Select the Right Sealless Pump

Understand the advantages and limitations of the two main options

By Amin Almasi, rotating equipment consultant

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Centrifugal pumps comprise over 90% of all pump installations in the chemical industry. They have proven to be the most economical pumps in various services; they require much less maintenance and operational efforts than other pump types. A centrifugal pump usually includes a casing (housing) having a cavity, a suction and a discharge; a shaft located in the cavity has an impeller (or impellers) positioned to receive liquid from the suction and exhaust that liquid to the discharge. Unfortunately, problems with shaft seals often arise — indeed, seals cause more than half of all unscheduled shutdowns of centrifugal pumps.

Many plant operations can’t tolerate any leakage of liquids for safety, environmental or economic reasons. Yet, some difficult services pose a nightmare for seal selection. This has spurred the development of sealless centrifugal pump technologies.

Sealless versions now are available for all common centrifugal pump designs: end-suction top-discharge used for single-stage pumps; top-suction top-discharge used for multi-impeller horizontal pumps; and multi-impeller vertical pumps (sometimes with 30 impellers or more) used for high-pressure applications.

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Magnetic drive units are the most common sealless pumps at chemical plants but submerged motor pumps also find wide use. Both types of pumps have proven themselves over many years in a variety of different services.

Magnetic Drive Pumps

These are close-coupled pumps that can be quickly and easily stripped and rebuilt in the field; most often they don’t require traditional alignment. Such units usually handle corrosive or difficult liquids; materials of wetted parts must suit the particular liquid.

Magnetic drive pumps have some limitations and disadvantages. For instance, they don’t come in large sizes and with high power ratings. Many internally circulate the liquid being pumped for bearing lubrication and cooling; so, those pumps aren’t appropriate for some applications, such as ones involving liquids susceptible to forming scale.

Magnetic drive pumps also should not run dry. While that’s more or less true for centrifugal pumps in general, a magnetic drive pump is more vulnerable to damage from dry running because the pump liquid provides bearing lubrication. Some manufacturers have developed bearing materials and coatings that are more forgiving of upset conditions and can run dry for a limited time; this also depends on the particular pump’s details and service. Upset conditions often result in some liquid remaining in the pump; this aids in bearing lubrication and prevents the bearings from breaking during brief dry-run periods. Hopefully, advances in bearing technology eventually will allow dry running for extended periods.

The magnet system transmits all the pump power and so requires special attention. A straddle-mounted design with bearings on either side of the inner magnet provides excellent stability and operation; this modern design reduces radial loading and allows the pump to better tolerate off-peak operation — and is far superior to the old-fashioned overhung inner magnet design.

Recirculation Flow

Every magnetic drive pump has a recirculation flow system — usually either discharge to suction or discharge to discharge. In the discharge-to-suction design, the fluid enters the magnetic coupling area at a high-pressure discharge point and returns to the bulk flow at the suction eye of the impeller. In the discharge-to-discharge design, the fluid enters the magnetic coupling area at a high-pressure discharge point and returns to the bulk flow at a point behind the rear shroud of the impeller. Each design has its advantages; so, make the choice between them on a case-by-case basis. Other recirculation flow paths and designs also are available in special magnetic drive pumps.

In the discharge-to-suction design, the flow is routed to the suction through either thrust balance holes in the impeller or through a hole along the axis of the pump shaft. The differential pressure between the recirculation inlet and return locations drives the recirculated liquid at high velocity. As the differential pressure rises, the internal flow rate increases but at a decreasing rate. The internal flow will reach a maximum beyond which any additional increase in differential pressure will have negligible impact. This occurs when friction losses begin to become the dominant factor affecting flow. The observed internal pumping effects primarily are caused by the action of the inner magnet ring and thrust washers.

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