Robinsons Ace Features:
All pumps use basic forces of nature
to move a liquid. As the moving pump part (impeller, vane, piston
diaphragm,etc.) begins to move, air is pushed out of the way. The
movement of air creates a partial vacuum (low pressure) which can be
filled up by more air, or in the case of water pumps, water. This is
similar to sucking on a straw. A partial vacuum is created in your
mouth when you suck on the straw. The liquid is pushed up the straw
because of the pressure differences between your mouth and the
atmosphere.
The pumping action is your mouth closing around the liquid (in your
mouth) and forcing it down your throat.
ATMOSPHERIC PRESSURE
At sea level, mother nature exerts a pressure of 14.7 psi all around
us. If one end of a tube is placed in water and a perfect vacuum is
applied to the other end, that 14.7 psi could hold a column of water
33.9 feet high. This is only obtainable at sea level and with a
perfect vacuum.
In reality, ALL centrifugal pumps can lift water no more than 26
feet at sea level. This drops off approximately 2 feet for each 1000
feet of altitude above sea level.
PRESSURE DIFFERENCES (suction)
In nature, movement is from more dense to less dense. Weather
systems are tracked as high pressures move toward low pressures. In
batteries, one end contains more positively charged particles that
move to the end with the negatively charged particles.
A liquid under high pressure will move to an area of less pressure
if a path is provided.
CENTRIFUGAL FORCE
The centrifugal pump works in the same way as sucking on the straw.
As the engine starts, the impeller turns which forces the water
around it out of the pump's discharge port. The partial vacuum
created, allows the earth's air pressure to force water up the
suction hose (straw), and into the suction (inlet) side of the pump
to replace the displaced water. When the water hits the rotating
impeller, energy of the impeller is transferred to the water,
forcing the water out (centrifugal force). The water is displaced
outward, and more water can now enter the suction side of the pump
to replace the displaced water.
SEALED SYSTEM
If a water pump is to create a partial vacuum in the pump housing,
three things must happen:
The pump must be primed. The water in the housing is essential to
lubricate the mechanical seal so that it won't wear and leak.
The suction hose, hose seals and all O-rings must be in good
condition so air can't be drawn in, losing the vacuum.
The impeller-to-volute clearance must be within specification to
achieve the proper vacuum.
HONDA PUMP TYPE DIFFERENCES
The size of the impeller and its vanes dictate what pressures,
discharge capacities and types of material that can pass through the
pump. The impeller material,and the size of the volute discharge
opening, determine what size material can pass through the pump
without damaging it.
STANDARD (WP, WD)
Deeper vanes will produce a larger discharge capacity.
MULTI-PURPOSE (WMP20X)
Specially designed pump to to allow transfer of certain industrial
and agricultural chemicals.
HIGH-PRESSURE (WH15X, WH20X)
A larger diameter impeller with more, shallower vanes will produce a
greater pressure.
TRASH (WT20X, WT30X, WT40X)
Deepest vanes produce largest discharge capacity.
Deeper vanes, incorporated with a large volute discharge opening
will pass larger debris without harming the pump components.
PUMP PERFORMANCE
The performance curves reflect standard testing. Pump manufacturers
typically calculate performance curves using a pressure gauge and a
flow meter connected to the discharge port. For any anticipated
total head, the discharge capacity can be determined.
PUMP PERFORMANCE CONSIDERATIONS
The performance curves are useful in selecting a particular water
pump. When a question regarding the performance of a specific pump
must be answered, refer to the pump specifications for the
particular model.
Determine how high the pump will sit above the water source (static
suction head). Determine how high the discharge end will be elevated
above the pump (static discharge head). Determine what the discharge
capacity (gpm) of the pump must be .
Given the total head (suction + discharge), the discharge capacity
can be estimated by referring to the performance curve.
Keep in mind, the actual discharge performance may be significantly
less than predicted by using static head alone because of fricvtion
losses in the system.
Pressure can be calculated for total head by multiplying total head
by .433. Pressure available at the end of the hose at zero flow for
a given total head (less then the maximum total head) can be
calculated by multiplying the total head by .433 then subtracting it
from the maximum pressure.
Example:
The maximum pressure for a WH20X is 71 psi (.433 x 164 total head in
feet). The maximum available pressure at a total head of 120 feet is
71 - 52 (120 x .433) = 19 psi at zero flow.
SPECIAL CONSIDERATIONS
The total static head is often only considered when selecting a
pump. However, because of frictional losses, this method can often
lead to large error, and in many cases, the pump performance will
not meet expectations. The selection process becomes even more
complicated when a nozzle or sprinklers are used.
In order to accurately predict the performance of a centrifugal pump
in a specific application, the total head losses must be considered.
These losses include, but are not limited to: total static head,
losses due to pipe size, length, and material, and losses due to
sprinklers or a nozzle.
Accurately predicting the discharge and pressure for a given pump in
a specific application requires tedious calculations and a lot of
trial and error.
Honda offers Pump Select®
software to make the difficult calculations for you.
DISCHARGE MATERIAL vs PERFORMANCE
(FRICTION LOSSES)
Another fact of nature, is that a liquid moving through a hose
creates heat due to the friction of the two surfaces (water against
hose). Steel pipe will produce more friction than will smooth PVC or
vinyl pipe. Friction INCREASES with INCREASED length of pipe, or
hose, or smaller diameter hose, and will DECREASE the discharge
capacity (GPM).
The roughness of the hose/pipe is considered in
Pump Select®
calculations.
SUCTION HEAD vs PERFORMANCE
Mother nature plays an important role by exerting only 14.7 psi on
any body of water at sea level. This limits the suction head of
centrifugal pumps to 33.9 feet. However, this would only be obtained
if we could achieve a perfect vacuum in the pump. In reality, the
suction head of centrifugal pumps is limited to about 26 feet. Pump
performance (capacity or pressure) is highest when the pump is
operated close to the water's surface. Increasing the suction head
will DECREASE the discharge head and consequently the discharge
capacity of the pump. Most importantly, suction head should be kept
to the smallest value possible to reduce the likelihood of
cavitation. Cavitation can also occur if the suction hose is
restricted. Never use a suction hose with a smaller diameter than
the suction port. Cavitation can quickly
damage a pump.
DISCHARGE HEAD vs PERFORMANCE
Mother nature plays an important role in how high we can push water.
Water is heavy; about 8.3 lbs per gallon. The old saying, "what goes
up, must come down" tends to want to bring the water back down to
its source. The mechanical energy of the impeller transmits its
force against the water coming in contact with it. This force can be
measured in psi at the pump discharge. As the pump discharge head
increases in height, the pump capacity (gpm) decreases and the
available pressure at the end of the discharge hose (if the flow is
stopped or a sprinkler/nozzle is used) will also decrease. At
maximum head, the capacity (gpm) will drop to zero and there will be
no pressure available at the end of the hose to run a sprinkler or
nozzle. If we measured the pressure at the bottom of the discharge
hose, we would read maximum head pressure which would be the result
of the pump supporting the water weight.
The performance curves show the relationship between discharge
capacity and total head.
DISCHARGE LENGTH vs PERFORMANCE
As the length of the discharge hose increases, the water comes into
contact with more hose surface. As discussed in hose material, the
inner wall of the discharge hose (in contact with the rushing water)
will cause friction to build up. The increase in friction will slow
the water, decreasing the discharge capacity.
Hose/pipe length is considered in
Pump Select® calculations.
RESTRICTION vs PERFORMANCE
Restrictions are like dams to the flow of water. When the water hits
the restriction, only a partial amount of the flowing water will be
allowed to pass through. A rule of thumb is to keep the discharge
hose as straight as possible and avoid reducing the size of the hose
whenever possible. Restrictions will INCREASE the friction and
DECREASE the discharge capacity at the end of the pipe.
ELBOWS vs PERFORMANCE
Elbows added to lengths of pipe break up the smooth flow of the
water. The turbulence created around these joints causes an increase
in friction which will DECREASE the discharge capacity
COUPLERS AND VALVES
As valves and couplers are added to lengths of pipe, the smooth flow
of the water is broken up. The turbulence created around these
joints causes an increase in friction which will DECREASE the
discharge capacity.
ALTITUDE vs PERFORMANCE (ATMOSPHERIC
LOSSES)
Engine performance DECREASES with altitude. The higher the altitude,
the less air there is available to support combustion. Maximum
engine power DECREASES about 3.5% per 1000 feet of altitude.
Less air also means there is less air pressure to push on the body
of water we are trying to draw into the pump. Because there is less
air pressure forcing the water into the pump, the maximum available
suction head is DECREASED. The reduction in engine power may also
result in a reduced discharge capacity.
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