Most failures of refurbished compressor gas seals result from lack of clean and dry buffer gas supplied to the compressor, according to a recent analysis conducted by a leading supplier of engineered sealing systems and associated products. These seals are used on most pipeline compressors that supply gas to gas turbines at power plants and in compression systems needed to boost gas pressure in power plant gas turbines.
According to Joe Delrahim, marketing manager for Morton Grove, Ill.-based John Crane Inc., this high percentage is significant because most companies in the power industry invest lots of money and effort into monitoring or recording gas seal leakage flow, yet devote little emphasis to reliability and problem prevention. More often than not, says Delrahim, seals fail due to lack of a continuous supply of clean, dry buffer gas that is critical to successful operation and seal longevity.
“Although the process gas may have impurities or contain liquid, it is essential to use clean and dry buffer gas for compressors equipped with dry running gas seals,” says Delrahim. “These seals are designed with no contacting faces, and the normal operating gap between the faces is approximately 2 microns. Therefore, any accumulation of particulate larger than 2 microns will cause the faces to contact each other, which will result in seal failure. Unfortunately, this extremely critical requirement is often ignored throughout the planning, commissioning and operating process. This shortcoming, particularly during the commissioning period, results in multiple seal failures, which cause operational loss and delay in start-up.”
Common control system designs for gas seals consist of filtration, regulation and monitoring. Although the filtration facet is rather simple, these control systems typically offer elaborate monitoring and regulation features. In essence, says Delrahim, standard filtration is used on all applications of gas composition and/or presence of liquid and condensation at any level of mixtures. According to documented project histories by John Crane over the last decade, this shortcoming has proven to be the root cause of most seal failures, particularly during commissioning and/or the operation period.
Delrahim says that getting to the root of the problem begins with the proper analysis of mechanical failure. This involves a review of gas composition, commissioning procedures and control system design, along with the interface between the seal, compressor and the control system. Further, it is essential to gain an understanding of the control system piping/instruments diagram and its related pre-established software that allows logic input for safe unit operation.
One factor commonly overlooked, he says, is determining when changes in gas properties such as pressure and temperature will cause condensation. For example: A Malaysian petroleum facility operated by a leading American company experienced frequent seal failure. An investigation revealed that the malfunction was caused by buffer fluid forming condensation across the pressure regulator valve located downstream of the filters. Knowing this, the maintenance team was able to move the pressure regulator upstream of the filter. Subsequently, the team added a heater and insulated the entire line. The problem disappeared.
Based on this experience, as well as numerous requests from the field, John Crane dedicated various resources to developing a gas conditioning unit (GCU). Designed with the intent of consistently delivering clean, dry, properly pressurized gas to seals, the GCU has proven to advance the performance of dry gas seals by solving critical buffer gas supply issues. Unlike conventional gas panels that incorporate only coalescing filters, the GCU features a knock-out filter/coalescer vessel that removes solid particles as well as free liquids and aerosols. A heater controller also monitors and maintains gas temperature. Maintaining fluid gas temperature above the dew point prevents condensation of aerosols in the process gas stream. Therefore, the collective features of the GCU effectively manage liquids to ensure that the cleanest and driest possible gas supply is available at all times.
“In start-up, slow-roll and settle-out, the GCU maintains adequate gas flow using a seal gas pressure intensifier,” said Delrahim. “A flow switch signals the intensifier control, which automatically activates and deactivates the intensifier as needed. The intensifier then provides sufficient seal gas flow to prevent unfiltered process gas from working its way back to the seal faces across the inboard labyrinth, clogging the grooves and causing failures. With minimal customer interface connections and self-controlled, self-regulated functions, the GCU meets the difficult sealing challenges faced by many in the industry.”
Lack of training is another common problem, says Delrahim. For example, many maintenance technicians in plants with dry running gas seals have never received essential training in seal operation and maintenance. “Maintenance technicians familiar with wet seals are used to flooding the seal cavity with no consequences,” he says. “However, dry running seals require no lubrication, which means the faces have to be isolated from bearing oil all the time.”
He warns that mechanical failure, the delivery of new equipment, or installation of a new seal are not the times to start a training program, let alone become acquainted with the compressor, pump or seal operation manual. Instead, training programs should be arranged in conjunction with supplier selection. “The compressor, pump and mechanical seal manufacturers, along with the engineering firm, should offer training as part of their service,” he says. It is also essential, he says, for the seal manufacturer to provide on-site technicians to help improve equipment reliability, mean time between change or repair, and overall plant productivity.
He cites an instance where the staff of PT Arun Company’s natural gas liquefaction plant in Northern Sumatra, Indonesia, experienced first-hand the benefits of component analysis and getting to the root of the problem. More than a decade ago, oil leakage into the process steam gas stream was adversely affecting heat exchanger performance and contaminating the liquefied natural gas.
“The wet seals we were using allowed seal oil into the process compressor, which disturbed the main heat exchanger used to liquefy feed gas that becomes liquid natural gas,” says PT Arun’s engineering coordinator Ismoyo Sumitro. “We know that the dry gas seal was the one option to solve the problem.”
Technicians from John Crane installed 10 pairs of Type 28 dry running gas seals at locations throughout the facility, which eliminated seal oil migration into the compressed process gas. Sumitro reports that a number of the dry gas seals have been in operation for more than a decade now without problems. Recently, as part of a compressor inspection, he removed a 10-year old seal and was amazed to see that it showed little wear. This success was attributed to extensive planning and training of personnel prior to implementing the sealing solution.