Design optimization of long district heating transmission pipelines
Mikael Jakobsson, 迈克尔·雅各布逊
District heating transmission pipeline, district heating, district energy, environment friendly
Introduction to long district heating transmission pipelines
In Northern Europe long district heating transmission pipelines have been applied for decades, being a technical solution to achieve multiple benefits, including but not limited to;
Utilization of surplus heat from remote industries and power plants
Balance available heat production capacity and heat demand, by merging district heating networks
Merging district heating networks to achieve a more optimized global production mix
The distance of a “long” district heating transmission pipeline is not defined in Northern Europe, and is a relative term. The feasibility of long district heating pipelines is project specific and will highly depend on the local conditions.
In Northern Europe some district heating transmission lines ranges more than 100km. Some are connecting remote heat sources such as surplus heat from industries or power plants, while others are somewhat more complex connecting two or several individual networks.
In China district heating transmission pipelines has been applied for decades as well – often larger (in terms of pipe dimensions) than the ones that can be found in Northern Europe. As the Chinese district heating systems are developing rapidly, aiming for global energy efficiency, utilization of district heating transmission lines for the very same purposes as in Northern Europe are increasing.
在中国，会合供热保送管道也曾经使用了数十年，通常比在北欧所用的管径更大。 随着中国会合供热条理的敏捷开展，以全体动力服从为目的, 与北欧一样，会合供热长输管线的使用逐步添加。
The abundant heat resources from remote power plants are being utilized in greater extent which allows heating areas to be expanded and local boilers to be demolished.
The technical- and financial feasibility of long district heating transmission lines has been widely debated. There is no single correct answer in regards to the feasibility of long district heating transmission lines, as it highly depends on the local conditions, design practice, chosen technologies, implementation, operation and maintenance etc.
Therefore, to copy a transmission line design from Northern Europe without evaluating the feasibility in the local Chinese context, is doomed to fail. Nevertheless, there are many lessons to learn from long district heating transmission line project in Northern Europe, that can be used as inspiration when developing solutions for the local conditions in China.
There are different approaches when designing district heating transmission pipelines.
The district heating transmission systems for merging networks in greater Copenhagen, VEKS and CTR, are independent systems separated from the local district heating systems with heat exchanger stations. VEKS and CTR has higher pressure- and temperature levels that the local district heating systems. The VEKS district heating transmission system is approximately 135km long and the CTR district heating transmission system is approximately 55km.
In greater Stockholm, however, most district heating transmission pipelines are directly connected with the local district heating systems and designed for the same pressure- and temperature levels. The greater Stockholm district heating system comprises several district heating transmission pipelines ranging up to approximately 80km individually.
In both Copenhagen and Stockholm, the local district heating system comprises primary and secondary networks, most often separated with building level substations. There are pros and cons with both approaches. In Stockholm the philosophy is to minimize OPEX and CAPEX, as large-scale heat exchanger stations are expensive, and will generate a temperature drop that will influence the efficiency of power plants, heat-pump facilities, flue-gas condensation etc. However, as the district heating system in greater Stockholm comprises several district heating companies, it is important that there is a well-developed cooperation model to avoid conflicts as events in one system will influence the others. This has been addressed with tailor-made planning- and operation tools, special trained operation optimization personnel, solid cooperation models, among others. In Copenhagen one of the arguments for separated systems are：clear system/ownership boundaries and that the systems can be individually designed depending on the needs. The figures below illustrate the greater Copenhagen district heating system and the greater Stockholm district heating system.
The maximum velocity in district heating transmission pipelines in Northern Europe will most often depend on a financial evaluation comparing OPEX (pump cost and heat losses) and CAPEX (pipeline investment) for different alternatives. Below, the left graph illustrates the principle of calculating annual total distribution cost. The right graph has consolidated the total cost, and added a third axis; temperature level.
北欧会合供热保送管网的最大流速通常取决于对差别备选方案的运转用度（泵的电耗以及热丧失）和投资本钱（管线投资）的财政评价。 下图左图表现了盘算年总分派本钱的原理。 右图兼并了总本钱，并添加了第三个轴：温度程度。
Additionally, hydraulic safety analysis is carried out in order to assure the safety of the system in case of i.e. pump maneuver or valve maneuver with the chosen maximum velocity. Noise is another factor that should be considered when deciding maximum velocity of pipelines, not least for pipes near consumers. It should also be noted that high velocities could be more critical in small dimensions, as larger pressure losses are generated than in pipes with larger dimensions.
别的，要停止水力平安剖析，以确保在所选最大流速时泵调理或阀调理等工况的条理平安性。 噪声是决议管道最大流速时应思索的另一个要素，尤其是对用户左近的管道。 还该当留意，在小口径管道中的高流速能够愈加值得存眷，由于小管径管道里发生的压降比大管径管线的压降更大。
Hydraulic safety (transient-state) analysis
Hydraulic safety (transient-state) analysis are frequently carried out in Northern Europe to assure that the district heating systems are safe. The analysis is carried out both during design of new systems, but also for existing systems to assure that any new operation modes are safe as the systems develop continuously.
Almost any system could have potential safety issues in case of i.e. pump trips, pump maneuver or valve maneuver, but for systems with high velocities, long distances or high elevation differences the importance of carrying out hydraulic analysis is even more critical.
In Northern Europe there are standards regulating maximum allowed pressure, 6 bar(g), 10 bar(g), 16 bar(g), 25 bar(g) and so on, but there are no standards regulating minimum pressures. Too low pressure can have even greater consequences than slightly too high pressures. At a certain pressure, depending on the temperature, the water will evaporate to steam. The steam formation can move unpredictable in the pipeline until it condense back to liquid and a huge pressure peak may occur. Therefore, it is important to assure that neither too high or too low pressure occur in the system, both in normal operation and in case of any failure.
北欧有规范规则最大容许压力，6 bar（表压力），10 bar（表压力），16 bar（表压力），25 bar（表压力）等，但没有规范规则最小压力。 太低的压力绝对于略微高一些的压力，能够会形成具有更大要挟的结果。 在肯定压力下，依据温度状况，水会蒸发成蒸汽。 而蒸汽构成后在管线中到挪动是不行预测到，在其冷凝为液体之前，都能够发生宏大的压力峰值。 因而，确保在正常运转和任何以障的状况下条理中不呈现过高或过低的压力，至关紧张！
Below a pipe that has been affected by a water hammer is illustrated (left picture) and a screenshot from the animation from the regular hydraulic transient analysis showing the transient events in Stockholm district heating system in case of pump trip (right picture).
To understand the theories behind hydraulic transient calculations, and thus understand the results and their origin, is of great importance when carrying out hydraulic safety analysis. This is not least important as hydraulic transients can be devastating, ruin assets for millions (not to say billions) due to broken pipes, compensators, heat-exchangers etc., but even worse; be a matter for personal safety as hot water can be released and harm both workers and the public. Only by understanding the source of critical hydraulic transients, feasible safe solutions can be developed and implemented – a software is only a calculation tool.
Depending on the origin and consequence of the hydraulic transient, there are many different solutions to solve such problems. The solutions could however differ a lot in terms of reliability, investment cost and operation cost. To illustrate this, an example is presented below：
水力瞬变的原因源和结果差别，则有很多差别的处理方案来处理这些题目。 但是，这些处理方案在牢靠性、投资本钱和运转用度方面能够存在很大差别。 为了阐明这一点，上面给出一个例子：
In a fictive Chinese city with valleys, a power plant is located outside the city at a higher elevation than the city center. A district heating transmission line, with a booster pump station, is constructed to supply heat from the power plant to the city center (to the right in the picture), which is located on a lower elevation than the power plant. The hydraulic steady-state analysis presented in the picture, shows that the system is safe in normal operation. In case of pump trip in the booster pump station, there is an obvious risk that one pressure wave hits the high pressure limitation at “A”, and that another pressure wave hits the low pressure limitation (for evaporation) at “B”. Hydraulic transient analysis will show if it is likely, possible or unlikely to hit any of the pressure limitations. However, no software is able to suggest the most reliable and/or cost effective solution to potential problem. In this specific case, possible solutions to the problem could be; i) increased dimensions to reduce the velocity, ii) increased pressure rating of the pipeline system and increase the holding pressure, iii) install a heat exchanger station instead of the booster pump station, iv) change pump head in CHP and booster pump station, v) change pump arrangement to symmetric pumping in the booster pump station, vi) install pressure vessels, surge tanks, steam release valves in strategic places, vii) construct a direct connected Thermal Energy Storage tank to act as combined holding pressure and pressure separator between CHP and the district heating system, among other solutions. It can easily be realized that the cost implication between the different solutions may vary dramatically. To construct a heat exchanger station, instead of just change the pump arrangement or even just adjust pump heads, could differ with tens (not to say hundreds) of million RMB in investment. To increase pipe dimensions in order to reduce velocity, would not only increase the pipe investment, but also increase the heat losses.
假定一个山区地带的中国都会，某发电厂位于都会之外，地形高于市中央。在两地之间建有装置了中继泵站的会合供热长输管线，未来自觉电厂的热量供给到海拔地位低于发电厂的郊区中央（图中右侧）。图中表现的稳态水力剖析表现，条理在正常运转中是平安的。在中继泵站中的泵跳闸的状况下，会有一个压力波触及“A”点的压力下限，而另一个压力波触及“B”点的压力上限（避免汽蚀），此危害不言而喻。动向水力剖析将表现能否有触及任何压力限值。但是，没有任何软件可以为潜伏的题目提供最牢靠和/或本钱效益最好的处理方案。在这种特别状况下，处理题目的能够方法是： i）添加尺寸以减小流速，ii）添加管道条理的压力品级并添加定压，iii）装置隔压站而不是中继泵站，iv）在热电联产厂和中继泵站中改动水泵扬程， v）将在中继泵站中的泵部署改动为对称设置泵组，vi）在紧张地位装置收缩罐，缓冲罐，蒸汽开释阀，vii）制作间接衔接的蓄热罐，既可以作为定压点和又能作为热电厂和会合供热条理的压力别离设置装备摆设。不难过知，差别处理方案之间的本钱投入能够差异很大。建一个热交流器站，而不是仅仅改动泵的部署或仅调解水泵扬程，可以会有几千（而不是几百）万元的投资差别。为了低落流速而添加管道尺寸，不只会添加管道投资，还会添加热消耗量。
This case presented above represents a relatively common district heating system in China, and illustrate some of the important matters to consider while both designing new safe systems, or implement safe solutions to existing systems. Merged district heating systems with several production sites, booster pump stations and other facilities will require more experience from the engineers carrying out the transient analysis in order to define, analyze and develop solutions for the most critical scenarios.
As a comparison the greater Stockholm merged district heating system should be mentioned, which comprise over 20 production sites, 3 different pressure levels, over 10 booster pump stations, over 100 meter elevation difference, 4 Thermal Energy Storage tanks and production costs that change on daily basis which change the operation modes and which direction the water is distributed.
Below a pressure diagram and heat load curve (inc. boiler priority) for a newly developed Chinese district heating system is illustrated, comprising 3 merged district heating networks, 2 booster pump stations, 5 production sites and 1 transmission pipeline.
Below pressure diagrams illustrates the hydraulic transient events along the pipeline route, in case of pump trip in one of the booster pump stations. The pressure diagrams are exported from the movies/animations, generated through hydraulic transient-state analysis, illustrating the entire pressure transient event in the system. First diagrams from the left illustrates the pressure levels at 0 seconds (normal operation, before pump trip). Second diagram from the left illustrates pressure levels at 5 seconds after pump trip. Third diagram from left illustrates pressure levels at 10 seconds after pump trip. Forth diagram from left illustrates pressure levels at 50 sec after pump trip.
上面的压力求阐明了在某中继泵站中的泵跳闸的状况下，沿着管线道路的水力动向工况。 该压力求从经过动向水力剖析天生的影戏/动画中导出，展现了条理中的整个压力瞬变事情。 右边的第一个图表表现了0秒（正常运转，泵跳闸前）的压力程度。 左起的第二个图示出了在泵跳闸之后5秒的压力程度。 左起第三个图示出了在泵跳闸之后10秒的压力程度。 第四张图表阐明了泵跳闸后50秒的压力程度。
From the diagrams, it can be seen that after 10 seconds, too low pressure appears in the second pump station (third diagram form left), and that after 50 seconds too low pressure appears near the CHP (forth diagram from left).
Nordic District Heating expertise present on the Chinese market
The article is written by Mr Mikael Jakobsson, who holds a M.Sc. degree in Engineering from the Royal Institute of Technology (Sweden), and has practiced hydraulic steady-state and transient-state analysis for over 15 years. Mr Jakobsson has worked 8 years continuously on the Chinese District Heating market, and carried out over 50 projects being stationed in Beijing and Nanjing. Mr Jakobsson started his career working for Stockholm Energy (Fortum), being responsible for operation and design optimization of the greater Stockholm District Heating system. Influenced by his father, who has been engaged in District Heating management and supervision of all his life, Mr.Jakobsson started his experience even before school, and worked with his father every day around the district heating sites.
Today Mr Jakobsson holds the position as Chief Marketing Officer, as one of the international competences in the Swedish engineering consultancy company Termoekonomi.
本文著作人Mikael Jakobsson老师，拥有瑞典皇家理工学院的工程学硕士学位，拥有15年以上静态和动向水力剖析经历。 Jakobsson老师延续8年深化中国会合供热市场，在北京和南京展开了50多个项目。 他从前在在斯德哥尔摩动力（Fortum 富腾团体）任务，担任大斯德哥尔摩地域供热条理的运营和设计优化。而遭到一辈子从事供热任务的父亲的影响，早在孩童时期，他就开端追随父亲在供热现场办理和监视整个会合供热条理了。
Termoekonomi has been active in China since 2004 providing engineering consultancy services within Thermal Power, District Energy and Large-scale Heat-Pump facilities. In China Termoekonomi operates through the fully owned subsidiary Beijing Ruitengmao Energy Conservation Technology Co. Ltd (RTM). In Sweden Termoekonomi acts as a design institute, while in China Termoekonomi acts as a compliment to the local Chinese design institutes. Having an efficient organization with a combination of international and domestic specialists, localized in China, have shown to be a successful way to provide professional services timely and cost-effective.
瑞典腾茂公司（Termoekonomi AG）自2004年以来不断活泼在中国，在热电、地区动力和大型热泵项目中提供工程征询效劳。 在中国，腾茂经过全资子公司北京瑞腾茂节能科技有限公司（RTM）运营。 在瑞典，腾茂是一家专业设计院，而在中国，腾茂是中邦本土设计院的增补。 拥有扎根在中国的国际和国际专家的高效构造，为外地市场实时并提供高性价比的专业效劳，是腾茂可以在当地市场获得乐成的次要缘由。
Below the areas of special expertise, services of expertise and Termoekonomi’s value propositions are listed.
For more detailed information about the article (both in Chinese or English), if you have any needs for design optimization, operation optimization, safety analysis, general due diligence etc. of your district energy system, or if you just need some general advice; don’t hesitate to contact Mr. Mikael Jakobsson at email@example.com, or China District Heating Association.