HYDRAULIC ANALYSIS OF OIL SPILL CONTROL SYSTEMS AT ELECTRICAL TRANSFORMER STATIONS

Transcription

1 HYDRAULIC ANALYSIS OF OIL SPILL CONTROL SYSTEMS AT ELECTRICAL TRANSFORMER STATIONS JAMES LI (1) & CELIA FAN (2) (1) Department of Civil Engineering, Ryerson University, Toronto, Canada, (2) Department of Civil Engineering, Ryerson University, Toronto, Canada, ABSTRACT Transformer oil (mixture of paraffins, naphthenes and aromatic compounds) is used to increase the efficiency of the electrical voltage transfer and reduce the moisture and air at electrical transformers. Each year, there is a high probability of spilling transformer oil accidentally from electrical transformer stations to the water environment in Ontario, Canada. In order to control these spills, oil spill control system are installed at electrical transformer stations. These on-site oil spill control systems, either electrically- or hydraulicallybased, use the concept of different specific gravities between oil and water for oil separation and containment. It is also assumed that the trapped transformer oil in the oil spill control system will be held for a short period of time and subsequent cleanup mechanism will remove the spilled oil. At a non-staffed transformer station in Ontario, Canada, a sudden surge of oil spills may be trapped by its hydraulically-based oil spill control system. However, the trapped transformer oil may not be cleaned up right away and may be flushed out during subsequent severe rain storms. The objective of the research project was to investigate the oil trapping performance of the proposed oil spill control system at a Hydro One s transformer station near the City of Burlington, Ontario, Canada. It focused on the analysis of oil trapping performance of this system during a major spill event as well as the potential flushing of trapped oil from the spill control system under severe storm events. Using a scaled physical model (1:12) and the actual transformer oil used at the station, the performance of this oil/water separator system was analyzed. The study results indicate that (1) major transformer oil spills can be trapped by the proposed API oil control system under dry and wet weather flow conditions; and (2) the flushing of the trapped oil out of the proposed system is low even by rainfall events more severe than the 100 year design storm. Keywords: oil spill control; electrical transformer stations; hydraulic analysis 1. INTRODUCTION Electrical transformers are generally located at several areas of a power system. Each electrical transformer sits on a spill containment with a storm inlet connecting to the underground drainage system (like a big bath tub with a drain). Although they may vary in sizes, their functions are to increase the electrical voltage from about 100 to 500 kilovolts for transmission purposes or to decrease the electrical voltage for industrial (480 volts) or residential (240 or 120 volts) uses. Transformer oil is applied at a transformer to increase the transfer efficiency and to reduce the moisture and air in an electrical transformer. Each year, there is a high frequency of accidental transformer oil spills into the environment in Ontario, Canada. According to a hydro spill study in Ontario (Weslake 2002), over 860,000 litres of spilled transformer oil were reported between 1993 and The largest volume of a transformer oil spill event between 1993 and 1999 was in 200,090 litres (Weslake Inc., 2002). Most of the transformer oil contains polychlorinated biphenyl (PCB). According to the Sewer Use Bylaw for the city of Toronto, the PCB concentrations for sanitary/combined sewer and storm sewer discharges must be less than mg/l and mg/l respectively. As a result, there is a need to control spilled transformer oil from entering sewer systems. A hydraulically-based oil spill control system uses the oil-water separation concept to trap the transformer oil spills from a spill containment and prevent spills from entering the downstream sewer system (Fig. 1). It is designed to provide adequate storage capacity for major transformer oil spills. The purpose of the long inflow and outflow pipes is to prevent the spilled transformer oil from escaping the system. When spilled transformer oil enters the system, it floats to the water surface due to the density difference between oil and water and traps in the system (transformer oil layer in Fig. 1). If the inflow is a combination of spilled transformer oil and rain water, the oil-water separation process will occur between the two vertical pipes (red dotted lines in Fig. 1). Runoffs after a transformer oil spill will pass along the bottom rainwater layer and exit to local sewers. The hydraulically-based control system requires less maintenance (e.g. removal of the transformer oil spills and grit, cleaning the maintenance hole, and the occasional inspection of the two pipe columns and maintenance hole for cracks and clogs to the structure) than that of the electrically-based control system (Fan 2004). However, it requires a sizable area to house the enormous underground structure. It is also assumed that the trapped transformer oil in the oil spill control system will be held for a short period of time and subsequent cleanup mechanism will remove the spilled oil. At a non-staffed transformer station, a sudden surge of oil spills may be trapped by its oil spill control system (designed according to the specifications of American Petroleum Institute for oily wastewater 1990). However, the trapped transformer oil may not be cleaned up right away and may be flushed out during subsequent severe rain storms. The objective of the research project was to investigate oil trapping performance of a proposed oil spill control system at a Hydro One s transformer station near the City of Burlington, Ontario 1

2 , using a physical model. It focused on the analysis of oil trapping performance of this system during a major spill event as well as the potential flushing of trapped oil at the system by subsequent severe storm events. Figure 1. A hydraulically based oil/water separator system. 2. PHYSICAL EXPERIMENTS OF OIL SPILL CONTROL SYSTEMS Based on a proposed oil spill control system for a Hydro One s transformer station near the City of Burlington, Ontario, Canada, a Froude-based physical model (1:12) was constructed to simulate different oil spill conditions at the station. As shown in Fig, 2, the upstream container represents the storm inlet at the spill containment, the middle container (690 mm x 260 mm x 265 mm) represents the oil/water separator, and the downstream container with a pump represents the storm sewer connection. All oil spill control systems at any transformer stations must meet three general operating conditions: normal, normal oil spill and major oil spills. The normal operating condition assumes that the inflow to the oil spill control system contain rainwater only. Therefore, the function of the oil spill control system is to provide an adequate flow capacity for proper drainage. The normal oil spill operating condition assumes that the inflow to the oil spill control system is a combination of minor oil spills and rainwater. Transformer oil spills may come from minor leakages at the transformer station resulting in small quantities. The function of the oil spill control system under minor spill conditions is the same as the normal operating condition. The small quantity of spilled transformer oil is supposed to be trapped in the oil spill control system. The major oil spill operating condition assumes that a sudden large surge of oil enter the oil/water separator. This condition is caused by an explosion and a fire at the transformer station due to lightening or by transformer oil oxidation. In this extended abstract, only the investigation of major oil spill condition is described. Other investigations can be found in Fan (2004). Traditional oil spill control systems are designed using the principle of oil/water separation of oily wastewater (American Petroleum Institute 1990). A transformer oil spill is a spillage of pure transformer oil. In order to test the performance of the proposed oil spill control system under major spill conditions, a major transformer oil spill was simulated based on previous largest spills in similar transformer stations. Three experiments were conducted to simulate the oil trapping performance of the proposed oil spill control system: (1) Experiment A focused on the trapping of a large 15 L (equivalent to 35,241 L in the proposed spill control system) of spill without water flow in the physical model (dry spills); (2) Experiment B focused on oil trapping of a large 15 L of spill with a moderate water flow in the physical model (wet spills); and (3) Experiment C focused on the flushing of a 36 L (equivalent to 84,580 L in the proposed spill control system) of trapped transformer oil by a severe rainfall event in the physical model. 3. RESULTS 3.1 Experiment A Observation When 15 L of transformer oil entered into the physical model quickly without pump operation (Fig. 3), large oil globules rose up along the vertical inlet pipe region due to the density difference between oil and water. The maximum diameter of the oil globules was around 15 mm and the density of the transformer oil equaled to 0.86 kg/l. Most of the oil globules were in irregular elliptical shape and traveled closely along the surface of the vertical inlet pipe. Since the oil globules were large in size, their rising velocities were fast and they traveled close to the vertical inlet pipe. The transformer oil spill in this experiment did not escape from the system. 2

3 Figure 2. The 1:12 physical model. 3

4 , Figure 3. A large spill trapped insider the spill control system under no rainfall condition. 3.2 Experiment B Observation When 15 L of transformer oil entered the system at a fast speed with pump operation, the shapes of the oil globules were similar to Experiment A. However, their sizes were little smaller than those of Experiment A, about 10 mm. The oil globules traveled along the outside of the inlet pipe instead of the surface of the inlet pipe in Experiment A (Fig. 4). This observation might be caused by the water inflow to the oil trap system which forced the oil globules to move further from the inlet pipe. However, no oil globules escaped from the oil spill system. Figure 4. A large spill trapped inside the spill control system under rainy condition. 3.3 Experiment C Observation After the oil spill control system was filled with 36 liters of transformer oil, the oil level reached just above the inlet pipe. When the pump was turned on to a maximum flow of 0.18 L/s, no waves were formed at the interface of oil and water (Fig. 5). There was no flushing of the trapped oil from the oil spill control system. The prototype flow corresponds to this 0.18 L/s is about 97 L/s. Using the 100-year design storm in the city of Burlington and the drainage area of the 4

5 Figure 5. No waves were formed at the oil water interface of the spill control system under severe rainy condition. transformer station, the maximum 100-year runoff rate was estimated to be 7.2 L/s. As a result, the chance of flushing out the trapped transformer oil during a 100-year storm was low. 4. CONCLUSIONS The physical model investigation has indicated that transformer oil spills can be trapped by oil spill control systems designed in accordance to the API recommendations. For typical major oil spills at transformer stations during dry and wet weather conditions, all the spilled oils can be trapped. Even with rain events more severe than the 100-year design storm, the runoff through the spill control system may not flush out any previously trapped oil spills. As a result, the API s oil/water separators can be used to control transformer oil spills. However, the API s oil/water separators require large space and may pose challenges at stations with limited space. ACKNOWLEDGMENTS The authors would like to acknowledge the support from Hydro One Inc. and Natural Science and Engineering Research Council for this project. REFERENCES American Petroleum Institute. 1 st Edition (February 1990). Design and Operation of Oil-Water Separators. Monographs on Refinery Environmental Control Management of Water Discharges. Fan, C. (2004). Hydraulic Analysis Of Oil Spill Control Systems In Transformer Stations. Master Thesis, Department of Civil Engineering, Ryerson University, Toronto, Ontario, Canada. Weslake, Inc. (2002). Hydro One Spill Study. Report prepared for Hydro One Inc. 5