Carver Pump GH - EFFECTS OF FLUIDS; NET POSITIVE SUCTION HEAD (NPSH); CHANGING PUMP SPEED; EFFECTS OF VISCOSITY

I-120
GH Pump 120 – 16.02.EN
5
Item 2 or 3 - Impeller. The pumps are equipped with
semi-open or enclosed impellers. The impeller is keyed
to the shaft (6) by the impeller key (32) and is locked in
place by the impeller capscrew (26) and impeller washer
(28).
Item 6 - Shaft. The shaft of the pump is designed to
provide stabilization to the rotor system when pump
operates away from the best efficiency point. A coupling
connect the pump shaft to the driver shaft.
Item 16 and 18 – Radial and Thrust Ball Bearings.
Deep groove Conrad type, C-3 internal fit ball bearing or
C-3 internal fit ball bearing are housed in the bearing
frame (19).
Item 19 - Bearing Frame. If equipped, the principal
function of the bearing frame is to carry the loads from
the liquid end of the pump to the base and to transport
power from the motor to the impeller (2). The bearing
frame has an outboard radial ball bearing (16) and an
inboard thrust ball bearing (18).
Item 90 – Mechanical Seal. A mechanical seal may be
supplied in place of a packed stuffing box when
specified by the customer. Many variations of types and
materials are available.
IV. EFFECTS OF FLUIDS.
A. NET POSITIVE SUCTION HEAD (NPSH). Any
liquid, hot or cold, must be pushed into the impeller of
the pump by some absolute pressure, such as the
atmosphere or the vessel pressure from which the pump
takes its action.
The head in feet of liquid necessary to maintain the
required flow into the pump is called the Net Positive
Suction Head (NPSH). This value is measured above
the vapor pressure of the liquid at the pumping
temperature.
NPSH is commonly expressed in two ways: the NPSH
required by the pump, and shown on the pump curve, is
the head needed to cover the losses in the pump
suction and the energy required to enable the liquid to
climb onboard the leading edge of the impeller vane.
The NPSH available is that inherent in the system,
taking into account friction losses in suction piping,
valves, fittings, etc. In all cases, the NPSH available,
measured above vapor pressure, must exceed the
NPSH required in order to push the liquid into the pump.
B. CHANGING PUMP SPEED. Changing the speed of
a centrifugal pump affects the capacity, total head,
NPSH required and the brake horsepower. In general
the capacity will vary in a direct ratio with the speed,
whereas the total head and NPSH required will vary as
the ratio of the speed squared. The brake horsepower
will vary as the ratio of the speed cubed.
C. EFFECTS OF VISCOSITY. The pump is designed
to deliver rated capacity at rated head for a liquid with a
particular viscosity. When pump is handling heavy
viscous liquid, the viscosity of the liquid must allow it to
be pumped easily. The liquid may have to be heated
prior to starting the pump. When contemplating
operation at some viscosity other than that for which the
pump was originally designed, check with Carver Pump
Company.
D. EFFECTS OF SPECIFIC GRAVITY. The capacity
and total head in feet of liquid developed by a centrifugal
pump are fixed for every point on the curve and are
always the same for the same speed. Neither capacity
nor total head will be affected by a change in the
specific gravity of the liquid pumped. However, since the
discharge pressure in Pounds per Square Inch (PSI)
and the brake horsepower required driving the pump are
functions of the specific gravity of the liquid, both will be
affected in direct proportion by any change in specific
gravity. Therefore, an increase in specific gravity will
raise the discharge pressure and is dangerous as it
might overload the pump's driver, or exceed the pump
casing allowable pressure.
V. TECHNICAL DATA.
Specifications and operating limits should be recorded
on a Service Record Page; an example is located in the
front matter of this manual. Record the necessary
information upon receipt of the pumping unit.
Stuffing Box. The stuffing box, Figure 1, is where the
packing or mechanical seal is located.
Materials of Construction. The listed material
specifications in Table 2 are referenced for standard
catalog options.
Mechanical Data. The Key Mechanical data in Table 3
lists the maximum allowable horsepower at the
maximum allowable pump shaft speed (3600 RPM).
Dimensions and diameters are reference and given in
inches.