A dry mechanical seal runs with no liquid between its faces, while a wet mechanical seal runs on a liquid film supplied by the pumped product, a flush, or a barrier fluid.
In compressors, “dry vs wet” means something else: a dry gas seal against an oil-film seal, which this article does not cover. A standard liquid-lubricated seal that has lost its film is also not a dry seal. It is a seal that is failing.
What Is a Dry Mechanical Seal
A dry mechanical seal is built to run with only gas or air at the faces, and it comes in two designs. The contacting dry-running seal presses a self-lubricating carbon face against a silicon carbide or tungsten carbide seat.
The gas-lubricated seal is non-contacting. Grooves in one face draw in gas, usually nitrogen, and lift the faces apart once the shaft turns.
Contacting dry-running seals go on top-entry agitators and mixers, vacuum dryers and dry-powder vessels, where the seal sits above any liquid. Gas-lubricated seals are mostly used as dual gas-barrier seals on pumps, fed with pressurised nitrogen.
What Is a Wet Mechanical Seal
A wet mechanical seal runs with a liquid film between its faces, and that film lubricates the faces and carries away their frictional heat. The liquid comes from one of three places: the pumped product itself, an external flush piped into the seal chamber, or a buffer or barrier fluid held between the two seals of a dual arrangement.
Single and double arrangements are both wet, as are nearly all centrifugal pump seals in service. The single, cartridge, double and metal bellows seals we build are wet seals by default.
The Difference Between Dry and Wet Mechanical Seals
The liquid film is the difference.
Lubrication and Heat at the Faces
A wet seal’s liquid film lubricates the faces and removes their heat. A dry seal either has rubbing faces with no film to take the heat away, or a gas film that generates almost no heat at all.
Frictional heat therefore limits a contacting dry seal to slow shafts.
Face Materials
Dry and wet seals use the same basic pairing. What differs is the grade of carbon and the finish of the hard face. Carbon against silicon carbide is already the API 682 default for ordinary wet pump seals, so ordering “carbon vs SiC” does not get you a seal that can run dry.
A contacting dry seal needs a carbon grade developed for dry running. Published guidance on dry-running carbon recommends a counterface roughness of 0.4 µm or less. Specify the carbon grade and the counterface finish on the order, not just the pairing.
Gas-lubricated seals can drop carbon and run silicon carbide against silicon carbide. Grade choice for dry duty is covered in materials for dry-running mechanical seals.
Speed and Pressure Limits
Contacting dry seals are rated for slow, low-pressure shafts, while gas-lubricated seals need pump speed to lift their faces and wet seals cover the normal pump range. The ratings below come from individual seal datasheets and the API 682 seal categories. Treat them as examples; they are not limits for each whole family.
| Seal design | Rated product or standard | Sliding velocity | Pressure |
|---|---|---|---|
| Dry, contacting | EagleBurgmann SeccoMix 461 dual agitator seal | 0–2 m/s | vacuum to 6 bar |
| Dry, gas-lubricated | EagleBurgmann CGSH-K single pump seal | 4–25 m/s | 25 bar |
| Dry, gas-lubricated | EagleBurgmann HRGS-DD dual pump seal | 4–25 m/s | up to 40 bar |
| Wet | API 682 Category 2 and 3 pump seals | no single published figure | up to 40 bar |
Below 4 m/s the faces of a gas-lubricated seal do not lift, so it cannot replace a contacting dry seal on a slow agitator.
Product Contamination
A dry seal keeps liquid out of the product, while a wet dual seal can leak its barrier fluid into it. In food and pharmaceutical service that barrier must be compatible with the product, and where no barrier liquid is acceptable a nitrogen barrier is the usual answer.
Dry seals still add a little to the product. A gas-barrier seal sends part of its barrier gas into the process. It normally vents downstream, but in a closed loop it can collect and gas-lock the pump.
A contacting dry seal sheds fine carbon dust as its face wears, and on a sanitary food or pharmaceutical vessel that dust has to be kept out of the product.
Containing Hazardous Fluids
Containment depends on whether the seal is a pressurised dual or a single, whichever lubricant it runs on. The Fluid Sealing Association states that pressurised dual seals, liquid- and gas-lubricated alike, leak only their barrier fluid to atmosphere and can hold emissions of the pumped fluid at zero.
The single contacting dry-running seal has no barrier, so whatever crosses its faces escapes to atmosphere.
For volatile or toxic chemical service, the choice is between a liquid-barrier dual and a gas-barrier dual.
Support Systems and Maintenance
A wet dual seal needs a barrier system: a reservoir or bladder accumulator with piping, pressurisation and cooling, sited close to the seal and refilled on a schedule.
A wet agitator seal also needs cooling coils or a jacket, because on an agitator the pumping ring that circulates barrier fluid turns too slowly to move useful flow.
A contacting dry seal needs no support system. What you budget for is face wear, with faces replaced on a schedule.
A gas-lubricated seal needs a Plan 74 panel that filters, regulates and meters the barrier gas, usually plant nitrogen. The panel needs no cooling water and almost no maintenance, but it depends on a reliable, clean gas supply. The full set of support arrangements is on our API plans page.
How to Choose the Right One
Choose by what is at the seal faces.
- Liquid at the seal, normal pump duty: a wet single or double seal, the default for centrifugal pumps.
- No liquid at the seal, slow shaft: a contacting dry-running seal, for top-entry agitators running above the liquid level, powder mixers and vacuum dryers.
- Product must not see barrier fluid, or the pump runs dry during tank unloading: a gas-lubricated dual seal, where a reliable pressurised gas supply is available.
- Hazardous or volatile product: a pressurised dual seal, with either a liquid or a gas barrier.
On cost, the contacting dry seal is the cheapest to buy and has no support system, and its running cost is face replacement. A dual gas seal may cost more to buy than a comparable liquid dual, but once the support system and energy are counted it may come out cheaper overall. A wet dual adds the cost of its barrier system and its upkeep.
The two can also be combined. Where a wet seal risks losing its flush, a dry-running carbon grade against silicon carbide or tungsten carbide lets it survive short spells of face contact.
The leakage figures and wet-to-dry retrofit cases that turn up under “dry vs wet seals” come from compressors. They do not transfer to pump or agitator seals.