Influence of Disk Friction on Turbine Pump Design

Abstract

Abstract The mathematical survey of the problem leads to these conditions for a minimum loss through disk friction: a Smoothness (polish) of both disk and casing. Roughness of either is equally detrimental. b Smallest possible surface of both. Excessive extension of surface is equally detrimental whether it is the surface of the disk or of the casing. c Outward indication of attainment of minimum is the fact that the waste-water rotates half as fast as impeller. A gyrostatic pressure is generated by the rotation of the waste-water and added to static pressure prevailing at the center of the impeller. From the influence of the width of the impeller it follows that it is important to keep the thickness of metal at the periphery as small as possible. Protruding rims are objectionable. The influence of the ordinary roughness of non-machined castings has no perceptible effect on the efficiency except with high lift pumps. Painting or japanning the surfaces generally seems less desirable than machining them with a medium heavy cut. High polish seems wasted. The experiments verify conclusions of mathematical survey. The influence of viscosity is proportional to its fifth root; yet, it is responsible for an improvement in the efficiency of hot-water turbine pumps. The effect of pumping heavy oil and tarry liquid is estimated. The influence of fluid density is almost exactly proportional to the specific gravity. The loss through disk friction constitutes a constant percentage of the normal useful power at all speeds in one and the same pump. Generally its percentic value grows with the value of head per stagecapacity at constant speed, and diminishes with increasing speed and constant ratio head per stagecapacity. High heads are more economically produced by high speeds or a greater number of stages than by increasing the diameter of the impellers, but the number of stages should be left to the discretion of the makers, not fixed by specifications. A steep angle between impeller blades and the tangent at the periphery serves very considerably to improve the efficiency owing to indirect reduction in disk friction losses, especially in high lift pumps. The disk friction reaches a minimum for a certain width of the waste water chamber, which is about ⅜ in. for disks of 12 in. diameter. The increase with increasing width is due a To the increase in retarding surface. b To the induction of secondary or induced hydraulic currents. Concentric ribs are advantageous; radial ribs are detrimental. In single-stage pumps the rotation of the waste-water reduces the tendency for leakage by about 20 to 35 per cent. In multi-stage pumps the same influence may even increase the leakage. Inequality in shape or roughness of the waste-water chambers on both sides of the impellers produces a gyrostatic axial thrust due to disk friction which can assume very considerable values. The direction of this thrust is indicated by the rule: “The impeller is drawn to the side where the waste-water rotates fastest.”