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Centrifugal Pumps for Wastewater Treatment from Kelair Pumps

Centrifugal pumps bear up under very heavy duty.

Kelair Pumps was asked to quote on centrifugal pumps for a wastewater treatment plant (WWTP) duty. This wasn't for a local council WWTP, but for an LNG plant. The specifications therefore were quite high in terms of pump construction, pump testing, material certifications, welding specifications etc.

The pumps required were to be built to ANSI B73.1, which is an extra heavy duty centrifugal pump specification, pump casing construction in ductile iron, with 316SS impeller, and cartridge mechanical seal, long-coupled to motors (motor manufacturer specified by site) and mounted on a mild steel-welded base frame.

Duties were mixed liquor (with sludge to 10g/L), ultra-filtered effluent, and aerobic digester with solids to 30g/L. The pumps were specified in such a way as to give extended service life. Each pump bearing housing contained two heavy duty back-to-back mounted 7300 series angular contact bearings, giving L10 life exceeding 100,000 hours. All bearing housings had premium wave type oil seals fitted, eliminating shaft grooving. John Crane type 4610 standard single cartridge mechanical seals were also fitted.

Maximum and minimum duty points generally accepted for centrifugal pumps are 110% of BEP and 50% of BEP. However, this is not a limita-tion with ANSI B73.1 specification pumps, which can operate continuously at such duty points. (Standard DIN 24255 specification and ISO 2858 centrifugal pumps are limited in their capacity to endure the radial loads occurring at these duty points. Continuous operation at that max/min level or beyond will generally mean shaft deflection causing spring hysteresis on internal mechanical seals thereby causing those seal faces to open and prematurely fail, and premature bearing failure also becomes a potential problem).

Pumps built to ANSI B73.1 are extra heavy duty which enables them to readily endure such radial loads imposed on impellers when the duty point is "far removed" from best efficiency point (BEP) according to the pump curve. Part of the reason for this increased ability is the short, thick pump shaft with the heavy duty bearings to overcome the radial loading on the overhanging impeller.

In addition to the robust build of these pumps, the baseplates were also highly specified. Baseplates were to be "ladder" type construction complete with welded draining trays (with screw thread plug to drain). Base plate welding had to comply with Australian Standard AS1554.1 Part 1. NDT weld testing and both ultrasonic and x- ray was also to be carried out.

Once the builds were completed, Kelair then had to hydro test each pump. When that was suc-cessfully completed, each pump was connected to Kelair's in-house test rig to performance test to AS2417 grade 2. Further, EN.10204 material certificates were also required for the pumps only. Finally, all the pumps and coupling guards were coated with 2-pack epoxy paint to suit the corrosive marine environment of the WWTP site.

Kelair has the capability to provide all manner of pump types (centrifugal and positive displacement) to a range of specifications for water and wastewater treatment plants.

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