Turbomachinery International Sep/Oct 2008

Source: http://findarticles.com/p/articles/mi_qa5385/is_/ai_n29494464

The recent ASME Turbo Expo (June 9-13, 2008), Berlin, Germany, featured presentations by academicians and industry experts. Two papers were highlights of the symposium. One presented a purchaser's perspective on API 617 as applied to centrifugal compressors...

Proactive Engagement

The API presentation was made by Mahdi Taherimoghaddam of Namvaran Consulting Engineers Company, Teheran, Iran. Taherimoghaddam notes that the API standard requires the purchaser to make several decisions on the compressor design and specs. The standards prescribe a minimum service life of 20 years and at least five years of uninterrupted operation. Achieving this goal demands a well-established purchasing specification. The performance integrity guaranteed by the manufacturer at the contract stage depends on the purchaser's specific project requirements reflected in material requisition. And the purchaser shares the responsibility for the performance of the design. Vendors may offer alternative designs or engineering solutions for the individual application if these designs improve the safety, reliability, mechanical integrity, or performance of the equipment. But exceptions to the standard or purchaser's specification must be explicitly stated in the vendor's proposal, says Taherimoghaddam. It should also be explained why the alternative design is superior. Cost alone may not provide sufficient reason for deviations, he adds.

For gas turbine-driven centrifugal compressors, the compressor vendor is usually selected as the vendor who has unit responsibility. This is due to the fact that the compressors are customized while gas turbines are usually off-the-shelf equipment. But Taherimoghaddam says that the compressor is only about 30% of the cost of the package, this arrangement often leads to commercial complexities and project financing requirements for the compressor vendor.

Gas properties should also be mutually agreed upon. The three equations of state that are used provide differing levels of accuracy across the range of applications and with various gas mixtures. These may lead to differences in calculated compressor performance. Taherimoghaddam recommends that that the purchaser and manufacturers should mutually agree on the appropriate equation of state and calculations.

API 617 introduces 15 tests on centrifugal compressors, of which are four are mandatory. They are:

─       Hydrostatic test

─       Mechanical running test

─       Impeller over-speed test

─       Assembled compressor gas leakage test

While the above are important for safe operation, the purchaser is also interested in the optional performance test, since that determines if the equipment meets performance criteria and serves as a means to determine fulfillment of manufacturer's guarantees. The compressor shall be performance-tested in accordance with ASME PTC 10-1997, ISO 5389 or other approved standards. A minimum of five points, including surge and overload, shall be taken at the normal speed. For variable-speed machines, additional points may be specified. The purchaser and vendor must agree on selecting the appropriate reference standard for performance testing. It should be finalized at the contract stage and before order placement, adds Mahdi Taherimoghaddam.