The central heating hot water pipe network of Tianjin Thermal Power Company (hereinafter referred to as hot water pipe network) takes Tianjin No.1 Thermal Power Plant as the heat source, with a total length of 66.4 kilometers and a total heating area of 3.08 million square meters. The heating type of hot water network is intermittent heating, with 322 thermal stations providing intermittent heating. Trunk lines can be divided into south trunk line and north trunk line. The topology plan of the pipe network is shown in the following figure:
Figure 1 Schematic Diagram of Topological Structure of Pipe Network of Tianjin Heating Company
Years of practical operation and adjustment show that the characteristics of hot water pipe network are as follows:
1) There are many thermal power stations, so it is difficult to adjust the primary network.
Although the total heating area of the hot water network is about 3 million square meters, it is not too much, but the total number of thermal stations is more than 300, and the average heating area of each thermal station is less than 1 000 square meters. Such a large number of thermal stations is extremely rare in China. The number of buildings corresponding to this kind of hot water network thermal station is small, and it is difficult to adjust the secondary network. However, in the primary network, due to the large number of heating stations, the characteristics of large inertia, large coupling and poor stability of the heating network, as well as the problems existing in the system and management of some heating stations, it is difficult to adjust the primary network and requires high technical content and management.
2) The pipe network is in good order and the heating area is clear.
The whole hot water network can be divided into two main lines: the south trunk line and the north trunk line, and three major areas: the power plant exit area, the north trunk line area and the Fifth Avenue area. The pipe network is well organized and the heating area is clear.
3) In the hot water pipe network, the diameter of the main pipe is relatively thick and the resistance consumption is relatively small. For example, in the Fifth Avenue area, except for the DN300 pipeline, the specific friction between the trunk line and the branch line is generally within 10 Pa/m, which makes the first-stage pressure heads of the thermal power stations in the Fifth Avenue area except Chongqing Road have little difference and the heating effect is close.
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2. The1is out of operation.
Pursuing the stability of thermal conditions, neither horizontal nor vertical dislocation, and making the room temperature of each heating room uniform is an important goal of heating system operation and regulation.
Due to the large number of thermal power stations in the hot water pipe network, and most of them are public thermal power stations, the public thermal power stations have a long operation life, most of the equipment is aging, and the management is relatively difficult, which also brings great difficulty to the operation adjustment, giving birth to the operation mode of "large temperature difference and small flow", which makes the actual operation condition of the hot water pipe network seriously deviate from the design condition.
The design working conditions are as follows: the design temperature difference between supply and return water of primary network is 30℃, and the design flow is 4300 t/h;
Actual operation [situation: the actual temperature difference between the supply and return water of the primary network does not exceed 20℃, and the total hot water flow during the severe cold period is about 6000 t/h.
In recent years, the operation mode of hot water pipe network in heating company has been "large flow and small temperature difference". Two circulating pumps (2 for use/kloc-0 for standby) in the heat source plant are running at full load, and the total hot water flow is about 6000 t/h, which seriously deviates from the design working condition, and the hydraulic working condition is seriously out of balance. The temperature difference between supply and return water under the maximum heat load is less than 22℃.
According to the past operation experience, among the two major heating areas, the power plant outlet area has the best heating effect, followed by the northern trunk line area, the fifth avenue area is the worst, and the Chongqing Road area in the fifth avenue area is the worst (affected by maliandao bottleneck, as shown in figure 1). Thermal power stations with flow ratio above 1.2 are generally distributed in the outlet area of the power plant, thermal power stations with flow ratios between 1.0 and 1.2 are generally distributed in the north trunk line area, and thermal power stations with flow ratio below 0.8 are generally distributed in the Fifth Avenue area.
The main reasons for this phenomenon are that the operation adjustment of heating network is not in place, the temperature difference and pressure difference of pipe network are not fully established, the near-end backwater temperature is high, the temperature difference is small and overheated, the far-end backwater temperature is low and the temperature difference is large and cold. The large flow operation mode cannot fundamentally eliminate the lateral imbalance of the system, that is, the problem of uneven heat distribution among users has not been solved. According to the actual operation data of the pipe network, the horizontal imbalance of the actual hot water pipe network is 2 1.4% (the well-regulated pipe network is about 8%- 15%, and the pipe network with automatic monitoring system is about 1%-5%), which is serious.
2.2 "Large flow and small temperature difference" hazards
The essence of "large flow and small temperature difference" operation mode is to reduce the heat dissipation capacity of the near-end heat exchanger and increase the heat dissipation capacity of the far-end heat exchanger by increasing the pipe network flow, which can alleviate the imbalance of thermal conditions to a certain extent, but it has great limitations, such as the corresponding energy consumption is very large, and the more the system flow increases, the more energy consumption. Under the large flow operation mode, the system operation adjustment has the following shortcomings:
1) High power consumption per square meter.
The high-flow operation mode is realized by increasing the system flow. The pump shaft power is cubic with the pump flow, and the pump consumption is doubled for every 25% increase of the flow. Therefore, the operation mode of large flow is at the expense of huge consumption of electric energy, which leads to large power consumption per square meter of heating system.
2) Large heat consumption per square meter
Because it is impossible to increase the flow indefinitely, it is often not enough to eliminate the uneven heat and cold of users through the operation mode of large flow, and there are still cold spots in the system. At this time, it is often necessary to increase the water supply temperature of the system to improve the average room temperature of the end users, thus improving the heating effect, which is at the expense of heat energy consumption, resulting in a large heat consumption per square meter of the heating system;
3) The adjustability of the system is reduced.
The greater the flow rate, the worse the regulation performance of the system, and it is difficult for the terminal heat users to reach the required ideal flow rate, because the transportation capacity required by the system exceeds the maximum lift provided by the circulating water pump.
The primary pipe network operates under the condition of "large flow and small temperature difference", which leads to serious imbalance of hydraulic conditions of the large network and obvious uneven cooling and heating of users. The heat exchange equipment of each thermal power station can not operate according to the rated parameters, which makes the secondary pipe network deviate from the design working condition and causes a vicious circle. Therefore, it is urgent to improve the economy of the hot water pipe network, improve the transmission and distribution capacity of the pipe network, improve the heating quality, save energy and reduce consumption. At the same time, it is urgent to change the operating conditions of the hot water pipe network to meet the development needs of the heating load of the hot water pipe network.
3. Solution to the problem
Large flow is a backward operation mode, so we should constantly explore experience in operation adjustment, gradually change backward operation conditions, establish temperature difference and pressure difference of pipe network operation close to design conditions, and promote a virtuous cycle of heating system design and operation. In the heating season of 2006, our company made a large-scale adjustment of the whole network. Because the automatic monitoring system of hot water pipe network has not been fully established, manual adjustment is still the main method. Therefore, technical analysis and management in the adjustment work are particularly important.
3. 1 theoretical guidance of operation
First of all, through repeated hydraulic calculation, our company obtained the design flow and capital head of each thermal power station, and measured the characteristic curve of the flow control valve mainly adopted by our company, thus calculating the valve opening of each station, as shown in the following figure:
According to this calculation result, in the hot water pipe network, the valve with constant flow adjustment mode can set the flow value, the valve with constant pressure difference adjustment mode can set the pressure value, and the thermal station with flow adjustment valve can realize the preliminary adjustment of the thermal station by adjusting the valve opening. In actual operation, due to the influence of the opening of valves related to the main pipeline and branch pipes of the pipe network, and the deviation of the resistance characteristics of each heating network element in calculation, it will be somewhat different from the guidance value and still need to be fine-tuned.
3.2. Implementation measures
As mentioned above, it is complicated to complete the regulation of 300 thermal power plants, and whether the implementation method is feasible is also the key to whether the regulation can be promoted normally. The implementation measures of this adjustment are as follows:
1) is carried out in stages and in a planned way.
The adjustment is divided into three stages, namely, the experimental stage, the preliminary adjustment stage and the fine-tuning stage.
The experimental stage was carried out at the end of heating in 2005. The main purpose is to change the experimental operation mode of hot water network and study the characteristics of the network. On the premise of a lot of experimental work by the previous pipe network and users, the power plant shut down a circulating pump, and the flow rate dropped from 6000 tons/hour to 4800 tons/hour. At that time, there was no pressure difference at both ends of binjiang road branch line, Hebei branch line and Xinhua Road of the northern line, which was fed back to Jinhui branch line and both ends of Fifth Avenue of the southern line. The water supply temperature of these stations without pressure difference was 5 or 8 degrees lower than normal temperature. According to this experiment, we have basically mastered the main characteristics of hot water pipe network.
The initial adjustment stage was carried out in the early stage of heating and cooling in 2006, according to the principle of flow distribution ratio. We carefully analyzed the data of 300 heat exchange stations and pipe networks, determined the direction of efforts to change the operation mode, recalculated and determined the flow ratio required by all more than 300 heat exchange stations and the north-south trunk line, and divided all users into three categories: 93 overheated stations are the first batch of adjustment heat exchange stations, 82 heat exchange stations with moderate temperature are the second batch of adjustment heat exchange stations, and the rest are the third type of heat exchange stations. Each adjustment is carried out in the order of three types of heat exchange stations: overheated-moderate-unheated, so as to minimize the influence of system coupling characteristics and make the adjustment in the right direction.
In the fine adjustment stage, after the experimental stage and the initial adjustment stage, the flow conditions of the whole hot water pipe network are basically established, and the fine adjustment work is mainly carried out for special thermal stations caused by high heat consumption and low heat exchange efficiency. The main principle is to adjust according to the temperature difference of each thermal station.
2) Scientific theoretical guidance, multiple test conditions in parallel.
In the whole debugging stage, a lot of data are collected for each adjustment, and the next adjustment scheme is determined according to these data, so that each adjustment scheme is meticulous and thoughtful, and the adjustment means and methods have scientific basis.
At the same time, in the debugging scheme, two sets of operation modes, ring network operation and branch network operation, are prepared for the south trunk line under different conditions. There are clear and detailed adjustment targets for the operation mode of branch pipe network and the opening and closing state of valves of branches and trunk lines.
3) The preparatory work is completed, which makes the overall adjustment go smoothly.
In this year's debugging, we give typical examples, focus on analyzing key households, eliminate defects and remove adjustment obstacles. For example, the primary flow control valve of DN200 in Golden Mansion has never been adjusted before, because the valve cover of the adjusting rod has been rusted to death for a long time, and the valve cover is relatively thick, and at least 800 pipe wrenches are needed to clamp the valve cover, not to mention the maintenance team, even the outsourcing team does not have such a large tool and the valve is made in China, and there is no special adjusting tool, resulting in the highest temperature difference between supply and return water in the station of only 5 ~ 6 degrees. This summer, we overcame various problems. The valve was repaired and a special adjusting wrench was made. After five times of station adjustment, the primary supply and return water temperature of the station reached above 25 degrees, and the valve that had been fully opened for 25 turns was adjusted to 5.5 turns. The problem of uneven flow distribution in high and low areas was found out, and the regulating valve was persuaded to be installed in high and low areas, which was understood and recognized by users. The flow control valves of five stations, such as Geely and Hayes Aviation Maintenance Department, were also overhauled. As early as this summer, we not only reformed the heat exchange station equipment, but also eliminated the problems existing in the pipe network, such as reforming Nantong Li, Gangjianli and l_ secondary network; The transformation of household system in Chongrenli and Nantong has eliminated the hidden danger of indoor secondary network and achieved good results. Some indoor heaters have been removed and improved because of the long-term scaling of heating pipes, and most of the heating fees owed before heating have been recovered, avoiding covering up the secondary defects with large flow.
3.3. Annual operating results
The change curves of supply and return water temperature in heating season in 2005 and 2006 are as follows:
Variation curve of supply and return water temperature in heating season in 2005 and 2006.
As can be seen from the figure, in the heating season of 2006, by controlling the flow rate of hot water pipe network below 5000 t/h, the temperature difference between supply and return water of primary network increased by nearly 5℃ under the same heat load. After nearly a heating season, the operation effect is much better than in previous years.
1, the output of the pipe network can reach 6000 t/h, but it is reduced to 5000 t/h after adjustment, so that the pipe network has a margin of 1000 t/h for external heating, which provides technical support for future development load.
2. Under the condition of 5000 t/h flow, the pressure difference between supply and return water in the pipe network increases, which makes the terminal capital head increase and the pipe network tends to a virtuous circle.
3. Due to the successful adjustment this year, the temperature difference between primary supply and return is widened, which improves the heat exchange efficiency of the heat exchanger and can give full play to its energy efficiency.
4. After the pipe network is adjusted to 5000 t/h, the water supply pressure of the power plant drops by 0. 1 MPa, and the leakage points of the pipe network are obviously reduced, which improves the reliability of the safe operation of the pipe network.
4. Conclusions and recommendations
Compared with the operating parameters of previous heating seasons, in the heating season of 2005-2006, we controlled the flow of hot water pipe network below 5000 t/h, and raised the water supply temperature of primary network and the temperature difference between supply and return water by 5℃ under the condition of constant heat load. Through the operation in the past heating season, the operation effect is obviously better than that in previous years, as follows:
1) The imbalance of hydraulic conditions has been greatly alleviated.
2) After repeated adjustment in the early stage of heating, the phenomenon of uneven hot and cold hardly appeared.
3) The heating effect is better than in previous years, and the number of calls reflecting insufficient heating has dropped significantly.
It can be seen that the large hot water pipe network runs under the parameters of "small flow and large temperature difference", the hydraulic conditions tend to be more balanced, the heating quality is greatly improved, and it has great potential for load development. In the future, we will further adjust the pipe network, increase the temperature difference between supply and return water while increasing the heating load to make it close to the design temperature difference, deeply tap the heating potential, further improve the transmission and distribution capacity of the pipe network, further improve the balance of the pipe network, and make the operation mode of large-scale hot water pipe network more reasonable and standardized, saving energy and reducing consumption.
References:
(1) He Ping, Sun Gang, Heating Project (M). China Building Industry Press, 1993.
(2) sokolov. Heating and heating network M. Beijing: Machinery Industry Press, 1988.