Ten golden rules for steam purity control in high-pressure boilers for power plants
In power plants, steam purity conditions impact specifications, operational and treatment practices, this is because the steam turbine is very susceptible to damage by corrosion mechanisms or formation of deposits on its components and when this happens it can have huge financial implications, more so in these times when the current electricity market situation requires power plants to maintain very high availability and efficiency.
The only practical approach is to ensure that the turbine is running at all times with acceptably pure steam and this means that the boiler (whatever its type) must maintain mechanical, operational and chemical conditions to generate steam that meets all purity specifications and for this the following basic rules must be followed:
1. All with the same objective. The water treatment specialist should establish a comprehensive program for chemical control in all facets of the system, but in addition the rest of the disciplines should be involved and focus their operational and maintenance practices so that the boiler can generate steam with a purity required by the turbine manufacturer and/or with the following specifications:
• Sodium (Na) < 3 ppb
• Silica (SiO2) < 10 ppb
• Chlorides (Cl) < 2 ppb
• Sulfates (SO4) < 2 ppb
• TOC < 100 ppb
2. Achieve maximum purity of make-up water. The raw water treatment should be adjusted in all its stages to satisfy the boiler water demand, but with a purity of make-up water that meets the same specifications of the superheated steam.
3. Use high purity (ACS Grade) chemical additives. All chemicals break down into by-products that can contaminate steam, increase blowdown demand and affect operating costs. Ammonium hydroxide, amine or passivators must meet the following conditions:
• Chlorides < 1.0 mg/kg (1 ppm).
• Sulfates < 1.5 mg/Kg (1.5 ppm)
• Sodium < 1.0 mg/kg (1 ppm)
• Iron < 0.2 mg/Kg (0.2 ppm)
• Silica < 0.5 mg/kg (0.5 ppm)
4. Effective control of air infiltration. The objective is to maintain condensate and feedwater with corrosive gases (such as oxygen and CO2) at tolerable levels for generator and turbine operation (O2 < 10 ppb, CC < 0.2 µS/cm). This requires:
a. Immediate repair of mechanical failures of the incondensables or deaerator system.
b. Immediate repair of air infiltrations.
c. Use of chemical additives (sequestrants) as a last option and as a temporary solution.
5. Condenser leakage control. Any pore or crack in condenser tubes, whatever its origin or size, can cause contamination of condensate and feedwater that will manifest itself with corrosion mechanisms in the tubular elements of the boiler or steam turbine. To minimize risks, efforts from different disciplines should be combined for the following:
a. Ensure effective chemical control of the cooling water to avoid erosion corrosion in the surface condenser tubes.
b. High precaution to avoid shocks on the condenser tubes during maintenance work.
c. Maximum vibration control in the condenser tubes.
6. Be careful with the temperature control water. If condensate contamination occurs, whatever the cause, the impurities will reach the turbine by the fast way, that is, through the temperature control water. In the event of any condensate contamination event, the first maneuver to consider is to shut down the temperature control system and proceed to repair the fault causing the contamination.
7. Mechanical drag control. In order for the steam generator to achieve maximum efficiency of moisture separation and maximum steam drying capacity, the steam generation capacity must be respected according to the manufacturer's specifications, as well as the stop and start ramps, but the following good practices must also be considered in the steam dome:
a. Good condition of primary and secondary separators.
b. Correct water level in the dome.
c. Immediate correction of disturbances or bubbling in the water mirror of the dome.
d. Avoid high level fluctuations due to load movements.
8. Control of volatile carryover. Since the conditions of pressure, temperature and PH of the dome water influence the specific volatility of chemical species, the following operating practices should be considered in the steam dome:
a. Establish maximum limits for specific conductivity, PH, sodium, silica, chlorides and sulfates in the dome water based on the type of treatment applied (phosphate, caustic or all volatile) and the maximum operating pressure of the steam dome.
b. Manage continuous blowdown of the boiler to always comply with the PH and specific conductivity limit and secondly to maintain control of silica, sodium, chlorides and sulfates.
9. Integral Chemical Treatment. All the programs offered today for the chemical conditioning of boiler water (condensate system and dome water) act by providing OH alkalinity to neutralize the corrosive action of impurities, but the best practice is that the chemical program, in addition to the chemical aspect, integrates mechanical and operational aspects to contribute to the improvement of the total operating costs of the plant, in this sense, the selection of the chemical program should consider the following criteria:
a. It must neutralize impurities in the boiler water with minimum affectation to steam purity.
b. It must achieve the best control of corrosion product transport (CPT).
c. It should help to reach steam conditions in less time.
d. It must help to optimize purge %.
e. It should provide high responsiveness and flexibility for chemical contingency control.
f. It should minimize chemical cleanings of the generator.
g. It must achieve the best steam generator conservation conditions (Lay Up).
10. Monitoring and contingency control plan (Troubleshooting). The water-steam cycle is a system that is prone to become contaminated at any day or time, therefore, power plants must be prepared with a plan that provides a quick response time before impurities cause permanent damage to the turbine or tubular elements of the steam generator. To confirm effectiveness, the plan should be evaluated regularly with drills. The plan should be documented with identification and definition of the following elements:
a. Sampling points at all stages of the cycle.
b. Critical and non-critical variables for each stage of the cycle.
c. Variables to confirm any chemical condition (Troubleshooting).
d. Chemical variables measured continuously (analytical instrumentation), plant laboratory and external laboratory.
e. Control limits for each chemical variable.
f. Alarm limits of 3 levels based on the severity of contamination.
g. Maximum permissible exposure time
h. Visible alarms in control room.
i. Routine and contrastive analytical procedures.
j. Frequencies of analytical routines
k. Field procedures for control of each contingency.