Pump Casing Casting: Volute Design, Materials & Process

Quick answer: Pump casings are cast because the volute is a spiral passage that grows in cross-section around the impeller, a shape that is slow and costly to machine. Small and mid-size casings, volutes and diffusers are often investment cast for better surface finish and accuracy; large casings are usually sand cast. Material follows the fluid: cast iron for clean water, CF8M stainless steel for mild corrosion and hygienic duty, and duplex grades for chlorides and abrasive slurries.

For pump designers and buyers, pump casing casting decides where the best efficiency point lands, how much load the bearings carry, and whether the unit passes hydrotest first time. Below: the volute features that matter in casting, material choice, casting routes, and defects to check before approving a sample.

What a pump casing does

A casing has two jobs. It holds pressure, and it turns velocity into pressure. The impeller throws liquid outward at high speed. The volute around it widens steadily, so the liquid slows down and its pressure rises before it reaches the discharge flange.

So the foundry must get three things right together: passage geometry, wall thickness, and sound metal.

Volute casing design: features that affect the casting

Single volute or double volute?

The Hydraulic Institute notes that the single volute is still the most common design because it is easier to manufacture and to inspect. The trade-off is radial thrust. Radial load is lowest near the BEP and climbs as flow moves away from it, and a single volute typically sees its highest radial thrust at zero flow.

A double volute adds a splitter rib that creates two opposed passages. The forces largely cancel, which allows a smaller shaft and longer bearing life. For the foundry, that rib is a thin internal wall, and the outer passage is hard to reach for cleaning and inspection. Core design and core removal become the hard part of the job.

Pump casing material: how to choose

Start with the fluid, then check pressure, temperature and solids. Treat this table as a starting point, not a corrosion review.

Two numbers help when comparing a stainless steel pump casing against duplex. The first is PREN (Cr + 3.3 Mo + 16 N), a common index of pitting resistance; duplex scores well above 316. The second is strength: ASTM A351 CF8M has a minimum yield of 30 ksi (205 MPa), while ASTM A890 Grade 4A (CD3MN) requires 65 ksi (450 MPa). That extra strength can allow thinner walls.

Cost moves the other way. Industry comparisons for end-suction pumps commonly put 316 stainless at roughly 1.5–2× the cost of cast iron and duplex at 2–2.5×, so duplex pays back mainly where 316 would fail early. For more on the most common grade, read our guide to CF8M investment castings.

Investment casting vs sand casting for pump casings

Investment casting for pumps and valves pays off where the passage finish directly affects efficiency: small end-suction volutes, diffusers, impellers, and valve bodies with complex ports. If your casing exceeds about 30 kg or our 400 mm envelope, a sand foundry is the better choice, and we will say so. Our pump and valve castings page lists the parts we produce today.

The investment casting process for a pump casing

How a volute casing moves through our investment casting process (new to the method? See what investment casting is):

1. Wax pattern with a soluble core. A water-soluble core forms the volute passage inside the wax die. It is dissolved out afterwards, leaving a clean internal spiral that a one-piece die could not release.
2. Tree assembly. Patterns are attached to a sprue. Gates are placed to feed the thick discharge and suction flanges, which solidify last.
3. Shell building and dewaxing. The tree is dipped in ceramic slurry and stucco in layers, dried, then dewaxed in an autoclave.
4. Firing and pouring. The shell is fired and the alloy is poured while the shell is hot, which helps fill thin sections such as the cutwater.
5. Heat treatment. Stainless and duplex castings are solution annealed. Skipping or rushing this step leaves harmful phases that cut corrosion resistance.
6. Finishing and machining. Shot blasting, then machining of flange faces, the impeller bore and seal seats to ISO 2768 (f).
7. Inspection. Spectro analysis for chemistry, X-ray radiography for internal soundness, CMM or laser scanning for volute geometry, and a hydrostatic test to the customer's pressure. API 610 process pumps, for example, call for a casing hydrotest of at least 1.5 times the maximum allowable working pressure.

Details of our in-house equipment are on the testing and machining capabilities page.

Common pump casing defects and how to prevent them

• Shrinkage porosity at flange-to-wall junctions. It often shows up only as a weep during hydrotest. Prevent it with solidification simulation, feeders on heavy sections, and even wall transitions.
• Core shift. This gives uneven volute walls and an unbalanced flow path. Check wall thickness at fixed points and review radiographs on first articles.
• Misrun or rough cutwater. It causes noise and local erosion. Control shell preheat and pour temperature.
• Non-metallic inclusions. In stainless steel, they become starting points for pitting. Use clean melt practice and ceramic filters.

What to send your foundry for a quote

• A 3D model and a 2D drawing with machined surfaces marked
• Material grade and standard (for example ASTM A351 CF8M)
• Fluid, temperature, design pressure and chloride level
• Annual volume and batch size
• Test needs: hydrotest pressure, radiography level, PMI, and EN 10204 3.1 certificates
• Required surface finish inside the volute

Building pumps for refinery or offshore service? See our oil and gas castings. Ready to discuss a drawing? Request a quote and our engineering team will review manufacturability before pricing.

FAQs

What material is a pump casing made of?

Usually cast or ductile iron, carbon steel, CF8M stainless steel or duplex stainless steel, chosen by fluid, pressure and solids content.

What is the difference between a volute casing and a diffuser casing?

A volute casing uses one spiral passage that widens toward the discharge. A diffuser casing uses fixed vanes around the impeller to slow the flow. Diffusers are common in multistage pumps; volutes dominate single-stage centrifugal pumps.

Why are double volute casings used?

Two opposed passages balance the radial force on the impeller, especially away from the BEP. This reduces shaft deflection and bearing load. The cost is a harder casting with a thin internal rib.

Is investment casting or sand casting better for pump casings?

Investment casting suits smaller casings where finish and accuracy matter. Sand casting suits large, heavy casings and low volumes.

Which stainless steel is best for a pump casing?

CF8M (cast 316) is the usual choice for general chemical and food duty. For chlorides above what 316 tolerates, or for abrasive fluids, duplex grades such as CD4MCuN or CD3MN (2205) last longer.

How are cast pump casings tested?

With chemical analysis, radiography, dimensional inspection of the volute, and a hydrostatic test above design pressure as the customer or standard requires.