Reliability Factors of Pumps

Topics: Fluid dynamics, Pump, Centrifugal pump Pages: 51 (18526 words) Published: August 9, 2013
Reliability Issues – Centrifugal Slurry Pumps
Introduction
Pumps were probably the first machine ever developed, and are now the second most common machine in use around the world, out-numbered only by the electric motor. The very earliest type of pump is now known as a water wheel, Persian wheel or “noria”, consisting of a wheel of buckets that rotates to pick up water from a stream and dump it into a trough. Another early pump was the “Archimedean screw”, similar to the modern screw conveyor except that the flights were often fixed to the tube so that the whole arrangement would turn together. Both of these devices are still used, most commonly in basic agricultural applications. Pumps are now produced in an enormous range of types and sizes, for a very wide scope of applications, and this makes it difficult for any individual reference document or organisation to cover “pumps and pumping” as a general topic. So the broad field of pumping is classified into sub-divisions and then dealt with at that level. In the mining industry, the upper end of the pump scale includes impellers with diameters over 2.5m, slurry lines 10km long, particle size up to 100mm, flow rates handling more than 7000tph, and motors over 10MW. Finer slurries of around 1mm particle size are pumped for hundreds of kilometres in some operations. There are many ways to classify pumps. This just one of them. Pumps  Displacement o Reciprocating  Piston  Diaphragm o Rotary  Single rotor  Vane  Piston  Flexible member  Screw  Peristaltic  Multiple rotor  Gear  Lobe  Circumferential piston  Screw  Dynamic o Centrifugal  Axial flow  Single stage  Multi stage  Mixed Flow  Single suction  Double suction  Radial flowl  Single stage  Multi stage o Special Effect  Jet  Gas lift  Hydraulic ram  Electromagnetic This document only addresses centrifugal pumps, with a focus on single-stage radial-flow slurry pumps. Centrifugal pumps are capable of meeting duties of up to 1.4 m /s at 30MPa, and higher volumes at lower 3 pressures. The maximum flow rate at low discharge pressure is about 180 m /s. Industrial applications requiring high delivery pressures generally use reciprocating fixed-displacement pumps, but they are limited in the amount of flow they can put out per unit. In general purpose applications, where different types of pumps 3

could all deliver the performance sought, centrifugal pumps are usually the preferred choice due to lower lifecycle costs.

Basic Requirements for Reliability
Assuming correct pump manufacture and installation, the basic requirements for reliable long-term operation of centrifugal pumps are: 1. Continuous operation at best-efficiency point (BEP) 2. Adequate net positive suction head (NPSH) 3. Low velocity fluid flow within the pump and throughout the system 4. Processing of fluids that are benign -- ie: a) Chemically and physically stable b) At near-ambient temperatures c) Free of particles likely to cause wear or blockage Pumps of a basic design satisfying all these requirements have run for 50 years and more without major component replacement. The first three requirements are satisfied by matching pump performance to expected duty. Where item 4 cannot be addressed through pre-treatment of the fluid, the pump configuration, geometry and materials must be optimised to give best results. Obviously, item 4.c) is a dominating issue for slurry pumps as it cannot be eliminated and must be managed.

Centrifugal Pump Construction
Centrifugal pumps have two main sub-assemblies – the rotating parts (impeller, shaft, bearings), and the fixed parts (casing, piping connections, stand, foundations. Pumps of all types may be single stage or multi-stage. Multiple stages are used where it is not practical to generate the necessary discharge pressure using a single impeller. The simplest way to imagine a multi-stage pump is as one pump with its discharge feeding straight into the suction of a second pump so that...

References: Pump Handbook Third Edition; McGraw Hill; New York et al; 2002 Slurry Systems Handbook; B.E.Abulnaga; McGraw Hill; New York et al; 2001 API Standard 610 (Centrifugal pumps for refinery, heavy duty chemical and gas industry services) www.mcnallyinstitute.com (heaps of technical articles) www.lawrencepumps.com – The seven deadly sins of pump ownership Weir Slurry Pumping Manual
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