Fire Pumps, Fire Apparatus, Fire and Defense Equipment

Just Ask Engineering - Pump Components and Corrosion

The_Darley_ZSF_all_bronze_pump
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Martin Simpson asked: What metals do you recommend on pump components that come in contact with water, and why?

The answer depends on the type of water being pumped: salt or fresh. With salt water or brackish water, Darley recommends an “all-bronze” pump. It consists of a sand cast C87500 silicon bronze pump casing, suction head, inboard head, discharge plumbing and impeller. The impeller shaft is either a wrought 316 austenitic stainless steel, or a wrought 17-4 participation hardened stainless steel.

These materials are best because bronze is the most corrosion resistant material commercially available. There are more corrosion resistant metals - gold or platinum - but a pump made from those materials would be so expensive no one could afford to buy it!

Using those particular wrought stainless impeller shafts comes from the need for more strength than what bronze material can provide. It’s easier to machine bronze than stainless steel, thus wherever possible, bronze is used. Machining an all-stainless-steel pump would also make it too expensive for many to purchase. Additionally, an all-stainless pump that costs the same as an all-bronze pump cannot be machined to the precision of the all bronze pump.

To prove these materials in a salt water pumping application, one need look no further than the deck of any modern Navy ship. Most equipment on deck is made of bronze material – it has that nice copper look. If it’s not bronze, it’ll be so thickly painted you won’t know what it is. That thick paint protects the base metal from contact with salt water. If you were a salvage diver, you’d know that almost all Navy ship’s screw shafts are made from these same wrought stainless-steel materials.

With respect to fresh water pumps, Darley recommends a “cast iron” pump. It consists of a sand cast ASTM A48 class cast iron pump casing, suction head, inboard and discharge head. We use a sand cast C87500 silicon bronze impeller and discharge ball valve body and a wrought 416 martensitic stainless steel for the impeller shaft.

The reason for using cast iron is because it is the best material when factoring in corrosion resistance, material strength and material cost. Cast iron corrodes. You see that familiar red rust and that rust does not easily wash away with time. Rust building up over time works the same way as that thick paint on a Navy ship. The thicker the rust, the slower the base material will corrode away.

Aluminum does not rust like this. Aluminum rusts quicker than cast iron - it’s a proven fact. Aluminum rust, also known as oxidation, easily rubs off the base material. In this manner, water simply washes away the aluminum oxidation exposing the base aluminum material to renew its oxidation again. Quite simply, aluminum pumps disappear over time while cast iron pumps get rusty.

The impeller is the heart of the pump; thus, we keep its material the same as a salt water pump. 416 stainless has slightly less corrosion resistance as compared to 316, but 416 is easier to machine, thus the reason it’s used for freshwater. Darley has used “all-bronze” and “cast iron” pumps in these applications for over 100 years with outstanding success.

Darley ZSF - IC 3000 - Marine Version