The key to long submersible motor life is good cooling. Most submersible pumps rely
on moving heat away from the motor by forced convection. The ambient/produced
fluid is typically drawn by the motor in the course of pumping to accomplish this task.
Submersible motors used in the water supply industry are typically designed to operate
at full load in water up to 30°C (86°F), provided the flow velocity can be maintained at a
minimum of 0.5 feet per second (fps).
Required Cooling Flow and Velocity
AWWA specifications state the maximum motor diameter and the minimum inside
diameter of the well shall be in such relationship that under any operating condition the
water velocity past the motor shall not exceed 12 fps (3.7 m/s) nor be less than 0.5 fps
(0.15 m/s). The AWWA specification are principally applicable to motors 6-inch and larger,
as most 4-inch motor designs are based on a minimum cooling flow velocity of 0.25 fps
(0.08 m/s) at rated ambient temperature. Table 8 relates flow, casing and motor size
requirements to accomplish minimum cooling velocity.
Table 8: Minimum Submersible Cooling Flow Requirements
Casing/Sleeve 4” Motor 6” Motor
I.D. (inches) (0.25 fps) (0.5 fps)
4 1.2 –
5 7.0 –
6 13 9
7 20 25
8 30 45
10 50 90
12 80 140
14 110 200
16 150 280
18 – 380
Notes: 1. Minor irregularities associated with motor shape and diameter variations
between manufactures are not accounted for in the table.
2. At the velocity specified in the table the temperature differential between
the motor surface and ambient water will range from 5° - 15°C (10-30°F).
MOTOR INFORMATION
(gpm)
Some submersible motor manufactures require no cooling fluid flow past the motor,
when the produced fluid temperature is 20°C (68°F) or less. Cooling by free convection
in such cases, is only permitted in the vertical position and is contingent on no adverse
operating conditions present such as; poor power, high stop/start frequency, presence
of incrustating deposits on the motor surface, etc. Detramental operating conditions are
difficult to identify or predict, and for this reason, the minimum cooling flow should be
provided whenever possible - regardless of the ambient fluid temperature.
7
To Next Page
Loading...
