Reasons for failure essential in fixing problem.
Getting To The Root
Repeated seal failures are common, even though sealing technologies are improving. For example, a gas pipeline station operated by a leading petroleum company was experiencing frequent seal failures. The mean time between failures was 200 hours. Upon investigation, the company determined that the malfunction was a result of an excessive compressor misalignment, which caused repeated balance diameter o-ring failures due to the unusual axial motions experienced at the operating speed. Until they conducted a proper analysis, the problem continued to perpetuate itself.
By pin-pointing the root cause of the seal failure, the maintenance manager was able to alleviate such frequent seal replacement. This knowledge obviously increases predictability and performance of the platform or compressor stations, as well as decreases costs in terms of labor, reduced product ordering and product loss due to compressor downtime.
Proper analysis of mechanical failure includes a review of gas composition, commissioning procedures, control system design, as well as 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 to be inputted for the safe operation of the unit.
One factor commonly overlooked is determination of when condensation will result from a drop in pressure. For example, a Malaysian petroleum facility operated by a leading American company experienced frequent seal failure. Upon investigation, a group of maintenance, control systems and other representatives from the plant determined that the malfunction was the result of buffer fluid forming condensation across the pressure regulator valve, which was located downstream of the filters.
With this knowledge, the maintenance team was able to move the pressure regulator upstream of the filter. Subsequently, they added a heater and insulated the entire line, so that the problem disappeared. Too often in this scenario, seals would simply be replaced with little regard to the root cause of the problem. But by determining the condensation point, the investigation team was able to resolve the problem.
Training Is Key
Another common challenge is the lack of training with, and knowledge about, the plant's equipment. For example, many maintenance technicians in plants with dry-running gas seals have never received the essential training in the seals' operation and maintenance.
Training has to be an integral part of the maintenance and operations process prior to the commissioning period. 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 up front 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. One should avoid partnering with a firm that does not offer, and insist upon, a solid educational component as part of its regular service.
It is essential that the seal manufacturer provide on-site technicians to help improve equipment reliability, mean-time between change or repair, and overall plant productivity. Be wary of any supplier that promises the best technology yet ships a box containing a product without delivering the proper support.
Finally, communication among all players is vital. Too often, finger-pointing between 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 is certain to be problems.
The staff of the PT Arun company's natural gas liquefaction plant in northern Sumatra, Indonesia has experienced firsthand the benefits of component analysis and strong communication with their suppliers. More than a decade ago, oil leakage into the process gas stream was adversely affecting heat exchanger performance and contaminating the liquefied natural gas (LNG).
Engineers at the John Crane Technology Testing Center simulated the operating conditions at the PT Arun plant, and measured the effects of factors such as pressure, temperature and friction which impacted the seal distortion, wear and service life.
"The wet seals we were using allowed seal oil into the process compressor, which disturbed the main heat exchanger used to liquefy feed gas which becomes liquid natural gas," said PT Arun's engineering coordinator, Ismoyo Sumitro. "We knew that the dry gas seal was the one option to solve the problem." Technicians 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 his dry gas seals have been in operation for more than a decade without problems. Recently, as part of a compressor inspection, he removed a 10-year-old seal and saw that it showed little wear-and-tear.
The key to minimizing seal failure is a review of the components of the plant and their impact on the total system. In order of importance, the following factors should be considered in review of machinery:
* Gas Composition: Understanding the actual gas composition and true operating condition is essential, yet often overlooked. For example, it is necessary to understand when and where condensation will result from changes in the sealing fluid property.
* Commissioning Procedures: How do you pressurize the compressor? How do you bring the speed up? At what point is this a go or no-go scenario? It is essential to detail the answers to these questions. You must also be familiar with the owner's manual for your compressor, pump, seal products and control system.
* Control System Design: Buffer gas filtration, conditioning and flow vs. pressure control. It is important to not only review the control system to ensure that problems don't arise, but also gain an understanding of the design philosophy of the control system itself to have a good feel for the manner in which the logic is addressed. Further, it is crucial to review the buffer gas conditions. Do you need to use a flow or pressure system? Which one is most suitable for this application? It is necessary to do some homework to find out which system is most suitable to ensure that the seal faces are exposed to clean and dry gas at all operating conditions.
* Interface between the compressor, seals and control system. Start-up and shut down sequence, alarm settings and flow measurement units. Be sure to recognize signs of problems within the start-up and shut-down points. Also, in terms of the logic and the interface, how do you want the control system to read? What control setting do you want? Remember that you cannot design one control system to fit every scenario.
* Plant specifications, including tubing vs. piping. pipe sizing, logic system and wiring diagrams. Sometimes the plant specification is totally different than a supplier's recommendation, but for good reason. For example, a plant may specify tubing instead of piping or a different type of welding procedure. Or, while a supplier may recommend a shut down to integrate a new park, the plant may opt for coordinated shut down to avoid process upset.
Successfully Commissioning The Compressor System
With so many factors necessary for a successful compressor start-up, 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, compressor manufacturer, 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 start up periods are a result of parties not fully understanding the responsibilities and roles of each of the critical partners.
Ironically, the time when everything must come together--the start-up--is often the only time the team physically comes together. Therefore, it is crucial to change this model by opening the lines of communication at the beginning of the project--not at its culmination when it is too late to effect change. The following guidelines have proven to aid in sustainability and increase productivity.
* Focus on creating a shared document--a strategic agenda--that leads participants to action, creates a forum for comments, and outlines ideas, concerns and possible bottlenecks. Goals and objectives must be commonly understood and agreed upon by all parties involved with the commissioning.
* Create a clear, agreed-upon process for reaching these goals. This process must include reviewing the compressor start-up checklist and assigning definitive deadlines and responsibilities. To be effective, requests should be recorded, tracked, and reviewed at each compressor start-up.
* Recognize that other critical partners have data crucial to successful implementation, so share information. This includes asking questions and communicating before the compressor start-up, as well as gathering any necessary materials. Be sure someone captures the agenda, discussion points and action items.
* Encourage each critical partner in the commissioning process to develop a sense of involvement and empowerment. Such an environment creates ownership, as well as fosters honesty and teamwork.
* Review the compressor start-up. There is always chance for improvement, so schedule time near the completion of the start-up to review what happened and acknowledge the effort. Such an effort will give you the unique opportunity to troubleshoot future issues that may occur based on the history, documentation and team approach.
this article was prepared by Joe Delrahim, Marketing Manager, John Crane.
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|Title Annotation:||Eliminating Compressor Seal Failures|
|Publication:||Pipeline & Gas Journal|
|Date:||Jun 1, 2004|
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