Niagara Falls 100.000-Hp. Development

 10] Niagara Falls 1 00,000 Hp. Development Details of the I. P. Morris Unit Vertical Shaft Single Runner Turbine—Draft Tube Is of the New Spreading Tube Type —Governor Regulation By Lewis F. Moody Assistant to vice president and consulting engineer William Cramp & Sons S. & E. B. Company, Philadelphia, Pa. TWO of the three turbine units in this installation were furnished by the I. P. Morris Department of The William Cramp & Sons Ship & Engine Building Co. These units involve in their design the most recent developments in this field of engineering, and include a number of interesting departures from previous practice. Every effort has been made by the builders of the turbines to provide the greatest possible reliabil- ity in operation and maximum obtainable efficiency. The turbines are of the vertical shaft single-runner type, having cast-iron volute casings imbedded in the concrete of the power-house substructure. They oper- ate at 150 r.p.m., and are designed to develop maximum efficiency when operated under an effective head of 213.5 ft. and discharging 1,500 sec.-ft. of water. Taking up first the hydraulic features, it may be noted that the engineers of the power company work- ing in co-operation with the engineers of the turbine builder have exercised every care to provide an efficient water passage from the point of entrance from the forebay into the penstock to the final point of discharge to the draft tubes into the tailrace. Preliminary tests have shown a very small loss of head in the penstocks, and indicate that excellent hydraulic conditions have been secured at all points in the water passage. Before entering the turbine the water passes through a large Johnson valve. In passing through this valve the velocity of the water is gradually increased, the penstock diameter being reduced at the outlet of the valve to a value equal to the diameter of the inlet to the turbine casing so that the velocity of discharge from the Johnson valve is maintained at a constant value between the valve and the turbine. Owing to the construction of the turbine casing of cast iron, it has been possible to preserve smooth surface contours for the internal surfaces of the casing and to avoid all local disturbances in the flow due to abrupt changes of direction or irregular surfaces. The diameter of the intake to the turbine casing is 10 ft. 6 in., involving a velocity of 17.3 ft. per sec. and a velocity head of 4.66 ft., which represents 2.18 per cent of the effective head of the turbine. The casing is designed for a gradual acceleration of velocity as the water passes around the volute. The casing is stayed across the distributor by ten curved vanes cast integrally in a cast-steel speed ring forming a part separate from the casing and built in halves. The vanes of the speed ring line up with alternate guide vanes, when the guide vanes are in the normal gate position corresponding to maximum efficiency. There are twenty movable guide vanes or wicket gates. Spreading Draft Tube After passing through the runner, the water enters a straight conical draft tube of cast iron built in sec- tions. From the lower end of the cast-iron section of draft tube the remainder of the tube is molded in the concrete substructure. The draft tube is of a new type, the “spreading draft tube.” The design of this tube has been care- fully calculated and verified by many tests of experi- mental models both under conditions of smooth streamline flow in the water entering the tube and under the actual conditions of flow experienced when the discharge is received from a runner. In the design of this tube it has been the object to turn the water as smoothly as possible along paths of gradual curva- ture from the vertical to the horizontal direction, and to preserve symmetry about the turbine axis to a point where the velocity has been reduced to so low a value that only a small amount of velocity head remains in the water. Care has been taken to maintain a gradual decrease in the velocity by properly varying the trans- verse area of the passage at all points in the tube, in order under all conditions of operation to avoid the formation of eddies or disturbances at any point in the stream. In many earlier installations under high and medium heads, particularly those involving long penstocks, the use of elbows in the draft tubes has resulted in severe vibration of the water column in the penstock and turbine. This vibration is believed to be due to the formation and breaking down of eddies in the draft tube. Another advantage of the form of tube used is the ability of such a draft tube to regain the energy of whirling components of velocity in the water leav- ing the runner. This property of the tube is of value in increasing the efficiency under part-gate and over- gate conditions, and in increasing the margin of power beyond the point of maximum efficiency of the turbine. Preliminary tests of the turbine indicate that both of these advantages have been realized, and in actual oper- ation the unit is remarkably free from vibration of any kind at all gate openings. Cast-Iron Runners and Casing The turbine runners are of cast iron, each one being in one piece. The runners are 10 ft. 6 in. in diameter in inflow. The specific speed of the runners is 158 metric or 35.5 in the foot-pound system. The throat of the runner or the diameter inside of the band is 9 ft. 10J in. At normal discharge the corrected veloc- ity of the water leaving the runner and entering the draft tube is 21.3 ft. per sec., corresponding to a veloc- ity head of 7.0 ft., or 3.3 per cent of the effective head on the turbine. Although the most valuable feature of the new draft tube is probably the elimination of vibration just mentioned, it is also estimated that this tube will improve the efficiency over that which could be obtained with tubes containing elbows by from 0.3 per cent to 0.4 per cent in the efficiency of the entire turbine under conditions if normal gate operation. The same type of draft tube when applied to turbines of high specific speed would, of course, be capable of producing much greater increases in efficiency than is possible in these turbines, which are of moderate specific speed. The final discharge velocity from the draft tube at normal gate is approximately 5 ft. per sec., corresponding to a velocity head of 0.175 per cent of the effective head on the turbine. The turbine casing is of cast iron built in six sec- tions. The use of a cast-iron casing, although involving a somewhat increased cost as compared to the use of plate steel, furnishes a superior design in the follow- ing respects: The casing is absolutely rigid and can be imbedded in the concrete so as to form a part ■HiiBKnHnBSSHSKHCiiiianaaBEMRMHsnaHBaffiraaRiinraEiBKnEKiaEfflHUMaRBnBnnmHsiiiHHMnMiMinMMaMHlI