MECHSOFT ENGINEERING DESIGN AND CONSULTANCY
Water Horse Power Calculation for a Fire Pump
How do we Define Horse Power?
→ 1 HP of Power is the unit of work necessary to move 33,000 lb a distance of 1 feet in
Min.
So,
HP =
𝑊∗ 𝐻
33,000∗ 𝑇
---------------(1)
Where;
W = Weight of Water in pounds
H = Distance water is moved in feet
T = Time Period in minutes
As we know, The Relationship between the elevation and the pressure of water is
P = 0.433 * H
Above Equation can be rewritten as,
H = P /0.433 -------------------------------------------------------------(2)
Since the water is incompressible, relationship between weight to volume is constant. Since
each pound of Water contains 0.12 Gallons, the following can be written as,
V = 0.12 W
Where;
V = Volume of Water (Gallons)
W = Weight of water (lb)
Mehboob Shaikh
MTech | B Eng. | CFPS | CFI
+919146861270
MECHSOFT ENGINEERING DESIGN AND CONSULTANCY
Above can be rewritten as,
W = V/0.12 ------------------------------------------------------------------(3)
Putting Eqn 2 & 3 in Eqn (1)
HP =
(V/0.12) ∗( P/0.433
33,000∗𝑇
-----------(4)
Rearranging,
(V∗P∗1)
HP =0.12∗0.433∗33,000∗𝑇
(V∗P)
HP =1714∗𝑇
Can also be written as,
As we know, Q = V/T
(Q∗P)
HP = 1714
(Q∗P)
HP = 598.8 ----------------(in KW)
Mehboob Shaikh
MTech | B Eng. | CFPS | CFI
+919146861270
MECHSOFT ENGINEERING DESIGN AND CONSULTANCY
Effect of Efficiency:
(Q∗P)
HPnet =1714∗𝐸
HPnet =
(Q∗P)
598.8∗𝐸
----------------(in KW)
Effect of Temperature and Altitude per NFPA 20
Diesel Engine are rated to operate with the air temperature of 77 F (25 C) and maximum
Altitude of 300 Ft. above sea level
Mehboob Shaikh
MTech | B Eng. | CFPS | CFI
+919146861270
MECHSOFT ENGINEERING DESIGN AND CONSULTANCY
Effect of Motor Service Factor:
The service factor - SF - is a measure of periodically overload capacity at which a motor can
operate without damage. The NEMA (National Electrical Manufacturers Association) standard
service factor for totally enclosed motors is 1.0
When the service factor is in excess of 1.0, the excess can be applied towards meeting the
horsepower demands of motor.
Mehboob Shaikh
MTech | B Eng. | CFPS | CFI
+919146861270
MECHSOFT ENGINEERING DESIGN AND CONSULTANCY
Worked Examples:
1. Consider a Driver that is connected to 1000 GPM pump rated at 115 psi and that
produces 78 psi at 1500 GPM with 65% Efficiency
Calculate;
A) Maximum horsepower demand in US Customary Unit
B) Maximum horsepower demand in SI Unit
Solution:
a) The pump is rated at 1000 GPM and 115 psi and needs the following power for this
rating:
(Q∗P)
HPnet =1714∗𝐸
(1000∗115)
HPnet = 1714∗0.65
HPnet = 103 HP
But it also must deliver 1500 GPM at 78 psi, which needs the following.
(1500∗78)
HPnet =1714∗0.65
HPnet = 105 HP
Mehboob Shaikh
MTech | B Eng. | CFPS | CFI
+919146861270
MECHSOFT ENGINEERING DESIGN AND CONSULTANCY
2. Consider a 92 hp electric motor with the service factors of 1.15 determine whether
this pump is capable of meeting the demand of a fire pump requiring.
a) 105 HP
b) 85 KW
Solution:
a) The motor can handle 105.8 hp ( 92 x 1.15) yes, it could supply such pump
b) The motor can handle 86.25 kw ( 85 x 1.15) yes, it could supply such pump
3. An Engine is Rated at 95 HP is used to drive a pump in yellow National Park, where
there is an elevation of 7000 Ft above the sea level and ambient temperature could
be 105 F(40.5 C) What is the adjusted rating of this Engine ?
Solution:
EHcorrect = ((0.80 + 0.973) – 1) x 95
EHcorrect = 73.435 HP
Mehboob Shaikh
MTech | B Eng. | CFPS | CFI
+919146861270