Page 46 - Engineering in Kenya Mag

Bearing Vibration
Analysis for 1000kW
Electric Motor in Kenya
Pipeline Company
K1.0 Introduction
ENYA Pipeline Company (KPC) Ltd operates a pipeline system for refined petroleum products from Mombasa into the hinterland. The pipeline system currently consists of 450 Km of Line-1 and 450 km of line 5 running from the port of Mombasa to Nairobi, 325km of Line-2 from Nairobi to Eldoret, 325km of Line-4 from Nairobi to Eldoret, 121km of Line-3 from Sinendet to Kisumu, and 121km of Line 6 from Sinendet to Kisumu. At intervals along the
Pipeline, there are Pump stations for boosting the line pressure. The equipment in each Pump station includes a set of redundant motor driven centrifugal pumps of various sizes and ratings. Each pump is driven by a 3300V or 6600V Motor. Fig 1 shows a pump station with a Motor and Pump set in the foreground.
Fig 1. KPC Pump station showing a Pump-set with a 1600kW Motor (left).
Each pump-set consists of a motor drive, fluid coupling for pump speed control, and a multistage centrifugal pump. To ensure safety of the pump, each pump-sets is equipped with a Bentley Nevada Pump monitoring system that monitors Motor winding temperature, motor bearing temperature, pump casing temperature, pump speed, fluid coupling temperature and fluid coupling vibrations.
2.0 The problem
A 3.3kV, 1007Kw (1350 HP), 2900 RPM Squirrel cage induction motor coupled to a centrifugal pump through a fluid coupling in the Kenya Pipeline Company pumping station at Pump Station 22, Ngema was noticed to be unusually vibrating and shaking the Motor foundations. The motor vibrations are not monitored by the pump monitoring system. Initial troubleshooting could not identify the cause of the failure. Each motor is fitted with 2 No. sleeve bearings, TYPE ENLB, with a 90 mm bore, one each at the drive end (DE) and Non-Drive end (NDE).
2.1. Motor bearing vibration
Induction motors are the most used electro- mechanical device in industry because of economic considerations and their rugged nature. One of the defects that motors exhibit during operation is high level of vibrations. Every motor vibrates, and usually within acceptable limits, but if excessive vibrations are left unchecked, equipment damage will result. Vibrations are characterized by amplitude and frequency. Amplitude, or displacement shows how strong the vibration is, while frequency shows the rate of vibration.
2.11 Vibration sources
Vibration is a repetitive movement around a point of equilibrium, characterised by its variation in amplitude and
ELECTROMAGNETIC
l Static magnetic field unbalance l Loose cable (single phasing) l Eccentric rotor -Broken rotor bar
MECHANICAL
l Imbalance l Bearings l Misalignment l Cooling fan l Loose
motor base frequency. The causes of vibration are outlined in Fig.2.
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Engineering in Kenya Magazine Issue 002
VIBRATION SOURCES IN ELECTRIC MOTORS