MUHAMMAD REHAN KHAN

CONTENTSBackgroundPVC II PolymerizerMan way specificationsSequence of eventAnalysis of failure Probable causesConclusionRecommendationsCost Analysis

BACKGROUND PVC II Polymerizer faces the VCM leakage most of the time when it get discharges and taken on hold for next batch charging for more than 4 to 5 hoursNOTE: All the failures are of same nature and follows the same sequence, this was the reason that led to the development of this Case Study and complete investigation.

DateEventObservationAction Taken23rd Nov 2010 VCM leakage detected from the man way of PLY-1EPolymerizer was on hold for batch chargingMan way opened and the O-ring was replaced8th January 2011VCM leakage detected from the man way of PLY-1EPolymerizer was on hold for batch chargingMan way opened and the O-ring was replaced1st February 2011VCM leakage detected from the man way of PLY-1EPolymerizer was on hold for batch chargingMan way opened and the O-ring was replaced2nd April 2011VCM leakage detected from the man way of PLY-1E as well as 2EBoth polymerizer were on waiting for batch charging from 48 hrs and 24 hrs respectivelyMan way opened and the O-ring was replaced2nd October 2011Pressure test of all three Polymerizers failed while they were hold on for more than 10 hours All the Polymerizers were on hold due to the power outage from GT BMan way opened and the O-rings were replaced

PVC II PolymerizerDesign Pressure & Temperature = 1.55MPa & 930C

Operating Pressure & Temperature = 0.85MPa & 550C

MOC: 16MnR (C-Mn-Si Steel) +304 (GB6654, A537M)

ID = 3810mm, H = 7068mm

Empty Weight = 44.7 M Tons

REACTION TEMPERATURES:

AFA Coating T = 850C

Batch Charging T = 650C

Reaction T = 590C

Discharge T = 580C

ABOUT MANWAYO-ring arrangement for man way

SEQUENCE OF EVENTS

ANALYSIS OF FAILURECHEWED/ CHIPPED APPEARANCE

PROBABLE CAUSES1. Difference in thermal expansion of the mating surfaces i.e. man way and its closure ring

2. Size of the O ring and its groove dimension3. Softening of Elastomer due to incompatibility4. Improper machining of O-ring gland i.e. Sharp edges (Not the case)5. Elastomer too soft (Not the case)

Thermal expansion of mating surfacesDATA:

Material = 0Cr18Ni9 (SS304)Ambient Temp. = 30 C Temp. Of Ring = 43 C

AFA Coating T = 850CBatch Charging T = 650CReaction T = 590CDischarge T = 580C

THERMAL EXPANSION

l = L * k * T (k = 17.2 X 10-6 m/m-K)

l = 0.2mmMANWAYCLOSING RING

SIZE OF THE O-RING AND GROOVE DIMENSION

O-rings dimensionsO ring diameter = 7.5mmOD = 633mmVolume of the O-ring (2pi2 Rr2) = 86,856 mm3 Grooves dimensionsActual W = 6.5mm, H = 5.72mmVolume of the Groove = 92,222mm3

Standard Groove DimensionsAs per AS568B for O ring diameter = 7.5mm,W = 6.2mm, H = 6.2mmVolume of the Std. Groove = 94,345 mm3Which is greater than the total actual volume

Softening of Elastomer due to incompatibility

OBSERVATIONS

SEQUENCE OF FAILUREO-RING INSTALLED (OVER COMPRESSED DUE TO UNDERSIZE GROOVE)VCM

CONCLUSIONAs per AS568B, the groove of the man-way O-ring is undersized which after closing the man way, over compressed the O-ring and cause it to tear from the edges

The different thermal expansions of man way and its ring causes the crushing of the O-ring from its edges

EPDM is not suitable with the VCM service . During investigation PVC powder found on the d/s of the O-ring which should not be a case in 100% sealed O-ring. This EPDM O ring was suggested by the TCC vendor

RECOMMENDATIONSReplaced the man way arrangement with a flanged arrangement like PVC I polymerizer

Use Teflon or FFKM O-rings instead of EPDM

Machined the groove for increasing its volume to suit with our O-ring dimensions

Use standard size of O-ring for this groove i.e. ID = 633.48 +- 2.92, d = 6.99+-0.15 as per AS568B # 474

COST ANALYSISTotal Outage time = 8-9 hours

Total loss of PVC / batch = 18 Tons1 Tons = $1,070 (Reference: www.icis.com)For one batch, we face the loss of 19,260$ (Rs 16,37,000)FFKM O-ring cost = $ 3,000

REFERENCESDISCHTOMATIC O-ring handbookParker O ring handbook Standard (AS568), Metric O-Ring sizesPVC II Process description by TCCB-reports of O-ring failure

QUESTIONS

This form of failure may also be found from time to time in static applications subject to high pressure pulsing which causes the clearance gap of the mating flanges to open and close, trapping the O-ring between the mating surfaces.*