, and aisles in the boiler room; direct air cooling System area, etc. When the ambient temperature is lower than 0 °C, the following situation will occur: the liquid medium in the sampling pipeline is frozen, which causes the medium parameters to be accurately measured; the compressed air pipeline of the pneumatic actuator generates condensed water, which causes the valve to fail to operate normally after freezing; There is accumulated water at the bottom of the process valve of the direct air cooling system, which causes the valve to fail to operate normally after freezing; the temperature of the direct air condensing water system and the vacuum system is too low or frozen, causing ice formation in the fin tube bundle.
In the northern part of the thermal power plant instrument sampling pipeline often uses electric heat tracing and steam heating two kinds of insulation methods, with the heat medium to release a certain amount of heat, through direct or indirect energy exchange to supplement the heat loss of the heat meter tube to achieve The normal working requirements of the medium in the instrument pipeline. The heat of steam is accompanied by the heat released by the steam heating pipe to compensate for the heat loss of the insulated pipe. Because the heat dissipation of the steam is not easy to control, the heat distribution of the entire pipe is uneven, and the localization may cause vaporization of the measured medium. Lower. The thermal measurement points of the steam and water systems in the power plant are scattered, and the sampling pipelines that require heat tracing are complicated, which makes it difficult to arrange steam heating pipes. In addition, the steam heating pipe often appears in the winter when running, "bubble, take, drip, leak" phenomenon, there are certain security risks. Compared with steam heating, electric heat tracing system has the advantages of high thermal efficiency, energy saving, long service life and automatic control. Therefore, the thermal instrumentation sampling pipeline of thermal power plant should use electric heat tracing as the preferred anti-freezing method.
1 electric heating principle
The electric heat tracing is to install a heating semiconductor polymer material between the heat insulating layer and the heat tracing pipe, and use the electric energy to compensate for the heat lost during the sampling process of the meter sampling line, thereby making the meter sampling pipeline The medium temperature is maintained at the most reasonable range. The typical electric heating cable style is shown in Figure 1.
The electric heat tracing cable is composed of a semiconductor polymer material and two parallel alloy bus bars plus an internal polymer insulating material, an alloy shielding mesh, and an outer polymer sheath (Fig. 2), wherein the semiconductor polymer material is processed by a special process. Cheng, is the main heating component.
The tin-plated copper alloy used in the electric heating cable can withstand up to 593 °C. The heat per unit length is constant, and it does not change due to changes in the external environment and the planting material. It has the characteristics of constant power. The fluoropolymer sheath provides additional protection for the heating cable and the braid, and is suitable for high temperature. Chemical and corrosive environments.
The heating process of the electric heating cable is greatly affected by the environmental tear. According to the principle of thermal expansion and contraction, when the surrounding environment of the heating cable is in a low temperature state, the conductive chemical polymer material will cause molecular shrinkage, connecting the carbon particles inside the polymer to form a loop, and then generating an electric current, and the heating cable starts to generate heat; When the surrounding environment of the heating cable is in a high temperature state, the conductive chemical polymer material generates micro-molecular expansion, so that the internal carbon of the polymer is separated, and the problem cannot be formed, thereby interrupting the circuit, increasing the resistance value, blocking the current, and outputting power. Reduced, the heat from the heating cable is also reduced accordingly. Since the conductive chemical polymer material is automatically adjusted by the ambient temperature, the temperature of the medium in the metering pipeline will fluctuate within the most reasonable interval.
2 electric heating system design
2.1 Several factors related to thermal calculation
2.1.1 Insulation thickness
Different insulation materials also have different thicknesses of the protective layer as the working conditions change. The thickness of the insulation layer of the metering tube is 10~30mm, the metering pipeline is marked as “30/25”, which means the thickness of the inner insulation layer is 30mm, the thickness of the outer insulation layer is 25mm; the sampling line of the instrument is marked as “25” (not required) Insulation). Under normal circumstances, when calculating the heat dissipation of the sampling pipeline, the thickness of the insulation layer is selected to be 25 mm.
2.1.2 Medium temperature in the pipeline
According to the characteristics of the thermal power plant process, the medium in the sampling tube can be considered in three working conditions: high temperature, medium temperature and low temperature. High temperature medium (temperature >200 °C) mainly includes superheated steam, reheated steam, economizer inlet feed water and superheater desuperheating water; medium temperature medium (65 ~ 200 °C) mainly includes condensed water, deoxidized water, etc.; Strong medium (maximum temperature is 65 ° C) is normal temperature water. According to the temperature of the medium in the sampling tube of different meters, different heating cables can also be used, which can meet the requirements of heat preservation and reduce the construction cost. The measuring and sampling pipeline of high temperature medium should adopt electric heating cable with constant power; the measuring and sampling pipeline of medium temperature and low enthalpy medium should adopt self-controlled temperature electric heating IQ and cable.
2.1.3 Thermal conductivity of thermal insulation materials
When the thickness of the protective layer of the metering pipeline is constant, in order to improve the heat preservation effect, an insulating material with a small thermal conductivity should be selected. The aluminum silicate insulation material is usually used in the design process, and its thermal insulation coefficient is 0.044 W/(m·°C).
2.2 Calculation Selection Example
Taking a thermal power plant in the north as an example, the desulfurization water pressure measurement sampling pipeline length L1 is 15m, the outer diameter d is 14mm, the medium temperature t1 of the measuring sample tube is 90°C, and the minimum ambient temperature t2 is -25°C, using silicic acid. Aluminum insulation material, insulation coefficient A is 0.044W / (m · ° C), insulation thickness δ is 25mm, pressure measurement sampling pipeline heat dissipation Q calculation formula is as follows:
Q=2πλ(t1-t2)/ln[(d+2δ)/d]=21W/m.
The electric heating cable can be laid by the parallel method and the winding method. If the calculated heat loss per unit length is less than the rated heating value per unit length of the electric heating cable, the parallel method is generally adopted, that is, the heating cable is directly attached to the pipe using aluminum tape, and the heat-resistant tape is used every 0.5 m. The electric heating cable can be wrapped; if the calculated heat loss per unit length is greater than the rated heating value per unit length of the electric heating cable, the winding method is generally adopted. The winding method needs to first determine the winding pitch S of the electric heating cable, and then calculate the length L of the electric heating cable according to S. The winding pitch S is calculated as follows:
S=πd/(K2-1)1/2 and
Where d is the outer diameter of the pipe, mm;
The length factor of the K-electric heating cable, K is equal to the heat dissipation Q of the sampling line divided by the meter power of the electric heating cable.
The length L of the electric heating cable is calculated as follows:
L=(L1/S)[S2+(πd)2]1/2
Table 1 lists the parameters of four types of CWH2 electric heat tracing cables.
Taking CWH2-20W electric heating cable and deaerator water pressure measurement, the heat dissipation amount Q of the sampling pipeline is 21W/m, which is calculated by the above formula: S=137.38mm, L=l7.3m. Inquire about the electric heating cable laying inventory in the construction phase of the power plant. The actual laying amount of the heating cable for the deaerating water pressure measurement sampling pipeline is 17.5m, the design error rate is only 1.14%, and the design accuracy rate is high.
When the electric heating cable is laid by the winding method, the amount of the electric heating cable is calculated by the estimation method in the past, that is, the length of the insulated sampling line is usually 1.5-2 times. If the 1.5-fold coefficient is selected, the 15m de-oxygen water pressure measurement sampling pipeline requires a 22.5m electric heating cable, which has a large deviation from the actual actual installation amount. It can be seen that using the above formula to calculate the length of the electric heating cable can accurately calculate the usage of the whole plant electric heating cable and reduce the construction cost of the engineering material.
2.3 power supply design
Since the electric heating cable needs 220V AC power supply, in the power supply design, it is usually necessary to set up a power supply cabinet for the power distribution of the main building electric heating cable to meet the power supply requirements of the electric heating cable in each area of the main plant. Under normal circumstances, the electric heat tracing power supply cabinet is arranged in the boiler operation layer, and the electric heat tracing power supply cabinet power supply comes from two 380V AC power sources, which are the MCCA segment and the MCCB segment of the boiler operation layer, and then pass the electric tour, automatic switching device, and the disk. Small busbars and fuses are allocated to the power supply circuit of the main plant instrument incubator. In the above selection example, the electric heating cable with the medium temperature t1 of 90 ° C and the power of 20 W/m is insulated by the length of 15 m, and the lOOOW electric heater and 60 W incandescent lamp are arranged in the incubator. The heating current of the heating cable is 1.37A, the capacity of the optional branch miniature circuit breaker is 2A, and the capacity of the main circuit miniature circuit breaker of the incubator should be selected as lOAa.
3 electric heating system installation precautions
(1) Master the requirements of the supplier's "Electrical Heat Tracing System Installation and Operation Manual", accept the professional installation training of the manufacturer's technical personnel, and conduct insulation performance test on the electric heating cable.
(2) Anti-rust, anti-corrosion, deburring and acute angle treatment of instrumentation pipes and valves.
(3) Reasonably install the inner insulation layer of high temperature medium pipeline (aluminum silicate insulation cotton), and the thickness is not lower than the design value to prevent direct contact with the burnt electric heating cable.
(4) After installing the electric heating cable, the outer insulation layer and the rhodium-plated tinplate, the insulation test shall be carried out to prevent the heat from being excessively lost and the personnel from being burnt.
4 application effects
The above method is applied to the design of the electric heat tracing system of a 2*330MW unit in a thermal power plant in the north. The total design of the heating cable for a single unit is 960m, and the actual electric heating cable is 994m. The design error rate is low. Practice has proved that the design of the electric instrument sampling tube electric heating system according to the design method described in this paper not only has good effect, saves investment, reduces engineering cost, and can fully utilize the advantages of electric heating system.
