Page 129 - Fluid mechanics, heat transfer, and mass transfer

PUMPS 107

. What is the function of suction piping? & There is always an unevenflow in an elbow, and when
one is installed on the suction of any pump, it
& The suction side piping of a pump is much more
introduces that uneven flow into the eye of the
important than the piping on the discharge side.
impeller.
& The suction piping is to supply an evenly distributed
& This can create turbulence and air entrainment,
liquid flow to pump suction with sufficient pressure to
avoid cavitation (detailed later) and the consequent which may result in impeller damage and vibration.
damage to the impeller. & When the elbow is installed in a horizontal plane on
the inlet of a double suction pump, uneven flows are
& An uneven flow distribution is characterized by
introduced into the opposing eyes of the impeller,
strong local currents, swirls, and/or excessive en-
upsetting the hydraulic balance of the rotating
trained air.
element.
& The suction piping should preferably straight with no
& Under these conditions, the overloaded bearing or
turns or flow-disturbing fittings close to the pump
mechanical seal will fail prematurely.
inlet.
. What is NPSH?
& Isolation valves, strainers, and other devices used
on the suction side of the pump should be sized & When the absolute suction pressure is low, cavitation
and located to minimize flow disturbance into the may be the result. To avoid cavitation, it is necessary
pump. to maintain a net positive suction head, NPSH, which
is the total head of the liquid at the pump center line
& The suction piping should at least have the same
minus vapor pressure of the liquid at the prevailing
diameter as the pump inlet nozzle and sized to ensure
temperature, expressed in equivalent of head units.
that the maximum liquid velocity at any point in the
inlet piping does not exceed 2.5 m/s (8 ft/s). & Some times NPSH is written as NPIP, net positive
inlet pressure. This terminology is used for positive
& The minimum straight pipe length before pump
displacement pumps for which the inlet conditions
suction, to ensure laminar flow to the eye of the
are traditionally defined in pressure terms rather than
impeller, should be
the head terms. NPIP is inlet pressure minus fluid
➢ 1–8 pipe diameters (for low suction energy/low
vapor pressure.
specific speeds)
➢ 3–16 pipe diameters (for high suction energy/high . What are NPSH A and NPSH R ? How is NPSH A
calculated?
specific speeds)
& NPSH A : Available NPSH.
➢ smaller multiplier would be used on the larger pipe
diameters and vice versa. & NPSH R : NPSH recommended by the pump
manufacturers.
. What problems can result from using improper suction
piping? & NPSH A must be equal to or greater than NPSH R for
the desired capacity.
& Noisy operation.
& NPSH A can be calculated as
& Random axial load oscillations.
& Premature bearing/seal failure.
NPSH A ¼ P a h s h f P 0 ð5:1Þ
& Cavitation damage to the impeller and inlet portions
of the casing.
where P a is the absolute pressure above liquid level in
➢ Cavitation occurs when the suction pressure of the
the source tank/vessel, expressed as units of liquid
pumped fluid drops below its vapor pressure,
head (m), h s is the static suction head (m), h f is the
leading to the formation of vapor bubbles. As the 0
head loss due to friction in suction line (m), and P is
fluid becomes pressurized again in the pump, these
the vapor pressure of the liquid at the prevailing
bubbles implode, leading to pitting of the impeller
temperature expressed as units of liquid head (m).
and other pump components.
& Petroleum products are not pure components but
➢ In addition, since vapor has a lower density than
mixtures of hydrocarbons, which do not have finite
liquid, cavitation leads to a reduction in the pump
vapor pressures. In such cases, Reid vapor pressures
capacity and efficiency. 0
(RVP) may be used in place of P in the equation for
& Occasional damage from liquid separation on the
NPSH A for estimation purposes. RVP determinations
discharge side.
are to be made at the liquid temperatures, particularly
. What is the effect of installing an elbow close to suction from the process streams being pumped directly from
side of a centrifugal pump? condensers without cooling.