- Insufficient NPSH
Without adequate pressure at the suction of a pump, the pump will cavitate. This cavitation may do damage
to the pump but it
also will interfere with the pump's ability to produce the desired head and flow. Net
Positive Suction Head (NPSH) is of particular concern when: 1) you have a suction lift
condition, and 2) when you are pumping hot fluid (greater than 140° F . and 3) when
fittings or accumulation of solids at the suction of a pump cause throttling (a pressure
drop) at the suction of the pump
- Air pockets in suction line
If your dry pit pump has any high points in the suction line, these
can capture air pockets, most piping systems provide vents at high points such as these,
to enable the air to be vented. See also pump not primed.
- Pump not primed
If water cannot get to the impeller of the pump, the impeller cannot
lift it. Most centrifugal pumps lack a significant suction lift capacity, if the water
level is not at least as high as the impeller, a lack of prime results.
- Gas binding of pump (gas trapped in eye
of impeller)
This problem is similar to lack of prime but is caused by gasses entrained in the
pumpaged collecting in the eye of the impeller during operation. This problem is generally
characterized as the pump operating normally for a while, then gradually the flow trickles
off to basically zero. We have found that the simplest method of combatting this problem
is to set up the controls for the pump to allow frequent "rest periods" of a few
minutes during which the pump does not operate. Pumps used in anaerobic digesters are
particularly prone to this problem.
- Suction air leaks
Suction air leaks can cause many problems such as cavitation, lack
of prime or gas binding. They are generally audible by listening around the suction
manifold / suction piping.
- Packing air leaks
This is the same problem as the one above, only a little more
subtle.
- Vortexing in pit at inlet
When the influent water level approaches the suction of the pump, a
vortex will appear. The center of this vortex is a path for air to enter the pump. This
will cause cavitation. This phenomenon gets more
pronounced in pumps with large amounts of flow, and the effects can be more severe in
large sumps (because the vortex is present for a longer period of time). This problem can
sometimes be solved by attaching a suction pipe to wet-pit pumps, or modifying the suction
piping of dry-pit pumps or setting the controls to shut the pump off before the vortex
appears.
- Intake openings blocked
See lack of NPSH
- Pump speed too high, or impeller dia
too large
This is a common problem, when
pumps are specified, the natural tendency is to be conservative. It would be very
embarassing (for the engineer who specified the pump's performance characteristics) to
install a pump only to find that it does not pump because it cannot overcome the static
head in the system. In addition, (and for the same reason) equipment manufacturers (ie
heat exchanger manufacturers) tend to overstate the head losses through their equipment.
The result is installing a pump that was designed to operate at 500 gpm @ 30' of head,
actually grossly cavitating at 900 gpm @ 6 or 7 feet of
head. From the owner's standpoint this is no better. The way to prevent this happening is,
if the pump is replacing an existing pump, to measure the pressure readings at both the
suction and discharge of the current pump, determine the difference, this is exactly how
much head the current pump is actually seeing. A little common sense is also helpful, if
your new pump is recirculating, (discharging into the same vessel that the suction is
drawn from) it is unlikely that the system head requirement is 50 or 60'. The solution for
the incorrectly-sized pump operating in a cavitating mode is to either rewind (or replace
the electric motor) to operate at a lower speed or replace the pump's impeller with a
smaller diameter one.
- Pump speed too low, or impeller dia too
small
If your pump is producing insufficient
flow the answer is to increase pump speed or install a larger impeller. Be aware though,
of the pump's ability to tolerate this higher flow. Pumps will self-destruct if asked to
run at too high flow.
- Fluid excessively hot
When a liquid reaches it's flash point, 212°F @ sea level
(14.7psia) for water, bubbles of vapor form in the liquid. As the pressure drops, this
"flash point" temperature also drops. Inside a centrifugal pump the eye of the
impeller is just such a low pressure area. If the temperature is sufficiently high, (about
140°F) this boiling will occur inside the pump. This is cavitation.
The solution is to reduce the pump speed or impeller dia.
- Pump too near wall or floor
Installing the pump too near the wall or floor in wet-well and
submersible pumps can set up resonance during operation. The solution is to move the pump.
- Pump rotation incorrect
Three phase motors have three wires going to them, reversing any two
wires will reverse the pump rotation. This should be checked carefully during
installation. Be aware that florescent lights operate at household frequency (60 Hz in the
U.S.) this "strobe" effect can fool you into thinking that the pump is turning
the opposite direction.
- Pumping against too little
backpressure
If the pump does not have
sufficient backpressure against it (also known as "running off the end of the
curve") it will cavitate. Some pumps will cavitate
worse than others, depending on impeller dia or horsepower. See also Too high rpm
- Pumping against too much backpressure
When the pump puts out very little flow,
compared to it's design capacity, the pump is operating near it's shutoff head. This can
damage a pump, (it is also not very efficient) see pump speed
too low.
- Excessive weight or strain on pump
flanges
PUMPS SHOULD NOT BE EXPECTED TO
SUPPORT PIPING. Suction and discharge piping should be independently supported to
prevent the pump being put into a bind. This is also why flexible pipe couplings should be
installed onto pumps, if the pipe does not line up perfectly, the pipe should be adjusted
to fit, do not move the pipe into position with a crowbar and hold it in place with the
flange bolts.
- Pumped fluid abrasive or corrosive
Fluid with suspended grit will eat the inside of a pump. It's only a
matter of time. Most manufacturers should be able to supply pumps with special materials
for the casing or impeller to mitigate the effects of grit or acids. Unfortunately, the
usual solution is to replace the parts on a regular basis.
- Specific gravity higher than expected
Horsepower required to drive a pump changes in direct proportion
to specific gravity. Specific gravity is the weight of a volume of liquid as compared
to the same volume of 60° F water. A pump running a given rpm will lift liquid to a
certain height, wether the specific gravity is 1.0 or 2.0, but the discharge pressure and
power requirement will be twice as high for the 2.0 sp.gr liquid as the 1.0 sp. gr liquid.
The reason the centrifugal pump world uses the term "head" instead of
"pressure" is because head is unaffected by specific gravity.
- Viscosity higher than expected
High viscosity liquids, like sludges and some petroleum products,
require more pressure to get moving. If the flow that the pump is producing is
insufficient, see impeller dia too small
- Operation at low flow
See operating against excessive
backpressure
- Improper parallel operation of pump
Parallel operation of pumps is common, for instance, a duplex lift
station where both pumps occasionally work at the same time, pumping into a common header.
The risk of this type of system is that since the duplex station only has the head
capacity of one pump (and the flow capacity of both) the common discharge system may not
permit enough flow for both pumps to operate "on the curve" (in a satisfactory
head flow condition) see operating against excessive
backpressure.
- Improper series operation of pump
Series operation of pumps is when two pumps are installed with the
discharge of the first leading into the suction of the second. This is a touchy
installation. The problem is that the resulting system has the flow capacity of the
smallest pump, and the total head capacity of the sum of the two pumps head capacity. If
the two pumps are not carefully matched, the booster pump can potentially have NPSH problems if the first-stage pump is of insufficient flow. Or
the resulting system can have insufficient backpressure if
the head requirement is underestimated.
- Loss of phase (poly-phase electrical
motors)
Have a competent electrician check
the power supply
- Low voltage
Local power companies are loathe to admit this, but occasionally
polyphase power is not up to snuff, check it if in doubt.
- Phase-to-ground leakage
This potentially dangerous condition should be checked by a
competent electrician.
- Pump discharge blocked
This problem rarely occurs in Vaughan pumps because of their
positive chopping action (influent is chopped into small enough bits that downstream
plugging rarely happens). To verify, observe the discharge pressure gauge, is the pump
producing shutoff pressure? If so, something is plugged or closed off downstream of
the gauge. If the gauge reads something less than shutoff head the problem is probably
upstream of the gauge.
- Misalignment of pump / driver
Proper alignment is critical, Vaughan pumps do not need alignment
because of the rabbeted fits used to mount the pump to the motor. But for other pumps, be
sure to align the shafts per the pumps (or coupling's) owners manual.
- Foundation not rigid
A pump that is not securely fastened to the floor, or is supported
by a flimsy base, will vibrate. Vibration wrecks pumps.
- Disintegrator tool wrapped with rags
Vaughan pumps are sometimes equipped with a "disintegrator
tool" on the outside of the pump to break up large solids or objects too big to get
into the pump. Fibrous material, like hair, wrags, string etc, can collect on the tool,
blocking the suction and causing vibration. In these rare instances, the tool should
simply be either replaced with a bolt (older models), or replaced with a set screw (new
pumps).
- Worn bearings
If you have access to a vibration analyzer, you can troubleshoot
worn bearings quickly. Replace bearings using the manufacturer's service manual.
- Loss of fresh water to stuffing box
Packing, and to a lesser extent, mechanical seals, need clean flush
water to lubricate them. If this supply is interrupted, even for a short period of time,
the pump can be damaged.
- Packing improperly installed
Carefully check the manufacturer's service manual for the correct
size, type and organization of packing, on some pumps, if the packing is not installed in
the correct order, the flush water may not work and the pump will be damaged.
- Improper packing material for
application
Packing comes in many different
materials, be sure to select the correct one for your service, call the pump manufacturer
or a packing supply house for help.
- Shaft sleeve worn
If the shaft sleeve is damaged, it is impossible for the mechanical
seal or the packing to prevent leakage. Replace the sleeve if necessary
- Packing gland too tight
Tightening the gland too tight will prevent flush water from cooling
the outer rings of packing, destroying them and eventually the packing sleeve and pump
shaft.
- Dirt or grit in sealing fluid
Contaminated flush water (or barrier fluid) can be as bad as no
barrier fluid. see loss of fresh water.
- Flooding of oil reservoir
Vaughan pumps do not need to have the oil changed unless the oil
gets contaminated.
- Overfilling of oil reservoir /
thermometer effect
When pumps run, they heat
up. When oil heats up, it expands. If the pump is FULL of oil when it is cold, the oil
will escape wherever it can. When the pump cools down, the oil level drops. If a
maintenance person sees it then, he is likely to add more oil, a vicious circle. Try to
determine the point of oil level equilibrium, and do not overfill.
- Mechanical seal failure
The first and most obvious evidence of seal failure is a drop in the
oil level. Vaughan pumps are equipped to detect this drop in oil level and shut off the
pump to avoid destroying the bearings and other internals. Clean or replace the seal.
- Bent shaft
Attach a dial indicator to the exposed portion of the shaft if
possible. Shafts are difficult to straighten and usually must be replaced.
- Cutter bar or impeller worn
Vaughan pumps chop by the action of the sharpened impeller passing
over the suction plate (cutter bar). When these parts get dull, chopping efficiency may
drop. The impeller and cutter bar are an extremely hard alloy steel that is case hardened
to Rockwell C 60 minimum, maintaining base strength for chopping. Replace these
parts only when problems occur with either chopping or pumping efficiency.
- Impeller damaged or loose on shaft
An impeller that is not secured properly to the shaft can lead to
premature pump failure.
- Lack of lubrication
See seal failure
- Improper Repair / Installation of
bearings
If the pump has been recently
rebuilt, it is probably a good idea to double-check the work against the factory service
manual. Or call the factory, they can help.
- Dirt in bearings
Contaminated oil is the number one cause of bearing failure.
- Impeller hitting internal cutter
This is specific to Vaughan pumps, but the cutter
bar has an internal cutter intended to increase chopping efficiency. If the cutter bar
is not spaced properly from the impeller, the impeller will hit the internal cutter,
causing a high frequency vibration. This vibration will clear with time, or should
be reshimmed if excessive.
- Cavitation
Cavitation is the phenomenon of boiling in a flowing liquid at
normal temperatures, which results from low pressure. The formation of the bubbles is
generally not too severe of a problem, but when the bubbles collapse, they do so
violently, research indicates pressures of as much as 100,000 psi in the immediate
vicinity of the collapsing vapor bubble. This violent collapsing can be heard outside the
pump as a sound like pumping rocks. Cavitation can wreck any pump in a short period of
time and should be avoided. The way to prevent this from happening is to measure the
pressure on the suction and discharge of the pump multiply the difference between the
gauge readings (in psig) by 2.31 to determine the head in feet that the pump is pumping
against. Compare this number to the curve for your pump/impeller combination. The pump's
curve should intersect the head value you calculated at a point on the solid (allowable or
safe) portion of the curve.
