The form of a purchase specification should be the one most familiar to,
and most commonly used within, the issuing organization; typical forms
are given in API standards 617 and 618. Here we shall consider the content
of the specification.

Performance criteria must be carefully defined for the end use that the compressor must have within the overall system. Following should be included in the specification:
• The range of mass and volumetric flow as influenced by variations in the inlet temperature, pressure, molecular weight, gas composition (vapor loading, compressibility factor, etc.), discharge pressure, temperature, and flow of cooling fluids (water, air, etc.)
• Startup, standby, and shutdown conditions of the compressor and of the entire system
• Mention of even traces of vapors, liquid droplets, dusts, or gases that may be minor items for the chemistry of the process but may cause fouling, gunking, seal problems, etc., either by themselves or when mixed with lubricants or sealing fluids (items such as these may appreciably influence the choice of the compressor type)
• Range of ambient temperature
• Altitude
• Area electrical classification
• Applicable codes and standards from such organizations as the Tubular Exchanger Manufacturers Association (TEMA) and the American Society of Mechanical Engineers (ASME)

Purchase specifications must define quality requirements for auxiliary equipment such as seals, piping systems (material and arrangement), type and quality of control elements and systems, level of redundancy, and shop testing (if any). Checklists for such items may be prepared based on the available knowledge within an organization, as well as on accepted references on the inspection of completed installations, such as Chapter X of the API “Guide for Inspection of Refinery Equipment.” Purchase specifications—or those prepared by the customer for use by an engineer-constructor—should not limit the bidders from using their knowledge and experience.

Required controls cover a very wide range of supply. The compressor specification should include all elements that directly measure and control any part of the compressor system. This includes local panels, receivers from external inputs, and any items to provide outputs to external devices. Devices for volume control as such—or those used to control mass flow and provide anti-surge control—must be carefully defined as to which elements are supplied as part of the compressor system and which elements are external. Thus, such items as inlet guide vanes are best included in the compressor system, while anti-surge and recycle devices are usually best considered as external to the system.

An increasing area of interest for controls consists of the types of diagnostic devices used to measure, indicate, alarm, and record vibration (velocity and displacement), axial movement, bearing temperatures, and drive-motor-copper temperatures. Axial-movement and motor-copper temperature indicators are best used for both alarm and shutdown. Other instruments are most suitable for alarm only and as trend indicators. The compressor supplier is in the best position to select the points of pickup and recommend types of pickup and readout devices. Controls for units to be attended only by remote or occasional local surveillance require very careful attention.

Job cost and completion time is improved with proper use of shop-assembled units. Typical packages, including skid-mounted units, comprise refrigeration (chilled water and low-temperature brines) and complete instrument and plant-air units. Purchase specifications should therefore call for or permit packaged units to be offered where feasible. The units should be such that they need merely to be set on simple foundations, and have the power, cooling-water, and supply and discharge piping connected.

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