In response to its growing acceptance as well as environmental demands, power plants have integrated natural gas as a means to run their gas turbines, which in turn power generators in order to produce power—all in a clean fashion. Keeping other components, such as the all-important compressor, free of contaminants is essential. As such, there is little argument that compressor reliability and uninterrupted operation are paramount and crucial to plant operation. In fact, an online search for “natural gas compressors” returned 327,000 hits in less than a second. Although there are countless vendors, and though compressor sizes continue to increase and their specifications advance, spare compressors, because of their cost, simply aren’t an option for most buyers. Further, operating requirements, such as pressure rating and speed, continue to expand beyond original intent, resulting in compressors being taxed beyond their design -capabilities.

One means to improve reliability is with the integration of dry gas seal technology. Proven as a solution for almost two decades on thousands of installed bases, the dry gas seal has been shown to improve compressor and plant reliability, as well as product quality. However, the key to success in achieving optimum reliability is the selection of the right dry gas seal and control system, as well as a commitment to training and commissioning/operational procedures.

Selecting the Right Seal
For more than 100 years, compressor seals have served as the main solution for emissions control. As our industries have advanced and the expected production level has increased, so has the duty experienced by a typical compressor. The industry has responded with larger compressors to meet the changing needs, yet the pressure and temperatures experienced are often still going beyond their limits, and balancing minimal or no emissions with the tremendous horsepower required to operate in these conditions is a challenge. Although the main function of a compressor seal is to confine the process within a compressor casing, the right seal also can protect the environment against process emissions. However, not all seals are created equal so it is crucial to gain an understanding of the different sealing concepts developed to accomplish this task.

To begin, a labyrinth seal provides the simplest method for sealing a rotating shaft. The labyrinth clearance is closely controlled to limit the process leakage or emission, although leakage can be vented if the process is non-hazardous gas. The leakage flow rate across the labyrinth, which may measure as high as 100 scfm, is a function of clearance, size, and sealing pressure (Figure 1) In contrast, carbon ring seals typically have a 50% lower leakage rate than labyrinth seals because of the smaller clearance between the rings and the rotating shaft. These types of seals are primarily used on low-temperature/-pressure applications.

Bushing seals are always used in conjunction with buffered oil, and the oil pressure must be maintained above the sealing pressure. The amount of oil leakage depends on shaft size and sealing pressure, but it may be as high as 10–20 gallons per day. The leakage will increase as the clearance between the bushing and shaft increases, due to the radial contact. Process-oil contamination and degassing the contaminated oil are major concerns when this device is specified. Pump bushing seals also use buffered liquid, such as oil or water. However, the buffer pressure must be higher than the process sealing pressure, and the leakage flow rate is determined by operating conditions and shaft size. Static leakage can be high until the shaft starts turning, and the leakage flow rate to the trap may be as high as 1– 5 gallons per day. Leakage is then degasified and returned to a reservoir.

Circumferential sealing devices contain a series of one or multiple segmented carbon rings held together with a spring. Unlike other types of seals, this seal operates directly with process fluid, and leakage is typically a function of pressure, speed, and the quantity of carbon segment in a series arrangement. Normal leakage is 1 scfm, and each segment is typically capable of sealing 100 psig. This type of seal may operate directly in a sealing process or buffer gas.

Contacting face seals require oil buffers at a pressure above the sealing process pressure. Oil leakage passes the seal face and is drained to a reservoir and separated from the gas. Speed and pressure are limited to the seal size, but oil leakage may be as high as 8 or more gallons per day. The seal-oil support system reliability is vital to the operation of this device. Process fluid contaminated by oil during the operation or massive seal failure can be a major reliability concern. Degassing or disposing of oil mixed with process gas has forced many plants to seek alternative solutions to this sealing device.

Despite the validity and proven success of the aforementioned sealing options, a newer concept has proven its worth as a solution for zero fugitive emissions from process pumps—dry-running, non-contacting spiral groove seals. This sealing concept was introduced to the compressor industry in the early 1980s in response to the growing demand for increased operating design, as well as for compliance with regulations focused on volatile organic compounds (VOC). As a result, discarding or degassing contaminated oil became expensive, and process material, such as ethylene fluid contaminated with buffer oil, was no longer acceptable. The dry gas seal(s) and its ancillary system led to significant reductions in operating and maintenance costs in the industries in which it’s now operating. Today, almost all new compressors are equipped with gas seals, which results in more successful and reliable long-term operation of centrifugal compressors. As a result, more and more plants are retrofitting their old wet seal compressors to dry gas seal.

Non-Contacting Spiral Groove Gas-Seal
Technology Ensures Zero Emissions
In addition to reduced maintenance costs, the implications of a zero-emission seal are obvious, as the technology ensures compliance with the EPA, OSHA, and other government and industry standards. Some of today’s regulations dictate an emission limit of less than 500 ppm for centrifugal compressors, while others are pushing for zero emissions—and non-contacting seals meet or exceed most of these emissions regulations. The fact that the seal faces are not in contact during operation also is significant, as no face wear or frictional heat is created. Therefore, a non-contacting seal requires no circulating cooling fluid at normal compressor speed, and no process contaminant particles are transferred into the product. Users of the gas-lubricated non-contacting seals also report that installation and maintenance are less expensive than with wet sealing systems.

The non-contacting, dry-running gas seal is based on face separation resulting from hydrodynamic lift. The seal face separation is the only way to eliminate the effects of friction due to contact, and the most efficient method to separate them is with the spiral groove design. The pressure distribution at the seal face is illustrated in Fig. 7, and the sealing dam at the inside of the seal face allows the gas to compress and provide a necessary force to separate the seal faces (Figure 8). This face separation results in slight leakage across the faces. This leakage flow rate is a function of sealing pressure and speed.

Non-contacting, dry-running seals are available in tandem and double seal arrangements. The tandem arrangement gas seals are commonly used on ethylene and propylene applications, as shown in. The first seal operates as a primary seal, while the second functions as a backup device. Process gas from the discharge line of the unit is piped to the control system supply line, and this control system then regulates and filters the buffer flow before it is injected to the primary seal. Leakage across the primary seal is piped to the flare. The pressure and/or leakage flow rate are monitored and recorded to ensure that the seals function properly. To comply with emission requirements, an intermediate labyrinth is installed between the primary and secondary seals. A controlled nitrogen flow rate of 1 ACFM at 2–3 psi above flare pressure is injected between the two seals. As a result, leakage flow across the secondary seals remains as inert nitrogen gas. This sealing arrangement meets and often exceeds the fugitive emission regulation issued by the EPA, as well as the 1991 California South Coast Air Quality Regulation Management District directive limiting fugitive VOC emission to 1,000 ppm.

In contrast, the double-seal arrangement consists of two seals facing each other. Nitrogen or inert buffer gas at 20 to 30 psi above sealing gas is injected between the two seals. Although the double-seal arrangement provides zero emissions, slight amounts of buffer gas are expected to leak into the process fluid. A tandem-seal arrangement is normally specified for refrigerant applications, such as propylene and ethylene, to avoid process being contaminated from buffer nitrogen leakage.

Justification for Retrofit
Problems associated with wet seals, such as oil usage, frequent replacement, high operating costs, and emission problems, have caused many to evaluate retrofitting to dry gas seals. If this is the case for your plant, the first step to a successful retrofit is to engage in a justification analysis. This process entails evaluating the power gain caused by absence of oil shear, power gain due to elimination of seal oil and fans, as well as savings from reduced seal gas decompression/venting and booster blowdowns—all crucial environmental considerations. Further, savings attributed to the absence of necessary additional power to overcome pipe oil contamination, oil lost through the seals, replacement of contaminated oil, and maintenance and downtime also should be considered. Elimination of fire hazards associated with wet oil systems also should be reviewed in terms of savings on insurance costs. A review of such factors is likely to easily justify the monetary savings associated with retrofitting wet seals to dry gas seals.

With the justification complete, the next step is retrofit design. It is important to select the seal design based on geometry and unit operating parameters. The review of the Corrective Action Request (CAR) on several returned seals by John Crane Inc.—the world’s largest supplier of engineered sealing systems and associated products—strongly suggests that optimal reliability is often sabotaged during initial construction and seal selection. Flooding the seals with trapped water and/or liquid during the compressor hydro-test, as well as field piping errors, are among the common and costly mistakes that result in seal failure at startup. These shortcomings, particularly during the commissioning period, result in multiple seal failures, which cause operational loss and delay in startup. Optimum seal life and compressor reliability begins in the early planning stage with a thorough review of plant flow and operations, specification compliance, safety codes, and local environmental regulations and factors. With this information in hand, review the intended process fluid, including gas composition, operating pressure, and temperature, liquid and contaminant level in the process, nitrogen availability, and the auxiliary buffer gas requirement. Finally, review the project parameters in terms of schedule and budget constraints.

Another thing to consider is the selection and design of a control system, since optimum seal performance requires that clean and dry buffer gas is available at all times. A recent analysis of refurbished gas seals used on compressors, conducted by John Crane Inc., validates that a majority of all seal failures result from lack of clean and dry buffer gas supplied to the compressor. This high frequency is significant because far too many companies invest their money and efforts in monitoring or recording gas seal leakage flow, yet devote little emphasis to reliability and prevention of problems in the first place. More often than not, seals fail due to lack of a continuous supply of clean, dry buffer gas that is critical to successful operation and seal longevity. Unfortunately, this extremely critical requirement is often ignored throughout the planning, commissioning, and operating process.

The modern control system equipped with a John Crane Gas Conditioning Unit (GCU) has proven to advance the performance of dry gas seals by solving critical gas-supply issues. Unlike conventional gas panels that only incorporate coalescing filters, the GCU features a knock-out/coalescing filter vessel that removes particulates as well as free liquids and aerosols. A heater controller also monitors and maintains gas temperature above its dew point, which prevents condensation of aerosols in the process gas stream. Additionally, a gas intensifier is included, which provides the proper flow of gas to the seals during transient periods of operation, such as startup, slow roll, and settle-out. The collective features of the GCU effectively manage the seal gas ensuring the cleanest possible gas, at the proper flow rate, is available at all times. With minimal customer interface connections and self-controlled, self-regulated functions, the GCU meets the difficult sealing challenges faced by many in the industry.

Training Is Key for a Successful Retrofit
Another common source of problems is lack of planning and training. To begin, it is important to perform a risk analysis by identifying possible problems that may result from or during the retrofit, evaluate the probability of the problem occurring, investigate alternatives and prepare backup plans. With regard to training, many maintenance technicians in plants with dry-running gas seals have never received any instruction in seal operation and maintenance. Technicians familiar with wet seals are used to flooding the seal cavity with no consequences; however, in contrast, dry-running seals require no lubrication, which means the faces have to be isolated from bearing oil all the time.

Training has to be an integral part of the maintenance and operations process prior to the commissioning period, and any shortcuts will likely result in costly plant startup. Contrary to common practice by many in the industry, mechanical failure or delivery of a new pump or seal is the wrong time to begin a training program, let alone acquaint oneself with the compressor, pump, or seal operating manual. Rather, training programs should be arranged as part of supplier selection. The compressor, pump, and mechanical seal manufacturers, as well as the engineering firm, should offer training as part of their service. Also, it is essential that your seal manufacturer provide onsite technicians to help you improve equipment reliability, meantime between change or repair, and overall plant productivity. Be wary of any supplier that promises the best technology, yet simply ships you a box containing a product without delivering the proper support.

Finally, communication among all players is vital. Too often, finger-pointing among the engineering firm, compressor technician, and the seal manufacturer occurs. Without a clear understanding and effort to bring all parties to the table at design onset, there are sure to be problems.

Successfully Commissioning the Compressor System
After performing the necessary planning and training, the all-important commissioning process occurs. However, once again, without a thorough review of the environment and all factors, even the best laid plans can result in seal failure. In fact, 99% of all dry gas seal failures that occur during commissioning are not caused by seal failure. Rather, failure results from a variety of factors that include a flooded compressor with oil/liquid or foreign contaminants in the seal’s cavity, or problems with the alignment, piping, or instrument.

With so many factors necessary for a successful compressor startup, it is important to develop a team of critical component experts for the commissioning. This team should include, yet not be limited to, the engineering company that selects the critical components, the compressor manufacturer, the seal manufacturer, and the plant owner/operator of the compressor. Clearly defined areas of responsibility for each party are essential, especially for the many steps that must occur in conjunction with another function or team member. More often than not, unproductive and lengthy startup periods are a result of parties not fully understanding the responsibilities and roles of each of the critical partners.

Final Thoughts for Success
Although it once seemed unthinkable, many members of the industry are now benefiting from 10-plus years of continuous operation of their compressor and seal arrangements. By selecting the proper dry gas seal and control system, as well as engaging in adequate training and a thorough commissioning process, reaching the beyond-40,000-operating-hours milestone is realistic—as is the resulting reliability and improved uptime. Further, there is little argument that emission regulations will continue to increase in stringency. Global and local environmental agencies continue to investigate the best available control technology to set a guideline for future fugitive emission standards. However, with an understanding of a seal’s environmental and tribological issues, and the acceptance of non-contacting, dry-running seal technology, users can experience a significant increase in rotating equipment meantime between failures without compromising the concerns for process emission and environmental regulation.

JOE DELRAHIM is marketing manager for John Crane Inc., a leading global supplier of engineered sealing systems and associated products.

DE - January/February 2006