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On our site You can find out all about what is Hydro-X process and what it is used for, as well as the methods to be implemented during the Hydro-X process ( conditioning boiler water, scale and corrosion removal, as well as preventing them, etc.)

 
 
 

New Danish Standards for Water Treatment on District Heating Systems

 

Correct water treatment is of vital importance to the proper operation of a district heating system. It affects the operating economy of the system as well as its preservation and should thus be given high priority on the maintenance schedule of the district heating utility.

 

It may be added that the average water loss from normal district heating systems in Denmark is 0,15% a day – slightly more in large district heating systems, less in the new decentralized combined heat and power plants. This very small water loss has been contained by combining intensive renovation with the installation of modem pipe systems and the implementation of correct water treatment. Today, a service life of 30 – 50 years is anticipated for district heating networks built according to present standards.

In 1999 Danske Fjemvarmeverkers Forening (Danish District Heating Association) updated the recommendations for the third time which applied to feed and circulation water in at district heating plants in Denmark. These standards apply to the temperature range from 35°C to 180°C. It must however be stressed that no aluminium should be allowed in the system. Aluminium corrodes at pH – values above 8,7. In the systems, a distinction is made between softened water and demineralised water used in the feed.

In tables 1 and 2 are the recommended values for application.

Compared with the former standard, which was much simpler, the above standard has been extended to include protection against corrosion problems and damage which has been experienced during the last ten years as changes also in the construction of new district heating systems, the use of new methods and materials has taken place.

Table 1: The following recommended values apply for feed water

 

Table 2: The following recommended values apply for circulating water

(*) It is not recommended to adjust the pH – value with ammonia, as the corrosion of copper alloys increases rapidly at pH – values above 9,0.

(**) as stainless steel however, is frequently used in a district heating systems today, it should be underlined, that chloride contents above 7 mg/l at elevated temperatures (94 or 108°C) will cause stress corrosion on AlSI 304 and 316 steel (see the curve elsewhere).

(***) be attentive however if the water has a smell of sewage, slime is found in the filters or an unusual increase in the consumption of chemicals.

Scale formation and deposits

It is a well – known fact that hardness of the circulation water leads to the formation of scale in boilers, heat exchangers or pipes. Deposits of scale will impede the operation of the boiler plant considerably. Figure 1 shows the increase in fuel consumption in % for each millimtre of scale. That is why the feed water must either be softened or demineralised.

The risk remains however, that raw water can penetrate the circulation system, from e.g. Leaking hot water tanks. This risk necessitates the water treatment being supplemented by adding chemicals, whereby the hardness precipitates as sludge.

Figure 1. Extra fuel consumption (%) in relation to boiler scale formation (in mm)

Fuel consumption diagramme

The consumption of fuel increases together with a reduction of the plant capacity. The scale increases the heating surface temperature however, often by up to 50%, causing an expansion of the iron that may lead to stress corrosion. It should be added, that recent investigations have shown, that deposits consisting of organic material (i.e. Bio films) have an insulation effect approximately 4 times higher than normal calcium scale.

Figure 2. heat transmission through heating surface with or without scale

Corrosion

Most types of iron corrosion are caused by the presence of oxygen. It is therefore of vital importance that the oxygen content of the circulating water is kept very low, i.e. Less than 0,02 mg/l. If oxygen is present in the water, the corrosion risk rises proportionally with increased salinity, especially if sulphate and chloride compounds are also present.

Oxygen

Oxygen is removed by passing the feed water through a deaeration plant before it enters the district heating system. Only relatively big district heating stations in Denmark have such a unit however. In fact this is also where the biggest relative water losses occur. Smaller volumes of oxygen can be neutralized by adding the correct chemicals to the circulation water. This is the mode of treatment chosen for smaller district heating installations.

 

Figure 3. Magnetite dissolution as a function of pH – value and temperature

The pH – value of the circulation water must be correctly adjusted at 9,8, and a margin of +/-0,2 is allowed. This narrow pH interval has been chosen because a high pH–value protects iron as can be seen from Figure 3, which gives the interaction between pH–value, temperature and solubility of magnetite, Fe3O4. The figure shows that the higher the pH–value, the lesser is the magnetite dissolution. The minimum pH–value should be 9,6.

The fact that a district heating system contains metals other than steel, such as copper and brass, means, however, that there is also a maximum value for pH, determined by the corrosion graph for brass (Figure 4). This figure shows that the corrosion minimum is found at pH 9,5 followed by a strong increase at pH – values above 10,0 since at this value there is an increased leaching of the zinc content of brass.

Figure 4. Brass corrosion in relation to pH – value

Stainless steel is used in smoke gas heat exchangers and plate heat exchangers and offers generally great resistance to corrosion, but it is sensitive on two counts, 1. temperature and 2. chlorides. Figure 5 makes it quite clear, where the two most commonly used types of steel – SISI 304 and 316 can be applied, since both of them are sensitive to chloride. The 304 is sensitive from 94°C, while 316 endure temperatures from 108°C before corroding as a result of the chloride content. If stainless steel is to be cleaned, never use hydrochloric acid, since this acid immediately ruins the passive membrane of stainless steel, leading to immediate damage to the steel. Sulfide corrosion is normally caused by sulfur reducing bacteria. Copper and brass are attacked and the corrosion products will appear as a black/grey crystalline layer in copper pipes, in pumps or at thermostat valves.

Figure 5. graphs for AISI 304 and 316 Stainless steel. Temperature in relation to chloride content

In Denmark we are allowed to add up to 15 mg/l of an efficient biocide named Rodalon. It will easily kill the circulating bacteria, but the destruction of a bio film that may eventually form might be more problematic.

It is prescribed that plastic pipes used in district heating must have an oxygen barring according to DIN 4726 or 4729, as an unprotected plastic pipe will allow oxygen to penetrate to the circulating water.

Filtration

Corrosion due to solid deposits is the term used for corrosion found e.g. Underneath precipitated sludge. It is extremely important to remove sludge as well as iron particles from the circulation, since this not only reduces corrosion, but also erosion and mechanical wear e.g. In pumps, as well as magnetite deposits in heat – exchangers and clogging of thermostatic valves and in addition leading to errors in the heat measurement systems. The recommendations specify that the circulation water is to be clear and sludge flee, which can only be obtained through a combination of mechanical and magnetic filtration.

The filtration is used to achieve economical and reliable treatment of the water in the system by efficiently removing sludge and magnetic particles from the network. A continual reduction of the sludge greatly reduces the sedimentation in the heat exchangers, pipes and radiators. Thereby maintaining a high degree of efficiency in the heat transfer and minimize corrosion due to sedimentation and erosion.

Part stream filters Figure 6 are mounted in parallel to the main return line and 5 to 15% of the flow is diverted through the filter and back into the main – line, thereby continuously filtering the water in the system, removing fine and medium size contaminating particles. The filtration degree should be in the range of 5 – 10 micron in a normal operational mode.

As the function is to remove the dirt from the system, the filter has to be cleaned at regular intervals. The filter is placed off – line; thereby it can easily be isolated and cleaned without diverting the main flow. To overcome the differential pressure of the filter, a small circulating pump is installed in front of the filter.

The part stream filters should always be installed in the return line and in the lower part of the system, as the sediments gravitate to this part and accumulate here.

The main reason for installing the part stream filter is to achieve one or more, if not all, of the following:

  • - To remove fine particle pollution from the system;
  • - To avoid blocking of heat exchangers;
  • - To extend lifetime on most installed components;
  • - To secure correct readings and measurements from gauges and heat meters;
  • - To avoid erosion on components;
  • - To minimize deposit corrosion;
  • - To minimize galvanic corrosion;
  • - To extend service time on pumps, thermostats, regulating valves and other components with moving parts.

Removal of the hard type particles as silicates and magnetite minimizes the erosion effect on the system and also adds to the life time of pump shaft sealing and bearings.

Filtration of feed water is achieved in feed filters. They are installed tin order to protect the parts of the system like pumps, heat exchangers, valves and pipes from physical damages caused by larger particles or elements. These filters also protects against clogging in case of laminated heat excangers as well.

Leakage and bacteria

The above standards came into effect at the end of 1999. the Danish district heating industry has passed through rapid development. New materials, new products and new concepts have been developed, and at the same time new problems have been observed.

Two types of additives to the district heating systems have been mentioned in the standards for the first time: coloring agents and tensides. The coloring agents (uranine and pyranine) can be used in danish systems, providing one solution that leaks can be found. The condition is that the consumer is informed in advance about the purpose of this treatment and what can be expected. It is not permitted in Denmark to use a coloring agent continuously in the district heating water. The tensides or the smooth water treatment is still under research and is not yet permitted for general use.

The new standards especially focus on the preconditioning of the feed water, deaeration, and adjustment of the pH-value. The recommendations for the circulating water are in practice the same but many comments for its use have been made:

  • - Be aware of oxygen penetration due to the use of accumulation tanks (use a nitrogen pillow instead of a steam pillow);
  • - be aware of eventual bacteriological growth (do not use sulfide as an oxygen scavenger, as it is a nutrient for eventual sulfur reducing bacteria);
  • - The use of reverse osmosis water is recommended in stead od normal soft water, control the ammonia content (ammonia will corrode copper in cases where only small amounts of oxygen is present);
  • - be aware of sludge formation in accumulation tanks and idle boilers.

Environment

finally the environmental aspects of using chemicals for the district heating systems has mentioned; the water treatment and the corrosion prevention must live up to the fact that district heating is an environmentally friendly may of production and has a high green profile. District heating must be environmentally approved and this approval also includes the water treatment. The use of chemicals should be reduced to a minimum, and the most environmentally friendly chemicals should be adopted.

An environmental evaluation must include the Following facts, that the hot water supply might be contaminated by the district heating water will leak to the soil, streams etc. Or simply, run to the sewage system.

Also the internal working environment must be evaluated; handling and eventual mixing of chemicals must be described to avoid accidents. Each single chemical must be described and declared, and EU – safety data sheets are at hand.

Figure 6. Filter unit in which 5 – 15% of the flow is pumped through a magnetic/bag filter and returned into the system. This unit has been extended by automatic pH – control, ensuring the correct dose of chemicals and continuous surveillance

 
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