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Home > Archive > Electrical Engineering > July 2007 > Unusual Diff. Pressure switch sought
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Unusual Diff. Pressure switch sought
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| Iconoclast 2007-07-14, 8:25 pm |
| I'm dealing with a control problem that involves a differential pressure
(D.P.) switch
mounted across the check valve after the discharge of a pump.The pump will
be one of four discharging into a common header. The check valve will have a
motor operator on it to open and close it. Pressure in the header will
normally be in the range of 70 - 100 psig.
Before the pump starts, header pressure will initially be higher than the
pump's
pressure, of course; but with the check valve closed, discharge pressure
will rapidly
build and soon exceed header pressure after the pump starts. The D.P. switch
will soon
sense the difference in pressure and the switch's contact will close, and
cause the valve to open. Once the valve's motor actuator gets an open
command, its own circuit will cause it to seal-in and drive the valve to the
full-open position, until a limit switch stops the motor.
Ideally, what I'd like to find, though, is a D.P. switch with a contact that
reopens
when pressure AFTER the check valve exceeds pressure BEFORE the valve (in
case the pump malfunctions mechanically, and there is backflow.)
Has anyone ever heard of such a D.P. switch? And if so, does anyone have a
manufacuter and model number?
I know I could do this with a D.P. transmitter, DC power supply and a signal
module that changes a contact's state when the transmitter's current reaches
a certain level, but
I'm hoping a simple switch could do it instead.
I hope someone can suggest something. But then maybe I'm hoping for too
much.
Iconoclast
| |
| Long Ranger 2007-07-15, 3:25 am |
|
"Iconoclast" <mj.hamill@sbcglobal.net> wrote in message
news:vmemi.46351$5j1.35081@newssvr21.news.prodigy.net...
> I'm dealing with a control problem that involves a differential pressure
> (D.P.) switch
> mounted across the check valve after the discharge of a pump.The pump will
> be one of four discharging into a common header. The check valve will have
> a motor operator on it to open and close it. Pressure in the header will
> normally be in the range of 70 - 100 psig.
>
> Before the pump starts, header pressure will initially be higher than the
> pump's
> pressure, of course; but with the check valve closed, discharge pressure
> will rapidly
> build and soon exceed header pressure after the pump starts. The D.P.
> switch will soon
> sense the difference in pressure and the switch's contact will close, and
> cause the valve to open. Once the valve's motor actuator gets an open
> command, its own circuit will cause it to seal-in and drive the valve to
> the full-open position, until a limit switch stops the motor.
>
> Ideally, what I'd like to find, though, is a D.P. switch with a contact
> that reopens
> when pressure AFTER the check valve exceeds pressure BEFORE the valve (in
> case the pump malfunctions mechanically, and there is backflow.)
>
> Has anyone ever heard of such a D.P. switch? And if so, does anyone have a
> manufacuter and model number?
>
> I know I could do this with a D.P. transmitter, DC power supply and a
> signal module that changes a contact's state when the transmitter's
> current reaches a certain level, but
> I'm hoping a simple switch could do it instead.
>
> I hope someone can suggest something. But then maybe I'm hoping for too
> much.
>
> Iconoclast
>You need a little micro-controller, and two analog pressure sensors.
| |
| Cameron Dorrough 2007-07-15, 8:25 pm |
| "Iconoclast" <mj.hamill@sbcglobal.net> wrote in message
news:vmemi.46351$5j1.35081@newssvr21.news.prodigy.net...
> I'm dealing with a control problem that involves a differential pressure
> (D.P.) switch
> mounted across the check valve after the discharge of a pump.The pump will
> be one of four discharging into a common header. The check valve will have
> a motor operator on it to open and close it. Pressure in the header will
> normally be in the range of 70 - 100 psig.
>
> Before the pump starts, header pressure will initially be higher than the
> pump's pressure, of course; but with the check valve closed, discharge
> pressure will rapidly build and soon exceed header pressure after the
> pump starts. The D.P. switch will soon sense the difference in pressure
> and the switch's contact will close, and cause the valve to open. Once
> the valve's motor actuator gets an open command, its own circuit will
> cause it to seal-in and drive the valve to the full-open position, until a
> limit switch stops the motor.
I've been around a lot of pumping installations over the past few decades
and can't recall seeing anything like you describe above (it actually sounds
a bit dangerous to me).
Pardon my ignorance, but why does the check valve need to be motor operated
when a simple swing-check would do??
> Ideally, what I'd like to find, though, is a D.P. switch with a contact
> that reopens when pressure AFTER the check valve exceeds pressure
> BEFORE the valve (in case the pump malfunctions mechanically, and
> there is backflow.)
>
> Has anyone ever heard of such a D.P. switch? And if so, does anyone have a
> manufacuter and model number?
>
> I know I could do this with a D.P. transmitter, DC power supply and a
> signal module that changes a contact's state when the transmitter's
> current reaches a certain level, but I'm hoping a simple switch could do
> it instead.
United Electric www.ue.com make a huge range of diaphragm-style DP switches
and we've specificied many of their J120K-series over the years on pumping
applications.
I hope this helps.
Cameron:-)
| |
| Iconoclast 2007-07-15, 8:25 pm |
|
"Cameron Dorrough" <cdorrough@nortonconsultants.com> wrote in message
news:f7e8nq$d90$1@news-01.bur.connect.com.au...
> "Iconoclast" <mj.hamill@sbcglobal.net> wrote in message
> news:vmemi.46351$5j1.35081@newssvr21.news.prodigy.net...
>
> I've been around a lot of pumping installations over the past few decades
> and can't recall seeing anything like you describe above (it actually
> sounds a bit dangerous to me).
Originally, I meant to have the valve open on high differential pressure
(switch contact closes) and then close on low D.P. as the contact opened.
The problem was, at low flows on these pumps, the D.P. across the valve
would be so low the contact would open. Since this was conceived, I've
learned that this valve, made by Golden-Anderson here in the U.S., will
close like a conventional check valve if pressure on its outlet exceeds
pressure on its inlet. As its closing, a (separate) high pressure switch
between the pump's discharge and the check valve will cause the pump to
trip.
> Pardon my ignorance, but why does the check valve need to be motor
> operated when a simple swing-check would do??
The client does not trust a conventional mechanical check valve in this
application (we'll be controlling pressure in the header downstream of four
pumps, each with a variable speed drive.) The maintenance supervisor
believes in this case, the pump will start, and do nothing for a bit, and
then flow will come gushing out in a burst, making pressure regulation hard.
These motorized check valves will open (and close) gradually.
>
>
> United Electric www.ue.com make a huge range of diaphragm-style DP
> switches and we've specificied many of their J120K-series over the years
> on pumping applications.
>
> I hope this helps.
>
> Cameron:-)
Thanks for the feedback.
Iconoclast
| |
| BFoelsch 2007-07-16, 3:25 am |
|
"Iconoclast" <mj.hamill@sbcglobal.net> wrote in message
news:Awymi.1533$Dx2.1116@newssvr17.news.prodigy.net...
>
> "Cameron Dorrough" <cdorrough@nortonconsultants.com> wrote in message
> news:f7e8nq$d90$1@news-01.bur.connect.com.au...
pressure[color=darkred]
the[color=darkred]
until[color=darkred]
decades[color=darkred]
>
> Originally, I meant to have the valve open on high differential pressure
> (switch contact closes) and then close on low D.P. as the contact opened.
> The problem was, at low flows on these pumps, the D.P. across the valve
> would be so low the contact would open. Since this was conceived, I've
> learned that this valve, made by Golden-Anderson here in the U.S., will
> close like a conventional check valve if pressure on its outlet exceeds
> pressure on its inlet. As its closing, a (separate) high pressure switch
> between the pump's discharge and the check valve will cause the pump to
> trip.
>
>
> The client does not trust a conventional mechanical check valve in this
> application (we'll be controlling pressure in the header downstream of
four
> pumps, each with a variable speed drive.) The maintenance supervisor
> believes in this case, the pump will start, and do nothing for a bit, and
> then flow will come gushing out in a burst, making pressure regulation
hard.
> These motorized check valves will open (and close) gradually.
>
have[color=darkred]
do[color=darkred]
>
> Thanks for the feedback.
>
> Iconoclast
Using pressure switches as you describe is a major hassle. Using the check
valves themselves to put the pumps on line is by far the cleanest way to do
this.
The standard way to address this combination of events is to design a
"shutdown" mode into the pump sequence.
As the pump accelerates, it pushes the check open and follows the command
from the pressure controller. When the pump is to stop, the VFD decelerates
the pump to a speed where there is about 10% backflow through the pump. This
backflow permits the check to close, and a limit switch detects the closure
of the check valve and only then completely stops the pump. If the check
valve doesn't close, the pump doesn't stop, and the 10% reverse flow
prevents the pump from overheating. If the pump is again needed, it simply
accelerates, the flow reverses and the pump picks up its load. Even though
the check is stuck the system stays in control. If the valve were to stick
and the pump were simply shut off, it would backspin, drawing large
quantities of fluid out of the header and very possibly damaging the pump.
Too, a backspinning pump is very difficult to restart. The shutdown scheme
avoids all this grief.
If you have never done it before, don't underestimate the issue of
sequencing and staging the pumps. Depending on the system curve, you may use
almost none of the speed range of the VFDs. Also consider that, when pumping
into a common header, pump speeds must be matched; running one pump at 100%
and the next one at any speed less than that will result in no flow from the
second pump! This is perhaps the origin of the maintenance supervisor's
concern, some doubtful schemes try to run one pump at full speed and an
additional one a bit slower to provide incremental additional flow. This
makes the second pump go from zero to full flow with a very small change in
speed, and is nearly impossible to control satisfactorily.
| |
| Jerry Avins 2007-07-16, 5:25 pm |
| Cameron Dorrough wrote:
...
> I've been around a lot of pumping installations over the past few decades
> and can't recall seeing anything like you describe above (it actually sounds
> a bit dangerous to me).
>
> Pardon my ignorance, but why does the check valve need to be motor operated
> when a simple swing-check would do??
I had the same questions when I read Iconoclast's post. It seems to me
that the plant designers want to make motor-controlled (butterfly?)
valves behave like check valves by adding instrumentation. As you say,
that seems to be -- expense aside -- a possibly dangerous and certainly
cumbersome approach.
...
Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
| |
| Jerry Avins 2007-07-16, 5:25 pm |
| Iconoclast wrote:
...
> The client does not trust a conventional mechanical check valve in this
> application (we'll be controlling pressure in the header downstream of four
> pumps, each with a variable speed drive.) The maintenance supervisor
> believes in this case, the pump will start, and do nothing for a bit, and
> then flow will come gushing out in a burst, making pressure regulation hard.
> These motorized check valves will open (and close) gradually.
Then bring the pump up to speed gradually. There's no need to close a
check valve slowly. It closes by itself when flow stops; that's
completely shockless. A motorized valve that closes slowly must brake a
backflow.
Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
| |
| daestrom 2007-07-16, 8:25 pm |
|
"Jerry Avins" <jya@ieee.org> wrote in message
news:r6KdnSBy4LRITAbbnZ2dnUVZ_r63nZ2d@rcn.net...
> Iconoclast wrote:
>
> ...
>
>
> Then bring the pump up to speed gradually. There's no need to close a
> check valve slowly. It closes by itself when flow stops; that's completely
> shockless.
Well, that's not true. With fixed speed pumps, the water-hammer from
check-valve slam can be quite severe. It's a function of how fast the pump
costs down and how far the valve disk has to travel from full-open to
fully-shut. If the valve travel is very large (like a reverse-angled seat),
the reverse flow can build up considerably before the valve reaches the
seat. And depending on the piping configuration you can get hydraulic
shocks of several hundred psi.
That is why VFD systems have the 'shutdown mode' that BFoelsch discussed.
By lowering the speed only partially, the reverse flow doesn't build up to
nearly the same flow rate and then when the valve seats there is much less
water-hammer.
daestrom
daestrom
| |
| daestrom 2007-07-16, 8:25 pm |
|
"Iconoclast" <mj.hamill@sbcglobal.net> wrote in message
news:Awymi.1533$Dx2.1116@newssvr17.news.prodigy.net...
>
> "Cameron Dorrough" <cdorrough@nortonconsultants.com> wrote in message
> news:f7e8nq$d90$1@news-01.bur.connect.com.au...
>
> Originally, I meant to have the valve open on high differential pressure
> (switch contact closes) and then close on low D.P. as the contact opened.
> The problem was, at low flows on these pumps, the D.P. across the valve
> would be so low the contact would open. Since this was conceived, I've
> learned that this valve, made by Golden-Anderson here in the U.S., will
> close like a conventional check valve if pressure on its outlet exceeds
> pressure on its inlet. As its closing, a (separate) high pressure switch
> between the pump's discharge and the check valve will cause the pump to
> trip.
>
So what you have is commonly referred to as a 'stop-check' valve. When the
stem is withdrawn, it acts like an ordinary check valve. However, the stem
can push/hold the disk in the shut position when run in.
Using D.P would be difficult in this sort of thing because of the VFD. Take
the case of one pump running at some speed supplying the fluid flow. The
D.P. across the full open check valve would be rather low. Now, you want to
start the second pump and place it in service. If you run it up to the same
speed as the running unit, the D.P across the shut stop-check will depend on
how much flow the running pump is supplying. If is supplying very little,
then the discharge pressure of the two pumps will be very nearly equal and
the DP across the starting unit will also be very low. If the flow of the
running pump is high, then there will be a difference in pressure between
the already running and starting pump. But exactly how much difference in
pressure depends on the pump curves. If the pump curves are fairly 'flat',
then there won't be much difference, if they are 'steep', then there will be
a difference.
So your use of DP will have reliability issues, it may open the valve
sometimes (enough difference in discharge pressure caused by flow on a
'steep' pump curve), or it may not.
You don't go into a lot of details about these pumps. I *assume* they are
the centrifugal type. If the pump motor starts, how likely is a mechanical
failure going to prevent the pump from developing head? Most designs don't
consider this sort of failure to be very likely. Many set ups that I've
seen will simply tie the valve operator to the pump start/stop logic. For
starting, first start the pump and after a short time delay open the valve.
For stopping, first shut the valve and when it reaches fully-shut, stop the
pump. This sort of logic is very easy compared to what you're proposing.
daestrom
>
> The client does not trust a conventional mechanical check valve in this
> application (we'll be controlling pressure in the header downstream of
> four pumps, each with a variable speed drive.) The maintenance supervisor
> believes in this case, the pump will start, and do nothing for a bit, and
> then flow will come gushing out in a burst, making pressure regulation
> hard. These motorized check valves will open (and close) gradually.
>
>
> Thanks for the feedback.
>
> Iconoclast
>
| |
| Mike Lamond 2007-07-16, 8:25 pm |
|
"BFoelsch" <BFoelsch@comcast.ditch.this.net> wrote in message
news:QrmdnSIkJfkwbwfbnZ2dnUVZ_hadnZ2d@giganews.com...
>
> Using pressure switches as you describe is a major hassle. Using the check
> valves themselves to put the pumps on line is by far the cleanest way to
> do
> this.
>
> The standard way to address this combination of events is to design a
> "shutdown" mode into the pump sequence.
>
> As the pump accelerates, it pushes the check open and follows the command
> from the pressure controller. When the pump is to stop, the VFD
> decelerates
> the pump to a speed where there is about 10% backflow through the pump.
> This
> backflow permits the check to close, and a limit switch detects the
> closure
> of the check valve and only then completely stops the pump. If the check
> valve doesn't close, the pump doesn't stop, and the 10% reverse flow
> prevents the pump from overheating. If the pump is again needed, it simply
> accelerates, the flow reverses and the pump picks up its load. Even though
> the check is stuck the system stays in control. If the valve were to stick
> and the pump were simply shut off, it would backspin, drawing large
> quantities of fluid out of the header and very possibly damaging the pump.
> Too, a backspinning pump is very difficult to restart. The shutdown scheme
> avoids all this grief.
>
> If you have never done it before, don't underestimate the issue of
> sequencing and staging the pumps. Depending on the system curve, you may
> use
> almost none of the speed range of the VFDs. Also consider that, when
> pumping
> into a common header, pump speeds must be matched; running one pump at
> 100%
> and the next one at any speed less than that will result in no flow from
> the
> second pump! This is perhaps the origin of the maintenance supervisor's
> concern, some doubtful schemes try to run one pump at full speed and an
> additional one a bit slower to provide incremental additional flow. This
> makes the second pump go from zero to full flow with a very small change
> in
> speed, and is nearly impossible to control satisfactorily.
>
>
About 10 years ago, I did the pump controls for a central sewage pump
station
that used a scheme more like the G-A check valves. The pumps were 5 x 200
HP, rated for 16MGD with VFD control and hydraulically actuated rotary check
valves on the 30" discharge lines. The pumps started on deadhead, and once
the
pump speed was high enough the discharge pressure closed a _gauge_ pressure
switch which triggered the check valve to open. On shutdown, one relay
dropped
out and triggered the valve to close,while a second relay kept the pump
running
until it was unlatched by the valve closed limit switch.
That worked, until the day the wetwell level reached an extreme high and
caused
at least one check valve to open with its pump still stopped. (No interlock
between
my pump PLC and the valve supplier's PLC.) The engineering firm wrote us a
change order to replace the switches with transmitters wired to my PLC and
add logic to open the check valve only when the pump was running and the
pump
discharge pressure to wetwell level was above a minimum.
I agree about the sequencing and staging. The sequence changed only when
just
one pump was running, and multiple pumps ran at the same speed.
Mike
| |
| Jerry Avins 2007-07-17, 9:25 am |
| daestrom wrote:
>
> "Jerry Avins" <jya@ieee.org> wrote
>
> Well, that's not true. With fixed speed pumps, the water-hammer from
> check-valve slam can be quite severe. It's a function of how fast the
> pump costs down and how far the valve disk has to travel from full-open
> to fully-shut. If the valve travel is very large (like a reverse-angled
> seat), the reverse flow can build up considerably before the valve
> reaches the seat. ...
Evidently there are more check valves that were dreamt of in my
philosophy. Thanks for the update.
Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
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