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Vol.2 No.3 previous GA 15 - AA 25 - 26 - SP4 ( 27 - 28 - 29 ) - NT 29 - 30 - 31 - 32nextVol.3 No.1
Vol.2, No.4, NT29
 
Condition-Based Maintenance of Motor-Operated Valves in Nuclear Power Plants
 
OKANO VALVE MFG. CO. and TOKYO ELECTRIC POWER CO,INC.
 
1. Technical summary

Classification
2 - A (Inspection Technology of Valve)

Nuclear power plants in Japan in general each have about 20,000 units of valves installed in them. Those valves had been mainly inspected based on time-based maintenance (TBM), where they are overhauled on prescribed time intervals. In January 2009, the Japanese government urged the nuclear power plants to streamline and optimize their maintenance activities. In line with it, a study is underway of the necessity to introduce in future a method considering condition–based maintenance (CBM), with which the timing and process of necessary repair is determined by measuring ongoing conditions of aging and damage.

Having predicted the growing importance in future of CBM, Okano Valve has successfully developed its own unprecedentedly innovative Ampere & Voltage Diagnostic System (AVD), to use in diagnosing motor-operated valves (MOV), which is expected to contribute to (a) enhancement of safety, (b) heightening of work efficiency and (c) improvement of working environment.

The AVD is a technology to diagnose a valve by measuring continuously the current and voltage of its motor at the Motor Control Center (MCC). Measured data are displayed as curves on the screen of the AVD apparatus. Those curves are interpreted to diagnose the ongoing performance of motor-operated valves. This innovative technology effectively precludes fieldworks such as valve remaking, use of strain gauges and installation of sensors which were indispensable in the conventional valve diagnostic method.

Being a condition monitoring and diagnostic system, where maintenance personnel are not required to come in contact with the valve under inspection, the system dismisses the possibility of erroneously damaging the valve while overhauling it. Especially, for nuclear power plants, the synergistic effect that radiation exposure of maintenance personal is reduced. The system requires no specific data transfer unit such as wireless, and also requires less number of sensors in data measurement, another advantage that simplifies working process, contributing to measurable reduction of working period of time, and minimization of personal error in measurement accuracy and of skill training of personnel.

 
2. Development phase

Phase 2 : Industrial Confirmation Phase

3. Scope
  1. (1) Components: Moto-Operated Valve
  1. (2) Location: Motor, Gear Case and Valve
  1. (3) Others: Condition monitoring and diagnosis, on-line or off-line
4. Features
 
The AVD valve diagnostic apparatus with its all composing parts neatly stashed in a carrying case is very easy to carry, allowing easy and prompt access to the working place. See Fig. 1.
 
Unlike the conventional method that was troublesome in handling as it required measuring sensors be installed directly on the target motor-operated valve, the AVD makes it possible to diagnose a motor-operated valve at the MCC that is remotely located from the valve as shown in Fig. 2, offering an advantage that diagnostic process can be performed under whatever environment the valve is installed in. Especially, preparatory setup takes as shorter as ten percent of the time our old method demanded.
 
The AVD technology further produces an extra added value when integrated into the MCC, i.e. with the system there is a possibility that even a valve that is installed in high radiation area can be measured or diagnosed irrespective of whether the plant is in operation or shutdown, enabling remarkable maintenance planning based on track records as well as reducing the effect of radiation exposure.

 

EJAM2-4NT29_Fig.1_AVD_Apparatus

Fig.1 AVD Apparatus


EJAM2-4NT29_Fig.2_Measurement_of_Motor_Current_and_Voltage_at_Motor_Control_Center

Fig.2 Measurement of Motor Current and Voltage at Motor Control Center


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Fig.3 How to use AVD Apparatus


The AVD technology enables estimation of the torque and speed of the target motor-operated valve based on its measured current and voltage. Fig. 4 illustrates a model of drive force transmission of a motor-operated valve.
 
Motor-operated valves are in general operated by having a gear mechanism to amplify the motor torque into a driving torque required to open/close the valves. It means that the ongoing operating condition and performance of a motor-operated valve can be known by measuring its driving torque, i.e. because the measured value of driving torque changes in synchronization with valve loading, it is possible to know time-series operating characteristics of a motor-operated valve by measuring its driving torque. Moreover, because, where the rotational speed of the motor is known, valve lift can be estimated with it, the combined measurement of driving torque makes it possible to analyze more precisely the ongoing state of the valve. See Figs. 5 and 6 that show characteristics of MOV torque, also See Figs. 7 and 10 that show the results of AVD validation tests.

EJAM2-4NT29_Fig.4s_Model_of_Drive_Force_Transmission_Mechanism_of_Motor-Operated_Valve

Fig.4 Model of Drive Force Transmission Mechanism of Motor-Operated Valve, and Layout of Measurement Process

 
EJAM2-4NT29_Fig.5s_Motor-Operated_Valve_Gate_Type

Fig.5 Motor-Operated Valve: Gate Type.
 
EJAM2-4NT29_Fig.6s_Characteristics_of_MOV_torque_and_FFT_waveform_after_motor_starts

Fig.6 Characteristics of MOV torque and FFT waveform after motor starts
 

EJAM2-4NT29_Fig.7_MOV torque vs. time after motor starts

Fig.7 MOV torque vs. time after motor starts (AVD Validation Test Data)

 

EJAM2-4NT29_Fig.8_Motor_speed_vs_time_after_motor_starts

Fig.8 Motor speed vs. time after motor starts (AVD Validation Test Data)

 

EJAM2-4NT29_Fig.9s_MOV_torque_curve_after_valve_opening

Fig.9 MOV torque curve after valve opening

 

EJAM2-4NT29_Fig.10s_Stem_Torque_Waveform_Right_before_Closing

Fig.10 MOV torque curve before valve closing

Cases of MOV Operational Errors and Failures
 

Case 1: Packing Failure

The valve has a grand packing installed in it to have it produce sealing effect on its moving parts. As the grand packing deteriorates, the friction resistance between the ground packing and the valve stem grows bigger. Therefore, a motor-operated valve with its deteriorated grand packing needs its elevated driving torque, i.e. a larger torque value to keep it running. See Fig. 11.

 

EJAM2-4NT29_Fig.11_Detection_of_Packing_Failure

Fig. 11 Detection of Packing Failure
(MOV torque comparison between normal and failure modes)

 

Case 2: Unseating Torque

A motor-operated valve with its maladjusted stop position may have its seating torque exceeding its design value and cause the gate valve to have its seat cracking, or the vessel to have its internal pressure rising irregularly. It is possible to discover the incidence by monitoring the unseating torque that develops right after the valve starts its opening motion. See Fig. 12.

 

EJAM2-4NT29_Fig.12s_Detection_of_Unseating_Torque

Fig. 12 Detection of Unseating Torque
(Unseating Torque comparison between normal and failure modes)

 

Case 3: Stem Nut Wear

The motor-operated valve has a part called "stem nut" installed in it. The role of the stem nut is to convert the revolving motion of its drive mechanism into the reciprocating motion that is transmitted to the valve stem. The stem nut has threads cut over its inner surface. The material of the stem nut is in general made of a copper alloy softer than the metal of the valve stem in order to prevent seizure between metallic threaded portions of both parts. As the lubricant coating deteriorates as time passes while the valve is in service, the stem nut has its threads worn out gradually. The more the threads wear out, the bigger becomes curve clearance right after the valve start-up. By monitoring this characteristic together with valve lift, the progressive wear of stem nut threads can be measured. See Fig. 13

 

EJAM2-4NT29_Fig.13s_Detection_of_stem_nut_wear

Fig. 13 Detection of stem nut wear
(MOV torque comparison between normal and failure modes)

 

Case4: Bearing Failure

Bearing failures not only in motor-operated valves but also in any other rotating drives lead to the waning reliability of the mechanical systems they are in. The AVD technology, which, analogous to general vibration analyses, can analyze the frequency of the waveform pulsation of the electric power that has been obtained by measuring electric current and voltage, is instrumental in detecting irregular motion of rotating parts including bearings. See Fig.14.

 

EJAM2-4NT29_Fig.14s_Detection_of_bearing_failure

Fig.14 Detection of bearing failure
(Motor power spectral comparison between normal and failure modes)

 

Case5: Gear Failure

Motor-operated valves in general employ a speed reducer with gear mechanism for their drives. The purpose is to amplify motor torque by reducing motor’s rotating speed. The gear mechanism is however vulnerable to damage because of likely inadequate lubrication or improper installation. As in Case 4 above, irregularities of the gear mechanism can be detected by means of frequency analysis of electric power. See Fig.15.

 

EJAM2-4NT29_Fig.15s_Detection_of_Gear_Failure

Fig.15 Detection of Gear Failure
(Motor power spectral comparison between normal and failure modes)

 
5. Examples of Application

Tokyo Electric Power Company has so far collected and evaluated data on approximately 2,000 units of motor-operated valves installed at its Fukushima Dai-ichi, Dai-ni, and Kashiwazaki-Kariwa nuclear power plants. At the same time, the company has launched the compilation of a database for them. Also, it is underway of studying the quantification of the standards to manage the condition monitoring maintenance of its motor-operated valves.

6. Reference
  1. [1]K.Nagaiwa, K.Tanaka, K.Oyama, S.Hikita, A.Kawamoto, Conditon Based Maintenance of the Motor Operated Valve in Nuclear Power Plant, The Proc. of 7th Annual Conference of the Japan Society of Maintenology, Omaezaki, pp.510-514, July 2010. (in Japanese)
7. Contact

Japan Society of Maintenology (ejam@jsm.or.jp)