Grundfos MS 4000, Product Manual

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20
MS Motors
Mechanical Installation
Submersible Motor Cooling
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. Grundfos
Submersible Motors are designed to operate at full load
in water up to 30°C (86°F) free convection, and 40C
(104F) provided the flow velocity can be maintained at
a minimum of 0.25 feet per second (fps).
Required Cooling Flow
and Velocity
AWWA specifications state the maximum motor diam-
eter and the minimum inside diameter of the well shall
be in such relationship that under any operating condi-
tion 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 specifications 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 1
relates flow, casing and motor size requirements to
accomplish minimum cooling velocity.
(Table 1) Minimum Submersible Cooling Flow
Requirements
Grundfos Submersible Motors require no cooling fluid
flow past the motor, when the produced fluid tempera-
ture is 30°C (86°F) or less. Cooling by free convection
in such cases, 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.
Water Temperature and Motor
Derating
As previously stated, the full motor capacity is a func-
tion of ambient fluid temperature and flow past the
motor.
When the ambient temperature exceeds the tempera-
ture at which the motor performance is based, the
motor must be derated and/or cooling velocity
increased. Table 2 provides typical derating criteria for
hermetically sealed/canned type submersible motors.
Such motors should not be used in applications which
exceed 60°C (140°F) regardless of any special provi-
sions incorporated into the system. AWWA specifica-
tions state that the motor temperature shall not exceed
the allowable operating temperature of the motor thrust
and radial bearings, and in no case shall it exceed the
temperature rating of the motor insulation system.
When the service duty exceeds 40°C (104°F) pumps
and motors fitted with NBR rubber components are
subject to reduced life if not replaced on a regular basis.
A minimum replacement interval of three years is
recommended.
FKM elastomers (rubber compounds) are recom-
mended any time the normal ambient fluid temperature
exceeds 104°F. Allowable % Max. Namplate Amps
Derated for Ambient Water Temp. @ .5 fps
( Table 2) Allowable % Max Nameplate Amps
Derated. for Ambient Water Temp at .50fps.
Casing/Sleeve
I.D. (inches)
4" Motor
(0.25 fps)
6" Motor
(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
Note: 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).
Water Temp. 0 - 3 Hp 5 - 15 Hp 20 - 40 Hp
30°C (86°F) 100% 100% 100%
35°C (95°F) 100% 100% 90%
40°C (104°F) 100% 90% 80%
45°C (113°F) 90% 80% 70%
50°C (122°F) 80% 70% 60%
55°C (130°F) 70% 60% 45%
60°C (140°F) 50% - -
Note: Derating % is based on an ambient fluid temperature of 30°C
(86°F) @ 0.5 fps, consult motor manufacture for specific maximum full-
load cooling water temperature without derating. Typical base abient fluid
temperature rating for various manufactures of submersible motors used
in the water supply industry range from 25°C to 40°C, with 30°C being
the most prominent.
MS.book Page 20 Wednesday, January 26, 2005 3:25 PM