C ryogenic pumps are essential components of liquefaction, storage, transportation, regasification, and distribution systems of liquefied gases, such as LNG. Therefore, end users expect high performance and reliability,

requiring operation for up to 40 000 hr without service or maintenance. The growth in utilising LNG as a transportation fuel will further increase the need for enhanced pump reliability and safety, especially as new functional requirements emerge due to variations in quantity in transit, frequency of fuelling, and supply infrastructure.

As shown in Figure 1, vertically-oriented cryogenic pumps are structurally and functionally complex. They operate at extremely cold temperatures with a submerged motor and may consist of up to 30 stages stacked around a long shaft.The mass profile along the axis of rotation is non-uniform because of concentration of mass near the top due to a heavy electric motor. Secondary flows are extracted

Yousef Jarrah, Toshi Hayashi, and Daniel McInnis, Nikkiso Cryo Inc., USA, review

typical roller bearing profiles.

RELIABILITY

Achieving cryogenic

pump

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from the main flow and delivered into narrow passages in order to continuously cool the motor and bearings, and to obtain proper axial thrust balancing. The rotordynamic response is different from gas-based turbomachines because the mass of the process liquid and hydrodynamic instabilities must be accounted for. Finally, stabilising mechanisms must be implemented to keep the rotating assembly physically constrained over the entire pump lifecycle. Therefore, essential to pump function and durability is the bearing system, which is deployed to restrain axial and lateral movements due to rotation-induced vibratory modes.

Axial thrust devices are fitted to transmit the build-up of hydraulic forces into the housing; otherwise, the rotating assembly would slide axially towards the pump inlet. Axial thrusts, with strong variations occurring during start-up as pressure profiles throughout are being established, can give rise to axial vibrations with unique frequencies and resonances. To achieve proper balance and to eliminate the need for thrust bearings, Nikkiso Cryo Inc. (NCI) pumps are designed such that axial displacements are produced and balanced by hydraulic processes, and a balance drum is utilised to restrain movements at all operating conditions.

Lateral vibrations are perpendicular to the axis of rotation and, if not properly controlled, can become excessive and cause bearing wear and even rotor contact with stationary parts. In order to constrain lateral displacements, the company positions displacement inhibitors around the rotating assembly, such as roller bearings, journal bearings (also called bushings), and wear rings. The purpose of this article is to review typical roller bearing profiles, which are utilised to maintain structural integrity by hindering growth in amplitudes of lateral vibrational modes.

Hydro-mechanical differentiators The dynamics of the pump are different from that of other rotating equipment, such as gas turbines and air-separation turboexpanders. Pumps run at low frequencies (speeds up to 8000 RPM), while turbomachines with gas as the process fluid usually run at high frequencies (speeds up to 200 000 RPM). The pump liquid is incompressible, and so hydrodynamic instabilities such as cavitation and rotating stall are felt immediately throughout the pumping system. The liquid density is high,

so its mass may not be neglected. For example, the LNG-to-air density ratio is 367. The vertical orientation of the cryogenic pump is stabilising because the equilibrium position of the rotating assembly, the position with the minimum potential energy, is when the pump is exactly vertical. Perturbations swaying the centre of the shaft from the upright position will be resisted by the action of gravity.

The mechanical and hydrodynamic differences are relevant to structural stability, and may not be trivialised or ignored. For example, the primary kinetic energy (KE) of the rotating assembly is proportional to the square of the speed. Therefore, going from a multi-stage LNG pump running at 5000 RPM to a multi-stage gas compressor of equal mass but running at 100 000 RPM, would increase the KE by a factor of 400. Because a tiny (less than 1%) portion

of the primary KE is supplied to create-and-maintain the secondary vibratory modes, it is clear that the energy available to feed and sustain the vibration energy is relatively low for cryogenic pumps.

Fundamental rotordynamics At the most fundamental level, the rotation generates instability modes or disturbances with discrete frequencies and mode shapes. Once born and depending on the rotational speed, the amplitude of the disturbances can grow to undesirable levels. The continuous supply of energy tends to sustain the disturbances, while design mechanisms are implemented to prevent and/or minimise their growth rates. This competition determines the structural quality of the pump. Without proper bearing distribution, the amplitudes of the instability modes grow and, in rare cases, can cause contact between the rotating and stationary parts due to large lateral displacements. However, an optimised distribution of bearings will hinder the instability’s rate of growth, and limit the maximum possible displacements to levels well below design clearances.

Rotordynamics codes, where the rotating assembly is converted into a lumped parameter mass model, are utilised to predict interactions between rotor natural frequencies, critical rotational speeds, stiffness, design clearances, and damping as functions of different distributions of journal bearings, roller bearings, and wear rings along the axis of rotation. The codes determine whether or not the selected roller bearing distribution

Figure 1. Typical cryogenic pump

Figure 2. Roller bearings distribution. (1)

Reprinted from February 2017

sufficiently minimises lateral displacements, especially near the operating speed.

The natural frequencies of the pump are determined by many factors, but two major factors, stiffness and damping, are provided by the shaft supports along the axis of rotation. By adding stiffness to the design, natural frequencies move to higher operating ranges. By adding damping, the effects of the natural frequencies are reduced. Roller bearings can be moved around or added to the design to adjust mode shapes or move natural frequencies. The main providers of damping in submerged cryogenic pumps are the wear rings and balance drum seals, which get their damping from a pressure drop across the sealing surface.

Another provider of both stiffness and damping are the journal bearings, which unlike wear rings and the balance drum seal, gain their stiffness and damping through hydrodynamic effects rather than a pressure drop across the sealing surface. These effects become present when a radial force from the shaft creates an increase in pressure on one side of the journal bearing moving it back to the centre, thus creating the stabilising effects of stiffness and damping. Due to the smaller diameters of the journal bearings and minor radial forces present in vertical machines, the stiffness and damping values provided from journal bearings are relatively low, but still play a role in controlling the critical speeds of the rotating assembly.

Using these supports, the goal is to have a subcritical design, meaning that the operating speed or speed range is below the first natural frequency. However, this is not always achievable. In some instances, the natural frequencies intersect with the operating speed or fall inside an operating speed range. When a natural frequency of the rotating assembly is reached by the operating speed of the pump and a sharp spike in radial displacement occurs, which would be a case of insufficient damping, it is defined as a lateral critical speed. These lateral critical speeds are not always a problem, as they can be controlled with sufficient damping, which limits the shaft displacements to acceptable levels. Occasionally, a design may have insufficient damping, which warrants

modifications to provide additional damping to allow pump operation near a critical speed without excessive vibration. Obviously, the preferred solution would be to avoid dealing with critical speeds in the first place, which is why NCI offers three distinct design solutions, which help navigate the critical speeds away from the operating speeds early in the design stage.

Distribution oneFor some applications, the bearing profile shown in Figure 2 is selected. The roller bearing structure consists of only two motor bearings – one above and one below the motor. This configuration can be applied successfully even in large high pressure pumps. The success of this bearing distribution is due to the shorter hydraulic stack length and tighter wear ring and journal bearing clearances that are present in these designs. The shorter stack length of the housings results in a shorter pump shaft, which, in turn, increases the natural frequencies of the shaft assembly. The tighter seal clearances create higher stiffness and damping. The higher stiffness generated by the tight clearances directly increases the natural frequencies, while the damping helps reduce the shaft displacements.

Distribution twoFor some applications, the bearing profile shown in Figure 3 is selected. The roller bearing structure consists of two motor bearings plus one bearing at the extreme lower end. In some cases, the pump design is analysed and a larger-than-acceptable displacement is found near the inducer end of the pump. Some solutions include reducing the mass of the inducer, thickening the pump shaft, or adding a bearing near the end of the shaft. Reducing the mass of the inducer is sometimes an option, but thickening the pump shaft is generally a considerable design change that is uncommon due to hydraulic standardisations and increased cost. The simplest and most robust option is to add a small roller bearing at the end of the pump shaft, typically between the inducer and the first impeller stage.

Distribution threeFor some applications, the bearing profile shown in Figure 4 is selected. The roller bearing structure consists of two motor bearings plus an additional roller bearing every four to six stages along the pump shaft. This design option is usually selected for high pressure pumps with a large number of stages, which, in turn, require a long shaft. Increasing

Figure 4. Roller bearing distribution. (3)

Figure 3. Roller bearings distribution. (2)

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the number of bearings along the pump shaft provides additional support, thus making the design robust and able to operate for long periods of time without maintenance. This profile is usually selected for high pressure pumps because long shafts are inherently receptive to deflection, operating clearances tend to increase at a faster rate during the pump lifecycle, and the separation between the motor-rotor centre of mass and the rotating assembly centre of mass is increased.

Energy fundamentalsThe governing equations of motion do not permit absolute and coherent stability of the rotating assembly because, once rotation is initiated, a finite amount of the primary rotational KE is transferred into a secondary work-type form of energy; the total energy of the system is the sum of the primary and the potentially destabilising secondary. The secondary vibrational energy, while less than 1% of the sum, manifests in leading to coherent patterns or mode shapes at discrete frequencies in the form of whirl, wobble, orbits, rolling, or displacements. The cellular-type motions vary along the axial, radial, and circumferential directions and, if not restrained, can grow in amplitude into unacceptable levels. The function of the bearing system is to prevent or hinder the growth of these secondary instability modes.

Physics allows the instability modes to be established, while bearings control and limit the maximum distortions

that these modes can cause. In other words, if the destabilising modes are considered as patterns of perturbations circulating the axis of rotation, then proper positioning of bearings render the axis of rotation as an ‘attractor’, which means that the trajectories of the instability modes are prevented from causing large shaft displacements, and a steady state motion is maintained.

The mechanism for the generation and subsequent containment of the instability modes is simple. The electric motor transmits power into the shaft, which, in turn, causes the establishment of primary rotational kinetic energy (big whorls). A tiny amount of energy is then transmitted into instability modes or wavy-type disturbances (little whorls), which could, depending on the magnitude of the rotational speed, grow in amplitude and cause large shaft movements. It is not possible to prevent the formation of the instability modes, but it is possible to hinder their growth by implementing bearing profile unique to the pump mass profile and functional requirements.

ConclusionThe preferred distribution of roller bearings depends on pump requirements and operating speeds. To achieve stability of the rotating assembly at all operating speeds, and accounting for potential future enlargements of geometric clearances, which may occur over the pump lifecycle, NCI customises, optimises, and precisely positions a select number of roller bearings along the pump shaft.

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