SUBJECT : The sealing of hot oil
The largest user of hot oil pumps is the heat transfer oil
customer. Many consumers use these products with oil temperatures
exceeding 500° Fahrenheit (260° C.) and 600° to
700° F. ( 315° to 370° C.) becoming common. Some
hotels have recently installed these systems in their laundry to dry
Heat transfer oils have many advantages over the steam that was
formally used in these applications.
- The product doesn't flash.
- No boiler blow down.
- No deaeration heat loss.
- No high pressure. This means it's not only safer but also
tends to leak less.
- No licensed boiler operator needed.
- The temperature can be kept uniform over a large processing
- You can heat and cool with the same system.
- These oils are excellent in systems that are water/ steam
- The product is kept in a closed system. This means that all
leakage can be stopped.
- There is less corrosion in the system.
In addition to these heat transfer oils, you'll encounter hot
petroleum oil applications in refineries and hot organic oil
applications in various other industries. There are several problems
associated with sealing these hot oil products and each of them has
to be solved if satisfactory seal life is ever to be obtained.
- High temperature oil is generally too hot for most
commercially available elastomers. (the rubber parts)
- The product "cokes". These coke particles form at the elevated
temperatures and coat them selves inside the system piping,
hardware and on the mechanical seal working parts. These "coke"
particles restrict the movement of sliding/flexing seal components
causing the lapped seal faces to open. The amount of coke that
forms is a function of time and temperature. In other words,
coking will be a more severe problem in a closed loop system than
it will be in the oil refining business. Contrary to popular
opinion testing has shown that air or oxygen is not needed for the
formation of coke. This means that seal designs that try to
eliminate the oxygen by quenching or some other method will not
work. The use of steam quenching is limited to its' cooling effect
- The product is always a fire hazard and depending upon the
type and brand you purchase, there could be toxicological
problems. Keep in mind that the seal is going to wear out or fail
at some time and the product will leak out to the atmosphere.
- Thermal growth of the pump parts will cause problems in
maintaining correct pump "wear ring" and impeller clearances, as
well as the correct seal compression.
- Misalignment between the driver and the pump, and between the
piping and the pump suction is a serious problem at elevated
- The product is costly. Leakage represents large monetary
losses and personnel danger as well as environmental
- Heat tracing must be provided throughout the system to prevent
the product from becoming too viscous during periods of prolong
shut down. No one ever heat traces the stuffing box.
- Vibration is always a problem with hot oil pumps because the
coke attaches to rotating components interfering with the dynamic
- You always end up pumping a slurry which means frequent
impeller adjustments, or wear ring replacement and excessive
vibration due to the imbalance caused by wear of the rotating
- As the coke builds up on the inside of the discharge piping
the pump will operate further off of its best efficiency point
(B.E.P.) causing shaft deflection, vibration, and excessive seal
movement. Coking on the inside of the suction piping can cause
Although there are many techniques available to the seal man to
address each of these problems, the combination of these problems
eliminates most of the common techniques and leaves the customer with
very few options to get good seal life. Regardless of the seal
selected you must address all of the problems or the seal life will
Oil refineries pump hot oil with closed impeller pumps and, as a
result, have to put up with the additional problems associated with
replacing "closed impeller" wear rings. Unlike the chemical industry
they cannot take advantage of the features of an open impeller design
that can be easily adjusted to maintain maximum efficiency. There are
two reasons why oil refineries chose closed impeller designs with
mechanical seals and A.P.I. glands :
- Fear of a bearing failure that could cause sparking as the
metal impeller contacted the metal volute. The soft non sparking,
metal wear ring on one end of the shaft and the carbon disaster
bushing installed in the A.P.I. Gland on the other, would insure
no hard metal contact if a bearing failed as the shaft was
- Shaft expansion or impeller adjustment could cause the
rotating, open impeller to contact the stationary volute. To
prevent sparking the impeller or volute would have to be
manufactured from a soft non sparking metal such as aluminum or
bronze and this would not be very practical.
To insure long seal life you must do the following:
The product has to be cooled in the seal chamber :
- The oil must be cooled to stop the coking. Coke is a function
of heat. Many years ago it was believed that oxygen had to be
present for coking to occur, but testing has shown that this is
not true. You can coke any petroleum product in an inert
atmosphere as long as the temperature is high enough. The finest
lubricating oil available will start to coke at 300° F
(150° C). The amount of coking that you get is determined by
the oil temperature and time.
- The oil must be cooled to prevent damage to any elastomers
that might be installed in the seal or shaft sleeve. Elastomers
that are subjected to high heat will first take a compression set
and then shrink in volume. They will eventually grow hard, crack
and leak excessively.
- The oil must be cooled to reduce the amount of heat that will
be transferred through the shaft to the bearing oil or grease.
This heat will reduce the viscosity of the lubricating oil or
grease and eventually cause premature bearing failure. The SKF
bearing company states in their lubrication literature, that the
life of bearing oil is cut in half for each ten degrees Centigrade
(18° F) increase in bearing oil temperature. They recommend
60° C to 70° C (140° F to 158° F) as an ideal
- The grease or lip seals are sensitive to any increase in shaft
temperature. A stainless steel shaft is a good choice in these
applications because stainless steel is a poor conductor of heat
compared to carbon steel. This is the reason there are no
stainless steel frying pans unless they are clad with either
aluminum or copper.
You must install a back up seal for the following reasons:
- The product is dangerous. Leaking hot oil can start a fire or
injure any personnel in the area. Many brands are toxic and some
have been identified as possibly carcinogenic.
- The product is too costly to tolerate even small amounts of
- Back up cooling is necessary if the primary cooling method
fails. A back up seal, with a cool barrier fluid system, can
provide this cooling
- If you elect not to use a back up seal, then be sure to
install an American Petroleum Institute (A.P.I.) type gland. Look
at the following illustration. The gland can perform several
functions for you:
- The disaster bushing can provide shaft support if you lose a
- The leakage will be directed to the quench and drain
connection when the seal wears out or fails.
- The quench connection will allow you to use steam for product
cooling, but do not use too much because it could penetrate into
the bearing case.
- You can connect steam to the quench connection and use it to
put out a fire, should it occur on the outboard side of the
- In this application the flush connection is not used. The
stuffing box is "dead ended" to take full advantage of the
heating/ cooling jacket.
A large diameter cooled sealing chamber should be installed on the
- To allow centrifugal force to throw solid coke particles away
from the seal faces and sliding or flexing components
- Misalignment is always a problem in these pumps. This shaft
displacement can cause the rotating seal to rub against stationary
parts in a conventional stuffing box.
- Vibration means movement . The seal must be free to move
within the seal chamber.
- When the pump stops gravity will pull solid particles to the
bottom of the stuffing box. A large seal chamber will almost
guarantee that the particles will not collect around the seal at
A Cartridge seal is necessary.
- Thermal growth will cause volute casing and shaft expansion.
Only a cartridge seal will compensate for this movement and allow
for the impeller adjustment that will be necessary.
- The wear caused by the slurry will cause frequent impeller
adjustments. The average pump used in these applications has
almost 0.250 inches (6 mm) of adjustment possible.
To compensate for misalignment you will have to :
- Use a "C" or "D" fame adapter to compensate for misalignment
between the pump and its driver.
- These adapters are available from all good pump companies
and will compensate for misalignment as the pump goes through
its temperature transients.
- No other method of alignment works any where near as well.
If you're going to do a conventional alignment with dual
indicators or a laser aligned be sure your calculations
compensate for thermal growth.
- Use a "centerline" wet end to prevent excessive wear ring wear
and pipe strain at the pump suction. If your pump did not come
equipped with this type of wet end it can easily be installed in
the maintenance shop. Look at the following illustration:
The illustration shows the centerline design. It will allow the
pump volute to thermally expand both up and down, and thereby
eliminate strain on the suction piping.
Now that we have discussed these important points lets take a look
at some solutions that are often offered, but that we should not
adopt as our solution. Here are the things that do not work well
Bad solution #1. Use a metal bellows seal to eliminate the need
for cooling in the seal area.
Comment: Although the metal bellows doesn't have rubber parts that
are sensitive to high temperature cooling is still needed for the
coking. Most bellow suppliers offer an A.P.I. type gland to provide
low pressure steam behind the seal for cooling purposes and thereby
eliminate the option of backup sealing. This quenching should be
limited to only a back up cooling status. If quenching is done with
water rather than steam, watch out for a calcium build up outboard of
the seal. This "hard water" build up can restrict the movement of the
flexing portion of the seal as it tries to compensate for face
If you substitute condensate for the quenching fluid the build up
can be eliminated almost entirely.
Bad solution #2. Run a line from the discharge of the pump through
a cooler and filter to cool down and clean up the oil going into the
- Comment: The problems with this solution are obvious. The
filter will clog and the cooler will become inoperative as coke
builds up on the tubes.
Bad solution #3. Use two seals and run a cool oil between
- Comment: You have addressed the cooling problem but you have
not addressed the problem of the slurry with this solution.
What then is the best solution that addresses all of the problems?
Look at the following illustration:
- Install a large jacketed sealing chamber. These bolt on
accessories are available from your local pump/seal supplier. Many
suppliers can provide a replaceable pump back plate with a large
seal chamber cast into the plate
- Dead end the stuffing box. In other words no lines coming into
or away from the inner seal chamber. Do not worry about the heat.
With a six to eight gallon per minute( 20 to 30 liters/ minute)
flow through the cooling chamber the cooling jacket can keep the
temperature down to 200° to 250° Fahrenheit (95° to
120° C.) without any trouble. If you have hard water in your
area, condensate may be the best choice to use as the cooling
medium. In some cases low pressure stream is satisfactory. If you
anticipate long periods of shut down, low pressure steam will be
your best choice as it will keep the heat transfer oil at the
proper low viscosity during these shut down periods.
- Install a cartridge dual seal that has the inner seal balanced
in both directions. If the pump doesn't have precision bearings a
double motion seal with the same features will work just as well.
"Two way" balance is necessary because the system and barrier
fluid pressure can and will vary.
- The dual seal is necessary to conserve the expensive product
and to provide a safety feature when the inboard seal wears out or
fails. It will also allow you time to schedule a seal
- Install a convection tank between the two seals and use cool
heat transfer oil as the barrier or buffer fluid. A lower pressure
or buffer fluid is preferred. A slight pressure on the tank will
allow you determine which seal has worn out or failed first. A
pumping ring or forced lubrication between the seals is
- Install a carbon restrictive bushing into the bottom of the
stuffing box to act as a thermal barrier. Applications have worked
without this bushing but not as well as with it. Any material that
has poor heat conductivity will work as well as carbon as long as
it's non sparking and dimensionally stable.
That's all there is to the application. Centrifugal force will
clean up the small amount of fluid in the sealing chamber while the
cooling jacket holds the temperature low enough to prevent coking and
injuring the seal elastomer.
The only problem with this system is that it works so well we
often forget to clean the cooling jacket on the pump. A small layer
of calcium inside this jacket will provide an insulation and destroy
the cooling affect of the jacket. Be sure to keep this jacket clean
or substitute steam or condensate for the cooling water, and then
don't worry about it.
Here are a few additional thoughts:
- A cartridge dual bellows seal can be substituted as long as
adequate vibration damping has been provided to prevent breakage
of the bellows. With metal bellows seals try to rotate the fluid
in the sealing chamber to prevent excessive wear of the thin
- The bearing grease or lip seals should be replaced with
labyrinth or positive face seals. The O.E.M. lip seals have a
design life of about two thousand hours (84 days) and they will
cause costly shaft fretting damage. These grease or lip seals will
also allow moisture to penetrate into the bearing case
dramatically reducing bearing life.
- If you eliminate these lip seals, you will be able to convert
to a solid shaft and improve the "stiffness ratio" enough to
prevent some of the shaft bending and vibration that's experienced
at start up, and as the pump runs off of its' best efficiency
- A cool oil flush with a restriction bushing installed into the
bottom of the stuffing box, is another choice. Be sure that the
flushing pressure remains at least one atmosphere (15 psi. or 1
bar) higher than the stuffing box pressure.
- Do not hydrostatically test the seal with water. Any moisture
left in the seal or trapped in a gasket will flash to steam when
the hot oil enters the seal. This could be dangerous.
- When using an A.P.I. type gland be sure to check that the
quench and drain ports have not been confused with the flush
ports. If these ports are connected incorrectly it could be very
- If you're using stationary bellows seals with a cool oil flush
be careful to direct the flushing fluid away from the seal face.
Since the bellows is not rotating the cooling on one side and the
hot system temperature on the other can cause the bellows seal
face to go "out of flat".
- Recent tests show that carbon faces always experience some
pitting in hot oil applications. In the past these pits were
ignored, but fugitive emission standards dictate that two hard
faces should be used in all hot oil applications.
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