Pump Magazine Publications

“The Essence of Equipment Failure Analysis -

Theory, Approach, and a Case Study”

by

Sourav Kumar Chatterjee

Manager, Rotating Equipment

Hindustan Petroleum Corporation, Ltd.

Mumbai Refinery, India

July 9, 2002

Abstract

Failure is incapability of an item to deliver desired level of service as specified by design/expected by user, under specified condition. A thorough analysis of Root Cause of Failure is followed by the detailed Field Case History of a seal failure of a pump at a refinery. Human factor, logistics and team assignment is analyzed, along with tracking technical aspects of a problem. Actual data for a pump operation around the failure period is related to mean time between failures (MTBF) and a follow-up monitoring plan, after problem evaluation and correction, is established. An interesting and informative case for practicing plant engineers, maintenance and operating personnel, to compare notes and learn.

Analysis

Analysis is a technique where a set of useful information on an event under consideration is compared with a set of design information of same areas and pertaining to same item involved in the event, - to find out the deviations, followed by a logical conclusion on cause of eventuality, using expert system which possess wide database on similar type of events or using human expertise.

Failure analysis

Failure analysis is an analytical technique used by professionals of all field at various functions to protect against potential problems in process & products.

What is a Potential Failure?

The identifiable & measurable physical condition of an item, which may be equipment / person / system, and which indicate that the functional failure is about to occur or in the process of occurring is known as potential failure.

The term potential implies strong probability of occurrence.

EXAMPLES:

Temperature of running equipment parts (bearing housing casing lube oil, etc)

Visible leaks and wear

Vibration level indicating potential bearing failure

Wear particles in gearbox oil showing imminent gear failure

What is a Failure Mode?

It is an event, likely the cause for the condition of each failure state. In other words, it is the manner in which an item could potentially fail to meet the functional requirement, or design intent, or both, as defined and/or acceptable to the end user.

Some typical failure modes:

* Bent * Incorrect adjustment

* Broken * Internal leak

* Contaminated * Jamming

What is a Failure Effect?

Failure Effect indicates the result of failure, and makes us realize the following:

Evidence that the failure has occurred.

Safety, environmental & social consequences

The way in which the production or operation or system is getting affected

The physical damage caused by the failure

Action to be taken to repair/revive /cure the system and arrest further deterioration

Some typical failure effects:

* Leakage of pumpage * Low pressure

* Low flow * No production

* Erratic operation * No control

* High vibration * Poor performance

* Rough finish * Unstable operation

* Operating parameter fluctuation * Intermittent operation

* Deterioration of product quality

Objectives of Failure Analysis

To find out Root Cause of failure and remedial actions

Recognize and evaluate the potential failure modes

Higher organizational, environmental, social and human security and safety

Identify actions, which could eliminate or reduce the chance of potential failure from occurring

Cost control

Higher productivity

Documentation of the process for future reference and monitoring

Core View of Failure Event

In an apparent assessment, though failure event leads to losses, hazard, despair, discrimination and all similarly negative notions, - but, ironically, there are few positive features also are inherent in it if followed by Root Cause analysis.

H A Z A R D		Loss

Despair

A formalized approach is of utmost necessity to carry out effective and successful failure analysis. Such concept generally comprises of five main activities.

Data collection

Formulation of probable cause areas

Analysis

Remedial measure

Documentation and corrective actions

Data Collection

The success of a failure analysis greatly depends on data collection. Out of so much data, the technique of picking up relevant data accurately is a highly skilled job. Many times the analysis sets back as concerned personnel become at a loss to understand what data are required, and how to get it. For an equipment failure following steps may be followed:

1- Identify the equipment & component

2- Find out potential failure mode or failure effect

3- Find out designed parameters (constructional & operational)

4- Note observations on operating parameters (during failure) & constructional parameter on dismantling

Formulation of Probable Causes

Type of equipment and accessories

Constructional features

Service condition

Type of component failed

Nature of failure

Potential failure modes observed before failure

Last maintenance details and MTBF

Probable causes

1)—

2)---

3)---

0---

Input

Analysis

Remedial Measures

Remedial measures are adopted based on area of root cause and feasibility study for implementation.

Design problem

Installation problem

Assembly problem

Mal-operation

Raw material/spare part problem

Documentation and Corrective Actions

Documentation is an arrangement / system to keep useful information in meaningful manner which can be retrieved easily whenever required. It is also information for concerned in relation to pertinent item or event, which is basic requirement for further development, and progress.

Documentation of entire failure analysis event must be done in designated item field and in prescribed format highlighting details of event and total observations, analysis considerations, justification for selecting appropriate measures, implementation details, effect and observations after implementation, update of P&ID / Datasheet / Drawings, indicating cause, date and agency involved.

_____________________________________________________________

Plant, site, or an enterprise modernization means continuous alteration of policies & approaches with the goal of making positive response to the needs, in terms of quality of product, its cost effectiveness, time, availability, and safety. In course of doing that, a careful study must be carried out to select appropriate measures and to identify its key aspects for successful operation.

Appropriate monitoring of performance of such new systems also has immense importance. Absence of mandatory accessories for operation and monitoring can lead to trouble and hazardous situation. This case study presents a situation where an ECS seal for emission control has failed creating hazard due to improper monitoring system and supporting accessories.

1A- Equipment type: centrifugal pump, back pull out design

Tag no.- 14P19

Location Cr. LPG

Service C3+C4 (Propane+Butane+propylene)

1B- Mechanical parameters:

Bearing type NU310/7310*2

Seal type ECS seal Double tandem

Flushing Plan 02,62 water quenching

Seal box venting to closed flare

Cooling Plan- Plan G

Lubrication type- Oil splash lubrication

Lube oil grade-- Turbinol-68

Suction and discharge nozzle size-6"*300 & 4"*300

MOC of Major parts- SS-410, SS-316, CS

1C- Operation parameters:

Service fluid: cracked LPG

Temp: 45 C

Flow: 115 M3/hr

Sp.gr: 0.49

Diff. head: 75M

NPSHr 1.0M

Suct.pr: 210 PSI

Disch. pr: 263.5 PSI

RPM: 1450

Min. flow: 28 M3/hr

Vapor pressure at p.t.: 200 PSI

1D- monitoring facility:

Online primary seal failure detection facility

Alarm / trip connected seal failure alarm

Failure detection probability: Fair

Incident

Failure mode and effect: both primary & secondary (ECS) seal leaked

Time of failure: 1st May 2002 @ 3 AM

Detected by: Operation personnel

Immediate action taken: Pump stopped and isolated immediately

Safety hazard: Yes

Environmental Hazard: Continuous leakage of LPG through seal.

Failure Reporting time: 1st May 2002 @ 10 AM

Input process conditions:

Suction condition trend: O.K.

Temperature trend: constant

Suction flow trend: N/A

Suction source level / pressure: suction drum pressure & level trend constant

Output process conditions:

Discharge flow trend: though the reflux flow trend found constant,

heavy fluctuations observed in LPG run down flow and back pressure.

Discharge Temperature trend: N/A

Discharge pressure trend: N/A

Observations at site:

Cooling/flush Line and jacket condition: cooling water lines found through and clear. Scaling found inside stuffing box jacket.

L.O Condition: Good No contamination observed.

Coupling condition: Good and intact

Foundation condition: OK

Alignment readings on decouple: within limit

Suction and discharge piping alignment: no piping stress

Piping Foundation condition: in order

ECS Seal system: flare vent line found plugged

Observations:

Bearing condition: bearings found good and intact, no radial and axial play observed.

Bearing housing condition: OK

Seal parts condition: heavy pitting on seal ring mating face. Seal ring packing ( "O" ring) totally burnt. Heat marks on Insert mounting burnt and damaged. Rotary unit springs found broken in pieces. Dust of carbon found around seal parts. Observations on secondary seal: wave springs broken, bellows found punctured. Rotary face and packing good and intact. Heat marks on shaft at sleeve sitting portion.

Shaft condition/runout: OK, runout 0.001"

Impeller / lock nut condition : lock nut intact, impeller found cracked at back shroud sleeve/ bush Clearances: wear marks on sleeve at steam purge bush position

Wearing conditions & clearances: rubbing marks on both wear rings. Clearances found: 0. 045" and 0.050" as compared with designed 0.026" & 0.030" front & back respectively (suction and discharge)

Condition of other related parts: coupling teethes well. Throat bush clearance also found increased by 0.015

MTBF: 12 months

Last PM & observations: 11th April, 2001 BCW lines were clear, coupling condition was good, bearing good, foundation bolts OK, alignment was off realignment was carried out, coupling run out OK.

Last failure details and cause: pump was removed for seal leak on 14/03/2001. Subsequently single seal

was replaced with ECS seal .

Last overhauling details with activities:

Bearings were changed, ECS seal was installed.

Parts used from OEM/local: OEM

Vibration trend since last O/H:

Starvation (loss of flow?)

Bend shaft

Bearing failure

Misalignment

Loose rotor assembly

Sealing system problem

Observations and conclusions were based on type of equipment and accessories, constructional features, service condition, type of component failed, nature of failure. Potential failure modes observed before failure are depicted in the chart:

Detail Analysis and Discussion

The heat mark on seal parts, sleeve and fatigue failure of wave spring bellow of 2nd seal and spring of primary seal, eventually reveal the parts were exposed to high temperature and high stress causing catastrophic failure. Moreover, the alarm on failure didn't activate which is major flaw in ECS seal system and calls for immediate rectification. It may be noted that this seal was installed during March 2001 and the vapor recovery line has been connected to flare system only on April 2002. During the operation of seal this was kept plugged, as LPG is prohibited item or releasing to atmosphere. It is evident from observations of failed parts that primary seal failed first which could not be noticed, as alarm system didn’t work. The seal kept on running on ECS seal and only on failure it both failures got exposed leading to hazardous situation.

Failure of Primary Seal

The flushing plan 02.62 (water quench) for this service, always has a tendency towards getting inadequate seal flush. This is because the pump design, which has back wearing and throat bush restriction to stuffing box along with impeller balancing holes. Due to this design, the stuffing box pressure always equals to suction pressure, which is very close to vapor pressure at process temperature. Hence rise in temperature at seal box can create vaporization at seal box and faces leading to loss of seal face lubrication. More dead end vapor recovery system also didn't allow the vapors at primary seal face and got accumulated at ECS seal box, pressurizing ECS seal box and increasing face loading on ECS seal. After some time the heat generated due to seal friction would add more heat to entrapped vapor causing the rise of pressure due to constant volume. This enhanced pressure will act on secondary seal box at O.D and on inner diameter of primary seal insert squeezing off any possible lubrication film, which was already constrained due to type of flushing plan.

Thus the compression units were subjected to abnormal stress due to increased pressure along with high heat due to lubrication less rubbing of seal faces. In this case the primary seal leak took place due to inductee seal flush (evident from heat mark and carbon dust) followed by reversed pressure, causing damage of o-rings and compression of unit springs. Pitting on the seal face appears to be due to blistering as a result of heat concentration The hairline crack on impeller surface across the radius is also due to corrosion fatigue as it was subjected to cyclic stress due to flow variation within corrosive environment as the H2S, which is present in LPG (15000 PPM).

Failure of ECS seal

This failure was the result of high load on wave spring due to vapor concentration at seal box and rapid wear due to high face loading and lack of lubrication. Actually this seal face has less contact area so that heat generation be less and designed for operating under minimum box pressure. Once first seal is failed, this seal provided service for short period, allowing planned (although urgent) shutdown for seal repair.

The wearing clearances increased due to temporary rotor bow at impeller end while operating under fluctuating load condition away from BEP. Scale formation in stuffing box jacket further caused poor cooling effect and heat dissipation.

Calculation of heat generation at seal faces:

Pressure-velocity facto (PV)

Heat Generation at seal: Q=C1 x PV x f(Ao), B.T.H/Watt

b= seal balancing ratio, 0.7

K=Pr. gradient factor, 0.3 for light liquid

Psp= spring pressure = 0.45 bar

Vm=velocity at mean diameter 3.14x65x3000/1000 x 60 = 10.5 m/sec

f= coefficient of friction, 0.07 for C/TC Combination

Ao=Seal face area of seal ring = .001 sq. m

C1= 1 for SI unit

PV = {(12-0.45) x (0.7-.0.3) + 0.45} x 10.5 = 53.025 bar m/sec

Q= 1x 53.025 x0.07 x0.001 =0.037 watt/sec=0,037/4.18=0.009 cal/sec *

Or Q= 0.009 x60 x60=31 cal/hr

* Cal= watt/J (J=4.18 Jules/sec)

This undissipated heat will cause rise in temperature of LPG vapor at constant volume and the rise per hour could be calculated by using gas law: P1 x V1 / T1 = P2 x V2 / T2

Root Cause of Failure

The improper flushing plan and lack of vapor escape feature is the Root Cause of failure of primary seal. The non-function of alarm system and absence of vapor recovery connection are the root causes for ECS seal failure. The lack of cooling due to jacket scale also a cause to accelerate the failure.

1. Seal flush system modification to API Plan 11 that will maintain higher stuffing box pressure and enough flush.

2. De-scaling of stuffing box jacket and thorough inspection during preventive maintenance to be carried out.

3. The diff. temperature of cooling water to be monitored for effective heat dissipation.

4. The vapor recovery line to be connected properly to flare header.

5. Rectification of alarm annunciation system for seal failure.

Timing Schedule and Team Assignment

Activity no 1,2,4 - by maintenance

Item 3 - by operation / PAD

Item 1 - in consultation with seal manufacturer during next available opportunity

Item 5 by Instrument section

Remaining items to be implemented immediately

Follow-up:

Six months observation

Document and update of records and history log to be maintained after corrective measures are implemented and continued during the following satisfactory operation of period of one year.

Readers Feedback, Questions, Discussion and the Author’ Comments: