Fluid handling – Processes – Involving pressure control
Reexamination Certificate
2000-07-18
2001-10-16
Chambers, A. Michael (Department: 3753)
Fluid handling
Processes
Involving pressure control
C137S486000, C137S487500, C137S557000, C340S611000
Reexamination Certificate
active
06302130
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to pressure-type flow rate controllers for a variety of fluids including gases, for use in the manufacture of semiconductors, chemicals, precision machine parts, and the like. More specifically, this invention relates to a method and apparatus for the detection of the clogging of the orifice in the flow rate controller.
2. Description of the Prior Art
In the past, mass flow rate controllers have been widely used for control with precision of the flow in fluid feeding systems for facilities for manufacturing semiconductors and chemicals.
However, the mass flow rate controller has presented various problems including: (1) relatively slow response in the case of the heat type flow rate sensor, (2) poor control precision in low flow rate region and lack of product-to-product precision uniformity, (3) frequent occurrence of operating troubles and low operation reliability, and (4) high price and expensive replacement parts, which means high running costs.
Seeking a solution to those problems, the inventors of the present invention conducted extensive research and succeeded in developing a pressure-type flow rate control system using an orifice as disclosed in Japanese patent application laid open unexamined under 08-338546.
The pressure-type flow rate control system works on this principle: It has been known that the flow velocity of a gas passing through a nozzle reaches the sonic velocity if the ratio of the gas pressure upstream of the nozzle to the pressure downstream of the same—P
2
/P
1
where P
1
=pressure on the upstream side and P
2
=pressure on the downstream side—falls below the critical pressure of the gas (in the case of air, nitrogen, etc., about 0.5). In such a state, a change in pressure on the downstream side of the nozzle is no longer propagated to the upstream side. The flow rate of the gas passing through the orifice will be proportional with the pressure P
1
on the upstream side of the orifice. As a result, it will be possible to obtain a stabilized mass flow rate corresponding to the state on the upstream side of the nozzle.
In other words, in the event that the orifice diameter is fixed, if the upstream pressure P
1
is held twice or more higher than the downstream pressure P
2
, then the downstream flow rate Q
C
of the gas passing through the orifice will be dependent on the upstream pressure P
1
only, with a linearity given by the equation Q
C
=KP
1
(K=constant) being established to a high degree of precision. And, if the orifice is the same, so is the constant K.
The construction of this pressure-type flow rate control system will now be described with reference to FIG.
12
.
The flow passage
4
upstream of the orifice
2
is connected to a control valve CV that is operated by a drive
8
. The downstream flow passage
6
is led to a fluid reactor (not shown) via a gas take-out joint
12
.
The pressure P
1
on the upstream side of the orifice
2
is detected by a pressure detector
14
, and then sent to an amplifier circuit
16
and displayed on a pressure indicator
22
. The output is then digitized by an A/D converter
18
and referred to a calculation circuit
20
where the flow rate Q on the downstream side of the orifice
2
is worked out with the equation Q=KP
1
(K=constant).
Meanwhile, the upstream temperature T
1
is detected by a temperature detector
24
and output to a temperature revision circuit
30
via an amplifier circuit
26
and an A/D converter
28
. There, the flow rate Q is revised for the temperature, and the calculated flow rate Q
C
is output to a comparison circuit
36
. The calculation circuit
20
, the temperature revision circuit
30
, and the comparison circuit
36
form a calculation control circuit
38
.
A flow rate setting circuit
32
outputs a set flow Q
S
to the comparison circuit
36
through an A/D converter
34
. The comparison circuit
36
works out a signal difference Q
Y
between the calculated flow rate Q
C
and the set flow Q
S
with the equation Q
Y
=Q
C
−Q
S
and outputs that signal difference Q
Y
to the drive
8
through an amplifier circuit
40
. The drive
8
regulates the control valve CV in such a way as to bring the difference signal Q
Y
to zero, that is, to equalize the downstream flow rate Q with the set flow Q
S
.
While the pressure-type flow rate control system has the advantage of controlling the downstream flow rate with precision by detecting the upstream pressure P
1
only, the drawback is that the tiny orifice tends to clog. The orifice is a hole of the order of microns and dust sometimes clogs the orifice hole, rendering the flow rate uncontrollable.
The piping for a fluid to pass through must be well clean. But cuttings, dust, or the like sometimes remain in the piping and clog the orifice. The clogging of the orifice could cause the flow rate control to fail and make the operation of the whole plant unreliable, turning out large quantities of faulty finished products. Furthermore, some gases could become involved in runaway reactions and trigger explosions. Placing a gasket filter in the piping was tried as a solution to the problem, but that did not work because it could have adverse effects on the conductance of the piping.
FIG. 13
shows flow rate characteristics exhibited when the orifice is clogged in comparison with the flow rate characteristics shown by an orifice after it is purged. Flow rate characteristics shown by the orifice after purging constitute normal performance that can be expected of a clean orifice. In
FIG. 13
, if the set value is 100 percent, for example, the N
2
gas flows at the rate of 563.1 SCCM as indicated by the line with circular marks. The subsequent reaction systems are all designed on the basis of the expected flow rate characteristics of the orifice with no clogging. In the example in
FIG. 13
, the flow rate given by the clogged orifice decreases to 485 SCCM as indicated by the line with box marks, and the designed reaction can no longer be expected. It is noted that SCCM is the unit of gas flow rate—cm
3
/minute—under the standard conditions.
As shown, clogging of the orifice can make the flow rate lower than the set value. In semiconductor manufacturing and chemical plants, overfeeding or underfeeding of gases as a starting material could trigger an explosion or result in large quantities of faulty finished products. For this reason, how to detect orifice clogging in those gas-using plants has been a major concern.
SUMMARY OF THE INVENTION
The present invention solves that problem. The present invention as defined in claim
1
provides a method of detecting clogging of an orifice in a pressure-type flow rate controller which has a control valve, an orifice, a pressure detector for measuring the upstream pressure P
1
therebetween, and a flow rate setting circuit, wherein with the upstream pressure P
1
maintained at about two or more times higher than the downstream pressure P
2
, the downstream flow rate Q
C
is calculated with the equation Q
C
=KP
1
(K=constant), and wherein the control valve (CV) is controlled by the difference signal Q
Y
between the calculated flow rate Q
C
and the set flow rate Q
S
, the method comprising: a first step of holding the set flow rate Q
S
at the high set flow rate Q
SH
; a second step of switching from the high set flow rate Q
SH
to the low set flow rate Q
SL
and determining the upstream pressure P
1
to obtain the pressure attenuation data P(t); a third step of checking said the pressure attenuation data P(t) against standard pressure attenuation data Y(t) measured under the same conditions but with the orifice not clogged; and a fourth step of turning on a clogging alarm when the pressure attenuation data P(t) deviates from standard pressure attenuation data Y(t) to or beyond a specific degree.
The present invention as defined in claim
4
provides a method of detecting clogging of an orifice in a pressure-type flow rate controller which has a control val
Dohi Ryousuke
Hirose Jun
Hirose Takashi
Ideta Eiji
Iida Seiichi
Chambers A. Michael
Fujikin Incorporated
Griffin & Szipl, P.C.
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