Allowable operating region is defined the portion of a pump’s hydraulic coverage over which the pump is allowed to operate, based on vibration within the upper limit of this International Standard or temperature rise or other limitation, specified by the manufacturer. {API 610}

A centrifugal pump’s operating flow range can be divided into two regions, one termed the best efficiency or preferred operating region, over which the pump exhibits low vibration, the other termed the allowable operating region, with the limits, both high and low, defined as those flowrates at which the pump’s vibration reaches a higher but still ―acceptable‖ level. Figure 30 illustrates the concept. Factors other than vibration, for example, temperature rise with decreasing flow or NPSH3 with increasing flow, can dictate a narrower allowable operating region. See also 6.1.12.

• Item 1 in the below photo defines the allowable operating region. The vendor specifies it. The lowest point of this region is called minimum continuous stable flow. And the maximum point is named maximum flow rate.

• Performance test is conducted by vendor within allowable operating region at 6 points that have been shown by API 610. The vendor shall take test data, including head, flowrate, power and vibration at a minimum of five points. These points will normally be

1. shutoff (no vibration data required)
2. minimum continuous stable flow (beginning of allowable operating region)
3. between 95% and 99% of rated flow
4. between rated flow and 105% of rated flow
5. approximately the best efficiency flow (if rated flow is not within 5% of best efficiency flowrate)
6. end of allowable operating region

• Pumps shall have a preferred operating region of 70 % to 120 % of best efficiency flowrate of the pump as furnished.
• Rated flow shall be within the region of 80 % to 110 % of best efficiency flowrate of the pump as furnished.

Comments:

• NPSH available shall exceed NPSH required by a margin of at least 1 meter at all flows from minimum continuous flow to maximum allowable flow.
• The pump Supplier shall specify and indicate on the data sheet and pump curve, the allowable operating region
• The allowable operating region shall be stated in the proposal. If the it is limited by a factor other than vibration, that factor shall also be stated in the proposal.