Subject: Seal partnering
The latest fad to hit the process industry is "partnering,"
allowing industrial academics to have a wonderful time thinking up
these new projects so they will appear to be doing something
productive and useful. Although "partnering" implies many lofty
benefits it always ends up being nothing more than the purchasing department
negotiating a large discount from the seal manufacturer. The really
important stuff that is costing you the most money, always takes
Modern mechanical seal designs are supposed to run until the only
sacrificial part (usually the carbon seal face) has worn away. In
better than 90% of the cases this never happens. Through some
convoluted thought process, or maybe a hard sell by their local seal
supplier these same academics have decided that purchasing these failing
seals at a discount will somehow make sense to whomever they have to
answer. I know they're not going to like hearing this, but mechanical
seals are really not a commodity. Anything with a 90% plus failure
rate would have a hard time being put into a commodity status by
If you serious about trying to finally getting some decent
mechanical seal life in your rotating equipment, you're going to have
to start looking for some very specific features in your mechanical
seals and some very knowledgeable people to deal with when it come to
application and troubleshooting. Consulting, application, and
troubleshooting expenses have traditionally been built into the price
of the mechanical seal. Large consumer discounts eliminate that
There are three things you would like to receive from your seal
- An excellent product
- A low price
- Good service, including: partial inventorying, application
engineering, and trouble shooting assistance.
Choose any of the above two. Isn't it kind of stupid to think you'll get all three!
We will start with the product. Decide on the type of seal you'll need :
- Stationary mechanical seals are better than their rotating
cousins. Some of these stationary designs are available with seal
wear indicators and you should take advantage of that feature.
Stationary seal design do not have rotating springs or
- Dual seals are the only reliable method you have of protecting
yourself against an unexpected seal failure, and all the problems
associated with product leaking out of the pump. With all the
current talk about fugitive emissions, dual seals are becoming a
necessity in many applications. They are available in both the
stationary and rotary version with liquid or inert gas buffer, or
barrier fluid circulating between the seals, and can be installed
in four different configurations:
- Back to back rotating seals are never acceptable. Back to
back stationary seals are OK because the sealing fluid is at
the inside seal outside diameter
- Tandem or series is the best choice for rotating or
- Face to face designs are acceptable
- Concentric, or one inside the other is alright but this
design takes a lot of radial room. Too much for a conventional
- If you're going to use single seals, the split version should
be your first choice. There is no sense in taking equipment apart
and spending all the money associate with disassembly, spare
parts, new gaskets, realignment, insulation, etc, if you do not
have to. In most cases the only problem with the pump is it is
leaking. Fix the leak, do not overhaul the pump. Remember your
grandfather's words "if it aint broke don't fix it".
- Cartridge seals not only make assembly easier and faster, but
they are your only alternative if you intend to adjust the
impellers used on the common open impeller, back pullout pumps. It
is unbelievable the number of companies that are failing to adjust
these impellers and as a result are running the pumps very
inefficiently. This can only be explained by the lack of knowledge
that industry has about how pumps work. Otherwise, it is being
done on purpose. Split seals can be cartridge mounted without too
- If your seals are going to incorporate elastomers make sure
you use O-rings. Stay away from wedges, chevrons, U-cups, etc.
They destroy (frett) shafts and are seldom needed in modern
- Metal bellows seals are good for eliminating the need for
elastomers. They make sense in cryogenic service and hot
applications such as polymers and resins. You can use them in any
fluid other than hot petroleum products that "coke" and fail seals
prematurely. Oils must be cooled.
Seal materials are an important consideration. With few exceptions
you should be able to install the same seal in every pump of the same
shaft size and reduce your inventory considerably. Make the decision
right now to quit using "mystery materials" identified only by a part
number or some generic term used to describe the material in your
expensive mechanical seals. How are you going to fix something if you
not know what it is? There is a lot to know about materials and it is
a subject that is always changing as new materials are developed.
- If you are going to use a carbon/ graphite face use the three
impregnate, non filled type in most of your applications. There
will be a couple of exceptions for hot dry air and cryogenic
service, but exceptions are always easy to handle.
- Hastelloy "C" springs and metal bellows plates are necessary
to prevent chloride stress corrosion problems associated with the
three hundred series of stainless steel. Never use stainless steel
springs or stainless steel metal bellows in any mechanical seal
- Remember that reaction bonded silicone carbide will be
attacked by high pH materials such as caustic. You will probably
be better off with the alpha sintered version of silicone carbide
in these applications.
- Your seal materials should not be subject to "shelf life"
problems. This is an on going problem with Buna N O-rings and
Look for non-clogging features in you seal designs because there
are solids present in most of your applications:
- The springs should be out of the fluid unless you have some
real good reason for putting them there.
- Dynamic elastomers (moveable O-rings) should always move to a
- Centrifugal force should throw solids away from the seal
moveable parts and lapped faces.
- Inconel bellows make a lot of sense in high temperature seal
- A non-stick coating on sliding components helps. Baked on
Teflon® works well on frying pans and seal components. The
coating will also reduce hysteresis problems with sliding or
flexing dynamic elastomer or O-ring.
There are other desirable features you should look for:
- Hydraulically balanced seal faces for vacuum and varying
stuffing box pressures.
- The inner seal of a dual seal should be hydraulically balanced
in both directions because pressures commonly reverse in these
- The seal design you specify should be able to seal if it
rotates either clockwise or counter clockwise. Most hydrodynamic
gas seals and some single spring liquid seals cannot do this.
- The seal should have good axial and radial movement
capability. This is especially important for mixer applications.
Specify excessive motion designs for mixers, agitators, sleeve
bearing equipment, etc. These designs incorporate:
- Wider hard faces so that the thinner carbon/ graphite face has
more radial movement capability.
- More internal clearance in the seal components.
- Axial movement capability without compressing or extending the
spring or springs and affecting proper face loading. You need at
least 0.125 inches (3mm) in both directions. More would be
- The seal design you select should be the thinnest and shortest
seal that satisfies the operating conditions. There is no
advantage in having the seal take up too much axial or radial
room. A high percentage of requests for special seals means
shorter or thinner designs.
- The seal should be installed close to the pump bearing to
eliminate the affect of shaft deflection problems.
The seal should be easy to install.
- No print should be required. Cartridge and split seal both
have that advantage.
- The design should be independent of shaft finish and
reasonable shaft diameter tolerances. In other words there should
be no dynamic elastomers or metal bellows vibration dampers
rubbing on the pump shaft or sleeve.
- The seal should not frett or damage the shaft or sleeve
through normal operation. This will eliminate all spring loaded
elastomers and many rotating seal designs.
- The seal should be packaged to protect the lapped faces.
Bubble packaging on a piece of cardboard should not be allowed.
The seal should be in its own box and protected by foam or a
similar protective material. The lapped faces should be dipped
into a protective coating that can be removed just prior to
installation. A boxed seal should be able to survive a one meter
(39 inch) drop without injury to the seal components.
The seal should generate very little heat.
- The seal should be hydraulically balanced for low face
- Specify two way balance for the inner seal of a dual seal
- Low friction faces are a necessity. A good grade of carbon/
graphite running against a hard face like silicone carbide will
generate very little heat.
- Use materials that conduct heat well. Silicone carbide is
- Never insulate the seal faces by elastomers or plastics if you
can avoid it. You want to conduct the face heat into the gland or
face holder when ever possible.
- Try not to position the dynamic elastomer in the seal face.
The dynamic elastomer is the one part of the seal that is the most
sensitive to heat.
- If you are sealing a non lubricating fluid (liquid or gas) use
a dual seal with a lubricant circulating between them. Avoid oil
as a buffer or barrier fluid, the specific heat of oils is too
high and they all have poor conductivity compared to other cooling
- Install a gland vent above the seal faces in all vertical pump
applications. Air will collect in the top of the stuffing box
causing the seal faces to run dry. You can drill a hole in the
upper corner, at the end of the stuffing box to vent the seal
cavity in a horizontal pump.
- Use a large inside diameter, bored out stuffing box to give
the seal plenty of clearance on its outside diameter. Avoid
tapered stuffing boxes.
- Use heavy duty seal designs for greater pressures. These
- Fiber or Teflon® back up rings to prevent elastomer
- Thicker cross section components to prevent face
- New thinner cross section configurations have become practical
since the introduction of finite element analysis programs.
- Lower spring pressure at the seal faces.
- A different balance ratio. Especially with fluids having a
specific gravity below 0.4.
- Do not glue carbon/ graphite seal faces into a metal holder.
The glue may not be compatible with the sealing fluid or the
solvent/ steam used to clean the lines.
- Avoid "shrunk in" carbon faces. "Pressed in" faces shear to
conform to the "out of roundness" of a metal holder.
Some cartridge seal feature to look for.
- The cartridge sleeve must be sealed at the inboard end to
prevent solids from lodging between the cartridge sleeve and the
pump shaft making seal removal difficult if not impossible.
- In some dual seal designs, if the cartridge gland is pushed to
move it against the stuffing box face, the friction between the
seal sleeve elastomer and the shaft outside diameter will cause
the inner seal to compress and the outer seal will unload. Look
out for this design flaw.
- Can the clips or whatever is setting the correct installation
length be reinstalled when an impeller adjustment has to be made?
In many designs it is very difficult to re-install these
- Can the assembly be rebuilt easily and at a sensible cost?
Does it have to be returned to the distributor or manufacturer?
New "right to know" laws are going to discourage returning
hardware that has been exposed to chemicals.
- The cartridge gland should have a flush port and vent built
into the gland. An API type gland would be ideal with its disaster
bushing and quench/ drain connection.
All split seals are not alike:
- Will the design seal both pressure and vacuum. Many designs
cannot do this.
- Be sure there are no glued elastomers, especially the dynamic
elastomer or O-ring that has to flex and roll to compensate for
face wear and shaft movement. The glue creates a hard spot that
- Stationary designs are the best. In this design the spring or
springs do not rotate with the shaft.
- Can it be mounted on a cartridge so that you can make pump
impeller adjustments or be able to remove the split seal with out
having to drain a side entering mixer?
- Does it have a wide hard face and lots of internal clearances
for radial movement? Many of these seals are used on mixers and
this is a necessary feature in a mixer application.
Once the proper mechanical seal has been specified for the
application, the really difficult part takes over. Someone has to do
the application for the individual chemicals and troubleshooting of
failed seals. Because these people are rare, we see consumers
throwing seals at the application hoping one of them will work. The
90% failure rate you presently experience is one of the results of
It would be wonderful if you could call in three or four seal
companies selling the same product, with the same level of expertise
and choose among them for your best price. The fact is you'll be
lucky if you can find one good supplier, and I will bet he will not
be the lowest price. He will likely have the highest inital cost.
Isn't it kind of naive to think otherwise?
I know you are not going to pay attention to this, so good luck
with your 90% + failure rate.
For information about my CD
with over 600 Seal & Pump Subjects
Link to Mc Nally home page