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Water As A Heating Agent

In industry today whenever possible boilers in electric and steam plants are given purified treated water. This is done in order to reduce corrosion scale formation which can result in structural damage to the boilers.

Boilers operate at super-critical pressures of 3200 psi and above.For improved efficiency water used should have a high level of purity. It should be noted that drum type boilers operating in the sub critical range are more common than once through boilers that operate in the super-critical range of 3200 psi and above.

Drum type boilers can have solids accumulate in the drums that can be removed with a fairly simple operation. In this instance there is little to no solid deposits on the turbine blades. With once through boilers this is not the case. On once through boilers the water passes into one end of a tube and exits the other as steam. This is how impurities get deposited  on the tube surfaces. This reduces the overall efficiency of heat transfer and fluid flow. In addition oxygen in the feed water can reacts with the metal surfaces creating corrosion. The deposits from this corrosion can deposited on the turbines. This will reduce their efficiency. This can also necessitate maintenance for the unit. To put it in perspective a once percent drop in the efficiency of a 200 megawatt turbine could cost a business $75,000 dollars or more annually.

In order to keep the pH level between 9 and 9.6 in this process ammonia is added to the steam condensate to minimize the impurity deposits. To combat metal deposits hydrazine is added as well. Although amounts of these two substances can vary with installation they typically are used at a rate of 0.4 ppm of ammonia and 20ppb of hydrazine over the oxygen concentration.

Steam that leaves the turbines is condensed then treated and recycled to the boiler and included in this is pre-treated make-up water. The bulk of this water used in this process is recycled so before feeding back to the boiler its treatment is very important. The requirements for this recycled water are particularly strict for once through boilers. These types of boilers often are required to have both filtration and demineralization. Make-up water goes through the same process typically and may undergo chlorination and coagulation.

Feed Water Quality for Supercritical Once-Through Boilers
Conaminant Concentration ppb
Total Dissolved Solids 500
Silica 20
Iron 10
Copper 10
Oxygen 5
pH 808 to 9.6

Numerous sources can contribute to impurities in feed water. Silica and metal salts can be in feed water even after treatment. If there are leaks in the condenser the condensate can pick up impurities in the process of the units function. Also steam steam and condensate can pick up metal oxides that form during the normal operation of a once through boiler.

Feed water for boilers can be treated in a variety of ways so as to reduce the scale formation corrosion and the harmful effects that accompany them. This is done by cold water softening. Cold water softening includes the addition of lime soda ash sodium aluminate or aluminum sulfate at the top of an open settling tank. The precipitates   that result from this are removed by the settling process. Then the water is further purified using pressurized filtration to remove the suspended solids. When applying heat this process results in a more rapid process and complete removal of hardness. Adding phosphates will even further the calcium carbonate removal in the system.

Turbine efficiency can be compromised by silica deposits. As a result silica must be removed in the case where high pressure boilers are used. To remove the silica magnesium hydroxide is used for its absorption properties of silica during the water treatment process.

Another method to soften boiler feed water is to use ion exchange resins. Sodium ion exchange resins usually are able to remove almost all of the hardness present in the feed water. Metal Cations Ca Mg Na  can be removed in their hydrogen form by using cation exchange resins. However the some troublesome carbonic hydrochloric and sulfuric acids will remain. Aeration will eliminate H2CO3 or carbonic acid. This acid can be removed by an anion exchange resin.The resulting feed water is comparable to distilled water in terms of its resistance.

There are numerous boiler compounds that claim to reduce or eliminate the challenges and problem associated with subcritical boilers and their operation. Some of these work and some of these dont. The ones that do contain organic elements that impede scale buildup on boiler surfaces.  It should be pointed out though that none of these would be a satisfactory solution for use in a supercritical boiler and its operation.

Primer : Water Use In Industry

  • Water as a Raw Material
  • Water As A Cooling Agent
  • Water As A Heating Agent
  • Water As A Cleaning Agent
  • Water As A Solvent
  • Water As A Transport Mechanism

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