IDENTIFICAZIONE DI MODELLI DI ATTRITO NELLE VALVOLE PER LA DIAGNOSTICA DI LOOPS DI REGOLAZIONE

Sviluppo di una procedura di modellazione di un loop di regolazione industriale affetto da attrito nella valvola. Identificazione dei parametri caratteristici del sistema e quantificazione dell'attrito. Applicazione su dati industriali, per la verifica e la programmazione della manutenzione degli attuatori

Stiction Quantification: A Robust Methodology for Valve Monitoring and Maintenance Scheduling

Valve stiction is one of the most common causes of poor performance in control loops. This paper presents a procedure which allows stiction quantification. The technique permits one to estimate the unknown real stem position, and moreover, it does not need any process knowledge and requires only the data normally registered in industrial plants. It is pointed out that the real problem consists of the lack of knowledge about the true value of stiction. A general methodology is proposed to discard data for which quantification is very likely to give wrong indications and to restrict its application to appropriate cases. Simulations show that several sources of perturbations can be eliminated, thus improving the reliability of stiction evaluation. Results are confirmed by application to industrial data: a significant number of valves are analyzed for repeated acquisitions before and after plant shutdown. The proposed procedure seems to be a valid methodology to monitor valve stiction and to schedule and check valve maintenance

Comparative performance analysis of an electric actuator for control valves

This article deals with the analysis of performances of electric actuators for control valves in industrial control loops. The objective of the recent collaboration between University of Pisa and CLUI AS is to assess potentials and benefits of control valve electric actuators, by testing and comparing devices of different typologies and manufacturers. As a premise, it should be noted that pneumatic actuators still represent the most commonly used actuation devices in the process industry, mainly because of high performance and fast response. In recent years, electric actuators, as a result of their enhanced features, are finding increasing applications in the area of process control. Anyway, some practical aspects, such as the degradation of the valve seat, an excessive tightening of the seal, and an expansion of metallic components due to high temperature operation, can cause malfunctions, and in particular, phenomena of wear and friction within a control valve regardless of the type of actuator. In fact, pneumatic and electric valves differ only in the actuation system; while the valve body, subject to most of the friction forces, is absolutely the same. In detail, the present work has been focused on the analysis of a recent electric actuator installed on an rotary control valve, and tested in the last biennium in an pilot plant, owned by ENEL in Livorno (Italy). Specific experimental tests were carried out, by collecting operating data in open-loop and closed-loop mode. The validity and effectiveness of the performance was verified in nominal and faulty conditions, in particular, by introducing a dead-band. Furthermore, performances of this electric actuator was compared with that of a conventional pneumatic actuator with positioner, coupled to the same valve, installed on the same plant line, and tested in equivalent experimental conditions. In general terms, it has been confirmed that the electric actuator for control valve is a promising technology, and its performance are fully comparable - if not superior - to those of the pneumatic actuator. In particular, some simple performance indices assume similar numerical values, and also the time trends of the positional error and the limit cycles registered on polar diagram between valve input and output signals are similar

Identification techniques for stiction quantification in the presence of nonstationary disturbances

The paper presents a detailed comparison of different identification techniques applied to valve stiction quantification, possibly in the presence of nonstationary unknown disturbances. The control loop with sticky valve is modeled as a Hammerstein system, in which the nonlinearity is identified using enumeration of the parameters’ space. Five different techniques for identification of the linear model are compared in terms of achievable performance. In particular, the capability to cope with the presence of nonstationary disturbances is analyzed. The techniques allow one to estimate the unknown actual valve position (MV), without requiring any process knowledge, being based only on data which are usually recorded in industrial plants: controller output (OP) and controlled variable (PV). Simulations show that external perturbations can be tolerated, thus ensuring a reliable evaluation of stiction in practical situations where external disturbances are usually present. Models which incorporate a time varying additive nonstationary disturbance grant a better process identification and a more accurate stiction estimation in the case of disturbance acting simultaneously with valve stiction. However, simpler models are the best choice when stiction happens to be the only source of loop oscillation. Results are confirmed by application to real data: pilot plant data are used to corroborate the effectiveness of the techniques

System identification applied to stiction quantification in industrial control loops: A comparative study

A comparative study of different models and identification techniques applied to the quantification of valve stiction in industrial control loops is presented in this paper, with the objective of taking into account for the presence of external disturbances. A Hammerstein system is used to model the controlled process (linear block) and the sticky valve (nonlinear block): five different candidates for the linear block and two different candidates for the nonlinear block are evaluated and compared. Two of the five linear models include a nonstationary disturbance term that is estimated along with the input-to-output model, and these extended models are meant to cope with situations in which significant nonzero mean disturbances affect the collected data. The comparison of the different models and identification methods is carried out thoroughly in three steps: simulation, application to pilot plant data and application to industrial loops. In the first two cases (simulation and pilot plant) the specific source of fault (stiction with/without external disturbances) is known and hence a validation of each candidate can be carried out more easily. Nonetheless, each fault case considered in the previous two steps has been found in the application to a large number of datasets collected from industrial loops, and hence the merits and limitations of each candidate have been confirmed. As a result of this study, extended models are proved to be effective when large, time varying disturbances affect the system, whereas conventional (stationary) noise models are more effective elsewhere

Model predictive control design for multivariable processes in the presence of valve stiction

This paper presents different formulations of Model Predictive Control (MPC) to handle static friction in control valves for industrial processes. A fully unaware formulation, a stiction embedding structure, and a stiction inversion controller are considered. These controllers are applied to multivariable systems, with linear and nonlinear process dynamics. A semiphysical model is used for valve stiction dynamics and the corresponding inverse model is derived and used within the stiction inversion controller. The two-move stiction compensation method is revised and used as warm-start to build a feasible trajectory for the MPC optimal control problem. Some appropriate choices of objective functions and constraints are used with the aim of improving performance in set-points tracking. The different MPC formulations are reviewed, compared, and tested on several simulation examples. Stiction embedding MPC proves to guarantee good performance in set-points tracking and also stiction compensation, at the expense of a lower robustness with respect to other two formulations

PERFORMANCE ASSESSMENT OF CONTROL LOOPS: controller evaluation for frequent set-point changes

The high seasonality and variability of electricity production from renewable sources has enormous impact on both production and distribution networks. Among the many aspects, several traditional power plants, especially, combined cycle plants, but also coal-fired plants, now operate intermittently with variable set points due to fluctuations of energy load which they are requested to deliver [1]. Thus, a specific analysis of control system performance during these transient cyclic phases, as reference changes, operations of start up and shutdown, is highly desirable. Generally speaking, monitoring and assessment of performance of control systems of industrial plants are important topics in process control. The deterioration in performance is, in fact, a fairly common phenomenon and manifests with sluggish or oscillating trends of control variables. Oscillations in control loops can cause many problems which affect normal operation of process plants. Typically, fluctuations increase variability of product quality, accelerate wear of equipment, move operating conditions away from optimality, and, in general, cause excessive or unnecessary consumption of energy and raw materials [2]. This paper introduces a technique for the analysis of performance of basic control loops when process is subject to changes of operating conditions. The method employs the well-established approach of Internal Model Control, IMC. After establishing lower limit for the absolute value of the integral (IAE) of control error and the total variation (TV) of control action, such limits are assumed as reference values for a control considered “optimal”, or anyway “good”. A performance index is thus based on IMC and is properly defined with respect to lower limit of IAE and TV. With this approach, the validity of tuning of PID-type controller in response to any reference change can be assessed. In particular, one can successfully evaluate closed-loop performance for setpoint changes, as steps, ramps, or generic trends, as for the common case of preset programs of variable load of power plants

A valve stiction tolerant formulation of MPC for industrial processes

This paper presents three different formulations of MPC to face static friction in control valves for industrial processes. A pure linear formulation, a stiction embedding structure, and a stiction inversion controller are designed. The controllers are derived for SISO systems with linear process dynamics, where valve stiction is the only nonlinearity present in the control loop. A novel smoothed stiction model is introduced to improve and fasten the dynamic optimization module of stiction embedding MPC. A stiction compensation method is revised and used as a warm-start to build a suitable trajectory for the predictive controller. The different MPC formulations are tested and compared on some simulation examples