Allan Hansen discusses how to ensure a service life beyond 30 years and how to reduce the service life costs for a district heating network
Today’s pre-insulated distribution network has an expected service life of 30 years, and it may even last up to 50 years. However, that long a service life can only be expected if you use quality products, ensure that only treated district heating (DH) water is used in the network, and that the pipe system is continuously maintained.
This article gives a brief overview on how to ensure a service life beyond 30 years and how to reduce the service life costs for a DH network.
In order to ensure a constantly low heat loss throughout the entire service life of the distribution network, it is essential to choose a pre-insulated pipe system with a good insulation thickness which has a low thermal insulation value. If – depending on the dimension – it is also chosen to have a diffusion barrier between the outer casing and the insulation, it is ensured that the good insulating properties remain high throughout the entire service life of the pipe.
A well-insulated distribution network results in a good operating economy throughout the entire service life of the distribution network.
|Cross-linked PE shrink joint|
The weakest link in a DH distribution network is the casing joints. It is therefore essential to use high-quality casing joints. They may be cross-linked PE shrink joints or weld joints, i.e., joints where the casing joint and outer casing are melted together in a computer-controlled welding process. To ensure that the casing joints are installed correctly, all fitters have to be certified in the installation of the casing joint type they are installing.
In order to ensure that the fitter is able to install the casing joint correctly, all the equipment used has to be qualified tools and the installations have to be supervised.
To ensure that the installation has been carried out in accordance with the installation instructions, the installation itself must be documented. This can be done by means of a casing joint report in which each activity is noted. As for weld joints, the welding process must also be documented. If the installation is carried out by a certified fitter and the installation instructions observed, then the risk of faulty installation of casing joints will be very small.
|The ‘WeldMaster’ welding machine|
As a minimum, all components in a DH distribution network have to meet the requirements in the EN standard related to the product.
Treated DH water
To prevent internal corrosion of the pipes it is imperative only to use treated DH water, i.e., water with a low oxygen content and a high PH value (9.5-10). There will always be a certain water loss in a DH distribution network, so makeup water must continuously be fed to the system, and in this connection it is essential that the makeup water has the proper quality so internal corrosion of the pipes is prevented.
A pre-insulated pipe system is usually buried, and therefore it is difficult to assess the condition of the pipe system physically. There are many possibilities of faults and damages to the pipe system. The most common fault is a welding fault to the service pipe and faulty installation of casing joints due to human error. These are faults which will typically appear immediately after or during the first years after commissioning. It is crucial for the service life and heat loss of the pipe that a fault is fixed as soon as possible.
There are many different kinds of damages which a buried pipe system can be subjected to.
If water enters the insulation either from the inside or the outside, there are methods which can visualize this.
If the pipe system does not have a built-in surveillance system, it is possible to assess the condition of the pipe system by making a thermographic measurement. This is done by measuring the temperature of the cover layer over the pipes. If the temperature at one spot is higher than the other cover layer temperatures, it indicates that the pre-insulated pipe is damaged and that water has entered the insulation at that spot. A fault which can be detected by means of thermography is a fault which should be repaired as soon as possible, because the damage may have existed for quite some time.
|Excavation damage (left), Penetration damage (middle), Tunnelling of fibre- optic cable (right)|
Thermographic measurements only give a snapshot of the condition of the pipes, and the method is only able to measure major faults in the pipe system. Thermography is measurements of temperature differences, so the measurements can be made during the winter where the differences are the largest.
If the pipe system does not have a surveillance system, it is necessary to make thermographic measurements of the network regularly in order to make the necessary repairs, before the damage increases and at the end reduces the service life of the pipe system.
The most widely spread surveillance system for a DH distribution network is the Nordic system. In all its simplicity it consists of two or more 1.5 mm2 copper wires, embedded in the insulation. These copper wires are used to measure the electric resistance between the service pipe and the copper wires and to measure the impedance of the insulation encircling the copper wires.
If moisture enters between the copper wire and the service pipe, then the total electrical resistance between the copper wire and the service pipe will diminish. This indicates a fault in the pipe run, but not in the section where the fault is. That is, after a fault has been registered, it must be localized.
The resistance measurement can be made manually or with a stationary detector. A manual measurement only gives a snapshot of the condition of the pipeline, so it is only possible to react on what you see and initialize a repair, if necessary.
If the surveillance wires are connected to a detector which is constantly measuring the resistance between the copper wire and the service pipe as well as continuously registering the measurements, then it is possible to observe and assess faults. On the basis of the assessment of the fault it is possible to plan the repair of the damage.
|Thermographic measurements of fault|
Impedance measurement is a more advanced measuring technique by which a pulse is emitted through the copper wires. It works like an echo, registering a possible change in the material around the copper wires, i.e., the PUR foam.
If water is present in the PUR foam, then the impedance of the foam will also change. Contrary to the resistance measurement, it is also possible to locate a fault by means of impedance measuring by calculating the distance to the fault on the basis of the speed of the pulse. The instrument which is used makes this calculation automatically.
Impedance measurement can be carried out with a portable pulse reflectometer or at a stationary detector with impedance measurement. The portable pulse reflectometer is used to find and localize faults, registered by means of resistance measurements, so they can be repaired. The stationary detector with impedance measurement also measures and registers the resistance between the copper wire and the service pipe.
A comparison of the continuous resistance measurement and the impedance gives a really good overview of the condition of the pipe, and in case of a fault it is possible to observe the spread of the fault by comparing the history of the pipe run and the new measurements. This gives a unique possibility of assessing the fault.
The detector which measures the impedance as well as the resistance in the copper wires can send the information via a GPRS system, so it is conveniently possible to monitor the DH distribution network from a PC via the internet. It is not necessary to go around to each and every detector to read the measurements. If there is a failure in the network, you will receive an e-mail and a SMS message and the location of the fault appears on your PC.
When measuring the distance to a possible fault with a pulse reflectometer or a stationary detector with impedance measurement, the wire distance including cables from the detector to the pipes – not the pipe length – is measured. It is therefore important that an as-built drawing of the cable and wire route, including a length specification, is available. Without these drawings it will be difficult to establish the correct localization of the fault, and there is a risk of excavation at the wrong location.
Valve and valve chamber maintenance
To ensure that a valve will also work when needed, it is important to follow the instructions of the supplier as regards the intervals at which to activate the valve. To prevent a valve from perishing in the valve chamber it is necessary to ensure that the valve is not flooded and that any sludge is removed from the chamber. The valve chambers are the only localizations of the DH distribution network where it is possible physically to see the component, so the condition of the valves should be checked at regular intervals.
Service life of the pipe system
A pre-insulated pipe may have a considerably longer service life than the 30 years stated in the standard, but it requires that:
• The operating temperatures are limited to the maximum temperature stated by the supplier;
• The static design of the pipe system is correct;
• Only treated water is used;
• The necessary repairs of possible leaks in casing joints or damages to the outer casing are made as soon as they occur;
• The valves are activated continuously in accordance with the instructions of the supplier and the valve chambers are maintained.
Heat loss in the distribution network
If there is no fault in the network and yet there is a discrepancy between the energy delivered from the DH plant, the energy sold, and the expectable heat loss, then it should be examined whether there are older pipe runs with huge heat losses which ought to be replaced.
An efficient DH distribution network is a network in which the temperature difference between the temperature from the DH plant to the end user is as little as possible, so it is possible to lower the temperature in the entire distribution network and thereby minimize the total energy loss. It may be financially advantageous to the complete network to replace individual sections in order to obtain the lowest possible temperature in the network.