Disc friction

Disc friction (P_Ldiscfric) in centrifugal pump engineering is the friction loss caused by the fluid between the impeller shrouds and the pump casing.

Among all losses caused by friction (losses in bearings and seals, disc friction loss), the power loss resulting from disc friction usually dominates (see Internal efficiency). 

As disc friction is the result of viscous friction between the fluid rotating in the side gap (at circumferential speeds differing from that of the impeller) and the outside impeller surfaces, disc friction is greater for a closed impeller than for an open impeller with one shroud.

The equation for disc friction loss is as follows:

c_M        Moment coefficient
ρ         Density of the fluid handled
u₁, u₂   Circumferential speed of impeller at D₁ and D₂
D₁        Internal diameter of impeller shroud
D₂       Outside diameter of impeller
n         Impeller speed

The moment coefficient (c_M) takes into account the influences of the Reynolds number, the surface roughness and the side gap geometry. As the equation shows, disc friction changes at constant circumferential speed with the square of the outside diameter of the impeller and at constant rotational speed with the fifth power of the outside diameter of the impeller. If the outside diameter of the impeller and the rotational speed are constant, the ratio of disc friction (P_Ldiscfric) to power input (P) depends on the flow rate (Q), i.e. this ratio is a function of the specific speed (n_s). 

Experiments have shown that the difference in surface roughness between a rough (cast) impeller shroud and a polished one reduces the disc friction loss by approx. 30 %. The influence of the side gap geometry can also make a difference of up to 10 %. Both changes should always be considered in conjunction with the specific speed. See Fig. 1 Disc friction

In the specific speed (n_s) range from 40 to 60 rpm the losses caused by disc friction only amount to between 1 and 2 % of the power input (P). From a specific speed of roughly 30 rpm downward, disc friction loss increases rapidly with decreasing specific speed, amounting to approx. 5 % at a speed of 20 rpm and as much as 10 % at a speed of 10 rpm. %.

This means that the surface roughness and side gap influences can hardly be measured for specific speeds higher than 40 rpm. Marked improvements can however be achieved at low specific speeds by using smooth impeller shroud and casing surfaces. As disc friction increases with the fifth power of the outside diameter of the impeller, it will suffice to machine only the outside portions (outside of approx. 0.7 of the impeller diameter).