2. Circulating Water System

Transcription

1 SUPPLEMENTAL OXYGENATION OF ELECIRIC GENERATING STATION COOLING WATER G. FRED LEE and GEORGE A. FREY* Department of Civil Engineering, Colorado State University, Fort Collins, CO 80523, U.S.A. (Received 14 February, 1979; revised 4 May, 1979) Abstract. The Delaware River Basin Commission established the requirement that the Philadelphia Electric Eddystone Generating Station shall add 5000 pounds (,,, 2.3 tonnes) of dissolved oxygen (DO) per day to the Delaware River as a condition for increased use of cooling water at this station. It has been found that the Eddystone Station normally adds from ~ 1 to 3 tonnes per day of DO to the river arising from entrainment of air into the cooling water discharge tunnel. Further additions can readily be accomplished by bleeding air into the condenser tailpipe. The Delaware River Basin Commission's requirements can readily be met without significant expenditure of funds and without altering normal plant operating conditions. 1. Introduction The Delaware River Basin Commission (DRBC) established that the Eddystone Electric Generating Station shall add 5000 lbs day -~ (,,, 2.3 tonnes day -~) of DO to the Delaware River as part of the conditions for Philadelphia Electric Company (PECO) to expand the station. This amount was chosen by the DRBC to offset a potential reduction in the River's biological oxygen demand (BOD) assimilative capacity due to the station's thermal discharge of water used for once-through cooling. A study was initiated in February 1973 to identify temperature and DO changes associated with the use of Delaware River water for once through cooling at the Eddystone Station. This paper reports on the results of these studies. 2. Circulating Water System The intake tunnels for the circulating water (condenser cooling water) system are located on the Delaware River at the downriver end of the PECO Eddystone property. Water is drawn in between elevation -5.7 m, the bottom of the intake tunnel, and elevation -1.9m, the bottom of the apron wall. There are four intake tunnels, each equipped with trash racks, stop logs, traveling screen, river water pump suction, and circulating water pump suction. Each circulating water pump is rated at ~ 6 m 3 s-~ and each river water pump at,,,0.6 m 3 s ~. One 2-m I.D. inlet pipe leaves underground from each circulating water pump. Two of the pipes terminate at the steel inlet pipes to the Unit 1 condenser which has split water boxes. The two 2-m I.D. inlet pipes for Unit 2 each split into two 1.4-m I.D. pipes. Unit 2 has two condensers with split water boxes so that one 1.4-m pipe from each inlet *United Engineers, Philadelphia, PA. Water, Air, and SoiI Pollution 12(1979) /79/ $00.75 Copyright ~ 1979 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.

2 136 r FRED 1.EL AND GEORGE A FRE~( pipe feeds one water box on each condenser. Each condenser is located above the mean high water level. The Delaware River in this area shows a tidal fluctuation of approximately 1.8 m. The condenser inlet and tail pipes are vertical to connect the condenser with the lower intake and discharge piping. Analysis of the circulating water hydraulic and energy gradients shows that the condenser tail pipes are always at less than atmospheric pressure. The vacuum condition is a function of river water level with the greater vacuum occurring at the lowest river water level. The Unit 2 condenser tail pipes discharge into a discharge tunnel which flows in an upriver direction under the plant where the discharge from the Unit 1 condenser joins this same tunnel. Flow continues on and then discharges to Crum Creek about 275 m upstream from the Delaware River. Just before the bulkhead at Crum Creek there is a discharge well where circulating water can be sampled. The common discharge tunnel for Units 1 and 2 is vented to the atmosphere near its origin by means of a 61-cm vent. Additional information on the characteristics of the Eddystone plant and the Delaware River in the region of the plant is provided in a paper by Lee (1979). 3. Experimental Procedure A YSI Model 54 DO meter was used. This instrument automatically compensates for temperature effects. A 15-m probe lead was used to allow direct measurements in the intake and discharge cribs. The instrument was calibrated with air-saturated water. 4. Sampling Program The objective of the sampling program was to determine DO changes in the circulating water system of Eddystone Units 1 and 2 between intake and discharge during normal operation and after air was bled into the discharge from the condensers. Locations sampled included the Delaware River in front of the intake structure, the intake structure between the trash racks and the discharge well. On 23 March 1973 and 15 November 1973 investigations were performed to determine the effects of bleeding air into the circulating water system. The addition of air to the system was accomplished by opening two 3-cm-diameter valves on each of the two Unit 1 condenser tail pipes. Data were taken beginning in February 1973 to determine circulating water DO changes during normal plant operation. Data were taken periodically from 13 August 1973 through 31 January In addition, on 9 October 1973 a field study was conducted on the characteristics of the cooling water thermal plume in the Delaware River.

3 SUPPLEMENI'AL OXYGENATION OF POWER STATION COOLING WATER Results 5.1. EFFECT OF NORMAL PLANT OPERATION ON DISSOLVED OXYGEN CONCENTRATIONS IN DISCHARGE WATERS Table I presents a summary of the changes in DO concentrations that are occurring during normal plant operation. On 8 August 1973, the Delaware River adjacent to the intake structure contained a mean DO content of 1.78 mg 1 -~ with a 22~ saturation. The circulating water between the trash racks and bulkhead contained a mean DO content of 1.62 mg 1 -] with a saturation. Sampling in the discharge well at Crum Creek showed a mean DO content of 2.64 mg 1 l with a 370/0 saturation. On 19 September 1973, the circulating water sampled between the trash racks and bulkhead from 10:10a.m. to 10:18 a.m. showed a mean DO value of 1.24 mg 1 l with a saturation. The discharge well sampled between 9:47 a.m. and 10:42 a.m. showed the circulating water to contain a mean DO content of 2.37 mg 1-~ with a saturation. The circulating water between the trash racks and bulkhead from 12:20 p.m. to 12:33 p.m. contained a mean DO value of 1.40 mg 1 -~ with a saturation. The discharge well sampled between 11:56 a.m. and 12:11 p.m. showed a mean DO value of 2.37 mg 1 with a saturation. On 9 October 1973, the circulating water sampled between the trash racks and bulkhead contained 1.00 mg 1 -~ DO with an 11% saturation. Sampling in the discharge well showed the circulating water to contain 2.10 mg 1 -~ DO with a saturation. The mean intake DO on 10 October 1973 was 1.31 mg 1 -~ with a 6070 saturation. The mean discharge DO was 1.6 mg 1 ~ with a corresponding saturation. On 15 November 1973 the intake structure between the trash racks and bulkhead contained a mean value of 2.54 mg 1 -~ DO with a saturation and the discharge well TABLE 1 Summary of dissolved oxygen data taken during normal plant operation of the Eddystone generating station Date Intake Discharge DO DO Mean DO % Saturation Mean DO o70 Saturation Increase Addition (mg1-1) (mgl-i) (mgl-i) (kg day i) August September , October i , i0 October l 15 November January ~-' 0 ~ 0 31 January ,,, 0 ~" 0 Note: Two data points for one day are a result of two data sets being taken.

4 138 Ci FRED LEE AND GEOROE A. FREY contained a mean value of 2.88 mg 1 i DO with a 32% saturation. The same locations sampled 1 to 2 h later contained a mean DO content of 2.30 mg 1 ~ with a 21% saturation and a mean DO content of 2.70 mg 1 -~ with a 29% saturation, respectively. On 3 January 1974, the data in Table I show that there was little detectable change in the DO content of the circulating water between intake and discharge. The temperature increase across the plant averaged about 7.3 ~ C. The DO increase from intake to discharge varied from a maximum of 1.13 mg 1 -~ in September to an undetectable amount in January. The percent mean DO saturation of the intake water varied from 11 to 82%. This shows that the Delaware River in the region of the Eddystone Station is grossly polluted by upstream discharges of oxygen demanding wastes. The percent DO saturation values for discharge waters during the studies increased from 7 to 18%. This is due to the increased temperature of the discharge and to the increase in DO arising from air entrainment in the discharge tunnel. Ordinarily it is thought that the passage of water through an electric generating station would tend to decrease the amount of oxygen present in the discharge waters. However, this situation applies only to conditions where the intake water is at or above the saturation value. In a situation such as that for the Delaware River near the Eddystone Station, the natural tendency for the station is to add DO to the river rather than remove it. Calculations were made to determine how much DO was being added to the Delaware River on the days of the study. The results based on a circulating water flow of gpm (28 m 3 s -s) are shown in Table I. On days when the percent saturation of the intake water was less than about 20%, 5000 lbs ( ~2.3 tonnes) or more of DO was being added to the river OXYGEN ADDIT1ON BY BLEEDING AIR INTO THE CONDENSER TAIL PIPE The increases in DO that normally occur across the station in circulating water arising from air entrainment in the discharge tunnel led to an investigation of the amount of oxygen that could be added to the system by bleeding air into the condenser tail pipe. As noted above, the condenser tail pipes are under reduced pressure as a result of the vacuum created by the water falling from the condenser outlet to the discharge tunnel. The results of the investigation on the effects of bleeding air into the circulating water system by means of two valves installed on each of the Unit 1 condenser tail pipes showed that the DO content of the Delaware River in the vicinity of the Eddystone intake structure was 6.5 mg 1 1. The circulating water in the discharge well contained 7.0 mg 1 -~. The discharge well circulating water was found to contain 7.5 mg 1 l DO when the condenser tail pipe valves were opened. The results of the 15 November study on the effects of bleeding air into the circulating water system showed a mean DO content of the discharge circulating water of 3.91 mg 1-1 when the condenser tail pipe valves were opened. The DO content dropped to 2.70 mg 1-1 when the valves were closed. The 1.21 mg 1-1 DO increase measured on 15 November represents an addition of 2886 kg day -1. Examination of Table I shows that on 15 November approximately

5 SUPPLEMENTAL OXYGENATION OF POWER STATION COOLING WATER! kg of 02 was added to the circulating water during normal plant operation. Therefore, the air bleeding could add about 1800 kg day 1 of 02 under the conditions that prevailed at that time. The 23 March and 15 November studies on the effect of bleeding air into the Eddystone Generating Station's circulating water system show that a significant amount of DO could be added by opening valves on the Unit 1 condenser tail pipes. Based on operating records, no noticeable increase in the pressure in the discharge side of the condensers was found during times when air is being bled into the condenser discharge tail pipe. This indicates that bleeding air into the system did not alter the hydraulic and cooling characteristics of the condensers. The DO increase across the plant will decrease as the overall water quality in the Delaware River improves since the driving force of 02 absorption is directly proportional to the degree of 02 undersaturation. This means that it is likely that at some time in the future as the overall quality of the Delaware River water improves, the Eddystone Station will not continue to add 2.3 tonnes of DO per day as required by the Delaware River Basin Commission. However, coincident with this situation is that the need for this DO addition will be decreased since the DO levels in the river will increase as a result of less O2 demand discharge from upstream sources. 6. Conclusions The Eddystone Station of the Philadelphia Electric Company naturally adds from 0.5 to 1.5 mg 1 -~ of DO to the cooling water as it passes through the plant. Additional 02 can readily be added to the cooling water by bleeding small amounts of air into the discharge side of the condensers without markedly affecting their overall hydraulic and cooling characteristics. The approach of bleeding air into the condenser tail pipe in order to satisfy the Delaware River Basin Commission's requirement of an addition of 5000 lbs (,-,2.3 tonnes) of 02 per day to the river, has proven to be an easily implemented and inexpensive approach for meeting the requirement. Acknowledgments This study was supported by a contract from the Philadelphia Electric Company, Philadelphia, Pa. The assistance of M. Piwoni, J. Lopez, C. Stratton, G. Frey and D. Cahoone as well as several other Philadelphia Electric Company personnel is acknowledged. Support was also given this paper by EnviroQual Consultants & Laboratories, Inc., of Fort Collins, Colorado. Reference Lee, G. F.: 1979, 'Persistence of Chlorine in Cooling Water Discharged to the Delaware River Estuary from the Philadelphia Electric Company Eddystone Generating Station'. Jour. Amer. Soc. Civil Eng., (accepted for publication).