Reason For Silica Deposition In Turbine

 

Silica Deposition in Turbine

How is silica coming to turbine?

There are two types silica :

1. Colloidal Silica: SiO2-O-SiO2-

 More prevalent in water from surface source pH>7,, this is insoluble and mainly removed by UF, Coagulation, Nano filtration.

2. Reactive Silica: (OH)3Si-O-

This is soluble  and removed by EDI (Electro deionisation), RO.

But Silica is coming in turbine through steam from Boiler. Silica in steam should be less than 0.02ppm . It can be increased due to following reason:

1.       High silica level in steam drum: : Silica in drum should be less than 2ppm upto 100ata drum pr ref fig below (drum pr vs silica level and pH).If it increases above this value, blow down (IBD and CBD ) should be operated to control the silica level.  Secondly it can be seen when boiler is not properly cleaned during commissioning time. It can also be seen due to lower steam separator efficiency in boiler drum.

2.       High steam drum level: This can be due to mismatch with DCS and local level in gauge glass ,sudden fluctuation of load or poor drum level control,

3.       Carryover of impurities : Due to priming and foaming in drum

4.       High silica level in DM water: Silica in DM water should be less than 0.02ppm and higher pressure of boiler it is should be less than 0.01ppm.

5.       High silica level in Condensate: It can be observed when there is hi silica level in steam or there is tube leakage in condenser.

6.       High silica level in Feed Water: This can be observed when makeup water silica level is high. So DM water quality should be rectified.

     7.   Poor quality of chemical in HP//LP dosing system 

 

Symptoms of Silica deposition in Turbine

If above parameters are high continuously, following symptoms can be observed at the turbine side :

1.       Higher wheel chamber or first stage pressure

2.       Higher extractions/bleed pressure

3.       Higher  specific steam consumption

4.       Lower Loading of the turbine

5.       Higher loading in the condenser

             6.       Higher axial shift of the turbine 

Boiler Drum Silica limit vs Pressure Vs pH

Removal of Silica:

Steam Washing: Low pr (11-12kg//sq.cm) steam at saturation temperature (185 deg.C) is passed through turbine and it rotates around 700 rpm . 

Sand /Alumina Blasting: Dismantle the turbine and clean the different parts i.e rotor, stator, guide blades, steam chest etc by sand / alumina blasting method.

Monitoring of Silica: Online silica monitoring instruments are to be installed and shift wise silica lab testing and corrective action are to be taken.