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Is the accuracy of a control valve determined primarily by the valve or positioner?

Is the accuracy of a control valve determined primarily by the valve or positioner

When regulating the accuracy of the adjustment is not allowed, many users are concerned about, are suspected of regulating the valve supporting the positioner is not an imported brand, or not the use of high-tech intelligent positioner. So, the precision of the control valve, as long as the matching on the intelligent positioner can be solved? Or by the valve other more aspects of the common to decide?

First of all, let’s learn the difference between the intelligent positioner and the general valve positioner, to facilitate future judgement and choice:

Valve positioner is the main accessory of the pneumatic actuator, it is used in conjunction with the pneumatic actuator, used to improve the positional accuracy of the valve, to overcome the friction of the valve stem and the influence of the media imbalance, so as to ensure that the valve in accordance with the signals from the regulator to achieve the correct positioning.

The use of a positioner is recommended in the following cases

A. When the medium pressure is high and the differential pressure is large;

B. When the calibre of the regulating valve is large (DN>100);

C. High temperature or low temperature adjustment valve;

D. The need to improve the action speed of the control valve;

E. When using standard signals and operating non-standard spring actuators (spring range other than 20-100KPa);

F. When used for split-range control;

G. When the valve is used to achieve reverse action (air-closed and air-open types are converted to each other);

H. When it is necessary to change the flow characteristic of the valve (the positioner cam can be changed);

I. No spring actuator or piston actuator, to achieve proportional action;

J. When the pneumatic actuator is operated by electric signal, it must be equipped with electric-pneumatic valve positioner.

Intelligent positioner control valve structure and ordinary control valves are the same, because of the attached intelligent valve positioner so that the control valve with intelligent functions, intelligent valve positioner and the main differences between the ordinary valve positioner are as follows:

1. The realization of the flow characteristics of the different ways

Intelligent positioner feedback part of the linear feedback, the required control valve flow characteristics are achieved in the setup loop.

Ordinary positioner feedback part is a different shape of the cam, by changing the shape of the cam to achieve the desired control valve flow characteristics.

2. Different input and output methods

Usually, intelligent valve positioner is intelligent electrical valve positioner. Compared with the general electrical valve positioner, the input signal of the intelligent electrical valve positioner is a standard 4-20mA or 1-5V electrical signal, which needs to be converted by analogue-to-digital as the input signal of the microprocessor.

While the general electrical valve positioner input signal is 4 ~ 20mA or l ~ 5V signal, but it does not require analogue-to-digital conversion, can be sent directly to the electromagnetic coil to generate electromagnetic force to achieve force balance.

Intelligent valve positioner output signal is a digital signal, it is usually sent to the piezoelectric valve group, through the switch of the piezoelectric valve group to regulate the air pressure sent to the control valve diaphragm head.

General electrical valve positioner output signal is amplified by the pneumatic amplifier gas signal.

3. The use of different control methods

Intelligent valve positioner and general computer control devices similar to the use of discrete control mode, therefore, in the sampling interval, the regulating valve opening does not change. During operation, the opening of the control valve shows a step change.

General valve positioner using continuous control, therefore, the entire control process, regulating the opening degree of change is continuous (in addition to the dead zone caused by the leap).

4. Different feedback signal detection and processing

Intelligent valve positioner valve feedback signal needs to be sent to the microprocessor after analogue-to-digital conversion processing, while the general valve positioner feedback signal directly as a feedback force (torque), does not require analogue-to-digital conversion to electrical signals.

Some smart valve positioner input signal using standard analog signals, in the same wire also transmits HART digital signals, composed of mixed-signal smart valve positioner, which does not belong to the fieldbus smart valve positioner, but still belongs to the smart valve positioner.

III. The control valve precision is not high and the reason for the short life of the analysis:

Then, the accuracy of the control valve control may also have what aspects of the decision? Next, we together again to analyse:

Reason 1:

Valve stem using packing seal, control valve in the working state, the need for non-stop back and forth proportional adjustment, packing is prone to a certain amount of friction resistance to the valve stem, will affect the control valve response lag and sensitivity, which ultimately leads to a decline in the precision of valve control. Endangered switch adjustment is also very easy to wear and tear of the packing consumption, resulting in leakage of short life. In addition, from the point of view of energy saving considerations, this is never allowed; and for high-temperature thermal oil conditions, but also can not be used, because the permeability of oil is about 50 times that of steam! High temperature hot oil will naturally leak out from the packing!

Solution:

Internal seal (valve flap and seat) in the 304 stainless steel on the basis of surfacing Stellite alloy, which is resistant to erosion, abrasion resistance of a special material, with precision machining process, enhanced valve flap valve seat erosion and abrasion resistance, life than ordinary valves longer than a number of times more.

Reason 2:

Steam is a high-temperature, high-pressure, high-flow rate of special fluids, especially prone to gas-liquid two-phase conversion, prone to flash evaporation, cavitation, cavitation, and this phenomenon is the biggest danger to the internal sealing surface caused by severe erosion wear.

Once the control valve reaches the set temperature, the valve opening will be very small, this frequent small opening operation, the amount of steam is also large and small, the steam flow rate is very large, can be as high as 400m/s, the degree of harm is also doubled, thus the valve sealing surface is also doubled the destruction of the valve valve and the valve seat sealing surface in a very short period of time, will produce internal leakage, which leads to a reduction in the accuracy of the control, the deviation increases. Deviation increases.

Generally most manufacturers of regulating valve sealing surface is the use of conical sealing, can not avoid this throttling when the serious cavitation, erosion and cavitation effect, which will soon lead to the valve closure can not completely cut off the medium, which in turn leads to the control accuracy is low.

Solution:

The use of curved surface shunt valve flap, completely cut off when the sealing surface of the plane seal. When the high-speed steam flows through the throttle port, the steam will flow along the tangential direction of the curved surface, rather than flowing directly through the sealing surface, which avoids repeated cavitation, erosion and cavitation on the sealing surface of the serious destructive effect, so that its sealing performance to achieve the best, you can really cut off the media completely, thus ensuring that the control valve control precision, and life is also several times longer than the same type of products. After reaching the set temperature, it will not produce the phenomenon of over-temperature, and its control accuracy can reach +0.5℃.

Reason 3:

The flow coefficient of the control valve, i.e. Kv value, is not calculated accurately, or even not selected according to the actual working condition parameters, which will also result in the inability to achieve accurate control, or even completely unusable. -Kv value calculation in the control valve selection is critical.

Solution:

Control valves in the pre-sale will have sales engineers, according to the site of the actual operating parameters and equipment process requirements, strict calculation of the Kv value, to help the user selection, and provide appropriate technical support to ensure the correct use.

Reason 4:

Instead of using equal percentage control, but the use of lower precision but low cost linear flow characteristics, and many manufacturers of adjustable ratio is only 30:1, which is not stable control of the direct cause of frequent jumps in the control process, which is a lot of heat transfer process can not be allowed.

Solution:

Equal percentage flow characteristics of the control valve spool, adjustable ratio of 50:1, can ensure the highest control accuracy, and smooth control of each heating process.

Comprehensive analysis of the above, affecting the control accuracy of the control valve, is determined by the valve several aspects of the joint decision, and whether the positioner needs to use intelligent positioner, can be selected according to the site-specific needs.

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