Operation & Maintenance of Power Plants
Optimising power plant efficiency by measuring silica
Pierre Guillou and Katrien Verhassel look at the problem of silica contamination in power plant water systems.
Fig. 1. Silica can deposit itself on turbine blades in a very short period of time. If it is not removed deposits can distort the shape of the blades.
A
mong many contaminants in the steam/ water circuit, silica plays an important role in process monitoring, mainly because it is highly soluble in steam and extremely difficult to remove from
steam/water. It is a contaminant that appears in many potential external and internal entry points. External contamination could arise from: raw water
ingress (demine plant) or drain line mix-up (wearing of seal or incorrect installation); use of silicon based lubricants and oils (result from leaky seals in the water system or turbine oil leaks or by silicon-based coatings on tubes used in replacement activity); feed water system (eg, un-reacted silicon) or chemical dosing of reagent problems (eg, caustic addition after resin bed regeneration). Internal contamination is caused by: the condenser dust (a build-up of paint, quartz and grease; overhaul materials such as gasket materials, silicone sealants and kitty litter are potential sources of the condenser dust); oil spill absorbent materials (kitty litter,
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diatomaceous earth); in case of required replacement an open boiler tube could cause fly ash contamination or refractory material; blasting material (need to clean the LP turbine of scale from the process); accidental misplacement of materials (caused by work practices and poor housekeeping).
Altered steam velocities If silica is not removed from the boiler feed water, it will concentrate itself on the drum and is carried over in steam to form adherent deposits in the steam passage way distorting the original shape of turbine nozzles and blades. Tis alters steam velocities and the pressure drops reducing the capacity and efficiency of the turbine. Severe conditions can cause excessive rotor thrust
while uneven deposition can unbalance the turbine rotor causing vibration problems. Turbine deposits can accumulate in a very short time when steam purity is poor and can only be removed by external service cleaning and blasting aluminium oxide on the surface.
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