Useful Information on Pump Slip
What is Pump Slip?
Slip is the loss of pumping capacity due to fluid leaking back through a pump from the discharge side to the inlet side. It can affect the efficiency of all types of pumps although the actual causes may be very different.
How does Pump Slip occur?
For centrifugal pumps, slip is more of a theoretical concept rather than a measurable quantity. For this class of pump, slip is the loss of efficiency from a theoretical ideal arising from design constraints and turbulence at the impeller vanes. This results in some fluid being misdirected with regards to the discharge outlet. The calculation of slip is a key factor in impeller design, including the number, diameter, angle and geometry of impeller vanes.
In reciprocating positive displacement pumps, back flow can occur because of poorly fitting valves or through delays in their operation. It is important to note that displacement pumps rely on the pumped fluid for lubrication and a certain amount of slippage is essential to ensure their smooth operation. Slippage can also be useful in flushing away solids that could cause abrasion in the small clearances around pistons, valves and bearings.
Gear pumps rely on the close tolerances between components for their efficient operation. The dynamic seals between the gears, bearings and case, in which the pumped fluid itself plays an important part, allow the parts to move whilst maintaining a seal between the inlet and outlet. Wear, and consequently leakages, can occur around the gear teeth, across the face of the gears and around the bearings (Figure 1). As wear continues, for example, due to an abrasive feed, there is increasing leakage of the pumped fluid from the discharge side back to the suction side.
What factors affect Pump Slip?
There are three main factors: differential pressure, fluid viscosity, and, in the case of positive displacement pumps, clearance dimensions.
Differential pressure
As the difference between outlet and inlet pressure increases, more fluid is forced back through the pump (increasing slip) and consequently the achievable flow rate is lower. This is the key factor in determining centrifugal pump efficiency since these do not have internal valves or gears to restrict backflow. With well-fitting and efficient valves, a reciprocating pump is only slightly sensitive to differential pressure. It is necessary for gear pumps to have some clearances to allow movement of the gears within the body of the pump so, when operated at a higher differential pressure the backflow will also be higher.
Fluid Viscosity
A more viscous fluid effectively seals the leak paths through the clearances in a pump, decreasing the amount of slip. The viscosity of liquids such as oil or water is a simple function of pressure and temperature so slip is predictable in these cases. On the other hand, non-Newtonian fluids such as corn syrup or paint change viscosity under shear stress so slip will be unpredictable and this will affect a pump’s ability to produce accurate dosing or metering.
Clearances
Within a positive displacement pump, slip is strongly dependent on the size of clearances (see “How is slip assessed?”, below) so, in practice, wear has a small effect until a critical point is reached, from which performance degrades rapidly. When this occurs, metering accuracy is affected and pump repair or replacement is necessary.
How is Pump Slip assessed?
The modelling of slip in centrifugal pumps is a complex problem. This involves calculating the theoretical power output, which assumes that the impeller consists of an infinite number of vanes and that the fluid is an ideal liquid. In this idealised situation, the liquid would be perfectly guided by the vanes and would leave the impeller at the vane angle. Slip is the loss in efficiency that occurs from this theoretical ideal, mainly due to the finite number of vanes and other design elements.
In a reciprocating pump, such as those used commonly in the oil industry, some slippage is important for lubricating the piston. However, greater pump clearances increase the amount of slippage, which may lead to inefficient operation. Empirical relationships can be used to predict slippage as a function of pumping speed, stroke rate, pump diameter and clearances.
Slip in gear pumps can be accurately modelled using the Hagen-Poiseuille’s law. Originally, this was devised to relate the pressure drop in a fluid to its flow when constrained by a long cylindrical pipe. The following version of the equation can be used to model slip in pumps:
It can be seen that a doubling of the height of a clearance gap causes an eightfold increase in leakage whereas leakage is only doubled when a pump is used with a fluid with half the viscosity.
In gear pumps, the amount of slip, as a percentage of total flow, is lower at higher operational speeds. A pump that can deliver the desired flow rate at the highest practical speed may experience less slip but, of course, component wear will be higher in such a system.
How is Pump Slip controlled?
To ensure constant flow, some form of feedback control system is used. For example, by using accurate flow measurement in conjunction with a variable drive motor, flow can be maintained at a constant rate by gradually adjusting the pump speed as slip varies. Alternatively, if it is important to run a pump at constant speed, a bypass line can be used to return a small amount of the overall flow to the source reservoir. By controlling the flow on the bypass line, the overall output can be maintained at a constant rate.
Of course, if slip primarily results from increased clearances within a pump, these methods can only be a temporary measure and replacement of the worn components will eventually become necessary.
Summary
Slip is the internal back flow of fluid in a pump returning from the discharge side to the suction side. In a centrifugal pump, slip is the loss of efficiency from a theoretical ideal arising from design constraints and turbulence at the impeller vanes. Displacement pumps, on the other hand, rely on the pumped fluid for lubrication and a certain amount of back flow and slippage is necessary to ensure smooth operation. This leakage increases with differential pressure and is indirectly proportional to the viscosity of the pumped fluid. In all positive displacement pumps, as components such as valves, gears, and bearings experience wear, which increases the internal clearances, slip increases substantially.
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