Sealing dangerous fluids
The sealing of dangerous fluids.13-5
The definition of fluids includes both liquids and gases and any of these fluids could be labeled dangerous for a variety of reasons:
- 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 pressure.(like caustic)
- Fluids sensitive to agitation. (cream becomes butter with agitation)
- Non-lubricating gases and liquids. (hot water is a poor lubricant)
- 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 piping.
- 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 tank.|
|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 include:
- 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 application.
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 that limitation.
- 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 choice.
- 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 applications.
- 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 gland.
- Bellows seals have the possibility of rupturing and causing a massive failure. This is especially true of rubber bellows seals.
- Try to incorporate as many non-clogging features into the seal as you can.
- Use designs where the springs are positioned out of the fluid.
- The elastomer should move to a clean surface as the carbon face wears.
- 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 lapped faces.
- On February 18, 2018