MECHANICAL SEAL SELECTION
NOTE: The blue links do not work on this
version of the tutorials. They only work on the
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
- 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 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 occurrence.
- 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 (BEP).
- Centrifugal pumps equipped with double volutes are not very 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
In the following paragraphs, whenever I use the word fluid, I am talking about either a liquid
or a gas. If I say 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 are 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 is 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 will 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 will 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 would like to be able to install the seal without having to
modify the pump. The seal should be the shortest, thinnest design
that will satisfy all of the operating conditions. Once you have the
shortest, thinnest design that will 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 will 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 is not enough room to install a standard seal.
- You are not allowed to use an environmental
control or no environmental control is available.
- Odd shaft sizes often dictate special seals.
- 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 are 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
- 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 will 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