JP3236407B2 - Flow measurement method for highly viscous fluid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring a flow rate of a highly viscous fluid such as a molten plastic.

[0002]

2. Description of the Related Art Generally, a flow rate Q of a highly viscous fluid is measured by measuring a pressure loss of a nozzle or a die provided in a flow path, that is, a differential pressure ΔP between an inlet side pressure and an outlet side pressure of the nozzle or the like. The calculation is performed by the formula.

Q = ０００３P / μK (1) where μ: average apparent viscosity of fluid K: resistance coefficient of flow channel

[0004]

However, the conventional measuring method described above has a problem that the measuring sensitivity coefficient λ is low, and the measuring error is liable to be affected by the nozzle and a considerable measuring error occurs.

That is, according to the above equation (1), the flow rate Q
Is inversely proportional to the average apparent viscosity of the fluid and is proportional to the differential pressure ΔP, but the apparent viscosity μ changes constantly depending on the molecular weight, composition, temperature, or shear rate of the fluid, and the fluctuation of the average apparent viscosity of the fluid. Is the largest noise. Also, the differential pressure ΔP, which is a directly measured amount, changes following the viscosity. Therefore, even if the flow rate Q is constant, the flow rate measurement system is regarded as having changed the flow rate, and a measurement error occurs.

Further, the conventional flow rate measuring method has a problem that there is no simple instrument calibration method, that is, a method of identifying the measurement sensitivity coefficient λ. The instrument calibration itself can be performed by measuring the flow rate Q and the differential pressure ΔP corresponding to the flow rate Q using another means, but it takes time and effort. In particular, as described above, since the apparent viscosity μ varies depending on the type of fluid and the shear rate, instrument calibration must be performed every time the operating conditions are changed. Therefore, the above flow rate measuring method is not always economical.

In view of the foregoing, the present invention provides a practical and accurate flow rate measuring method for a highly viscous fluid such as a molten plastic, that is, a high S / N ratio (Signal / Noise) and simple instrument calibration. It is an object of the present invention to obtain a flow measurement method capable of performing the measurement.

[0008]

According to a first aspect of the present invention, a rotational speed N of a positive displacement pump in which the volumetric efficiency of a flow rate changes due to a back pressure flow or a screw pump equivalent thereto, and an inlet pressure P _{1 of the} pump. And measuring the outlet side pressure P ₂ , and using these measured values, the flow rate Q of the fluid passing through the pump is calculated by the following equation.

Q = αN−β / μ · ΔP where α: theoretical discharge amount per rotation of the pump in the state of ΔP = 0 (cm ³ ) N: rotation number of the pump β: back pressure flow coefficient (cm ³⁾ Μ: average apparent viscosity of the fluid (kgf-s / cm ² ) ΔP = P ₂ -P ₁ (kgf / cm ² ) In the second invention, the parameter α in the flow rate arithmetic expression is set as It is characterized in that the parameter β / μ is calculated by the following equation, using a specification value at the time of designing the pump or an actually measured value under a predetermined condition such as ΔP = 0.

Β / μ = −1 / α · (△ Pa- △ Pb) /
(Na−Nb) where ΔPa: differential pressure between the pump outlet pressure and the inlet pressure when the pump rotational speed is Na ΔPb: differential pressure between the pump outlet pressure and the inlet pressure when the pump rotational speed is Nb.

[0011]

[Effect] If α, β / μ is known, it can be directly measured immediately N, ΔP
The flow rate Q can be easily measured by calculation from
Moreover, the measurement sensitivity coefficient is extremely large, and high-precision measurement is possible. Also, the parameter β / μ is set to −1 / α · (△
Since the calculation is performed according to Pa− △ Pb) / (Na−Nb), the pump pressure is slightly changed while the Q is kept constant, and the differential pressure between the pump inlet side pressure and the outlet side pressure is measured. The calibration of the instrument can be performed very easily only by performing the calibration.

[0012]

FIG. 1 is a block diagram showing a schematic configuration of a dual extruder to which the present invention is applied.
A gear pump 2, a filter 3, and a die 4 are sequentially arranged. In this type of apparatus, in order to stabilize the production amount or the product thickness over a long period of time, a change in the supply amount of the raw material supplied to the twin-screw kneading extruder 1, that is, a change in the characteristics of the feeder is detected and corrected. There is a need.

Therefore, the present invention is to precisely detect a change in the feeder supply amount by the differential pressure ΔP before and after the gear pump. That is, in the state of ΔP = 0, α is the theoretical discharge amount per rotation of the pump, N is the number of rotations of the pump, β is the back pressure flow coefficient of the pump, μ is the average apparent viscosity of the fluid, and P is the inlet pressure of the pump. ₁ , if the outlet pressure is P ₂ , the differential pressure is ΔP, and the pump flow is Q,
The characteristics of the gear pump can be expressed as follows: Q = αN−β / μ · (P ₂ −P ₁ ) = αN−β / μ · ΔP (2)

Therefore, if α, β / μ are known,
The flow rate can be directly measured from the measured quantities N and ΔP by calculation.

Incidentally, the characteristics of the above equation (2) can be represented by an equivalent model diagram as shown in FIG. According to this equivalent model diagram, the flow rate Q ₂ to which flow through the bypass is very small compared to the overall flow rate Q, also variation δQ of the flow rate Q is believed that all flows to the bypass side. That is, Q ₂ / Q is generally 1 to 5%, at most 1 to 15 (volume efficiency 99 to 85%). Then, as described above, since all the fluctuation amount δQ of the flow rate Q flows to the bypass side, the flow fluctuation rate δQ / Q ₂ of the bypass becomes a very large value. For example, if Q ₂ / Q = 2% (volume efficiency 98%) and δQ / Q = 2%, δQ / Q ₂ reaches 100%. Therefore, since the differential pressure amount δ (△ P) before and after the gear pump shows a large value due to the slight flow rate change δQ, very sensitive and lightweight measurement becomes possible according to the flow rate measuring method.

On the other hand, from the above equation (2), ΔP = β / μ · (αN−Q) (3) Therefore, assuming that the measurement sensitivity coefficient is λ,

[0017]

(Equation 1) Is obtained. Therefore, in order to express the characteristic equation of the gear pump as a function of the measurement sensitivity coefficient, λ = −
By substituting μ / β, Q = αN + 1 / λ · △ P (5) Therefore, in order to measure (calculate) Q from N and △ P, it is necessary to accurately identify the two unknowns α and λ.

Here, α is a constant specific to the gear pump, and the numerical value is large. Therefore, the numerical value in the specifications of the manufacturer may be set as it is. On the other hand, the value of the back pressure flow coefficient β is much smaller than α, and tends to vary depending on the gap between the gear and the casing. Since λ varies depending on the fluid viscosity, it must be accurately identified each time the operating conditions are changed.

From the above, in the present invention, Q
The pressure difference ΔPa, ΔPb corresponding to two different rotation speeds Na, Nb is measured while being kept constant. That is, Q = αNa + 1 / λ · △ Pa (6) Q = λNb + 1 / λ · △ Pb (7) When λ is obtained from the above equations (6) and (7), λ = −1 / λ · (△ Pa − △ Pb) / (Na−Nb) (8), and λ = −μ / β can be calculated by the above equation (8), and this λ can be used for the flow rate measurement. Therefore,
With Q constant, the gear pump speed is slightly changed to detect the differential pressure between the inlet and outlet pressures of the pump, and the instrument calibration can be easily performed simply by calculating λ from equation (8) based on those values. it can. FIG. 3 shows a flowchart of the instrument calibration.

Next, the experimental results regarding the extrusion molding of the amorphous polyester resin will be described. In this case, the same apparatus calibration example as shown in FIG. 1 was used. The amorphous polyester resin used had a melting specific gravity γm of 1.15, an apparent viscosity of about 1500 poise, and a theoretical discharge rate α of the gear pump of α = 297,328 cm ³ .

Therefore, the feeder supply amount (flow rate) Q = 12
At 8 kg / H, the rotational speed of the gear pump Na = 6.42r
pm, Nb = 6.52 rpm, and the difference ΔP before and after the corresponding gear pump was measured. Then, the measured values were ΔPa = 21.9 kgf / cm ² , ΔPb = 34.
It was 1 kgf / cm ² . These values were applied to equation (8) to obtain λ = −24.6 kgf-s / cm. The formula for calculating the flow rate Q was set as follows.

Q = 297.328N−ΔP / 24.6 (9) FIG. 4 shows simulation data under the same conditions. Therefore, the feeder supply amount was increased by 1% stepwise, and the flow rate was measured by the method according to the present invention. The results are as follows.

That is, the differential pressure before and after the gear pump was ΔPa = 17 kgf / cm ² when the flow rate was 128 kg / H, but ΔPb when the flow rate was increased to 129.28 kg / H (equilibrium state). = 9.5 kgf / cm ² .

FIGS. 5 and 6 show the simulation data and the actual test data. Then, as a result of applying ΔPb to the equation (9), Qb = 130.7 kg / H was obtained. Therefore, the measurement error was 0.6%.

[0025]

As described above, in the present invention, the flow rate of the highly viscous fluid flowing through the positive displacement pump is represented by Q = α
Since the calculation is performed by N-β / μ · ΔP, the measurement can be performed with high accuracy, and the instrument calibration can be easily performed.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of a combined extrusion device to which the present invention is applied.

FIG. 2 is an equivalent model diagram showing characteristics of a gear pump.

FIG. 3 is a flowchart of instrument calibration.

FIG. 4 is a view showing simulation data of instrument calibration.

FIG. 5 is a diagram showing simulation data for measuring a change in flow rate.

FIG. 6 is a view showing test data of the above.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-58-19511 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01F 1/00-9/02

Claims (2)

(57) [Claims] 1. A and the rotational speed N of the displacement pump or screw pump analogous to flow volumetric efficiency of the back pressure flow is changed, the pump inlet pressure P ₁ and the outlet pressure P ₂
And measuring the flow rate Q of the fluid passing through the pump according to the following equation using the measured values. Q = αN−β / μ · ΔP where α: Theoretical discharge amount per rotation of the pump in the state of ΔP = 0 (cm ³ ) N: Number of rotations of the pump β: Back pressure flow coefficient (cm ³ ) μ: Average apparent viscosity of fluid (kgf-s / cm ² ) ΔP = P ₂ -P ₁ (kgf / cm ² ) 2. The parameter α in the equation for calculating the flow rate is a specification value at the time of designing the pump or an actually measured value under a predetermined condition such as ΔP = 0, and the parameter β / μ is calculated by the following equation. 2. The method for measuring the flow rate of a highly viscous fluid according to claim 1, wherein: β / μ = -1 / α · (△ Pa- △ Pb) / (Na-N
b) However, ΔPa: Differential pressure between the pump outlet side pressure and the inlet side pressure at the time of the pump rotation speed Na ΔPb: Differential pressure between the pump outlet side pressure and the inlet side pressure at the time of the pump rotation speed Nb.