Maximizing Reliability in Sealless Pump Operation (2)
The monitoRing and reliability Issues

In this second part of our article about the practice of working with sealless centrifugal pumps, we concentrate on the specific monitoring and reliability aspects.

While the primary advantage of sealless pumps is the complete, hermetic sealing of the process liquid, this can also lead to one of the biggest frustrations with pumps of this design: You can’t see what’s going on inside. Whereas normal seal wear in an ANSI pump can be detected visually, normal bearing wear occurs with no outward indication. If preventive maintenance is not conducted in a timely manner, more serious and expensive peripheral damage is likely.

Core Issues for Reliable Operation
The top causes of unreliable sealless pump operation are:
- bearing loads, design and material options
- process fluid lubrication
- cooling and "the other NPSH" value
Proper control of bearing loads is basically within the control of the pump designer. Look for thrust balancing designs that can reduce the hydraulic loads imposed on the bearings. The more conservative the load placed on the bearing, the better. No single bearing material is right for all liquids and pump operating scenarios. As stated, softer materials are more tolerant of the abuses of liquid flashing or intermittent dry running, but they provide a shorter useful life under normal operation. Harder materials such as SiC provide in-creased wear resistance to abrasive particles, and will yield longer life under ideal conditions. However, these materials are intolerant of dry running and may result in more extensive peripheral damage if they are run to failure.
Process fluid lubrication of the bearings is, by definition, dependent on the properties of the fluid pumped. Bearings require two things for reliable operation: liquid (not vapor) between the bearing surfaces, and that this liquid be relatively clean and non-abrasive. Even with this obvious constraint, much can be done to create an improved bearing environment. For fluids containing abrasive solids, a secondary clean fluid flush can be injected into the bearing area. With volatile liquids, pre-cooling or internal flow schemes that boost and maintain the liquid pressure above the bubble point are available.
Cooling is a relative term as applied to sealless pumps. Modern rare earth magnetic drive pumps function well without external cooling to approximately 260°C. Torque ring design mag drive pumps can tolerate temperatures to 450°C without external cooling. Canned motor pumps featuring ceramic motor insulation systems can operate up to 410°C without supplemental cooling.
Bear in mind, however, that these temperatures are based solely upon the technical capability of the individual pump design. The requirement of limiting the fluid temperature within the pump may be based on maintaining the characteristics of the process fluid required for reliable bearing life: liquid (not vapor) and no abrasive solids. Therefore, in the case of certain fluids such as polymers, cooling may be required to prevent localized hot spot polymerization within the bearings. When pumping volatile liquids such as chlorine, cooling may be required to ensure that the liquid does not flash off to a gas within the bearings.
With high temperature applications such as heat transfer oils, canned motor pumps with conventional organic resin insulation systems may require cooling to protect the motor stator core from overheating. The "other NPSH value" is a widely misunderstood phenomenon. In sealless pumps the process fluid is circulated within the motor (or mag coupling) and through the bearings to remove heat. The amount of heat generated varies widely from design to design and is generally less in non-metallic lined mag drives due to the absence of eddy current generation across the non-metallic containment shroud. Metallic mag drives rank second in heat generation. The heat from electrical inefficiency of the drive motor is removed through air cooling rather than by the process liquid. Canned motor pumps generally add the most heat to the pumpage.
As noted earlier, there are special considerations for volatile liquids. It is not solely the added heat that is of concern, but the combined effect of heat vs. pressure and the operating point on the fluid vapor pressure curve. The internal flow paths within the pump must also be considered.
The most common internal flow scheme used in sealless pumps takes a side stream of liquid from the pump discharge (or at least a point within the pump volute where partial discharge pressure has been generated) and circulates it across the rotor, between the bearings and back to suction. It is the pressure differential between discharge and suction that causes the stream to flow. With this setup, liquid at an elevated temperature is being introduced directly into the pump suction. If the liquid is prone to flashing at this lower pressure and higher temperature, it will do so in the pump suction eye. Thus, even though the NPSH of the pump is determined to be a lower value, cavitation can still occur. Be sure to consider this phenomenon when selecting a pump. Alternate internal flow schemes are available to avoid this problem. You can maintain higher internal pressures or return the side stream directly to the suction vessel rather than the impeller eye.

Improving Reliability in Sealless Pumps
Look for various configurations to control the bearing environment. Variations in internal flow paths, centrifugal separation, screens or filters, auxiliary heat exchangers, flush injection points, flush restriction devices and auxiliary impellers are available to assure that the bearings operate in as ideal an environment as possible. This will result in long, reliable life.
Bearing materials should be determined based on fluid properties and the anticipated "real world" operating range. The hottest place in a sealless pump moving corrosive liquid is the bearings. A wide variety of carbon graphite binders are available to suit the corrosive nature of most process fluids.
Avoid building in unnecessary "safety factors" in establishing the design flow rate of the pump. Look for suppliers with a wide range of hydraulic designs to match your required conditions. Some vendors have hydraulic designs for low specific speed (low flow, high head) services that help you avoid selecting too large a pump and operating well back on the curve. These designs help minimize radial loads imposed by off-design operation.
Use monitoring devices appropriately. Fortunately, many good products for sealless pumps are available. One of the simplest and most economical ways to protect a sealless pump from operational abuse is by using a power control monitor. This measures input Kw and enables the user to detect potentially damaging operation of the pump, such as low or high flow (beyond recommended operating range) and dry running (resulting from improper venting and/or loss of flow). Upsets in fluid viscosities are also easily detected by an increase in Kw. Power control monitors are readily available for use on mag drive pumps, where it may be impractical to use other types of direct sensing bearing wear monitors.
Several manufacturers of canned motor pumps offer a variety of bearing monitors, including direct-acting mechanical units that sense axial and radial rotor position, mechanical monitors with electrical switches to provide a remote alarm/ shutdown feature, and electronic units that can provide a progressive indication of the rotor’s position within the motor. The most advanced of these monitors can determine precise rotor positioning independently at both radial bearings, as well as axial rotor positioning, and they can identify thrust direction, direct detection of two phase (gas/liquid) flow at the bearings, and direction of rotation. Several user inter- faces are available, including intuitive local displays using LEDs to indicate rotor position. Automatic, remote monitoring is available through relays and a 4-20 mA signal. Two manufacturers offer RS485 serial port communication links to the user’s DCS system. This enables the operator to access a wide range of data from the monitor’s host software and use it for advanced rotor dynamics analysis including shaft orbital plotting, time waveform analysis and spectrum frequency analysis. All the monitors are non-invasive and maintain the integrity of both the primary and secondary liquid containment boundaries.
Given the enclosed "black box" nature of sealless pumps, it is vital that one of these technologies is included in all sealless installations. All sealless pumps should have some sort of monitoring device, if only a power control monitor. These devices are inexpensive and will pay for themselves quickly by reducing maintenance costs. For critical applications, extremely hazardous liquids, special pump designs or larger, relatively expensive pumps, consider including an electronic diagnostic monitor. Choose from the available technologies of canned motor pumps, non-metallic lined mag drives and metallic mag drives based on application. And avoid "one technology fits all" decision making. You wouldn’t put the same seal on all applications, would you?