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A step-by-step guide on the turbomachinery specification process, including considerations for unusual conditions and indoor vs. outdoor installation.
Before beginning specification, the turbomachinery should be sized. Final sizing will be done by the vendor/manufacturer. But a preliminary sizing is needed for a quality specification: such as specifying a single-stage turbo-compressor when a multistage turbo-compressor is not appropriate. There is often more than one turbomachinery type suited to a defined task.
For instance, in many modern compression services both integrally-geared turbocompressors and conventional-type turbocompressors could be used. Once the different turbomachinery types are reviewed, various factors sway the decision. The key point is this: It is quite difficult to write a comprehensive multipurpose specification. Therefore, some decisions should be made at an early stage such as the type of turbomachinery. The specification can then be written based on these decisions. That specification outlines what the turbomachinery needs to accomplish and how reliable it should be.
Operational conditions should be divided into a set of normal condition(s) and those other than normal (such as alternative, emergency, etc.). Off-design or unusual conditions such as abnormal gas molecular weight (as might be encountered in a startup), a holding period where part of a plant is not in operation, and similar situations should be accurately included and explained. If anything is missed it could lead to future problems.
The purchaser generally should know a lot about the plant, the processes involved, and its operation. In particular, the purchaser should be much more aware of unusual conditions, involved risks, and potential upsets. It is unwise to assume that a vendor is completely knowledgeable about the plant or process.
Reference check any previously successful turbomachinery references in similar facilities. However, each plant or unit has its own set of challenges and specific requirements. Note that turbomachinery manufacturers are specialized in machinery design and manufacturing. Their staff, background, structure, and business are not focused on processes or plant operation.
Off-design or unusual conditions should be included in any specification.
If the vendor’s engineers have experienced a problem in a process or facility similar to the plant under discussion, they may raise a question. However, this is not often the case. Take fluid temperature runaway potential in hot units. A specification should note some expected maximum temperatures and ask the vendor about the maximum temperature that the turbomachinery can handle.
Past experience of operation could provide guidance concerning requirements such as the material selection. Stating minimum material requirements can help a vendor in part selection. On the other hand, a specification should not be so rigorous that it seriously limits the vendor. Any specification should specify the minimum material requirements and invite manufacturer input based on their experience.
A large number of unscheduled shutdowns are traced back to vendor design, manufacturing, material selection or component selection such as seal problems, bearing issues, excessive fouling, high degradation, corrosion, and erosion. However, many of these issues could be prevented if the purchaser communicated turbomachinery requirements, expected conditions, and technical specifications properly.
One example could be the selection of austenitic stainless steel. This is a premium material, but cannot be used if chlorides are present due to inter-granular corrosion and subsequent cracking problems. All traces of corrosive, sour or problematic materials should be accurately specified by the purchaser to prevent the selection of wrong material.
There have been many discussions whether to install machinery outdoors or indoors. If in doubt, specify an external location without roof or shelter. But later stages can add shelter or a roof if there is a need to increase overall reliability.
Turbomachinery design parameters, such as turbomachinery speed, should be given as the allowable values after enough research and checking into the most recent technologies and modern turbomachinery installations. These maximum allowable values should be insisted upon to keep bidding compatible.
More time is often spent specifying accessories than is spent on the core turbomachinery, mainly because options are so numerous. Some engineers reported that more than 50% of their discussions and communications with vendors have only been about instrumentations, condition monitoring, electrical systems and control items. All items related to electrical, control and instrumentation should be fully specified early in specifications and clarified at the earliest opportunity. Otherwise, lengthy and challenging discussions are to be expected. Reference: Royce N. Brown, Compressors Selection and Sizing, Third Edition, 2005 (Gulf Publishing, Elsevier).