The term minimum continuous stable flow, or MCSF, is one of the frequently
discussed, yet less understood, topics on centrifugal pumps. If you asked three
pump engineers to evaluate a pump performance curve and recommend its
MCSF it would be likely that three different answers would be given.

Minimum continuous stable flow (MCSF) refers to the lowest flow rate (e.g. GPM)
in which a pump can operate continuously without exceeding the vibration limits
specified for that pump by its end-user, purchaser, or engineering contractor.
(For simplicity the U.S. customary units are used in this article.)

The vibration limits are usually in accordance with widely accepted industry
standards, such as those issued by ANSI, API, ISO, ASME, etc., or with the
customer's own pump specifications that may be a variation of an industry
standard that was modified to be consistent with their specific service needs or
experience.

If, for example, an API pump is specified, then by default, the pump should
comply with the vibration limits set forth in API 610 standard. The vibration limits
are imposed to ensure the design integrity of the pump and improve its reliability
or mean-time- between-failures (MTBF).

Although the main focus of specifying MCSF is to ensure stable flow to limit the
pump vibration level, its consequent effect to a lesser degree is also to meet
environmental regulations with regard to noise abatement; a pump operating in
its stable flow region does not have issue with low flow recirculation, does not
surge, and does not produce cavitation-like noise, hence the pump operates
quieter.

If a pump were to operate below the manufacturer's recommended minimum
continuous stable flow, it may be necessary to provide a low flow by-pass line to
recirculate part of the flow that is equal to the difference between the MCSF and
the lowest flow rate at which the pump is expected to run.


Ways of determining  MCSF

There are different ways by which a manufacturer determines the recommended
MCSF for a specific pump, each one of which has it advantages and
disadvantages. It may be based on:

  • Test results specific to the pump
  • Historical vibration experience
  • Hydraulics that go into the pump design
  • Pump mechanical design and construction
  • Characteristics of the pumped liquid and energy density of the pump.

[These are discussed further in the full version of this article.]


Continuous operation

What is continuous operation in the context of MCSF? Pump standards do not
address this subject; manufacturers, vendors, and engineers define continuous
operation differently. Some consider continuous as a 24/7 operation, others an
8-hour run, and still others consider a few hours as
continuous if the pumps are
intended to be used regularly in intermittent service.

Operating a pump above its recommended MCSF is intended to protect a pump
from damage or premature failure arising from high pump vibration level. High
vibration level can damage the pump mechanical seals and thrust bearings,  
cause wear parts to seize or gall, and in some extreme situation damage the
shaft.

Damages and failures can occur even if the pump ran for just an hour, or two,
such as when a pump is undergoing testing in a test facility. It does not have to
be a 24/7, or 8-hour, or a short 4-hour operation. From the context of protecting
a pump from damage or failure,
continuous operation should be defined as any
operation other than start-up, shutdown, coastdown, switchover between main
pump and standby pump, or an instantaneous upset condition.


Minimum flow

Many pump people consider MCSF as hard number - not so, that is why it is
considered a recommended number. Most MCSF numbers are rounded-up to
add a safety margin. Operating a pump just below its recommended MCSF does
not mean the pump will automatically suffer from damage or failure, rather, the
pump vibration may start to be elevated.

Pumps that are provided with instrumentation that include probes for vibration
alarm and shutdown can be operated well below its recommended MCSF as
long as its vibration level remains above the alarm setting.


Is MCSF affected by impeller cut diameter?

Two of the most common phenomena affecting MCSF are suction flow and
discharge flow internal recirculation. Between these two, suction recirculation is
a prevalent phenomenon whereas discharge recirculation is seldom a dominant
factor. Suction recirculation can be a cause for concern in all centrifugal pumps
but concern for discharge recirculation is usually found in pumps with high
specific speed (NS) with excessive impeller cut diameter.

If a recommended MCSF is based on suction recirculation, then cutting or
trimming the diameter of an impeller would not affect the MCSF because it does
not change the impeller suction parameters or the pump suction hydraulics. The
recommended MCSF is not reduced by reducing the impeller diameter.

If, on the other hand, the recommended MCSF is based on discharge
recirculation, then cutting the impeller diameter would affect the MCSF, either
positively or negatively, depending on its effect on the impeller discharge vane
area.

Is MCSF affected by viscosity?

The EMA Project has no information on any test study that shows a definitive
effect of viscosity on MCSF but a common sense approach seems to indicate
that a recommended MCSF can be reduced for the effect of viscosity to the
extent that a pump performance is reduced in both capacity and head due to the
viscosity of the pumped liquid.
CENTRIFUGALPUMP.COM
Minimum Continuous Stable Flow (MCSF)
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