Monitoring and evaluation of the status of Motor-Operated Valves in nuclear power plants

To address the inability to monitor the performance status of Motor-Operated Valves (MOV) during the operation of nuclear power plants, develop the MOV status evaluation model and design a valve online monitoring system. During valve operation, the data acquisition unit (DAU) installed in the distribution cabinet of the motor control center (MCC) can capture the voltage, current and switch signals. On the basis of the aforementioned parameters, the active power curve and characteristic points of the valve can be determined, implemented monitoring and evaluation of the status and performance of MOV.


Introduction
Many countries around the world now recognize nuclear power as a safe, clean, and efficient source of energy.Nuclear energy's peaceful development and use has not only given new impetus to human development, but has also played an important role in guaranteeing energy security and combating climate change.The most significant distinction between nuclear power and conventional energy sources such as thermal power and wind power is that their functioning necessitates safe operation, and the foundation of safe operation is equipment reliability.
MOVs have the advantages of high driving torque, brief action time, simple remote control, a variety of protection functions, etc., and are used in a variety of process systems of nuclear power plants, with some of the valves performing crucial safety functions.In addition, the complex structure and electric drive mode make it simple to cause motor overheating, gear wear, and other defects in the event of frequent switching or regulation.Monitoring and diagnosing the performance status of electric valves, and promptly addressing valve faults, are crucial for enhancing the unit's safety level.

MOV Diagnostic Technology
A MOV consists primarily of three components: an electric actuator, a bearing chamber, and a valve.Electric actuator is to provide driving force for the valve mechanism, by the motor, deceleration mechanism, worm gear / worm gear and hand wheel; bearing room is the motor output torque is converted to the valve stem thrust mechanism, by the stem nut, thrust bearings, etc.
MOV diagnostic technology is presently the most prevalent method for understanding the status of MOVs.This diagnostic technology involves the installation of mechanical and electrical sensors on the valve without its disassembly, as well as the use of specialized valve diagnostic testers and data analysis software to test and analyze the performance characteristic parameters of the valve in order to determine the performance status of the electric valve [1].Unfortunately, the above diagnostic methods require close proximity to the valve, which has the following drawbacks: some valves are inaccessible or have non-rising stems, making diagnostic testing impossible; The test valve is located in a high dose area, increasing the test technicians' exposure to radiation; The diagnostic work prolongs the overhaul period, which increases the cost of maintenance.This article proposes using the active power parameter of the electric actuator to evaluate the status of a valve in order to surmount the limitations of the previously described diagnostic methods.This technique possesses the following qualities: It can capture current and voltage parameters at the power distribution position of the actuator without needing to be in close proximity to the valve; the active power consumed by the motor is consistent with the mechanical state of the valve after losses have been subtracted.This method can be used as a supplement or replacement for diagnostic testing "atthe-valve" in nuclear power plant MOVs performance monitoring.

Status Evaluation Model for MOVs
For in-service MOVs, two aspects must be evaluated on a regular basis: operability and mechanical condition.The former evaluates whether the actuator can output the required torque to drive the valve to complete the opening and closing actions, thereby conducting or cutting off the medium in the pipeline; the latter evaluates whether there is wear or insufficient lubrication in the mechanical components of the valve, particularly the worm [2]. Figure 1 depicts the general procedure for evaluating the status of MOVs.During operation of the nuclear unit, only the MOV current and voltage, as well as the active power calculated based on both, are obtainable.According to Granjon P, there is a good correspondence between the active power of the motor and the mechanical state of the valve [3]; therefore, it is essential to comprehend the characteristic curve of the active power of the valve during the opening and closing stroke [7].The figure 2 depicts the active power curves of a globe valve which closing controlled by torque and opening controlled by limit in a static state (without medium or no pressure difference in the pipeline),and the left curve of the figure 2 depicts the opening stroke, while the right curve depicts the closing stroke.

Operability Evaluation
The evaluation of the operability of MOVs consists of the following steps: first, determine the upper and lower limits of the actuator's output torque; Complete the baseline testing of the valve during the unit's overhaul to determine the power-torque relationship; Obtain the active power of the valve during the nuclear unit operation and estimate actuator output; Lastly, compare the actual output to the actuator's limit value, evaluate the operability, and compute the functional margin.

Determination of the limits of the actuator's output torque
In order to protect the valve, the torque value of the actuator torque switch trigger point has a maximum value.
First, the upper limit of the torque switch action point should be less than the maximum torque that can be provided under the rated voltage drop condition, because the power supply system of the power plant can only provide the voltage after the drop under the accidental condition, and if the value of the torque switch trigger point setting is too high, the torque switch cannot be operated, and the power supply of the motor cannot be cut off, which will result in damage [2].The torque value is determined by the formula below.
Where: MST: Motor rated starting torque, N.m; OAR: Overall ratio (dimensionless)----OAR is the ratio of motor turns to stem nut turns.It is equal to the helical gear ratio times the worm ratio; AF: Application factor(dimensionless)----AF is used to account for the variability of voltage from the nominal values, and other application-specific limitations, 0.9; : Actual available voltage, Volts;   : Rated voltage, Volts; n =2 for voltage less than 90% of rated voltage but not less than 70%; =0 ±10% of rated voltage (90%-110%) when sizing the actuator; Second, the upper limit of the torque should be less than the structural limit of the valve's weakest component, otherwise, the valve will be damaged.This includes the mechanical structure of the actuator, valve stem, valve disc, and valve seat, among others.The upper limit value is the minimum of the two elements listed above.
The lower torque limit (N.m) of the actuator is the minimum torque required to operate the valve.The lower limit of torque is calculated based on the minimum thrust required to open and close the tested valve.The thrust mainly includes differential pressure force, packing friction force, Piston effect force and sealing force.Determine the lower torque limit using the following formula (2): =   *  (2) Where, T req is the minimum thrust required to operate valves (N), FS is the stem factor (mm).In actual measurements, it is necessary to account for uncertainties such as load rate, lubricant deterioration, and torque switch repeatability, and to adjust the above limit values accordingly.

Baseline Test
The purpose of the baseline test is to determine the relationship between the active power of the motor and the output torque of the actuator in order to facilitate the subsequent estimation of the actuator's output torque when only power data are available, using the following estimation formula (3). =   (  −  ℎ ) (3) Where, M is the output torque of the actuator (N.m), C c is the conversion factor (N.m/kW), P is the power when the torque switch is triggered(kW), and P hold is the no-load power (kW); In order to reduce errors, the thermal losses of cables and motor windings should be accounted for when testing and obtaining the active power of the actuators.If feasible, the mechanical losses of gearbox mechanisms such as worm gears/worms should also be considered [4].
Typically, the torque at the characteristic point and the active power measurement with correction are used to determine the conversion factor [5]. Depending on the available calibration equipment and the type of calibration measurement, the following methods may be used: 1) When the actuator is not coupled to the valve, torque calibration apparatus and other devices can undergo baseline testing.There are two approaches based on the data recording function of the calibration device: Method 1: If the calibration testing device can record the time-domain data of the active power and torque signals simultaneously, the power and torque curves can be obtained simultaneously, as depicted in Figure 3.  where, t 1 : Start loading the load; t 2 : When the control switch tripped; t3: When the active power reaches its maximum value before power outage; t4: Maximum torque.The conversion factor   is determined using data at t1 and t2, and the formula ( 4) is as follows: Where, ( 1 )/( 2 ) and ( 1 )/( 2 )represent the torque and power at t1 and t2, respectively; Method 2: If the testing apparatus cannot record power and torque signals in real time, the load device must be loaded in stages and the power and torque of each stage must be recorded manually.Points corresponding to the number of loading stages are used to establish the functional relationship between torque and power, as depicted in Figure 4.According to formula (5),   can be inferred: Where,  1 / 4 and  1 / 4 represent the torque and power of the actuator during the first and fourth loading; 2) When the MOV is located on-site and the power parameters of the valve can be obtained, but there is no torque acquisition device available.And the torque value of the actuator is accurately set, the complete power signal of the valve opening and closing procedure can be recorded in order to calculate the conversion factor using the following equation (6).The conversion factor   is determined using the formula (6) as follows: =     − ℎ (6) Where,  ℎ is the no-load active power, kW;   is actuator's set torque, N.m;   is the measured and corrected active power when the torque switch is triggered, kW.

Operability evaluation
After completing baseline testing and determining the power-torque relationship, the valve can be evaluated further by collecting data at the MCC.As long as there is no significant change in the setting of the MOV, this power torque relationship can be used indefinitely.And the significant changes generally include changing the torque value setting of the actuator, replacing the motor, renovating the electric actuator, adjusting the gear ratio or replacing the gear, and altering the actuator's lubricant type 1) Estimation of output torque of actuator.We use the following equation ( 7) to determine the actuator's output torque when Cc is known: Where,   ′ is the predicted output torque of the actuator when the torque switch is triggered (N.m),   ′ is the measured power when torque switch triggered (kW), and  ℎ ′ is the measured no-load power (kW).
2) Operability evaluation: If   <   ′ <   , the operability of the actuator meets the requirements for valve operation;If   >   ′ or  <   ′ , the actuator does not meet the valve operation requirements; 3) When the actuator meets the requirements for valve operation, the following equation ( 8) is used to evaluate the operability margin: G=(  ′ −   )/  *100% (8)

Mechanical condition Evaluation
The evaluation of the mechanical condition of a valve consists primarily of evaluating the valve's critical mechanical components and operating parameters.The evaluation method is obtaining the numerical values of characteristic points based on the power characteristic curve and comparing them to the standard curve and values [6].

Trend Analysis
It is not feasible to observe small changes in valve performance for the majority of valves based on a single data point.In this situation, it is necessary to conduct trend analysis on historical test data for valves and predict the performance development trend.The method consists of extracting valve characteristic curve feature points and displaying them on the same graph, observing the trend of pertinent parameters' development, and completing an analysis of the valve's performance trend.Figure 6 depicts the change in torque value when the torque switch is triggered.Based on the trend of torque change, the valve can be maintained prior to its limit value being reached.Note: MTup is the upper torque limit of the actuator; MTlow is the lower torque limit of the actuator;

Development and application of Valve Monitoring System
To complete online monitoring and status evaluation of MOVs, a valve online monitoring system was developed, and the valves were monitored to achieve monitoring and evaluation of MOVs.

Example of performance evaluation for MOVs
A nuclear power plant has installed an online monitoring system for MOVs to monitor and evaluate the status of critical valves.Some valves were discovered to have defects such as inadequate actuator output and incorrect limit setting.The monitoring and evaluation of a few valves are introduced in the following section.

Insufficient output torque of a MOV
A motor-operated shut-off valve whose opening stroke is controlled by limit and its closing stroke is controlled by torque, its basic information is shown in (1) The limits of the actuator's output torque The upper limit value of the output of the actuator is determined using the method in section 2.1.The output torque of the electric actuator under pressure drop conditions is 205N.m.After considering uncertainty, the maximum output torque of the actuator under reduced voltage conditions is 174N.m.The weakest structure of a valve is typically the valve stem's thread position.Based on the yield limit of the material and the size of the valve stem, the utmost force that the valve stem can withstand is 544000N, which is converted into a torque value of 1046N.m.After accounting for unpredictability, the maximal output torque of the actuator is determined to be 910 N.m based on the mechanical limit of the valve.The upper limit of the actuator's output is the lesser of the two values listed above, which is 174N.m.The lower limit value of the actuator's output is determined as well using the section 2.1 method.Based on the minimal thrust and stem factor required to operate the valve, the minimum torque required to operate the valve switch is 84.4 N.m.Taking into account the uncertainty, the lower limit of the output of the actuator is 95N.m.
(2) Baseline testing Before installing the valve in the system, torque verification will be performed on the actuator to ensure that the specified torque value is appropriate.Here, the power curve of the first action of the valve is used as the reference curve to calculate the power torque conversion coefficient.Figure 8 depicts the initial power curve that was gathered.The torque value of the valve is 140N.m, the no-load operating power is 0.239kW, and the power when the closing torque switch triggered is 0.884kW.The power torque conversion coefficient is calculated to be 217N.m/kW based on the method described in Section 2.1.

Operability evaluation during operation of the unit
During the power operation of the unit, the valve failed to open.The valve power curve is shown in Figure 9.According to the power curve, the no-load power is 0.246kW and the power while the closing switch triggered is 0.459kW.Based on the conversion coefficient, the calculated output power is M=46Nm.Visibly, the output torque of the valve is less than the lower limit value of the torque, and the output of the actuator is insufficient.During the overhaul of the unit, an inspection of the actuator revealed that the torque setting spring was defective, resulting in a drift of the torque setting value.

Limit switch setting incorrectly
After the limit switch is triggered on limit-controlled valves, the valve should shut off the power supply, and the final value of the valve stroke power curve should not exhibit an increasing trend.However, there is a significant increase in power at the conclusion of the valve B opening stroke.Typically, an error in the opening limit setting or a failing of the opening limit is the cause of this phenomenon.After technical personnel examined the actuator, it was determined that the limit cam was improperly set.Figures 10 and Figure 11 depict the power curves of the valve before and after limit adjustment, respectively.

Conclusion and further study
This paper proposes a method for evaluating the status of MOVs based on active power, including valve operability evaluation, mechanical status evaluation, etc., in response to the deficiencies of MOV diagnosis technology.Developing an online monitoring system for valves capable of collecting and processing the voltage, current, and switch signals of actuators at the distribution cabinet.In a nuclear power plant, some MOVs were monitored, and the application result was positive.The application of online monitoring and diagnosis systems for valves can surmount the limitations of offline diagnostic methods, detect potential valve defects in a timely manner, and take corrective action, which is crucial for the safe and stable operation of nuclear power plants.Due to the fact that the output torque of the actuator is estimated by the active power and the accuracy of the model is limited, there is a certain discrepancy between the estimated value and the actual value.Future research is required to enhance the accuracy of torque output prediction for electric actuators [8].

Figure 2 .
Figure 2. Active power curve of the opening and closing stroke of the globe valve.Where, each characteristic point corresponds to the following conditions of the valves: O: No load power; C: Running power of closing stroke; D: Running power of opening stroke; M: Switch off power; A: Max power; B: Max.relaxation power; S: Valve running time; E: Actuator running time; Tab: Shut off delay time; Pullout efficiency(dimensionless)----PE is the efficiency of the gear train during pullout conditions (that is Zero-speed);

Figure 3 .
Figure 3. Power and torque curve using method 1.

Figure 4 .
Figure 4. Power and torque curve using method 2.

Figure 5 .
Figure 5. Power and torque curve using method three.

Figure 6 .
Figure 6.Graph of the power trend when the torque switch is triggered.

Figure 7
depicts the online monitoring system's measurement module structure, which consists primarily of collectors, current transformers, voltage transformers, and signals.The collector can initiate the collection action based on the signal from the relay switch and transmit the signal to the upper computer via the Ethernet network in order to complete data processing and display.

Figure 7 .
Figure 7. Schematic diagram of online monitoring and collection.

Figure 8 .
Figure 8.Initial power curve after valve operation.

Figure 9 .
Figure 9.The power curve of the valve.

Figure 10 .
Figure10.The power curve of the valve before limit adjustment.

Figure 11 .
Figure 11.The power curve of the valve after limit adjustment.

Table 1 .
Table 1 outlines the principal evaluation criteria and methodologies.Mechanical Condition Evaluation Method.

Table 2 .
Basic Information of Valve A.