Centrifugal Compressor Shaft End Seals Vasanth Bhat Singapore Refining Company Pte Ltd.

Presenter/Author bios

Vasanth Bhat is a Integrity Manager at Singapore Refining Company ( A joint venture refinery between Chevron & Petro China ). During his last 6 years with SRC, his main role has been support all technical and asset management issues with regards to rotating machinery in the refinery. Prior to this, Vasanth had over 19 years of experience in various roles as consultant , Sales , Reliability & Maintenance engineer in organizations in India , Middle east and Singapore . Vasanth has a B.E. degree (Mechanical Engineering, 1989) from Manipal Institute of technology – India and is a certified Category IV Vibration specialist by Vibration Institute USA . He is currently pursuing a Masters Degree in Rotating Machinery From University of Zaragoza ( Spain)

Centrifugal Compressor Shaft End Seals • Types

– Labyrinth seal – Carbon segmental seals – Floating ring circumferential

wet seals – Face contact wet seals – Dry Gas seals

Labyrinth Seals • Simplest seals ( multiple orifices) • Low pressures . • Could be made of generally of softer material to prevent

damage to rotor . Non metallic materials such as PEEK gaining popularity.

• Bearing housing seals and separation seals in dry gas seals are another commonly seen applications.

• In early times labyrinth seals were used as main seals with the help of barrier gas to prevent process gas to come out to the atmospheric side eg Wet Gas compressor application in FCC units.

Pros Cons High Reliability , long life

Higher leakages compared to other seals

Lower Cost $ Limited to low pressure applications

Easy to install & maintain in most cases

In most cases would need inert gases if applied to applications other than air

Support System requirements are simpler

Carbon Segmented seals • In recent times actively replacing labyrinth seals in the

typical applications. • The clearances are smaller and result in lower leakages. • Now commonly used in air compressors and low pressure

gases .In case of hazardous gas application a barrier gas such as nitrogen is provided to prevent leakage of process gas into atmosphere.

• Very commonly applied in low pressure application such as blowers and now commonly used in dry gas seals as tertiary seals or barrier seals to prevent lube oil contamination into secondary seals.

Pros Cons Quiet Reliable Not suitable for higher

pressures Lower Cost $ compared to other seals

In most cases would need inert gases if applied to applications other than air

Easy to install & maintain in most cases with some requirements on mating shaft surface

Leakage higher compared to other seals

Support System requirements are simpler

"Image provided courtesy of Flowserve Corporation, all rights reserved.”

Wet Seals • Wet seals need liquid barrier ,

in most cases lubricating oil to prevent the gas from leaking to atmosphere.

• They are of circumferential and face type seals in various arrangements.

Pros Cons Can be applied to low to high pressures

Not suitable for higher pressures as it would need a seal oil system of higher then sealing pressure

Lower leaks compared to carbon segmented seals

Life is dependent on quality of Seal oil ,operation and reliability of seal oil system

Don’t fail in a catastrophic manner when gas gets contaminated

Will be expensive system and could increase exponentially with higher pressures

Mixing of lub oil system with seal oil system is not a major concern

If seal oil contaminates process gas that would be a limitation to the seal application

Start up & shutdowns are easily managed and don’t need addition of equipment to maintain ideal conditions for the seal

"Image provided courtesy of Flowserve Corporation, all rights reserved.”

Face Contact Wet Seals • Many OEMS would offer this seal design as it

has lower leakage rates . • Works on same principle as pressurized

barrier mechanical seals in pumps with the challenge of the speeds being much higher compared to pumps.

Image Courtesy Elliott Ebara

Dry Gas Seals • Currently the default seal

selection for most centrifugal compressor application.

• Capable of handling any type of applications as long as there is clean dry gas available for the seals.

"Image provided courtesy of Flowserve Corporation, all rights reserved.”

Dry Gas Seals

Pros Cons Can be applied to almost entire pressure range The cost of the seal is highest in most cases for the

similar application Lower leaks compared to other seal types best to meet most difficult emission requirements.

Life dependent on quality of seal gas and how it is maintained through out the operation of the compressor

As compressor gas is used in most cases as sealing media the system is relatively simpler

Migration of lube oil to seal side could be pose a serious concern and need special barrier seals

Life of dry gas seals is generally better compared to other seals when operated and maintained well.

Failures in many cases are catastrophic in nature resulting in sudden outage of machines . Sensitive to compressor gas composition changes

Why Compressor End Seals are important ?

• An analysis of un planned trips and events of single line critical compressors in a typical refinery indicates above 20% of incidents are due to seal leaks .

• Some of these events could have quick recovery but majority may involve outages in the duration of 7 to 15 days resulting in significant losses.

• The presentation explains some of these events and share their behavior and lessons learnt from these incidents

Case Study 1 : Recycle Gas Compressor Wet Seal leak

• Compressor Introduction : – A 5 Stage horizontal Single Case barrel

compressor. – Max operating speed -11,000 rpm – Driven by a 650 KW turbine – Combined lube oil & seal oil system – Floating ring circumferential low

pressure seal with guaranteed leak of 100 l/day for the case.

• For a year post TAR the seal leakage was between 2.8 -3.2 l/hr

• There was decrease in seal oil leakage to 0.2-.6 l/hr for about 6 months post which increase in seal oil leakage rate from 5-7 l/hr for about a year post which increase in seal oil leakage rate was observed to 7-10 l/hr

• As the compressor seal oil top up rate was getting difficult to manage compressor was taken for seal replacement found gas seal rings damaged.

Observations During Overhaul • No major damage to the rotor . • Both the Gas seal rings ( suction and discharge side ) found with the

babbit melted off. Compared to the required clearance of 50 microns ( diametrical ) the clearance was more than double.

• Atmospheric side seal rings found in good condition.

16 November 2015 12

Seal Details and Failure Mechanism • It is observed that the clearances on these

seals are on the lower side to ensure the guaranteed seal leakages. The surface temps computed are in excess of melting point of the Babbitt metal . If the gap is uniform and passages are clean there are no issues . But if the seal surface gets contaminated affecting heat transfer there is high chance that localized hot spots will emerge and melt the white metal.

• Also it was observed if the seal oil has varnishing potential this could impact the heat transfer process more and result in premature seal failure.

Lessons Learned • Floating ring seals require a minimum

leakage for safe operation. • A drop in seal leakage by 1/3 rd to 1/5th of

normal leakage is cause of concern as the seal is likely to fail.( leak heavily). The drop in leakage indicates clearance passage is getting clogged.

• Regular monitoring of leakage rate is essential to avoid being caught off guard.

• No serious damage to the seal and machine even though the machine was operating for a close to a year with high leakage rate.

16 November 2015 14

Case 2: High Pressure Recycle Gas • Compressor Details:

– High pressure 6 stage compressor. approximately 150 barg seal pressure. Turbine driven

– High pressure seal oil system – Machine speed :13000 rpm.

• Event 1 : Heavy seal leak just after overhaul Findings:

– Seal oil was losing at rate of about 1 drum per hour.

– Seal oil delta pressure (seal oil/reference gas) control system was unstable – fluctuating.

– Discharge sour seal oil trap was empty and level control valve was open.

– Suction sour seal oil trap was full and level control valve was ‘hunting’ – close/open

16 November 2015 15

Incident 1 : What did we find ? Action taken: • Compressor was shut down for investigation. • Both suction & discharge side seal ring did not

indicate any wear though some deposit was found at the seal.

• Discharge side sour oil trap level control valve transmitter diaphragm was found damaged resulting in malfunctioning of the control valve.

Consequences of level loss in the sour seal oil trap • Gas breakthrough to degassing drum and waste

oil drum • Upsetting of seal oil pressure control system

– Fluctuation of reference gas pressure – Fluctuation of seal oil supply pressure

• Excessive sour oil flow/leakage: – To degassing tank – To process side compressor/piping

16 November 2015 16

Image Courtesy Dresser Rand Co : Olean

Second Failure • Seal leak found very high post a emergency trip of

the compressor . • On dismantling found the securing bolts (part no 17)

sheared and the Babbitt on the gas seal ring melted. • No damage to rotor although seal was damaged.

Image Courtesy Dresser Rand Co : Olean

Lessons Learnt & Corrective Action plan Likely Cause : • Failure of the securing bolts due to sudden

depressurization caused the seals to loose orientation this could have lead to gas seal clearance going down on one side and resulting that to fail.

• The bolt failure was attributed to lag in seal oil system response during depressurization ( control valve issue). Age of the bolts was another factor , followed by concentricity of the seal housing to the main barrel & tightening of securing bolts.

Corrective Action Plan • Considering to upgrade the seal to cartridge and

windback design to reduce leakage and eliminate assembly issues.( as seen in this component seal)

• Improvements in seal oil system instrumentation and filtration of buffer gas system for attending some of the issues seen in both the incidents

Image Courtesy Dresser Rand Co : Olean

Case 3: Refrigeration Compressor leak Machine Introduction: • Motor driven four stage, single casing

centrifugal compressor through a speed increase gearbox at 6000 rpm.

• Utilizes face contacting wet seal. • Has a combined lube oil and seal oil

system. What was the issue ? • After about 3 years of service post

overhaul the seal leak was reported high. The adjoining picture shows the leakage trend of the compressor seals

• Post this incident there have been multiple occasions when the seal leak would be high.

• The case study explains the findings and what is our current practice.

Image Courtesy Elliott Ebara

What did we do ? • Stabilized the buffer gas pressure. • Reduced pressure regulator setting to a lower

value in the allowable range. • Inspected drainer vent gas orifice for choking • Seal Oil Inlet pressure reduced from

3.9kg/cm2 to 3.5 kg/cm2. • The seal leakage stopped at the suction side . • Drum top up rate reduced.

• Typical recommendations : – Seal leak likely due to seal hang up. – Could be due to poor buffer gas quality. – Stop the compressor and increase the

seal oil pressure to double the existing value .

– Start the machine and gradually bring the pressure back to normal .

– Monitor the seal leakage and if not dropped try this a few times to get it back to normal.

16 November 2015 20

What was the cause & What was Found • During an pit stop the compressor seals were replaced. • What did we find ?

– Found both seals had scoring at the mating faces. – The labyrinth seals were found dirty resulting in

contamination to the seal and the possible hang up . – Post this incident we have seen multiple occasions of leak but

most of them were resolved when the compressor was started post a stop or when we increase the seal oil pressure and bring it back to normal.

• Actions for future: – Upgrade to cartridge design which are designed for lower

hang up tendencies – Installed filters on the buffer gas so that clean gas is present

on the seals – Change oil type to a less varnishing type so that varnishing

tendencies are reduced and could extend seal life.

16 November 2015 21

Case Study 4: Wet Gas Compressor Compressor Introduction : • 2 Stages opposed to each other with 4

impellers per stage running at 7700 rpm. • The compressor is driven by a steam turbine

close to 5 MW rating. • The compressor was initially provided with

floating ring wet seal as shown in case study 1 but due to poor reliability it was retrofitted with a Dry gas seals.

• Due to the low pressure and need for external gas source a double seal with nitrogen as seal gas was used.

• In order to avoid issues due to Oring ( life ) PTFE rings were used as static and dynamic O rings

Failure Sequence

#1 (R ) ( (PC 73188.OP-50.0)*2.2/23/2012 14:55:14 72.27 (Firs t @1 Min) N o D es c r iption D efined**#2 (R ) SI75300.PV 2/23/2012 14:55:14 6353.53 R PM (R aw ) WGC SH AFT SPEED**#3 (R ) PD C 75A51.OP 2/23/2012 14:55:14 33.41 % (R aw ) SEAL GAS D IFF PR ESSU R E**#4 (R ) PD I75A52.PV 2/23/2012 14:55:14 1.14 KG/C M2 (R aw ) SEAL GAS D P D R IVER SID E**#5 (R ) PD I75A54.PV 2/23/2012 14:55:14 0.49 KG/C M2 (R aw ) SEAL GAS D P TH R U ST SID E**#6 (R ) ZI75300B.PV 2/23/2012 14:55:14 0.14 MM (R aw ) C OMPR AXIAL D ISPLAC E**

150.00

-50.00

9000.00

-1000.00

150.00

-50.00

2.50

-0.50

2.50

-0.50

0.40

-0.30

2/23/2012 14:46:32 2/23/2012 15:06:14

New Plot Title @ 19m42s • During a sudden decrease in speed the seal DP was lost and the compressor tripped.

• A seal hang up was suspected as after a slow roll of the machine the DP normalized.

• As we had spare seals we did a quick hystersis check and found the losses were much higher compared to normal . This was rectified.

• Initially based on requirement the DP for the seal gas was set at 2 barg . Due to this issue of higher drag friction the DP was increased to 4 barg and since then the machine is running without issues although there have been multiple events when the compressor was stopped and restarted . The increase in seal leak was minor for the increased DP.

Lessons Learnt • It is important to review the Inspection and test plan from

vendors and ensure all the necessary checks related to the part are done .

• Look for the nature of the trip and analyze before you conclude on the cause of the failure.

• The tuning of control systems is critical a good balance in control valve opening to ensure stable operation of the machine in various conditions.

• For double seals it might be worth considering higher DP compared to norm of 2 barg above seal chamber pressure to ensure reliable operation.

Case Study 5 : Recycle Gas Compressor Dry Gas seal Failure

• 8 stage compressor • ~11,000 rpm • Ps = 50 barg (725 psig) • Pd = 80 barg (1160 psig) • Tandem arrangement dry gas seals • External seal gas from Hydrogen Make-up

compressors (lubricated recips) as primary source, with the compressor discharge as back up in case of Make-up compressor trip.

• Compressor was commissioned in 1995 and had no failures prior to 2011.

First Failure • Compressor was started post turnaround. • No compressor work was done during turnaround. • After about 13 hours of running the compressor

tripped on high primary vent pressure on the suction-side seal.

Cause of Failure : • Seal system found heavily contaminated with

water-like liquid. • Suction drum steaming for vessel entry during

turnaround done with compressor suction and discharge ESVs left open resulting in condensate building up in the compressor and seal gas system.

• Seal gas take-off from casing bottom allowed for additional condensate to reach the dry gas seal, shattering the faces.

Compressor Flow Scheme

27

FI

PDCV ESV

Filter

ESV

Primary Dry Gas From Hydrogen Make up Compressor Discharge Header Comp

Back-up Dry Gas From Compressor Suct

ion

Disc

harg

e

Suction Drum

Recycle Gas To System

(Piping pocket)

(Piping pocket)

To Compressor Dry Gas Seal

Challenges in restoration • Dry gas seal rotating face fragments (WC) caused

damage that impeded compressor disassembly. • OEM’s previous experience with similar failure: 4

weeks to grind out the stuck sleeve with pencil grinder.

• Spare rotor allowed for partially destructive drilling method for sleeve removal. Rotor was removed within two days and compressor was restarted in 11 days.

Second Failure • Compressor tripped on high primary vent pressure on suction

side exactly one month after the first post-failure start up. • Damage similar to first failure with shattered faces and seal

stuck on the rotor. • No liquid found. Cause of Failure: • Presence of particulate contamination in the system due to

improper cleaning following first failure. • Seal gas system high flow excursion due to DP transmitter

failure. • Dry gas seal system vulnerabilities to contamination (both

solids and liquids).

NDE Primary Seal

NDE Secondary Seal

Restoration Challenges & Post Failure Work • No spare rotor & no spare seals. • Sleeve was drilled using same

technique as the last time but with added precaution to prevent damage to rotor.

• Rotor was repaired at the seal seating area using API recommended rotor repair techniques.

• The seal vendor was able to re groove the discharge end seal from first failure, test and return the seals within 4 days.

• Ensure the root cause is identified and mitigate risks by completing as many recommendations as practical.

• Compressor was restarted in 13 days from date of failure.

Post Failure Work : • Complete review of the compressor and dry gas seal system

identified major gaps from evolving industry and company practices. New gas seal panel and system changes provide improvements and new capabilities: – Gas treating (separation, heating) – Filtration, coalescing, drainage – Seal gas supply availability (booster)

• Upgrades to the seals done in terms of groove design and material in line with latest industry trends.

Key Lessons Learned • Today there are multiple options available for sealing . Reliability and cost determine the

final choice. • The five events highlight the various ways the seal types behave and how these were

analyzed and managed. • Seal designs are constantly evolving and so we should always look at ways to upgrade them

on timely basis to take advantage in terms of lower leakage rates and higher reliability. • Ensuring clean sealing medium is the key immaterial of the typical centrifugal compressor

seal type immaterial if the compressor is running or while maintaining it. • A machine without failures does not mean it has no gaps. Constant review of systems and

latest industry practices helps to ensure equipment reliability. Dry gas seal system design has evolved to consider off-design and unanticipated conditions.

• Challenging situations like these are best overcome with strong partnerships with vendors who can provide unique and timely solutions.