JPS5858612B2 - pressure detection device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pressure detection device, and particularly to a pressure detection device suitable for use in controlling a process with a short time constant.

When measuring the pressure of fluid in a pipe, the detector is rarely directly attached to the point of the main pipe to be measured, but is often connected through an instrumentation pipe.

This is because the environment of the place to be measured, such as temperature, humidity, vibration, etc., is often not suitable for installing the detector, or the conditions are unfavorable for maintenance and inspection. , a certain length of instrumentation piping is required.

However, instrumentation piping itself tends to generate pressure vibrations with its own unique frequency.

The reason for this will be explained next. In FIG. 1, 1 is a main pipe, 2 is an instrumentation pipe, 3 is a pressure detector, and 4 is an output signal.

The pressure within the main pipe 1 is guided to the pressure detector 3 via the instrumentation pipe 2 and converted into an output signal 4, but pressure oscillations occur within the instrumentation pipe 2 as follows.

Figure 2 shows how the pressure distribution inside the instrumentation piping changes over time. The horizontal axis shows the coordinates of the position in the instrumentation piping from the main piping to the detector, and the vertical axis shows the pressure. Each graph represents the pressure distribution within the instrumentation piping at a certain time.

As time passes, the pressure distribution changes in the direction indicated by the arrow as shown in the lower figure.

That is, the pressure inside the main pipe, which was in a steady state, is at time t.

If the odor changes in a step-like manner, a pressure wave corresponding to the width of the step advances in the instrumentation piping from the main piping side toward the detector side as shown by the arrow.

When the pressure wave reaches the detector side at time t1, the pressure inside the instrumentation piping becomes constant, which is equal to the pressure on the main piping side.
Since the instrumentation piping on the detector side is closed, the pressure wave is reflected as is (the pressure wave with a height of +1 [in the direction of pressure increase] is +1) and is reflected from the detector side to the main piping side. Proceed in the direction of the arrow at time t2.

Then, at time t3, when the pressure wave reaches the main piping side, the instrumentation piping is considered to be an open end, so its polarity is reversed (the pressure wave with a height of +1 becomes a pressure wave with a height of -1) and is reflected. Then, it moves toward the detector again (time 1+).

Thereafter, the pressure wave repeats reciprocation and returns to the original state after two reciprocations, but the amplitude of the pressure wave attenuates due to loss etc. as it travels, and eventually the vibration component disappears and the pressure wave inside the main pipe Match the pressure.

FIG. 3 is a diagram showing the time response when this situation is observed on the detector side.

When the pressure within the main pipe changes in a stepwise manner, damped oscillations having a unique frequency component occur in the pressure signal due to the above-mentioned reasons.

In this way, the output signal of the detector has a specific natural vibration component superimposed on the original signal due to the influence of the instrumentation piping, so problems occur when controlling using this signal. .

In other words, when looking at the frequency characteristics of the detector including the instrumentation piping, the gain at the natural vibration frequency is high, so there is a possibility that sustained vibration will occur when the closed loop gain of the control system is increased. .

Therefore, the gain may have to be lowered even though the controllability of the control system deteriorates.

Therefore, in order to obtain good control characteristics, it is necessary to eliminate the influence of the above-mentioned natural vibration.

There are several ways of thinking about how to remove the effects of natural vibration.

One is to eliminate the influence of instrumentation piping by some method, and the other is to eliminate the influence by processing the output signal of the detector.

One idea for the former is to put a restriction in the instrumentation piping, but there is a possibility that there will be a delay if you try to put in an effective one.

Another method is to fill the inside of the instrumentation piping with a high-density liquid such as water.

In general, the pressure wave propagation velocity in liquids is much higher than that in gases unless it deviates significantly from atmospheric pressure and room temperature (pressure wave propagation velocity in water at atmospheric pressure and room temperature: 15
00m/s1 Pressure wave propagation speed in air: 340r'r1
/s).

Therefore, if the inside of the instrumentation piping is filled with a high-density fluid such as liquid, the effects of natural vibration can be practically ignored.

However, with the exception of special cases, high-density fluids (which should be filled into instrumentation piping) that are physically and chemically stable under the environmental conditions such as the temperature and pressure of the fluid in the main piping. ,
There are few suitable ones.

There is also a method of partitioning the main piping and instrumentation piping using a diaphragm or the like and filling the inside of the instrumentation piping with a high-density fluid, but this method is not common due to construction and maintenance considerations.

On the other hand, the latter method uses an electrical filter.

This is a method that uses a band rejection filter that reduces the gain of the natural vibration frequency and adjusts the overall frequency characteristics of the detector and filter, including the instrumentation piping, to be flat.

However, this method requires adjustment of filter parameters when fluid conditions change.

High-frequency response of the control system becomes a problem when the plant is started or stopped, and the fluid conditions change significantly at that time, so parameters need to be adjusted, but even when operators are at their busiest, Therefore, a mechanism is needed to automatically adjust parameters in response to changes in plant conditions.

Therefore, the device may become complicated. As described above, it is not easy to eliminate the influence of the natural vibration of the pressure waves generated in the instrumentation piping, and the conventional technology requires advanced technology, resulting in an increase in cost.

An object of the present invention is to provide a pressure detection device that has a simple configuration and can eliminate the effects of natural vibration of pressure waves in instrumentation piping.

The present invention focuses on the mechanism by which natural oscillations of pressure waves occur, and uses two pressure detection means to make the phases of the pressure waves generated in each instrumentation tube opposite to each other, and to synthesize them. This is used as a pressure signal.

Therefore, the influence of natural vibration can be removed without requiring any special additional equipment.

FIG. 4 shows one embodiment.

In the figure, 21.22 is an instrumentation pipe, 31.32 is a pressure detector, 4L42 is an output signal, 5 is an averager, and 6 is a pressure signal.

In this system, the pressure of the fluid in the main pipe 1 is converted into output signals 41 and 42 by a pressure detector 31 or 32 via an instrumentation pipe 21 or 22, respectively.

The respective output signals are then added together in an averager 5, and the averaged signal is output as a pressure signal 6.

It is desirable that the instrumentation pipes 21 and 22 and the pressure detectors 31 and 32 have the same structure and be placed in approximately the same environment.

Also, how to determine the interval between connecting the instrumentation piping to the main piping will be explained next.

The pressure wave propagation velocity C in the fluid in the pipe is expressed by the following equation.

where ρ is the density and ω is the angular frequency of the natural vibration of the pressure wave in the instrumentation piping.

is expressed by the following equation. However, CI is the pressure wave propagation velocity lI in the instrumentation piping, and the pressure in the main piping changes as pressure changes propagate upstream or downstream.

Therefore, for example, if a pressure wave is traveling inside the main piping from the side of the instrumentation piping 21 (in the direction of the arrow in the figure), the effect of pressure changes on the instrumentation piping 22 will be due to the The distance IM between the pipes 21 and 22 lags behind the instrumentation pipe 21 by the time the pressure wave travels.

Therefore, it is a good idea to choose the distance between the instrumentation pipes 21 and 22 equal to the distance that the pressure wave travels in the main pipe during a time corresponding to half a period of the natural vibration of the pressure wave in the instrumentation pipe. Recognize.

Therefore, we obtain the following equation. However, the subscript ■ is a constant for the instrumentation piping, and the subscript M is a constant for the main piping. The pressure oscillations are in opposite phase, and FIG. 5 shows the state of the detector output signal at that time.

When a pressure wave advances from the side of the instrumentation pipe 21, the inlet pressure of the instrumentation pipe 21 changes.

The effect is that the pressure wave reaches the detector 31 after a delay of the propagation time in the instrumentation piping, that is, a time corresponding to the period of the natural vibration, and causes vibration, so the output signal 41 has a response as shown in the figure. Become.

In addition, the pressure wave in the main pipe reaches the connected part of the instrumentation pipe 22 after a delay of the time it takes to travel the distance expressed by equation (3), that is, a time corresponding to a half period of the natural vibration. Since the signal reaches the detector 32 with a delay of a time corresponding to the cycle, the population pressure of the instrumentation pipe 22 and the output signal 42 have the responses shown in the figure.

As a result, the output signals 41 and 42 have a time difference corresponding to a half period of the natural vibration, and therefore have opposite phases, and by adding them, it is possible to remove the influence of the natural vibration of the pressure wave as in the pressure signal shown in the figure.

Note that although the above explanation has been made on the assumption that the pressure waves are traveling from the instrumentation piping 21 side, the same thing is true even if the pressure waves are traveling from the opposite direction.

Additionally, since instrumentation piping is usually wrapped in heat insulating material or other measures to prevent the temperature from differing greatly from that of the main piping, the value of equation (3) will vary greatly even if the operating conditions change. There's not much to do.

ρ and k are so-called physical property values, which change when the temperature changes even if the pressure is constant.

However, since the temperature of the instrumentation piping can be kept constant by wrapping the heat insulating material, the value of equation (3) does not change significantly depending on the operating conditions.

Therefore, the effect does not change even when operating conditions change.

Note that (Equation 3 assumes that a condensable gas such as water vapor exists in the main piping and a water vapor condensate exists in the instrumentation piping.

On the other hand, if the fluid in the main piping is water vapor, drain may accumulate on the detector side of the instrumentation piping.

FIG. 6 is a diagram explaining this case, and shows that there are 7 lumps of condensed water in the instrumentation pipes 21 and 22, and the length of the instrumentation pipes is JI.
Assume that the length of the portion that remains as water vapor is l'.

In such a case, the pressure wave propagation velocity in the instrumentation piping is
The water vapor part and the water part are different.

However, the propagation speed of the water portion is considerably faster than that of the water vapor portion.

Therefore, it is necessary to use an effective length in consideration of the difference in propagation speed in the term of the length ll of the instrumentation piping in equation (3).

That is, instead of lI in equation (3), CII is the pressure wave propagation velocity of the water vapor portion, CI2
is the propagation velocity of the water part.

It is sufficient to use lr obtained by .

The above equation ignores the effect of reflection of pressure waves at the interface between water vapor and condensate in the instrumentation piping, since this effect is actually small.

The reason for this is explained below.

In instrumentation piping, pressure waves are reflected at the gas-liquid interface, and 1
Vibration waves are generated in each region of length IrlI' and (lI-l').

The gas-liquid interface in the instrumentation piping is formed by the phase change of the same substance, and the interface is determined by the balance between the condensation of water vapor and the evaporation of condensed water.

This interface is not uniquely determined like the interface between water and oil, but changes as either condensation or evaporation becomes dominant depending on pressure changes within the instrumentation piping.

Therefore, at the interface within the instrumentation piping, the transmitted component of the pressure wave is sufficiently thicker than the reflected component.

Therefore, the influence of pressure wave reflection at the interface can be ignored.

In addition, the opening reflection (when the pressure wave returns) at the junction of the instrumentation pipe with the main pipe is reflected when the main pipe diameter is sufficiently larger than the instrumentation pipe diameter, and is reflected at the interface. Much larger than reflection.

In a normal instrumentation piping, the gas phase part is sufficiently shorter than the liquid phase part, so the period of the pressure wave at the length l is the longest and the amplitude is also large.

Further, in this case, it is also possible to use only one detector.

That is, instead of averaging the two signals after converting them into electrical signals with a detector, it is conceivable to average them at a stage before converting them into electrical signals.

An example in this case is shown in FIG.

In FIG. 7, 7 is condensed water, 8 is an orifice, and the others are the same as before.

As shown in the figure, the instrumentation pipes 21, 22 are connected immediately before being connected to the detector 3, and an orifice 8 is provided in each instrumentation pipe immediately before the connection point.

This orifice is provided to prevent the flow of condensed water 7 and to guide only the pressure signal.

As described above, in this case, the same effect can be obtained by using only one detector, so it is possible to reduce costs.

According to the present invention, the natural vibration of pressure waves generated in the instrumentation piping can be easily removed despite the simple configuration using only two commonly used pressure detectors.

Furthermore, it is effective as is even when operating conditions change, and has the effect of eliminating natural vibrations of pressure waves under a wide range of operating conditions with a simple configuration.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram illustrating an example of the installation of a normal pressure detector;
Fig. 2 is a diagram explaining the generation of natural oscillations of pressure waves in the instrumentation piping, Fig. 3 is a response diagram of the detector output signal to a step change in pressure in the main piping, and Fig. 4 is a diagram of the configuration of the present invention. A schematic explanatory diagram, FIG. 5 is a response diagram of each part of the apparatus according to the present invention, and FIGS. 6 and 7 are other embodiments of the present invention. Explanation of symbols, 1... Main piping, 21, 22...
... Instrumentation piping, 31, 32 ... Pressure detector,
5... Averager.

Claims (1)

[Claims] 1. Two instrumentation pipes of equal length are connected to a main pipe through which fluid flows at a predetermined interval in the axial direction of the main pipe, and a pressure detection means is coupled to each of the instrumentation pipes, In a pressure detection device configured to detect the pressure of the main piping, a means for combining and outputting signals obtained from the two pressure detection means is provided, and an interval between the two instrumentation pipings connected to the main piping is provided. are arranged so that the phases of pressure vibration waves in the two instrumentation pipes differ by 180°.