Specifications

A specification should outline to the bidding vendors just what it is you want to accomplish with the equipment and how reliable it must be. Compressor reliability can be achieved if it begins prior to vendor design and fabrication. Therefore, a truly reliable piece of equipment begins in the specification.

Basic Data

Enough data must be supplied to rate the compressor. For example, the mass flows, inlet and outlet pressure, inlet temperatures, type of gas or gas physical constants need to be itemized. The basic data must be very clearly stated and complete to the extent necessary to achieve a common understanding between the user and vendor.

Operations

Very few pieces of equipment operate at a single set of conditions. Thus, operating conditions must be divided into a set of normal conditions and those other than normal. The entire anticipated range of operat ing conditions should be defined either by range limits or alternate operating conditions. To establish guarantee points and nameplate rating points, unique conditions within these parameters must be selected. After defining the normal operating conditions, consideration should be given to "off-design" conditions. For example, these would include abnormal molecular weight deviations as might be encountered in the startup of a machine on a gas other than the design gas. Another example might be encountered in an equipment holding period where part of a process is not in operation.

In describing abnormal, unusual, or off-design operating conditions, even insignificant items should be mentioned, if for no other reason than to establish conversation at a later date. While these conditions may frighten the vendors at the time, it is better to point them out in the specifications rather than to use the "Oh, by the way" method to the service representative at startup. Examples of little potential vendor surprises are hydrogen sulfide or chlorides in the gas, even though they are in the ppm levels. Many times it is erroneously assumed that a vendor's knowledge of the process is more than your own, and therefore, the vendor should have been aware of these unusual conditions. The same reasoning can be applied to known potential process upsets. If the vendor has had an unfortunate experience with a process similar to yours and recognizes it in your process, he may well anticipate the problem. However, you should be much more aware of your potential upsets than the vendor. An example of this might be the temperature runaway potential in the gas being fed to a hot gas expander. At least the specification could be used to question the vendor on the degree of over temperature tolerance that his machine can handle. Another example is the potential of the sudden dead ending of a centrifugal or axial compressor during switching operations of batch-type reactors.

Some consideration of surge characteristics should be made, and unless yuur own plant design includes antisurge controls, it should definitely be mentioned in the specification. Possibly, the vendor can quote antisurge equipment with his offering. At a later date, the specification will serve as a good reminder to both the vendor and purchaser if some mention of compressor control is made. The details may be worked out ai any convenient point, such as at the bid review or possibly later in a coordination meeting.

One way to decrease your reliability and cause problems is to not mention fouling potential in centrifugal compressor specifications if process experience exists. The experience may exist but not be recognized because it did not come directly from operating centrifugal compressors but from a smaller plant operation with reciprocating equipment. By mentioning a fouling potential in the specification, it may be possible for the vendor to include some form of washing in his offering. Alternatively, he may be able to allow additional head margin in his design. By working together, the period of time before cleaning a unit can be maximized.

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