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Thu 29 Jul 2010 04:00 AM

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Ask the expert: Jim Williams

If you have a question you want answered, or a topic discussed, please send it to daniel.canty@itp.com.

Ask the expert: Jim Williams
Jim Williams, AESSEAL.


If you have a question you want answered, or a topic discussed, please send it to daniel.canty@itp.com .


Question: Dry Gas Compressor Seal Repair: Is there a Viable Alternate to the Seal OEM?


Expert: Jim Williams , AESSEAL plc


The use of centrifugal compressors for gas transportation has increased over the past decades. In these presumed "clean gas" services, many modern gas movers are now fitted with dry gas seals (DGSs) in preference to the older conventional wet and labyrinth seal configurations. Dry gas seals operate by creating and maintaining a very thin fluid film between two mating surfaces. Maintaining this fluid film under all operating conditions is essential to ensure reliable seal operation.


How do I know if I have the right size fluid film?


The fluid film is so small that the most efficient method of demonstrating its existence is to perform a dynamic test. For this reason, all dry gas seals are dynamically tested at the time of manufacture and after repair. AESSEAL have two dedicated DGS test facilities, capable of testing at shaft speeds up to 45,000 rpm and pressures up to 350 bar (5000 psi).


What are the advantages of DGS technology?


Dry gas seals contribute to significantly improved overall machine reliability. Advanced seal designs reduce overall maintenance requirements; they also save power and gas consumption and thus contribute to sizeable operating cost savings in gas transmission services. It has been shown over the last 2 decades that upgrading from the traditional compressor seals to advanced dry gas seals is an opportunity to gain economic benefits from advances in technology.


What are the DGS repair options?


Primary emphasis from seal OEMs on the manufacture of new dry gas seals has resulted in a short fall in after-market capability with regards product engineering and repair support for DGSs that have been in use for several years.


As with all industries where demand for new products outstrips the capacity to support existing products, both pricing structure and lead time for component repairs have come under scrutiny. In 2005, AESSEAL recognized this issue and has invested in facilities aimed at the repair and testing of all types and makes of dry gas compressor seals. This is supported by a comprehensive global refurbishment reference list.


Control of the AESSEAL test activities is achieved through specially designed test panels and software. The test panels control all gas flow in and out of the test environment and are designed to enable any part of seals cartridge to be individually pressurized or vented, as appropriate. Test speeds and temperatures are controlled via an integrated software program. This software also facilitates the data capture and logging of parameters such as speed, pressure, gas consumption and temperature when experienced from critical locations around the seal during the test regime. AESSEAL have the ability to meet all customer verification testing requirements, including over speed, shuttle and reverse rotation.


What makes AESSEAL DGS service different from others?


The ability to test DGSs to high standards is only one aspect of a full repair service. The first element of the repair process is usually an initial price quotation which can also be based upon electronically transferred images or historical repair knowledge of the seal's condition. Based on this information, and before a seal leaves the customer premises, AESSEAL issue DGS repair budget quotations within days of receiving the relevant information, where seal OEM's / industry standard is often weeks.


When a seal arrives at the AESSEAL DGS service facility, a detailed damage assessment is carried out. The scope of work required to effect the repair is then documented to provide full information to the owner of the equipment to highlight areas where the operating methods can be improved to extend seal operating life of the seals. Firm price and delivery commitments are offered together with details of the dynamic test protocol, normally conducted in accordance with API 617.


Following customer approval, the repair process commences. Where needed, steel components are repaired or recoated and seal faces relapped / recut to the OEM face profile.


Replacing consumable items is standard. Such replacement includes all springs and fasteners and secondary seals where all O-rings fitted to DGS repairs are explosive decompression-resistant grades. Furthermore, whenever polymer seal components require replacement, the original selections are replaced with spring-energized PTFE-equivalent sealing devices.


Prior to reassembly and dynamic testing all non ductile components are subjected to a full over speed test, this is conducted at 23% over the maximum speed rating of the seal. The component is subjected to twice its normal rotational stress. Also, whenever applicable, full dynamic balancing is done on all rotating assemblies to ISO standard 1940. Certification for materials of construction is available on request.


What about the External Barrier Device?


Since most DGSs are supplied with external barrier devices, these components can also be repaired in parallel with the seal cartridges, by AESSEAL. Labyrinth components or contacting/non contacting segmented bushes can be supplied as part of the repair program.


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