Slow roll, or slow rolling, is a condition in which a pump runs at a low rotational
speed (RPM) due to an unusual or upset condition, far lower than its normal
Examples of unusual conditions in which a pump may slow roll, or slow
- When a pump in critical service is on "hot standby" where it has to be
available for quick start up in a matter of second when the main pump
breaks down or trips off line.
- A pump in high temperature application may also be in slow roll condition
to prevent its shaft from drooping or sagging during periods of non-
operation; both the pump and its driver (turbine or VFD) are typically slow
rolled for a period of time.
- A pump driven by variable frequency drive (VFD) is in extended soft start
or shutdown condition.
- When the driver is de-energized or shutdown and the pump is rotating
freely due to the high suction pressure acting on the pump suction nozzle.
- A centrifugal pump running in reverse rotation, as a hydraulic power
recovery turbine (HPRT), will slow roll and become a drag to the system if
its suction pressure dropped significantly due to an upset condition.
(Read: When does an HPRT becomes a drag to the system?)
The safe rotational speed by which a pump may be put into a slow roll
condition will depend on several factors:
- The pump, at slow roll speed, should generate enough differential
pressure to provide adequate lubrication and cooling to the wear rings,
sleeves, and mechanical seals to prevent damage to those parts.
- The rotor mechanical seal face must generate adequate hydrodynamic
forces that separate the seal faces during operation at slow roll. The
hydrodynamic forces are proportional to the rotational speed of the shaft
and can be significantly reduced if the seal is run below the lower
rotational speed limit of the design.
- Obviously the pump will generate a low differential head, well below the
system head. Its discharge flow has to be recirculated elsewhere,
probably to the pump suction where it could result in unwanted
temperature rise of the liquid.
A pump in slow rolling condition is different from a pump that is coasting down. A
pump in coast down situation loses driver power and its rotor starts to coast
down from its rated speed down to zero, or to a speed where it will rotate freely
solely on the power exerted by its suction flow. The time it takes for the rotor to
slow down from its normal speed to rest (or its final speed) is referred to as the
coast down time, and is affected by the rotor's moment of inertia.
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The following definitions are intended to give basic understanding of the terms
commonly used in the pump industry. Some industry standards, or customer
specifications, may have specific meaning of certain terms that may be different
from their common usage - in such case the terms should be interpreted in the
context of the standards, or specifications, being referred to.