The Results of Silica on Steam Turbines

Steam and water analysis system. Steam and water analysis system (SWAS)(1) is a system dedicated to the analysis of steam or water. In power stations, it is usually used to analyze boiler steam and water to ensure the water used to generate electricity is clean from impurities which can cause corrosion to any metallic surface, such as in boiler and turbine. (3)

Another important area of concern as far as silica deposition is concerned is boiler tube. Silica scale is one of the hardest scale to remove. Because of its low thermal conductivity, a very thin silica deposit can reduce heat transfer considerably, reducing efficiency, leading to hot spots and ultimately ruptures.


Video advice: The explosion of a steam turbine

The explosion of a steam turbine


Corrosion and erosion are major concerns in thermal power plants operating on steam. The steam reaching the turbines need to be ultra-pure and hence needs to be monitored for its quality. A well designed Steam and Water Analysis system (SWAS) can help in monitoring the critical parameters in the steam. These parameters include pH, conductivity, silica, sodium, dissolved oxygen, phosphate and chlorides. A well designed SWAS must ensure that the sample is representative till the point of analysis.

EDI for Power Plants – pPower plantsuse high purity water to drive steam turbines. Cogeneration plants are highly efficient, using both exhaust turbines and high pressure steam turbines. In the …/p.

To keep power efficiency high, maximize uptime and minimize turbine maintenance, boiler makeup water must be ultrapure (17+ Megohm. cm) and have very little silica (SiO2). Silica levels required are in the range of 5-20 ppb, depending on the boiler pressure. Under the boiler/turbine conditions, dissolved silica will form a glass on surfaces like the turbine blades.

Corrosion – Steam turbines are machines that convert the heat energy of steam from a water boiler into rotary motion. Their interior consists of a series of blades that capture the steam and provide a rotational force. As it rotates within a magnetic field, the turbine generates electric power. This principle constitutes 80 percent of the means of electric power generation worldwide. The purity of steam that passes through the turbine is crucial to its function and efficiency. Mineral and organic contaminants are present in the reservoir and river water that provides the steam source. These can be silica, detergents from urban waste or salts such as sodium chloride and sodium sulfate. Silica Silicon is the world’s most abundant element after oxygen. It does not occur as a sole element but in compounds with oxygen, forming silicon dioxide or silica, and iron, potassium, aluminum, magnesium and calcium. The natural waters used in power stations contain large amounts of dissolved silicates. Carryover Carryover is any contaminant that leaves the power station boiler within the steam that flows into the turbine.

Silica Scaling in Boilers

Home Applications Process water Boiler feed water Silica Scaling in Boilers Silica, in amounts ranging from less than 1 to over 100 ppm, is found in all natural water supplies. In rain hail and snow, silica content range from 1 to 2. 8 ppm. In the analyses of various surface and ground waters, silica content range from 1 to 107 ppm. This refers to soluble silica content and not to the silica that may be present in the suspended matter. Suspended matter may be removed from a water supply by coagulation and filtration; such processes have little effect in reducing the soluble silica content. Silica is the only boiler water salt which vaporizes at pressures below 2400 psig. It can vaporize at pressures as low as 400 psig. This has caused deposition problems in numerous turbines. The solubility of silica in steam increases with increased temperature: silica becomes more soluble as steam is superheated. Deposits are formed when steam is cooled by espansion. Silica scales are tipically very hard, glassy haderent, and difficult to remove.

Optimising power plant efficiency by measuring silica

Pierre Guillou and Katrien Verhassel look at the problem of silica contamination in power plant water systems.

Among many contaminants within the steam/water circuit, silica plays a huge role in process monitoring, simply because it is extremely soluble in steam and very hard to remove from steam/water. It’s a contaminant that seems in lots of potential exterior and internal entry ways. Exterior contamination could arise from: raw water ingress (demine plant) or drain line mix-up (putting on of seal or incorrect installation) utilization of plastic based lubricants and oils (derive from leaky seals within the water system or turbine oil leaks or by plastic-based coatings on tubes utilized in substitute activity) feed water system (eg, united nations-reacted plastic) or chemical dosing of reagent problems (eg, caustic addition after resin bed regeneration). Internal contamination is because: the condenser dust (a build-from paint, quarta movement and grease overhaul materials for example gasket materials, silicone sealants and cat litter are potential causes of the condenser dust) oil spill absorbent materials (cat litter, diatomaceous earth) in situation of needed substitute a wide open boiler tube might cause fly ash contamination or refractory material blasting material (have to clean the LP turbine of scale in the process) accidental misplacement of materials (brought on by work practices and poor housekeeping).

Removing Silica from Industrial Water

If you run an industrial facility or manage industrial systems, you know that keeping silica out is important. Learn more about how to remove it!

Wherever your water originates from, silica will probably be a significant component — especially in groundwater. Silica may cause a variety of unwanted effects that necessitate effective cleaning practices through industrial water treatment. Without one, you’ll likely see early degeneration of apparatus and lack of functionality because of:

  • Granular Silica
  • Colloidal Silica
  • Reactive Silica
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How to Remove Silica From Boiler Water

Membrane fouling: Silica particles can get lodged in membrane pores in reverse osmosis and nanofiltration systems. They block the flow of liquids through that membrane and can cause early tearing. Reduced heating and cooling efficiency: Scale deposits build up on the side of equipment when silica deposits precipitate out of the water and concentrate on the heat transfer surface of the equipment. Scale is a good insulator, reducing the efficiency of the heat transfer mechanism and the equipment’s performance. It can also slough off and create pieces that interrupt the flow of water. Effects on pressure ranges: Those deposits and pieces can impact a system’s ability to operate at optimal pressure ranges. Abrasion: Silica is an abrasive substance — it’s even used for sandblasting and carving applications. It will bring its abrasive effects to the inside of a boiler and eat away at it, causing premature wear and tear and costly repairs.


Video advice: Silica Scale deposits in Steam turbine blades


Silica in boiler water

Steam contamination (solid particles in the superheated steam) comes from the boiler water largely in the carry‐over of water droplets. The need for extreme purity of steam for use in high pressure turbines has prompted the development of highly satisfactory devices for separating steam and water in a boiler drum. Consequently, steam contamination has been steadily reduced. Troublesome turbine blade deposits may occur with surprisingly low (0.6 ppm) total solids contamination in steam. In the 3.5–6 MPa range, however these deposits are usually water soluble and can be removed by periodic washing. In the 4 to 10 MPa range, however, silica deposits predominate and these deposits are not easily removed by water washing. With operating pressure of 13 MPa and above insoluble deposits do occur which may be controlled by residual water washing. Before the unit is returned to service, the deposits should be removed by air or water‐driven turbine cleaners or by chemical cleaning.

Abstract – Steam contamination (solid particles within the superheated steam) originates from the boiler water largely within the carryover water tiny droplets. The requirement for extreme wholesomeness of steam to be used in ruthless turbines has motivated the introduction of highly acceptable devices for separating steam and water inside a boiler drum. Consequently, steam contamination continues to be continuously reduced. Difficult turbine blade deposits can happen with surprisingly low (. 6 parts per million) total solids contamination in steam. Within the 3. 5–6 MPa range, however, these deposits are often water soluble and could be removed by periodic washing. Within the 4 to 10 MPa range, however, silica deposits predominate which deposits aren’t easily removed by water washing. With operating pressure of 13 MPa and above insoluble deposits occur which can be controlled by residual water washing. Before readily stored away came back to service, the deposits ought to be removed by air or waterdriven turbine cleaners or by chemical cleaning.

On-Line Silica Measurement for Steam Generating Systems

ABSTRACTSilica is an important parameter for measurement and control, particularly in systems where steam is sent to turbines. Silica monitoring is important because it is difficult to remove from water, volatilizes with steam, can rapidly foul turbines and the resulting deposits are difficult to remove. This paper will discuss the importance of silica measurement, where on-line measurement should be used and the basics of silica monitors. INTRODUCTIONIn systems that do not contain a turbine or other steam driven rotating equipment, the limitations on silica in boiler water are generally less restrictive but they are still very important because they serve to prevent the deposition of silica on boiler tube surfaces. Silica does have a limited solubility in boiler water but on-line monitoring of silica is not routinely practiced in these systems. In high pressure systems that do contain a turbine or other steam driven rotating equipment, the concern becomes silica transport due to the volatility of silica.

The Importance of Boiler Water and Steam Chemistry

To maintain boiler integrity and performance and to provide steam of suitable turbine or process purity, boiler feedwater must be purified and chemically conditioned. The amount and nature of feedwater impurities that can be accommodated depend on boiler operating pressure, boiler design, steam purity requirements, type of boiler water internal treatment, blowdown rate, and whether the feedwater is used for steam attemperation.

Steam PurityPurity or chemistry needs for steam is often as simple like a specified maximum moisture content, or may include maximum concentrations for various chemical species. Frequently, for low pressure building or process heater steam, merely a maximum moisture submissions are specified. This can be up to . 5% or as little as . 1%. On the other hand, some turbine manufacturers specify steam condensate maximum cation conductivity, pH, and maximum concentrations for total dissolved solids, sodium and potassium, silica, iron and copper. Turbine steam must have total dissolved solids under . 050 parts per million, and perhaps under . 030 parts per million. Individual species limits might be even lower. If steam will be superheated, an optimum steam dissolved solids limit should be enforced to prevent excessive deposition and corrosion from the superheater. This limit is usually . 100 parts per million or fewer. Even where steam wholesomeness needs aren’t enforced through the application, dissolved solids concentrations under 1.

Water Handbook

The development of modern, high-efficiency steam turbines has led to an increase in deposition, erosion, and corrosion problems.

Whenever a turbine becomes fouled with water-soluble salts because of boiler water carryover or attemperating water contamination, turbine capacity can frequently be restored by water washing. Because it may cause severe turbine damage, water washing ought to be supervised carefully and also the recommendations from the turbine vendor ought to be adopted.

  1. Figure 18-1. Contaminated steam attemperating water fouled this 30-MW turbine, increasing stage pressures and decreasing capacity by over 5% during a 15-month period.
  2. Figure 18-2.
  3. Figure 18-3. Effect of silica and boiler water pH on the volatility of silica.
  4. Figure 18-4. Maximum boiler water silica allowable to maintain less than 0.02 ppm silica in the steam.
  5. Figure 18-5. Solubility of silica in steam for conditions found in a steam turbine.
  6. Figure 18-6. Sulfide contributed to stress corrosion cracking of this turbine disk.

Although several factors influence the formation of deposits on turbine components, the general effect is the same no matter what the cause. Adherent deposits form in the steam passage, distorting the original shape of turbine nozzles and blades. These deposits, often rough or uneven at the surface, increase resistance to the flow of steam. Distortion of steam passages alters steam velocities and pressure drops, reducing the capacity and efficiency of the turbine. Where conditions are severe, deposits can cause excessive rotor thrust. Uneven deposition can unbalance the turbine rotor, causing vibration problems.


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dangerous silica in steam on steam turbine, power system generation, steam turbine , power station


[FAQ]

What is the effect of silica in steam boiler?

Silica is the only boiler water salt which vaporizes at pressures below 2400 psig. It can vaporize at pressures as low as 400 psig. This has caused deposition problems in numerous turbines. The solubility of silica in steam increases with increased temperature: silica becomes more soluble as steam is superheated.

Why dissolve silica is harmful in sense of boiler and turbine performance?

Capacity Reduction. Silica deposits on the blades and other elements of turbine restrict steam flow to it from the boiler. This results in a loss of output from the turbine and a reduction in the turbine's electricity generation capacity.

Which impurity in steam affects the turbine performance the most?

Of these, amorphous silica (SiO2) is the most prevalent. Table 18-1. Silicate deposits found in steam turbines.

Why does silica increase in boiler drum?

Due to silica content in the feed water goes to steam drum where separation of wet steam and water takes place and in this way silica content increased in Boiler drum.

What causes high silica in water?

Water passing through or over the earth dissolves silica from sands, rocks and minerals as one of the impurities it collects. The silica content in natural waters is commonly in the 5 to 25 mg/L range, although concentrations over 100mg/L occur in some areas.

Erwin van den Burg

Stress and anxiety researcher at CHUV2014–present
Ph.D. from Radboud University NijmegenGraduated 2002
Lives in Lausanne, Switzerland2013–present

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