JPH0463239B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for measuring the flow rate of a pump, particularly a centrifugal pump.

Note that in this specification, the term "pump" includes a liquid pump and a blower.

(Conventional technology) Conventionally, the method of measuring the flow rate of a pump is as follows:
Normally, a method is used to measure the flow rate by installing a flow rate measuring device, such as a type that calculates the flow rate based on the pressure difference before and after an orifice, on the upstream or downstream side of the pump. However, due to structural limitations of the pipeline, etc., it may not be possible to install the flow rate measuring device as described above. If it is not possible to install such a flow rate measurement device, or if you do not want to install a flow rate measurement device, for example, a calibration test should be performed in advance to determine the relationship between the pump suction side pressure, the discharge side pressure, the pressure difference, and the flow rate. During actual pump operation, the pump suction side pressure and discharge side pressure are detected and the flow rate is calculated based on this pressure difference. .

(Problem to be solved by the invention) However, with the above method, if the pressure difference between the suction side pressure and the discharge side pressure of the pump changes slightly with respect to the flow rate, it is difficult to calculate the flow rate from the pressure difference. Become. That is, when the above-mentioned conventional relationship between the pressure difference and the flow rate is shown in the pump head curve in Fig. 2a, with the pressure difference between the suction side pressure and the discharge side pressure on the vertical axis, and the flow rate on the horizontal axis, the X curve In the region where the flow rate is small, the discharge pressure does not change much depending on the flow rate, and in extreme cases, as shown in the X curve in Figure 2b,
In some cases, the pressure is upward to the right. The reason for this is that in a centrifugal pump (vortex pump), the fluid exiting the impeller outlet generally has pressure energy and velocity energy, and the velocity energy is due to cross-sectional changes in the guide vanes, volutes, etc. This is converted into pressure energy, and the pump discharge port exhibits a higher pressure (static pressure) than the impeller outlet. However, in areas where the flow rate is low, vortices or collision losses occur in the flow path, and This is because the conversion of velocity energy into pressure energy is not sufficiently performed. Therefore, at the impeller outlet, the total water head, which is the sum of pressure head and velocity head, is smaller when the flow rate is low than when the flow rate is high. Although it is theoretically and experimentally high, the pressure (static pressure) at the pump discharge port does not change much in the region of low flow rate and slopes upward, similar to the curve above. There are also things.

Therefore, especially at small flow rates, calculating the flow rate (horizontal axis) based on the pressure difference (vertical axis), which does not change much as described above, may cause a large error, or depending on the pump characteristics, the same discharge pressure may be calculated. In practice, there are cases where it is impossible to measure small flow rates using the above method.

(Means for Solving the Problems) In order to eliminate the drawbacks of the prior art described above, the present invention provides a pressure difference-flow rate curve with a downward slope as pronounced as possible over a wide range from low flow rates to high flow rates. A plurality of pressure detection points having a pressure difference exhibiting a curve are found, and the flow rate is calculated from the pressure difference detected from these points. To this end, the present invention provides a first pressure detection means in the casing or conduit upstream of the inlet of the pump impeller, and detects the pressure near the outlet diameter of the impeller at a location other than the vicinity of the water cutter. A second pressure detection means for detecting the pressure through a hole provided in the casing in approximately the same direction as the rotation axis of the pump is provided, and a third vertical force detection means is provided in the casing of the pump discharge port to detect each pressure. The pressure is detected from each means, and the first
The flow rate is calculated based on the pressure difference between the pressure detected from the second pressure detection means and the pressure detected from the second pressure detection means (referred to as A means), such as a head curve, and the first pressure detection means. It is equipped with a means (referred to as means B) such as a head curve for calculating the flow rate based on the pressure difference between the detected pressure and the pressure detected from the third pressure detection means, and in the low flow rate range, the means A is used. The method is characterized in that the flow rate is calculated by the above-mentioned means B in the large flow rate range.

(Function) As described above, the present invention includes a first pressure detection means for detecting the pressure at the suction port of the pump, and a pressure detection means for detecting the pressure at a location other than the vicinity of the water cutter and near the outlet diameter of the impeller by rotating the impeller. The second pressure detection means detects the pressure through a hole provided in approximately the same direction as the shaft, and the third pressure detection means detects the pressure at the discharge port of the pump. The pressure detected by the detection means measures the pressure (static pressure) before velocity energy is converted into pressure energy, or in a state where only a small amount has been converted. It is not affected by losses in the channel and corresponds mainly to the pressure head applied to the water in the impeller, and theoretically this is highest at low flow rates.

In addition, in the actual flow inside the pump, loss also occurs at the impeller inlet and in the impeller, and in particular, in an impeller whose inlet diameter is close to the outlet diameter, the proportion of collision loss at the inlet is large. This is a phenomenon characteristic of large Ns impellers. However, in a typical centrifugal pump, the impeller inlet diameter is smaller than the outlet diameter, and therefore the collision loss at the impeller inlet is smaller than the overall energy. Furthermore, when the flow rate decreases, the collision loss at the blade inlet also increases, but the loss after the blade exit becomes even larger. In this way, even if the collision loss at the impeller inlet is large, by detecting the pressure in a place that is less affected by the loss after the impeller outlet, that is, near the impeller outlet, it is possible to detect You can get the gradient.

In fact, the pressure difference-flow rate curve showing the relationship between the difference in pressure detected by the first and second pressure detection means and the flow rate is always a curve with a downward slope to the right over a wide range from low flow rate to high flow rate. This has also been experimentally confirmed.

On the other hand, a pressure difference-flow rate curve (head curve) showing the relationship between the difference in pressure (static pressure) detected from the first pressure detection means and the third pressure detection means and the flow rate.
does not change much during the flow rate because, as mentioned above, there is a loss in the conversion of velocity energy into pressure energy or pressure energy in the flow path of the guide vanes and the head of the volute chamber, and sometimes it shows an upward slope to the right. be. However, as the flow rate increases, the lift curve based on the pressure difference detected by the first and second pressure detection means begins to exhibit a further downward slope to the right.

During actual pump operation, the pressure or pressure difference is detected from each of the pressure detection means, and the flow rate calculated based on these pressure differences is in a low flow rate range, for example, on the lower flow rate side than a predetermined flow rate. when,
The flow rate is calculated based on the pressure difference between the pressure detected by the second pressure detection means near the pump impeller outlet, and when the flow rate is in a large flow range, for example, on the large flow side than the predetermined flow rate, the flow rate is calculated from the pressure difference between the pressure detected by the second pressure detection means near the pump impeller outlet. The flow rate is calculated based on the pressure difference between the pressure detected by the third pressure detection means and the pressure detected by the third pressure detection means.

(Example) Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a longitudinal cross-sectional view of a centrifugal pump used to carry out the flow rate measurement method of the present invention, in which a casing body 1 has a suction port 2 and a discharge port 3,
The point that the impeller 4 is supported by the rotating shaft 5 inside is the same as the conventional one.

In this embodiment, the first pressure outlet hole 10 is located at a position close to the inlet 2 of the casing on the upstream side of the pump impeller inlet 4a, and the casing main body 1 is located near the outlet diameter corresponding to the outlet 4b of the pump impeller 4.
A second pressure outlet hole 11 is provided in the casing body 1 near the discharge port 3 of the pump, and a third pressure outlet hole 1 is provided in the casing body 1 near the discharge port 3 of the pump.
2 are provided respectively.

The second pressure extraction hole 11 in this pump
The relationship between the pressure difference between the pressure near the impeller outlet and the suction port pressure from the first pressure extraction hole 10 and the flow rate can be determined as follows. That is, the flow rate of the pump is controlled by a flow rate control valve 26 provided on the pump discharge side, as shown in the experimental equipment of FIG. 28. In addition, the pressure difference between the pump suction side pressure and the pressure near the impeller outlet corresponding to each of these flow rates is determined by the difference in pressure detected from the first pressure extraction hole 10 and the second pressure extraction hole 11, respectively. , determined by calculation or by a differential pressure gauge. Such an operation is repeated a plurality of times (n times) while changing the flow rate set value, and a table is created based on each data, or it is plotted as a head curve. Furthermore, the above operations can be converted electrically and processed by a microcomputer.

When the relationship between the pressure difference and the flow rate determined as described above is displayed using the pressure difference-flow rate curves shown in FIGS. 2a and 2b, a Y curve is obtained. As can be seen from this curve Y, the relationship between the pressure difference between the suction pressure detected from the first and second pressure extraction holes 10 and 11 and the pressure near the impeller outlet and the flow rate is as follows.
It is expressed as a curve with a downward slope to the right over a wide range from small flow rates to large flow rates.

Similarly, when the relationship between the pressure difference between the pressure at the pump discharge port from the third pressure take-off hole 12 and the pressure at the suction port from the first pressure take-off hole 10 and the flow rate is expressed by a lift curve, the X curve is can get. This curve X is the same as that obtained in the calibration test of the conventional pump described above, and in the range, the pressure difference does not change much depending on the flow rate (Fig. 2a), and sometimes shows an upward slope to the right. (Figure 2b). However, in the high flow area, it exhibits a markedly downward slope to the right.

As mentioned above, since the Y curve has a downward slope to the right over a wide flow rate range, the flow rate can be uniquely calculated by measuring the pressure difference in any flow rate range using the Y curve. However, the greater the change in differential pressure due to flow rate, the greater the slope of the side curve, the smaller the error when calculating flow rate from differential pressure, and the higher the accuracy of the calculated flow rate. Therefore, in this embodiment (the present invention), in order to calculate the accuracy of the flow rate over the entire flow range, a Y curve with a large slope is used on the low flow rate side, and an X curve with a large slope on the high flow rate side is used. The flow rate is calculated from the differential pressure using the following.

Figure 3 illustrates the flow rate measurement range based on both the X and Y curves in the above case, and shows the flow rate measured at a predetermined flow rate x ₀ (at this time, the difference between the pressure near the impeller outlet and the suction pressure is y In the low flow area to the left of _x0 , the flow rate is calculated using the Y curve, and in the large area to the right of _x0 , the flow rate is calculated using the X curve.

Note that methods for calculating the flow rate based on the pressure difference include a method of directly determining the flow rate from the head curve, or a method of using the table, reading the flow rate from the pressure difference, and correcting it by calculation. Also,
These operations can also be converted electrically and processed by a microcomputer.

An example of an experimental flow sheet for determining the flow rate based on the pressure difference from the second pressure take-off hole using the above-mentioned microcomputer will be explained with reference to FIG. ₇ .
The internal pressure P ₂ is extracted as an electrical signal by the pressure sensors 22 and 23, the current is converted to voltage (I/V conversion), and the signal is read by the pressure indicator needles 24 and 25, and the multiplexer 30 of the microcomputer C Enter. On the other hand, the discharge amount of the pump is
It is controlled by a flow rate control valve 26 connected to an opening setting device 27, and the flow rate at that time is detected by a flow meter 28.
The signal is input to the multiplexer 30 via the flow rate indicator 29, and the relationship between the pressure difference and the flow rate is stored and processed in the computer. Next, when calculating the flow rate during operation, the pressures detected from both pressure sensors 22 and 23 are input, the pressure difference is calculated, and the corresponding flow rate is calculated from the table stored as described above. is displayed on the display.

The fact that the pressure difference-flow rate curve detected from both the first and second pressure extraction holes exhibits a downward slope to the right over a wide range as described above is confirmed by the following experimental results. There is.

FIG. 4 is a longitudinal sectional view of the centrifugal pump used in the above experiment, and FIG. 5 is a sectional view of the casing section taken along line V-V in FIG. Codes indicate similar parts.

In the figure, A is the pressure extraction hole provided in the outer peripheral wall (volume chamber) of the casing 1 at the position V-V in FIG. 4, or B to F are the pressure extraction holes provided in the casing side wall, Especially B
is provided near the outlet diameter position of the impeller 4. These extraction holes are distributed radially with respect to the rotation axis as shown in FIG.

FIG. 6 is a performance curve diagram showing experimental results when the pumps shown in FIGS. 4 and 5 were operated. As can be seen from the figure, pressure curves A- to A- taken from the outer peripheral wall of the casing (volume chamber)
It can be seen that B- and B- taken out from the side wall near the impeller outlet have a more pronounced downward-sloping curve close to a straight line.

Furthermore, since the impeller has a point-symmetric shape,
Originally, the pressure at the outlet should be equal, but
Since there is an asymmetrical spiral chamber around the outer periphery of the impeller, the actual pressure is influenced by this and becomes asymmetrical, and this effect is particularly strong near the water cutout (tongue) where the cross-sectional shape of the spiral chamber suddenly changes. Avoid this draining part because the pressure may behave differently from other parts and may not slope downward to the right.

In the above embodiment, a case was described in which the pressure difference was determined from the pressure detected from each pressure outlet hole and the flow rate was calculated based on this pressure difference. The pressure difference from both pressure extraction holes is directly detected using a differential pressure gauge, etc., which introduces a vertical force fluid into each and detects the pressure difference by the displacement of the partition wall, and the flow rate is determined based on this pressure difference. Of course, it is also possible to calculate
Moreover, instead of selecting the X curve and the Y curve depending on the magnitude of the flow rate as in the above embodiment, the pressure difference obtained from the pressure detected from both the first and second pressure outlet holes can be used. , or Y if the pressure difference directly determined by the differential pressure gauge is larger than the predetermined pressure difference, that is, the range of pressure difference y ₀ in Figure 3.
It is also possible to calculate the flow rate using a curve, and when the pressure difference is smaller than the above range, to calculate the flow rate using an X curve.

Note that the present invention can be applied not only to centrifugal liquid pumps but also to centrifugal blowers having similar characteristics.

(Effects of the Invention) As explained above, according to the present invention, the first pressure detection means for detecting the suction pressure and the pressure in the vicinity of the outlet diameter of the impeller at a location excluding the vicinity of the water cutter and the pressure in the vicinity of the outlet diameter of the impeller are detected. A second pressure detection means detects the discharge pressure through a hole provided in substantially the same direction as the rotation axis of the The flow rate can be calculated from the pressure difference between the pressure detection means and the pressure difference between the first pressure detection means and the third pressure detection means, and on the small water flow side, the first pressure detection means and the second pressure detection means The flow rate is calculated from the pressure difference between the means, and on the large water flow side, the flow rate is calculated from the pressure difference between the first pressure detection means and the third pressure detection means, so accuracy is high over a wide flow range. Furthermore, regardless of the head curve of the pump, the flow rate can be calculated without using a conventional flow rate measuring device.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-sectional view of a centrifugal pump used to implement the method for measuring the flow rate of a pump according to the present invention, and FIG.
Figure 3 and Figure 2b are curve diagrams showing the relationship (X) between the difference between the suction pressure and discharge pressure and the flow rate, and the relationship (Y) between the difference between the pressure near the impeller outlet and the suction pressure and the flow rate, respectively. is a diagram showing the flow rate measurement range by both curves X and Y in Figure 2a, and Figure 4 is a diagram showing the pressure difference due to both the first (suction port) and second (impeller outlet) pressure extraction holes related to the present invention. A vertical cross-sectional view of the centrifugal pump used to experimentally determine the relationship with flow rate, Figure 5 is a cross-sectional view taken along line V-V in Figure 4, and Figure 6 is the head curve of the pump showing the experimental results. Figures 7 and 8 show experimental flow sheets and flowcharts using a microcomputer. 1... Pump casing, 4... Impeller, 4a
... Impeller outlet, 4b... Impeller inlet, 10...
1st pressure extraction hole, 11...second pressure extraction hole, 12...3rd pressure extraction hole.

Claims (1)

[Claims] 1. In a method for measuring the flow rate of a pump, the first pressure detection means detects the pressure at the suction port of the pump, and the pressure at a location other than the vicinity of the water cutter and near the outlet diameter of the impeller is detected from the rotation axis of the impeller. A second pressure detection means for detecting the pressure through a hole provided in the casing in substantially the same direction as the first pressure detection means and a third pressure detection means for detecting the pressure at the discharge port of the pump are respectively provided. means (referred to as means A) for calculating the flow rate based on the pressure difference between the pressure detected by the second pressure detection means and the pressure detected by the second pressure detection means;
Means for calculating the flow rate based on the pressure difference between the pressure detected by the first pressure detection means and the pressure detected by the third pressure detection means (referred to as means B).
A method for measuring flow rate, characterized in that in a low flow rate range, the flow rate is calculated by means A, and in a high flow rate range, the flow rate is calculated by means B.