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Pressure application
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BobG
science forum beginner


Joined: 12 Sep 2005
Posts: 7

PostPosted: Tue Jun 20, 2006 10:19 pm    Post subject: Pressure application Reply with quote

Hi all,
Thought I would ask for opinions in this newsgroup regarding a new
project I'm involved in. I'm a PLC programmer who has been tasked with
programming a new clean in place (CIP) project for a large spray dryer and
it's associated duct work. There are 135 nozzles split unevenly between 11
defined "zones". Each zone is to wash for a predefined period of time by
sequentially actuating ball valves off the main CIP header. Differences in
elevation between the zones differ by as much as 110 feet, and there are
several pressure transmitters to be installed to detect pressure at each
zone. I'll move the PT signals into the PID instruction when the appropriate
zone is washing. Each nozzle is designed to operate at 100 p.s.i.. At the
designed pressure the nozzles will use 12 g.p.m.

The pump is a centrifugal pump and a frequency drive has already been
specified to control pressure in each zone. Having been through the
extremely unpleasant experience of having to control pressure with a drive
and a centrifugal pump I raised the possibility of using a modulating valve
to either control pressure, or deliver the designed flow rate, for ex: 12
nozzle at 8 g.p.m. for 96 g.p.m.

I was told by the chemical engineer who's running the job (he *seemed*
like a knowledgeable fellow) that a modulating valve wouldn't work and the
frequency drive was the only way to go. Because of the head. After much
discussion (most of which I didn't understand Smile ), he convinced me that as
much as I hated to hear it the drive was the way to go.

The project was approved and materials were already being delivered when
I became involved. Was he yanking my chain about the drive vs. valve issue?
Or does the head on the pump demand a frequency drive in this application?
I'll also add that there's 160 P.S.I. direct steam injection controlled by
another PID loop to the mixing chamber of a Pick heater to heat the water to
160 Deg. F. Anyone who has had to control pressure with a centrifugal pump
will understand my trepidation.
Back to top
sQuick
science forum beginner


Joined: 25 Sep 2005
Posts: 14

PostPosted: Tue Jun 20, 2006 11:30 pm    Post subject: Re: Pressure application Reply with quote

"BobG" <BobG@comcast.net> wrote in message
news:6dCdnV9m3oqo6QXZnZ2dnUVZ_oednZ2d@comcast.com...
Quote:
Hi all,
Thought I would ask for opinions in this newsgroup regarding a new
project I'm involved in. I'm a PLC programmer who has been tasked with
programming a new clean in place (CIP) project for a large spray dryer and
it's associated duct work. There are 135 nozzles split unevenly between 11
defined "zones". Each zone is to wash for a predefined period of time by
sequentially actuating ball valves off the main CIP header. Differences in
elevation between the zones differ by as much as 110 feet, and there are
several pressure transmitters to be installed to detect pressure at each
zone. I'll move the PT signals into the PID instruction when the
appropriate
zone is washing. Each nozzle is designed to operate at 100 p.s.i.. At the
designed pressure the nozzles will use 12 g.p.m.

The pump is a centrifugal pump and a frequency drive has already been
specified to control pressure in each zone. Having been through the
extremely unpleasant experience of having to control pressure with a drive
and a centrifugal pump I raised the possibility of using a modulating
valve
to either control pressure, or deliver the designed flow rate, for ex: 12
nozzle at 8 g.p.m. for 96 g.p.m.

I was told by the chemical engineer who's running the job (he *seemed*
like a knowledgeable fellow) that a modulating valve wouldn't work and the
frequency drive was the only way to go. Because of the head. After much
discussion (most of which I didn't understand Smile ), he convinced me that
as
much as I hated to hear it the drive was the way to go.

The project was approved and materials were already being delivered
when
I became involved. Was he yanking my chain about the drive vs. valve
issue?
Or does the head on the pump demand a frequency drive in this application?
I'll also add that there's 160 P.S.I. direct steam injection controlled by
another PID loop to the mixing chamber of a Pick heater to heat the water
to
160 Deg. F. Anyone who has had to control pressure with a centrifugal pump
will understand my trepidation.




I've done several CIP systems on a plc in large spiral freezers etc. If its
piped correctly
there is no reason why you cant do it with a modulating return valve or VSD.

i'm sure you are aware of the non linear effects of VSD on centrifugal
pumps.

Personally I would prefer a variable/modulating valve.

sQuick..
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Herman Family
science forum Guru Wannabe


Joined: 14 Jun 2005
Posts: 173

PostPosted: Wed Jun 21, 2006 4:39 am    Post subject: Re: Pressure application Reply with quote

"sQuick" <squickdrill@hotmail.com> wrote in message
news:1150846231.21075.0@proxy02.news.clara.net...
Quote:

"BobG" <BobG@comcast.net> wrote in message
news:6dCdnV9m3oqo6QXZnZ2dnUVZ_oednZ2d@comcast.com...
Hi all,
Thought I would ask for opinions in this newsgroup regarding a new
project I'm involved in. I'm a PLC programmer who has been tasked with
programming a new clean in place (CIP) project for a large spray dryer
and
it's associated duct work. There are 135 nozzles split unevenly between
11
defined "zones". Each zone is to wash for a predefined period of time by
sequentially actuating ball valves off the main CIP header. Differences
in
elevation between the zones differ by as much as 110 feet, and there are
several pressure transmitters to be installed to detect pressure at each
zone. I'll move the PT signals into the PID instruction when the
appropriate
zone is washing. Each nozzle is designed to operate at 100 p.s.i.. At the
designed pressure the nozzles will use 12 g.p.m.

The pump is a centrifugal pump and a frequency drive has already been
specified to control pressure in each zone. Having been through the
extremely unpleasant experience of having to control pressure with a
drive
and a centrifugal pump I raised the possibility of using a modulating
valve
to either control pressure, or deliver the designed flow rate, for ex: 12
nozzle at 8 g.p.m. for 96 g.p.m.

I was told by the chemical engineer who's running the job (he *seemed*
like a knowledgeable fellow) that a modulating valve wouldn't work and
the
frequency drive was the only way to go. Because of the head. After much
discussion (most of which I didn't understand Smile ), he convinced me that
as
much as I hated to hear it the drive was the way to go.

The project was approved and materials were already being delivered
when
I became involved. Was he yanking my chain about the drive vs. valve
issue?
Or does the head on the pump demand a frequency drive in this
application?
I'll also add that there's 160 P.S.I. direct steam injection controlled
by
another PID loop to the mixing chamber of a Pick heater to heat the water
to
160 Deg. F. Anyone who has had to control pressure with a centrifugal
pump
will understand my trepidation.




I've done several CIP systems on a plc in large spiral freezers etc. If
its piped correctly
there is no reason why you cant do it with a modulating return valve or
VSD.

i'm sure you are aware of the non linear effects of VSD on centrifugal
pumps.

Personally I would prefer a variable/modulating valve.

sQuick..


I think I'd rather the valve also. The pump will have to work a little

harder if you use the valve, but that's life. There may be a problem
getting the right sized valve to handle the entire control range with the
pressures required.

I'd seriously consider using a positive displacement pump and a dump valve
for this application. Use a dump valve to lose the extra pressure. Each
zone can have their own dump valve so that it only needs a shutoff valve to
run. That makes things quite easy.

Michael

Michael
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Tim Wescott
science forum Guru Wannabe


Joined: 03 May 2005
Posts: 292

PostPosted: Wed Jun 21, 2006 3:37 pm    Post subject: Re: Pressure application Reply with quote

BobG wrote:
Quote:
Hi all,
Thought I would ask for opinions in this newsgroup regarding a new
project I'm involved in. I'm a PLC programmer who has been tasked with
programming a new clean in place (CIP) project for a large spray dryer and
it's associated duct work. There are 135 nozzles split unevenly between 11
defined "zones". Each zone is to wash for a predefined period of time by
sequentially actuating ball valves off the main CIP header. Differences in
elevation between the zones differ by as much as 110 feet, and there are
several pressure transmitters to be installed to detect pressure at each
zone. I'll move the PT signals into the PID instruction when the appropriate
zone is washing. Each nozzle is designed to operate at 100 p.s.i.. At the
designed pressure the nozzles will use 12 g.p.m.

The pump is a centrifugal pump and a frequency drive has already been
specified to control pressure in each zone. Having been through the
extremely unpleasant experience of having to control pressure with a drive
and a centrifugal pump I raised the possibility of using a modulating valve
to either control pressure, or deliver the designed flow rate, for ex: 12
nozzle at 8 g.p.m. for 96 g.p.m.

I was told by the chemical engineer who's running the job (he *seemed*
like a knowledgeable fellow) that a modulating valve wouldn't work and the
frequency drive was the only way to go. Because of the head. After much
discussion (most of which I didn't understand Smile ), he convinced me that as
much as I hated to hear it the drive was the way to go.

The project was approved and materials were already being delivered when
I became involved. Was he yanking my chain about the drive vs. valve issue?
Or does the head on the pump demand a frequency drive in this application?
I'll also add that there's 160 P.S.I. direct steam injection controlled by
another PID loop to the mixing chamber of a Pick heater to heat the water to
160 Deg. F. Anyone who has had to control pressure with a centrifugal pump
will understand my trepidation.


The closest I've come to your problem is with itty bitty air pumps and

valves, small enough to mount on a PC board. So I don't know enough to
share your trepidation, but I've seen other smart people trepidating
about this sort of thing on this group, so I trust that it is, indeed, a
hard nut to crack.

It seems that your problem is both a technical one and a political one
-- technical because you are going down a path that you fear may not
succeed, and political because you are being set on that path by someone
you are working with.

I can think of some things that I might do to resolve this sort of issue.

* Line up a bunch of guys with experience who say it's a bad idea (or
who tell you how to do it right). This is what you seem to be doing
here, I hope it's all you need.

* If the guy you're working with is generally reasonable but gets stuck
on things every once in a while I'd take the 'dumb yokel' approach: "gee
Mr. Chemical Engineer, I'm just a dumb PLC programmer. Every time I or
any one of my friends tries to do this it fails. Could you please
detail how you make it work?" If he's done it before with success grill
him (nicely, naively) on how that job was like this job, and why the
differences won't make a difference. If he gets over your head revert
back to "I'm just a _dumb_ PLC programmer" and make him simplify.
Hopefully he'll start to explain, stop in the middle and say "oh sh--"
and hurriedly order a valve for you.

* An alternative to the "dumb PLC programmer" is the "prima donna PLC
programmer": "I'm sorry sir -- I don't believe that I can make this
work, and I refuse to guarantee my results. I'll take your money for
spending time on it, but you won't get any refunds if it fails." In my
experience this is a battle you want to choose carefully -- you can
really get people's back up doing this, and make them stick to their
guns even after it's obvious that it's going to be a screwup.

* Another thing I'd consider doing would be to prototype the hard part
-- the pump with a head on it -- and (a) see if I could control it and
if not (b) show the setup to him and get _him_ to try to make it work.
This approach has the difficulty that you have to get working pieces:
either you need to get the pump in question going without a bunch of
effort, or you need to get a small pump and then argue about how
representative it is.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html
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Steve Cothran
science forum beginner


Joined: 28 Jun 2005
Posts: 24

PostPosted: Thu Jun 22, 2006 3:15 am    Post subject: Re: Pressure application Reply with quote

Are you an employee or a contractor? If your'e an employee, sorry for
your luck. If you are a contractor, don't dive into a job that you
know to be a sure-fire cluster-f*ck. While SuperChemist aims the shots
the target will be your backside.

I've had those jobs...where at the end I would have had more dollars
in the bank if I had wrote the customer a check for $5000 and told him
"screw you, I don't want the job!".
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MLD
science forum beginner


Joined: 04 May 2005
Posts: 15

PostPosted: Thu Jun 22, 2006 1:57 pm    Post subject: Re: Pressure application Reply with quote

"Herman Family" <ecalptsudwaseht.in.reverse@frontiernet.net> wrote in
message news:_34mg.4030$Oh1.571@news01.roc.ny...
Quote:

"sQuick" <squickdrill@hotmail.com> wrote in message
news:1150846231.21075.0@proxy02.news.clara.net...

"BobG" <BobG@comcast.net> wrote in message
news:6dCdnV9m3oqo6QXZnZ2dnUVZ_oednZ2d@comcast.com...
Hi all,
Thought I would ask for opinions in this newsgroup regarding a new
project I'm involved in. I'm a PLC programmer who has been tasked with
programming a new clean in place (CIP) project for a large spray dryer
and
it's associated duct work. There are 135 nozzles split unevenly between
11
defined "zones". Each zone is to wash for a predefined period of time
by
sequentially actuating ball valves off the main CIP header. Differences
in
elevation between the zones differ by as much as 110 feet, and there
are
several pressure transmitters to be installed to detect pressure at
each
zone. I'll move the PT signals into the PID instruction when the
appropriate
zone is washing. Each nozzle is designed to operate at 100 p.s.i.. At
the
designed pressure the nozzles will use 12 g.p.m.

The pump is a centrifugal pump and a frequency drive has already
been
specified to control pressure in each zone. Having been through the
extremely unpleasant experience of having to control pressure with a
drive
and a centrifugal pump I raised the possibility of using a modulating
valve
to either control pressure, or deliver the designed flow rate, for ex:
12
nozzle at 8 g.p.m. for 96 g.p.m.

I was told by the chemical engineer who's running the job (he
*seemed*
like a knowledgeable fellow) that a modulating valve wouldn't work and
the
frequency drive was the only way to go. Because of the head. After much
discussion (most of which I didn't understand Smile ), he convinced me
that
as
much as I hated to hear it the drive was the way to go.

The project was approved and materials were already being delivered
when
I became involved. Was he yanking my chain about the drive vs. valve
issue?
Or does the head on the pump demand a frequency drive in this
application?
I'll also add that there's 160 P.S.I. direct steam injection controlled
by
another PID loop to the mixing chamber of a Pick heater to heat the
water
to
160 Deg. F. Anyone who has had to control pressure with a centrifugal
pump
will understand my trepidation.




I've done several CIP systems on a plc in large spiral freezers etc. If
its piped correctly
there is no reason why you cant do it with a modulating return valve or
VSD.

i'm sure you are aware of the non linear effects of VSD on centrifugal
pumps.

Personally I would prefer a variable/modulating valve.

sQuick..


I think I'd rather the valve also. The pump will have to work a little
harder if you use the valve, but that's life. There may be a problem
getting the right sized valve to handle the entire control range with the
pressures required.

I'd seriously consider using a positive displacement pump and a dump valve
for this application. Use a dump valve to lose the extra pressure. Each
zone can have their own dump valve so that it only needs a shutoff valve
to
run. That makes things quite easy.

Michael

Michael

Depending on the flow and the gain of the valve, operating on the pump

relief valve is not a desirable condition. Heat, valve chatter, pressure
oscillations are typical results. Most of all, pump flow can drop off a
cliff once the valve starts to open. Having said that, the temperature rise
across a centrifugal pump can sky rocket at low flows. Also, controlling
flow with a centrifugal pump can be a challenge since the pressure rise
across the pump is a function of pump speed, system flow demand and the
system line loss. It can get complicated---typically, a metering valve (or
orifice) along with a pressure regulator to keep the Delta P across the
metering valve constant is used to control (or vary) flow using a
centrifugal pump.
MLD
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bruce varley
science forum beginner


Joined: 31 May 2005
Posts: 38

PostPosted: Fri Jun 23, 2006 11:03 am    Post subject: Re: Pressure application Reply with quote

BobG <BobG@comcast.net> wrote in message
news:6dCdnV9m3oqo6QXZnZ2dnUVZ_oednZ2d@comcast.com...
Quote:
Hi all,
Thought I would ask for opinions in this newsgroup regarding a new
project I'm involved in. I'm a PLC programmer who has been tasked with
programming a new clean in place (CIP) project for a large spray dryer and
it's associated duct work. There are 135 nozzles split unevenly between 11
defined "zones". Each zone is to wash for a predefined period of time by
sequentially actuating ball valves off the main CIP header. Differences in
elevation between the zones differ by as much as 110 feet, and there are
several pressure transmitters to be installed to detect pressure at each
zone. I'll move the PT signals into the PID instruction when the
appropriate
zone is washing. Each nozzle is designed to operate at 100 p.s.i.. At the
designed pressure the nozzles will use 12 g.p.m.

The pump is a centrifugal pump and a frequency drive has already been
specified to control pressure in each zone. Having been through the
extremely unpleasant experience of having to control pressure with a drive
and a centrifugal pump I raised the possibility of using a modulating
valve
to either control pressure, or deliver the designed flow rate, for ex: 12
nozzle at 8 g.p.m. for 96 g.p.m.

I was told by the chemical engineer who's running the job (he *seemed*
like a knowledgeable fellow) that a modulating valve wouldn't work and the
frequency drive was the only way to go. Because of the head. After much
discussion (most of which I didn't understand Smile ), he convinced me that
as
much as I hated to hear it the drive was the way to go.

The project was approved and materials were already being delivered
when
I became involved. Was he yanking my chain about the drive vs. valve
issue?
Or does the head on the pump demand a frequency drive in this application?
I'll also add that there's 160 P.S.I. direct steam injection controlled by
another PID loop to the mixing chamber of a Pick heater to heat the water
to
160 Deg. F. Anyone who has had to control pressure with a centrifugal pump
will understand my trepidation.

Your concerns are valid. There's a particular problem controlling flow with

centrifugal pump speed where the static head is high, in that the gain
between flow and speed can be very high, leading to (possibly)
uncontrollable instabiity. One possible solution is to add some hydraulic
resistance into the system, there may already be enough via the nozzles
themselves. If not, then just putting some manual throttling valves in the
lines may reduce the gain enough to make control using pump speed
acceptable.

Prior analysis may or may not give you a clear answer on whether things are
going to be OK. It would be worth planning for some manually adjustable
throttling in the lines, eg. allow for easy isertion of valves, ensure
there's some excess discharge head available from the pump. The system will
certainly become more controllable as the resistance increases. The point at
which things become 'OK' is probably something to find out live.
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Peter Nachtwey
science forum addict


Joined: 06 Nov 2005
Posts: 68

PostPosted: Sat Jun 24, 2006 6:22 pm    Post subject: Re: Pressure application Reply with quote

BobG wrote:
Quote:
Hi all,
Thought I would ask for opinions in this newsgroup regarding a new
project I'm involved in. I'm a PLC programmer who has been tasked with
programming a new clean in place (CIP) project for a large spray dryer and
it's associated duct work. There are 135 nozzles split unevenly between 11
defined "zones". Each zone is to wash for a predefined period of time by
sequentially actuating ball valves off the main CIP header. Differences in
elevation between the zones differ by as much as 110 feet, and there are
several pressure transmitters to be installed to detect pressure at each
zone. I'll move the PT signals into the PID instruction when the appropriate
zone is washing. Each nozzle is designed to operate at 100 p.s.i.. At the
designed pressure the nozzles will use 12 g.p.m.

The pump is a centrifugal pump and a frequency drive has already been
specified to control pressure in each zone. Having been through the
extremely unpleasant experience of having to control pressure with a drive
and a centrifugal pump I raised the possibility of using a modulating valve
to either control pressure, or deliver the designed flow rate, for ex: 12
nozzle at 8 g.p.m. for 96 g.p.m.

I was told by the chemical engineer who's running the job (he *seemed*
like a knowledgeable fellow) that a modulating valve wouldn't work and the
frequency drive was the only way to go. Because of the head. After much
discussion (most of which I didn't understand Smile ), he convinced me that as
much as I hated to hear it the drive was the way to go.

The project was approved and materials were already being delivered when
I became involved. Was he yanking my chain about the drive vs. valve issue?
Or does the head on the pump demand a frequency drive in this application?
I'll also add that there's 160 P.S.I. direct steam injection controlled by
another PID loop to the mixing chamber of a Pick heater to heat the water to
160 Deg. F. Anyone who has had to control pressure with a centrifugal pump
will understand my trepidation.
I have not experience with the CIP projects but I do with hydraulic

motion, pressure and force control. I too have seen difficulties in
controling a pump to control flow or pressue. due to the slow response
compared to quickly changing loads.. It is far better to have an
accumulator on the output of the pump and a flow control valve after
that. The accumulator will slow down the pressure changes which will
make the control of the pump less critical and much easier. The valve
will have a relatively constant pressure source and is much faster
responding than the pump motor. I know it cost more money but it can
be thought of as insurance.

After you get into the project a bit you should find the RPM the pump
needs to run at for each valve combination. You should use this
knowledge to provide a feed forward or bias for your PID. This way you
don't need to rely on the integrator winding up or down to match the
new flow requirements for the new valve combination. If done right you
should be able to run the pump with the bias and proportional gain.

Use the past to predict the future. Make your system learn.
Hopefully the pump will find a constant RPM to run at for each
combination. Update your bias data using this RPM. You may wish to
filter the changes a bit. This way you system will almost self tune
and adapt as the system ages.
Back to top
BobG
science forum beginner


Joined: 12 Sep 2005
Posts: 7

PostPosted: Tue Jun 27, 2006 9:12 pm    Post subject: Re: Pressure application Reply with quote

"Peter Nachtwey" <peter@deltacompsys.com> wrote in message
news:1151173328.958689.11870@y41g2000cwy.googlegroups.com...
Quote:

BobG wrote:
Hi all,
Thought I would ask for opinions in this newsgroup regarding a new
project I'm involved in. I'm a PLC programmer who has been tasked with
programming a new clean in place (CIP) project for a large spray dryer
and
it's associated duct work. There are 135 nozzles split unevenly between
11
defined "zones". Each zone is to wash for a predefined period of time by
sequentially actuating ball valves off the main CIP header. Differences
in
elevation between the zones differ by as much as 110 feet, and there are
several pressure transmitters to be installed to detect pressure at each
zone. I'll move the PT signals into the PID instruction when the
appropriate
zone is washing. Each nozzle is designed to operate at 100 p.s.i.. At
the
designed pressure the nozzles will use 12 g.p.m.

The pump is a centrifugal pump and a frequency drive has already
been
specified to control pressure in each zone. Having been through the
extremely unpleasant experience of having to control pressure with a
drive
and a centrifugal pump I raised the possibility of using a modulating
valve
to either control pressure, or deliver the designed flow rate, for ex:
12
nozzle at 8 g.p.m. for 96 g.p.m.

I was told by the chemical engineer who's running the job (he
*seemed*
like a knowledgeable fellow) that a modulating valve wouldn't work and
the
frequency drive was the only way to go. Because of the head. After much
discussion (most of which I didn't understand Smile ), he convinced me that
as
much as I hated to hear it the drive was the way to go.

The project was approved and materials were already being delivered
when
I became involved. Was he yanking my chain about the drive vs. valve
issue?
Or does the head on the pump demand a frequency drive in this
application?
I'll also add that there's 160 P.S.I. direct steam injection controlled
by
another PID loop to the mixing chamber of a Pick heater to heat the
water to
160 Deg. F. Anyone who has had to control pressure with a centrifugal
pump
will understand my trepidation.
I have not experience with the CIP projects but I do with hydraulic
motion, pressure and force control. I too have seen difficulties in
controling a pump to control flow or pressue. due to the slow response
compared to quickly changing loads.. It is far better to have an
accumulator on the output of the pump and a flow control valve after
that. The accumulator will slow down the pressure changes which will
make the control of the pump less critical and much easier. The valve
will have a relatively constant pressure source and is much faster
responding than the pump motor. I know it cost more money but it can
be thought of as insurance.

After you get into the project a bit you should find the RPM the pump
needs to run at for each valve combination. You should use this
knowledge to provide a feed forward or bias for your PID. This way you
don't need to rely on the integrator winding up or down to match the
new flow requirements for the new valve combination. If done right you
should be able to run the pump with the bias and proportional gain.

I had to do just that in the previous case I mentioned. After much headache
it was the only way to reliably control the process. I like to use standard
control methodology whenever possible and hate to complicate a process, but
sometimes the initial designer leaves you no other choice :)

Quote:
Use the past to predict the future. Make your system learn.
Hopefully the pump will find a constant RPM to run at for each
combination. Update your bias data using this RPM. You may wish to
filter the changes a bit. This way you system will almost self tune
and adapt as the system ages.

Years ago I installed a nutating disc flowmeter that had an electronic head
that received pulses from a reed switch. The unit controlled a pneumatic
ball valve downstream. The program in the unit had a parameter that
compensated for valve wear, air pressure variations, or anything else that
might cause the batch to overrun set point. I thought it useful and have
implemented that particular type of logic in a PLC for a couple of different
applications. Useful indeed.
Back to top
George11
science forum beginner


Joined: 13 Dec 2005
Posts: 24

PostPosted: Tue Jul 04, 2006 11:27 am    Post subject: Re: Pressure application Reply with quote

Either a VFD pump or a pump/valve system can be capable of meeting your
needs. The problem that you will face is the problem if non-linearity.

For example, "the zones differ by as much as 110 feet", means roughly
45 psi of difference in head. Since your nozzle pressure requirements
are 110 psi, you can expect at least 40% difference in process gain
from this alone. Factor in longer 135 different pipe runs, with
different bends, and you can expect a highly non-linear system
response.

The non-linearity will cause the pressure/flow controls to appear slow
and sluggish for some nozzles, and unstable for others. The trick is
to find the "right" tuning for this system. If the system is highly
non-linear, you may also need to apply a characterizer to the control
loop.

Ideally, you could find the tuning by doing a "bump and tune" for each
nozzle. But I am going to assume that this is unlikely, due to time
constraints. You should try to find "robust" controller tuning, that
will keep the system stable under all conditions.

This article from Control magazine may be helpful to you:
http://www.expertune.com/ArtConMay99.html

You can find some more articles on tuning, using "Relative Response
Time", at http://www.expertune.com/Articles.html#PIDOpt


Good Luck!

-George

BobG wrote:
Quote:
Hi all,
Thought I would ask for opinions in this newsgroup regarding a new
project I'm involved in. I'm a PLC programmer who has been tasked with
programming a new clean in place (CIP) project for a large spray dryer and
it's associated duct work. There are 135 nozzles split unevenly between 11
defined "zones". Each zone is to wash for a predefined period of time by
sequentially actuating ball valves off the main CIP header. Differences in
elevation between the zones differ by as much as 110 feet, and there are
several pressure transmitters to be installed to detect pressure at each
zone. I'll move the PT signals into the PID instruction when the appropriate
zone is washing. Each nozzle is designed to operate at 100 p.s.i.. At the
designed pressure the nozzles will use 12 g.p.m.

The pump is a centrifugal pump and a frequency drive has already been
specified to control pressure in each zone. Having been through the
extremely unpleasant experience of having to control pressure with a drive
and a centrifugal pump I raised the possibility of using a modulating valve
to either control pressure, or deliver the designed flow rate, for ex: 12
nozzle at 8 g.p.m. for 96 g.p.m.

I was told by the chemical engineer who's running the job (he *seemed*
like a knowledgeable fellow) that a modulating valve wouldn't work and the
frequency drive was the only way to go. Because of the head. After much
discussion (most of which I didn't understand Smile ), he convinced me that as
much as I hated to hear it the drive was the way to go.

The project was approved and materials were already being delivered when
I became involved. Was he yanking my chain about the drive vs. valve issue?
Or does the head on the pump demand a frequency drive in this application?
I'll also add that there's 160 P.S.I. direct steam injection controlled by
another PID loop to the mixing chamber of a Pick heater to heat the water to
160 Deg. F. Anyone who has had to control pressure with a centrifugal pump
will understand my trepidation.
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