A gas barrier seal is a dual seal whose faces float apart on pressurized gas; a water flushed seal’s faces run in contact on clean water fed from outside. Running without contact, the gas seal makes almost no heat and uses no water, but it needs a clean product, steady shaft speed and an uninterrupted gas supply.
The water flushed seal copes with solids and slow running. On a single seal, though, its flush water ends up in the product, and on a dual seal the water barrier needs a cooler and regular refilling.
What Is a Gas Barrier Seal
A gas barrier seal is a dual pressurized mechanical seal that runs on gas instead of liquid. Pressurized gas is fed into the space between the inboard and outboard face pairs, and grooves or similar patterns cut into the faces pump it into the gap as the shaft turns. The pressure this builds pushes the faces apart, so in normal running they float on a thin gas film without touching or wearing.
It is a barrier because the gas is held above seal chamber pressure; unpressurized gas between the seals is called a buffer. The gas is usually plant nitrogen.
API 682 lists its support system as Plan 74, a small panel of valves and instruments that cleans the gas, regulates its pressure and measures the flow going to the seal.
On pumps the same design is also called a pump gas seal or a dual gas-lubricated seal.
What Is a Water Flushed Mechanical Seal
A water flushed mechanical seal is a seal whose faces run on clean water piped in from outside the pump. Flushing means feeding a clean fluid to the seal to keep its faces lubricated, cool and clear of solids. Plants use the name for two set-ups.
The first is a single seal on an API Plan 32 flush. Clean water is injected into the seal chamber behind a close-clearance throat bushing, flows across the faces and leaves past the bushing into the pumped product.
The second is a dual pressurized seal with water as the barrier liquid, held above seal chamber pressure and circulated between the inboard and outboard face pairs (Plans 53A, 53B, 53C and 54).
The Difference Between a Gas Barrier Seal and a Water Flushed Mechanical Seal
A gas barrier seal is a non-contacting seal, and a water flushed seal, single or dual, is a contacting one.
Face Lubrication and Heat
Water-lubricated faces run partly in contact and make frictional heat that the water has to carry away, whereas gas faces run apart and make almost none. A gas seal’s face friction can be as little as 6 W at normal pump speeds, against up to 300 W for a single liquid-lubricated seal in similar conditions.
Where the water’s heat goes depends on the set-up. A Plan 32 flush takes it into the product. A water barrier carries it back to the reservoir, where a heat exchanger has to remove it. The gas seal needs no cooler.
What Enters the Product
A Plan 32 flush sends all of its water into the product, while a dual seal, whether its barrier is water or gas, puts in only what leaks across its inboard faces. A Plan 32 flush without flow control can easily run 20 to 30 gallons a minute.
A gas seal’s leakage is measured in standard cubic feet per hour, and only about a third of it goes inboard. In an open process the nitrogen is carried along with the fluid and vented further downstream, and it is typically not an issue. It does matter in closed loops and vessels, where the gas can collect until the pump cavitates or vapor-locks, and in processes sensitive to chemical reaction, catalyst contamination or dilution.
Utility Supply
A gas barrier seal needs only clean gas at pressure and no water at all. A water flushed seal needs water, and on a dual seal a pressure source and cooling as well.
The barrier gas has to stay about 30 psi above the pump’s maximum seal chamber pressure, and the exact minimum depends on the seal maker and seal type. Where the plant nitrogen header cannot reach that pressure, a pressure amplifier boosts it into a receiver that feeds the panel. Banks of bottled nitrogen are not recommended, because someone has to keep swapping empty bottles for full ones.
A Plan 32 flush draws clean, filtered water for as long as the pump runs, and you pay for that water twice, once as supply and again as wastewater to treat.
A water barrier keeps its water in a loop and only needs topping up for what leaks away. On Plan 53A the pressure comes from plant nitrogen, so a pump on 53A already has nitrogen piped to it, and the barrier cooler usually runs on plant cooling water.
Operating Limits
A gas barrier seal is limited by speed, solids and sticky or viscous products. Water at the faces is limited by temperature and, on a Plan 32 seal, by whether water is allowed into the product.
- Speed: The faces lift only above a minimum circumferential speed, typically 250 ft/min (1.3 m/s) in standard designs. Speeds below that come with slow roll at shutdown, long VFD ramps and steam-turbine drives kept on slow roll; special low-speed designs exist but use more gas.
- Solids: Slurries above 5 percent solids are a problem unless a clean flush or an anti-particulate seal chamber keeps them away from the inboard faces.
- Sticky, viscous or high-freezing-point products: Film-building fluids such as paint can stop the seal from moving freely, and products with a high freezing point can solidify where ambient-temperature barrier gas enters the pump.
Water works in a narrow temperature window. It freezes in winter, and when hot its viscosity falls too low to lubricate the faces well.
A Plan 32 flush is usually much colder than the process, so on hot duties it also cools the product and costs process heat.
Support System Maintenance
A Plan 74 gas panel needs almost no maintenance, while both water set-ups need regular attention.
On the gas panel the main job is the filter element. Inspect it after the first month of running, then every six months, and replace it within three years at most, sooner if the supply gas is dirty. Record the barrier gas flow on routine rounds.
A water barrier’s working volume is typically sized to cover about a month of seal leakage between refills. The barrier cooler also fouls on its cooling-water side, and in hard-water areas it needs frequent descaling.
A Plan 32 flush has no reservoir or cooler, and its upkeep is flow control and filtration.
Loss of Supply
Either seal loses its face lubrication when the supply stops: a gas seal’s faces close and run in contact, and a water flushed seal’s faces run dry or in the pumped product itself.
If barrier pressure falls below its set point, or process pressure rises higher than planned, most gas seals are built to hold the reversed pressure, so the product does not leak into the barrier space or to atmosphere. Gas seal faces running in contact do not last long at pump speeds, and even a partial loss of pressure can damage them.
If a water barrier’s level is not kept up, both the inboard and outboard faces run dry, overheat and fail early.
When a Plan 32 flush stops, nothing keeps the pumped fluid and its solids out of the seal chamber.
How to Choose the Right One
Choose a gas barrier seal for clean, steady-speed duties where water should stay out of the product; keep a water flushed seal for solids, slow or variable speed, and products that can take the water.
Nitrogen is inert and does not contaminate or degrade most products, so gas seals are used on food and pharmaceutical pumps where purity comes first.
Switching is easiest to justify where the pump needs a dual seal anyway. Water and wastewater pumps on a dual seal qualify too, but VFDs are common there, so review the duty with the seal maker if the pump has one or handles solids.
An industry energy reference rates each support plan’s impact on electrical, thermal and water efficiency:
| Support plan | Set-up | Electrical | Thermal | Water |
|---|---|---|---|---|
| Plan 32 | External flush, single seal | Minor | Major | Major |
| Plan 53 | Pressurized liquid barrier, dual seal | Minor | Minor | Minor |
| Plan 54 | Liquid barrier circulated by an external system, dual seal | Moderate | Moderate | Moderate |
| Plan 74 | Gas barrier, dual seal | None | None | None |
The Plan 53 and 54 rows cover any barrier liquid, water included.
On purchase price, a dual gas seal can cost more than a comparable liquid-lubricated seal, but with its support system counted it can come out cheaper than a seal on a liquid barrier system. Against a single seal on a Plan 32 flush, it adds a second face pair and a panel, and the water and heat it saves have to cover that cost.
Gas and water make no difference to emissions on a dual seal, since any pressurized dual seal leaks only its barrier fluid to atmosphere.
If you stay with a liquid barrier, water is a good choice wherever its temperature window allows. Outside that window, glycol mixes and oil-based barrier fluids are the alternatives.
