1 Importance of the HRSG water and steam quality monitory
Electric Power Research Institute (EPRI) indicated that of the top five modes of HRSG tube failure, four (flow-accelerated corrosion, corrosion fatigue, under-deposit corrosion, and pitting) had links to or could be influenced by cycle chemistry, HRSG water and steam monitory is key for the combined cycle power plant operation, The lifeblood of the combined cycle plant is its water chemistry program
There are four main reasons why water quality is so important. Impurities in water form scales.
- Water contains dissolved salts, which upon evaporation of water forms scales on the heat transfer surfaces. Scales have much lower heat transfer capacity than steel: the heat transfer coefficient of the scales is 1 kcal/m/°C/hr against 15 kcal/m/°C/hr for steel. This leads to overheating and failure of the boiler tubes. Scale also reduces flow area, which increases pressure drop in boiler tubes and piping.
- Low pH or dissolved oxygen in the water attacks the steel. This causes pitting or lowering the thickness of the steel tubes, leading to rupture of the boiler tubes. Contaminants like chlorides, a problem in seawater cooled power plants, also behave in a similar way.
- Flow assisted corrosion occurs in the carbon steel pipes due to the continuous removal of the protective oxide layer at high flows.
- Impurities carried over in the steam, causing deposits on turbine blades leading to reduced turbine efficiency, high vibrations, and blade failure. These contaminants can also cause erosion of turbine blades. Silica at higher operating pressures volatilizes and carries over to the turbine blades.
2 ESEC HRSG internal water treatment program
As top power plant, as fig 1, ESEC online monitory steam water cycle quality as displayed on DCS page, to monitory the HP, IP. LP steam conductivity, cation conductivity, degas conductivity, and sodium, the sample take from drum and superheating steam, monitory the HP/IP/LP drum PH, cation and specific conductivity, sodium, for the HP feed water monitory the PH , cation and specific conductivity, sodium and dissolved oxygen and also monitory the condensate before and after chemical feed water quality.
In the meaning time, each shift will grip sample for different point to test silica, phosphate, and iron.
The test the phosphate is once per day at night shift, test PH and conductivity per each shift, each shift test the HP and IP steam and condensate after chemical feed and demin water. Every week test the Iron to understand the FAC situation.
For water treatment program, ESEC select to add amine to condensate water to increase the PH as below component. Amine is part of the volatile treatment, which is easy to volatile at LP drum to cause the LP specific conductivity over range.
NH3 + H2O ⇌ NH4+ + OH−.
ESEC steam cycle system is not have any copper-alloy material, so add ammonia to system will not worry the copper alloy corrosion issue. Add tri-phosphate to HP drum the cascade to IP drum, phosphate can condition the drum water and prevent scale and sludge formation, to prevent corrosion cracking, which can lead to pipe failure, and under deposit corrosion, also reduce heat transfer, lessen flow rate, and cause material degradation, the phosphate to add to HP drum can increase HP IP drum PH.
2Na3PO4+3CaSO4—Ca3(PO4)2 + 3Na2SO4
HP drum phosphate control range is 1 to 2 ppm, and IP drum phosphate control range is 2-5ppm. Plant has a connection to permanent condensate polishers, but not installed
FAC causes localized wall thinning (metal loss) in carbon steel piping, tubing and vessels exposed to flowing water (single-phase) or wet steam (two-phase). If undetected, the degraded component can suddenly rupture, releasing high temperature steam and water into neighboring plant areas and this often includes locations where personnel may be present. The escaping fluids can injure plant workers, sometimes resulting in fatalities, and damage nearby equipment. To prevent FAC corrosion, increase the PH level at condensate to 10 which is using the specific conductivity 20 as control target, also weekly test iron to monitory FAC situation.
Iron test records
Silica, phosphate, test use HACE DA3900 .
3 Operation challenge and solution
3.1 Cation and degas conductivity monitory:
Existing Cation Columns does not effectively remove conductivity caused by ammonia, CO2 and amine.
Existing Cation Columns do not remove ammonia conductivity effectively. As a result elevated cation conductivity readings prevents us from running the Steam Turbine due to Mitsubishi requirements.
The ammonia treatment we are using flashes off in the LP drum immediately so shows as high cation conductivity in LP steam even though it is not harmful to the Steam Turbine. The degas units would remove the ammonia and give us a more accurate and lower cation conductivity so we could run the Steam Turbine sooner. The economics of this is justified because the analysers would pay for themselves in days if not hours IF it allows us to run in combined cycle sooner.
Figure 1, new installed degas conductivity panel
3.2 Water simple panel reliability
Replace the existing analyzers for the boiler sample panel with more up-to-date/reliable analyzers/monitors Update the boiler sample panel DO analyzers (4) and pH analyzers (7) with the M4 Knick style memorial option. This option provides operations with reliable monitoring as well as maintenance the ability to calibrate all the sensors with one device.
The front panel would also require modification as the existing analyzers will be removed and replaced with a single Panel Mate display device. All of the analytical equipment will be within the cabinet installed on DIN rail. The existing HACH silica analyzer will also be replaced as part of this project
– Field construction/placement/installation on going with the lead EI&C technician providing support. It was decided that with this project complexity he will be utilizing this project for his Capstone project
The existing DO analyzers and sensors at the Boiler Sample panel have become obsolete. The sensors are not available from the OEM anymore and the recommendation from them is to replace both the analyzers as well as the sensors. The project looks to replace the existing DO analyzers with a technology that is more robust, reliable, and suitable for the industrial application, There are three common technologies for dissolved oxygen measurements: polographic, galvanic and optical. as well as replacing unreliable high purity pH probes at the same time.
3.3 HRSG steam and Water false alarm:
Review and adjust Set Points and alarm limits for Condensate and HP/IP/LP drums, (steam and water). ESEC controls pH in the HRSG feedwater using aqueous ammonia. Ammonia is only injected at a single point in the Condensate system. There are no downstream injection points, such that protective ammonia must cascade down through the LP, IP and HP Drums. Some ammonia is lost to the vapour phase in the LP Drum. This means that to provide adequate protection to the IP and HP Drums and their feedwater systems, the ammonia must be injected at high concentration at the front end. This causes two unintended consequences, namely:
- High pH and specific conductivity in the Condensate and Boiler Feed-water systems
- High specific conductivity in the LP Steam (due to ammonia volatility
These parameters often are in excess of established limits and trigger alarms.
This MOC aims to review the specified alarm limits to determine their applicability to ESEC’s process and adjust – provided that there is evidence that such actions will not compromise safe operation or equipment integrity.
Persistent alarms for high pH and conductivity in Condensate and BFP Discharge and high conductivity in LP Steam are likely giving false representation that a problem is occurring. Without additional ammonia injection locations, it may be impossible to balance the pH between the LP section of the HRSG and the IP and HP sections. Either there will be high pH in the LP section or low pH in the IP and HP section. Low pH in steam drums and feedwater systems is known to be harmful, while higher pH may be beneficial (in terms of corrosion protection). Existing HRSG pH limits need to be reviewed against operating data and current industry best-practice guidelines to determine if they can be raised.
Conductivity measurement is designed as a surrogate method to warn of harmful ions in the HRSG steam and/or water. Persistent high conductivity alarms may be giving false indication that ions are contaminating the system. Thus, it is important to consider potential alarm limit changes in the context of the purpose of conductivity measurement and ensure that readings are being interpreted correctly. Additional/alternate contaminant monitoring may be necessary to achieve this.
Install high and low PH alarms for drums and feedwater. High alarms on all conductivity meters for specific/ cation and Degas conductivity. High alarms on silica for make up water. get high silica and low resistivity alarms set up on mixed bed flow but those would need to be smart alarmed to only be active when R.O. is on and permeate forwarding pump is on as well, Better warning of mixed bed expiry and provide a second line of defense for water quality during busy start ups. Example- the LP steam degas conductivity has to be below 1.0 for the ST start up.
Suggestion solution:
ESEC use the pure amine to increase the condensate PH which is the amine is
Every amine has a vapor-to-liquid ratio (V:L), representing the distribution of a particular amine in a steam/liquid mixture. For instance, an amine with a high V:L ratio prefers to go with the vapor phase relative to the liquid phase, while an amine with a low V:L ratio prefers to remain in the liquid phase. To further complicate the matter, V:L ratios change with pressure.
ETA has a low V:L ratio at all HRSG operating pressures. For comparison, ammonia is 10 times more volatile than ETA at LP operating temperatures and pressures. To inject the blending ammonia with ETA to decrease the vapor to liquid ratio, another way to increase the phosphate according to EPRI, phosphate in the drum keep up to 8.0 ppm which if not add the blending ammonia increase the phosphate concentration can increase drum PH.
3.5 Hp drum saturation steam sample point leakage
The HP drum steam simple line is temporary plugged due to the steam leakage at connector which will be recovered at next outage
5 Conclusion
HRSG steam and water quality monitory is very important for the combined cycle power plant, ESEC through MOC increased the water analysis reliability.