A test facility employing a magnetic bearing for mechanical element tests

1. Introduction

Industrial machines such as multistage centrifugal pumps including boiler feed water pumps and pumps for circulating cooling water play an important role in electric utilities and chemical plants. Such machines are used, for example, in nuclear and thermal power plants, as well as in oil refineries. The reliability of such machines is paramount since a failure can shut down operations causing monetary loss.

The principal contributor to pump failure is uncontrolled vibration leading to breakdown of critical machine elements. Such vibration resulting from fluid discharge is characteristically due to the high viscosity and density of the operation fluid. The underlying science which enables the prediction and helps avoidance of this type of vibration is an important subject. Additional complications arise as the designer attempts to improve efficiency to obtain energy savings. Such improvements often require, for optimal operation of the pump, increases in the range of flow control. This, in turn, requires increases in the range of rotational speed and/or valve opening. The pump designer is required to provide a trouble-free design, and must guarantee the performance of the pump over the extensive operation range.

The annular pressure seal, which serves to prevent leakage of the fluid being pumped, has an impact on the vibration characteristics, and has to be chosen carefully. The desire to obtain a good seal has to be balanced with the desire to reduce shaft vibration since the pressure seal characteristics necessary for meeting these requirements run counter to each other.

The most serious vibration problems often arise when cavitation occurs at low flow rates. It is important to conduct an experimental simulation in order to identify the cause of vibration and initiate countermeasures. Our company has developed an apparatus for testing elements of a pump (impeller, annular pressure seal, bearing, mechanical seal, etc.) under large static and dynamic loads. The system can evaluate the performance of the machine elements with sufficient accuracy.

We employ a magnetic bearing to realize the testing performance at an appropriately high level (shorter test time, lower cost and good measurement accuracy). Reasons underlying the use of a magnetic bearing include:

(1) The non-contact state between the rotor and casing can be preserved.
(2) The direct excitation of rotating shaft is possible by magnetic bearings. Moreover, it is easy to control amplitude, phase, and the excitation frequency accurately.　 Displacement and shaft excitation along 5-axes including radial and axial direction can be controlled independently.
(3) A magnetic bearing can serve as a load detector. Accordingly, this apparatus can evaluate the static and dynamic load acting on the bearing directly.

2. Introduction of the machine element testing equipment

Our company has two systems with different capacities for the examination of various machine elements. These facilities, developed and manufactured using in-house technology, employ an appropriate magnetic bearing unit with independent control along all axes in both radial and axial directions except in the rotational direction as shown in Fig. 1. (figure shows the medium sized test equipment)
As shown in Table 1, the performance of magnetic bearing and the power of electric motor depends on the capacity (size) of the test apparatus. The performance and capacity was chosen according to the size and capacity of the machine element under test. The test procedure was also tailored appropriately.

 

 

 

The test equipment for medium-sized machine element was developed in 1999. The equipment is designed for evaluating the quality, dynamic properties and the noise characteristics of the non-contact annular pressure seal installed in an industrial pump^[1].

The test stand was subsequently modified (in 2002 and 2004)to be able to measure the vibration characteristics of centrifugal impellers in relation to the change of flow rate or cavitation number. Additional modifications to the system was made to be able to:


1. Determine and evaluate inducer performance and dynamics for achieving both high suction performance and low vibration (in 2004 and 2006)

2. Assist the development of hydrostatic bearings (2009). The facility could perform endurance tests for submerged bearings with a surface temperature higher than 200ºC under boundary lubrication and overload condition (2010).



The greatest advantage of the test stands is that they can control the rotating shaft directly with magnet bearing actuators, as shown in Figure 3, and perform the control operations of displacement and excitation in an easy and reproducible manner. For the control of excitation to determine the vibration characteristics of machine elements to be measured, the test stands provide displacement and constant load controls for such elements, as well as frequency and attitude controls. By selecting from a variety of control methods according to the test purpose or procedure, they support a wide range of tests for machine elements, such as general performance testing (shaft sealing capability of annular pressure seals, sliding characteristics and load capacity of submerged bearing, etc.) and reliability evaluation/verification testing, including vibration tests and destructive tests. Figure 4 shows a typical vibration test block diagram.

The magnitude and phase of the force acting on the bearings can be identified based on the control current, displacement, and amplitude monitored by the magnet bearing control system on a real time basis. By using this control mechanism to measure the static and dynamic loads produced by machine elements, the test stands can determine the performance of such elements, including load capacity and dynamic stiffness of bearing. The magnetic bearing control system has been designed optimally taking into account the dynamics of various machine elements to be tested (control targets); thus, it allows rigorous tests like high-load sliding (destructive) tests while maintaining the stability of the rotating shaft/bearing system of the test stands.

A non-contact annular seal is a machine element that significantly affects the vibration characteristics of a pump’s rotating shaft system. For pumps with low specific speed (NS), better shaft sealing performance improves the pump efficiency. However, the shaft sealing performance is inversely related to the vibration characteristics, and the evaluation of both parameters is required for practical applications. Therefore, the test stands evaluate the performance of non-contact annular seals based on the parameters regarding general performance, vibration performance, and acoustic characteristics (only for grooved seals), as shown in Figure 5 ^[2]. Figure 6 shows a cross-sectional view of each test stand under seal test conditions.

The magnitude and phase of the force acting on the bearings can be identified based on the control current, displacement, and amplitude monitored by the magnet bearing control system on a real time basis. By using this control mechanism to measure the static and dynamic loads produced by machine elements, the test stands can determine the performance of such elements, including load capacity and dynamic stiffness of bearing. The magnetic bearing control system has been designed optimally taking into account the dynamics of various machine elements to be tested (control targets); thus, it allows rigorous tests like high-load sliding (destructive) tests while maintaining the stability of the rotating shaft/bearing system of the test stands.