August 2020

Special Focus: Valves, Pumps and Turbomachinery

Improve API and ASME centrifugal pumps reliability with permanent magnet shaft couplings

Many factors reduce centrifugal pump reliability. Internal factors include pump design aspects, manufacturing deficiencies, maintenance deficiencies such as lubrication, or process related problems. Other factors are external: the shaft coupling is one of the external elements causing reduced mean-time between failures (MTBF) and pump life through misalignment, friction and unbalance forces.

Alkhowaiter, A., Machinery Consultant

Many factors reduce centrifugal pump reliability. Internal factors include pump design aspects, manufacturing deficiencies, maintenance deficiencies such as lubrication, or process related problems. Other factors are external: the shaft coupling is one of the external elements causing reduced mean-time between failures (MTBF) and pump life through misalignment, friction and unbalance forces.

When the alignment of such couplings is precise and the balance concentricity of couplings is accurate, then a minimum of forces and vibration are transmitted to the driven pump. However, the majority of couplings used are torsionally rigid and will transmit high startup acceleration torque to the pump shaft and mechanical seals when driven by motors (FIG. 1). This startup acceleration torque can fatigue the M-seal face drive pins, and can cause chipping of faces when the faces have excessive sticktion. High vibration in a well-designed and manufactured pump originates from external causes, such as the pump coupling, piping strain, hydraulic (such as cavitation and flow issues), or foundation issues. Centrifugal pump failure statistics are shown in FIG. 2.

FIG. 1. Typical flexible metal coupling.
FIG. 2. Centrifugal pump failure statistics.

Pumps are frequently exposed to coupling unbalance or hot operating alignment that are far from ideal (FIG. 3). This results in the following failure modes in mechanical seals and bearings:

FIG. 3. Pumps are frequently exposed to coupling unbalance or hot operating alignment, resulting in failure modes in mechanical seals and bearings.
  • Coupling misalignment causes radial forces that lead to shaft bending or deflection at each revolution, affecting the mechanical seal faces tracking at the inboard side of the pump. This forces the faces open, as the seal springs have difficulty reacting in time to the shaft bend per revolution and higher multiples of vibration frequency. Breakdown of the lubricating film between the seal faces results in face wear and fretting.
  • Coupling misalignment forces produce an axial movement and axial vibration of the pump shaft, which then tends to open the inboard and outboard M-seal faces and leads to gradual seal life reduction as solid particles pass through the faces. This axial movement also damages rolling element bearings, such as thrust bearings.
  • Fretting wear of the shaft sleeve-mounted drive mechanism pins, lug and mating slots, grooves and holes results in the loss of axial tracking and damage to the seal rings.
  • Vibration induces loosening of the drive collar set screws, causing a loss of the spring tension on faces and loss of torque transmission to the rotating components.
  • Coupling unbalance and misalignment forces cause high radial alternating stresses at the inboard bearing, reducing its lifetime; and high bearing wear leads to shaft vibration, thus affecting the M-seals. The same forces also impact the driver itself, whether that is a motor or steam turbine. Motors with rolling element bearings experience early failure due to the coupling unbalance and alignment related forces.

These damaging forces can be transmitted by conventional dry flexible metal couplings and lubricated couplings, such as gear or spring-type couplings. However, they are eliminated when utilizing modern permanent magnet couplings in API-610 or ASME B73.1 pumps. This does not refer to seal-less magdrive pumps, but rather to separate shaft couplings that take the place of standard flexible couplings. Permanent magnet couplings are considered an improvement over conventional dry flexible metal couplings when used in the range of 1,000 hp and below, especially in pump services. The author does not advocate the use of magnetic couplings in all applications, but the superior benefits compared to conventional couplings should be reviewed by users who can make their own judgement.

Magnetic shaft couplings in refineries and gas plants

Permanent magnet couplings were introduced to the market to transfer torque from one machine to another using a rotating rotor and non-contacting mating magnets. Since then, they have been installed around the globe. Fixed-gap magnetic couplings (FIG. 4) have advantages compared to standard flexible metal, lubricated gear or spring-grid couplings, including:

FIG. 4. Magnetic fixed-gap coupling.a Source: Tritec Fluxdrive.
  • They do not wear or fatigue like flexible metal couplings, such as shim-pack type, or gear and grid couplings. They cannot fail due to misalignment stresses and lack of maintenance.
  • They cannot overstress the shafts and cause shaft and bearing fatigue failures. The magnetic field strongly damps and isolates torsional vibration.
  • No precision alignment of driver to driven with laser or dial indicators is required. The typical internal axial clearance gap is 3 mm, so even machinery movement with time cannot cause coupling problems.
  • Up to 1,000 hp of power for air-cooled models can be transmitted without requiring external cooling devices. However, maximum speed is limited to 3,600 rpm.
  • A major reduction can be achieved in shaft and bearing housing vibration levels of both driver and driven compared to standard couplings, ranging from 25% minimum up to 70% reduction.
  • Magnetic couplings have the ability to vary driven machine speed simply by adjusting the axial gap at the coupling, thus producing a slip that translates to reduced centrifugal pump speed and, therefore, a reduction of power demand at the motor. Oversized pumps speed can be readily adjusted manually to provide optimum flow and pressure range in their curve without losing energy in the discharge PCV.
  • For a refinery’s 120-hp, fuel oil-loading pumps, the original centrifugal pumps were exposed to very high torsional stresses during pump shutdowns due to hydraulic fluid transients from frequent loading-unloading cycles. The sudden pressure transients caused a reverse torque that occurs before check valve closing, resulting in extremely high reverse torque back into the drivetrain. The original pumps with dry flexible shim pack couplings (and later elastomeric type ) experienced frequent broken couplings and broken pump shafts. This was not misalignment related. New ANSI pumps with magnetic couplings were installed in 2017. With these couplings, all transients are highly damped torsionally in the drivetrain so no coupling or pump shaft fatigue occurs. The new pumps are now operating smoothly.
  • For two smaller, 75-hp centrifugal pumps retrofitted with magnetic couplings in 2011, the M-Seal MTBF increased from 9 mos to a minimum of 3 yr. Bearing life was increased 500% due to this coupling retrofit alone.

Retrofit of magnetic couplings

Retrofit guidelines for retrofitting conventional couplings with magnetic couplings in petroleum and petrochemical facilities are detailed here.

The following design and installation criteria have been developed to ensure a problem-free retrofit of magnetic couplings into an existing conventional coupled pump skid. The same guidance is required for new pump installations when specifying magnetic couplings in process plant services.

  1. The API-610 Standard regarding magnetic couplings (API-610, 11th Ed.) states in paragraph 7.2.2 that, “Unless otherwise specified, all-metal flexible element, spacer-type couplings manufactured in accordance with AGMA 9000 Class 9 shall be provided.” This means that API-610 is mandating conventional flexible metal couplings, unless the user has specified alternative designs as written in the specification or pump data sheet. Although not directly stated, this leaves the choice of magnetic couplings to the user, provided that the coupling installation meets other criteria of paragraph 7.2.
  2. The fixed-gap couplings (FIGS. 5 and 6) are suitable for retrofit in centrifugal pump applications of 3,600 rpm and below. It is practical to utilize magnetic couplings in horizontal pump services up to a 500-hp pump limit on normal machinery, and up to 1,000 hp for bad actor pumps with frequent coupling related failures. This is because larger power rating couplings become bulky with overhung weight; this is a reasonable limit until future improvements prove advantageous for larger API pumps.
    FIG. 5. Fixed-gap magnetic coupling on fuel oil pump, Red Sea refinery, 2017. Source: MagnaDrive.

    FIG. 6. Fixed-gap magnetic coupling on geared centrifugal pump. Source: Tritec Fluxdrive.
  3. Request fixed-gap magnetic couplings, unless a continuous variable-speed drive is needed. Fixed-gap couplings are air-cooled models limited to about 1,000 hp in 40°C ambient air temperature. Notice that fixed-gap couplings can still provide speed variation that is performed manually at an installation by increasing the rotors’ separation gap. This introduces slip, which allows speed variation in centrifugal machines. However, this does not apply to screw type or reciprocating pumps that have a constant torque curve at varying speed.
  4. Send the existing coupling drawings to magnetic coupling manufacturers and request a quote. Send side-view dimension drawings of the pump skid with clear dimensions of the driver and pump, as the manufacturer must ensure that there is enough radial height to fit. Ensure that the new coupling OD clearance to the skid is not less than 3 in. It may be necessary to raise both driver and driven to allow for the height of the new magnetic coupling.
  5. Request that couplings be temperature rated for the plant summer maximum ambient air temperature or a minimum of 40°C, whichever is higher. This impacts the design of the rotor, which heats up and is cooled by ambient airflow.
  6. Pumps operating with viscous liquids can require very high torque at startup in cold conditions. Calculate the highest hp required at the lowest temperature, then convert this to torque required at starting conditions. Add a 20°F negative factor to account for incorrect operation as a safety factor.
  7. A magnetic coupling should be torque rated to 1.25 x driven machine-rated torque. Above 200 hp, this factor can be reduced to 1.2. When pumps are down for extended periods, the breakaway torque can be high, so the coupling must have higher torque rating or it will spin in place and overheat.
  8. Coupling materials: Coupling hubs should be manufactured from steel alloys only. For coastal application plants, request full stainless-steel (SS) bolting using hardened SS bolts and nuts.
  9. Balancing: Require the proper API grade of balancing for rated speed of coupling.
  10. Fabricate or purchase a new coupling guard from a magnetic coupling OEM, since the new coupling diameter is larger than conventional.
  11. The coupling manufacturer should confirm if the existing axial space between the pump and driver is sufficient to fit a magnetic coupling. On such retrofit applications, there can be insufficient distance between shaft ends (DBSE) between drivers and driven, so a retrofit will not be possible unless the driver or driven has the available space on the skid support to push backward and allow more axial DBSE.
  12. For pump applications, couplings should be spacer type with a minimum DBSE or spacer length of 8 in. to allow for M-seal removal.
  13. Hub fit to shaft: Manufacturers typically use a tapered bushing design, which is acceptable per API-610. Alternatively, the user can specify keyed hubs and manufacturers will comply. Note: Some vendor’s standard hub is a split hub—this split hub is not recommended for API applications. All manufacturers can provide a solid hub option with keys or other drive methods.
  14. In hydrocarbon pumping applications, magnetic couplings can overheat and melt the rotor in the rare instance of driven machine seizure. This is the only weakness of magnetic couplings, whereas the prime weakness of conventional couplings is that they can fatigue and broken parts can fly out. The failed conventional couplings can produce severe unbalance, causing sudden M-seal breakage, leaking and fires. Some magnetic coupling users install thermal sensors inside the magnetic coupling guard to monitor excess coupling heat and provide a shutdown command to the driver. If installed, these should not only give alarm, but should also provide automatic shutdown. Alternative control solutions are to utilize flowmeters or flow switches to indicate a zero flow condition, which then shuts down the driver after a 5-sec time delay. Magnetic coupling manufacturers have heat sensors available as an option, with fully wired coupling guards, if needed. Installing heat sensors on coupling guards in non-flammable fluid applications is unjustified.
  15. Latest development: One manufacturer is developing a synchronous magnetic coupling that will not overheat with driven machine seizure; it is expected to be on the market in early 2021. HP

The Author

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