NOTE: Some of the blue links do not work on this
version of The Tutorials.
They only work on the
In the following section we'll be looking at overviews of several
pump and seal subjects:
In these tutorials I am attempting to put each of the subjects
into perspective. You'll use the tutorials for multiple purposes:
- To learn the terms we use for each of these individual
- To see how the various subjects fit together.
- To find out how much you know about any one of the
- And you can use the tutorials as an outline to teach the
subjects to other people.
I suggest that you read the entire tutorials and then go back and
look up the details of any unfamiliar words or subjects. Any word or
phrase in blue and underlined is a link
to a detailed explanation of the subject (The links only work on
version) Most of the time I
have tried to use the link only for the first mention of the word or
phrase; otherwise the narrative would be full of links.
CENTRIFUGAL PUMP SELECTION. HOW TO PICK THE CORRECT SIZE PUMP
FOR YOUR APPLICATION.
We'll begin by deciding what operating conditions our pump has to
meet and then we'll approach pump suppliers to see how closely they
can satisfy these needs. Unfortunately no comprehensive theory which
would permit the complete hydrodynamic design of a centrifugal pump
has evolved in the many years that pumps have been around, so the
pump manufacturer will be doing the best he can with the information
you supply to him.
To clearly define the capacity
and pressure needs of our system
we'll construct a type of graph called a system
curve. This system curve will then be given to the pump suppliers and
they will try to match it with a pump curve that satisfies these
needs as closely as possible.
To start the construction of the system curve I'll assume you want
to pump some fluid from point "A" to point "B". To do that
efficiently you must make a couple of decisions:
- Decide the capacity you'll need. This means the gallons per
minute or cubic meters per hour. You must also consider if this
capacity will change with the operation of your process. A boiler
feed pump is an example of an
application that needs a constant pressure with varying capacities
to meet a changing steam demand. The demand for boiler water is
regulated by opening and closing a control valve on the discharge
side of the pump with a discharge re-circulation line returning
the unneeded portion back to a convenient storage place, or the
suction side of the pump. Remember that with a centrifugal
pump if you change its capacity you change the pressure also. A
rotary or positive displacement
pump is different. It puts out a constant capacity regardless of
- For other centrifugal pump applications, you're going to have
to calculate how much pressure will be needed to deliver different
capacities to the place where you'll need them. You'll need enough
pressure to :
- Reach the maximum static
head or height the fluid will have to attain.
- Enough discharge pressure to over come any pressure that
might be in the vessel where the fluid is discharging, such as
the boiler we just discussed. This is called the pressure
- Overcome friction resistance in the lines, fittings and any
valves or hardware that might be in the system. As an example:
high-pressure nozzles can be tricky, especially if they clog
up. This resistance is called the friction
- Will you be needing any special materials for the pump
- The pump manufacturer will try to choose pump metal
components that are chemically compatible with what you're
pumping as well as any cleaners or solvents that might be
flushed through the lines. If the temperature of the pumpage
changes, the corrosion rate can change also. His choice of
materials could have a serious affect on your spare parts
inventory. Will he be selecting universal and easily obtainable
materials? Unless you have a great deal of experience with the
product you're pumping, do not select the metal components by
using a compatibility chart. Metal selection is a job for
metallurgists, or your own experience.
- If the product you're pumping is explosive, or a fire
hazard, you should be looking at non-sparking materials for the
pump components. Don't depend totally upon the pump
manufacturer to make this decision for you. If you're not sure
what materials are compatible with your product, how will the
pump man know? Also, keep in mind that some of the fluids
you'll be pumping could be proprietary
products known only by their trade name.
- Dangerous and radioactive materials will dictate special
- Food products require high-density seal and pump materials
that are easy to clean.
- If there are abrasive solids in the pumpage you'll need
materials with good wearing capabilities. Hard surfaces and
chemically resistant materials are often incompatible. You may
have to go to some type of coating on the pump wetted parts or
select an expensive duplex
- Occasionally you'll find an application where metal is either
not compatible, or not practical. There are many monomer and
polymer materials available for these applications, but their cost
is generally higher than comparable metal parts. Be aware that if
you're using a mechanical seal in a non-metallic pump, the seal
cannot have metal parts in contact with the fluid, for the same
reasons the pump was manufactured from non-metallic materials. Use
a non-metallic seal in these
Since we're just getting into the subject, one of the first things
we should learn is that centrifugal pump people do not use the word
pressure. As mentioned in an earlier paragraph they substitute the
word "head", so you'll have to calculate the three kinds of head that
will be combined together to give you the total head of the system
needed to deliver the required capacity. Here are the three kinds of
head you'll be calculating:
- The static head or maximum height that the liquid will reach.
We must also learn how to compensate for the siphon
affect from down running pipes on the discharge side of the pump.
Remember that if you fill a tank from the bottom instead of the
top, the static head will continually increase. This is not a good
application for a centrifugal pump because the capacity is
decreasing with an increasing head. If you must fill from the
bottom, or if you'll be using the pump as an accumulator, a
rotary positive displacement pump
will be your best choice as long as it can meet the needed
- The pressure heads are next, if the container we are pumping
to, or from, is pressurized. We'll have to learn how to convert
pressure units to head units because, later on, we will need this
conversion knowledge to read the manufacturers pump curve. Pump
gages are labeled in psi or bar. Pump curves
are labeled in feet of head, or meters of head.
- The friction head is the last one that we'll have to
calculate. This head tells us how much friction or resistance head
there is in both the suction and discharge piping, along with the
fittings and valves in the piping system. And to make the job a
little tougher this head changes dramatically as the pump capacity
You'll be calculating these heads on both the suction and
discharge side of the pump. To get the total head you'll subtract the
suction head from the discharge head and that will be the head that
the pump must produce to satisfy the application. It'll become
obvious in the calculations, but I should mention here, that if the
suction head is a negative number, the suction and discharge heads
will be added together to get the total head. If you subtract a minus
number from a positive number you must add the numbers together. As
an example: 4 - (-2) = + 6
The total head of a pump seldom remains static. There are a number
of factors that can change the head of a pump while it's operating,
and you should become familiar with most of them.
All of this head information is calculated from piping, valve, and
fitting, along with friction graphs you'll find in the index. This
head data will be plotted on a set of coordinates called a system
curve. Since we'll not be operating at a single point all of the
time, we'll make the calculations for a range of different capacities
and heads that we might expect to encounter. This range is described
as the operating window we need
to satisfy the application.
Making these calculations is not an exact science because the
piping is seldom new; pipe inside diameters are not exact, and the
graphs you'll be consulting cannot compensate for corrosion and any
solids built up on the piping, valve and fitting walls.
Life is never simple. This is the point where most people start
adding in safety factors to compensate for some of the unknowns.
These safety factors will almost always guarantee the selection of an
oversized pump that will run off of
its best efficiency point (BEP) most
of the time.
The final calculations are then plotted on the system curve that
describes what the pump has to do to satisfy the requirements of the
application. You can learn to do all of this by referencing the
- Calculating the total head in metric
- Calculating the total head in USCS
- Making a system curve, S111
The pump manufacturer requires a certain amount of net positive
suction head required (NPSHR) to
prevent the pump from cavitating. He shows that number on his
pump curve. When you look at the
curve you'll also note that the net positive suction head required
(NPSHR) increases with any increase
in the pump's capacity.
You'll also be calculating the net positive suction head available
(NPSHA) to be sure that the pump you
select will not cavitate. Cavitation
is caused by cavities or bubbles in the fluid collapsing on the
impeller and volute. In the pump business we recognize several
different types of cavitation. :
Pump cavitation is recognized in several different ways
- We can hear cavitation because it sounds like the pump is
moving rocks or ball bearings.
- We can see the damage from cavitation on the pump's impeller
- The operator can sometimes tell if the pump is cavitating
because of a reduction in the pump's capacity.
- The main problem with cavitation is that it shakes and bends
the shaft causing both seal and bearing problems. We call all of
this shaking and bending shaft
Remember that the net positive suction head required (NPSHR)
number shown on the pump curve is for fresh water at 68°
Fahrenheit (20°C) and not the fluid or combinations of fluids
you'll be pumping.
When you make your calculations for net positive suction head
available (NPSHA) the formula you'll
be using will adjust for the specific gravity of your fluid.
- In some cases you can reduce
the NPSH required. This is especially true if you are pumping hot
water or mixed hydrocarbons.
- You may have to install an inducer
on the pump, add a booster pump,
or go to a double suction pump
design if you don't have enough net positive suction head
When the pump supplier has all of this in-exact information in his
possession he can then hopefully select the correct size pump and
driver for the job. Since he wants
to quote a competitive price, he is now going to make some critical
He might begin with the type of pump he'll recommend:
- If the capacity is going to be very low he would recommend a
rotary, or positive displacement
- Between 25 and 500 gpm (5 m3 /hr - 115
m3/hr) he'll probably select a single stage
end suction centrifugal pump. It all depends upon the
supplier. At higher capacities he may go to a double suction
design with a wide impeller, two pumps in parallel
or maybe a high-speed pump.
- You might need a high head, low
capacity pump. The pump supplier has several options you
should learn about.
- Will he recommend a self-priming
pump? These pumps remove air from the impeller eye and suction
side of the pump. Some operating conditions dictate the need for a
self-priming design. If you don't have a self-priming pump, and
you are on intermittent service, will priming become a problem the
next time you start the pump?
- How will the pump be operated?
- If the pump is going to run twenty-four hours a day, seven
days a week and you're not going to open and close valves;
you'll not need a heavy-duty
pump. It's easy to select a pump that'll run at its best
efficiency point and at the best efficiency point (BEP)
there is very little shaft displacement
- Intermittent service is the more difficult application
because of changing temperatures, vibration levels, thrust
- Intermittent pumps require a more robust, heavy-duty design
with a low L3/D4
- How important is efficiency
in your application? High efficiency is desirable, but you pay a
price for efficiency in higher maintenance costs and a limited
operating window. You should be looking for performance,
reliability, and efficiency in that order. Too often the engineer
specifies efficiency and loses the other two. The following
designs solve some operation and maintenance problems, but their
efficiency is lower than conventional centrifugal pumps.
- A magnetic drive or
canned pump may be your
best choice if you can live with the several limitations they
- A vortex or slurry
pump design may be needed if there are lot of solids or
"stringy" material in the pumpage.
- A double volute
centrifugal pump can eliminate many of the seal problems we
experience when we operate off the pump's best efficiency
point. The problem is trying to find a supplier that will
supply one for your application. Although readily available for
impellers larger than 14 inches (355 mm) in diameter they have
become very scarce in the smaller diameters because of their
less efficient design.
- The supplier should recommend a centerline
design to avoid the problems caused by thermal expansion of the
wet end if you are operating at temperatures over 200°F
- Will you need a volute or circular casing? Volute casings
build a higher head; circular casing are used for low head and
- Do you need a pump that meets a standard?
ANSI, API, DIN, VDMA or ISO are some of the current standards. You
should be aware of pump standards problems that contribute to
premature seal and bearing failures. An ANSI
(American National Standards Institute) standard back pullout
design pump has many advantages but presents problems with
mechanical seals when the impeller clearance is adjusted, unless
you're purchasing cartridge seals.
- The decision to use either a single or multistage
pump will be determined by the head the pump must produce to meet
the capacities you need. Some suppliers like to recommend a high
speed small pump to be competitive, other suppliers might
recommend a more expensive low speed large pump to lessen NPSH and
There are additional decisions that have to be made about the type
of pump the supplier will recommend:
- Will the pump be supplied with a mechanical seal or packing?
If the stuffing box is at negative pressure (vacuum) a seal will
be necessary to prevent air
- If he's going to supply a mechanical seal, will he also supply
an oversized stuffing box and any
environmental controls that might be needed?
- Will he specify a jacketed stuffing box so that the
temperature of the sealed fluid can be regulated? How does he
intend to control the stuffing box temperature? Will he be using
water, steam or maybe a combination of both? Electric heating is
sometimes an option.
- How will the open or semi-open impeller
be adjusted to the volute casing or back plate? Can the mechanical
seal face loading be adjusted at the same time? If not, the seal
face load will change and the seal life will be shortened.
- If the pump is going to be supplied with a closed
impeller you should have some means of knowing when the wear
rings have to be replaced. If the wear
ring clearance becomes too large the pumps efficiency will be
lowered causing heat and vibration problems. Most manufacturers
require that you disassemble the pump to check the wear ring
clearance and replace the rings when this clearance doubles.
- Will he supply a "C" or "D" frame adapter,
or will the pump to motor alignment
have to be done manually using dual indicators or a laser aligner
to get the readings? A closed-coupled
design can eliminate the need for an alignment between the pump
- What type of coupling will he
select to connect the pump to its driver? Couplings can compensate
for axial growth of the shaft and transmit torque to the impeller.
They cannot compensate for pump to driver misalignment as much as
we would like them to. Universal joints are especially bad because
they have to be misaligned to be lubricated.
- He may decide to run two pumps in parallel
operation if he needs a real high capacity, or two pumps in
series operation if he needs a
high head. Pumps that run in parallel or series require that they
are running at the same speed. This can be a problem for some
- An inline pump design can solve many pipe strain and thermal
- The pump supplier must insure that the pump will not be
operating at a critical speed or
passing through a critical speed at start up. If he has decided to
use a variable speed drive or motor this becomes a
- We all want pumps with a low net positive suction head
required to prevent cavitation problems but sometimes it's not
practical. The manufacturer has the option of installing an
inducer or altering the pump
design to lower the net positive suction head required, but if he
goes too far all of the internal clearances will have to be
perfect to prevent cavitation problems. This modification of the
impeller to get the low net positive suction head required (NPSHR)
and its affects will be explained when you learn about
suction specific speed.
- The difference between
specific speed and suction specific speed can be confusing but you
should know the difference.
- Shaft speed is an important decision. Speed affects pump
component wear and NPSH requirements, along with the head,
capacity, and the pump size. High
speed pumps cost less initially, but the maintenance costs can
be staggering. Speed is especially critical if you are going to be
specifying a slurry pump.
- The ratio of the shaft diameter to its length is called the
shaft L3/D4number. This ratio will have a
major affect on the operating window of the pump and its initial
cost. The lower the number the better, but any thing below 60 (2
in the metric system) is acceptable when you are using mechanical
seals. A low L3/D4 can be costly in a
standard long shaft pump design because it dictates a large
diameter shaft that is usually found only on expensive heavy-duty
pumps. A short shaft with a smaller outside diameter would
accomplish the same goal, but then the pump would no longer
conform to the ANSI or ISO
standard. We often run into L3/D4problems
when you specify, or the pump supplier sells you a low cost,
corrosion resistant sleeve, mounted on a steel shaft rather than a
more expensive solid, corrosion resistant shaft.
There are multiple decisions to be made about the impeller
selection and not all pump suppliers are qualified to make them:
- The impeller shape or specific
speed number will dictate the shape of the pump curve, the
NPSH required and influence the efficiency of the pump.
- Has the impeller configuration been iterated
in recent years? Impeller design is improving with some of the
newer computer programs that have become available to the design
- The suction specific speed
number of the impeller will often predict if you're going to
experience a cavitation problem.
- The impeller material must be chosen for both chemical
compatibility and wear resistance. You should consider one of the
duplex metals because most
corrosion resistant materials are too soft for the demands of a
- The decision to use a closed
impeller, open impeller, semi-open, or vortex design is another
decision to be made.
- Closed impellers require wear
rings and these wear rings present another maintenance problem.
- Open and semi-open
impellers are less likely to clog, but need manual adjustment
to the volute or back-plate to get the proper impeller setting
and prevent internal recirculation.
- Vortex pump impellers are great for solids and "stringy"
materials but they are up to 50% less efficient than
- Investment cast impellers are
usually superior to sand cast versions because you can cast
compound curves with the investment casting process. The compound
curve allows the impeller to pump abrasive fluids with less vane
- If you're going to pump low specific
gravity fluids with an open impeller, a non-sparking type
metal may be needed to prevent a fire or explosion. You'll be
better off choosing a closed impeller design with soft wear rings
in these applications.
- The affinity laws will
predict the affect of changing the impeller speed or diameter.
You'll want to be familiar with these laws for both centrifugal
and PD pumps..
Either you or the supplier must select the correct size electric
motor, or some other type of driver
for the pump. The decision will be dictated by the specific gravity
of the liquid you'll be pumping along with the specific gravity of
any cleaners or solvents that might be flushed through the lines. The
selection will also be influenced by how far you'll venture off the
best efficiency point (BEP) on the capacity side of the pump curve.
If this number is under-estimated there is a danger of burning out
some electric motors.
- How are you going to vary the pump's capacity? Are you going
to open and close a valve or maybe you'll be using a variable
speed drive like a gasoline or diesel engine. Will the regulating
valve open and close automatically like a boiler feed valve or
will it be operated manually? The variable speed motor might be an
alternative if the major part of the system head is friction head
rather than static or pressure head.
- The viscosity of the
fluid is another consideration because it will affect the head,
capacity, efficiency and power requirement of the pump. You should
know about viscosity and how the viscosity of the pumpage will
affect the performance of the pump. There are some viscosity
corrections you should make
to the pump curve when you pump viscous fluids.
- After carefully considering all of the above, the pump
supplier will select a pump type and size, present his quote and
give you a copy of his pump curve. Hopefully you'll be getting his
best pump technology. To be sure
that is true, you should know what the best
pumping technology is.
- At this stage it is important for you to be able to read the
pump curve. To do that you must
- Best efficiency point (BEP)
- Shut off head.
- How to convert pressure to head so you can reference pump
gage readings to the pump curve. When you learn the three
formulas you'll get the conversion information.
- Brake horsepower (BHP)
- Water horsepower (WHP)
- Net positive suction head required (NPSHR)
- How to calculate the net positive suction head available
(NPSHA) to the pump to insure
you'll not have a cavitation problem.
If all of the above decisions were made correctly, the pump
supplier will place his pump curve on top of your system curve and
the required operating window will fall within the pump's operating
window on either side of the
best efficiency point (BEP). Additionally, the motor will not
overheat and the pump should not cavitate.
If the decisions were made incorrectly the pump will operate where
the pump and system curves intersect and that will not be close to,
or at the best efficiency point, producing radial
impeller loading problems that will cause shaft deflection, resulting
in premature seal and bearing failures. Needless to say the motor or
driver will be adversely affected also.
With few exceptions pump manufacturers are generally not involved
in mechanical sealing. You'll probably be contacting separate seal
suppliers for their recommendation about the mechanical seal.
Mergers between pump and seal companies unfortunately does not
produce the instant expertise we would like sales and service people
CENTRIFUGAL PUMP INSTALLATION
Some one has to install the pump and all of its associated
hardware. The quality of this pump and driver installation will have
a major affect on the performance and reliability of the pump,
especially if it's equipped with a mechanical seal.
The pump will be installed on a baseplate.
The baseplate will be attached to a foundation
and grout will be placed between the
baseplate and the foundation to transmit any vibrations from the pump
to the foundation.
Once the pump and driver are firmly on the foundation it'll be
time to connect the piping. Be sure to pipe from the pump to the pipe
rack and not the other way, so as to avoid pipe strain that will
interfere with the operation of the mechanical seal and bearings.
There are many piping recommendations that you should be familiar
with. The leveling, and pump to driver alignment
can be made at this point, but you should check the alignment after
the pump has come up to its operating temperature because metal parts
expand and contract with a change in temperature.
If this is a new piping system some people like to install packing
in the pump and run on packing until the new piping has been cleaned
of slag or any junk that might be left in the piping system. If it's
not a new installation, and there is a mechanical seal in the
stuffing box, then installing the mechanical seal environmental
controls will come next.
If the pump has an open or semi-open impeller it's time to make
the initial impeller clearance setting. The final clearance can be
set when the pump comes up to its operating temperature. It's
important to note that if you do not have a cartridge
seal installed in the pump the seal face loading will change as you
make both the initial and subsequent impeller settings and there is
nothing you can do about it.
You'll now want to do a proper venting
of the pump. If it's a vertical installation you'll have to pay
particular attention to keeping air vented from the stuffing box
while the pump is running and be sure to vent the space between dual
seals if they've been installed.
After you have done all of the above, it's time to check out the
mechanical seal environmental
controls to be sure they're working properly. In most cases the
environmental control will continue to run after the pump has
stopped. Be sure the operators understand this or they might be
tempted to shut the control off when the pump is between batches.
Seal quench is always a problem with
operators because the steam or water dripping out of the seal gland
looks like the seal is leaking.
A constant monitoring of the pump is a good idea. Are you familiar
with some of the more popular monitoring methods? Unlike vibration
analysis, monitoring can tell you if
some part of the pump is getting into trouble before the vibration
CENTRIFUGAL PUMP MODIFICATION
If you find that your present centrifugal pump is not satisfying
the application and running as trouble free as you would like, and
you have checked:
- All of the internal tolerances are correct.
- There is no excessive pipe strain.
- The open impeller has been adjusted to the volute or backplate
after the pump came up to operating temperature.
- The pump to driver alignment was made.
- The rotating parts were dynamically balanced.
- The wear-ring clearance is within manufacturers
- The pump is running at the correct speed, in the right
direction, with the correct size impeller.
Then you may have to purchase a different centrifugal pump, or you
might want to consider modifying the existing pump to get the
performance and reliability you are looking for.
Here are a few modifications and pump upgrades you can
- Modifying the impeller
diameter could get you closer to the best efficiency point. The
affinity laws will predict the
affect of substituting a larger impeller, or the trimming the
present impeller will have on the pump's head; capacity, net
positive suction head required (NPSHR), and horsepower
- Converting to an impeller with a different specific
speed number will change the shape of the pump curve, power
consumption and the NPSH required.
- Changing to a heavy-duty power end can stop a lot of shaft
deflection, and with some pump manufacturers get you the pilot
diameter you need to install a "C or D" frame
adapter to eliminate pump alignment.
- Converting from a sleeved to a solid, corrosion resistant
shaft will often reduce or stop shaft deflection problems caused
by operating off the best efficiency point (BEP). If you're using
mechanical seals be sure that you're using the type that prevents
fretting corrosion. Most original
equipment manufactured (OEM) seals damage shafts, and that's one
of the main reasons they supply a sacrificial sleeve.
- Reducing the overhung shaft length can solve many shaft
deflection problems. You should
be able to get the L3/D4 number down to less than 60 (2,0 metric) by either reducing the
shaft length or increasing the shaft diameter.
- Changing the wet end to a double volute configuration will
allow the pump to operate in a larger window
without the danger of deflecting the shaft too much.
- You can drill a hole in the end of the stuffing box, at the
top, to increase stuffing box
- Change the flushing or recirculation connection from the top
lantern ring connection to the bottom of the stuffing box to
insure a better fluid flow through the stuffing box. Try to get
close to the seal faces.
- Enlarging the inside diameter of the stuffing box or going to
an oversize stuffing box can
solve some persistent seal problems.
- Converting the wet end of the pump to a centerline
design might solve some pipe strain problems by compensating for
radial thermal growth.
- Increasing the impeller to cutwater clearance could stop a
- Installing a sight glass in the bearing case can help you
maintain the correct oil level and prevent overheating problems in
- Replacing the bearing case grease
or lip seals with either labyrinth
or positive face seals for
bearings will keep moisture out of the bearing case and eliminate
a lot of premature bearing failure.
- Converting the radial bearing retention snap
ring to a more rugged holding device will eliminate many of he
problems associated with axial movement of the shaft.
- Converting the packed pump to
a good mechanical seal will reduce power consumption and product
- Converting solid mechanical seals to split
mechanical seals can reduce the time it takes to change seals and
eliminate the need for other trades to become involved in the
process of disassembling a pump and bringing it into the
MECHANICAL SEAL SELECTION
- In the following pages I'll be using the word "pump" to
describe the piece of equipment that you'll be sealing. If your
equipment is anything other than a single stage centrifugal pump
with an over hung impeller, the information still applies with a
couple of exceptions:
- Mixers, agitators and similar pieces of equipment sometimes
have severe axial thrust and
shaft deflection problems due to their high
L3/D4 numbers (The ratio of the shaft
length to its diameter).
- Sleeve or journal bearing
equipment allows more axial movement of the shaft than those
pieces of equipment provided with precision bearings. Axial
movement is a problem for mechanical seals because of the
changing face load; especially at start up when the axial
thrust reverses in a centrifugal pump.
- Open impeller pumps require impeller adjustment
that could cause excessive axial movement of the shaft that
will affect the seal face loading. Depending upon the severity
of the abrasives being pumped, this could be a frequent
- Multi-stage pumps are
seldom as sensitive to operating off the best efficiency point
(BEP) as single stage centrifugal pumps. The opposing cutwaters
in these pumps tend to cancel out the radial forces created
when the pump is operating off of its best efficiency point
- Centrifugal pumps equipped with double
volutes are not too sensitive to operating off the best
efficiency point (BEP), but do experience all of the other
types of shaft deflection.
- Specialized equipment such as a refiner in a paper mill
will experience a great deal of axial travel as the internal
clearances are adjusted.
Whenever I use the word fluid, I am talking about either a liquid
or a gas. If I say either liquid or gas, I am limiting my discussion
to that one phase of the fluid.
Any discussion of mechanical face seals requires that you have
many different types of knowledge. The first is, "should you be
converting packed pumps to a mechanical seal?" Seals cost a lot more
money than conventional packing and unless you're using split
seals, they can be a lot more difficult to install. There is a
packing conversion down side.
Assuming you have made the decision that the mechanical seal is
your best choice for sealing, you must know how to select the correct
design for your application. There are many different kinds of seals
to choose from:
- Rotating seals where the
springs or bellows rotate with the shaft.
- Stationary seals where the
springs or bellows do not rotate with the shaft.
- Metal bellows seals used
to eliminate elastomers that can have trouble with temperature
extremes or fluid compatibility.
- Elastomer type seals utilizing O-rings
and other shape elastomers.
- Single seals for most applications.
- Dual seal designs for
dangerous and expensive products or any time back up protection is
- Inside mounted designs that take advantage of centrifugal
force to throw solids away from the lapped seal faces.
- Outside seals. Usually the non-metallic
variety for pumps manufactured from non-metallic materials.
- Cartridge seals to ease
installation and allow you to make impeller adjustments without
disturbing the seal face loading.
- Split seal designs that allow
you to install and change seals without taking the pump apart and
disturbing the alignment.
- Hydrodynamic or
non-contacting seals used for the sealing of gases.
- Hydrostatic designs are
another version of non-contacting vapor seals.
There are some very desirable design features that you should
specify for your mechanical seals:
- The ability to seal fugitive emissions without the use of
dual seals, other than having the
dual seal installed as a "back-up" or spare seal.
- Will the seal dynamic elastomer damage or cause fretting
corrosion of the pump shaft? Almost all-original equipment designs
do. Spring-loaded Teflon® and graphite are notorious for shaft
destruction. There are many seal designs available that will not
cause fretting corrosion or
damage shafts and sleeves, and that is the kind you should be
- The seal should have built in non-clogging
features such as springs out of the fluid.
- The seal should be able to compensate for a reasonable amount
of both radial and axial movement of the shaft. There are special
mixer seal designs that can
compensate for axial and radial travel in excess of 0.125 inches
(3 mm) and you should know about them
- The seal should be designed to be positioned as close to the
bearings as possible to lessen the affects of shaft deflection.
Ideally the seal would be located between the stuffing box face
and the bearing case with a large diameter seal gland allowing
plenty of internal radial clearance for the seal.
- The seal should generate only
a small amount of heat. Seal face heat generation can be a problem
with many fluids and there's no advantage in letting the seal
faces, or the fluid surrounding them get hot
- Any heat generation between the seal faces should be
efficiently removed by conduction away from the lapped faces
and dynamic elastomer. Check to see if your design does it
- Any dynamic elastomer (an
O-ring is typical) should have the ability to flex and then roll,
or slide to a clean surface as the carbon face wears.
- The seal face load should be adjustable to compensate for open
impeller adjustments and axial growth of the shaft. Cartridge
seals do this very well.
- Can you use universal materials to lower your inventory costs
and avoid mix-up problems? All of the seal materials should be
clearly identified by type and grade. You'll need this information
if you have to analyze a premature seal failure. Some seal
companies try to make everything a secret. Do not tolerate
- Will the seals be hydraulically balanced
to prevent the generation of unwanted heat between the lapped
faces? What is the percentage of balance? If you are using dual
seals will the inner seal be a double balanced seal that is
hydraulically balanced in both directions? Pressures can reverse
in dual seal applications.
- You'll want to become familiar with the effects of heat on:
- The seal faces, especially the carbon and plated or coated
- The elastomers, especially the dynamic elastomer
- Excessive corrosion of the seal components.
- The product. It can change with heat. It can vaporize,
solidify, crystallize, coke or build a film with an increase in
the product's temperature.
- Internal tolerances of the seal especially face flatness
and elastomer squeeze. Heat causes thermal growth of these
components that will alter their critical tolerances.
We'd like to be able to install the seal without having to modify
the pump. The seal should be the shortest, thinnest design that'll
satisfy all of the operating conditions. Once you have the shortest,
thinnest design that'll satisfy the operating conditions, there is
seldom a need to modify any seal design.
The specific sealing application will dictate which seal design
you should choose. If your seal application falls within the
following parameters any stationary or rotating, "off the shelf"
balanced, o-ring seal should be able to handle the application
without any serious problems:
- Stuffing box pressures from a one Torr vacuum to 400 psi. (28
bar). Note that stuffing box pressure is normally closer to
suction than discharge pressure
- Stuffing box temperature from -40°F to 400°F.
(-40°C to 200°C)
- Shaft speed within electric motor speeds. If the surface speed
at the seal faces exceeds 5000 fpm. (25 m/sec) you'll have to
select the stationary version of the seal.
- Shaft sizes from 1 inch to 4 inches. (25 mm to 100 mm)
You may have to go to a special
seal design if your application falls into any of the following
- Stuffing box pressures in excess of 400 psi. (28 bar) require
heavy duty seals.
- Excessive shaft movement of the type you find in mixers,
agitators, and some types of sleeve or journal bearing
- The seal must meet fugitive
- No metal parts are allowed in the system. You need a
- Nothing black is allowed in the system because of a fear of
color contamination. You cannot use any form of carbon face; you
must use two hard faces.
- There's not enough room to install a standard seal.
- You're not allowed to use an environmental
control or no environmental control is available.
- Odd shaft sizes often dictate special
- If the seal components must be manufactured from an exotic
If any of the following are part of the application, you may need
a metal bellows design that eliminates all elastomers.
- You're sealing a non-petroleum fluid and the stuffing box
temperature exceeds 400°F (200°C) Petroleum fluids have
coking problems that require cooling in the seal area.
- Cryogenic temperatures.
You should go to a dual seal application if your product falls
into any of the following categories:
- You need two seals to control the seal environment outside the
- To control the temperature at a seal face to stop a product
from vaporizing, solidifying, crystallizing, or building a
- To prevent a pressure drop across a seal face that can cause a
liquid to vaporize.
- To eliminate atmospheric conditions outboard of a mechanical
seal when there is a possibility of freezing water vapor in the
- To break down the pressure in a high-pressure application, by
inserting an intermediate pressure between the seals. Two lower
pressure seals can then be used to seal a high-pressure fluid that
would normally require a very expensive high-pressure mechanical
- To provide a lubricant if one is needed to prevent slip stick
between lapped seal faces. This is always a problem when you are
sealing a gas or non-lubricating liquid.
You need dual seals as a protection for personnel in the area if
your product is any of the following categories:
- A toxic liquid or gas.
- A fire hazard
- A pollutant
- A carcinogen
- A radioactive fluid
- An explosive fluid
The other places we use dual seals are:
- Expensive products that are too valuable to let leak.
- You cannot afford to be shut down in the middle of a batch
- You do not have a standby pump and experience shows that the
seal failure is your highest probability of an unexpected shut
In the Sealing
Application section you'll learn:
- How to choose the correct seal materials.
- How to classify the fluid into specific sealing
- The environmental controls you might need to insure the seal
will not fail prematurely.
For information about my CD
with over 600 Seal & Pump Subjects
Link to Mc Nally home page