Radial Thrust
The three drawings below illustrate how radial thrust occurs in volute type pumps
As you examine the illustrations look for the following:
The size of the blue arrows surrounding the impeller indicate pressure intensities, large blue arrows indicating higher pressure than small blue arrows. Non-Uniform pressure intensities surrounding the circumference of the impeller, cause radial thrust. If the impeller is balanced, and fluid pressures were uniform around the impeller periphery, there would be no radial thrust.
The red arrow inside the box on the right hand side of each illustration, together with the two red arrows at the top and bottom of the impeller, indicate the general steady state direction of thrust at each different flow rate.
The direction of thrust is perpendicular to the shaft, or in other words, radial thrust pushes sideways against the shaft, as opposed to Axial Thrust which acts upon the impeller shaft in line with the shaft.
The illustration above shows fluid pressures (large and small blue arrows), surrounding the circumference of the impeller, when the pump operates at the BEP flow rate.
At BEP, pressure surrounding the impeller is more balanced than at any other flow rate, with a small amount of radial thrust away from the cutwater, with maximum hydraulic efficiency, and lowest levels of vibration, than at any other flow rate.
The illustration above shows fluid pressures (large and small blue arrows), surrounding the circumference of the impeller, when the pump operates ABOVE the BEP flow rate.
Above BEP, pressure surrounding the impeller is less balanced than at BEP, resulting in shaft deflection towards the volute enlargement, lower hydraulic efficiency, and higher levels of vibration, than at BEP.
The illustration above shows fluid pressures (large and small blue arrows), surrounding the circumference of the impeller, when the pump operates BELOW the BEP flow rate.
Below BEP, pressure surrounding the impeller is less balanced than at BEP, resulting in shaft deflection in the direction opposite the volute enlargement, lower hydraulic efficiency, and higher levels of vibration, than at BEP.
Pump Features Affecting Radial Thrust
Diffusers
Diffusers slow the high velocity fluid exiting the impeller and thereby convert fluid velocity energy into pressure energy, resulting in fluid movement by diffusion (the source of the name Diffuser), a slower and more orderly movement of fluid. Because diffusers surround the impeller they maintain a more stable hydraulic environment surrounding the impeller, resulting in more even balance of hydraulic pressure forces surrounding the impeller, resulting in less radial shaft deflection, and therefore lower vibration levels. Diffusers therefore widen the Operating Range or Window for any pump.
Double Volute Pumps
Double Volute pumps have two cut-waters or tongues 180 degrees apart. This lowers pump efficiency slightly but helps achieve a more balanced hydraulic environment surrounding the impeller, and therefore less radial shaft deflection and less vibration, widening the Operating Range of the pump. Actually, based on studies by Karassik and others, Double Volute pumps are markedly superior to Single Volute pumps. Double Volute pumps are an excellent choice for installations where the pump will be operated for long periods below BEP. The only problem with Double Volute pumps is that they may be hard to obtain, many manufacturers have stopped making them.
Concentric and Modified Concentric Volute Pumps
These types of pumps maintain a concentric casing surrounding the impeller for approximately 270 degrees from the cutwater in the direction of rotation before an enlargement leading to the discharge nozzle. Center Discharge pump cases are often, but not always, modified concentric cases. Like the double volute, the intent of the modified concentric casing is to reduce hydraulic unbalance surrounding the impeller, thus reducing radial thrust unbalance, and therefore reducing the pump's reaction against off BEP flow rates. Also like the Double Volute, efficiency is lower than a single volute pump.
Turbine Pumps - Concentric Flow with Diffusers
Turbine pumps experience almost no radial thrust at any flow rate for two reasons.
CONCENTRIC FLOW - The fluid flow path through a turbine impeller is concentric, fluid flows evenly through all impeller vanes simultaneously and continuously, as opposed to a scroll pump where the fluid is "sheared" away from the impeller by a cutwater. The amount of fluid exiting each individual impeller vane passage, is the same at all times as the impeller rotates. In volute pumps however, fluid moves through each impeller vane passage at a non-uniform rate, depending on where the vane passage is in relation to the cutwater.
DIFFUSER - Fluid moving through a turbine impeller immediately enters a diffuser, slowing the fluid down, resulting in pressure recovery and diffusion flow, which is more orderly and less turbulent, thereby producing a uniform pressure environment surrounding the impeller.
Specific Speed
The lower the pump specific speed, the wider the natural operating range of the pump. Pumps with low Specific Speeds (Ns <2000) have radial flow impellers allowing the pump to operate smoothly at flow rates well off BEP and some can operate smoothly all the way down to shut-off and also at flow rates higher than BEP. This is why low specific speed pumps make good jockey pumps. It must be noted however that radial flow pumps still exhibit low efficiency and may encounter recirculation cavitation at low flow rates, while at flow rates above BEP suction cavitation may become a problem.
Tribal Knowledge
The following discussion concerns only single volute, close coupled, horizontal end suction pumps driven by JP or JM frame motors. JP and JM motors were designed for pumps. As such, the front radial bearing in these motors is designed to accept the radial thrust from the pump impeller. Those familiar with these types of pumps and motors may have observed that the front bearing is much larger and more massive than the rear bearing, in order to accept radial thrust unbalance.
When these types of pumps experience high levels of radial thrust, the component most likely to fail first is the rear motor bearing, the bearing furthest away from the impeller. This is surprising but can be explained as follows.
The very large front motor bearing withstands the radial thrust because it is designed to accept those forces. However, severe vibrations can be transmitted down the shaft, through the front bearing, all the way to the back bearing, which may not be able to withstand the vibrations.
Pump Control Services (PCS) has observed this phenomena repeatedly. The failures observed involved failure of the rear bearing and bearing housing, an aluminum cup holding the bearing. The bearing "wobbles" in this cup, resulting in the loss of the entire motor. In one instance the rotor wobbled enough so that the rotor touched the stator plates and forced one of the plates to cut into the windings.
