A. Rama Rao

Vibration based Diagnosis of a Centrifugal Pump

The centrifugal pump has a long history of frequent failure of anti-friction bearing since its commissioning in 1985. Vibration based conventional condition monitoring has regularly been used to identify the progressive nature of the bearing failure, but has failed to identify the root cause. Modal tests have been conducted on the pump assembly to understand the dynamics of the complete assembly. A typical case of the resonance of the bearing pedestals with 2X component (two times the pump RPM) of the response during pump operation mainly due to nonlinear interaction between the pump foundation and the concrete floor has been identified as the main source of the bearing failure. The results, observations, and the diagnosis to identify the root cause are discussed here.

Diagnostics of direct CT - PT contact of the coolant channels of PHWRs

It has now been realised that the garter springs which maintain the gap between the pressure tube (PT) and calandria tube (CT) of a PHWR can get displaced significantly from their deign position in many channels. It has also been recognised that the large unsupported span of the PT restricts the life of the channel due to premature contact of the PT with the CT making it susceptible to delayed hydrogen cracking. This paper reports the details of a non-intrusive diagnostic technique based on vibration measurement for detecting the contacting channels.

Significance of analytical modelling for complete interpretation of experimental modal analysis: a case study

The reliability of the dynamic qualification of structural components of a nuclear power reactor totally depends on the dynamic characterization, i.e. identification of natural frequencies, mode shapes and damping of the components. Often, the correct identification of these parameters by the experiment or analysis alone may be difficult for many cases. In this paper, the strength of the analytical modelling in understanding the experimental results and interpreting them are presented through a case study.

Use of an unconventional technique for seismic qualification of equipments

There is a great deal of equipment in nuclear power stations which is required to withstand predefined levels of earthquakes. Such equipment is generally qualified analytically or experimentally by shake-table tests. However, some equipment is so complicated that an analytical simulation is very difficult. This equipment could also be so large and heavy physically that shake-table testing may not be possible in many cases. One typical example of such equipment is the Diesel Generator (DG) sets of Nuclear Power Plants (NPPs). For functional qualification of such equipment, the use of railway track unevenness to induce stationary random vibrations is being put forward as an economical and conservative alternative. This article also brings out the feasibility of using such a technique for all difficult to model and/or test equipment both in a passive and an active state.

Diagnosis and cure of Dhruva fuel vibration

Abstract Dhruva is a high flux research reactor with a nominal thermal power of 100 MW. The fuel for the reactor is in the form of seven-pin cluster of metallic natural uranium clad with aluminium. The optimisation from the physics and thermal hydraulic considerations has resulted in this design of small diameter, long pins arranged hexagonally ensuring a minimum specified clearance between the pins. The clearance is maintained throughout the length by a number of spacers located at regular intervals. This seven pin cluster is assembled inside an aluminium flow tube and the assembly goes into coolant channels made of zircaloy. The fuel assembly is constrained radially (i.e. in the horizontal plane) by the bulges at the two ends of the flow tube. The fuel was endurance tested in an out-of-pile flow test facility for many thousands of hours without any visible damage. However, on loading them in the reactor, many of the fuel pins failed due to fretting wear at the spacer locations. The maximum wear- was on the outer pins near the mid-length of the fuel assembly. The paper gives the details of the measurement and analysis carried out to understand the causes. The solution adopted was to make the supporting bulges flexible - the bottom one by cutting axial slits to obtain a collet type fixture and the top by a sleeve with slits to obtain leaf spring type support. With these design changes, the fuel performs satisfactorily.

Development of Technology for Detachment of Calandria Tube Rolled Joint of Pressurised Heavy Water Reactors

Coolant channel assembly of PHWRs consisting mainly of pressure tube and calandria tube undergoes sag due to irradiation enhanced creep during its life time. Typical service life of the pressure tube is limited to about 15 full power years due to such ageing related degradation. On account of the degradation the pressure tube needs replacement more than twice during the licensed period of reactor operation. As the sag in the calandria tube is also excessive, it has to be replaced to facilitate installation of new and straight pressure tube during retubing and to avoid interference of sagged calandria tube with horizontal reactivity devices. The calandria tube and the tube sheet are joined by sandwich type rolled joints. For detaching the joint an improved technique has been developed that helps to detach the joint without affecting the groove geometry and integrity of the tube sheet. The system works on the principle of fast thermal cycle of heating and cooling of the insert and calandria tube in the rolled joint region while applying axial load to the calandria tube. Though rolled joint detachment by induction heating technique has been in use in CANDU reactors for removal of pressure tubes and calandria tubes, the present design incorporates a liquid nitrogen for faster cooling and simultaneous application of load with a gripping module for detachment and removal of calandria tube and the insert from the tube sheet of end shield in shorter time. This paper describes the methodology of rolled joint detachment, challenges involved in the design, features of the detachment system, findings of experimental trials and observations.