Canned motor pumps vs sealed pumps: when sealless is worth it

A canned motor pump is worth it when a leak would be dangerous, costly or reportable, as with toxic, flammable, corrosive or volatile fluids. It removes the mechanical seal entirely, so there is no dynamic seal to wear or leak. For water, mild chemicals and services where a drip is acceptable, a sealed pump usually costs less.

How does a canned motor pump work?

A canned motor pump combines the pump and motor into one sealed unit with no shaft passing through to the atmosphere. The motor rotor sits inside a thin, corrosion resistant metal can, and the stator windings sit outside a second can. Pumped fluid flows through the motor to cool it and lubricate the bearings.

Because the impeller and rotor share one shaft inside the pressure boundary, there is nothing to seal against the outside world. Key features of the design:

• Rotor and stator liners: thin cans, often a nickel alloy, isolate the windings from the fluid.
• Product lubricated bearings: sleeve bearings, typically carbon or silicon carbide, run in a film of the pumped fluid.
• Secondary containment: the stator housing acts as a second barrier if the stator liner is ever breached.
• Bearing wear monitoring: many designs, including Teikoku Chempump, offer a monitor that warns of bearing wear before it causes damage.

Teikoku Chempump canned motor pumps, including the ANSI dimensioned LE and LE X Series, are built around this principle. You can review them on our Teikoku page.

Where do mechanical seals fail?

Mechanical seals fail most often from running dry, poor lubrication of the seal faces, solids or crystals in the fluid, misalignment, vibration and shaft deflection. Every sealed centrifugal pump has a rotating shaft passing through the casing, and the seal faces are the only thing keeping the fluid inside. When those faces wear or crack, the pump leaks.

Common seal failure causes include:

• Dry running during start up, after a tank runs low, or with entrained vapor.
• Abrasive particles that score the faces.
• Fluids that crystallize or polymerize between the faces.
• Operating far from the best efficiency point, which raises shaft deflection.
• Poor installation, piping strain or coupling misalignment.
• Seal support system problems, such as a failed flush or a barrier fluid that runs out.

Double seals with barrier fluid systems reduce the risk of process leaks, but they add a seal pot, instruments and their own maintenance. They lower risk without removing the leak path.

Why do leaks and emissions matter so much?

Leaks matter because they affect worker safety, environmental compliance, product loss and plant uptime. Pumps with mechanical seals are a recognized source of fugitive emissions, and many facilities handling volatile organic compounds or hazardous air pollutants must monitor and repair leaks under US EPA leak detection and repair programs.

A sealless pump changes the conversation. With no dynamic seal, there is no seal to monitor, no seal leak to report and no seal support system to maintain. For plants handling chemicals such as chlorine, hydrofluoric acid, phosgene, ammonia, refrigerants or heat transfer fluids, that can simplify compliance and lower the chance of a serious incident.

Even for less hazardous fluids, a leak still costs money in lost product, cleanup, housekeeping and corrosion of nearby equipment and floors.

How does maintenance compare?

Canned motor pumps generally need less routine maintenance than sealed pumps because there is no seal, no coupling to align, no separate motor bearings to grease and no seal support system. The main wear parts are the internal bearings, and a bearing monitor helps you plan their replacement instead of reacting to a failure.

Sealed pumps need more regular attention:

• Seal replacement and rebuilds.
• Coupling alignment checks.
• Motor and pump bearing lubrication.
• Flush plans, seal pots and barrier fluid levels on double seals.

The trade off is that canned motor pumps are less forgiving of poor operation. Dry running, extended operation at closed discharge and vapor in the motor section can damage bearings quickly. Good instrumentation and operator training matter.

What do they cost over their life?

A canned motor pump usually costs more to buy than a comparable ANSI sealed pump, but it often costs less over its service life in critical services. Savings come from fewer seal repairs, less downtime, reduced emissions monitoring and no separate motor, base plate or coupling to buy and maintain.

When you compare costs, include:

• Purchase price of the pump, motor, base and coupling.
• Seal support systems and instrumentation.
• Expected seal repairs per year and the labor for each.
• Lost production during unplanned shutdowns.
• Leak monitoring, reporting and cleanup.
• Energy use at your actual operating point.

The Hydraulic Institute publishes guidance on pump life cycle cost that is a useful framework for this comparison. In many plants, a single avoided seal failure on a hazardous service pays a large share of the price difference.

When is a mag drive or sealed pump the better choice?

A mag drive or sealed pump is often the better choice when the fluid carries solids, has very poor lubricity, runs at high viscosity or when a leak is a minor concern. Mag drive pumps are also sealless and are often chosen when you want a standard motor or a lower first cost than a canned design.

Consider a mag drive pump when:

• You want sealless containment but prefer a standard, replaceable motor.
• The service is a moderate size, clean chemical transfer.
• Your plant standardizes on frame mounted motors.

Consider a sealed pump when:

• The fluid is water or a mild, nonhazardous chemical.
• The fluid carries abrasive solids or slurries.
• Viscosity is high enough to cause problems for product lubricated bearings.
• Budget is tight and a small drip is acceptable.

Gear pumps follow the same logic. For viscous fluids, a sealless option such as an Eclipse mag drive gear pump can remove the seal without moving to a centrifugal design.

How do the options compare side by side?

The table below summarizes typical strengths of canned motor, mag drive and sealed designs. These are general guidelines, and the right answer always depends on your fluid, temperature, pressure, solids content and duty cycle. Use it to narrow the field, then confirm the choice with your actual operating data.

How should you decide?

Decide by starting with the consequence of a leak, then checking whether the fluid suits product lubricated bearings. If a leak would be hazardous or reportable and the fluid is reasonably clean, a canned motor pump is usually the strongest choice. If not, compare mag drive and sealed options on cost.

1. Rate the leak consequence. Is the fluid toxic, flammable, corrosive, volatile or expensive?
2. Check the fluid. Note solids, viscosity, vapor pressure and temperature.
3. Review the operating range. Know your normal, minimum and maximum flow, and how close the system runs to its vapor pressure.
4. Compare life cycle cost, not just purchase price.
5. Plan protection. Add low flow, dry run and bearing monitoring where needed.

PCT can review your process data and recommend canned motor, mag drive or sealed options from our pump lines. We also build pump skid packages and provide field service, start up and maintenance training.

Sources

• Hydraulic Institute
• US Environmental Protection Agency

Not sure if sealless is right for your service? Send us your fluid, flow and pressure and we will help you choose. Request a quote.