The principal reasons for the popularity of this between-bearing configuration include:
1. The rotor can be removed by just taking off the upper casing half, without disturbing the suction or discharge piping, or moving the motor
2. Less shaft deflection due to the between-bearing design.
3. Lower NPSH requirement due to the fact that each impeller eye only handles one half of the total pump flow rate.
4. Virtually no axial hydraulic thrust, due to the back-to-back / double suction impeller design.
5. Higher efficiency because power is not lost to balance the hydraulic thrust.
6. Relatively high allowable nozzle loads due to the rigidity of the lower portion of the casing.
Although split case, between-bearing models are quite popular, and have many advantages as listed above, the pumps are not without some drawbacks.
1. Due to the large flanges required for the split casing sealing joint, these pumps are normally heavier and cost more than comparable end suction pumps, especially in higher alloy and higher pressure applications.
2. While double-suction impeller pumps have lower NPSHr values then comparable end suction pumps, the through-shaft reduces the impeller eye area. This requires an increase in the suction eye diameter (higher eye tip speed and suction energy). Also, in order to keep the shaft bearing span to a minimum for critical speed purposes, the compressed inlet passages result in tight turns for the liquid entering the impeller eye. The gating Suction Energy and required NPSH margins tend to increase as a consequence. This phenomenon has caused field problems with high and very high suction energy pumps, when operating at low NPSH margins and/or when operating in suction recirculation (See October 2007 column). It should be noted that some of the newer split case pumps do have improved casing inlet designs (less inlet turbulence), which allows them to approach the higher suction energy performance (gating values) of end suction pumps.
3. Double-suction pumps are more sensitive to the orientation and geometry (radius) of elbows in front of the pump inlet (see April 2009 column). Suction elbows should be perpendicular to the plan view of the shaft. This reduces the tendency of unequal flow quantities reaching the two impeller eyes. Suction elbows in the plan of the shaft can cause uneven flow patterns which can upset the axial thrust balance, causing high bearing loads and shorter bearing life. Uneven flows to each impeller eye also tends to increase the NPSHR, and/or potentially put one half of the impeller into suction recirculation.
4. Between-bearing pumps need two shaft seals, whereas only one seal assembly is required for an end suction pump. This could increase both initial and maintenance costs in services that require expensive mechanical seals.
5. The axial split complicates the radial gasket sealing and can result in the mismatch of the casing halves at the joint.
6. This construction also limits the mechanical seal chamber options, since most split case pumps have the stuffing boxes integral with the casing halves.
