Liquefied gases and volatile chemicals are some of the hardest liquids to pump. They arrive at the pump at or near their boiling point, they often can’t be allowed to leak, and many applications need high pressure at low flow. A sealless magnetic drive pump solves the leak problem. But on near-boiling liquids, a mag drive brings its own risks. Here’s what to watch for, and how Roth’s one-foot NPSHr regenerative turbine mag drive pumps are designed around them.
Why Choose a Sealless Pump for Volatile Liquids?
A conventional pump seals its rotating shaft with a mechanical seal, and every mechanical seal eventually leaks or fails. For water that’s a maintenance item. For anhydrous ammonia, hydrofluoric acid, vinyl chloride or propane, a seal leak is a safety, environmental and emissions problem.
A magnetic drive pump has no shaft seal at all. The motor turns an outer ring of magnets, which drives an inner magnet rotor through a sealed containment shell (the “can”). The liquid stays completely enclosed, with only a static O-ring or gasket between the containment shell and the pump casing. That makes mag drive pumps the natural choice for:
- Liquefied gases such as ammonia, propane, butane and refrigerants
- Hazardous and toxic liquids
- Caustic and corrosive liquids
- Liquids that react when released to the atmosphere
The Challenge: Mag Drive Pumps and Near-Boiling Liquids
Removing the seal doesn’t remove the physics. Three things make volatile liquids hard on a magnetic drive pump:
- Little or no NPSH available. A liquid stored at its boiling point, like propane in a pressurized tank or ammonia in a refrigerated vessel, has a vapor pressure equal to the pressure above it. The only NPSH available comes from the few feet of liquid above the pump. If the pump needs more NPSH than that, the liquid flashes to vapor and the pump cavitates. (See our chemical boiling point and specific gravity chart and NPSH Calculator.)
- Heat inside the can. Eddy currents in a metal containment shell and friction in the internal bearings add heat to the liquid circulating inside the pump. With a liquid already near its boiling point, a small temperature rise can turn it to vapor.
- Product-lubricated bearings. A mag drive’s internal bearings are lubricated by the pumped liquid. If that liquid flashes to vapor, the bearings run dry and can fail quickly. Cavitation-induced vibration adds to the damage.
Low flow makes all three worse. Many liquefied gas and chemical transfer applications need only a few gallons per minute at high pressure, which is exactly where conventional centrifugal pumps run least efficiently and add the most heat.
How Roth Mag Drive Pumps Handle Near-Boiling Liquids
Roth sealless pumps combine a magnetic drive with Roth’s regenerative turbine design. The result is built specifically for continuous high-pressure pumping of liquefied gases.
1 ft (0.3 m) NPSHr across the full range
Every Roth mag drive model requires only 1 ft (0.3 m) of NPSH. That’s often the difference between a pump that works on a liquid at its boiling point and one that cavitates.
A non-cavitating regenerative turbine impeller
Roth’s floating, self-centering regenerative turbine impeller (U.S. Patent No. 5,137,418) develops high pressure at low flow: up to 1,500 ft (457 m) of head at flows up to 180 GPM (40 m³/hr). The non-cavitating design keeps destructive vibration away from the bearings.
A cooling flush that keeps the liquid in liquid phase
Liquid at discharge pressure is routed through the pump’s inboard cover to the inside of the containment can. It flows around the inside of the can, then through the alpha-grade sintered silicon carbide radial and thrust bearings, before returning to a lower-pressure point in the pump and rejoining the pump output. This flush circuit:
- Removes heat generated by eddy currents, controlling the liquid’s vapor pressure
- Forces liquid through the bearings, which have lubrication grooves to keep the surfaces wetted
- Keeps the bearing flush in liquid phase, because the regenerative turbine’s high discharge pressure provides a wide margin above vapor pressure
Low-loss containment shells
The standard Hastelloy C containment shell reduces eddy current losses, which means less heat added to the liquid, lower power consumption and higher torque capability. It’s rated for pressures up to 362 psi (24.9 bar). For applications above 25 HP (18.6 kW), a high-efficiency segmented 316 stainless steel containment shell is rated to 660 psi (45.5 bar). The primary containment shell is self-venting back to the supply vessel.
Materials for corrosive service
Pump cases are available in ductile iron, 316 stainless steel and Hastelloy C. Internal bearings and shaft sleeve are alpha-grade silicon carbide, with duplex stainless steel inner rotor and bearing housing.
Proven in Real Applications
Roth mag drive pumps are in service on a wide range of volatile and hazardous liquids. A few examples from our application list:
| Liquid | Flow (GPM) | TDH (ft) | Temperature (°F) |
|---|---|---|---|
| Anhydrous ammonia | 22 | 700 | 11 |
| Butane and propane | 10–12 | 763 | 60 |
| Anhydrous hydrofluoric acid (HF) | 21 | 616 | 70 |
| Vinyl chloride monomer (VCM) | 9 | 579 | Ambient |
| Refrigerant (Genetron AZ-50) | 50 | 185 | −65 |
| Vinylidene fluoride | 30 | 1,385 | 16 |
| Methyl chloride | 4 | 215 | 14–104 |
Other applications include aqueous ammonia, F152a, propylene oxide, trimethylamine, caustic, isoprene and sour water.
Installation Tips for Mag Drive Pumps on Volatile Liquids
Good piping is just as important as the pump. Roth recommends:
- Slope the suction line uphill toward the supply vessel, with no reverse loops, so any vapor can rise back to the vessel instead of collecting at the pump.
- Route the flush and vent lines back to the supply vessel, also sloped uphill with no reverse loops or dead legs.
- Vent the pump before start-up. Open the vent valve for initial start-up and close it afterward (an optional solenoid valve with time delay can automate this).
- Protect against over-pressurization with a suitable discharge bypass valve, returned to the supply vessel rather than directly over the suction outlet.
- Consider a flow switch on the flush line to confirm cooling flow.
The complete recommended system layout is in our Sealless Magnetic Drive Pump Bulletin M105R3.
Find the Right Mag Drive Pump for Your Liquid
If you’re pumping a liquefied gas, a hazardous chemical, or any liquid close to its boiling point, Roth’s 1 ft NPSH sealless magnetic drive pumps are built for the job. Send us your liquid, temperature, suction pressure, flow and required discharge pressure, and we’ll recommend a model. Contact Roth Pump, call 1-888-444-ROTH, or find your local Roth representative.