by Randal Ferman, Vice President Ekwestrel Corp
Minimum flow is a subject that many pump professionals will give a lot of time and consideration to. Here, we look at the ultimate purpose of minimum flow, how it should be established, and how it differs depending on the pumping application in question.
Over the years, the use of the term minimum flow has evolved. Decades ago, industrial centrifugal pump manufacturers would often quote a single, relatively low value for minimum flow in order to prevent users from running their pumps to destruction.
The term minimum flow generally meant the lowest continuous flow at which the pump was permitted to operate. These values did not take into account duration, vibration level or other factors.
Today, we have minimum flow values for continuous operation, for intermittent operation and for permissible temperature rise.
Figure 1 shows the relationships between various off-design pump phenomena and minimum flow conditions. This head versus flow rate curve is based on S. Gopalakrishnan’s curve from his well-cited paper, ‘A New Method for Computing Minimum Flow’.
As an aside, I recall Gopal (everyone knew him by that name) made a local technical presentation using the now well-known chart, before it was published. Evidently someone copied the chart from a handout of the overhead slides and it was quickly pirated by someone else, then others. Copies or variants of this chart are now found widely in papers and presentations on pumps.
The minimum flow quoted for continuous operation is usually referred to as minimum continuous stable flow, or its more common abbreviation, MCSF. A pump’s MCSF is the flow below which the pump should not be operated continuously. The primary purpose of MCSF is to achieve satisfactory bearing and seal life. However, MCSF may also be based on other considerations.
Any of the following factors may be considered in establishing the MCSF:
• Manufacturer’s experience
• Rule of thumb
• Calculated onset of suction recirculation or discharge recirculation
• Radial thrust
• Temperature rise
• Cavitation erosion intensity
• Maximum permissible pressure rise (for system purposes)
• Maximum permissible power rise (high specific speed and axial flow pumps)
• A combination of the above factors or others not listed.
For API 610 specified hydrocarbon processing pumps, the value of MCSF normally coincides with the lower flow limit of the allowable operating range, as shown in Figure 2, where a specified vibration limit must not be exceeded.
The MCSF value can range from roughly 10-80 per cent of best efficiency point (BEP) flow, depending on pump size and type, operating speed, impeller suction geometry, liquid density, and other factors.
A size 2” (50mm) discharge single-stage process pump may have an MCSF as low as ten per cent of BEP flow. MCSF is often in the range of 30-60 per cent of BEP flow for process pumps with discharge sizes 3” (75 mm) and larger. Large mixed flow vertical pumps and very high head-per-stage centrifugal pumps may have an MCSF greater than 60 per cent of BEP flow. Axial flow pumps have a power curve that rises toward shut-off and minimum flow may be limited by the power rating of the driver.
On certain high energy pumps, the minimum flow is governed by cavitation erosion damage. Minimum continuous flow for 40,000-hour impeller erosion life is where the system NPSH available curve intersects the pump’s NPSH required curve, at lower-than-BEP flow.
Intermittent minimum flow, when specified, is usually given as a percentage of MCSF. On some applications, the governing value may be based on temperature rise. On large high energy pumps, the value of intermittent minimum flow could be, for example, 70 per cent of MCSF and not to exceed 100 hours per year.
For some applications, a thermal minimum flow or minimum continuous thermal flow is specified based on permissible liquid temperature rise. MCTF is usually, but not always, lower than MCSF.
Although a pump’s thermal minimum flow is not always specified, the end user can readily calculate its value based on input mechanical power heating up the liquid. The limiting temperature rise is based on a safe margin to prevent flashing of the pumped liquid to vapour, which can result in pump seizure.
Thermal minimum flow is not normally a concern at pump start-up, as long as the closed discharge valve is set to begin opening right away. However, if the margin of system NPSHA above pump NPSHR is minimal, then the temperature rise conditions at pump start-up should be checked carefully.
A few pump applications, such as the use of a vertical turbine jockey pump for maintaining pressure in a large fire sprinkler system, can potentially operate continuously at shut-off while pump suction recirculation mixes with the water in the sump in which it operates. The sump acts as a heat sink and a minimal water temperature rise is not a problem. This example is a rare exception to an almost invariable restriction on operating the pump continuously at shut-off.
The purpose of minimum flow is generally to prevent undue wear and tear or damage to the pump. In the real environment of a process or utility plant, a pump is operated at just about any condition demanded by the situation at hand. Thus there are different pump minimum flows for different purposes.
For an independent evaluation of a pump minimum flow issue, contact an experienced consulting engineer who can help with your specific application.
About Randal Ferman
Randal has more than 35 years of experience in the pump industry. During 32 years with global pump manufacturer Flowserve, he was responsible for assignments in order engineering, field technical services, hydraulic design, training and product development, involving a broad range of pump types and applications. In 2009, Randal became a full-time principal of Ekwestrel Corp in Los Angeles, where he provides independent, objective engineering consulting services on new and existing pumping equipment and systems.