What is slip and how does it affect pump performance?
Slip is the fluid lost by leakage through the pump clearances. The direction of slip will be from the high pressure to the low pressure side of the pump i.e. from pump outlet to pump inlet.
It can be calculated as follows:
Slip = Total volume displaced by pump –Flow from pump discharge
The amount of slip is dependent upon several factors.
Increased clearances will result in greater slip. The size and shape of the rotor will be a factor in determining the amount of slip. Slip varies directly with clearance.
The amount of slip will increase proportionally as pressure increases as shown below.
The amount of slip will decrease as fluid viscosity increases. The decrease is not proportional. At low viscosities, small changes in any operating parameter can cause significant changes in flow rate.
Pump speed effect
Slip is independent of pump speed. This factor must be taken into consideration when operating pumps at low speeds with low viscosity fluids as flow will be zero until speed is reached where the volume displaced is equal to the slip.