ATA'S "THE EXPERT" NEWSLETTER - ISSUE OF NOVEMBER 2019




In this Issue:

Case Study: Pump Fire Root Cause Failure Analysis

Propeller Torque and Single Point Failures in Boat Steering

This Issue's Toolbox Feature - Amusements Industry

































CASE STUDY: PUMP FIRE ROOT CAUSE FAILURE ANALYSIS


This case study has been provided by KnightHawk Engineering, Inc. (KHE), a new primary associate of our ATA Petrochemical Team. KHE is a specialty engineering company with over 28 years in business. They are world-wide leaders in Static and Rotating Equipment, Field Services, Litigation Support, Failure Analysis, and Reverse Engineering.

Problem: A plant experienced a fire at one of their centrifugal pumps that pumps oil. The fire was safely put out, but the plant required a root cause failure analysis to determine why the fire occurred.


Fig 1: Failed Pump Impellor with Significant Rubbng Damage

Solution: KHE applied its Integrated Systems Approach to this project employing process, mechanical, and metallurgical analyses. While the failed pump parts displayed significant wear damage to the impeller and impeller housing, KHE metallurgical analysis was able to trace the failure to a defective bearing. Analysis of a bearing ball revealed the presence of aluminum oxide inclusions in the material, which are not expected in bearing material. The analysis also found significant signs of sub-surface cracking, and rolling contact fatigue in the bearing balls as a result of these inclusions.

KHE concluded that once the thrust bearing failed, due to the rolling contact fatigue, the ensuing heat of friction that resulted from the rubbing of the impeller face on the housing ignited the pumped medium and caused the fire

The metallurgical analysis also found indication that cavitation may have occurred in the pump. This corresponded well with the process and mechanical assessments, which had found that under certain operating conditions the pump was permitted to operate with high flow and low head conditions, which would result in cavitation. Pump cavitation results in excessive loading of various pump components, including bearings. Such conditions may have contributed to the premature failure of the defective bearings.

Result: KHE recommended an investigation of the source of the defective bearings and implementation of improved quality control measures for pump bearings at the facility. KHE also recommended a plant wide process control review of centrifugal pumps, and implementation of process control measures to prevent pump cavitation.


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PROPELLOR TORQUE AND SINGLE POINT FAILURES IN BOAT STEERING



On any boat powered by a single outboard motor, the spinning propeller not only drives the boat forward, it also generates a torque on the motor which will turn both the motor and the boat if that torque is not restrained somehow. A propeller rotating clockwise, which is typical for most propellers, generates an inherent, propeller-induced “steering” torque that will turn a boat to the right as it moves forward, if that steering torque is given free rein.


With simple, tiller steering, the propeller-induced steering torque on the motor can be felt by the boat’s operator as force acting through the motor’s tiller handle. That force must be resisted if the boat is to be successfully controlled by the operator. Likewise, with most single-cable steering systems, the operator can usually feel the steering force acting through the steering wheel via its cable connection to the motor. Even if he can’t feel the force, if the operator releases his grip on the steering wheel, he will see the force at work in spontaneous turns of both the wheel and the boat. On the other hand, the propeller-induced force usually can’t be felt through the steering wheel of most hydraulic steering systems, and, if the steering wheel is released, there will be no spontaneous turn of the wheel or the boat. This is because the reciprocating action of a piston pump in the helm of most hydraulic systems typically isolates the rotary motion of the steering wheel from the linear motion of the hydraulic cylinder acting on the motor’s tiller lever. This arrangement does not eliminate the propeller-induced steering force, however. It merely puts a check on that force which persists in the form of unbalanced port and starboard pressures in the hydraulic steering system and which will be ready to act on the boat whenever it has the opportunity.



Like all mechanical assemblies, boat steering systems are subject to various malfunctions. If such malfunctions permit the propeller-induced steering torque to turn the boat abruptly, they can have disastrous consequences. A boat’s operator and passengers can be ejected from a boat if the boat undergoes a sudden, unexpected turn while under way, and the probability of such ejections increases with increasing boat speed.

Commonly seen malfunctions in cable steering systems occur in the telescoping rod and tube assembly which passes through motor tilt tube at the boat’s transom and then connects to the motor’s tiller lever. Corrosion of a cable-to-rod coupling inside the assembly can result in complete binding of the steering system if corrosion products swell the outside diameter of the connector too much. That situation is typically discovered after a boat has been out of service for a long while, and it usually prevents the boat from being used again until the whole cable assembly has been replaced. A more dangerous situation occurs when erosive corrosion of the connector weakens it to the point of failing in service, allowing the steering cable to completely separate from rod and tube attached to the motor’s tiller lever. In that circumstance, an abrupt boat turn and the ejection of operator and/or passenger(s) are likely to occur. The investigation and diagnosis of each of these malfunctions can typically be accomplished only by way of industrial radiography or destructive disassembly, since cable steering systems are usually factory-assembled by irreversible means.

Hydraulic steering systems are also subject to malfunctions which can result in abrupt boat turns. Given that, in normal operation, a hydraulic steering system typically conceals the presence of the propeller-induced steering force, abrupt turns related to hydraulic system malfunctions can be, in some sense, more unexpected and so, more hazardous than turns related to cable system failures. Hydraulic system failure modes include catastrophic breaches of hydraulic fluid pressure due to abrasive chafing of flexible tubing against rigid boat structures or the sudden release of pressure through improperly assembled tubing-to-fitting connections. Another failure mode originates in the design of one low-cost hydraulic steering system which uses non-redundant threaded fasteners at the ends of an assembly which confines the lateral movement of the actuating cylinder attached to the motor’s tiller lever. In-service loosening and loss of one or both of those fasteners can result in loss of steering system control over the motor and an abrupt boat turn even though the fluid pressure retaining envelope of the steering system remains intact and leak free.








Hydraulic Steering System Connection Testing


In an investigation of a boat operator's ejection in an incident where a hydraulic steering system failure occurred, ATA pressure tested connections in an exemplar steering system. If properly assembled, the pressure carrying capabilities of tubing-to-fitting connections exceeded the bursting strength of the plastic tubing. If not properly assembled and tightened, connections were seen to fail abruptly by slipping at pressures well below the tubing burst pressure, without any prior warning such as a tell-tale leak.


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For more information on ATA's expertise and background, visit our Marine and Maritime Services web page.


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This issue’s featured topic for ATA’s Toolbox is the Amusements Industry. We have been performing forensic investigations on amusement park rides and facilities for nearly 40 years and have worked on a wide array of rides from traditional roller coasters to water-borne amusements and rider ejection incidents.

For more information on this topic, visit the ATA Toolbox/Amusements Industry drawer.


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