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 hours

NOTE: 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.

Date Event Observation Action Taken

23rd Nov 2010 VCM leakage detected from the man way of PLY-1E

Polymerizer was on hold for batch charging Man way opened and the O-ring was replaced

8th January 2011 VCM leakage detected from the man way of PLY-1E

Polymerizer was on hold for batch charging Man way opened and the O-ring was replaced

1st February 2011 VCM leakage detected from the man way of PLY-1E

Polymerizer was on hold for batch charging Man way opened and the O-ring was replaced

2nd April 2011 VCM leakage detected from the man way of PLY-1E as well as 2E

Both polymerizer were on waiting for batch charging from 48 hrs and 24 hrs respectively

Man way opened and the O-ring was replaced

2nd October 2011 Pressure 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 B

Man way opened and the O-rings were replaced

PVC II Polymerizer Design 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

HP SEAL WATER

CATALYST

DISPERSANT SHORT

STOPSTEAM POLISHED

WATER

VCM

FLUSH WATERMODIFIER

SHORT STOP

STEAM

COATING AGENT

INJECTION WATER

HP FLUSH WATER

ABOUT MANWAYO-ring arrangement for man way

EPDM O-RING

SEQUENCE OF EVENTSPolymerizer get discharge and hold on 4 to 5 hours for next batch charging

VCM leak from the man way which is detected by the VCM detectors and then soap test

Poly get depressurize completely (2.5 hours) and the o ring is replaced (1 hour)

V/A and Leak test done (5.5 hours)

Poly takes on service and found OK (Total outage 8-9 hours)

ANALYSIS OF FAILURE

CHEWED/ CHIPPED APPEARANCE

CHEWED/ 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 dimension

3. Softening of Elastomer due to incompatibility

4. Improper machining of O-ring gland i.e. Sharp edges (Not the case)

5. Elastomer too soft (Not the case)

Thermal expansion of mating surfaces DATA:

Material = 0Cr18Ni9 (SS304)

Ambient Temp. = 30 C Temp. Of Ring = 43 C

 AFA Coating T = 850C Batch Charging T = 650C Reaction T = 590C Discharge T = 580C

THERMAL EXPANSION

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

▲ l = 0.2mm

MANWAY

CLOSING RING

SIZE OF THE O-RING AND GROOVE DIMENSION

O-ring’s dimensions O ring diameter = 7.5mm OD = 633mm Volume of the O-ring (2pi2 Rr2) = 86,856

mm3 Groove’s dimensions Actual W = 6.5mm, H = 5.72mm Volume of the Groove = 92,222mm3

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

Softening of Elastomer due to incompatibility

OBSERVATIONS

SEQUENCE OF FAILURE

POLY GETS ON HOLD FOR MORE THAN 5 HOURS AND THE COOLING OF MAN WAY LEFT THE SPACES IN THE O RING

SWELLING OF O-RING FURTHER CONTRIBUTES TO THE COMPRESSION

VCM STARTS PENETRATING AND CAUSES O-RING TO SWELL

THERMAL EXPANSION OF MAN WAY COMPRESSES IT MORE WHICH CAUSES CRUSHING

O-RING INSTALLED (OVER COMPRESSED DUE TO UNDERSIZE GROOVE)

AFTER WHICH THE O RING DO NOT SEAL ANY MORE AND VCM LEAKAGE OSERVED

VCM

CONCLUSION

As 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

RECOMMENDATIONS

Replaced 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 ANALYSIS

Total Outage time = 8-9 hours

Total loss of PVC / batch = 18 Tons

1 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

REFERENCES

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

QUESTIONS