Integral gear CO2 compressors offer high reliability and efficiency

This article was written and submitted by a competent subject matter expert Bob DeMaria, shortly before he unexpectedly succumbed to a deadly form of cancer. He left us in 2015 and we wanted to see his article published as a testimonial to a true professional who handled his assignments in an impeccable fashion.

In 1998, Bob DeMaria began researching suitable technology for compressing 95 MMSCFD of “bone dry” (-100 ^OF dew point) CO₂ from 16.7psia to 2,710psia. The compressed CO₂ is delivered through a 205-mile pipeline from North Dakota to operations near Weyburn, Saskatchewan, Canada. The oil fields use the gas in enhanced oil recovery.  As an added benefit to the environment, virtually all of the injected CO₂ is expected to remain permanently sequestered in the depleted oil fields long after these will have been abandoned.

Three compression options were closely considered for this project:

(1) Motor + Gear increaser + LP compressor + MP compressor + HP compressor

(2) Motor + Gear increaser + LP compressor + MP compressor and Motor + Pump

(3) Motor + Integral Gear Compressor with 4 pinions (8 stages of compression),

having a maximum pinion speed in excess of 26,000rpm

Implementation of Option 3 commenced in 2003. Results were evaluated as of 2003 and when Bob wrote the article, system operation had been observed for over three years. His company in North Dakota had confirmed that integral gear compressors in dry CO₂ service achieved a number of important benefits:

• Least capital cost for compressor purchase
• Reduced footprint, resulting in least installation cost
• Between-stage cooling and 4 optimum pinion speeds result in highest overall efficiency and lowest operating cost

• Mechanically simplest design and high availability
• Bull gear-driven main lube oil pump simplifies the lube oil system by eliminating the need for an oil run down tank in the event of loss of lube oil to the compressor
• Two 50% capacity 20,000HP machines (Ref. 1) can be selected to allow for partial continuous production in the event of one machine being down for maintenance
• Highest compressor flow turn-down achieved through use of inlet guide vanes

Manufacturers’ capability, experience and reputation were considered in selecting the compressor train supplier. The owners took steps to ensure the success of this previously untested design. Budgetary and personnel resourcing were aimed at achieving high availability. With that in mind, a reliability design audit, life cycle cost analysis and sub-supplier preference reviews were conducted.

During this comprehensive audit process, the shaft seals were given considerable attention and long-term reliability upgrades were made the focus of Bob DeMaria’s discussion with all parties. Agreement was reached on the final design; cost considerations drove the use of carbon ring seals on all stages. However, that the seal housings would be capable of accepting dry gas seals in the event carbon ring seals proved unsatisfactory. Negotiating that space be provided in the design for future upgrading to dry gas seals was a superb demonstration of up-front Machinery Quality Assessment (MQA) by Bob DeMaria (Ref.1). A factory performance test with CO₂ was performed; it ascertained that aerodynamic performance was achieved. Of course, mechanical run test data were captured and closely analyzed.

(Numerous six- stage integrally-geared turbocompressors preceded the 8-stage machines presently used for high-pressure CO2 service. Source: MAN Turbo, Oberhausen, Germany; also Ref.1)

Other mechanical features of these successful designs included “Flexure Pad” bearings, hydraulically fitted coupling hubs, and diaphragm spacer couplings at the driver connection. The motors are of synchronous across-the-line starting design with liquid cooling. An OEM-designed control system manages ramping up to speed and continuously monitors machine condition. The control system is integrated into the plant’s digital control system (DCS), along with shaft vibration, thrust position, and bearing temperature monitoring --- all arranged for automated alarm and shutdown.

Within the control system, care has to be taken to ensure that no liquid forms in the below-1,100 psig compression sections during startup and normal operation. Air-flooded inter-stage gas coolers are used. A glycol/water solution is circulated through the oil cooler and motor for cooling. Heat is rejected from the glycol/water solution through an air exchanger.

In 2006, an identical 3^rd integral gear compressor was purchased to support additional sales of CO₂. All three machines were converted to synthetic lube oil; mineral-base lubricants had been used earlier. The change-over was prompted by the finding that mineral oil was being contaminated by certain sulfur constituents in the impure CO₂ stream. It was evident that the carbon ring seals allowed gas to leak into the lubricant. However, the owner’s reliability-focus prompted rigorous in-service testing and comparison of the different oils under varying load and operating conditions. With the synthetic oil, a minimum efficiency gain of 2% was realized, in addition to the new oil having higher resistance to degradation. The incremental cost of the synthetic oil was paid back within a few months. Moreover, as of 2012 the synthetic oil had been in service for over 6 years with no replacements necessary. The continued suitability of the synthetic oil was ascertained by periodic oil analysis.

Carbon ring seal life on High Pressure Stages 7 and 8 met the manufacturer’s warranty. However, the owner experienced a measure of unscheduled downtime and recompression of higher-than-anticipated rates of CO₂that quite obviously leaked back to suction because of seal wear. The original seals fell short of the owner’s goal of 20 years between seal repairs. However, carbon ring seals for stages 1 through 6 have proven satisfactory.

In 2010, Bob DeMaria’s employer, in partnership with John Crane and German manufacturer MAN Turbo, began the design and development of dry gas seals for this application. These seals have been designed, built and tested for the 7^thand 8^th stages. Bob had planned for their installation whenever the next opportunity would arise. He had looked forward to a discussion of this conversion to dry gas seals as a future full-length article.

Overall, these 8-stage integrally-geared compressors have met and exceeded expectations and have resulted in cost savings of more than 10% compared to the initial investment that would have resulted from the next-best option. These machines demonstrably have the lowest operating cost and have certainly achieved at least 96% availability.

With the recent increased interest in CO₂ sequestration projects, enhanced oil recovery and urea production, this technology clearly merits consideration. Needless to say, Best of Class companies prosper because they allocate time, effort and talent to up-front Machinery Quality Assessment.

(Reference: Bloch, Heinz P.; “Petrochemical Machinery Insights,” (2016) Elsevier Publishing, Oxford, UK and Waltham, MA; ISBN978-0-1280-9272-9)