Basic Design

At first glance, it may appear unnecessary to specify design parameters. This might appear to be the vendor's responsibility. While the vendor does take care of the basic design, there are optional areas where he must receive instructions (by way of the specification) to be able to complete the design.

The data sheet is used to convey the process design parameters, capacity, temperature, pressure, and gas data. The verbage can explain or better define any of the specifics. For example, if gas is to be removed from the compressor and cleansed, cooled, and otherwise modified by the owner's equipment, this is the place to provide the interfacing parameters and brief explanation for the vendor as to where or how to react. This information can be used to establish the diaphragm pressure differential for a centrifugal compressor. This in turn affects the mechanical design. Also, statements concerning momentary flow interruptions that may occur with switching exchangers should be conveyed.

If the decision for split-case or barrel construction is to be left to the vendor, enough details of the gas must be outlined. There is no reason the user can't make these determinations on his own and specify them in the scope.

If casing limitations are fixed by user-supplied relief valves, this information should be conveyed to keep the vendor from rating the compressors on other data. Evaluations can be more of a problem if the same design basis isn't universal with all vendors. Startup and shutdown consideration influence various components, shaft end seals, seal system pressures, and even thrust bearings in some instances. The use of an alternate startup gas, or the desire to operate a gas compressor on air to aid in plant piping dryout should be covered.

if there are speed limitations, such as piston speed in reciprocating compressors, these must be stated. While it would seem a range would be good to give latitude, it really works in another manner. Whatever is given as the high number will be the target, with the vendor testing the approximate value given and exceeding it by some amount to see if a concession can be gained. As a practical step, values should be given as maximum allowable and then held at that value to keep the bidding compatible.

Other options, such as replaceable cylinder liners, must be covered. API mandates replaceable liners with some variations allowed when they are not available. It would be beneficial to state the acceptable alternatives. This approach is probably useful in all areas.

Cylinder connection locations should be defined, as well as the use of air cooling. API 618 requires jacket cooling unless the user specifically states otherwise. Rider rings or wear bands, which are useful in helping extend ring life, are an option in API 618 and normally will not be furnished unless specified. If there is any doubt about a feature, rather than blindly specifying, a call for vendor comments can be made at that point in the specification to bring out later discussion. The price can be included as a breakout or adder.

For centrifugal compressors, nozzle orientation must be specified. If the compressor is on an elevated foundation, the down-nozzle orientation makes maintenance much easier, because major piping does not have to be removed to lift the casing upper half. Figure 11-1 shows an unusual case where all the nozzles are in the upper casing half.

Figure 11-1. This steam turbine-driven centrifugal compressor, shown just prior to shipping, has all nozzles in the upper half of the casing. Note the non-vertical orientation of the inlet nozzle. (Courtesy ofA-C Compressor Corporation)

In reciprocating compressors, the use of explosion-relief devices must be stated if desired. The type and style of distance piece must be specified together with the connections for the collection and disposal of leakage gas. API 618 gives a rather complete coverage of all options.

A basic item on the reciprocating compressor is the lubricated cylinder. The vendor will need guidance from the user advising whether lubrication can be used.

For the screw compressor, in many cases the application will dictate a flooded or dry type compressor. The user can either state this specifically if possible or give enough information for the vendor to make a choice. Care must be exercised in that all vendors cannot furnish both. The user may be required to evaluate the two types for the same service.

There are many other options that can be specified. The appropriate API standard can be consulted. Paragraphs that contain user decisions or choices are marked to help the user in writing the specification. Vendor literature generally gives guidance in those areas where the vendor has options available, which is a help for non-API equipment.

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