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Q: My background is in electronics, and I need to simulate the behavior of a valve in Simulink (MATLAB). Both the inlet and outlet pipe diameter to the valve are 2 inches. How do I select the diameter inside the valve? Should it be more or less than the size of inlet? Because I already have the model in MATLAB, I just need to choose the diameters of the duct inside the valve and the inlet and outlet ducts connecting the valve. My valve is a straightforward one run by a stepper motor. I don't know the exact name of it in mechanical terms, but I believe that the output from the motor (cross-sectional area) is fed to the valve as an input based on which valve will open or close. Is there any site where I can get some ready-made simulated valves that can be run in Simulink?
A: This question reflects today's culture of the "black box." This culture suggests that all it takes to gain answers to complex questions is to push some buttons on a box or to plug in a number, and the canned software will provide the answer. This is wrong! The question, "What port size is required if the pipe is 2 in?" is similar to asking, "What size shoe should I buy if my waste is 34?"
If process control in general or control valve simulation in particular were that simple, we would not need process control engineers! Gadgets like black boxes are only as good as the software inside, and if the programmer did not understand the problem, the software is useless. The rule of "garbage in, garbage out" will apply! In other words, if the instructions for a valve simulator require only to provide the inside diameter of the valve, that simulator model is useless.
This does not mean that there is anything wrong with Simulink. It is a commercial tool for modeling, simulating and analyzing multi-domain dynamic systems. It is used in the model-based simulation of the control of both simple processes, such as the thermostat control of a home, or as complex processes as the simulation of the automated transport vehicle (ATV) used in the international space station. The problem is not with the capabilities of this software; it is with the understanding of what information it needs to make the simulation meaningful.
In order to simulate the behavior of a control valve station, first the purpose of the simulation must be defined, and then both the characteristics (the "personality") of the valve and the nature of the process must be described. The process description includes both the nature of the installation and the properties of the flowing fluid, including its Reynolds number. As to the valve/actuator/positioner package itself, both its steady state and dynamic behavior should be described.
Its steady-state behavior (characteristics) describes the relationship between valve position and flow as a function of pressure drop (on the left of Figure 1). This inherent characteristic (linear, equal-percentage, hyperbolic, quick opening, square root, etc.) is affected by the variation in the system pressure drop, which determines the distortion coefficient (Dc on the right of Figure 1) and results in the actual characteristics of the valve.
The dynamic behavior of the valve station describes the relationship between the controller output signal (desired valve position) and the actual valve position, which is affected by the dead band and velocity limits of the installation. Naturally, these effects are reduced if the valve is provided with a positioner.
The dynamic behavior of a control valve may be represented by a time lag of first or second order, with a limited velocity in stem movement. The time lags are simulated by lag or lag-lead elements. Velocity limiting may be expressed by the differential equation below,
where x is the stem position, vL denotes the velocity limit, and xideal is the input position of an ideal, unconstrained valve. When the velocity exceeds the limiting value, the gain of the actuator decreases, and its phase lag increases.
The electrical engineer asking this question probably did not understand what I wrote here, but it should show that simulation is a complex and sophisticated field of engineering that requires process control knowledge, and if somebody suggests that all you need to simulate a valve is to plug in the port diameter, that person does not possess that knowledge. If you want to learn about valve simulation, read Chapter 8.11 in the 2nd volume of my handbook, the many ISA documents (such as 75.25) on valve simulation, or see my 2007 article on the subject at www.controlglobal.com/articles/2007/451.html.
A: The simulation you are using might not be adequate.
First your request does not say why the simulation is being done. That is, is it to learn flow dynamics, control dynamics or simply to estimate flows and pressure drops under a static situation? Each objective will require a different approach.
For the valve flow, see the ISA valve sizing program if the valve is to be used over a wide range. Flow relationships will vary with pressure drop and be linear versus stem position at very small openings; go into a region where the square root of pressure difference sets flow; and then may well go into a choked-flow regime where the flow is constant regardless of pressure changes. Also, the valve flow coefficient might not be linear with stem position.