Pumps and Systems, February 2007
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 liquids are highly unstable and can lead to "micro-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).
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.
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.
Centrifugal mag-drive atomic particle accelerator cooling pumps.
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.
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 mechanical seal configurations requires special attention to 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.
An axially balanced turbine impeller.
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 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, control-valves 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. CO2, 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-in (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, unstable geographic regions are ideal for close-coupled mag-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.
An alloy mag-drive turbine pump can be used for chemical applications.
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 150-deg C 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 150-deg C to +650-deg F; 650-deg F to 840-deg F (with heat exchanger)
Thermoplastic mag-drive turbine pumps can be used for corrosive applications.
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.