(1)
José Carlos Lauria
Escola de Engenharia Mauá
Praça Mauá, 1
09580-900 São Caetano do Sul – SP
Brazil
Phone +55 11 4239-3066 Fax +55 11 4239-4041
E-mail: jclauria@maua.br
Abstract:
“GED – GEneralized Diagrams” is a PC-based
software tool designed for computer-aided investigations on the fundamental
aspects of the operating behaviour of installed valves. The program plots
diagrams in a parametric form to provide means for evaluating the advantages and
limitations of each tentative valve according to a structured, stepwise
approach. It is adopted the pattern of the valve behaviour being represented by “Generalized Diagrams”, a proposal to determine the flow
behaviour of valves as elements integrated with their piping system. They
comprise three dimensionless families of curves drawn from the
energy-dissipation features of valves as components alone.
Keywords: valves; flow characteristics; operating behaviour
With the increasing requirements for hydraulic systems to be operated more efficiently and economically, the design and planning stages are becoming critical factors in determining their performance. In the specific case of valve-controlled installations, the process of selecting valves is carried out chiefly under two extreme conditions: on one hand, use of capacity coefficients for the element alone; on the other hand, complex flow-transient simulations to determine the optimum closure schedule.
No single valve is equally suited for the wide range of performance requisites encountered in the flow-control field. So it is necessary to examine the strengths and weaknesses of their basic types before specifying one for a given application. Defining valves, in fact, is a process of elimination and, as it has long been known, the first step to determine correctly their effective flow-characteristics.
Studies have been developed by the author to graphically analyse the operating behaviour of valves as installed elements in a structured, stepwise manner so as to support the correct choice.
Although it is possible to define the effective flow-characteristic by means of simple equations, plotting diagrams can be very time-consuming. Fortunately, computers make it possible to simplify the task and, facilitating sensitivity studies, to expand the way of understanding the valve functions and applications.
Having this goal in mind, the GED (GEneralized Diagrams) program was developed. The first version was reported on in 1992 (Lauria [1]). The aim of the present paper is the communication and description of GED’s third version, second release.
As the purpose of the work is to discuss concepts related to representation of the operating behaviour of valves, the program is distributed free of charge to those interested on the subject.
Valve flow characteristics are the most used means to assist the analysts to understanding the operating behaviour of valves. Knowledge of the effective flow characteristics is widely recognized as a necessary preliminary means to properly select valves as flow-controlling devices.
Over the past decades much has been said on flow characteristics of control valves, but mainly by means of idealized patterns or in case-by-case terms. Many references provide excellent conceptual basis on valve operating behaviour but, when it comes to valve flow characteristics, they restrict themselves to the inherent one, leaving a wide space for controversies.
To discuss the piping-system influence on valve operating behaviour, a great deal of works picks up a standard inherent flow characteristic as reference and sketches the effective ones to be expected as the percentage of the total pressure drop available for the valve is changed. The procedure only gives us the information that the more the piping-system resistance is, the more the effective characteristic flattens out at higher flowrates.
The subject remains open to debate and considerable discussion is still verified in the current literature regarding the best procedure for anticipating the operating behaviour of installed valves.
For the last 15 years the author has been developing studies on the operating behaviour of control valves in a dimensionless, parametric way. The simplest instance of valve application with practical meaning: valve equal to the duct size modulating fully turbulent, incompressible gravity feed, named here as basic case, is taken as the starting point for approaching the subject.
Describing the piping system by a dimensionless factor and the valve by its dissipative feature against aperture and supposing quasi-steady conditions, a dimensionless capacity index is put forward to quantify the relative importance of valves as installed elements, making it possible to define three generalized charts as a means of supporting their selection.
A brief summary of this study is present in the sequence. More details can be obtained in, for example, Lauria [2], [3], [4] or [5].
The problem of describing the operating behaviour of installed valves in a generalized form can be divided into two parts: the valve effects, represented by the loss coefficient KV, and the piping system influence, which is expressed associating the tube dissipative feature (fL/D) and the sum of the loss coefficients for all singularities excepting the valve (SKs), through a loss factor
(1)
For the basic case stated above, the available head is constant and no Reynolds number effects are present, so that the flowrate depends only on the valve aperture. The ratio between generic flowrate and the maximum flowrate, to be called reduced flowrate, results in
(2)
having for the fully
open valve
.
The reduced flowrate corresponds to a dimensionless index of capacity for the valve as an installed device as its aperture is changed, setting the participation of the valve (Kv) in comparison with the piping-system influence (Fp) as to the flow control. Besides, this parameter ranges from zero to one and is calculated from specific experimental data for elements alone.
Applying
into eqn (2), a second dimensionless parameter, or limiting
factor, is obtained to outline the reduction in the carrying capacity of the
installation as a result of the valve action
(3)
Comparing the generic flowrate to the flowrate with no valve in the installation, the limiting factor measures the relative importance of the valve with respect to the energy dissipation in the flow control. A low limiting-factor value means that the valve is the dominant part, and the importance of the valve decreases as the factor approaches one.
Depending on the style, there are several ways of describing the open area of a valve (angle, distance, area). For purposes of uniformity, a reduced aperture Ar is used to refer to the position of the valve closure element, corresponding to the ratio between generic aperture to the maximum one, and having fixed limits in the range from zero to one.
The parameters referred above associated with the experimental data of the loss coefficient against aperture for a specific valve style make it possible to represent the valve behaviour by families of curves in three generalized diagrams (for the valve and piping system), tools for evaluating the advantages and limitations of the valve in a given installation.
Corresponds to the Cartesian representation of the reduced flowrate (eqn 2) versus reduced aperture, having the loss factor (eqn 1) as parameter.
It registers the effective flow characteristic of the valve (Fig. 2) and, therefore, provides information on controllable flowrate range, convenient aperture limits or critical operating conditions.
It is the plot of the reduced flowrate as a function of the loss factor, for each reduced aperture (Fig. 3).
A family of curves describes the proportional changes on flowrate due to valve movements. The roles of the loss factor and the reduced aperture are inverted in relation to the operation diagram, so as to more easily identify the working range of the valve.
Obtained from graphical representation of the limiting factor (eqn 3) against the loss factor and taking the reduced aperture as parameter (Fig. 4).
The curves on this diagram compare the head loss caused by the valve with the head loss due to the piping system, allowing to identify when the valve starts to act as a dissipative component.
Although the generalized diagrams provide a conceptual insight into what a valve means to the flow control from an engineering standpoint, plotting them manually is time consuming and can be cumbersome if several possibilities are to be investigated. Spreadsheets can significantly reduce the amount of required work, but the ideal solution is a specific computer program.
With the aim of automating the process of drawing the generalized diagrams the GED software was developed.
GED is a comprehensive tool for analysing the operating behaviour of valves as installed elements and its application is associated with the planning, design, operation and maintenance of valve-controlled hydraulic installations.
GED is planned to make easier the regular usage of the generalized diagrams not only for supporting the selection of valves but also to give the analyst the opportunity of developing knowledge, attitudes and understanding on what constitutes de foundation of flow modulation.
GED is written in Turbo C++ language for personal computer environments and has been tested under MS-DOS 6.2, Windows 3.1 and Windows 98.
GED itself is composed by a 170-kbyte executable file and is accompanied by a reference manual recorded to a 40-kbyte text file.
Via menu selection, GED presents multiple graphical output of curves related to valve operating behaviour, provides tabular display of data sets and accepts input of valve performance data. Numerical outputs can be stored on floppy or hard disks and printed out.
The program is supported, to date (February 4, 2000), by 404 text files on 44 valve styles registering experimental data obtained from the open literature for a broad range of valve styles. Each data file occupies about 0,5 kbyte. Four text files (total of 40 kbytes) give information on the data source, valve features, and codification of both valve and source.
Two utility programs were written to display (40 kbytes) and to print out (40 kbytes) the content of all the text files.
The total storage area is of approximately 420 kbytes.
Besides the available experimental data, the user may supply additional information or create new files.
GED deals only with valves represented by their dissipative characteristics in the standardized form of a distinction coefficient
(4)
so that its values range
from zero to one for all kind of valve.
GED operates interactively and the user is prompted to all required inputs. The program begins by checking the video card adapter and the graphics driver. Then, the directory that contains the executable file is identified and kept as a reference from which other files will be read.
In the sequence, the validity of the user’s identification file (GED.USR) is checked out. If the file is not found or is invalid, the program is terminated. In the case of a positive result, a presentation screen is displayed, followed by a screen identifying the user, to finally the command screen to be shown.
GED terminates any time on both text or graphics mode when the combination ALT_X is typed.
The command screen (Fig 1) consists of three parts: the main line that describes the root leading to each one of the tasks possible to be performed; a communication window with secondary m enu tables, whose options are activated by pulldown menus; and a help line.
Fig. 1 Command screen
Main
menu line
GED is organized around seven chief options, as
seen in Fig 1:
(1) VALVE. Segment that deals with the various forms of entering the experimental data required to the generalized diagrams to be drawn.
(2) GENERALIZED DIAGRAMS. The launching point for drawing the three kinds of generalized diagrams: Operation (Fig. 2); Application (Fig. 3), and Limitation (Fig. 4).
When a specific diagram is chosen, GED plots the curves for the parameters that have been input. Up to ten curves can be specified. In case of being the first call of the option, curves for two reference limits are drawn. At the graphics stage the user may either return to the command screen or enter new parameters.
(3) COEFFICIENTS. Used for plotting the curve against aperture for either the distinction coefficient or the loss coefficient.
(4) DISPLAY. Designed for a tabular presentation of the co-ordinates of the generalized diagrams, distinction coefficient or loss coefficient.
The values for the loss coefficient are converted from the data input into the form of the distinction coefficient.
Fig. 2 Screen for operation diagram
Fig. 3 Screen for application diagram
Fig. 4 Screen for limitation diagram
(5) PRINT. Prints out matrices containing numerical information on the generalized diagrams or valve performance coefficients.
(6) WRITE. Similar to the PRINT option, records all the pertinent data to disk files. In addition, it is possible to specify a reference path through which the data will be recorded and a reference extension for the files.
(7) OPTIONS. Allows the graphics presentation to be set as colour or black-and-white output.
Nowadays a wide assortment of control devices is available, suggesting a high chance that an appropriate valve for each specific situation exists. However, valves need to be evaluated individually and compared according to the installed flow characteristic they offer.
The effectiveness of the control action performed by valves has its limit within a particular range of loss factors and it may be clearly established by making use of the generalized diagrams.
As a working tool, GED supports the conceptual description of the role played by valves as installed components and has much to add to the analysis, understanding and development of solutions in the design, operation and maintenance of valve controlled installations.
By its features, GED may be employed to simplify the synthesis/analysis activity of solving flow-control problems, to provide clear insight into the limits of controllability of installed valves and to avoid pitfalls in the process of decision making.
References
[1] Lauria, J. C. GED: A computer program to describe the operating behaviour of control valves. Proc. of Hydrosoft‘ 92, Valencia, Spain, pp.409-420, 1992.
[2] Lauria, J. C. A Procedure for Selecting Valves to Regulate Hydraulic Systems (in Portuguese). Master´s Thesis. University of São Paulo, 1986.
[3] Lauria, J. C. Fundamentals of valve operating behaviour in Gravity driven systems. Proc. of the 24th IAHR Congress, Madrid, Spain, 1991, pp. D-361-D-370.
[4] Lauria, J. C. Diagnosis of modulation capability and valve selection. Proc. of the BVAMA International Conference on Valves and Actuators for Fluid Control, Birmingham, United Kingdom, 1992, pp. 271-286.
[5] Lauria, J. C. Parametric Study of the Effective Flow Characteristic of Control Valves. Monograph. São Paulo, 1992.