Glossary S
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.

Sag boring

Scallop -

Seal -

Sealless pump – a pump that does not require sealing element to prevent the
pumped liquid from leaking into the atmosphere because its internal rotor, with
the integral impeller, does not extend outside of the pressure containment

Seal flush piping Plan 11 -

Second critical speed - see first critical speed for explanation.

Secondary flow -

Self-priming pump -

Self-venting pump - a pump whose casing can be flushed of entrained gas by
simply opening its suction and discharge control valves and rotating the shaft
manually prior to starting. The pump casing does not have a vent connection. In
a multistage pump, only the first stage volute needs to be self-venting. API
Standard 610 considers a pump to be self-venting if the nozzle arrangement and
casing configuration permit adequate venting of gases from the first-stage
impeller and volute area to prevent loss of prime during the starting sequence.

Semi-open impeller - a single suction impeller with no front or suction shroud
but has a back shroud enclosing the impeller vanes. A semi-open impeller is
typically used when pumping dirty liquid, or liquid containing slurry or small solid

Series operation - a set-up whereby two or more pumps operate in a manner
where one pump discharges into the suction of the next pump (and so on into
the next pump.) The capacity of each pump is the same for all pumps in the
series, but their total head is the sum total of the heads of the individual pumps
which could be different from one another. (See parallel operation.)

Series stage -



Sewage -

Sewage pump -

Shaft - the part of the pump on which the impeller is mounted and through which
the input power is transmitted from the driver to the impeller, or impellers. The
position of the shaft, either horizontally or vertically, defines a horizontal pump or
a vertical pump.

Shaft bow

Shaft deflection -

Shaft flexibility factor (SFF) - an index or dimensionless number indicating the
relative shaft flexibility of one pump with another pump of similar size and design.
SFF is calculated using the formula SSF = (L^3 ) / (D^4). A lower SFF value
means that a shaft is likely to have less deflection, less vibration, and will have a
better mean time between failures (MTBF).

Shaft sleeve - a short, thin, tubular piece placed on part of a shaft to protect it
from wear and corrosion. A spacer shaft sleeve is used as positive lock to
prevent a part, such as impeller, from moving axially along the shaft, or provide
spacing between adjacent parts mounted on the shaft. When used on a
destaged portion of a shaft it is used to protect the keyway from damage. Shaft
sleeve is also used to fit a mechanical seal assembly for ease of assembly or

Shockless entrance flow


Shrink factor -

Shroud – that portion of the impeller consisting of circular plate that holds
together the impeller vanes.

– the axial back and forth movement or motion of a pump part.

Shut-off head – the pump head at zero flow, or when the discharge control
valve is totally closed. This term includes the suction head. The term becomes
shut-off differential head if the suction head is excluded.

Single stage – a pump designed with only one impeller.

Single suction – The term single suction can apply to either an impeller
design, or pump design. A single suction impeller is one where liquid enters on
one side only (as opposed to both sides). A single suction pump is one where its
impeller is of a single suction design.

Single volute – a pump with only one volute splitter and throat area, as
opposed to a double volute pump two volute splitters and two volute throat area.

Slabbing – to remove or skim off some metal on a part either to increase an
area or to reduce a thickness.


Slow roll (or slow rolling) - a condition whereby a pump is freely turning at a
slow speed (RPM) to keep it on hot standby condition so that it can be activated
quickly in a matter of few seconds should the main pump is shut-down or kick off
line. In critical service a standby pump is kept in slow roll or slow rolling mode to
prevent damaging its parts (wear rings, sleeve, bearings, mechanical seals, etc.)
if it is started and put into service quickly. See
coast down condition.


Slurry pump – a pump specifically designed to handle slurry in the liquid.

Soft foot - A condition whereby the mounting feet of a machine - motor, pump,
compressor, turbine, or similar equipment - are not in the same plane and not in
proper contact with their mounting pads on the baseplate. Click on link to read

Spalling - bearing damage characterized by breaking out of small splitters or
chips on its surface due to improper lubrication, mechanical damage, material
defect, or material fatigue.

Specific gravity – the ratio of the density of a liquid to the density of water at
39.2 degrees Fahrenheit (4 degrees Centigrade). It is also known as relative

Specific heat

Specific volume -

Specific speed – see separate discussion on specific speed.

Speed-torque curve - A plot of the pump speed, ranging from zero to full
speed, drawn in the x-axis, with the corresponding required torque drawn in the

Splitter – See suction splitter, or volute splitter.

Standby service - A service where an equipment is in idling mode ready for
immediate start-up and continuous operation to switch with, unload, or share the
load of, another equipment. A standby unit in hot temperature service should be
maintained sufficiently warm to prevent thermal shock when it starts up

Static balancing - the correction, or reduction, of residual unbalance to comply
with specified or acceptable unbalance limit by removing (preferred method) or
adding weight in one correction plane only. It is also referred to as single plane

Static head - The head develop due to the difference in elevation between two
reference points.

Straddle – to be located in between two points such as if it were specified that ”
the BEP shall straddle the normal and rated flows” .

Strain -

Strainer – see suction strainer.


Stub shaft

Suction A-B area – a control area adjacent to the suction of the impeller (or the
first stage impeller in a multistage pump.)

Suction angle – the angle of the suction vane of an impeller.

Suction bay area -

Suction bell

Suction flange -

Suction flow recirculation -

Suction flow straightener -

Suction nozzle

Suction piping -

Suction specific speed -

Suction splitter – that part of a pump casing, usually in the form of a short rib,
whose function is to break the pre-rotation of the liquid in the casing suction bay
and funnels or directs the liquid into the suction of the adjacent. In a single
stage pump the suction splitter is typically cast into the casing whereas in
multistage pumps the splitters in each stage are typically separate pieces that
are bolted-on.

Suction strainer

Suction vortex -

Sump -

Sump pump

System head -

System-head curve – a curve or graphical representation showing the system
head varies with the changes in flow in a given system or installation.
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