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«PINSTECH-158 VIBRATION ANALYSIS OF PRIMARY INLET PIPE LINE DURING STEADY STATE AND TRANSIENT CONDITIONS OF PAKISTAN RESEARCH REACTOR-1 S.K. AYAZUDDIN ...»

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PINSTECH-158

VIBRATION ANALYSIS OF PRIMARY INLET PIPE LINE

DURING STEADY STATE AND TRANSIENT CONDITIONS OF

PAKISTAN RESEARCH REACTOR-1

S.K. AYAZUDDIN

A A. QURESHI

T. HAYAT

Reactor Experiment Group

Nuclear Engineering Division

Pakistan Institute of Nuclear Science & Technology

P.O. Nilore, Islamabad

November 1997

PINSTECH-158

VIBRATION ANALYSIS OF PRIMARY INLET PIPE LINE

DURING STEADY STATE AND TRANSIENT CONDITIONS OF

PAKISTAN RESEARCH REACTOR - 1

S.K. AYAZUDDIN A.A. QURESHI T. HAYAT

REACTOR EXPERIMENT GROUP

NUCLEAR ENGINEERING DIVISION

PAKISTAN INSTITUTE OF NUCLEAR SCIENCE AND TECHNOLOGY

P.O. NILORE, ISLAMABAD NOVEMBER 1997 ABSTRACT The Primary Water Inlet Pipeline (PW-IPL) is of stainless steel conveying demineralized water from hold-up tank to the reactor pool of Pakistan Research Reactor-1 (PARR-1). The section of the pipeline from heat exchangers to the valve pit is hanger supported in the pump room and the rest of the section from valve pit to the reactor pool is embedded. The PW-IPL is subjected to steady state and transient vibrations. The reactor pumps, which drive the coolant through various circuits mainly contribute the steady state vibrations, while transient vibrations arise due to instant closure of the check valve (water hammer). The ASME Boiler and Pressure Vessel code provides data about the acceptable limits of stresses related to the primary static stress due to steady state vibrations. However, due to complexity in the pipe structure, stresses related to the transient vibrations are neglected in the code.

In this report attempt has been made to analyze both steady state and transient vibrations of PW-IPL of PARR-1. Since, both the steady state and transient vibrations affect the hangersupported section of the PW-IPL, therefore, it was selected for vibration test measurements. In the analysis vibration data was compared with the allowable limits and estimations of maximum pressure build-up, deflection, natural frequency, tensile and shear load on hanger support, and the ratio of maximum combine stress to the allowable load were made.

LIST OF CONTENTS

Abstract List of Contents

1. Introduction 1

2. Description of the Primary Cooling System of PARR-1 2

3. Vibration Acceptance Criteria for a Piping System 3

3.1 Steady State Vibrations 3 3.2

–  –  –

9. Acknowledgment 13

1. INTRODUCTION The Pakistan Research Reactor-1 (PARR-1) is a swimming pool type, originally designed to operate at a full power level of 5 MW with highly enriched uranium (HEU) fuel. Due to nonproliferation resistance.policy adopted by the fuel exporting countries, the availability of HEU fuel virtually became impossible. It was decided to convert it to commercially available low enriched uranium (LEU) fuel. Changing experimental needs and requirement for isotope production demand higher neutron flux levels. Therefore, conversion to LEU was availed as an opportunity to upgrade the reactor power to 9 MW. During conversion and upgradation, most of the reactor systems were modified and several additional facilities were provided. Major modification and additions were carried out in the reactor cooling system. These involved installation of new primary pumps, a set of heat exchangers, a cooling tower and new primary pipelines. To check the systems integrity it is essential to perform vibration tests. Results of the tests performed on the primary pumps and the core support structure comply with the requirements for pre-operational and initial startup vibration testing of nuclear power plant (NPP) systems [1,2]. Attempt is now being made to investigate steady state and transient vibrations of the cooling system.

The Primary Cooling System (PCS) of PARR-1 is a closed loop, circulating demineralized water through the core and the heat exchangers, to remove the heat generated in the core. The cooling system has been designed in accordance with the American Society of Mechanical Engineers (ASME) and American Standards Association (ASA) codes [3,4], considering the effects of various loads such as pressure, weight, thermal and seismic loads. The PCS is subjected to various dynamic loads due to the flow-induced vibrations. The steady state vibrations are mainly contributed by the reactor coolant pumps, which drive the coolant through various circuits. These pumps generate periodic and random disturbances in the fluid, which in turn vibrate the system. The transient vibrations in the pipeline arise due to an instant closure of the check valve(s), initiating a water hammer, while switching to new operating condition. Other sources of vibrations are located at the singular points such as cross-sectional enlargements, bends, valves and T-junctions. The presence of these singular points increases the possibility with which such disturbances can excite in resonance beam type flexural modes of the pipe lengths, and may induce vibrations of lower order frequency [5], Water hammer initiating from startup or shutdown of pump causes built-up of pressure/velocity waves traveling upstream from the point of origin and reflected from the subsequent boundary. It may result in very high amplitude vibration levels. These vibrations can cause the degradation of the piping structures and supports and hence lead to fatigue failure.





Therefore, for the integrity of the piping system, it is important to estimate the dynamic stress associated with such vibration modes.

Tests were performed to assess the vibrational levels of the Primary Water Inlet Pipeline (PW-IPL) under steady state and transient operating conditions. Preliminary investigation and subsequent vibration measurement followed by data analysis indicates that at most of the monitoring positions the peak vibration levels are within the permissible range. A detailed analysis of water hammering was conducted to demonstrate the structural integrity of the piping system under transient displacement loads. This report gives the details of the vibration analysis of the inlet pipeline of the PCS of PARR-1.

2. DESCRIPTION OF THE PRIMARY COOLING SYSTEM OF PARR-1

The PCS comprises plenum, a hold-up tank, two sets of heat exchanger assemblies, two identical pumps PW-P1 and PW-P2, valves and piping. The cooling water flows downward under gravity through the reactor core, grid plate and plenum into the hold-up. Subsequently, water is drawn from the hold-up tank by the circulating pumps and pumped through the shell side of the heat exchangers and PW-IPL back into the reactor pool. The hold-up tank, heat exchangers and pumps are located in the pump room adjacent to the reactor containment hall. The flow diagram of PCS is shown in Fig. 1.

The reactor pool level is maintained by equalizing both inlet and outlet coolant flow rates at 900m3/h for full power operation (9MW). The stall and open end outlet lines are combined into one after butterfly valves V01 and V02 in the valve pit. Similarly, the inlet lines for the two sections of the pool are combined into one before the butterfly valves V03 and V04. The piping before the valves is embedded in the reactor hall floor while the piping after the valve pit is mounted on wall supports in a pipe tunnel. At the discharge ends of PW-P1 and PW-P2, two independent check valves NV-06 and NV-32 are installed. After these check valves, the piping are combined to a common header, which provides separate coolant entrance to each of the heat exchanger. The PW-IPL carries the coolant from the outlet of the heat exchangers to the reactor pool, is supported from the roof by the hangers. Since, both the steady and transient vibrations affect PW-IPL; therefore, it has been selected for vibration measurements. The vibration monitoring points are shown in isometric of PW-IPL depicted in Fig. 2.

3. VIBRATION ACCEPTANCE CRITERIA FOR A PIPING SYSTEM

A piping system is subjected to variety of stresses, some introduced by initial fabrication and erection and some due to dead weights (such as pipe, fittings, insulation etc.), contents of the pipe line, earthquake and water hammering. Internal/external pressure and the restraint of thermal expansion may introduce further stresses. Usually, the dead load effects are always maintained, while earthquake and hammering effect will be variable and reach maximum design value occasionally. Pressure and temperature changes usually occur simultaneously and may be relatively uniform for entire service periods. The Pressure Vessel and Piping (PVP) codes contain tables of allowable stresses related only to the primary static stresses present owing to the dead loading. Due to the lack of adequate analysis and complexity many transient stresses are neglected in the PVP code.

Using ASME/ANSI code [6], the portion of the primary piping system to be tested has been classified into Vibration Monitoring Group (VMG). The PARR-1 piping is placed in more stringent VMG-2 group, due to its shorter length compared to the NPP and the type of equipment available for vibration measurements. However, the pipe section classified into one group for steady state vibrations may be classified into another group for transient vibrations.

3.1 Steady State Vibrations The steady state vibration is determined by measuring the vibration level during normal operating conditions of the piping system. If the vibration level exceeds an acceptable limit, it may be evaluated by measuring displacement or velocity. The velocity method was adopted due to the availability of the test instrumentation and the convenience of the vibration measurement.

The allowable peak velocity was used to determine the level in steady state vibration.

Consecutive measurements were made along the pipe length to locate the points exhibiting the maximum vibratory velocity. The criterion for acceptability is;

Vmax f; Vallowable (1)

–  –  –

liquid, £ ' is the modulus of elasticity of the pipe material, p is the density of liquid and AVis the change in velocity of the liquid.

Considering a check valve connected to the pump at one end and a T-junction at the other end (Figure 1). When the valve is suddenly closed due to the shut down of the pump, a compression wave (surge wave) travels upstream with the velocity Sv to the end junction and a pressure at the valve is AP plus the discharge pressure Ps of the pump that existed at the check valve before it was closed. When the compression wave reaches the junction, it is reflected as an expansion wave. After the expansion wave reflected, the pressure at the valve is AP below the static pressure Ps, Compression and expansion waves would travel up and down the pipe alternately and indefinitely if there were no friction. The time taken for the pressure wave to make a round trip is t = 2L/ S v, L is the length of the pipe. The pressure ahead of the valve as a function of time is shown in the figure given below. The friction will reduce the amplitude of the pressure wave until equilibrium is attained.

–  –  –

When the closure time is greater than 2L/SV, the pressure history at the valve can be estimated by the relation AP«2pVL/Atc^ where Atc^ is the closure time of the valve.

4. TEST INSTRUMENTATION AND SIGNAL PROCESSING

The vibration monitoring system employed for field measurements comprises single- and triple-axis accelerometers, charge amplifies, high/low pass filters, vibro-meter and dual channel vibration analyzer. The single-axis accelerometer has the maximum input parameter range up to 5 g with the frequency span of 2-6000 Hz and has a sensitivity of lOOmv/g, while the triple-axis accelerometer has the frequency span of 1-600 Hz and sensitivity of 1 V/g. The vibro-meter is able to measure peak velocities over the range lO'-lO3 mm/s, same range for displacement in urn (p-p) and peak acceleration from 0.3-30 g. For accurate measurements over the wide amplitude ranges specified above, the meter is provided with several fixed gain adjustment. Gain normalization of the accelerometer output scale factor is incorporated to read out directly in absolute velocity and displacement units. Maximum frequency range of the dual channel vibration analyzer is 100 kHz. It can analyze vibration, sound, noise and other waveforms by performing time function, auto/cross correlation, coherence, phase impulse response function etc.

Low noise cable with a remote charge amplifier was used between transducers and signal conditioning units to avoid cable noise pickup or signal attenuation due to longer length. The DC component of the signal and extremely low frequency components of the vibration were eliminated with switch selected high pass filter. Anti-aliassing low pass filter was also available at the upper end of the vibration band to eliminate unwanted high frequency noise. Therefore, the proper amplitude function (rms, peak, peak-to-peak) consistent with the acceptance criteria for the measured variable were carefully selected.

On-line signal analyses include peak velocity and displacement measurements, real time signal traces, square root of sum of squares (SSRS) of two signals and auto power spectra.

During the analysis of the vibration signal the sampling frequency was 4096 Hz with the frequency bandwidth of 200 Hz. For additional off-line studies and processing, the data was recorded on a FM instrumentation tape recorder and analyzed on a PC based signal analyzer. The experimental set-up for the vibration measurement is shown in Fig. 3.

5. VIBRATION MEASUREMENTS AND ANALYSIS



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