Page 78 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 2 .9 Pump Best Practices
Efficiency

V 2
H= H = energy expressed in feet of liquid where: The pump efficiency is at its maximum at the pump design point
2g
m kgf ft - lbf using the maximum diameter impeller. Refer to Figure 2.9.1.
H = Head in The pump design point, often referred to as the BEP e best
kgM lbM
efficiency point e is the flow where minimum losses occur in the
V = velocity in m/s (ft/s) pump stationary passages and the impeller. At offedesign flows,
2
2
g = acceleration due to gravity in m/s (ft/s ) separation losses (low flows) and turbulence losses (high flows)
QxHx ρ xg increase internal produced head losses and reduce pump
Power = (33,000 if in BHP)
η x 3.6 x10 6 efficiency.
Where:
Power – kW (BHP)
3
Q = Flow in m /hr (GPM) Horsepower
3
3
ρ = Density in kg/m (lb/ft )
η (Efficiency) = ratio of power output to power input
The horsepower required by a centrifugal pump varies directly
= energy required to avoid vaporization in pump suction
NPSH R
with the specific gravity of the pumped liquid. Horsepower is
passage.
the only parameter on a typical centrifugal pump curve that is
affected by the specific gravity of the pumped liquid. Most
Fig 2.9.2 Centrifugal pump performance definitions pump curves present the horsepower curve based on water SG ¼
1.0. For pumped liquids of any other SG value, the horsepower
on the pump curve must be multiplied by the actual specific
gravity.
Head produced
NPSH R
The head produced by a centrifugal pump varies inversely with
the flow rate. The curve head rise is a function of the impeller The net positive suction head required by a centrifugal pump
inlet and discharge blade angles. Typical centrifugal pump head varies approximately with the square of the flow rate, since it is
rise values from design point to shutoff are 5e15%. a measure of the pressure drop from the pump suction flange to
When the head required by the process exceeds that which the eye of the first impeller.
can be produced by a single stage centrifugal pump, multistaging The NPSH R is also influenced by the pump rotational
is used to produce the energy required by the system. Multi- speed, and varies somewhat less than the rotational speed
staging is simply two or more impellers acting in series within squared.
a single casing to produce the total head (energy) required. It is
common practice for each impeller to produce an equal amount
of energy (refer to Figure 2.9.3).
The limits of the centrifugal pump curve

The centrifugal pump curve has high and low flow limits, which
can cause significant mechanical damage to the pump if not
avoided. At the low flow end of the curve, flow recirculation can
total net system energy (ft. head) – 366 meters (1200 ft.)
number of impellers selected – 4 damage a pump, while at the high flow end, excessive
NPSH REQUIRED , horsepower and choke flow can result in me-
energy (ft. head) 366 M (1200 Ft) 91.5 M
– = chanical damage to impellers, casing, shaft, bearings and seals.
produced for impeller 4 (300 Ft)/Impeller
Each of these factors is discussed below.
Fig 2.9.3 Example e multistaging
Low flow operation

Flow As we examine these factors we can see that oversizing a cen-
trifugal pump will result in low flows through the impeller.
The flow rate of a centrifugal pump varies inversely with the A portion of the flow will reverse itself and set up turbulence as
head (energy) required by the process. For a given impeller it reenters. The abrupt change in direction and very high ac-
design operating at a constant speed, increased process head celeration can result in cavitation on the back side of the im-
requirements will reduce centrifugal pump flow rates. Since the peller vane (refer to Figure 2.9.4).
typical head rise values for centrifugal pumps are 5e15%, Oversizing an impeller can significantly affect performance
a relatively small change in process head requirements can result and mechanical reliability, as indicated in Figure 2.9.5.
in significant flow reductions and possible impeller recirculation Pumps are designed to operate at minimum radial thrust
on operation near zero flow (shutoff). loads at their best efficiency point. Low flow operation results in

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