Subject : The sealing of dangerous
The definition of fluids includes both liquids and gases and any
of these fluids could be labeled dangerous for a variety of
- The fluid is a fire hazard.
- The fluid is an explosive.
- The fluid can be toxic to people.
- Pollutants are dangerous.
- Radioactive material is dangerous.
- Hot or cryogenic liquids that could injure personnel if they
came into contact with them.
- Toxic fluids are dangerous
- And the list goes on...
My seal application technique involves classifying these fluids
into one or more of seven specific categories. Dangerous fluids are
just one of these categories, but dangerous fluids could be placed
into one or more of the other six categories also. Here are the other
six categories in case you don't know them:
- Fluids sensitive to a change in temperature and/or
- Fluids sensitive to agitation. (cream becomes butter with
- Non-lubricating gases and liquids. (hot water is a poor
- Film building fluids.(paint as an example)
- Products that react together to form a solid.(Styrofoam)
- Clean lubricating liquids.(cold water)
Dangerous fluids are just like any other fluid you'll be sealing,
you must always make two decisions:
- Choose corrosion resistant materials for the seal components
and be sure the seal materials are also corrosion resistant to any
cleaners or solvents that might be flushed through the pump and
- If necessary, apply the correct environmental control to
ensure that the product stays in either a liquid or gaseous state
and does not solidify, vaporize, crystallize, build a film, etc.
Paper 2 -12 discusses these
classifications in great detail. Paper
3 -2 will teach you about the various environmental controls
we use to prevent the fluid from changing state.
In every instance you'll need some type of seal back-up protection
to protect personnel in the area when the seal wears out or fails. In
most cases it'll mean installing dual
seals, but here are some other options:
An A.P.I. gland with a disaster bushing (DB), and a
quench (Q) and drain connection that will direct most of the
mechanical seal leakage to a collecting tank or out to a
flare where it can be burned.
Dual seals with a convection tank supplying liquid to the
second seal. The convection tank is filled with a high
pressure barrier fluid to ensure that there will be no
pressure drop across the inner seal face to the convection
Here is a picture of a typical convection tank. The tank
can be either purchased or manufactured in your shop.
Purchased tanks must meet the boiler makers code, meaning
that the high pressure requirement might make them very
expensive for your application.
Some other back up options
- Dual seals installed with a convection tank, filled with a low
pressure buffer fluid to ensure that there will be no product
dilution if the inner seal fails.
- Dual, hydrodynamic gas seals with a
high pressure inert gas barrier fluid to ensure that no barrier/
buffer liquid will get into the pumpage.
- Dual hydrostatic gas seals
with a high pressure inert gas barrier fluid for the same
Some additional things to consider when
you are sealing dangerous fluids.
- Most hydrodynamic gas seals are uni-directional. Be sure the
shaft is turning in the correct direction to generate the
necessary hydrodynamic force. Hydrostatic gas seals do not have
- Avoid un-necessary pipe taps. Heat exchangers and coolers
installed between the pump discharge and the stuffing box require
a minimum of four pipe connections. You would be better off with a
jacketed stuffing box. A high temperature, high pressure boiler
feed pump is typical of this application.
- If you are sealing an intermittent service pump remember that
any environmental controls such as heating/cooling, or stuffing
box pressure control will probably have to function when the pump
is stopped as well as running.
- A low
L3/D4 pump shaft is critical with
dangerous products. Shaft deflection is a major cause of seal
damage and seal face openings. Shaft deflection also raises the
possibility of rotating and stationary parts contacting, causing
potentially dangerous sparking.
- Reduce stuffing box heat generation as much as possible:
- Avoid "discharge throttling" if you are pumping dangerous
fluids sensitive to increases in product temperature. The extra
heat generated by the internal recirculation could cause an
explosion, fire, etc. This is an instance when suction throttling
may be appropriate.
- Selecting low friction, non sparking seal faces with good heat
conductivity would be a wise choice. Carbon/graphite vs. silicon
carbide would be such a combination
- Try not to use oil as a buffer or barrier fluid in a
convection tank. Oil has a low specific heat and a poor
conductivity that will almost guarantee the generation of unwanted
heat between the dual seals. If oil is absolutely necessary be
sure to use a pumping ring between the dual seals because
convection will probably not be efficient enough to provide the
proper circulation and cooling.
- Hydraulic balance is very
necessary for low heat generation between the seal faces, and
"two way balance" for the
inboard seal of a dual seal arrangement would be a wise
- A large inside diameter stuffing box will reduce heat
generation in the seal area.
- A good flow through the stuffing box is another way to remove
unwanted heat. Suction recirculation is your best choice for most
- If sparking could set off an explosion or fire, a closed
impeller pump with non-sparking wear rings and a non-sparking
disaster bushing in the seal gland would be a good choice. Most
oil refineries have elected this design; it's called an API
- Bellows seals have the possibility of rupturing and causing a
massive failure. This is especially true of rubber bellows
- Try to incorporate as many non-clogging features into the seal
as you can.
- Use designs where the springs are positioned out of the
- The elastomer should move to a clean surface as the carbon
- The carbon face should wear towards a clean surface. This can
be a problem with some outside mounted seals and the inner seal of
some dual seal arrangements.
- Put the rotating parts of the seal into the fluid to take
advantage of centrifugal force that will throw solids away from
the lapped faces.
- Do not use seal designs that will frett
or damage the shaft or sleeve. This damage becomes a potential
leak path or a place to restrict seal movement, and open the
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