Valves in heating networks

When we think about the energy transition, the first thing that comes to mind is usually solar panels, wind turbines or heat pumps. But a crucial part of this transformation often remains hidden away beneath our roads, in shafts and technical facilities: district heating networks. They form the backbone of a sustainable heat supply in cities and communities. And at the centre of it all, inconspicuous yet essential, valves are hard at work. By controlling and shutting off the fluid handled, they ensure that the heat arrives reliably and efficiently exactly where it is needed.

This article highlights the key role played by valves in modern heating networks, explaining the variety of designs available and the requirements valves need to meet with regard to quality, durability and sustainability. Because without valves, heating networks simply would not be possible – and neither would the climate-friendly heating of future. 

The attached schematic diagram illustrates the structure of a modern heating network. It shows how the different heat sources – such as combined heat and power plants, large heat pumps, industrial waste heat, data centres and environmental heat from rivers or lakes – all feed into a common district heating system. The system is supplemented by a thermal store, which serves as a buffer and supports the demand-driven supply of heat.

The generated heat is transported to various consumers via a branched network of pipes. These include municipal facilities such as residential and public buildings as well as industrial consumers requiring large amounts of heat. The diagram distinguishes between generators (red) and consumers (green), highlighting the direction in which thermal energy flows in the network.

This visualisation underlines the versatility and flexibility of modern heating networks, which allow both centralised and decentralised feed-in. It also shows how sustainable and efficient operation can be achieved by integrating different energy sources, in particular renewable and recovered heat.
 

The first district heating networks were laid in New York, USA, around 150 years ago. The idea of transporting and distributing heat over long distances was very popular, as it offered many advantages for prosperity. Buildings could be connected to a heating network and were supplied with external heat for heating and hot water. In those days, decentralised hand-fired coal stoves were the standard source of heating in buildings. In Europe, Germany was the first country to install heating networks around 1900. Since then, heating networks have developed over the course of four generations of technology.

The first generation used steam as a heat transfer medium at temperatures of 130 to 200 °C, which led to high losses, inefficient systems and the risk of scalding. The second generation operated at temperatures of 100 to 130 °C and the third at around 95 °C. Lower temperatures were supported by better thermal insulation of the buildings, which also reduced the risk of accidents caused by scalding. Today, hot water networks are primarily used and only a small number of steam networks still exist. Most heating networks are based on the second, third and fourth generation of technology, with the fourth generation using temperatures of around 70 °C.

There is already talk of the fifth generation of heating networks for the future which will operate at network temperatures of 10 to 30 °C. Decentralised heat pumps will then be used to increase the temperature up to consumption level. This technology enables the use of many different heat sources and promotes the expansion of renewable and environmentally friendly energy sources. Another advantage of these cold networks is that they can extract heat from buildings on hot summer days and thus cool them down. However, lower temperature levels within the network need to be regulated, requiring the precise control and distribution of the heat transfer media, which is where various valves come into play.

Many different types of valves are used in heating networks which are distinguished by their function or design characteristics. On the generator side, consultants often use butterfly valves, gate valves or globe valves, as these are easily accessible for maintenance purposes and are more cost-effective than ball valves for large nominal sizes. Butterfly valves and gate valves are used for shutting off fluids. These valves are usually in an open or closed position and must function perfectly and close tightly when required. Control valves are used on the generator side for flow control and mixing. Various poppet valves with linear movement are used for this purpose, which are controlled by an electric actuator or the process fluid, depending on the application. At high temperatures, metal-seated valves are generally used, consisting of a metal seat and closing element. At lower temperatures below 80 °C, soft-seated valves are the better choice, as they are cheaper to manufacture and more resistant to possible particles in the heat transfer medium.

In heating networks, low-maintenance valves with low hydraulic resistance are preferred. Ball valves with a single-piece body and butt weld ends are ideal as they consist of just a few components and offer a full flow cross-section in the open position. They are operated underground in a valve pit or via a valve trim.

In so-called zone controlled HVAC systems, special valves are used that are operated according to demand, ensuring hydraulic balancing. The control characteristics of these valves are particularly important here. At transfer stations, valves are needed for shutting off and, if necessary, regulating the fluid handled. Furthermore, check valves, air valves and safety valves are also found in heating networks.

Valves in district heating networks need to be particularly durable, as they are often difficult to access and would incur high costs if they were to malfunction. The quality of the valves plays a major role for operators, which is why proven valve types and manufacturers are mainly used. Most specifications require the current standards such as DIN EN 13941-1, DIN EN 488 and EN 253 and stipulate the construction materials. When selecting the material, it is important to consider not only its resistance but also the long-term thermal exposure.

Shut-off valves with butt weld ends are used for underground pipes, as they form a perfect positive-locking connection with the pipe and no additional sealing elements or screw elements are required, minimising the risk of leakage. These welded joints also offer greater safety in the event of deformation underground.

As valves should be actuated as slowly as possible to avoid pressure surges in the long steel pipes, the high flow velocity in the intermediate positions of the valves can cause damage to sealing elements. Manufacturers have developed special solutions for this in terms of the choice of materials and the design of the individual components.

Although ball valves are technically preferred for large nominal sizes, they are expensive as they require a lot of material and sophisticated machining. Proven alternatives are butterfly valves and gate valves. Gate valves have a lower hydraulic resistance, but a high centre-to-top height, which requires more space. Butterfly valves generally have a greater flow resistance due to the valve disc, but require less space as the shaft rotates and does not rise. Due to their simpler design, butterfly valves are lighter and cheaper to purchase for larger nominal sizes.

The quality of the products is also reflected in the sealing to atmosphere over many decades. District heating water is specially treated and additives are added to ensure that the system lasts a long time, does not corrode and produces fewer deposits. If this fluid were to escape due to a leak, it would have to be refilled and result in additional costs. In addition, it would be detrimental to soil and environmental protection if the fluid were to escape and seep into the ground in large quantities.

District heating offers huge opportunities for achieving climate targets. Operators are switching to environmentally friendly technologies and reducing the need for fossil fuels. The transition is not only good for people and the environment, but also offers economic opportunities for companies to develop new solutions and make them available on the market. In addition to the high quality requirements, valves for heating networks increasingly feature intelligent solutions in which individual components exchange information and valves are able to do more than just open and close. The future will require valves that further improve the overall system, are particularly durable and are hydraulically optimised.