Ductile Iron Pump Bodies and Housings Cast to Drawing

A pump body is a pressure vessel with a shape designed to convert rotation into flow. That combination makes it one of the harder castings to get right: the wall has to hold pressure, the volute has to be smooth enough not to waste energy, and the sealing faces have to be flat enough not to leak.

A Pump Body Is a Pressure Vessel With a Shape

The pressure is the discharge pressure, which on a centrifugal pump at 50 metres head is about 5 bar, and on a multistage pump can be 40 bar or more.

The shape is the volute: a spiral passage that expands from the impeller outlet to the discharge flange, converting velocity into pressure. The volute's cross-section has to grow smoothly, and any step or rough patch in the casting costs efficiency.

So the casting has two requirements that pull in different directions: thick enough for pressure, smooth enough for flow. A casting that meets the pressure test and has a rough volute will pass inspection and disappoint the customer on power consumption.

Volute Geometry and What Casting Can Hold

The volute is formed by a core, and the core has to be located precisely and removed cleanly. A core that shifts by 2 mm changes the volute's cross-section on one side and the clearance on the other.

The surface finish inside the volute is set by the core. A shell core gives a smoother surface than a green sand core, which is why higher-efficiency pumps often use shell cores or a coated core.

The cutwater, the lip where the volute meets the discharge, is the most sensitive feature. It has to be sharp and in the right place; a rounded or displaced cutwater produces recirculation and a drop in head.

Wall Thickness and the Corrosion Allowance

The wall is calculated from the pressure, the diameter and the material, plus a corrosion allowance. On a water pump, the allowance is typically 3 mm; on a chemical pump, it can be 6 mm or more.

The allowance buys life as much as thickness. A pump body with no allowance will fail by corrosion long before it fails by pressure, and the failure will be a pinhole in a wall that was 8 mm when new and is 3 mm after ten years.

We will ask what the fluid is, because the allowance and the coating depend on it, and a body designed for clean water is not adequate for brine.

Sealing Faces: the Flange, the Wear Ring and the Gland

Three sealing surfaces decide whether a pump leaks.

The suction and discharge flanges have to be flat and square to the bore. A flange that is 0.2 mm out over 150 mm will leak at the gasket edge.

The wear ring seat, where the impeller runs, has a clearance measured in tenths of a millimetre. The casting has to hold the seat diameter and squareness, and it has to be sound, because porosity at the seat is a leak path into the casing.

The gland or mechanical seal chamber has to be concentric with the shaft. A chamber that is 0.1 mm off-axis will wear a mechanical seal in weeks.

Pressure Testing Before Machining or After

A pump body is tested hydrostatically at 1.5 times the design pressure, and the question is when.

Testing before machining finds casting defects while the part is still a raw casting, which is cheaper: a defective casting is scrapped before machining cost is added. That is the normal sequence.

Testing after machining finds defects that were opened by the machining, which is a second, smaller risk. Some customers ask for both, and we quote them separately.

The test pressure and duration come from the standard or the customer's specification. We record the result against the heat number.

Material Choice by Fluid

Ductile iron covers water, wastewater, most slurries and many mild chemicals. It is not adequate for strong acids, chlorinated seawater or high-chloride brines, where the chloride attacks the iron and a duplex stainless or a bronze is needed.

Grey iron is used on low-pressure pumps where cost matters and the fluid is benign, but its brittleness makes it a poor choice for a pressurised casing.

Where the fluid is abrasive, the wear areas are often fitted with a harder liner or a hardened insert, which changes the casting design rather than the material.

Cavitation and the Surface It Attacks

Cavitation happens when the local pressure drops below the vapour pressure and bubbles collapse against the metal. It removes material by fatigue, and it attacks the areas where velocity is highest: the impeller inlet and the volute near the cutwater.

A casting with a rough surface cavitates earlier than a smooth one, because surface irregularities create local low-pressure zones. That is another reason the volute finish matters.

Where a pump is known to cavitate, the answer is usually a design change upstream, not a harder material. A harder casting delays the damage and does not fix the cause.

Repair, Overhaul and the Second Life

Pump bodies are rebuilt rather than replaced in many plants, and the casting has to allow that.

A body with adequate wall thickness can be re-machined at the wear ring seat, with an oversize ring fitted. A body with no allowance cannot.

Corrosion damage is often repaired by welding or by a metal-filled epoxy, which is why the material's weldability matters. Ductile iron welds with difficulty; a repair is possible but it is not the same material, and we will say that rather than recommend it for a pressure boundary.

Designing for a second life is a small decision at the drawing stage and a large one for the operator twenty years later.

NPSH, Suction Layout and What the Body Cannot Fix

Most pump problems that get blamed on the casting are suction problems.

Net positive suction head available is set by the system: the static head on the suction side, minus the friction loss, minus the vapour pressure. If it is lower than the pump needs, the pump cavitates no matter how good the body is.

The suction layout decides most of it. A short, straight run into the suction flange, with no elbow immediately before it, gives an even velocity profile. An elbow bolted directly to the suction flange produces a swirling flow that loads one side of the impeller and reduces the effective NPSH.

The pump body can help marginally, with a well-shaped suction passage and a smooth bore. It cannot compensate for a suction line that is too long, too narrow or badly routed.

When a customer reports cavitation damage, we look at the pattern of damage first. Damage on one side of the impeller points to a suction layout problem, not a casting one.

Rebuilding an Old Pump Around a New Body

A pump body is often the last part of a pump to be replaced, and the first to be unavailable.

The impeller, shaft and bearing frame may still be serviceable, and the body has worn at the volute or corroded through. Replacing the body means casting to the original drawing, or reverse engineering from the worn part plus the dimensions of the parts that must fit it.

The critical dimensions are the impeller clearance, the wear ring seat and the flange positions. Everything else can be adjusted; those three cannot.

We have made bodies for pumps whose manufacturer had stopped supporting them, using the old body and the customer's impeller as the specification. It costs more than a catalogue part, and it keeps a pump running that would otherwise be scrap.

Reference Price Range and Minimum Order

Reference price range: US$0.10 to 100.00 per piece, FOB Qingdao. Minimum order quantity: 500 pieces per item. The final unit price is quoted against your drawing and depends on part weight, material grade, moulding process, machining allowance and tooling amortisation. Reference prices are valid until 31 December 2027.

Frequently Asked Questions

What pressure can you test a pump body to?

Up to 40 bar on the castings we pour, depending on the wall and the size. Above that the wall moves outside our range.

Can you cast the impeller as well?

Yes, in ductile iron, with the vane passages cored. It is a more difficult casting than the body and we will quote it separately.

Which coating is used inside a water pump?

Usually none, or an epoxy for potable water. Cement lining is not practical in a volute.

How do you check the volute core position?

By sectioning the first article and measuring the wall on both sides. After that, by the core print design and the mould check.

Can you supply a body with an oversize wear ring seat for rebuilding?

Yes, and it is worth specifying at order rather than discovering later that there is no allowance.

What lead time applies?

Five to six weeks on a repeat pattern with machining and test included. A new pattern runs seven to nine.