Double Suction Split Case Pumps | Selection & NPSH Guide

Double Suction Split Case Pumps: Where They Earn Their Keep (And Where They Don't)

A pump manufacturer's honest take on when a split case pump beats an end suction pump, when it doesn't, and what to put on the duty sheet before you ask for a price.

When an engineer sends us a duty sheet with a flow north of 1,500 m³/h and a head around 25 or 30 metres, we usually know what the answer is before we open the curve software. It's almost never a single-stage end suction pump, and it's rarely a multistage. More often than not, it's a double suction split case pump.

Customers sometimes push back on that. A split case pump costs more than a basic end suction model, it takes more floor space, and on paper both will move the water. So why do we keep recommending it for the big-flow, low-head jobs? The short answer is that the two designs behave very differently once the flow gets large, and the differences show up in noise, seal life, electricity bills, and what happens the first time you have to open the pump in a hurry. This is the long answer.

A horizontal axially split case pump built for continuous high-flow duty.

What "split case" and "double suction" actually mean

Two separate ideas get bundled into one pump, and it helps to keep them apart.

Double suction describes the impeller. Instead of a single eye where water enters one side, a double suction impeller takes water in from both sides at once. Picture two single-suction impellers mounted back to back on one shaft, sharing a common discharge. That's the hydraulic heart of the pump.

Split case describes the casing. On the horizontal machines we build most often, the volute is split along the horizontal centreline into a top and a bottom half. Unbolt the upper half and lift it off, and the entire rotating assembly, impeller, shaft, seals and bearings, sits exposed while the suction and discharge flanges stay bolted to the pipework.

Most of the pumps people mean when they say "split case pump" are both. But the two features solve different problems, which is worth understanding when you compare quotes.

Double suction impeller inside a horizontally split volute: water enters from both sides along the shaft.

The two suction eyes fix two problems at once

This is the part that decides most high-flow selections. Feeding the impeller from both sides does two useful things at the same time.

A double suction impeller is effectively two single-suction wheels back to back on one shaft.

Lower NPSH, and fewer cavitation headaches

NPSH, net positive suction head, is the pressure margin the water needs at the pump inlet so it doesn't flash into vapour bubbles. Lose that margin and you get cavitation, which sounds like someone pouring gravel through the casing and leaves the impeller eye and vane inlets pitted like a rough casting after a few months. On a big pump running 24/7 it's one of the fastest ways to destroy an impeller.

With a double suction impeller, the total flow is split between two eyes. Each eye only has to pass half the volume, so the inlet velocity is lower and the pump demands noticeably less NPSH than a single-suction wheel moving the full flow through one eye. We don't quote a fixed percentage because it varies with the specific speed and eye geometry, but on large flows the gap is wide enough that a double suction pump will often run happily from a sump or suction arrangement where a single-suction pump of the same rating would already be cavitating. For a municipal intake or a cooling-water pump drawing from a low-level channel, that alone can settle the choice.

Cavitation damage at the impeller eye: the classic result of running with too little suction head.

Axial thrust mostly balances itself

A single-suction impeller generates hydraulic thrust along the shaft, toward the suction side, and something has to absorb it, usually bearings, balance holes, or a back-pump-out arrangement. Because the double suction wheel is symmetric and loaded from both faces, those axial forces cancel each other out to a large degree. A small residual thrust still exists in the real world, no cast impeller is perfectly identical on both sides, so the bearings carry that, but the steady load is far smaller.

In practice that means cooler bearings, less shaft deflection, and a mechanical seal that isn't constantly working against a heavy axial load. On a pump expected to run for years between overhauls, that's worth real money.

Why the efficiency keeps climbing on big impellers

Large split case pumps are some of the most efficient rotodynamic machines you can buy, and the reason is mostly geometry. The internal losses in a centrifugal pump, disc friction, leakage through the wear rings, hydraulic friction in the channels, don't grow as fast as the useful work as the impeller gets bigger. A big, slow-turning, generously proportioned impeller with wide waterways loses a smaller fraction of its energy than a small high-speed one.

That matters because high-flow pumps consume serious power, often tens or hundreds of kilowatts, and run continuously in water treatment plants, power stations and irrigation districts. A couple of percentage points of efficiency on a 200 kW motor is the difference between a pump that pays for itself and one that quietly drains the operating budget for ten years. The double suction layout lets designers use a large-diameter impeller at the flow where it works best, which is exactly the high-flow, low-head zone these pumps live in.

The split in the casing is really about downtime

If the double suction impeller is about hydraulics, the split casing is about the maintenance crew. On a conventional end suction pump, getting at the impeller and seal means pulling the motor back, breaking the coupling, and on a back pull-out design removing the whole rotating assembly from behind. You can do it without touching the pipework on a good back pull-out unit, which is why we still build and sell plenty of long-coupled end suction pumps, but you're still working around the motor and realigning on the way back.

On a split case pump you remove the upper casing half with an overhead crane or lifting frame and the rotor is right there, accessible from above. Suction and discharge flanges, the bottom casing half, and the motor all stay put. Wear rings, impeller, seals and bearings can be inspected or replaced and the pump closed up again without disturbing a single pipe joint or redoing the shaft alignment.

Lifting the upper casing half gives full access to the rotor without breaking a single pipe joint.

One honest caveat from the workshop: the horizontal joint face has to seal against full discharge pressure, so the casing bolts need to be torqued in the correct diagonal sequence and to the right value. We've seen pumps leak at the split line after a field overhaul simply because the bolts were run down in order around the flange instead of cross-tightened. It's a twenty-minute detail that saves a messy cleanup.

Where a split case pump is the wrong call

We sell these pumps, but they are not the answer to everything, and a salesman who tells you otherwise is doing you no favours. Pick something else in these situations.

• Low flow, high head. If you're moving a small volume against a lot of pressure, a boiler feed or RO duty for instance, a single-stage split case is the wrong tool. That's multistage pump territory, where several impellers in series build the head you need.
• Flow below roughly 300 to 500 m³/h. As a rule of thumb this is where split case pumps start to make sense. Below it, an end suction pump is usually cheaper, simpler and entirely adequate, and the efficiency gap is too small to justify the extra casing.
• No floor space and no crane. A horizontal split case pump is wide and you need overhead lifting to open it. If the plant room is tiny or there's no way to lift the top half, look at a vertical inline pump or a vertical double suction model built for a compact pit or station.
• Heavily abrasive or solids-laden fluid. The wide passages tolerate some silt, which is why these pumps work in irrigation and drainage, but slurry, grit and mine tailings call for a purpose-built slurry or dredge pump. A clean-water split case pump will wear fast in that duty.
• Tight capital budget on a non-critical duty. If it's a small standby pump that rarely runs, the cheapest sound end suction pump is the sensible buy. Save the split case budget for the pumps that carry the plant.

Horizontal or vertical, and does it need to be marine grade?

Most split case pumps are horizontal because that layout gives the easiest top access and suits a conventional pump house with a crane rail. The horizontal CW series covers the widest flow range, up to 20,000 m³/h on our builds. Go vertical when the station geometry forces it, underground stations, retrofits where the footprint can't grow, deep pits where the pump has to sit close to the suction water. The hydraulics are the same; only the installation and the access change.

Horizontal split case pumps in a pumping station: wide flow range and top access for scheduled overhaul.

Material is the other fork in the road. Cast or ductile iron is fine for clean fresh water and is the default for municipal and HVAC duty. The moment seawater or brackish water is involved, iron is a mistake. For shipboard central cooling, ballast and seawater service we build marine double suction pumps in nickel-aluminium bronze, AISI 316/316L or Duplex stainless, with silicon carbide seals, because chloride pitting and marine growth will eat an iron pump in short order. Land-based plants taking cooling water from the sea need the same metallurgy even though they never leave the dock. If the pump is for a classified vessel, tell us the classification society and flag state up front; we build to the class requirements and arrange the surveyor to attend and witness the tests on a per-order basis, which adds scheduling time but gets you the inspection certificate the vessel needs.

What we actually need on the duty sheet

If you want a quote that's right the first time rather than a pump that's right on paper, these are the points our selection engineer will ask about anyway.

• Flow and head as a range, not a single point. Give us normal, maximum and minimum flow and the corresponding heads. Pumps rarely run at one fixed point, and the range tells us where the operating window sits relative to the best efficiency point.
• The NPSH available. Suction water level, atmospheric pressure, pipe losses and temperature set the NPSHa. It has to clear the pump's NPSHr with margin, and it's the number that most often gets left off the enquiry.
• What's in the water. Clean fresh water, treated cooling water, seawater, silt-laden river water, mildly corrosive process fluid, each points to a different casing and impeller material and a different seal.
• Temperature and pressure rating. Hot water changes the seal and gasket choice; the casing pressure class has to match the shut-off head, not just the design head.
• Flange standard and drive. DIN, ANSI or JIS flanges; electric motor or diesel engine for off-grid irrigation and fire duty; fixed speed or VFD.
• Rotation and access. Which side the suction and discharge sit on, and which way the rotor turns, matters for retrofit drop-in replacements.
• Inspection and certification. Whether a classification society, an owner's inspector or an independent third party needs to witness testing.

Send those over and we'll come back with the selected model, the performance curve with your duty point marked on it, the dimensional drawing and the material schedule, usually within a day.

Every selection comes back with the duty point marked on the curve so you can see where it sits relative to the BEP.

Questions buyers ask us most

What is the difference between a double suction and an end suction pump?
An end suction pump takes water through one eye on a single-suction impeller and usually sits with a horizontal suction and vertical or top discharge, while a double suction pump feeds one impeller from both sides inside a split casing. Once the flow climbs past roughly 300 to 500 m³/h, the double suction design generally gives lower NPSH, better efficiency and easier overhaul, at the price of a bigger footprint and higher cost. Below that range a good end suction pump is usually the more sensible buy.

Why does a double suction pump need less NPSH?
The flow is divided between two suction eyes, so each eye passes only half the volume at a lower inlet velocity. Lower velocity at the eye means less pressure drop and a lower NPSH requirement than a single-suction impeller handling the full flow. The exact margin depends on the impeller geometry, so always read the value off the specific curve rather than assuming a fixed figure.

Are split case pumps only built horizontal?
No. The horizontal version is the most common because it gives the easiest top access and suits a pump house with a crane, but vertical double suction split case pumps are made for underground stations, deep pits and retrofits where there is no room for a horizontal footprint. The hydraulic end is the same; what changes is the installation and how you reach the rotor.

What flow range do these pumps cover?
Our horizontal CW range runs from about 50 up to 20,000 m³/h and the vertical builds from roughly 80 up to 12,000 m³/h, with heads typically in the low-to-moderate band. If your head is high and your flow small, that points to a multistage pump rather than a split case. Tell us the duty and we will confirm which range it falls in.

Can a split case pump handle seawater?
Not in standard cast iron. For seawater, brackish water or shipboard cooling and ballast duty we build the wetted parts in nickel-aluminium bronze, 316/316L or Duplex stainless with silicon carbide seals. You can see the marine builds on our marine double suction pump page.

The short version

If the job is big flow, low to moderate head, clean or lightly silted water, and continuous running, a double suction split case pump will usually give you lower NPSH, balanced running, peak efficiency and the easiest major overhaul in the business. It gives up ground on price, footprint and very small or very high-head duties. Read the duty sheet honestly, match the pump to where it actually sits on the curve, and the choice is usually clearer than the catalogue makes it look.

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