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，集中供热输送管道也已经应用了数十年，通常比在北欧所用的管径更大。 随着中国集中供热系统的迅速发展，以整体能源效率为目标, 与北欧一样，集中供热长输管线的应用逐渐增加。
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.
北欧集中供热输送管网的最大流速通常取决于对不同备选方案的运行费用（泵的电耗以及热损失）和投资成本（管线投资）的财务评估。 下图左图显示了计算年总分配成本的原理。 右图合并了总成本，并添加了第三个轴：温度水平。
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.
此外，要进行水力安全分析，以确保在所选最大流速时泵调节或阀调节等工况的系统安全性。 噪声是决定管道最大流速时应考虑的另一个因素，尤其是对用户附近的管道。 还应当注意，在小口径管道中的高流速可能更加值得关注，因为小管径管道里产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.
北欧有标准规定最大允许压力，6 bar（表压力），10 bar（表压力），16 bar（表压力），25 bar（表压力）等，但没有标准规定最小压力。 太低的压力相对于稍微高一些的压力，可能会造成具有更大威胁的后果。 在一定压力下，根据温度情况，水会蒸发成蒸汽。 而蒸汽形成后在管线中到移动是不可预测到，在其冷凝为液体之前，都可能产生巨大的压力峰值。 因此，确保在正常运行和任何故障的情况下系统中不出现过高或过低的压力，至关重要！
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：
水力瞬变的起因源和后果不同，则有许多不同的解决方案来解决这些问题。 然而，这些解决方案在可靠性、投资成本和运行费用方面可能存在很大差异。 为了说明这一点，下面给出一个例子：
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.
假设一个山区地带的中国城市，某发电厂位于城市之外，地势高于市中心。在两地之间建有安装了中继泵站的集中供热长输管线，将来自发电厂的热量供应到海拔位置低于发电厂的市区中心（图中右侧）。图中显示的稳态水力分析显示，系统在正常运行中是安全的。在中继泵站中的泵跳闸的情况下，会有一个压力波触及“A”点的压力上限，而另一个压力波触及“B”点的压力下限（防止汽蚀），此风险显而易见。动态水力分析将显示是否有触及任何压力限值。然而，没有任何软件能够为潜在的问题提供最可靠和/或成本效益最好的解决方案。在这种特殊情况下，解决问题的可能办法是： i）增加尺寸以减小流速，ii）增加管道系统的压力等级并增加定压，iii）安装隔压站而不是中继泵站，iv）在热电联产厂和中继泵站中改变水泵扬程， v）将在中继泵站中的泵布置改变为对称设置泵组，vi）在重要位置安装膨胀罐，缓冲罐，蒸汽释放阀，vii）建造直接连接的蓄热罐，既可以作为定压点和又能作为热电厂和集中供热系统的压力分离设备。不难得知，不同解决方案之间的成本投入可能差别很大。建一个热交换器站，而不是仅仅改变泵的布置或仅调整水泵扬程，可以会有几千（而不是几百）万元的投资差异。为了降低流速而增加管道尺寸，不仅会增加管道投资，还会增加热损耗量。
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.
下面的压力图说明了在某中继泵站中的泵跳闸的情况下，沿着管线路线的水力动态工况。 该压力图从通过动态水力分析生成的电影/动画中导出，展示了系统中的整个压力瞬变事件。 左边的第一个图表显示了0秒（正常运行，泵跳闸前）的压力水平。 左起的第二个图示出了在泵跳闸之后5秒的压力水平。 左起第三个图示出了在泵跳闸之后10秒的压力水平。 第四张图表说明了泵跳闸后50秒的压力水平。
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.
本文作者Mikael Jakobsson先生，拥有瑞典皇家理工学院的工程学硕士学位，拥有15年以上静态和动态水力分析经验。 Jakobsson先生连续8年深入中国集中供热市场，在北京和南京开展了50多个项目。 他早年在在斯德哥尔摩能源（Fortum 富腾集团）工作，负责大斯德哥尔摩地区供热系统的运营和设计优化。而受到一辈子从事供热工作的父亲的影响，早在孩童时期，他就开始跟随父亲在供热现场管理和监督整个集中供热系统了。
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年以来一直活跃在中国，在热电、区域能源和大型热泵项目中提供工程咨询服务。 在中国，腾茂通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，腾茂是中国本土设计院的补充。 拥有扎根在中国的国际和国内专家的高效组织，为当地市场及时并提供高性价比的专业服务，是腾茂能够在本地市场取得成功的主要原因。
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.