Figure 200-13 presents a centrifugal compressor performance map, using API 617 nomenclature. The family of curves depicts the performance at various speeds where N represents RPM, and:
• Vertical axis—Head: polytropic head, pressure ratio, discharge pressure, or differential pressure; and
• Horizontal axis—Inlet Capacity: called “Q” or “Q1” shown as actual inlet volume per unit of time ACFM or ICFM where “A” is actual, or “I” is inlet.
Note that inlet flow volume, or capacity, is based on a gas with a particular molecular weight, specific heat ratio, and compressibility factor at suction pressure and temperature.
The curve on the left represents the surge limit. Operation to the left of this line is unstable and usually harmful to the machine.
A capacity limit or overload curve is shown on the other side of the map. The area to the right of this line is commonly known as “stonewall” or “choke”. Operation in this area is, in most instances, harmless mechanically, but the head-producing capability of the machine falls off rapidly, and performance is unpredictable. Surge and stonewall should not be confused. Although machine performance is seriously impaired in either case, they are entirely different phenomena. These are covered in more detail later in this section.
Terms frequently used to define performance are “stability range” and “percent stability”. Referring again to Figure 200-13, the rated stability range is taken as QD – QS where QD is the rated point and QS is the surge point along the 100% speed line. The percent stability expressed as a percentage is: