DE102017009709A1 - Control valve for the linearization of working curves - Google Patents

Abstract

A characteristic for control valves is specified in which the resulting working characteristic curve is exactly linear. The corresponding function is:

Description

Preliminary remarks:

wobei k der Durchflussfaktor ist.In a flow obstacle, the relationship between the flow q and the differential pressure Ap is given by $$q=k*\sqrt{\mathrm{\Delta }p}$$

where k is the flow factor.

(Note: We limit ourselves here to the medium of water as a fluid, since gases or vapors or liquids with different densities do not change significantly in this task.

Especially for control valves, the name k _v has become common. $$q=k_v*\sqrt{\mathrm{\Delta }p}$$

The k _v value of a control valve must of course be changeable, that is the purpose of a control valve. In general, the relationship between a manipulated variable y and the k _v value is the following: $$k_v=f\left(y\right)*k_{vs}$$

k _vs is the maximum opening. The function f (y) is called the characteristic of the valve. Especially for linear valves: $$k_v=y*k_{vs}$$

There is no common abbreviation for the flow factor of general flow resistances. We arbitrarily use k _w for: $$q=k_w*\sqrt{\mathrm{\Delta }p}$$

In an example of a controlled system is shown. There are a control valve and another flow resistance (eg a heat exchanger) connected together. The flow factor of the heat exchanger is half as large as that of the valve. Since the flow condition must be the same everywhere (incompressible water), the ratio of differential pressure across heat exchanger and valve is 4 to 1.

In the series connection of two flow resistances (here: valve and heat exchanger), the resulting flow factor k results from a simple calculation to: $$k=\sqrt{\frac{{\mathrm{<figure-callout\; id="2"\; label="k\; v"\; filenames="DE102017009709A1\_0011.png"\; state="\{\{state\}\}">k\; </figure-callout>}}_v^{}*k_w^2}{\left({\mathrm{<figure-callout\; id="2"\; label="k\; v"\; filenames="DE102017009709A1\_0011.png"\; state="\{\{state\}\}">k\; </figure-callout>}}_v^{}+k_w^2\right)}}$$

In conjunction with the equations (1) and (4), this results in the in illustrated relationship between manipulated variable and flow for a linear valve.

Problem:

Striking and reason for the present investigation is the convex curvature of the curve. This curvature is dynamic dynamics extremely uncomfortable, since the variable slope of the working curve directly enters the gain of the control loop and thus changes the dynamics of the control.

State of the art:

As a remedy, the linear characteristic according to Eq. (3) by a characteristic curve with concave curvature. One hopes to be able to compensate for the curvature.

Although there are many (more precisely: infinitely many) concave curved curves, so far only the equal percentage characteristic has prevailed. $$f\left(y\right)=k_0*e^{y*\text{ln}\left(\frac{1}{k_0}\right)}$$

The characteristic is in , the result of compensation in shown.

As you can see, the result is not really satisfactory. The curves "do not fit together properly". In the lower area too much is compensated, in the upper one too little.

In addition there is a special problem of the equal percentage characteristic: It does not go through the zero point. In addition, it is far too shallow to allow a safe closing of the valve. Therefore, you bend the curve in the lowest range and steers it directly to the zero point. This leads to a completely abnormal behavior in the lowest range. In practice, regulation in this area is hardly possible by normal means.

Task:

The invention has for its object to provide a characteristic curve for a control valve, which allows complete compensation of the curvature.

Solution:

mit $$\boxed{k_0={\left(\frac{k_{vs}}{k_w}\right)}^2}$$

erfüllt diese Aufgabe.The characteristic $$\boxed{f\left(y\right)=\sqrt{\frac{y^2}{\left(\left(1-y^2\right)*k_0+1\right)}}}$$

With $$\boxed{k_0={\left(\frac{k_{vs}}{k_w}\right)}^2}$$

fulfills this task.

Of course, as with all other characteristics, the two constants k _vs for the maximum opening and k ₀ for the curvature must be adapted to the respective track.

In is the characteristic curve for the value k ₀ = 4, that is for the in illustrated case shown. shows the result of the compensation.

As you can see, the compensation with this characteristic is completely successful. The resulting working characteristic is linear everywhere!

Claims (3)

Control valve, characterized in that by suitable measures in the internal structure, for example by appropriate shaping of the cone, a characteristic gem. the equations (8) and (9) is generated. Control device for a control valve, characterized in that a relationship k _v = f (y) * k _vs between a reference variable y and the opening of the valve k _{v is} generated, which corresponds to the equations (8) and (9). Algorithm in a process control system for a control valve, characterized in that a relationship k _v = f (y) * k _vs between a reference variable y and the opening of the valve k _{v is} generated, which corresponds to the equations (8) and (9).

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