Page 132 - Fluid mechanics, heat transfer, and mass transfer

110 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS

& For centrifugal pumps, specific speeds are in the . What are BHP and FHP?
range of 400–10,000 or over. & The gross power delivered to the pump by the driver
& For reciprocating pumps specific speeds are much is known as break horsepower (BHP).
lower than those for centrifugal pumps. & The net power delivered to the fluid by the pump is
. What are the ranges of specific speeds of centrifugal the fluid or hydraulic horsepower (FHP).
pumps in relation to the type of impeller used? & The difference between BHP and FHP is due to
& Radial flow impeller: 400–1000. internal mechanical and volumetric losses in the
& Mixed flow impeller: 1500–7000. pump.
& Axial flow impeller: >7000. . Give the three affinity laws/similarity relationships
among impeller speed, N, capacity, Q, head developed,
. “Axial flow centrifugal pumps are of (i) high/low flow
H, and BHP for a centrifugal pump.
and (ii) high/low head type, and have a (iii) low/high
2
3
specific speed.” Give correct answers for each of the Q / N; H / N ; and BHP / N : ð5:4Þ
cases sited above.
i. High flow. Capacity: Q 1 =Q 2 ¼ N 1 =N 2 ¼ D 1 =D 2 : ð5:5Þ
ii. Low head.
2
2
iii. High specific speed. Head: H 1 =H 2 ¼ðN 1 =N 2 Þ ¼ðD 1 =D 2 Þ : ð5:6Þ
. “Purely radial pumps are of (i) high/low flow and (ii)
3
3
high/low head and have a (iii) high/low specific speed. ” BHP: ðBHPÞ =ðBHPÞ ¼ðN 1 =N 2 Þ ¼ðP 1 =P 2 Þ :
2
1
Give correct answers for each of the above cases.
ð5:7Þ
i. Low flow.
ii. High head. 2
Head-capacity: ðQ 2 =Q 1 Þ ¼ H 2 =H 1 : ð5:8Þ
iii. Low specific speed.
. “The head developed by a centrifugal pump is propor-
. What is required power/brake horsepower?
tional to the speed of the impeller.” True/False?
& Required power or brake horsepower (BHP) is the
& False. It is proportional to the square of speed.
power needed at the pump shaft. This is always
. What are the normal ranges of capacities, heads higher than the hydraulic power due to energy losses
developed and efficiencies of different types of in the pump.
pumps? & Required power is expressed as
& Table 5.1 lists the normal ranges of capacities, the
heads developed, and the efficiencies of different v T; ð5:9Þ
types of pumps.
. What happens if a pump pumping water is switched to where v is shaft angular velocity and T is shaft torque.
pumping a petroleum product under the same piping . What is hydraulic power for a pump?
and tank arrangement? & The theoretical energy required to pump a given
& There will be no change in head developed. quantity of fluid against a given total head is known
& Power required to pump a lower specific gravity as hydraulic power, hydraulic horsepower, or water
liquid such as a petroleum product will be reduced. horsepower.

TABLE 5.1 Capacities, Heads Developed, and Efficiencies of Different Types of Pumps
3
Pump Type Capacity m /min (gpm) Maximum Head m (ft) Efficiency (%)
Centrifugal, single stage 0.057–18.9 (15–5000) 152 (500) a
Centrifugal, multistage 0.076-41.6 (20–11,000) 1675 (5500)
Axial flow 0.076–378 (20–100,000) 12 (40) 65–85
Rotary 0.00378–18.9 (1–5000) 15,200 (50,000) 50–80
6
Reciprocating 0.0378–37.8 (10–10,000) 300 km (1 10 ) b
a 3 3 3
45% at 0.378 m /min (100 gpm); 70% at 1.89 m /min (500 gpm); and 80% at 37.8 m /min (10,000 gpm).
b
70% at 7.46 kW (10 HP); 85% at 37.3 kW (50 HP); and 90% at 373 kW (500 HP).