Split Case Pump: Design, Selection and Suction Checks

A split case pump is a centrifugal pump with a casing that separates into two major sections. In the common horizontal, axially split design, the joint runs along the shaft centerline: the upper casing lifts away to expose the rotating assembly. “Axially split” describes the casing joint—not the direction of water flow through the impeller. KSB defines the split by its position along the shaft centerline.

The practical attraction is maintenance access. On a design with suction and discharge connections in the lower casing, the upper half can be removed without disconnecting the main piping. Bell & Gossett’s e-HSC specification describes this arrangement. But the casing arrangement does not establish flow capacity, pressure rating or suction performance. Those still require the exact pump’s curve and specifications.

Split case and double suction are different features

A common configuration combines an axially split casing, a double-suction impeller and bearings on both sides of the impeller. These describe separate parts of the design:

• Split case: how the pressure casing opens.
• Double suction: liquid enters the impeller from both sides, commonly through one shared external suction nozzle.
• Between bearings: the impeller sits between the shaft’s bearing supports rather than overhanging one support.

The two impeller entries share the flow. This arrangement helps accommodate high flow rates and balance axial thrust—the hydraulic force acting along the shaft. It does not mean twice the discharge pressure. KSB describes double-suction flow paths as operating in parallel, increasing capacity at a constant head. KSB double-suction reference

Check the sectional drawing rather than assuming every split case pump is double suction or single stage. Casing split, number of stages and suction arrangement are separate design choices. KSB volute-casing reference

When the layout is useful

Split case, double-suction pumps deserve consideration for water-supply and larger circulating-water duties where both hydraulic capacity and overhaul access matter. Examples include chilled-water distribution and district heating or cooling. Bell & Gossett’s e-HSC range illustrates the HVAC application, while KSB identifies water supply as a double-suction application. Xylem application list

The service advantage only works if the installation preserves it. Allow overhead lifting space, a route for moving the casing and rotor, and access around the bearings and coupling. A pump squeezed beneath permanent pipework may be easy to open on paper but difficult to overhaul in place. The e-HSC installation manual, section 4.1.1, calls for space around and above the pump for service and lifting.

Nor is split case the only serviceable layout. Some end-suction pumps use a back-pull-out bearing bracket. Compare the actual maintenance procedure and required clearances, not simply the pump category. KSB construction reference

Service access also varies by seal configuration. For example, the e-HSC manual notes that its high-pressure MR1 seals require upper-casing removal, unlike the seals covered by its casing-in-place replacement procedure. Manual, sections 6.5.1 and 6.5.6

Select by duty point, not flange size

Specify the required flow at total head, then evaluate the exact pump size, impeller diameter and operating speed. The operating point occurs where the pump curve meets the system curve; the nameplate capacity alone cannot predict what a different piping system will deliver. System-curve explanation

Before accepting a selection, ask for:

1. The full operating range. Include normal duty, minimum demand, peak demand and any parallel-pump operation—not just one design point.
2. BEP and operating-region limits. The best efficiency point (BEP) is a useful reference for hydraulic efficiency and reliability. The preferred operating region (POR) is a range around it, not a universal percentage for every pump.
3. Power across that range. Verify that the motor can carry the load at every intended operating condition.
4. Liquid and pressure limits. Confirm temperature, chemistry, solids or abrasives, casing material, seals, elastomers and maximum working pressure. Casing pressure must be checked against the system pressure at the installation, not merely the pump’s differential head.
5. Service details. Obtain the sectional drawing, lifting weights, clearances, seal arrangement and model-specific maintenance manual.

Operating well away from BEP can create recirculation, additional loading, vibration and cavitation. The allowable operating region may extend beyond the POR, but its limits depend on the design and application. Hydraulic Institute operating-region guidance Bell & Gossett’s e-HSC specification likewise calls for project-specific curves, pressure ratings and clearances.

Suction piping can defeat the double-suction advantage

Balanced construction does not guarantee balanced inlet flow. An elbow close to a horizontal double-suction pump can send more water toward one impeller entry than the other, depending on its orientation. The resulting unequal thrust can overheat bearings, accelerate wear and impair hydraulic performance. The e-HSC manual specifically warns about this mechanism. Installation manual, section 4.4.2

Review elbow orientation, the required straight inlet run, reducers, air pockets and suction-line losses against the manufacturer’s instructions. Do not impose one straight-pipe distance on every installation, and do not throttle a suction isolation valve to control capacity. Manual, sections 4.4.2–4.4.3

Also calculate net positive suction head available (NPSHA) for the limiting inlet conditions, including low source level, high liquid temperature and suction losses. NPSHA describes the inlet pressure above the liquid’s vapor pressure, expressed as head. Compare it with the manufacturer’s NPSH required (NPSHR) across the intended flow range, with an appropriate margin. Without sufficient inlet pressure, liquid can vaporize locally; the resulting bubbles can collapse farther into the impeller and cause damage. Hydraulic Institute NPSH explanation

Merely matching the published requirement is not a cavitation-free guarantee. The Hydraulic Institute’s explanation of the 2024 ANSI/HI 9.6.1 revision uses manufacturer-supplied NPSHR as the margin reference; that value must be at least the tested NPSH3, where cavitation has already caused a 3% head reduction. Margin recommendations depend on the application. NPSH-margin guidance

Before blaming the rotor

For low output or new vibration, record suction and discharge pressures, flow, speed, motor current and source level. Compare those readings with the correct curve before dismantling the pump.

Useful first checks include:

Symptom	Conditions to investigate
No delivery after startup	Incomplete priming, wrong rotation, blocked inlet or excessive system head
Low flow	Suction air leaks, restrictions, insufficient NPSH, low speed or impeller damage
Fluctuating pressure and noise	Cavitation, trapped gas or unstable operation
Hot bearings or repeated bearing failures	Alignment, lubrication, pipe strain and unequal flow into the double-suction impeller

These are investigation paths, not diagnoses from sound alone. The e-HSC manual covers inlet-flow imbalance, gauge behavior, lubrication and troubleshooting in sections 4.4, 4.5, 6.2 and 7.

Before opening the casing or removing plugs, qualified personnel must prevent the drive from restarting, lock out its energy supply, isolate the pump, relieve pressure and allow it to cool. Verify isolation before dismantling and follow the approved lifting procedure. A split case simplifies access; it does not eliminate stored-pressure, rotating-equipment or lifting hazards. Manual safety and disassembly precautions