Fluid slip is a common term used to describe reverse fluid flow inside a pump or other turbomachinery. Slip is affected by internal clearances of the parts, temperature, pressure and viscosity.
In a positive displacement pump, slip can be easily calculated just by looking at the flow being produced while in operation and subtracted from the nominal flow rate of the pump per one revolution.
Another way to determine the amount of slip is looking at the pump curve. At 0 psi, the pump is producing its nominal flow at the RPM. Notice as the pressure increases running at the same RPM, the flow will decrease. The difference between the operating pressure and 0 psi is the amount of fluid slip the pump is experiencing. This method is more of an approximation whereas the above measured flow minus the nominal flow per revolution is more accurate.
Why Should we Care About Slip?
Most importantly, too much slip can increase wear and decrease pump life, especially if the fluid is abrasive or has solids. Abrasive fluid passing through the clearances of the internal parts has the same effect of sand blasting. This opens up the clearances even more, amplifying the issue.
Next, too much slip can increase the cost of operation and loss of efficiency. In a positive displacement pump, if you are not getting the desired flow rate the first thing you do is turn up the speed, but doing this also increases the required power needed to meet same flow and pressure.
Excessive amounts of slip will introduce heat into the pump. This is critical in both the Progressive Cavity and Internal Gear Pump lines. Elastomers in the Progressive Cavity’s stator, or any rubber, has a set life limit based on how much heat it can absorb. The more heat it absorbs, the shorter its life.
The clearances in the Internal Gear Pump are extremely tight and with extra heat introduced can cause the rotor and idler to expand and potentially lock up against the head or casing. As a side note, if the safety relief valve is being used as a flow throttling mechanism, it can also cause the rotor and idler to expand as well.
How do you Decrease Slip?
When sizing positive displacement pumps, our guidelines is to keep slip under 15% of the desired flow rate. One option to achieve this is to choose a smaller pump, but keep in mind RPM restrictions. When sizing Progressive Cavity pumps, our guideline is nothing faster than 300 RPM, to minimize the amount of heat generated to maximize stator elastomer life.
“Summit Pump Man, I don’t want to change my existing pump size,
what else can I do to minimize slip?”
Reduce the pressure of the system. Reduce the all unnecessary fittings, increase pipe diameter, operate at a lower flow rate, ensure all filters and pipe runs are clean of debris, shorten the distance the fluid has to run.
A convenient feature of the Progressive Cavity pump is the ability to simply change the stage of the pump. For example, increasing a 2-stage rotor and stator to a 4-stage will reduce the amount of pressure per stage the pump experiences. Ultimately, reducing the amount of slip by a factor of the stage change. As with everything, there is always a tradeoff, torque and power required will increase and resulting factors need to be examined, such as motor size, pump frame size and location space available.
We all know that proper alignments between the pump and the driver are critical. Did you know that the bearing load increases in direct proportion with the misalignment? But even more importantly, a bearing load increase will decrease the bearing life by a cubic function. Simply stated, if the bearing load increases by a factor of two, due to misalignment, then the bearing life decreases by a factor of eight.
Warning: All pumps must be final aligned in the field. No matter how precise the alignment from the factory, it will be lost when the unit is shipped and installed.
Baseplates: Flat and Level
Baseplate flatness has become an issue in recent years as customers are looking to simplify installation and alignment and reduce MTBF (mean time between failures), while attempting to also reduce costs.
Most of us understand that the pump baseplate has to be level. The biggest reason for that criterion is the oil level in the bearing housing. It is possible to have too much oil on one bearing (for example the radial bearing) and not enough on the other bearing (thrust bearing). If the proper oil level is the middle of the bottom ball then it doesn’t take a whole lot of “being un-level” to miss this crucial mark.
What is flat? A typical fabricated steel baseplate for an ANSI or standard industrial pump will be flat to within 0.005 inches (0.127 millimeters) per foot (0.3048 meter).
Example: If the distance between the pump mounting pad and the motor mounting pad is 4 feet (1.2192 meters), then the motor pad can be 0.020 inches (0.508 millimeters) higher or lower than the pump pad. If a pump pad is one foot (0.3048 meter) long, it can be 0.005 inches (0.127 millimeters) lower or higher on one end.
Process Industries Practices (PIP) (and API610) bases for the same model can be 0.002 inches (0.0508 millimeters) per foot (0.3048 meter), or less than half of the standard baseplate. Benefits
A flat surface will allow the installer to more effectively level the baseplate prior to grouting and then align the motor to the pump by shimming under the motor feet. When attempting to align the pump and motor shaft to within 0.002 inches (0.0508 millimeters) TIR (Total Indicator Run-out), it is easier to start out on a level playing field rather than have unequal shim stacks under each foot of the motor.
Good millwrights and pump technicians/mechanics know to check for *soft-foot before they start the pump to driver alignment.
*“soft-foot” is where one or more feet are not in contact with the mounting pad when the unit is in the unbolted condition (this should be corrected by shimming, not by tightening the bolt which may distort the motor frame).
If you had a soft-foot of 0.016” and then bolted the motor down anyway; you have just introduced 0.008” of strain/stress into the driver. There is now potentially 0.008” of offset between the front and rear bearings, regardless of the motor size. If you do a similar offset to the pump, again you have introduced an internal stress on the bearings and shaft. Even without soft-foot on either pump or motor, if the baseplate is not flat, you have just done the same thing to the driver and or motor. Bases must be level and flat.
Once a year I attempt to remind all Summit Pump distributors of the “Plug and Play” myths
that unfortunately persist in the pump universe, like fake moon landings and that the earth is flat.
Please make sure you and others on your staff know these 5 key points:
1) OIL: A Pump shipped from the factory does NOT have oil in the bearing housing.
Someone at the pump system site must set the proper amount of the proper oil prior to startup. It is a violation of several federal laws to ship oil in the pump, as oil is considered a hazardous substance.
2) IMPELLER clearance: Final impeller clearance must be set prior to startup. The factory sets the clearance at a nominal setting for the pump type and size, based on ambient temperature liquids, as the factory does not know the specific fluid temperatures or properties
3) MECHANICAL SEAL: A pump shipped from the factory does NOT have the mechanical seal set, in hopes of preventing damage to the sealing faces.
The seal should be set only after adjusting impeller clearance, pump alignment, and rotational checks have been completed
4) ROTATIONAL DIRECTION: A pump shipped from the factory will NOT have the coupling spacer installed because you must first complete the driver rotational check. Additionally having the coupling removed helps in the process to set the impeller and seal. We have a 50% chance of guessing your local electrical phase rotation. If we are wrong, the pump becomes scrap metal.
5) ALIGNMENT: A pump shipped from the factory will NOT be precisely aligned to the driver. The factory conducts/logs a rough alignment check during Assembly.
Even if we precision laser aligned the driver to the pump in accordance with NASA and USA Space Force standards, the very Nano-second the skid is picked up by a forklift or other device that alignment will disappear.
Note, that industry best practices (*1) dictate that a driver to pump alignment be checked/adjusted at least 5 times prior to startup. If you don’t know or are unsure about these 5 alignment stages, please check with your Regional Sales Manager.
A warning tag is attached to each pump to communicate these 5 key steps to the end user/installer. Of course these steps have always been stated in the IOM. The IOM is included with every pump, and can also be downloaded from our website in at least 5 languages.
Retort / Conclusion
Several people have retorted that the competition does these 5 things and so their pumps are “plug and play”.I have checked with several knowledgeable and key sources at these competitive firms and that URBAN MYTH is simply NOT true.
As a matter of fact, the other OEMs state they have the same errors / issues with their end users not heeding the warnings on installation and startup.
Exceptions: I will venture to state that perhaps some distributors may offer these 5 key steps as part of their value package. If you do then you are best in class and get a gold star.
More than a minute… Extra Credit Post Script: On a recurring basis we have people rotate ANSI pumps backwards, consequently that trips the motor on overload. Why? Because the impeller will unscrew and “mate” with the casing. The operator subsequently corrects the directional issue (phase rotation), but does not disassemble the pump to check and correct the resultant damage.
Please note that if the impeller has “mated” with the casing there is a very high probability (99%) that the impeller will require replacement, repair and or rebalance, the casing will also require repair, and the shaft is now bent beyond specification, further the bearings and mechanical seal have been mechanically shocked.
Rotation in the wrong direction is a costly mistake.
In addition to my monthly column “Common Pumping Mistakes” the Pumps & Systems editors have asked me to write an article on the common ways that the systems can also reach out and bite you.
you own a home, a car or for that matter any machine—from a lawn mower
to a dishwasher—you already know that all things left to themselves
never get better. It is a law of our universe captured eloquently by the
word entropy (for the engineers I am using a loose second definition).
those of you that already own, operate or maintain a mechanical,
pneumatic, steam, hydraulic or electrical system, you carry all the
battle scars from these real life lessons. As a review for those that
already know and as a primer for those that are new to this area, I
offer in no particular order, some discourse on the subject.
system responsibility—whether a building with HVAC systems, a power
plant, a chemical processing or manufacturing facility, a paper mill, a
brewery/distillery, wastewater treatment or a manufacturing center—is
comprised of machinery: electrical, mechanical, steam, pneumatic and
hydraulic. Every system and component has a finite life.
All machinery, operating or idle, will eventually fail. Machinery fails for the following basic reasons: corrosion, erosion, stress or impact. Your job is to prevent it from failing or failing at the wrong time. When the machine does fail, we hope to manage it in a predictable and safe manner.
Ask yourself if you and your staff have the proper training and
equipment for flash protection, confined entry, general electrical
safety, chemical handling, radiation exposure, noise,
noxious/carcinogenic or poisonous gases, safety chains, ladders,
rigging/lifting equipment, signage, rotating machinery and OSHA guards.
in all of these areas is very important. Additionally, consider whether
the current training is adequate for new personnel or is simply a
review of procedures for the seasoned employee. Note that most injuries
occur during non-routine or emergency evolutions.
Manage by Walking Around
you do not manage the system, it will manage you. Choose a new system
every day and grab the system logic process diagram for review. Do you
really know how the system operates? If you do not know how the system
operates, you will not know if anything is wrong until it is too late.
So, the prudent thing to do is learn the system. With a flashlight, rag,
clipboard and a camera, walk and trace the system “hand over hand” to
look for issues and changes.
Look for leaks, wear, erosion,
corrosion, abrasion and discoloration. Look for unauthorized additions
and alterations. Use all of your senses including smell and hearing.
Look for what has changed in the system operation, starting with
different pressures, flows or levels. Check and review system logs for
Most plant issues occur when there is a change in the status quo.
The biggest “corrupter” of heat exchangers is fouling and corrosion. (From a dictionary aspect, corrupter
is not the correct word choice to describe the issue, but I insist in
this case it is appropriate). Both fouling and corrosion lead to a
reduction in performance, and in some cases to erosion with
consequential and undesired leaks. Determine the exchanger performance
or heat balance by measuring the flows, differential pressures and
temperatures on both sides of the unit. Compare readings and thermal
balance results to prerecorded, new or clean parameters. Heat exchanger
maintenance or replacement is time consuming and costly, so do not wait
for catastrophic failures when a few simple checks can help you predict
the need for repair or replacement.
Because my heat exchanger
experience is with nasty fluids at high pressures and temperatures, I
have a prejudice for shell and tube exchangers. I am also aware that
plate and frame exchangers offer many advantages and that the newer
designs have improved capabilities and reliability.
On shell and
tube type exchangers, the tubes can plug from the process fouling,
corrosion or debris. From past tube failures, there may also be tubes
that are manually plugged and taken out of service. An accepted common
industry maximum level of tube plugging is 10 percent, but I caution
users to always consult the manufacturer since some designs will demand a
lower percentage. According to Tubular Exchanger Manufacturer
Association (TEMA), the acceptance level for “U” tube designs is only 1
percent. Whatever the acceptable range is, at some point plugged tubes
will need to be replaced or the heat exchanger performance derated. You
can also clean the tubes and shell side as a maintenance procedure.
and frame heat exchangers are less susceptible to fouling due to
designs that yield higher fluid velocities and turbulence in the
channels. Plate and frames are also easier to clean, maintain and change
performance dynamics. My caution to personnel adding, changing or
cleaning plates is to be very careful of the plate orientation and
design. The most common issue I witness is how easy it is for plates to
be installed incorrectly—in the wrong order or the wrong plate
altogether. The other issue is with gasket leaks due to inadequate
torque or mishandling.
Control and throttling valves have exposed trim, stems and seats that will wear and corrode, which not only changes the pressure drop across the valve, but pieces of the valve/trim will lodge in downstream machinery. Another issue is with zealous mechanics that tighten the packing so tight the valve actuator cannot develop sufficient torque to operate the valve. The end result is a system not operating properly and the expensive actuator destroyed.
Note that proportional programmable logic control (PLC) valves will
drift and the settings will change with time, temperature and vibration.
When was the last time the valve settings were checked?
valves in the suction line of self-priming pumps will become jammed open
or closed with debris, destroying an expensive pump and then causing a
loss of service. Other styles of non-return valves/check valves can fail
or wear, allowing process flow to reverse direction. Pumps running in
reverse are a common cause of broken shafts. How often do you check that
your non-return valves are working and the spring factors are correct?
first two universal laws are that; 1) concrete will crack and 2) flat
roofs will leak. The third law is that strainers will clog and starve
the downstream equipment, typically a pump. I am simply amazed by the
number of strainers I observe with no means of determining the
differential pressure. A simple inexpensive duplex gauge will indicate
when the strainer is clogged and can save thousands of dollars.
induction motors draw high amps that force the motor to operate above
nameplate temperatures. This condition will rapidly degrade the
insulation and the motor will burn out. The quality and integrity of the
power supply such as voltage excursions and issues with blocked
ventilation or frequent starts and stops will also affect the insulation
life. As a general rule of thumb, insulation life doubles for each 10 C
of unused insulation temperature capability.
Operators/owners that believe it is perfectly acceptable to operate motors in the service factor range will quickly become the favorite annuity for the motor sales people. You can easily and safely determine your operating motor stator temperature with an infrared thermometer.
There are several motor health tests that can easily be conducted
such as electrical resistance (Ohm resistance between phase legs),
insulation resistance (megger test), running amps and voltage. Perhaps
better left to trained technicians for more in-depth testing are surge
frequency and partial discharge tests. Several companies offer motor
analyzers and training.
Note that some technicians consider the
megger test as a potentially destructive test. Depending on the voltage
level used, you can possibly force a failure of the insulation system.
motors, if operated in the proper pressure and temperature range with
clean fluid will perform very well for a long time. Like everything else
in a hydraulic system cleanliness is paramount. I cannot overstress
this point. These systems will run reliably for a long time as long as
the oil is clean and cool. An abundance of caution is required to keep
all dirt and contamination from the system.
motors will perform well if the air quality is consistent to the motor
design requirements. The air supply to the motor must be clean and dry
which means filters and moisture separators must be maintained properly.
Additionally the lubricators that are critical for motor efficiency and
long life (prevent corrosion) must be in working order with an adequate
supply of the correct oil.
No matter if it is a small dry insulated 1 kilovolt or a large 1,000 megavolt FOA (forced oil and air) cooled transformer, the insulation will degrade consistent with age and loading. Power factor and insulation resistance tests can be conducted to determine insulation integrity and life assessment. Oil cooled units must have an oil analysis conducted on a regular basis for water and gases (gases such as carbon monoxide and carbon dioxide, hydrogen, oxygen and methane). You need to be especially concerned with the combustible gases acetylene, hydrogen and methane as these are indicators of a serious condition called corona (arching and short circuiting in the windings) and will, if left unfettered, lead to catastrophic failure.
What good is an emergency system if it does not
work in an emergency? Battery backups for ancillary and tertiary
uninterrupted power supplies (UPS) and direct current (DC) controls must
be in a fully charged and ready condition. Batteries of all
technologies, but especially lead acid types, have well defined life
spans that are severely shortened if not maintained properly. Specific
gravity and fluid levels must be correct. Dead cells need to be
bypassed. Amps draws should be periodically checked and corrosion on
connections cleaned and addressed. Most lead acid batteries should have a
specific gravity of 1.265 to 1.280 at an ambient temperature of 78 F.
full safety precautions and certified training—inspect electrical
enclosures, conductors and terminals for tightness, discoloration and
heat using infrared technologies. Aluminum connectors are a special
consideration and must be checked for proper torque and treated for
oxidation on a periodic basis. Aluminum conductor terminals will loosen
with time due to the heating and cooling cycles associated with loading
and unloading. This cycle of expansion and contraction will cause the
connection to loosen and will subsequently require re-torqueing.
Many of the older breakers contain asbestos arc chutes. Asbestos is the best material to handle the extremely intense heat of arc interruption but presents other carcinogenic problems when handled during maintenance.
SF6 (sulfur hexafluoride) gas is found in many of the
higher voltage breakers because of its unique and inherent dielectric
properties. SF6 is a colorless, odorless, tasteless and nontoxic heavy gas. The danger of SF6
gas is that it displaces oxygen (air), and therefore can potentially
cause suffocation. From an environmental perspective, it is potentially a
very dangerous greenhouse gas, but because it is so heavy compared to
air (does not naturally rise to high altitudes like Freon) and is
confined to pressured spaces, any deleterious effect is minimized.
you have one or more high horsepower compressors (greater than 150 hp)
operating around the clock, then you already know that the cleanliness
of the air intake filters is critical and the unloader and dryer
maintenance is essential. For the smaller systems that operate some
fraction of the time the receiver tank must be (manual or auto) drained
daily, the filters and the oil changed regularly. The most common issue
after moisture elimination and air quality is a lack of oil changes. If
the compressor operates around the clock, that is 8,760 hours for the
year. If you drove your car at 60 miles per hour (mph) for the same
amount of time, that equals 525,600 miles (262,800 if you drove at 30
mph). How often would you change the oil in your car for that mileage?
air is—with rare exceptions—the most expensive utility you have in the
plant. Hire a consultant to conduct an air (leak) audit. You will be
surprised how much money is being wasted.
Since my monthly column is on pumps, I simply suggest you look over my articles of the last 3 years that can be accessed on the Pumps and Systems webpage.
Probably the most important issue I see in the field is that operators do not know where the pump is operating on the pump performance curve. The second and a compounding issue is little to no knowledge of the system curve. The intersection of the system curve and the pump curve is where the pump will operate.
In general, 90 percent of all pump problems are on the suction side
of the pump. You should think of the system as three separate systems:
the suction system, the pump itself and the discharge system. It is the
responsibility of the suction system to deliver the fluid to the pump.
It is not the responsibility of the pump to reach out and pull the fluid
to the pump, since that is not possible.
Instrumentation & Data Acquisition
acquisition systems often translate to imply data overload by an
exponential factor. Consequently, the task is to sort through the
morass of data and figure what is important and what is not. Data needs
to be translated into useful information to be of any tangible benefit.
You may require professional assistance to get started. Do not be afraid
to ask, and there are several companies that will train you or
subcontract the task.
Instrumentation and controls: if you have
them, be glad you do. Of course these systems require calibration and
maintenance just as the main systems they manage. If left to themselves,
they will also drift and fall out of calibration.
calibration programs are paramount, and yet I see few plants that have
gauges in most systems. If they do have gauges, there is no system to
maintain or calibrate the instruments. Standard type gauges will drift
over time due to bourdon tube stress, spring vitality, vibration and
shock. There are alternatives to gauges such as pressure transducers and
there are safe remedies for those other nasty locations. If you do not
measure it, you cannot manage it. I am constantly amazed by the lack of
system parameter measurement at many plants. They have no pressure
gages, no flow measurement, no level indication and no temperature
indicators and yet will be very upset when their system fails.
Lubrication and the associated system cleanliness are extremely important. Dirt, contamination and water are the biggest killers of ball bearings. Just 250 parts per million of water in the oil will reduce bearing life by a factor of four.
(standby electrical) need to operate with some periodicity to exercise
the unit and check for potential issues. Lubrication must periodically
be reintroduced to the bearings and cylinders and condensed moisture in
the fuel needs eliminated. When the main power goes out is not the time
to test the generator. One interesting issue I commonly run into when
troubleshooting generators is that the frequency settings will be set
for 50 hertz on a 60 hertz system. The other issues are lack of coolant
or coolant quality, failed block heaters and bad fuel.
are simple and basic tips to help you manage your systems. If this
article is of interest to the readers we can discuss in future articles a
host of other subjects that did not fit into this one, such as: steam
turbines, steam traps, fans, blowers, vacuum pumps, vibration analysis,
freeze damage, heat trace (electric or steam), freeze protection,
couplings, belts, sheaves, drive chains, gaskets, O Rings, elastomers
Ask a few questions about recent changes.
Did the operator change?
Was the system worked on, and if so what was the job scope?
Was the scheduled maintenance was actually performed, performed correctly and documented?
From an outsourcing perspective, did your company change vendors, suppliers or technicians?
Jim Elsey helps you avoid common centrifugal pump mistakes.
I have been writing “Common Pumping Mistakes” for Pumps & Systems
for more than three years. Typically the hardest part of the job is
topic selection so it will be fresh, educational and interesting. This
month, I am writing on a collection of shorter subjects and baking them
up into one article. Instead of a meal, we will have hors d’oeuvres.
Hopefully it will satisfy your appetite. If you have been reading my
column, many of these tidbits will be a review. These comments are based
on single-stage overhung centrifugal pumps moving ambient temperature clear water, except when otherwise noted.
Pumps are really designed to operate at only one point.
That hydraulic condition of one point of head and flow is the best
efficiency point (BEP), also known as the best operating point. Anywhere
else on the published set of curves is simply a commercial compromise.
It would be too expensive for most end users to have a pump designed and
built for their unique set of hydraulic conditions.
Pay attention to the published pump curves.
Manufacturers’ pump performance curves are based on clear water at
approximately 65 F, unless stated otherwise. They will not be corrected
for fluid viscosity. The horsepower stated may or may not be corrected
for specific gravity or viscosity.
When the manufacturers’ published pump curve stops at some point of flow and head, it is for a good reason.
Do not operate the pump at the end of the curve; if there was more
performance to be generated from the curve beyond that point, the
manufacturer would have extended the curve. Operating at or near the end
of the curve will be fraught with performance issues.
Pumps are stupid. A centrifugal pump is simply a machine, where for a given set of fluid properties, impeller geometry and operating speed it will react to the system in which it is installed. The pump will operate (flow and head) where its performance curve intersects the system curve. The system curve dictates where the pump will operate.
Understand the system curve. The system curve
represents all of the friction, static and pressure head baked into the
system. Velocity head is also present, but typically too small of a
component to be concerned about.
Pumps do not suck fluids.
This is a common misunderstanding, but realize that some energy source
other than the pump must supply the energy required for the fluid to get
to the pump. Normally these are gravity and/or atmospheric pressure.
Lastly, fluids do not have tensile strength. Consequently the pump
cannot reach out and pull fluid into the suction.
The maximum realistic suction lift is about 26 feet.
See the previous section where pumps do not suck. If you are at sea
level the atmospheric pressure will be 14.7 pound per square inch
absolute (psia), which translates (multiply by 2.31) into about 33.9
feet of absolute head. So, in a perfect world, if there was no fluid
friction or vapor pressure working against the system you might be able
to lift cold water 33 feet.
In reality, fluid friction and the
negative consequences of vapor pressure will work against you and
preclude fluid lifts of much more than 26 feet. Always calculate the net
positive suction head available (NPSHA) and compare to the pump’s net positive suction head required (NPSHr) value. The higher the margin, the better.
A pump running backwards does not reverse the flow direction. The flow will still go in the suction and exit from the discharge nozzle. Depending on the specific speed (Ns) of the pump (think impeller geometry), the flow and head will be reduced by some significant amount because the pump is much less efficient. For lower specific speed pumps the flow will be approximately 50 percent of rated and the head will be 60 percent of rated. An American National Standards Institute (ANSI) pump running backwards will cause the impeller to unscrew from the shaft and lodge itself in the casing.
You cannot vent air from the impeller eye of an operating pump.
A pump is in many ways like a centrifuge, and so the heavier water is
expelled to the outside diameter and the lighter air remains in the
middle or center. The pump should be at rest to be properly vented.
Pumps with centerline discharges are essentially self-venting.
Industrial pumps do not come from the factory ready to “plug and play.”
There are exceptions to this comment, but never assume. The pump will
require oil to be added to the bearing housings. The impeller clearance
must be ascertained and set for the fluid (temperature) to be pumped.
The driver will need to be aligned to the pump. Yes, the alignment may
have been performed in the factory, but the second the unit was moved
for transport the alignment was lost.
You will need to check alignment again after the piping is installed, and again when the base is grouted in. The direction of rotation should be ascertained and matched to the phase rotation on the motor driver.
The mechanical seal will need to be set after these other steps are completed. Most manufacturers do not install the coupling at the factory because it will just need to be removed for all of these aforementioned reasons.
Almost all pump problems occur on the suction side.
There is a common and pervasive misunderstanding about how pumps work.
Refer to above as a reference. Think of any pump system as three
separate systems when trouble shooting issues in the field. The suction
system, the pump itself and the system downstream of the pump. In my
years of working on pumps and solving issues, 85 percent of pump issues
occur on the suction side. When in doubt, it is a great place to start
looking for the solution.
Always, always, always calculate the NPSHA.
This is likely the most common and the most expensive mistake I witness
in the field. People will erroneously think that because they have
plenty of suction pressure or a flooded suction there is no reason to do
these calculations. A few feet of friction or additional losses due to
vapor pressure can wipe out that NPSH margin you thought you had. Insufficient NPSHA will result in cavitation in the pump impeller.
NPSHr has nothing to do with the system and is determined by the pump manufacturer. NPSHA
has nothing to do with the pump and should be determined or calculated
by the system owner or end user. I recently heard a phrase that the
“pump becomes grumpy and grouchy” when there is an insufficient NPSH margin.
Understand cavitation. Cavitation is the formation of vapor bubbles in the fluid stream due to a drop below the vapor pressure of the fluid. The formation of the bubbles typically occurs just in front of the impeller eye since this is typically the lowest pressure in the system. The bubbles subsequently collapse downstream as they enter a region of higher pressure. The bubble collapse is what causes the damage to the pump impeller.
Cavitation causes damage. If the bubbles collapse in
the middle of the fluid stream there is almost no damage. But when the
bubbles collapse near or at the metal surface, they collapse
asymmetrically and cause a small microjet. This collapse occurs on a
nanoscale (1.0 x 10-9 or billionth). Local pressure forces involved can
be higher than 10,000 pounds per square inch gauge (psig) (689 bar) or
more, plus there is heat generated. This phenomenon can occur at
frequencies up to 300 times per second and at speeds near the speed of
sound. Note the speed of sound in air is approximately 768 miles per
hour (mph) (1,236 kilometers per hour [k/h]) and varies somewhat with
humidity levels. The speed of sound in water is 4.4 times faster at
about 3,350 mph (5,391 k/h or 1,490 meters per second [m/s]). Because I
started my career in the submarine world, I have to point out that the
speed of sound is even faster in salt water.
Cavitation damage can occur at different locations on the impeller.
“Classic” cavitation damage will occur approximately one-third of the
distance downstream of the eye on the underside (low pressure side or
the concave side) of the impeller vane. “Classic” because it is due to
Cavitation damage may manifest at other locations on the impeller, but
those instances usually are due to recirculation issues that are caused
by operating the pump away from its design or BEP.
Cavitation is audible in the lower ranges.
If you hear the cavitation noise (sounds like pumping gravel), it is
likely cavitating. Just because you don’t hear the noise means nothing,
since the majority of the noise range is outside the range of human
hearing. Perhaps we should train dogs to help us detect cavitation? Cold
water is typically the worst fluid for the consequential damage from
Hydrocarbons have minimal effect from a damage aspect.
Hydrocarbon correction factors exist and are based on empirical data.
The rules for correction factors are covered in the Cameron Hydraulic Data book.
NPSHr is NPSH3. When a manufacturer states that the pump requires a certain amount of NPSHr at a given point, realize that the pump is already cavitating at that point with a 3 percent head drop because that is how NPSHr is measured. All the more reason to assure you have adequate margin.
Critical submergence is necessary to prevent vortexing. The vertical distance from the surface of the fluid to the pump inlet is the submergence level. The distance required to preclude air ingestion due to vortexing is the critical submergence level.
To preclude the ingestion of air, do not operate the pump when the
fluid level is below the critical submergence. The vortexing phenomena
is a direct function of the fluid velocity. You can preclude vortexing
by the use of baffles and/or larger pipe diameters such as bell flanged
inlets. There are numerous reference charts on submergence to use when
looking at the suction side design. The best one would be from the
Hydraulic Institute. A conservative rule of thumb is to have one foot of
submergence per foot of fluid velocity.
Pumps cannot efficiently
move fluids mixed with air if the percentage is greater than 4 or 5
percent. Most pumps start to lose performance around 2 to 3 percent air
entrainment. Almost all pump designs will cease to perform at around 14
percent entrainment. Exceptions can be disc pumps, self-primers and some
vortex or recessed impeller type pumps.
My pump bearing feels hot.
This is a common comment, but it is subjective, not objective. It is
difficult for the typical person to hold their hand on a bearing housing
that is over 120 F.
It is perfectly normal for a bearing to be
operating at 160 to 180 F. Use a thermometer or infrared device to
measure the temperature and deal in facts.
Viscosity is the kryptonite of centrifugal pumps. Most centrifugal pumps become too inefficient or exceed their horsepower (hp) limits in a viscosity range between 400 and 700 centipoise that depends on pump size. Always check with the manufacturer when pumping viscous fluids for corrected curves and power limits for the frame, bearings and shaft.
Horsepower requirements progressing along the pump curve change for different impeller geometries.
Low and medium specific speed pumps require more hp the farther out on
the curve you operate, which is fairly intuitive reasoning. For high
specific speed pumps (axial flow), the highest hp required will be at
the lower flows. This is also why it is common to start up these types
of pumps with the discharge valve open so as to not overload the driver.
There is a simple way to think of specific speed.
Specific speed (Ns) is a tool used by designers to look at the
performance and geometry of a hypothetical impeller. Don’t want to get
all caught up in the math involved? A low specific speed impeller will
have the flow enter parallel to the shaft centerline and leave the impeller at 90 degrees to the centerline. A medium specific speed impeller will enter parallel to the shaft and exit the impeller at 45 degrees to the centerline.
A high specific speed impeller will operate with the flow entering parallel to the shaft centerline and leave parallel to the centerline.
Some people still think they can align a pump with a straight-edge, in lieu of a laser or dial-indicator method. They are incorrect. While a straight-edge alignment can potentially kill your bearings, seals and couplings, it also consumes extra horsepower.
Recent studies show that a simple 0.050” offset, which is about the best straight-edge alignment possible, will consume 9% more power. An angular misalignment of 0.015” gap per inch of shaft diameter will consume 6% more power.
At least once a month, we will receive a complaint that our pump is not aligned to the driver upon site arrival. We try to advise our distributors that the final alignment must be performed in the field. An initial rough alignment is done at the factory to prove that the unit is able to be aligned, but as the pump is transported, the unit will become out of alignment. Pump units should be alignment checked five different times during the course of the installation. (We will cover that subject another time.)
Also, note on existing installations you would never move the pump, only the driver. Although, on new installations before the piping is brought to the pump, it is perfectly acceptable and sometimes necessary to move the pump within the room allowed by the bolt holes.