Method for monitoring and selectively sampling a fluid flow stream

An improved method and apparatus for monitoring, in real time, the quality of a flowing fluid stream and of automatically taking samples therefrom when the level of particulate matter in the flow stream exceeds an acceptable limit for a predetermined period of time. The apparatus includes a particle monitor and at least one three-way valve electronically linked via a programmable logic controller to the particle monitor. Not only is the latter an instrument for continuously measuring, on-line, numbers known as particle indices which are indicative of particulate matter concentrations in the flow stream, but also the particle monitor generates electronic signals. Both these signals and the particle index are proportional to fluctuations in the intensity of a light beam traversed by particles which are present in a portion of the flow stream during a fixed interval of time. Importantly, the particle monitor can detect relatively short-lived "spikes" in the level of 0.50 micron or larger particles which may be present in the flow stream. Utilizing this sensitivity of the particle monitor to such "spikes", the apparatus can be set to automatically take samples when excursions in the levels of particulate matter occur rather than at random intervals. As a result, the method can facilitate the diagnosis of an impending process equipment breakdown/malfunction in its early stages, while its observable effect is only an infrequent "spike".

BACKGROUND OF THE INVENTION 
This invention relates to the monitoring of particulate matter in fluid 
flow streams and to the collection of samples from such streams for 
chemical analysis. 
Automated samplers which can periodically collect discrete samples of 
liquid from a fluid flow stream are well known in the prior art. Unless 
the time interval between sample collections is kept relatively short and 
the number of samples correspondingly large, major excursions or "spikes" 
in the levels of contaminants in a flow stream can occur without samples 
being taken when these levels are elevated. Unfortunately, when such 
excursions go undetected because of their infrequent occurrence, 
information can be lost which, if these excursions had been detected 
earlier, could have prevented a plant shutdown or even catastrophic 
failure. 
On the other hand, discrete batches or samples of a fluid need not actually 
be collected in order to detect occasional "spikes" in certain 
contaminants. Specifically, the relative numbers of insoluble particles in 
a fluid flow stream, even when they occur only as relatively short-lived 
"spikes", can be measured, on-line, with the use of a particle monitor. 
Conveniently, output from the particle monitor, at any given moment of 
time, can be related to a single number known as the particle index, 
facilitating data interpretation. To determine each particle index, the 
particle monitor measures the intensity of a light or infrared radiation 
beam transmitted through, and perpendicularly to, a suspension flowing 
through a transparent tube. Fluctuations in the intensity of this beam 
signal the passage of undissolved material in the flow. From intensity 
measurements made over a fixed time interval, the particle monitor then 
computes a ratio equal to the intensity fluctuations, manipulated into a 
single number, divided by the average intensity of the beam. The particle 
index for a given time interval is then proportional to the ratio 
determined for that interval. 
In addition to generating particle indices, the particle monitor produces 
electronic pulses and an analog 4-20 milliamp signal which varies with the 
particle index and is proportional thereto. Particle monitors which both 
display the particle index and produce such electronic signals are 
available commercially. The latter signal, when fed into a suitable 
controller, can be used to regulate the opening and closing of an 
electronically-actuated valve. One use of such a valve, according to 
Bryant and Veal in U.S. Pat. No. 5,578,995, is to selectively dump the 
entire flow stream from a condensate return system. 
As Bryant and Veal also disclosed in the cited patent, a method combining 
the steps of continuously monitoring particulate matter in a flow stream 
with a particle monitor and automatically diverting the flow when the 
measured particle index exceeds a preset value can be used not only to 
protect process equipment but also to conserve energy and material 
resources. In a steam condensate return system, for example, this method 
can be used to eliminate guesswork as to when the condensate flow needs to 
be dumped and when it can be safely recycled. 
Unfortunately, many advantages of this method have gone unrecognized for a 
long time. Indeed, the usefulness of a particle monitor lies in very high 
sensitivity to detect particles larger than 0.5 micron in size or larger. 
Another instrument more sensitive to the presence of the smaller (less 
than 0.5 micron) insoluble particles--the turbidimeter--has long been 
considered the instrument of choice for detecting particulate matter in a 
fluid flow stream. This instrument decreases in sensitivity as particle 
size increases. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide an improved method and 
apparatus for automatically taking discrete samples during excursions or 
"spikes" in the levels of insoluble contaminants in a fluid flow stream 
without having to take samples when these levels are not elevated. 
Utilization of such a method and apparatus facilitates the collection and 
analyses of samples only when they are needed, greatly reducing costs. 
A still further object of the present invention is to provide means for 
automatically collecting discrete samples of fluid from a process flow 
stream so that the fluid can be analyzed for specific contaminants at the 
earliest stages of process equipment breakdown/malfunction, without 
necessarily collecting discrete samples prior to such malfunction. 
Information from the analyses of specific contaminants can then be 
employed to help identify their source(s) and to pinpoint exactly where 
problems are occurring in a complex system. 
In accordance with the present invention, there is provided an improved 
method which includes the step of automatically diverting, from its normal 
course, a portion of a fluid flow stream into a sample container only when 
the particle index of the fluid has reached a preset level and maintained 
this level for a predetermined interval of time.

DETAILED DESCRIPTION OF THE INVENTION 
The improved method according to the present invention includes the steps 
of monitoring the level of particulate matter in a clear fluid flow stream 
and of automatically collecting at least one discrete sample from such a 
stream during, but only during, each excursion or "spike" in which this 
level exceeds, for a predetermined time interval, a preset value. Although 
there are upper limits of particle concentration for which this method is 
useful, these limits vary widely from one fluid flow stream to another, in 
part, because of variances in particle sizes and light 
absorbing/reflecting properties between different systems. However, the 
method was worked satisfactorily when used on raw river water at turbities 
up to 30 NTU and ultrapure waters with less than one particle per 
milliliter of 2 micron size. 
Referring to the drawings, an apparatus, denoted generally by the reference 
numeral 10, comprises a 3-way solenoid valve 15, a particle monitor 
equipped with a sensor 20 which is located upstream of the valve, a 
controller 24 for electronically actuating the valve, and a power switch 
34 common to both the particle monitor and the controller. In use, a small 
fluid flow stream B is continuously directed through transparent vinyl 
tubing 26 and into the sensor 20 where both the tubing and flow are 
intersected by a narrow, but intense beam of radiation B (FIG. 3). 
Generated by a light emitting diode (LED) 21, this beam of radiation is 
preferably about 0.4 mm wide, with a wavelength of about 850 nm, that is, 
just in the infrared. Only a small fraction (about 20%) of the total flow 
in the tubing 26 actually passes through the beam. It is assumed that this 
fraction is representative of the entire flow. 
The transmitted radiation is monitored by a sensitive photodetector 22 
(FIG. 3). The latter measures fluctuations in the intensity of the light 
beam as individual particles pass through it and cast shadows. Changes in 
these fluctuations indicate changes in the concentrations of insoluble 
particles present in the flow stream. 
In addition to the sensor 20, the particle monitor comprises a signal 
processor, a RMS-DC converter, a voltage-to-frequency (V/F) converter, and 
a square-wave pulse counter. Preferably, the signal processor, RMS-DC 
converter, V/F converter, and square-wave pulse counter are all components 
of a single circuit board 23 (FIG. 5). Upon receiving an electronic signal 
from the photodetector 22, the board 23 can generate the following 
outputs: a 4-digit number known as the particle index which is presented 
on a digital display panel 31, a pulsed output (typically of about 5 Volts 
in magnitude) recognizable by a microprocessor and a 4-20 milliamp signal. 
In the preferred embodiment, each of these outputs is derived from a very 
small fluctuating component or AC value of the electronic signal from the 
photodetector 22, which must be separated from a much larger steady 
component or DC value, a measure of the average transmitted intensity of 
the beam A (FIG. 3). Separation of the AC signal from the DC is performed 
by the signal processor. Not only does the signal processor separate the 
AC signal but also it amplifies the AC value and then passes it to the 
RMS-DC converter. There the amplified AC signal is converted to a DC 
voltage equal to its root mean square (RMS) value. From the RMS-DC 
converter, the RMS signal is then passed to the V/F converter where it is 
processed into a succession of square-wave pulses whose frequency is 
proportional to the RMS value. Finally, in the square-wave pulse counter, 
these pulses are counted over a fixed time interval, in preparation for 
the pulse count being ultimately presented on the digital display 31 as 
the particle index. 
In this way, each particle index is generated from measurements made 
continuously over a specific time interval of a fixed length. Experience 
has indicated that the apparatus 10 performs satisfactorily when the 
length of the time interval for counting these pulses is set at about 10 
seconds. With a time interval of this duration, the various output signals 
from the particle monitor are smoothed; otherwise, a few very large 
particles (or bubbles) would momentarily cause the particle index to 
become very high. 
Moreover, in the preferred embodiment, the particle monitor utilizes a 
sensor 20 with a LED feedback circuit. Such a feedback circuit 
automatically adjusts the average intensity of the light beam traversing 
the tubing 26 so that this average intensity is maintained at a constant 
level at the photodetector 22. Preferably, the DC component is kept 
constant, by way of example, at 5.7 volts. Alternatively, in the absence 
of the LED feedback circuit, means for dividing the root mean square of 
the AC value to produce a ratio must be provided. Each particle index is 
then proportional to this ratio, a measure of the fluctuations in 
intensity of the light beam divided by the absolute intensity of the light 
beam, during a fixed interval of time. 
In the preferred embodiment, the particle monitor responds to fluctuations 
in the intensity of the light beam A, rather than to its absolute 
intensity, so that the particle monitor is not susceptible to electronic 
"drift" caused by DC noise, light source variances, optical surface 
fouling and fogging at elevated temperatures. Changes in the efficiency of 
the LED or in the transmissivity of the tubing 26, for example, are 
automatically compensated. 
Nevertheless, at some point, the transmissivity of tubing 26 becomes too 
low; and the tubing must be replaced. To help an operator determine when 
the tubing 26 must be replaced, the apparatus 10 is preferably equipped 
with an indicator 32 having a series of LED segments (FIG. 1). As the 
tubing 26 becomes coated with particles, it transmits less light, causing 
the diode 21 in the sensor 20 to draw more current. Simultaneously, the 
indicator 32 responds with more and more illuminated LED segments, a 
visual reminder of the degree of fouling in the tubing 26. 
As is best seen in FIG. 2, the sensor 20 includes means for holding a 
section of the tubing 26 in such as way that this section, which is 
otherwise round in transverse cross-section, is generally flattened on the 
sides of the tubing where it is traversed by the light beam, including the 
sides of the tubing which are proximate with the light source 21 and the 
photodetector 22, respectively. The flattening of these sides reduces 
reflection of the light beam from the walls of the tubing 26. 
Means for so flattening the tubing 26 includes an elongated pin 28 and a 
housing 19 for the sensor 20 (FIG. 2). The housing 19 defines an elongated 
aperture which is approximately rectangular in transverse cross-section 
and which is sized for receiving both a section of the tubing 26 and the 
pin 28 in juxtaposed relation. As it is being slideably inserted into the 
aperture, the pin causes the walls of the tubing section to flatten 
against three sides of the aperture, as well as against the pin itself. In 
use, both the tubing section and the pin 28 remain pressed together. When 
the tubing 26 must be replaced, the pin 28 can be readily removed by 
grasping a loop 27 affixed thereto (FIG. 2). 
A particle monitor which has been found to be satisfactory for this 
application is the Chemtrac Model PM 3500RSS, available commercially from 
Chemtrac Systems, Inc. of Norcross, Ga. General specifications for this 
model are as indicated below: 
______________________________________ 
Self Diagnostics Sample cell tubing LED 
Sample Cell Type Flow through 
Sensor Response Time 
Instantaneous 
Materials Contacting Sample 
Clear vinyl 
Ambient Operating Temperature 
32-120 degrees F. 
Sample Temperature 32-120 degrees F. 
Sample Delay Time (seconds) 
0-3600 
Sample Time (seconds) 
0-3600 
Sample Tubing Size 1/8 inch I. D., 3/16 inch O. D. 
Sample Flow Rate 100-500 ml/min. 
Sample Flow Control 
Constant Head Type 
Particle Size Range 
1 micron and above 
Minimum Particle Size Detection 
0.5 micron 
Particle Index Range 
0-9999 
Signal Output Isolated 4-20 mA (proportional 
to particle index with adjustable 
span) 600 ohm load Max 
Particle Index Readout 
10 seconds 
(Averaging) Interval 
Sample Flow Rate 100-500 ml/min. 
PLC Reset Momentary switch (lighted to 
indicate sample been taken) 
Recorder Single pen (standard) 
______________________________________ 
Simultaneously, as it displays a particle index on the panel 31, the 
particle monitor generates pulsed output which communicates the same 
particle index to a microprocessor within the controller 24. Upon 
receiving this pulsed output, the microprocesssor then compares it with a 
setpoint known as the "particle index threshold". The latter corresponds 
to the minimum particle index which the particle monitor must measure in 
the flow stream before the controller 24 initiates a sequence of events 
which may result in a sample being collected. Alternatively, the particle 
monitor can transmit the 4 to 20 milliamp electronic signal to a 
controller, subject to deadband control. In the latter case, the 
controller actuates a timer, once the 4 to 20 milliamp signal reaches a 
level corresponding to an upper setpoint (the particle index threshold), 
and continues to actuate this timer as long as this signal remains above a 
lower setpoint (a particle index which in a typical situation measures 
about 10-20 percent of the particle index threshold). As long as the 
particle index remains within this deadband, settings on the timer 
determine the conditions under which an electronic signal can be sent from 
the controller 24 to the 3-way solenoid valve 15. 
In the preferred embodiment, the controller 24 includes a a microprocessor 
with a programmable timer. A suitable controller is the model Z-104 
available commercially from Z-World Engineering in Davis, Calif. Upon 
receiving a signal from the microprocessor that the particle index exceeds 
the particle index threshold, the programmable timer initially blocks 
transmission of an electronic signal which would otherwise be sent from 
the controller 24 to the 3-way solenoid valve 15. With the timer, the 
controller 24 waits to transmit this signal until the particle index has 
exceeded the particle index threshold for a predetermined time interval 
known as the "delta time". Provided the particle index threshold is still 
met when the "delta time" has expired, the electronic signal from the 
controller 24 actuates the valve 15, causing its normally open portal to 
close and its normally closed portal to open. As a result, the flow stream 
is diverted from tubing section 12 to tubing section 13 fluidly connecting 
the normally closed portal to the sample container 16 (FIGS. 1 and 2). The 
timer is also used to block this signal but only after the flow stream has 
repeatedly flushed the container 16, filling it and the tubing section 13 
connecting it to the 3-way solenoid valve 15 before discharging through a 
tubing section 14 to the drain. Thus overfilling of the container 16 is 
averted. In the preferred embodiment, the container 16 measures, by way of 
example, 250 ml in volume and is rinsed at least 3 times before the 
normally closed portal is closed and a sample is actually collected. A 
second sample container 16', a duplicate of container 16, is preferably 
held in reserve (FIG. 2). 
As is also illustrated in FIGS. 1 and 2, the 3-way valve 15 is fluidly 
connected by flexible, transparent tubing 26 to a heat exchanger 41 and 
indirectly to a process pipe 25. Alternatively, the tubing 26 is fluidly 
connected directly to the pipe 25. Preferably, the flow rate through the 
tubing 26, which, by way of example, is 3/16 inch OD vinyl tubing with an 
internal diameter of 3 mm, is in the range of 100 to 500 ml per minute. 
In use, determination of a suitable "delta time" for a given fluid flow 
stream is facilitated by feeding the 4 to 20 milliamp signal from the 
particle monitor to a strip chart recorder 33 (FIG. 1). As example of a 
strip chart record generated by a particle monitor monitoring a steam 
condensate flow stream is shown in FIG. 4; this record reveals three 
excursions, over a 6 hour period, of the particle index. Each of these 
excursions lasted at least 15 minutes. However, no excursion was detected 
by a state-of-the-art turbidimeter which simultaneously monitored the same 
condensate flow stream over the same time period. 
Using a strip chart record, such as that shown in FIG. 4, generated by 
monitoring a particular flow stream, an operator can then make reasonable 
selections of the "delta time" and of the particle index threshold which 
can be used to optimize the collection of samples from that stream. The 
delta time and particle index threshold, once selected, are entered into 
the memory of the programmable logic controller 24 through its keyboard 44 
(FIG. 6). 
As the strip chart record shown in FIG. 4 illustrates, the particle monitor 
can detect relatively short-lived "spikes" of particles which are be 
present in the flow stream. Utilizing this sensitivity of the particle 
monitor to detect such "spikes", the apparatus 10 can be set to 
automatically take samples when excursions in the levels of particulate 
matter occur rather than at random times. Analysis of individual 
constituents in such samples can help an operator diagnose an impending 
process equipment breakdown/malfunction in its early stages, while its 
observable effect is only an infrequent "spike". 
Upstream of the sensor 20, a valve 40, when fully opened, allows flow to 
pass through the tubing 26 at a maximum rate of 500 ml per minute. The 
valve 40 comprises means for adjusting the fluid flow rate in the tubing 
26 (FIG. 1). This flow rate, however, is not critical as long as it is 
maintained fairly constant, that is, as long as it stays within about 10 
percent of its mean value. Generally, however, higher flows are preferable 
because they tend to minimize deposit formation on the walls of the tubing 
26. Nevertheless, for high flows with high particulate matter content, the 
particle index may exceed the maximum reading (9999) for the 
monitor/sampler 10; in such cases, the flow rate must then be lowered 
accordingly. 
Also located upstream of the sensor 20 is a heat exchanger 41 (FIG. 1). 
With use of the heat exchanger 41, the apparatus can accommodate flows 
such as steam condensate, hot oils or the like which must be cooled in 
order to protect the tubing 26. The heat exchanger 41 is used to lower the 
temperature of such flows to 120 degrees Fahrenheit or less. Moreover, 
upstream of the heat exchanger 41, tubing which fluidly connects it to the 
pipe 25 is preferably fabricated from corrosion resistant materials such 
as stainless steel. The latter are needed to protect surfaces which come 
into contact with hot fluid. 
Optimum performance of the apparatus 10 is obtained by keeping the tubing 
26 as short as possible. Indeed, the longer the tubing 26 is, the greater 
is the likelihood of deposits being formed therein which can restrict flow 
and/or slough off, causing meaningless, confusing "spikes" in the data. 
Generally, any erratic phenomena which would appear to the particle 
monitor within the apparatus 10 to be insoluble particles are to be 
avoided. In this regard, it is also helpful to locate any flow control 
devices, except the valve 40, downstream of the sensor 20. Otherwise, if 
fluid is forced through a pipe constriction or the like, gases can come 
out of solution as bubbles, interfering with particle index readings. 
It is understood that those skilled in the art may conceive other 
applications, modifications and/or changes in the invention described 
above. Any such applications, modifications or changes which fall within 
the purview of the description are intended to be illustrative and not 
intended to be limitative. The scope of the invention is limited only by 
the scope of the claims appended hereto.