Petrorabigh Vgo Pump Mechanical Seal Failures Analysis

PetroRabigh P-011 A, B Pump Failures March, 2010 Abdulrahman Alkhowaiter, Consulting Engineer, Saudi Aramco, CSD-Tel-03-876-0122

GOULDS

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PetroRabigh P-011 A, B Pump Failures March, 2010 The P-011 A, B pumps at PetroRabigh facilities are 1200 HP Gould’s pumps in VGO residual oil service. The pumps were purchased to API-610 8th edition and 31-SAMSS-004 Saudi Aramco standard. They are of single stage, double suction, operating at 1800 rpm, with pumped liquid maximum temperature at 360 Deg.c, and discharge pressure of 350 psig. The two pumps are of 100% capacity each, with one motor driven unit, and the other a steam turbine driven unit. The pumps started full operation in February 2009, and so far have had three pump thrust bearing failures (three failures on P-011A), and mechanical seal failures of two to three failures on each unit. The purpose of this report is to identify all root causes of the recent and past failures on these two pumps. In addition, direct solutions are given to eradicate the root causes. Some of these solutions are design modifications, some are instrumentation setting changes, and others are improved monitoring.

Thrust Bearing Failures on the P-011A Pump: The thrust bearings are Kingsbury type tilting pad bearings. The P-011A turbine driven unit has had three thrust bearing failures so far. One thrust failure has been directly linked to a major water contamination of the lube oil system in the turbine driven unit caused by an installation error in the carbon seals steam leak off piping. This

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PetroRabigh P-011 A, B Pump Failures March, 2010 has been corrected. A second failure occurred but its root cause was not determined fully. The third thrust bearing failure occurred in March-2010. Our analysis of this third failure points to the following root cause:

1. Startup of the P-011A auxiliary lube oil pump showed correct feed to the radial bearings, but zero flow to the thrust bearing. This proved that a blockage was in the inlet line or bearing housing orifices. We found the orifices clean but a rag was stuck inside the 0.75 inch feed pipe. The source of this rag is either from the recent cleaning/flushing of the oil system, or from original commissioning. 2. For the second thrust failure possibilities, a look at the overall thrust bearing design in the NDE bearing housing showed that there are two oil inlet orifices of only 3 mm diameter. This size orifice is not recommended because it can plug easily due to debris in the lube oil. Another problem is that these small size orifices are not the only orifices; there is a top outlet drain from the bearing housing and that uses a 90 degree turn plus a ½ inch drain hole several inches long. This is an additional pressure drop that resists proper oil flow. From my experience, the 3 mm orifice is only suitable for ISO-VG-32 lubrications oils which have lower viscosity, and thus less pressure drop through the bearing. In your case, the pump uses ISO-VG-46 medium turbine oil of greater viscosity (150% higher). Therefore, the optimum lubrication orifice is 4 mm diameter for this thrust bearing and will provide sufficient cooling oil flow to the bearing, providing a long life. This modification required for both pumps. 3. The lube oil inlet hole bored into the bearing housing is supposed to be plugged with a stopper plug bolt. This was not found. When the backplate of the bearing housing is bolted, this is supposed to seal the hole, but this is not professional as internal leakage can occur. Therefore, fabricate two plugs and install in both pumps NDE housings.

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PetroRabigh P-011 A, B Pump Failures March, 2010 4. The pump vendor has made a mistake in not providing a shaft coupling pre-stretch gap on the P-011A turbine driven unit, because both coupled machines have a thrust bearing in the NDE end. Therefore, thermal growth of both shafts will oppose each other and cause a new thrust force toward the bearings. This increases thrust loading. The motor driven pump unit P-011B does not need pre-stretch because the motor has no thrust bearing. The desired pre-stretch gap at coupling is: Growth Into coupling = Pump shaft growth + Turbine shaft Growth + Coupling Spacer Growth. Note: Pump shaft Avg. temp is only 300 DEGF due to external parts of shafts. Turbine shaft average temperature is taken as 200 DEGF. Also, due to casing thermal growth in opposite direction, actual pump/turbine shaft growth into coupling is typically one half from experience. Total Growth = Pump shaft length from thrust collar to hub x 0.5 x ∆T x Coefficient of expansion + Turbine Shaft length x 0.5 x ∆T x coefficient of expansion + Coupling spacer length x ∆T x coefficient of expansion Total Growth = 63 inch x 0.50 x 0.0000065 in/in DegF x (300 F- 80 F Ambient) + 40 Inch x (200 F-80F ) x 0.0000067 in/in F + 7.0 inch x 0.0000067 x (180 F- 80 F) = 0.065 inch. Recommend to use 0.050 inch gap on P-011A. This is accomplished by moving one machine away from the other until the coupling gap = coupling spacer length + pre-stretch gap of 0.050 inch. 5. The axial monitoring system did not provide sufficient protection as the thrust bearing was completely wiped out. Also, we noticed that the thrust bearings are only protected by drain oil temperature sensors which are a very poor indicator of pump bearing temperature. We recommend resetting of the pump thrust bearing axial probes to the following: Total Mechanical Float Required= 0.009 to 0.010 inch maximum, by shimming bearing. Alarm axial setting = ½ x float + 10 mills= 15 mills Shutdown setting = ½ x float + 15 mills = 20 mills 6. The lube oil sampling frequency is 6 months. This does not provide adequate protection of machinery from contaminated oil damage. Change to a two month frequency.

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PetroRabigh P-011 A, B Pump Failures March, 2010 Mechanical seal failures on both pumps & poor MTBF: A failure analysis was performed on one of the failed Burgmann seals, and we found the following: 1. The seal design is API-682 plan 53B double seal with pressurized barrier fluid. The primary faces are Silicon Carbide vs. Carbon face. The rotating sleeve has two static seals: A graphite carbon ring at inner location, and FFKM O-ring Elastomer at the outer sleeve. The primary & secondary seal spring is a single ply bellows type. 2. The primary bellows had an internal leak due to cracking. There was also a small leak from the sleeve elastomeric O-ring, as this was found to have compression set due to the high pump temperatures; it was no longer round, but was found with a square cross-section. 3. The primary rotating face was in excellent condition. The primary stationary carbon face was not scratched, but was beginning to chip on its inner/outer edges after only two months in service. This means that its face life would not have exceeded one year. 4. Based upon the above observations, we recommend the following: The mechanical seal life will continue to be low, not exceeding 3 to 5 months maximum unless the following design changes are made: 1- Upgrade all stationary faces from Carbon to Silicon Carbide so that the face combination is SIC vs. SIC. We have proven that this combination will provide in excess of five years MTBF when the liquid sealed has lubricating properties, such as this service, both buffer fluid and the pumped fluid. See attached book section on this. 2- Upgrade the primary/secondary seal bellows to double bellows design, preferably of Inconel-625 alloy which has proven long life in such services. 3- Request manufacturer to upgrade existing FFKM Elastomer on sleeve to a higher temperature type with at least 50 DegF higher temperature range. 5. After the recent startup in March-2010, the P-011B pump experienced some loss of barrier fluid into the pump. A study of the mechanical seal drawing showed that large thermal growths are occurring to the seal cartridge due to the 360 Deg.c pumped fluid. With such extreme temperatures, unusual leaks happen. In this case, a loss of axial spring tension on the primary bellows spring is occurring, about 1 mm maximum loss of spring tension on NDE. This leads to low primary face loading and subsequent buffer fluid leakage through faces. Please see the attached drawing with thermal growth calculations on seal unit. The recommendation for both pumps is to increase the existing seal sleeve S-dimension by 2 mm to compensate for the thermal growth seal face compression force losses and transient pump temperatures. Drill new dimples on shaft. Please note that the worst condition is when the pump fluid is heated by process during startup cycle, while the shaft is colder because it has a lower temperature rate response. The result is sudden axial expansion of pump casing, which makes the bellows lose tension (colder shaft=shorter) leading to leakage of seals. After this, the seal leak rate should be significantly reduced. A third reduction in bellows tension is due to casing barrel pressure expansion in axial direction, but this is a small percentage.

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PetroRabigh P-011 A, B Pump Failures March, 2010 Seal Elements Thermal Expansion in Various Directions Casing Datum

Seal Flange 316 SS Thermal Growth Outward Shaft Datum

Shaft Sleeve 316 SS Thermal Growth Inward DE Seal, Shaft Growth

NDE Seal, Shaft 410 SS Growth

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PetroRabigh P-011 A, B Pump Failures March, 2010 Thermal Growth Calculations of Seal Assembly Non-Drive End Mechanical Seal Combined Thermal Growth leading to Primary Spring Relaxation: Thermal Growth of 316 SS Seal Components = 0.0000095 in/in/DegF, Growth of 410 SS Shaft = 0.0000065 in/in/DegF Pump Shaft Avg. temperature from thrust collar to NDE Seal= 200 DegF, Length of this shaft section = 18 inches appx. Pump Fluid Temperature: 570 Deg F , Seal Flange Temp= 500 F, Seal Sleeve Temp = 500 F Shaft thermal growth = 0.0000065 x 18.0 in x ½ (casing growth reduction) x (200 F- 80 F) = 0.007 inch. Positive Seal Flange growth = 0.0000095 x 4.0 in (length) x (500 F – 80 F) = 0.016 inch Negative Seal Sleeve Growth = 0.0000095 x 7.8 in (length) x (500 F – 80 F) = 0.031 inch Negative Total Differential Growth= + 0.007 – 0.016 – 0.031 = - 0.040 inch = 1 mm reduction in spring tension Conclusion: Add 2 mm extra spring tension to sleeve setting.

Drive End Mechanical Seal Combined Thermal Growth leading to Primary Spring Relaxation: Thermal Growth of 316 SS Seal Components = 0.0000095 in/in/DegF, Growth of 410 SS Shaft = 0.0000065 in/in/DegF Pump Shaft Avg. temperature from thrust collar to DE Seal= 400 DegF, Length of this shaft section = 40 inches appx. Pump Fluid Tem. = 570 Deg F , Seal Flange Temp= 500 F, Seal Sleeve Temp= 500 F Shaft thermal growth = 0.0000065 x 40 in x ½ (casing growth reduction) x (400 F- 80 F) = 0.042 inch. Positive Seal Flange growth = 0.0000095 x 4.0 in (length) x (500 F – 80 F) = 0.016 inch Negative Seal Sleeve Growth = 0.0000095 x 7.8 in (length) x (500 F – 80 F) = 0.031 inch Negative Total Differential Growth = + 0.042 – 0.016 – 0.031 = - 0.005 inch = 0.12 mm reduction in spring tension Conclusion: Add 2 mm minimum extra spring tension to sleeve setting. Note that the DE seal is subjected to least thermal growth problems..

Spring Metal Bellows Tension Relaxation : The primary seal springs also experience a reduction in spring constant (k) due to thermal growth from 80 Deg F Ambient, to 500 Deg F actual operating temperature. Therefore, the spring force acting on M. Seal faces is also reduced. Note that the bellows is not a hardened steel spring, so it is relatively soft. This is why its recommended to increase the spring tension by 2 mm overall for both seals.

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PetroRabigh P-011 A, B Pump Failures March, 2010 Temperature recordings for pump P-011B by Laser Gun

Seal flange inboard taken at 4 points

172°C 175C

156°C 126°C

Seal flange outboard taken at 4 points

166°C

215°C

222°C 159°C

Case inboard

251°C 315°C

305°C 310°C

Case outboard

226°C 191°C

183.9°C 217°C

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PetroRabigh P-011 A, B Pump Failures March, 2010

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PetroRabigh P-011 A, B Pump Failures March, 2010

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