Mike Beckerdite

Every centrifugal pump needs a way to prevent the product liquid from leaking past the rotating drive shaft. The sealing method can be as simple as packing rope impregnated with graphite into a “packing box” or as complex as enclosing the entire pump in a chamber (mag drive or canned motors). The most common sealing method is a mechanical seal.

What is a Mechanical Seal?

A mechanical pump seal is a leakage control devices typically found on rotating equipment. They prevent the leakage of liquids from escaping into the environment.

The centrifugal pump mechanical seal relies on the product liquid to cool and lubricate the seal faces, preventing damage, with small amounts of vapor escaping. For abrasive or non-lubricating liquids, a “barrier fluid” may be needed to protect the seal faces. 

Basic Components of a Mechanical Pump Seal

The simplest mechanical seal consists of a “stationary face” attached to the pump housing and a “rotating face” attached to the rotating shaft. Both faces must be sealed to the pump via flexible “boots,” o-rings, or other means to prevent leakage. These boots are typically made of elastomers such as rubber (BUNA), EPR, or Viton. In some cases, flexible metal bellows or Teflon (though difficult to install) may be used.

How Does a Mechanical Seal Work?

Metal components keep the seal intact and provide spring pressure to maintain face contact. These parts are usually 316SS but can be made from other corrosion-resistant metals when needed.

A vast majority of centrifugal pumps will pump water in some varied conditions, possibly in combination with an antifreeze and some antibacterial additives. Since water by itself is not a very good lubricant (especially at high temperatures), the seal faces need to provide some lubricity of their own so that when they touch at startup or shutdown, the faces will not tear themselves up. This is usually accomplished by making one of the seal faces out of carbon or by some material impregnated with carbon. The other seal face is often a harder material that will resist wear from the carbon face, such as a ceramic or silicon carbide, or solid metal.

Note that although water is the most common liquid pumped, the different additives used can make a huge difference in the type of seal required. Relatively clean water, even when combined with glycol, is easy and inexpensive to seal, whereas dirty water or water with lots of additives can be very difficult. The difference is in the abrasiveness and lubricity of the liquid. If when you rub a sample between your fingers you can feel the lack of lubricity, or if your liquid has lots of salts or other dissolved solids, you may need a more complicated and expensive mechanical seal.

Selecting the Right Mechanical Seal for Your Application

Clean Water Mechanical Seal

For non-abrasive water, a basic seal is sufficient. The stationary face is typically ceramic, sealed with a Buna boot or o-ring. The rotating face is often carbon/graphite, also sealed with a Buna boot or o-ring. The spring and metal parts are 304SS or 316SS.

Water with Salts, Abrasives, or Additives

Water with any salts, abrasives, or additives is much harder on mechanical seal faces. When the mechanical pump seal is operating properly, small amounts of liquid enter the space between the two faces. As the liquid travels from the high-pressure side of the face to the low-pressure side, the liquid heats up and vaporizes. While this vaporization is needed to cool and lubricate the faces, it also leaves any salts or other solids in the space between the seal faces. These deposited solids are very abrasive and will accumulate over time and may quickly erode the faces (particularly the softer carbon face).

The simplest and cheapest solution for mildly abrasive liquids is to upgrade the seal faces to materials that can resist abrasives better than the inexpensive carbon/ceramic combination used for clean liquids. The least expensive harder material is usually silicon carbide, but other ceramics/metals are used. The downside of silicon carbide or any other hard material occurs when they run dry, even momentarily during startup and shutdown. Any period of dry running can cause wear and heat buildup and may cause the mechanical pump seal to fail. (In severe cases, the heat will melt the elastomeric parts). This dry running problem is significantly reduced when the silicon carbide faces are impregnated with carbon/graphite, but the problem is not eliminated. Depending on many variables, the seal with impregnated faces can only run dry for a short time before failing, but eventually, the seal will fail from heat buildup.

Other solutions for sealing abrasive liquids include flushing the seal faces with a clean liquid, but this option requires special seals (such as a double seal) and special flushing piping. The cost of this option gets expensive fast and may require a much larger, more expensive pump and seal combination.

Water at High or Low Temperatures

As water temperature gets lower than 32°F or higher than 212°F, the mechanical seal materials may need to be upgraded. Low temperatures usually need EPR elastomers to handle the cold, and higher temperatures may need Viton. Extreme temperatures may also require that the standard ceramic be replaced with the much tougher silicon carbide or other metal faces. Ceramic faces are very susceptible to thermal shock, and a sudden change in the temperature of the liquid can shatter the seal faces.

Food Grade Seals

Centrifugal pump seals used in food processing must meet sanitary requirements. These typically use Viton elastomers, 316SS metal parts, and other food-safe materials. Many food-grade seals are positioned outside the pump for easy daily cleaning, requiring specialized pump designs.

Liquids other than Water

Non-water liquids may require specialized centrifugal pump seals. Toxic or flammable liquids often demand double seals, barrier fluids, and specific flushing systems to protect both the seal and the environment. These advanced types of mechanical seals for pumps increase complexity and cost but are essential for safety and compliance. Consulting with a pump or seal specialist is recommended for these applications.

Pumps That Do Not Require a Mechanical Seal

If your liquid is very hard to seal or very dangerous, you may want to consider a “sealless” pump. The three most common types of sealless pumps are:

1. Mag Drive Pumps
2. Canned Rotor Pumps 
3. Vertical Cantilever Pumps

Mag Drive Pumps

A Mag Drive Pump uses magnets to drive the pump in a contained shell. One set of Magnets spins on an outside cylinder attached to the motor. Another set of magnets spins on an inside cylinder attached to the pump. The two sets of magnets are separated by a thin, non-metallic shell that keeps the pump liquid inside the shell. 

The problem with this arrangement is that there are lots of bushings and sleeves needed to support the pump shaft and the impeller. These bushings and sleeves are exposed to the pumped liquid, so any abrasives can kill the pump quickly. Repairing or replacing these bushings and sleeves is very difficult and very expensive and only adds to the cost of an already expensive pump. This pump is very useful for clean but dangerous liquids.

Canned Rotor Pumps

Canned rotor pumps take the sealless concept further by enclosing both the motor and pump in a sealed housing, eliminating the need for external magnets. However, they are highly sensitive to abrasive liquids, which can cause expensive damage. These pumps are ideal for certain hazardous or high-temperature applications.

Vertical Cantilever Pumps

Vertical cantilever pumps are well-suited for handling abrasive liquids. The motor is positioned above the liquid, and a long shaft extends down to drive the impeller. Bearings are kept out of the liquid, preventing wear. However, these pumps require additional space and structural support, and longer shaft lengths can reduce efficiency.

Contact American Stainless Pumps

At American Stainless Pumps, we specialize in high-quality stainless steel pumps designed for durability and efficiency. We also offer pump motors, pump components, seal kits, and more.

Contact us today to speak to a pump specialist.