The casing, or volute as it is sometimes called, captures the fluid energy generated by the impeller and directs it out of the pump through the discharge nozzle.
As with impeller designs, manufacturers each have their own “special design” for casings, but most can be described by how they fall into a few short categories. The first is how they are “split.” Within this category, you have axially or radially split.
Axial split casings have mating surfaces running on the same axis as the shaft. This design allows the maintenance department to open the pump for inspection without disturbing the bearings, seals, or piping.
The pump on the left is often referred to as a “Horizontal Split Case Pump,” leaving out the term “Axial.” Although not technically correct, the word horizontal, in this case, denotes both the orientation of the shaft and the direction of the split. (“Axial Split Case Horizontal Pump” might have been a better designation.”)
Due to the sheer numbers, the term “Axial split case” is most often associated with horizontal pumps, but the design is not limited to this single arrangement. The illustration below is an example of a vertical split case design.
Radial split casings have mating surfaces that are at 90 degrees on the axis of the shaft, as the picture below shows.
This concept is used extensively by the manufacturers of both rubber and metal-lined pumps as it greatly simplifies the changing of worn parts.
Tangential discharge casings have a discharge flange centerline that is in a plane offset from the pump’s centerline. The smooth path out of the pump leads to higher pump efficiencies and less wear. The offset design, however, requires cast-in lifting lugs to handle the offset weight. It also calls for special offset piping connections that may need independent support structures.
This casing design is only self-venting when in a “top left” discharge configuration.
The centerline discharge casing has the discharge flange center in the same plane as the pump’s centerline.
The advantages this design offers include self-venting for vertical discharge arrangements, the casing hanging straight when lifted by the discharge flange, and the weight of the casing being equally distributed to both feet.
The additional change in flow direction as the fluid exits the pump does result in a very slight loss in efficiency. This may be seen as a problem by some engineers; however, others prefer working with a centred discharge flange, which simplifies piping layouts.
Most small and general industrial pumps are tangential, not centerline.
Recessed impeller casing, as the name denotes, has the impeller recessed back into the casing. With the impeller in this position, it allows the casing to pass very large particles and or “stringy” slurries.
The max particle passageway is often similar to the discharge flange opening dimension.
Arguably, I have reviewed the most common terms you will hear when discussing casings. There are, however, a host of others. Terms such as double suction, double discharge, split volute, diffusers, end suction, top discharge, and self-priming just to name a few. Alas, subjects for another day.