1 General
Close cycle cooling system (CCCW) is important system in power plant which provide the cooling for the gas turbine and steam turbine generator and auxiliary rotation equipment, Power plant operation equipment cooling system is identify to two type of system, the one is close loop cooling system and another one is open cooling system, close cooling system water has high quality cooling water, each customer heater not have the contamination issue. Operation will more reliability than open cooling system.
2 ESEC Close Cycle Cooling System
The Closed Cycle Cooling Water System (CCCW) at ESEC includes two 100 percent capacity, centrifugal, horizontal, closed cycle cooling water pumps with motors, three 50 percent capacity closed cycle cooling water heat exchangers, one closed cycle cooling water expansion tank, and all piping, valves, and instrumentation necessary to transport closed cycle cooling water within the closed cycle cooling water system. A chemical pot feeder is provided for adding corrosion and scale inhibiting chemicals into the system
The closed cycle cooling water heat exchangers are designed to reject the heat from the closed cycle cooling water system. Each heat exchanger has a heat transfer area of 782.92 sq. meter , and a total heat transfer capacity of 39,762,863 kJ/hr (37,687,923 Btu/hr) with 531 plates provided.
3 CCCW system operation Challenge and solution
3.1 cooling water pump suction strainer cleaning frequently
Since commissioning, the ESEC has experienced higher than expected pressure differentials and short duration between cleans for its CCW pump suction strainers 0-CCW-STR-001/002. The basket-style strainers consist of 1/8” perforated plate with a 40-mesh screen. According to the cut sheet, the pressure differential across the strainers should be 14.5 kPa at 50% fouled. Experience has shown a clean dP of 40 kPa. The CCW pump 1 strainer lasts longer than 2, however run times peak at around 2 weeks before the dP reaches 60 kPa. There have been occurrences where run times are only a few days between cleanings. During start-ups, the strainers have required cleaning multiple times per shift. As part of a troubleshooting exercise, ESEC completed flow testing with a portable UT meter which confirmed the strainer flow is less than design and the dP is higher than design at the lower flow.
Engineering assessment determined that the likely cause of the elevated dP is turbulence induced by the upstream 90-degree elbow. The OEMs noted that the elevated pressure drops of 40 to 60 kPa are acceptable and do not put the equipment at risk of failing if monitored properly (the “burst pressure” of the basket is 69 kPa). An assessment of the CCW system determined that the system pressures are acceptable and the CCW pump NPSH margin is sufficient, ESEC Operations is looking to further investigate this issue with hopes of reducing the frequency of strainer cleans. This would benefit the plant by reducing the number of pumps starts, saving operator time and limiting the time when the CCW system is reduced to a single pump.
ESEC has two replacement strainer baskets with a finer 60-mesh(0.25mm) screen in its auxiliary warehouse. According to the specification sheet, these were intended for use during system start-up.
Based on input from Engineering, the current strainer mesh size is typical for this application, but was not based on a detailed assessment. In this ESECtion, various components within the CCCW system will be assessed to determine what strainer size would be required to provide adequate protection.
After detail study for the entire system found the minimum gap is below and the other heater is not mention at this table is tube cooler.
- Air Compressor Coolers Non-soluble suspended solids are recommended less that 1mm
- CT & ST H2 Seal Oil Coolers Flow path between plates = 2.1mm
- CT & ST Lube Oil Coolers Flow path between plates = 3.0mm
- Recommended mesh size of 2.1mm or less
- CCW Heat Exchangers Flow path between plates = 3.0mm
- CCW Pumps Impeller minimum passage size = 41.2mm
Changing to a strainer with an 18-mesh screen would provide adequate system protection from Engineer suggestion while allowing for increased runtime. The 18-mesh screen would provide protection for particles greater than approximately 1.0mm. This is roughly half the size of the smallest known clearance in the CCW system.
Finally Modify the replacement basket strainers in the auxiliary warehouse to use a 20-mesh screen. Based on typical mesh-to-micron conversion tables, this will increase the maximum particle size in the system from approximately 0.400mm to 0.841mm
3.2 CCCW heat exchanger fouling
In the winter time, two heat exchangers can provide enough cooling capacity to CCCW system, but in summer time due to the small flower powder fly in the air, it fills to the cooling water to plug the strainer very quickly, in the summer time the strainer plugged every week. So, in the summer time three heat exchanger must in service.
DP from 72 kpa decrease to 68 kPa after cleaning, plant set up the white board on the local CCCW heat exchanger to show the strainer cleaning date, build DCS trend and build in DCS alarm if the DP is bigger than 72 kPa. And reinforced the backflash to help flush out inside of the plate debris, overhead chin hoist set up on each heat exchanger to help remove the heavy strainer end cap, at outage time use chemical cleaning.
In summer time slightly increase the STG control valve from 40 degree to 42 degree to control the STG heat exchanger opening, in the meaning time to monitory the STG bearing temperature which not reach the high limit
3.3 CCCW system corrosion monitory
Corrosion can be controlled either by inhibiting the cathodic or anodic reaction. In open cooling water systems, typical chemistry is designed to inhibit both. But, in closed cooling water systems, anodic inhibition is the typical method
During the strainer cleaning time, take sample form strainer to send to lab the results is show the iron oxide is major component to blog the strainer as below:
Loss on Ignition @ 650° C 6.8% Calcium, as CaO 2.7% Magnesium, as MgO 1.4% Iron Oxides, as Fe2O3 81.4% Copper, as CuO 0.2% Sodium, as Na2O 2.0% Aluminum, as Al2O3 0.5% Manganese, as MnO 0.7% Phosphate, as P2O5 0.2% Carbonate, as CO2 <1.0% Sulfate, as SO3 0.8% Silica, as SiO2 2.7%
Weekly test corrosion inhibitor, requests PH >9.0 , Corrosion inhibitor is use ChemTreat CL6034, the Physical State and Appearance: Liquid, Light Straw, Clear Specific Gravity: 1.208 @ 20°C pH: 13.9 @ 20°C, 100.0% Freezing Point: 21°F, the component is :
Operation control parameter is
| pH | Units | 9.00 | 11.00 |
| Specific Conductivity | uS/cm | 4000 | |
| CL 6034 Inhibitor | ppb | 2000 | 4000 |
| Iron | ppm | 2 | |
| Copper | 0.5 | ||
| Microbio | CFU/ml | 0 | 1000 |
4 Operation results
After the pump suction strainer change from 60 mesh to 20 mesh strainers, the cleaning frequency extend from 2 weeks to 3 months, successfully to solved the problem, the plant reliability also increased due to avoid the single point trip the plant, one close cooling pump provide the entire plant cooling capacity no spare pump is high risk operation
For the CCCW heat exchange contention issue, through operation coordination and management support to increase the cleaning time and plus adjust the most heave customer STG cooler set point, successfully to operation over 6 years, the plant output never has derate due to CCCW system. the CCCW system corrosion rage is less than <0.1 mpy.
5. Conclusion
ESEC successfully overcome the challenge of the CCCW system operation. The heat exchanger cleaning to help plant stabile operation, team is looking better solution to minimize the cleaning. Weekly test the CCCW water quality to ensure that the concentration remains within the control range, to add extra chemical to meet the water criteria to minimize the pipe corrosion is good practice