Entering the Hot Zone: Mag-Drive Pumps
Joseph D. Warrender, Warrender, Ltd. and Adriano Mischiatti, 3M-Pumps
Sealless magnetically coupled ("mag-drive") regenerative turbine pumps can provide high
reliability in low flow/high head hazardous chemical process systems.
During the mid-1970s, Dr. Adriano Mischiatti, Ph.D.,
C.E., conceptualized a magnetically coupled turbine
pump in his college thesis. The concept has proven
extremely reliable, addressing low flow applications
with the perfectly balanced mag-drive design.
Hydraulically balanced impellers are a key design
aspect to both low flow turbine and high flow
centrifugal process pumps.
As applied to nuclear processes, balanced impellers
are critical when pumping cooling liquids such as
titanium dioxide saturated with helium gas. Gaseous A cross-sectional view of an alloy mag-drive
liquids are highly unstable and can lead to "microturbine pump.
cavitation." In conventional designs, centrifugal
impellers generate varying degrees of axial thrust across the performance curve when operating
apart from the best efficiency point (BEP) (see Table 1).
Unbalanced impellers are more susceptible to damage from axial
thrust, high frequency modulation and cavitation. The
effectiveness of variable orifice impeller balancing can be
diminished by abrasive wear and/or particle deposition. Some
centrifugal mag-drive designs employ pump-out-vanes (back
vanes) to counter balance axial thrust forces, simulating the
turbine effect. Thus, perfect impeller balancing can be achieved
at specified duty points for critical services.
A highly engineered seal-less pump design has significant
benefits over mechanically sealed counterparts in radioactive
applications. Longevity, breach resistance and rapid
maintenance procedures within "hot zone" areas are vitally
important. Heavy duty, mag-drive containment shells have
withstood rigorous destructive testing without leakage.
Table 1. Approximate axial thrust
calculation for centrifugal
impellers. This table indicates
how to identify each element of
thrust, including axial momentum
terms. In order to make the
calculation, the static pressure at
the impeller OD must be known.
Ten-minute overall requirements are met with quick-change
cartridge assemblies and registered fits. Additionally, rugged
alloy mag-drive pump designs, free of fluoroplastic coatings or
linings, are preferred in radioactive liquids for long-term reliability, without risk of material
degradation from the penetrating power emitted by the nucleus of radioactive substances.
Mag-drive pumps in general (i.e. turbine, centrifugal and
positive displacement designs) isolate the power frame
bearings from hydraulic radial and axial shaft loads.
Inherently, the shorter internal shaft of the turbine and
centrifugal mag-drive designs significantly reduces radial
shaft loads by means of a between-bearings impeller, as well
as low overhung mag-drive designs.
Consequently, high overhang in centrifugal and turbine
Centrifugal mag-drive atomic particle
mechanical seal configurations requires special attention to
accelerator cooling pumps.
power frame upgrades to meet bearing life projections. The
opposed double row turbine impeller features zero axial shaft
loads, thereby providing a forgiving pump mechanism, even during system upsets or arduous
operating conditions (e.g. entrained gas, vortexing, cavitation, pseudo-cavitation, etc.). This type
of turbine pump, equipped with a hydraulically balanced impeller-magnet assembly, is floating or
suspended within a liquid film barrier and can operate for extended time periods - service-free.
Turbine vs. Centrifugal Performance Characteristics
Under normal operation, near BEP, liquid passes through a
centrifugal impeller only once. Even though liquid flows
through the turbine impeller one time, it is pressure boosted
within each impeller cavity along a helical flow path.
The turbine impeller essentially operates like a peripheral
multi-stage pump that can develop five times the differential
An axially balanced turbine impeller.
head over centrifugals with comparable impeller diameters.
Liquid enters the side suction port and is compressed by the
multi-blade, dual action, horizontally opposed impeller faces. The turbine impeller, capable of
mixing gases into a process stream, works to pressurize vapors to keep them in a liquid state and
can handle up to 20 percent entrained gases.
However, performance curves should be de-rated by the percentage of gas. Some mag-drive
turbine designs allow for electrically reversible rotation that often avoids excess piping, controlvalves and additional pumps, depending upon system requirements.
In a centrifugal pump, liquid enters at the suction end, at the impeller eye, and is expelled
centrifugally as it expands through the impeller vanes and reaches the highest pressure point at
the casing discharge nozzle. The expansion of liquid through the impeller causes a pressure
reduction at the eye where flashing may occur with unstable hydraulic conditions or liquids.
Turbine pumps, along with centrifugals, are rotodynamic, fixed displacement designs, allowing for
flow control with minimal need for pressure relief or by-pass systems.
Liquefied Gases or Refrigerants
Mag-drive turbine pumps are highly effective for pumping liquefied gases (e.g. CO 2, LPG,
propane, butane, liquid nitrogen, etc.) due to their inherent ability to carry entrained gases in the
fluid stream.
Refrigeration systems are often configured with compact piping or tubing, requiring low flow/high
head performance. Oversized centrifugal pumps will induce heat from the friction of recirculating
liquid, resulting in high head cavitation. Consider the optimal operating range for centrifugal
pumps is near BEP and the excessive flow in a low flow process is recirculated within the pump.
Another source of heat generation is at the mechanical seal faces, further elevating the process
liquid temperature on closed loop systems and risking vaporization, particularly with low boiling
point liquids.
Solids Handling
Mag-drive turbine pumps require suction strainers that correspond to the minimal internal
clearances between the impeller and channel rings in processes that may contain solid
particulates. Mag-drive designs typically have more ample clearances (0.005-in) that are two
times to three times wider than mechanical seal configurations. This clearance corresponds to a
100-mesh strainer.
Temporary strainers or filters can be used for removal of pipe welding slag and proper flushing of
the system on start-up. Solids handling with mag-drive centrifugal pumps can be accommodated
by means of wider clearances and various options for API type flush plans.
Intermittent Dry-Running
The side suction, top discharge mag-drive turbine design maintains a volume of process liquid
inside the pump cavity (similar to a liquid-ring vacuum pump). Thereby, the internal sleeve
bearings remain wetted by the process liquid, even during a loss of suction.
The absence of a spring-loaded mechanical seal reduces heat generation, allowing ample time
(several minutes) for monitoring equipment to detect the adverse condition prior to damage.
Additionally, the pumping action in a mag-drive turbine pump, once filled with liquid, provides a
suction lift of 20-ft (H2O) that will work to draw air pockets through the pipeline with minimal risk
of vapor locking. However, properly adjusted power monitors can protect centrifugal pumps from
damage due to system upsets.
Process Control & Rise to Shut-Off
The turbine pump performance provides the highest rise to shut-off, far exceeding the 8 percent
to 10 percent required by many plants. Systems with varying head conditions and unknown
system heads are accommodated with minimal risk of cavitation due to insufficient head.
For example, a mere 10-ft to 20-ft of understated or overstated head in a centrifugal pump
installation often results in either dead-heading or a run out condition, respectively. Conversely,
such variance in differential head has minimal effect on flow or risk of damage in a turbine pump.
High efficiency centrifugal impeller designs should provide low suction-specific speed values and
the maximum rise to shut-off.
Motor Mounting & Maintenance
Magnetic drive pumps, with the isolated hydraulic end, are
perfect for close-coupling, thereby eliminating ball bearing
sump maintenance, emissions, and potential coupling
misalignment. Pump field realignment is often necessary due
to jarring during transit.
Also, seismic tremors, greater than 1.9 on the Richter scale,
can knock long-coupled pumps out of alignment. Therefore,
An alloy mag-drive turbine pump can
unstable geographic regions are ideal for close-coupled mag- be used for chemical applications.
drive pumps. The mag-drive assembly not only shields the
outboard motor bearings from hydraulic forces it allows for servicing of the motor without
exposing process fluid to atmosphere.
Extensive Range of Designs for Exacting Requirements
Mag-drive designs are available in cast or machined alloys, single or multi-stage, low NPSHa,
high system pressure units, high temperature, low temperature or cryogenic, and machined
thermoplastics for cost effective corrosive handling in standard and self-priming configurations.
Mag-Drive Turbine Pump Performances




Flows up to 60-gpm
Heads up to 3,250-ft
System pressures from vacuum up to 7,250-psig
Temperature from minus 238-deg F to +650-deg F
Mag-Drive Centrifugal Pump Performances




Flows up to 4,400-gpm
Heads up to 1,650-ft
System pressure from vacuum up to 7,250-psig
Temperature from minus 238-deg F to +650-deg F; 650-deg F to 840-deg F (with heat
exchanger)
Pump Power Monitoring
Linear power monitors (vs. non-linear ampere sensors) are
ideal for protecting mag-drive pumps from damage due to
system upsets and adverse process conditions. Low
flow/high head mag-drive regenerative turbine pumps draw
maximum power at high differential heads (or shut-off).
Conversely, centrifugal pumps will register maximum power
with high flow conditions.
Thermoplastic mag-drive turbine
pumps can be used for corrosive
applications.
Joseph D. Warrender is the general manager of Warrender, Ltd., 28401 North Ballard Drive, Suite H, Lake
Forest, IL 60045, 847-247-8677, Fax: 847-247-8680, joseph@warrender.com, www.warrender.com.
Adriano Mischiatti, Ph.D., C.E., is founder of 3M-Pumps, s.r.l., Via del lavoro, 40 - 45019 Taglio di Po
(ROVIGO), Italy, +39 426 346304, Fax: +39 426 349126, Info@3mpumps.com, www.3mpumps.com.