it should be furnished with the cartridge sealed and locked in the open position.
• Gas seal vents piping. The primary and secondary gas seal vents can be carbon steel. However, suitable isolating and drain valves should be installed. A check valve is recommended if the vent is connected to the flare system.
Manufacturing factors. There is the
possibility of hidden defects in both com- pressors and DGSs. Such conditions can create many risks to the performance and operation of the DGS system. There are five DGS channels in the compressor end walls, as shown in FIGS. 4 and 5. These
are connection points for the two supply gases, the two vent gases (primary and secondary), and a separation gas (FIG. 4).
The channels inside the compressor end walls (FIG. 5) should be inspected
with a borescope during manufactur- ing and commissioning for cleanness, machining defects and residual material.
FIG. 6 shows the findings during the com-
missioning of large compressors. There is evidence of incomplete machining and drilling from the manufacturing stage at the compressor end-wall channels. In- spections of these channels with a profes- sional borescope during manufacturing and commissioning are highly recom- mended. Residual defects may damage the DGS, and it is very difficult to remove these residual materials at the site.
DGS test factors. Each cartridge must
be dynamically tested under operating conditions at the DGS vendor shop and then disassembled and inspected. After rebuilding and balancing, a static test is done at the DGS vendor’s shop.
After a successful, four-hour compressor mechanical running test at the compressor vendor’s shop, the DGS cartridge may be sent back to the vendor for disassembly, in- spection, rebalance and replacement of the soft seals, followed by a static test.
Packing and storage factors. It is rec-
ommended to ship the DGS cartridges separately and install them during the commissioning of the compressor. The cartridges should be stored in durable box- es filled with O2-free N2 during shipments.
Failure to do so can incur several risks:
• Delivery of the DGS cartridges with the compressor can reduce installation tasks; however, the cartridges may be damaged during lifting, shipment and transportation, especially in a dusty construction atmosphere or during sea shipment
• Vacuum packages may be opened by customs during clearance checking; poor preservation during shipment and construction are key factors in DGS failure during startups.
Construction factors. Several site work
practices can impact DGS reliability: Pipe welding work. To minimize misalignment between the compressor nozzles with piping, fitters and welders, it is normal to temporarily bolt up the pip- ing flange to the compressor nozzle flange and then weld the piping flange to the pip- ing. This is a common mistake. The wrong welding practices can pose significant risks to the compressor bearings and DGS car- tridges due to tight clearances. The weld- ing current can travel through parts, and local welding can be created between parts of DGS cartridges and/or bearings with shafts or housing, damaging them.
Tubing work. Special attention to shop and field tubing work should be conducted to prevent any liquid trap and pressure drop that can cause error in in- strument readings or that creates pressure and temperature drop.
Piping hydrostatic test. Prior to hy- drostatic testing of the piping to the com- pressors, necessary and reliable isolation should be performed. When removing the DGS cartridge at the site, check for any leakage from the inside of the com- pressor casing to the stuffing box space.
Piping internal rust. Check all piping material, instrumentation and equipment in the DGS system for rust with a professional borescope (FIG. 7). Rust can result due to:
• Hydrostatic testing at the vendor’s shop. Ask the vendor to dry piping material by proper means— e.g., hot air blowing, N2 blowing,
vacuum pump, etc.
• Rust resulting from field hydrostatic testing.
• High humidity during sea shipment and/or onsite construction. Use O2-
FIG. 3. Rust-free piping material.
FIG. 4. DGS piping to compressor end-wall channels.
FIG. 5. DGS piping to compressor end-wall channels, end walls.
FIG. 6. Manufacturing defects in compressor end-wall channels.
68JANUARY 2015 | HydrocarbonProcessing.com
Rotating Equipment
free N2 to preserve the DGS piping
and instrumentation during ship- ment and construction.
• N2 preservation. Check, moni-
tor and reinject N2 inside the
compressor casing and DGS pip-
ing material during construction, when required.
• External and internal rusting in SS material of the DGS system can occur during construction. Request acid pickling when required (FIG. 8).
• Temporary piping material. Use SS piping similar to the DGS piping for temporary work, e.g., during flushing operation. The temporary use of car- bon steel piping for flushing SS is not recommended.
Commissioning factors. Commission-
ing practices are equally important: DGS piping flushing. Do not use steam to flush the DGS piping during commissioning. Use dry N2 and mechani-
cal means for required internal flushing and cleaning. Consult with a materi- als specialist for using suitable chemical cleaning, when required.
Oil flushing considerations. Before oil is sent to the bearings for cleaning and flushing, provisions for activating the separation gas must be made. If N2 is
not available, instrument air may be used as the separation gas during the bearing flushing. In case temporary separation gas is lost during flashing, the lube oil flow to the bearings must be immediately stopped (FIGS. 9 and 10).
Alignment. During alignment of the compressor with the driver, special care to prevent damage to the DGS should be considered based on the vendor’s recom- mendation.
Slow run consideration. During steam turbine warmup or slow roll con- ditions, special care should be taken to minimize bowing the rotor and to prevent damage to the DGS, based on the ven- dor’s recommendation.
Suction and discharge chemical cleaning. Chemical cleaning is normal- ly done for suction pipe and knockout drums during the precommissioning
phase. However, there are many opinions regarding chemical cleaning of the dis- charge pipe. The discharge pipe must be chemically cleaned up to, and including, the seal gas supply pipe, which is connect- ed to the DGS panel and the anti-surge line, which recycles gas to the compressor inlet. These lines must be preserved by N2
immediately after cleaning.
Other factors. Other causes of DGS
failure include soft engineering factors, such as a lack of employee training and/ or poor or inadequate planning.
TAKEAWAY
Engineering/design, lack of train- ing, and poor planning are the top three causes for DGS failure. It is recommended to check DGS channels in the compres- sor end walls and all piping material, in- struments and equipment installed on the DGS panel at the manufacturer’s site during the commissioning stage by us- ing a professional industrial borescope to confirm that the system is clean and free of residual manufacturing material, rust, particles and oil/grease.
The system should be carefully cleaned and dried. After hydrostatic tests, DGS car- tridges should be shipped to the site sepa- rately from the compressor and installed on the compressor as close to the end of commissioning as possible. Preservation of both the compressor and the DGS system by N2 during the shipping and construc-
tion phases is highly recommended. ACKNOWLEDGMENTS
The author is very grateful to all of the people who supported and gave input for gathering the data and finalizing the findings of this article, especially work- ers, clients, consultants, practitioners and vendors. The author is also very grateful to his family, for their keen, committed and warm encouragement during the preparation of this piece.
LITERATURE CITED
1 American Petroleum Institute, API 614: Lubrication,
shaft-sealing and oil-control systems and auxiliaries,
5th Ed., April 2008, Errata, May 2008, Part 4.
2 Chan, D. W. M. and M. M. Kumaraswamy, “An
evaluation of construction time performance in the building industry,” Building and Environment, Vol. 31, No. 6, 1996.
SHAHAB ZARDYNEZHAD is a registered senior mechanical engineer in Alberta, Canada, with 22 years of experience in large oil, gas and petrochemical projects. He holds a BS degree in mechanical engineering, an MS degree in industrial engineering and an MEng degree in project management.
FIG. 7. Rust on piping material.
FIG. 8. Rust on SS components.
FIG. 9. Separation gas. Black arrow shows a permanent N2 connection; orange arrow shows
temporary N2 or instrument air.
FIG. 10. Separation gas; oil leakage to DGS end walls without separation gas.