GCI TECH NOTES ©

Elastomers for use as o-rings and gaskets

Nitrile rubber should be considered as the absolute minimum for resistance to solvents and waste fuels.

Viton-E (Dupont trademark) or equal, a partially fluorinated elastomer, is very good for gaskets and o-rings but certain solvents will cause them to swell. Consequently, a gasket or o-ring which was in good condition may swell upon removal. Frequently, when left to dry out they will shrink back into shape.

Kalrez (Dupont trademark), another partially fluorinated elastomer, is excellent for a wide variety of solvents, particularly for the chlorinated hydrocarbons and the "super solvents" such as gama-butyl lactone or normal methyl pyrilodone. This material is very expensive and should be reserved for critical applications such as pump mechanical seals.

Teflon (Dupont Trademark) encapsulated rubber - This is a rubber such as nitril or buna-n which has polytetrafluoroethylene (PTFE) or equal covering. The PTFE resists virtually all chemicals and will work well in applications where the gasket o-ring is placed in service and undisturbed for a long period. The rubber interior gives good sealing properties without the requirements of an excessively tight bolting of the flange.

Teflon (Dupont trademark) or PTFE - This is the white, stiff plastic material most people are familiar with. It comes in a number of thicknesses and can be cut into any flat gasket shape. Teflon can be used as a pipe flange gasket, but requires a very tight bolting of the flange to ensure a seal. This will work well for raised face flanges but poorly for flat face flanges.

Fiber gasket materials - These were originally made of asbestos. The current fibrous material that replaces asbestos will work well in applications which are not subjected to movement or vibration. For example, such materials would be poor choice for tank trucks.

Metal Flex gaskets - These are fabricated gaskets made up of a flat metal ring and composed of flexible metal layers. This gasket can only be used once. Once the flange has been tightened, the metal flex gasket must be replaced if the flange is loosened. When installing this gasket, a torque wrench should be used to properly tighten the flange bolts. The metal flex gaskets are made of a number of corrosion resistant metals, 304SS, 316SS and others. They are expensive but are excellent for hot fluids at high temperatures.

Other elastomers - Many vendors have special "house brand" elastomers which have characteristics of the best chemical resistant materials. If a vendor offers a substitute material insist on a chemical resistance chart for that material.

Pump and agitator shaft mechanical seal materials

The stationary and rotary materials in mechanical seals are extremely flat highly polished disks that are pressed together to create a seal which will not pass liquids when rotated past each other. Unfortunately the springs holding the faces together cannot be too tight or the surfaces will become hot and destroy the mechanical seal. Consequently, the faces may occasionally spread open slightly, especially during pump or agitator startup, and allow fluid between the seal faces. If this fluid has gritty solids, such as sand, the sand will be trapped between the surfaces and damage the seal faces. For this reason a mechanical seal consisting of carbon rotary and a ceramic stationary will often fail in hazardous waste service. A tungsten carbide/silica carbide seal is recommended. The metallic parts of the seal are recommended to be constructed of 316SS including any springs or screws. A "bellows" seal is preferred over the multiple spring activated seals. The "bellows" seal is less subject being filled with solids which then harden due to heat and prevent the seals from moving freely.

Non-metallic materials are frequently used in valves as sealing materials. Nearly any of these are suitable for solvent or waste fuel use but, are subject to thermal shock and impact shock. These materials are not recommended for valves subject to temperature extremes or where torquing of the valve body is possible such as the valve connected to a transfer hose.

Transfer Hose Materials

The best all-around hose is a Dayco (brand) chemical hose with a cross linked polyethylene tube surrounded by layers of fabric and rubber. This hose is resistant to most chemicals, but to get the longest life from the hose it must be drained after each use.

For hot solvents the best hose is a Teflon inner tube covered by a stainless steel braid.

The standard unleaded gasoline transfer hose can be used on an interim basis but must be examined daily.