Although Virginia Power’s nuclear plants have continued to operate extremely well, the company’s commitment to keeping its nuclear units competitive in a deregulated market has allowed it no let-up in efforts to improve economic performance.
The company’s two nuclear stations, Surry and North Anna, both operate at high levels of reliability and load factor.* Having reached these levels, the plants have turned their attention to squeezing more megawatts out of the thermal energy generated in the plant.
One area identified was the reduction in thermal efficiency that occurs in summer due to increased condenser fouling. At Surry, the condenser cooling wate, which comes from the James River, is still brackish at this location, well inland from the Atlantic Ocean. This provides a vigorous marine growth environment leading to micro and macro fouling (eg from hydroids, slime, scale) and non-marine growth (mud, silt, aquatic species) on condenser tubes, reducing the flow rate and the heat transfer across the tubes, degrading the plant’s thermal efficiency. This also makes them difficult to keep clean. Because of the Surry plant’s once-through cooling system and internal plant elevations, continuous automatic cleaning systems cannot be utilised. The stations’s inlet pumps are also subject to suction-side restriction by marine growth, adding to the loss of cooling efficiency.
“When we recognised that Surrey was not hitting the 99.5 thermal performance indicator (the industry standard set by INPO†), we saw an opportunity to improve,” said Bryce Shriver, manager of operations and plant support at Surry. “This is not to say that our 99.4 figure was bad, it was just below our own target of always being better than the industry standard.”
Plant management realised that waiting for the fouling to build before action was taken to clean the tubes was not the most effective way to deal with it. According to Bernard Sloan, Surry steam cycle systems manager, “We could expect to lose some 300-400 megawatts during the summer. To win back some of that would require positive action.”
ACTION REQUIRED
A more aggressive approach was needed to avoid the loss of megawatts and a decision was taken to implement a pre-emptive strategy. Rather than wait for the problem to build up, the engineering, maintenance and outage managers together devised a cleaning schedule to check the build up of material while minimising output losses during the operation.
Cleaning requires taking the water boxes which contain the tubes out of service, one at a time, and having the tubes scraped. Tube scraping involves shooting plastic or metal scrapers through the tubes with a high pressure water gun at 700-800 psi. As each of the plant’s four water boxes contains 17 832 tubes, of 7/8 inch inner diameter and over 40 ft in length, this is quite a labour intensive job.
Under the new regime the cleaning operations were carried out during the summer (May through October) on a 4-6 week cycle over most weekends when the load is at its lowest and reducing power does not have a major impact on plant economics. Two water boxes can be cleaned over a weekend, involving about 10 people working 12 hour shifts.
The results so far would appear to justify the action. Instead of the previous summer loss of between 300 to 400 megawatts, the reduction now totals only 20-25 megawatts.
Improving condenser flow
Condenser performance also depends on water circulation. The flow, driven by eight large pumps, is affected by accumulation of barnacles, hydroids and other biota on travelling water screen roughing bars or trash grates. A diver is used to clear the trash grates and restore optimum flow rates. It was also decided to upgrade the pumps to increase pump flow 5-10%. This will also mean that the pumps will draw colder water from much deeper in the river which will also increase thermal efficiencies.
RESULTING PRODUCTION RISE
The improvement in thermal performance during the year is equivalent to raising each unit’s gross capacity from about 820 MWe in 1997 to 840-850 MWe.
The thermal performance indicators for both units through October of this year are 99.7, compared to 99.4 for 1997. They will likely remain at that value through the end of the year, although unit 2 could reach a yearly average of 99.8 depending on the last two months’ performance.
“If we assume that unit 2 remains at 99.7, the difference between 1997 and 1998 generation provides about and additional 2.57 MW per hour, or 22 513 MWh for the year for each unit,” said Shriver. “Using a value of $18.5/MWh, the performance gain equates to more than $416 000 per unit or a total for the station of $832 000.”
FUTURE IMPROVEMENTS
As tube scraping is a labour intensive operation, keeping them clean using a chemical treatment is being considered. An EPRI survey was performed of plants with similar cooling water systems. This survey was used as a basis for chemical treatment consideration. A trial with two of the water boxes over a six month period is set to start at Surry before the spring of next year.
Plant improvements can be seen as part of a long term life management programme that looks beyond the current 40-year licence period. This past year, Virginia Power began working full out on its preparations to apply to extend its operating licence another 20 years.
