Question { 7754 }

if a tank is contaning oil and water than how can we
measure the oil and water level seperately

Answer

In a case like this “how to measure a interface level?” is
not a straight forward answer since I cannot know what the
vessel looks like, know what type of tap-off points is
available and so on so I will give you a generic rule of
thumb explanation on interface levels and tell you what is
the perfect solution to all interface level problems. I will
also tell you what you must not do.

Since this is a very confusing subject to a lot of
technicians and engineers, although very few will admit it,
so it might be worth the effort to do a detail explanation
on the subject.

First of all regardless of what anyone tells you, you NEVER
use a differential pressure transmitter for a oil/water
interface level application. Neither a piped nor a capillary
type DPT is suitable to measure interface level. Believe me
I have tried as well, and have done the calculations front
to back and back to front and saw that it is theoretically
possible to do, but believe me in practice it just doesn’t work.
The only application where it might have a small chance of
success is if you work on a vessel with a very stable and
constant top product overflow. In other words your top
product is always at exactly the same level and you only
measure the variance of the bottom product. I don’t thing a
application like this exist but anyway this is the theory of it.
The moment the top product level changes, even by a couple
of mm, your whole calibration is invalid and therefore your
level control becomes unstable. So if you have a application
like this you can try it but other than, that stay away from
the DPT to measure interface levels.
Further more this is whole installation is dependent on
product density so you can image for yourself what will
happen if the top or bottom product density changes so this
is just not the way to do it, it’s to unstable. It is like
standing on a knife edge. Yes you might be able to stand
straight and upright for a couple of seconds, but not for long.

a Lot of people will tell you the capacitance probe it the
answer, it’s not. The capacitance probe can only measure one
product. There is a special interface capacitance probe
available on the market but most people are not even aware
that the probe they are working with is not a interface
measuring probe and that it is just a single measuring probe.
100% of the bottom product is 0% of the top product and then
100% of the top product is 0% of the bottom product they
would say is the way to do it. In theory that makes sense
but in practice it is not so easy to do.
Image you are on a live plant and you now want the
production to fill up the vessel to 100% with only the top
product and then ask them to drain that and fill the vessel
up with the bottom product again. They will most probably
start laughing at you, thinking you are joking. I have tried
this the one time since the cap probes I was working on was
installed in a separate stand pipe on the side of the vessel
so I thought if I can fill this standpipe I will not
interfere with operations but eventually this didn’t work
either since production will not allow you to open a
pressurized vessel that is online with single isolation
only. So in theory something might sound easy to do but in
practice it is not always so easy. The normal capacitance
probe calculations are also just for measuring a single
product in a vessel under perfect conditions so they are
useless as well.

Don’t trust your sight classes either since with interface
levels most sight glasses do not work accurately all the
time. These sight glasses were designed to measure a single
product and not the variance of two products in a vessel.
Draw yourself a vessel and vary the levels of the two
products and see for yourself how it will reflect on the
sight glasses. You will find that at various points the
sight glass cannot reflect the levels accurately due to the
sight glass tap-off points. Yes I know about the overlapping
type of sight glasses but even they do not reflect the
product levels accurately all the time.

But even if they do, you still sit with a problem of
blockage when working with crude and other high viscous
products and big vessels where the level changes very
slowly. Is this now a accurate level indication or is the
sight glass blocked again?

So in theory, if you can trust your sight glass and your
sight glass is installed in a stand pipe attached to your
vessel and you can open this stand pipe up to fill and drain
it as you please, you can set these probes up like that, but
this is a lot of if’s.

The best result I eventually got with these Cap probes was
to setup my TDR radars (radars and cap probes installed on
same vessel for control and ESD) perfectly according to the
design specs and then calibrate the capacitance probes to
the same zero and span positions as the radars. This
eventually resolved the whole problem. If I had my way I
would have thrown them out and installed a TDR in their
place as well.

THE TDR RADARS ARE THE BEST INTERFACE MEASURING INSTRUMENT
ON THE MARKET TODAY.

You can accurately and reliably (up to mm) measure the top
and bottom product continuously and give two separate 4 to
20mA outputs to the CCR, one for the level of the top
product and one for just the interface level in the vessel.
The CCR is normally interested in both these readings. These
radars are not influenced by density changes, temperature
changes, pressure changes or condensation or vapor changes.
They even still work perfectly after the product have
attached itself and started building up on the probes after
long periods of time.. Most other instruments will stop
working in a case like this or start to become inaccurate
even cap probes don't like it.

The only thing that will influence them, but only slightly
is changes in the dielectric constants of the products.
The chances of these DC changes is very remote and will
hardly ever occur, so it is not a major concern. Even if
they do occur it takes two minutes to make a adjustment on
one of the parameters and it will be accurate again.
The negative side of these radars is that most technicians
and engineers find them very difficult and confusing to work
with and I know about instances where the people struggled
for three years and still could not get them working
properly and reliably. This gives a negative name and
reputation to the TDR radar, but the truth is the same as
with anything else, unless you calibrate them correctly you
cannot expect them to work correctly.
TDR is new technology and you cannot use your previous
experiences to help you understand them. You need to study
and learn them from scratch, there is no easy way. They were
specially designed to resolve the decades of problems of
measuring interface levels and they work like a charm if you
set them up properly. I would recommend you go this way as
well.

There are other devices as well like a magnetic float that
will float only on the bottom product and not on the top
product and work by magnetically closing reed switches in
series with resistors inside a guide pipe but again they
will only measure only one product per instrument.
To install one for the top and one for the bottom product
might be a solution if you need both readings but they tend
to get stuck on the guides from time to time due to product
build up, but this might be possibility if you have a clean
product. Definitely not suitable for crude. Keep in mind
these are also density dependent instruments but they will
not be as unstable as a DPT. If the products have a lot of
turbulence, you can also install them inside a round damping
standpipe opened to the product, this will make the readings
a bit more readable and stable. I would also not consider
them for critical applications only for more or less low
priority applications and just for indication.

Below is a previous write up I have done for another
question from someone struggling to understand the TDR. If
you read it the first time it will be very confusing but is
you study it for a while you will see that it is not that
difficult, and once you have actually done it in the field,
you will find it is actually quite easy to do. The main
thing is to do it the way I have described below. The
manuals is sometimes a bit confusing so I have developed my
own way to set them up and this is much easier to understand
than the procedures in the books. The ones I have worked on
is the Khrone BM100A (2x S/S rods probe type) as well as the
BM100C (coaxial probe type)
Good luck


Re: ON RADAR INTERFACE LEVEL TRANSMITTER IF DI-ELECRIC IS
WRONG WITH CONTINIUS PROCESS THAN HOW CAN INDENTIFY OR HOW
CAN I PUT NEW DI-ELECTRIC VALUE?(ROSEMOUNT-TDR-3300-COXIAL
PROBE TYPE) Answer
# 1
You said interface level so I must assume you are measuring
oil and water. My experience is on the Khrone BM100 A but
your radar might be similar since the both use TDR technology.
The dielectric constant of crude is about 2 to 4 and water
is 80. The variance in dielectric constant will have a small
effect, so in order to find the right valve one quick way is
to use your sight glass to set the dielectric constants so
that the radar reads the same as the sight glass on the
water and oil. a Good average is normally 2,5 and 80.
Unfortunately this will not work unless you have setup your
Z/S parameters correctly.
If you are still having problems you need to do a complete
setup from scratch. In order to do this you need to get hold
of the design engineer's internal vessel drawings and look
what the calibrated span should be. What you are interested
in is the exact mm measurements from vessel bottom to Z/S
points or positions. From there it is just a matter of
taking exact measurements in the field of your vessel and
your radar installation and make yourself a neat, accurate
detailed drawing to indicate the design spec positions and
the actual position of your probe in relation to the vessel,
and put these measured values in the output 1 and output 2
parameters, keeping in mind zero position is measured from
the probe bottom up and 100% is also from the probe bottom
up and not from zero position up. So if you can see where
the design engineer have said zero should be and you can see
in exactly what position your probe is in relation to those
points it is a matter of calculating how high you need to
measure up from probe bottom to get to those Z/S points
marked by the design engineer. It takes a bit of
trigonometry to do but is easy enough.

Typically these parameters should look something similar to
this. Output 1 (Top product level) 4mA = 150mm, 20mA =
2500mm, Output 2 (Bottom product) 4mA = 150mm, 20mA = 2500mm.
Look strange I know but we have found it is better to set
them both the same instead of trying to set each one to it's
individual span. Both Spans are setup in reference to actual
vessel level %. Interface is also a actual level measurement
in relation to the whole vessel and not to just half the
vessel where for instance to where your weir plate is. If
you do this it will have the effect that you have a very
sensitive interface measurement to a slow level measurement
making your control very difficult. We have done it at one
point and it worked eventually, but it was very difficult to
optimize and get stable control, and even the smallest upset
will cause everything to go crazy again.

In this example the 150mm might be the mm you need to
measure from the probe bottom up to get to the Zero position
of the actual vessel as indicated by the design drawing and
the same with the 2500mm. Obviously just examples. This is
where your exact measurements in the field comes in.
NB!!
Also make sure you have the right probe length in the "tank
Height" parameter and not the real vessel height.
This probe length is normally stamped on the little spec
plate on the head by the supplier.
Good luck!

Question { 5195 }

How can we loop powered for a transmitter.How we can do
wiring

Answer

There are only three ways to wire any instrument in the
field. We called them two, three or four wire systems.
Loop powered is the most common and widely used everywhere
to connect the field instruments to PLC's or DCS.

2 wire = 1 wire Pos 24VDC (4 - 20mA signal)(loop Powered)
1 wire 0V

3 wire = 1 wire Pos 24VDC (Instrument supply V)
1 wire 0V (Common for supply and signal)
1 wire Pos 24VDC (4 - 20mA signal)

4 wire = 1 wire Pos 24VDC (Instrument supply V)
1 wire 0V (for supply)
1 wire Pos 24VDC (4 - 20mA signal)
1 wire 0V (for signal)

Also in 4 wire system:

4 wire = 1 wire Live 110VAC (Instrument supply V)
1 wire Neutral 0V (for supply)
1 wire Pos 24VDC (4 - 20mA signal)
1 wire 0V (for signal)

Good Luck

Question { 6117 }

how to calibrate the radar transmitter for measuring the
exact tank level if tank is having agitator moving
continiously because due to agitator tank surface level
will vary every time.

Answer

Just increase the damping on the radar's 4 to 20mA output.
This should stabilize the output, even if the local display
still jumps around.
In the DCS there is also damping that will help with the
input from the radar.
You will never be able to measure an "exact" level in a
turbulent application like this so the best you can do is to
use the average over a period of time and that is what the
damping does. It smooths out the ripples in the output by
taking samples of the level measurement and then give a
average output.
Good luck

Question { 9695 }

how to define range of dp level xmeter in open tank

Answer

The question can be a bit confusing in that the word "range"
sometimes seems to mean different things to different people.
So let's just first of all clarify what we are referring to
when we use the words "range", "span" and "zero"

Range is the size or capability of the transmitter. This
means that if the specs on the transmitter says "range =
-300 to +300UOM (units of measure) it means you can
calibrate the transmitter for a total span of 600UOM and not
more. It also means you can calibrate the transmitter for
any span within this 600UOM range.
For instance you can calibrate this transmitter for -300 to
+250UOM or +140 to +300UOM and so on. Any values as long as
it falls within the range or capability of the transmitter.
You cannot in this case calibrate it for -400 to +100UOM
even though the top value is still withing the range the
bottom value is below the negative capability of the
transmitter. The area that you have calibrated is called the
calibrated span. In other words the value from zero to span
is also called the span.
a Bit confusing in that we also refer to the 100% value as
the span value and then we call the area from zero to full
scale the calibrated span as well. On Smart transmitter it
is less confusing since we here refer to the LRV and URV
instead of the zero and span values. Anyway the main thing
is to understand the difference between the range and span
values on a transmitter.
So, using the above transmitter again, you can say for
instance that you have a differential pressure transmitter
with a range of 600UOM but it has only been calibrated for a
total span of 350UOM. To give a more detail description of
how you have done this calibrated you can also say that zero
is -100UOM and span is +250UOM so the total calibrated span
is 350UOM.

Ok so back to the above question. Looking at it again I must
assume this person is trying to find out how to select the
correct range for a DP transmitter for a level application.
Obviously very important to be able to do but keep in mind
this procedure below is ONLY for a dry leg or open tank
level application and NOT for wet leg or capillary DPT's
selections.

The fist thing you need to do is measure the height of the
tank in mm. The next thing is to find the density of the
liquid you will be measuring and the third thing is to
decide where the transmitter will be installed. The first
two is easy enough to find but with the third one have one
or two extra things you want to consider. It is always
better to install any DPT below the bottom tap off point in
a close or open tank and you also need to think about the
Instrument Tech that needs to do the installation or do
maintenance on it later on. Try and install the transmitter
in a place that is easy accessible on a 2" pipe or something
like that. Also look at where the tubing or capillaries will
run, cable rack and details like this.
So finally if you have all the information you can now make
a small calculation to see what the maximum calibrated span
might be on a installation like this.
Measure the distance from the transmitter to the bottom of
your vessel and add this value to the total height of your
vessel. Let's say in total this distance is 3500mm. You
then miltiply this 3500mm with the density of the liquid you
want to measure and the answer will be the maximum
calibrated span that can ever be done on a instalation like
this. So in this case say the liquid is diesel with a sg of
0,85. The calculation will be 3500 x 0,85 = 2975mmH2O
Now you can convert this final mmH2O into any value you
prefer to work in. Sometimes the manufacturer might indicate
the range of the transmitter in Kpa or PSI so all you need
to do is convert you calculated mmH2O into that type of UOM
and you can then see how big your transmitter needs to be.

This maximum calculated value should fall in between 60 to
80% of the maximum positive capability of the transmitter
you select. If your transmitter is to big it will be very
insensitive and if it is to small you will not be able to
calibrate the correct span on it.
NB!!
There is one more important thing to remember when working
on open tanks or when you do a dry leg installations. The
calibration can only be in the positive so the range of the
transmitter you select must be marked as for exp in this
case: - 3200mmH2O to +3200mmH2O and not give a total range
of 3200 like -1600 to +1600mmH2O.
The calibration in open tanks or dry legs are always done
from atmospheric zero up into the positive area of the
transmitter and the negative area of the transmitter is not
used at all.
Watch out for this.
Good luck

Question { 5091 }

what is different betwen Process zero and atm zero?

Answer

Atmospheric zero is when you check the zero of your
transmitter with HP and LP sides of your Tx open to atmosphere.
Process zero is when you check the zero of your transmitter
with the actual process pressure, where this transmitter
will be used, applied to the HP and LP side of your
transmitter. In both cases the transmitter should read zero.
So if you will be using this Tx on a vessel with a 100Bar
process pressure you need to apply 100bar on both sides
simultaneously and make sure it still reads zero, before you
start your calibration.
With such a high pressure there will definitely be a zero
shift so you need to compensate for it during the
calibration. The lower the process pressure the smaller this
zero shift will be. If your process pressure is below 5 Bar
you do not have to concern yourself about doing a process
zero check since the zero shift is so small you may use it
as is and just do the atmospheric zero check.
If you are not sure how to do this let me know.
Good luck

Question { 6304 }

regarding level tx
i would like to know how to select the range of the level
transmitter in a closed drum(steam drum or deaerator)?

Answer

The way to find out what the right size DPT is for any level
application wet or dry, open or closed vessel is actually
very simple. It is a matter of looking at the application
where the transmitter will be installed and make a decision
what type of level installation is best suited for that
specific application. Once you know and understand how this
is done the transmitter selection process is very easy to
understand.

If you are not sure how to decide what type of DPT level
installation is best suited for what type of application,
let me know and I will do a write up on the subject.

Ok so once you know or have made your decision on the type
of installation, there is only the two ways you need to know
about in order to select the correct size transmitter. The
way to do it for a dry-leg or open tank is in a previous
writeup below, and the procedure for a wet-leg and capillary
type installation is much the same, except that based on
your total measurements taken of the tank height, including
how far the DPT will be installed below the tap off point,
you will now look at a transmitter's total range (exp.
-1200mmH2O to +1200H2O) and not just at the positive area of
the range as with dry leg and open tank selection.

The reason is that when you do the zero, the reading will go
into the maximum negative for the application due to the
wet-leg on the LP side.
Also if you still want to do the wet-leg installation in the
old fashion way, where you change the HP and LP legs around,
this range sizing method will work as well.
Also consider this method of selection when you install a
transmitter ABOVE the bottom tap-off point, and yes, it can
be done!!
This sizing method is also suitable for any capillary type
level DPT's selection.

This zero negative reading could be, as a worst case
scenario, as much as the vessel height plus the distance
from the transmitter, installed below the bottom tap off
point, to the bottom of the vessel, times the density of the
liquid in the vessel. So this total height times your fluid
density must be smaller than the negative range value of the
transmitter you select then you will be safe.
Sorry I don't think I am doing a very good job in explaining
this. an Example might be better.

Expample:
Vessel height measured from the bottom to the top = 3000mm
The height from the transmitter,(installed below the bottom
tap-off point), to the bottom tap-off point = 300mm
Density of liquid inside vessel = 0,97
So 3000 + 300 = 3300mm x 0,97 = 3201mmH2O
So the transmitter you are looking for must be bigger than
-3200 to +3200mmH2O.
Tip:
Do your calculation in mmH2O as above and see what you need.
Convert these values into the UOM that the manufacturer used
to specify the ranges.

NB.
Be careful not to confuse the level Tx's required
calibration span as the range size needed. Look at the
example below.

Example: - Wet-leg installation using smart DPT:
Vessel height ID = 3000mm
The distance from the transmitter to the bottom tap-off
point = 300mm
Density of liquid inside vessel = 0,97
Bottom tap-off point is 100mm up from vessel bottom ID
Top tap-off point is 2800mm up from vessel bottom ID.
Required zero position is 1000mm up from vessel bottom ID
Required span position is 2200mm up from vessel bottom ID
Process pressure = 20Bar
Wet-leg fluid = same as process fluid

Process zero will be about -2910mmH2O with LP leg filled and
ONLY process gas pressure(20Bar) applied to both legs.
(2800 - 100 + 300 x .97) = -2910mmH2O)
This value will be slightly different from zero at
atmospheric pressure due to the normal "zero shift under
high pressure" problem, but it doesn't matter. You use this
process zero in your calibration since this is the pressure
the DPT will work on.

Zero position on vessel from DPT = 300 - 100 + 1000 = 1200mm
Span position on vessel from DPT = 300 - 100 + 2200 = 2400mm

LRV = -2910 + (1200 x 0.97)
= -2910 + 1164 = -1746mmH2O
URV = -2910 + (2400 x 0.97)
= -2910 + 2328 = -582mmH2O

So as you can see even though your calibration is well
within the range the process zero is the factor that really
influence your selection of the correct size transmitter so
be careful with this.

All info above is suitable for steam drum or any other
pressurized vessel level applications.

Good luck

------------------------------------------------

The question can be a bit confusing in that the word "range"
sometimes seems to mean different things to different people.
So let's just first of all clarify what we are referring to
when we use the words "range", "span" and "zero"

Range is the size or capability of the transmitter. This
means that if the specs on the transmitter says "range =
-300 to +300UOM (units of measure) it means you can
calibrate the transmitter for a total span of 600UOM and not
more. It also means you can calibrate the transmitter for
any span within this 600UOM range.
For instance you can calibrate this transmitter for -300 to
+250UOM or +140 to +300UOM and so on. Any values as long as
it falls within the range or capability of the transmitter.
You cannot in this case calibrate it for -400 to +100UOM
even though the top value is still withing the range the
bottom value is below the negative capability of the
transmitter. The area that you have calibrated is called the
calibrated span. In other words the value from zero to span
is also called the span.
a Bit confusing in that we also refer to the 100% value as
the span value and then we call the area from zero to full
scale the calibrated span as well. On Smart transmitter it
is less confusing since we here refer to the LRV and URV
instead of the zero and span values. Anyway the main thing
is to understand the difference between the range and span
values on a transmitter.
So, using the above transmitter again, you can say for
instance that you have a differential pressure transmitter
with a range of 600UOM but it has only been calibrated for a
total span of 350UOM. To give a more detail description of
how you have done this calibrated you can also say that zero
is -100UOM and span is +250UOM so the total calibrated span
is 350UOM.

Ok so back to the above question. Looking at it again I must
assume this person is trying to find out how to select the
correct range for a DP transmitter for a level application.
Obviously very important to be able to do but keep in mind
this procedure below is ONLY for a dry leg or open tank
level application and NOT for wet leg or capillary DPT's
selections.

The fist thing you need to do is measure the height of the
tank in mm. The next thing is to find the density of the
liquid you will be measuring and the third thing is to
decide where the transmitter will be installed. The first
two is easy enough to find but with the third one have one
or two extra things you want to consider. It is always
better to install any DPT below the bottom tap off point in
a close or open tank and you also need to think about the
Instrument Tech that needs to do the installation or do
maintenance on it later on. Try and install the transmitter
in a place that is easy accessible on a 2" pipe or something
like that. Also look at where the tubing or capillaries will
run, cable rack and details like this.
So finally if you have all the information you can now make
a small calculation to see what the maximum calibrated span
might be on a installation like this.
Measure the distance from the transmitter to the bottom of
your vessel and add this value to the total height of your
vessel. Let's say in total this distance is 3500mm. You
then miltiply this 3500mm with the density of the liquid you
want to measure and the answer will be the maximum
calibrated span that can ever be done on a instalation like
this. So in this case say the liquid is diesel with a sg of
0,85. The calculation will be 3500 x 0,85 = 2975mmH2O
Now you can convert this final mmH2O into any value you
prefer to work in. Sometimes the manufacturer might indicate
the range of the transmitter in Kpa or PSI so all you need
to do is convert you calculated mmH2O into that type of UOM
and you can then see how big your transmitter needs to be.

This maximum calculated value should fall in between 60 to
80% of the maximum positive capability of the transmitter
you select. If your transmitter is to big it will be very
insensitive and if it is to small you will not be able to
calibrate the correct span on it.
NB!!
There is one more important thing to remember when working
on open tanks or when you do a dry leg installations. The
calibration can only be in the positive so the range of the
transmitter you select must be marked as for exp in this
case: - 3200mmH2O to +3200mmH2O and not give a total range
of 3200 like -1600 to +1600mmH2O.
The calibration in open tanks or dry legs are always done
from atmospheric zero up into the positive area of the
transmitter and the negative area of the transmitter is not
used at all.
Watch out for this.
Good luck

Question { 9395 }

What is the relationship between Sensor

Answer

Sensors do the actual measurement of a process like
pressure, temperature, vibration, speed ext. This process
measurement is then converted to a proportional electrical
signal and send to the transmitter. The transmitter transmit
this signal again to a distant receiver.
a Instrument can be a combination of both a sensor and a
transmitter or the two can be separate.
Good luck

Question { 4437 }

there is one orifice plate already installed on 3" line, but
my previous engineer(who is not working here) removed that
tx, but now i want to take it in to the process, so now what
i have to do now? flow fluid is hytherm oil of temp. 260deg
& pressure is 3bar.

Answer

In any industrial plant there will always be good records
and documentation on all equipment as well as all
instrumentation installed on the plant. The easiest in your
case is to find the FT tag number on the P&ID drawings or
the CCR faceplate and look at the data sheet of the missing
transmitter. The original calibration in differential
pressure will be on there as well as the model number of the
transmitter you need.
All you then have to do is calibrate the new DPT with the
diff press as per the data sheet and it will work as before.
In most cases the squire root extraction is done in the DCS
and not in the transmitter so try it just like this first.

The faceplate in the CCR will normally be set up to display
the min and max flow so that will not help you to find the
calibration diff pressure of the transmitter.

You can also try and find the original transmitter that was
removed and put power on it ans see what it was calibrated
for before. Even if the transmitter is faulty it might still
be able to display information. The is just as a last resort
since you cannot be sure if this calibration is what it
should be and it is better to double check it with the
documentation. This is just a method to double check
information.

As a desperate measure you can also measure the upstream and
downstream pressure as it is now during normal operation and
calibrate your transmitter to 1,5 that differential pressure.
For example:
Say your upstream pressure before the orifice is 300Kpa and
downstream it is 270Kpa you therefore have a DP over the
orifice of 30Kpa during normal operation.
Calibrate the transmitter for 0 to 45Kpa.
The flow indication inside the CCR will indicate the correct
flow since the extraction and the display is already setup
as it was originally. Have a look at what the % the flow is
indicated on the CCR faceplate. Since it is normal
operations it should be about 50% of the scale on the
faceplate. If it is not, modify the DPT calibration until
the current flow display about 50% of the full scale in the
CCR. You should be pretty close then.
This is not a very good or accurate way to do it and you
should keep on looking for the original design specs and
then re-calibrate the DPT when you found it.
Good luck

Question { Reliance, 130612 }

What is the basic principle of vibration measurement by
BENTLY NEVADA vibration measurement system? and Why it
gives the -ve voltage output?

Answer

This is a very good, accurate writeup done by someone else
for a similar question earlier on. Hope it helps.


The vibration sensors used in turbine, generator etc are all
same type i.e proximity type of sensors. The sensor are
mounted at the proximity of the Turbine and generator rotor.
From the sensors one coaxial cable runs to the proximater,
which is mounted in a separate JB nearby. From TSI Monitors,
this Vibration probes gets +24 Volts DC. The proximater
gives a signal 0 to -22.4 Volts DC depending upon the
vibration measured by the sensor. Generally the distance
between the probe tip and the rotor is maintained such that
it gives -10 Volts DC when the turbine is not running.

The sensor works on eddy current principle. As the rotor
vibration increases, the eddy current generation remains
constant, but the absorption of power in terms of eddy
current increases. This loss of power is sensed by the
proximater and DC voltage signal is transmitter to TSI
monitor for Measurement, Controlling & Indication purpose.
Good luck

Question { 5067 }

how the presure switches are calibrated? and what happen
inside?

Answer

a Pressure switch is just a small diaphragm that moves as
the pressure increases. This diaphragm is pushing a small
pin upwards and the pin is pushing a micro switch to close.
Obviously if the pressure decreases again, the pin goes down
and the switch will open again. So based on this information
it is easy to see that you just need to supply pressure to
this diaphragm with a hand pump, put your multimeter on the
switch contact in order to see when the switch open or
closes and and make the adjustment on the switch until it
switches at the point where you need it to switch. The
adjustment is normally in the form of a spring.
Increasing or decreasing the tension on this spring will
change how high or low pressure is needed to move the diaphragm.
The only thing that you need to remember when working on
pressure switches, is that if you want to calibrate the
switch as a pressure switch low you need to pump the switch
up higher than your required switching point and then
decreases the pressure slowly until the switch changes
state. The same if you want to use it as a pressure switch
high, you then need to start with a low pressure and
increase your pressure slowly until the switch changes state.
Good luck

Question { 9923 }

i want know about capillary type LT transmitter installation. we
can install any location in between to HP and LP leg?

Answer

Yes you can and don't let anyone tell you different since I
have done it myself and it works perfectly.
This is also the only type of DPT that can be installed
above the bottom tap off point.
Personally I would not install it higher than 50% between
the tap off points.

To calibrate a DPT installation like this:
Once the installation is done, and the HP and LP pad cells
are still open to atmosphere, have a look what the
transmitter display. It will be a neative value if you are
still below 50% of the distance between the tapoff points.
What ever this value is is your atmospheric zero value. Put
process gas pressure on both HP and LP pad cells
simultaneously and have a look is there is a shift in this
zero value. If there is, use this new process zero value for
your calibration.

LRV is determined by measuring from the bottom tap off point
up to your zero point on the vessel and URV is determined by
measuring from the bottom tap off point to the 100% point on
your vessel. Add these measured values to the process zero
value, times the sg. of your liquid. UOM must be mmH2O if
you took your measurements in mm.

The above procedure of determining the LRV and URV is the
same when the DPT is installed below the bottom tap off
point as well, so the calibration and calculations are the
easy part, the difficult part is to get your process zero right.

To do the process zero might be a bit difficult if you have
not prepare for it in the original design. So if your vessel
pressure is below 5 to 10Bar don't bother doing it and use
the atm zero in your calibration calculation. With 10bar and
up the shift becomes significant enough that the process
zero becomes a must.

We normally install S/S flushing rings before each pad cell
and then link these two flushing rings with a piece of half
inch tubing and three needle valves. You only need one, the
other two is just for extra protection and isolation. You
should also install a needle valve on the top flushing ring
pointing upwards and on the bottom flushing ring pointing
downwards for draining purposes.
{Use the pipe spec of the vessel to determine the specs of
what size and pressure rating the flushing rings must have.}

To then do the process zero is a simple matter of closing
the bottom tap off main isolation valve, and draining the
piece of pipe before the pad cell. Then you open the needle
valves and the process gas from the top tap off flushing
ring will pressurize the bottom pad cell and you then have a
equal amount of process gas pressure on both pad cells.
Drain the bottom again to make sure there is no liquid on
the bottom padcell. Read off your process zero value on the
DPT and compare it to the atm zero value. Always use the
process zero for your calculation since this process
pressure is where the DPT will work and not on atm. pressure.

Ever since we discovered that we still have static alignment
problems, even with SMART technology, this type of capillary
installation has become the new standard through out the
industry. If possible do this type of installation on every
capillary DPT installation above 10Bar. Everyone will thank
you for it later since it can be quite hazardous to do this
process zero with temporary fittings, valves and a flexible
piece of high pressure tubing on a vessel running at
150bar. I had to do it once and believe me it's scary.
As a rule of thumb, the higher the vessel process pressure
the higher the priority to design the installation from day
one with this flushing rings, needle valves and tubing
installed. Try and implement this wherever you can, since it
is a improvement on accuracy and safety of the installation.
Good luck

Question { 9057 }

how we can calibrate split range control valve?

Answer

Below is a previous discussion on the subject. In your
question you do not give enough detail so I cannot give a
more detailed answer than below.
If you need more detail post a new question with more detail
about your application like valves where and how installed,
valve, actuator and line sizes, type of positioners, process
details ext. Also note that split range is done on two valve
installed in parallel in the same line and not just one as
in your question.



Question
Explain Split Range Control In Control Valve Briefly.
Question Submitted By :: Jaydeep_6494u I also faced this
Question!! Rank Answer Posted By Re: Explain Split
Range Control In Control Valve Briefly. Answer
# 1
split range control is which the output of a controller is
split to two or more control valves. For eg,
For 0% controller output,valve A fully open & B fully
closed.
For 25%, Valve A 75% open,B 25% open
For 50%, Valve A& B- 50% open,
For 75%, Valve A 25%open, B-75%open..
For 100%, Valve A fully closed & valve B Fully Open..

Like this, d controller output action can be set depending
upon the application...

Is This Answer Correct ? 5 Yes
1 No

0
Arunnatraj
Re: Explain Split Range Control In Control Valve
Briefly. Answer
# 2
In addition wt ANS.1-
in split range control the control signal is split in two
ranges according to the application .for eg,4-20MA.signal
can be used as 4-12 MA. FOR VALVE A OPEN i.e.50%.& 12-20MA
for vlv b open.vice-versa.

Is This Answer Correct ? 4 Yes
0 No

0
Nilesh Karpe



Re: Explain Split Range Control In Control Valve
Briefly. Answer
# 3
Split range is as per answer 2 and not answer 1. Answer 1 is
using two valves, one is a fail open and one is a fail
close. The 4 to 20mA control signal is then controlling the
two valves but in opposite directions. This can also be done
by calibrating the one positioner as a direct acting and the
other as a reverse acting.
Yes this is 100% correct and can be done and the two valves
will work perfectly and in exactly opposite directions to
each other. In a case like this the two valves will have to
be identical as well otherwise the balancing act you are
trying to perform with this setup might not work.
Never seen a application where you would need this but if it
needs to work like that, it can be done.

Split range means you are splitting a 4 to 20mA control
signal in two.

This is very useful in applications where the product flow
increases very rapidly and then falls away again. a Good
example will be before a slug catcher where the incoming
product from the well heads is coming in suddenly very fast
and then falls away again after a couple of minutes.
Another application is to control the steam from a boiler.
At times you need a lot of steam and other times you only
need a small amount.
How do you size the valve needed to control these very big
changes. a Small valve is needed to control the normal flow
but if the flow increases the small valve will open fully
and it will still be to small to control the now very high
flow or pressure. If you install a big valve to control the
big pressure or flow the valve will only open 2 to 5% most
of the time and it will be impossible to do stable control
with such a valve opening. Say nothing about the plug and
seat that will only last a couple of days.
To control these very high variance in flow and pressures of
the process we install two valves in parallel in the same
line. Normally the one is a small valve and the other is
about twice the size of the small valve.
The design and process engineers will decide what minimum,
normal and maximum conditions are and do the valve sizing
accordingly.
We calibrate the small valve to open from fully close to
fully open with a signal of 4 to 12 mA. We then calibrate
the second bigger valve to open from fully close to full
open with a signal of 12 to 20 mA.

In a situation where you use identical valves you can take
the signal from one controller and send it to both valves
but in the above application it is better to use two
controller but with the same input from one pressure or flow
transmitter. Each valve will then be controlled individually
from its own controller and on it's own PID tuning set. I am
sure you can see that it will be quite impossible to find a
PID tuning set that are appropriate for both the big and the
small valve and it is therefore better to use two
controllers but with the same input.

During normal operations the small valve will control the
process without any problems based on the input from the
pressure or flow transmitter, and the bigger valve stays
close all the time. If the process changes to something
bigger than what the small valve can handle the second
bigger valve needs to help, and will start opening up, once
the small valve is fully open. Once the demand falls away
the big valve will start to close and then the smaller valve
until the process is back to normal operating conditions..
Both valves receive a full 4 to 20mA signal from it's own
controller, but will only react based on the 4 to 12 or 12
to 20mA calibration that was done on each valve's positioner.

In other simpler applications you can use two identical
vales and set their positioners as above and send the one
controller output signal to both. One PID tuning set will
also work for both valves and conditions.

The valve sizes will depend on what the conditions are so
they can be the same or they might need to be different sizes.
Good luck

Question { Tata Power, 9564 }

how we use the positioner feed back in split range control
valves?

Answer

The feedback positioner on any control valve is just a "nice
to have" and is not necessary for any control loop to work.
Don't get confused between a feedback control system and the
feedback positioner. The controller receives it's feedback
on how to make valve adjustments from the transmitter in the
field, not this feedback positioner. You can therefore
disconnect any feedback positioner in the field permanently,
and everything in the control loop will still work.

Some plants use it only as confirmation that the valve is
working and reacting as the controller instructed it to. In
other words if the controller send a signal out to the valve
to open 30%, the feedback positior will should send a signal
back that will indicate 30% when the valve has reached the
instructed 30% position in the field.
As you can imagine control room operators like feedback
positioner very much, and they need them to be pin point
accurate all the time.

Anyway, so in the case with split range valves, nothing
changes since you still want to see that the valve has
opened from zero to 100% regardless what signal (4 to 12mA)
it uses to do so.
In the writeup below you will see I have used two separate
controllers working in a split range configuration from a
common transmitter. So this is your solution as well if you
need to get these feedback's working. Each valve has it's
own controller output and feedback. Read the previous
writeup below and you will see what I mean.

Also note there is another way, but I am a bit reluctant to
tell you about it since I don't think this is a very good
way of doing it but anyway decide for yourself:
Some plants do not use feedback positioners at all.
Where FB positioners are not used, the position indication
on the DCS is the same as the output signal to the valve.
Yes I know it is wrong, and should not be done like this but
I think the decision was made based on the amount of call
out we had just to repair these continues discrepancies
between the output and the feedback's.
So you don't need it to do control, but if you have to have
a indication on your DCS faceplate use the same signal as
the output signal to the valve.



Question

Explain Split Range Control In Control Valve Briefly.

Question Submitted By :: Jaydeep_6494u
I also faced this Question!! Rank Answer Posted By

Re: Explain Split Range Control In Control Valve Briefly.
Answer
# 1

split range control is which the output of a controller is
split to two or more control valves. For eg,
For 0% controller output,valve A fully open & B fully
closed.
For 25%, Valve A 75% open,B 25% open
For 50%, Valve A& B- 50% open,
For 75%, Valve A 25%open, B-75%open..
For 100%, Valve A fully closed & valve B Fully Open..

Like this, d controller output action can be set depending
upon the application...


Is This Answer Correct ? 5 Yes 1 No

0
Arunnatraj

Re: Explain Split Range Control In Control Valve Briefly.
Answer
# 2

In addition wt ANS.1-
in split range control the control signal is split in two
ranges according to the application .for eg,4-20MA.signal
can be used as 4-12 MA. FOR VALVE A OPEN i.e.50%.& 12-20MA
for vlv b open.vice-versa.


Is This Answer Correct ? 4 Yes 0 No

0
Nilesh Karpe



Re: Explain Split Range Control In Control Valve Briefly.
Answer
# 3

Split range is as per answer 2 and not answer 1. Answer 1 is
using two valves, one is a fail open and one is a fail
close. The 4 to 20mA control signal is then controlling the
two valves but in opposite directions. This can also be done
by calibrating the one positioner as a direct acting and the
other as a reverse acting.
Yes this is 100% correct and can be done and the two valves
will work perfectly and in exactly opposite directions to
each other. In a case like this the two valves will have to
be identical as well otherwise the balancing act you are
trying to perform with this setup might not work.
Never seen a application where you would need this but if it
needs to work like that, it can be done.

Split range means you are splitting a 4 to 20mA control
signal in two.

This is very useful in applications where the product flow
increases very rapidly and then falls away again. a Good
example will be before a slug catcher where the incoming
product from the well heads is coming in suddenly very fast
and then falls away again after a couple of minutes.
Another application is to control the steam from a boiler.
At times you need a lot of steam and other times you only
need a small amount.
How do you size the valve needed to control these very big
changes. a Small valve is needed to control the normal flow
but if the flow increases the small valve will open fully
and it will still be to small to control the now very high
flow or pressure. If you install a big valve to control the
big pressure or flow the valve will only open 2 to 5% most
of the time and it will be impossible to do stable control
with such a valve opening. Say nothing about the plug and
seat that will only last a couple of days.
To control these very high variance in flow and pressures of
the process we install two valves in parallel in the same
line. Normally the one is a small valve and the other is
about twice the size of the small valve.
The design and process engineers will decide what minimum,
normal and maximum conditions are and do the valve sizing
accordingly.
We calibrate the small valve to open from fully close to
fully open with a signal of 4 to 12 mA. We then calibrate
the second bigger valve to open from fully close to full
open with a signal of 12 to 20 mA.

In a situation where you use identical valves you can take
the signal from one controller and send it to both valves
but in the above application it is better to use two
controller but with the same input from one pressure or flow
transmitter. Each valve will then be controlled individually
from its own controller and on it's own PID tuning set. I am
sure you can see that it will be quite impossible to find a
PID tuning set that are appropriate for both the big and the
small valve and it is therefore better to use two
controllers but with the same input.

During normal operations the small valve will control the
process without any problems based on the input from the
pressure or flow transmitter, and the bigger valve stays
close all the time. If the process changes to something
bigger than what the small valve can handle the second
bigger valve needs to help, and will start opening up, once
the small valve is fully open. Once the demand falls away
the big valve will start to close and then the smaller valve
until the process is back to normal operating conditions..
Both valves receive a full 4 to 20mA signal from it's own
controller, but will only react based on the 4 to 12 or 12
to 20mA calibration that was done on each valve's positioner.

In other simpler applications you can use two identical
vales and set their positioners as above and send the one
controller output signal to both. One PID tuning set will
also work for both valves and conditions.

The valve sizes will depend on what the conditions are so
they can be the same or they might need to be different sizes.
Good luck

Question { 7173 }

in split range control system we are using two control
valves,but we have only one positioner and one feed back
link.so how we use this one feed back link for two control
valves?

Answer

If I read correctly between the lines, it seems that you are
doing a modification. The “one positioner” and “one
feedback”, I have to assume must be from the original valve
that was in there. You therefore only have two cables
available to work four signals. One cable from the CCR for
the control and another one for the feedback signal back to
the CCR.
I understand why you would prefer not to pull in any extra
cables. These kind of modifications are big and troublesome
and pulling in new cables will mean documentation updates
and DCS software modifications and all the rest.

Ok so, I will do my write up based on the assumptions that
you only have the two cables, one positioner and one
feedback positioner to work with and that you have already
installed two valves in parallel in the same line. I will
also assume the original valve is one of the valves you have
uses and we will call that the small valve. The second valve
which should be a bigger valve is installed in a bypass line
in parallel with the original valve in the same line, and we
will call that the big valve.

All stand alone FB positioners work with a variable
resistor(VR). Movement of the valve stem will turn this VR
and the resistance will change of this VR. This resistance
is then converted by the electronics of the FB positioner
into the 4 to 20 mA signal back to the CCR controller. You
need to measure this resistance and also see if it increases
or decreases during the upward movement of the valve stem.
Based on this you need to replace this VR with a smaller or
bigger VR in order to get only half or double the resistance
of the original full stroke value.
In other words if the resistance value in the original valve
changed from 0 to 1000 Ohm during a full, 0 to 100%, stroke
of the valve, the resistance will only be 500 Ohm at 50% of
the valve stroke.
Assuming the resistance value increases with upward movement
you need to change to VR with a 50% smaller VR (0 to 500
ohm) and install a identical VR in the big valve as well. If
you now connect them in series they will give you a total of
1000 Ohm which is the same as your original feedback
positioner resistance when you used only one valve. Assuming
the calibration of this FB positioner is still as it was in
the original valve, at 100% stroke the FB positioner will
only see 500 Ohm and the output to the CCR will only be
12Ma. As the big valve starts to open up the resistance of
the VR in the big valve will start to add itself to the now
fully open small valve’s resistance and the mA out to the
CCR will change accordingly until both valves are fully
open. In which case the total resistance will be as per your
original valve’s resistance, 1000 Ohm, and therefore your mA
out will be 20mA.

To do this:
1)
Small original valve:
Install the original FB positioner on the valve and connect
feedback signal cable to CCR as per normal for a one valve
installation. Do a calibration on it as normal. Stroke it up
and down and measure the VR’s resistance change and then
disconnect the variable resistor and remove it completely.
Replace it with the newly calculated VR.
2)
Valve Big:
Either install a new small JB on the valve, with the newly
calculated variable resistor in it or a complete new FB
positioner, if available. If you install a new FB
positioner, disconnect the original variable resistor inside
and remove it completely. Replace it with the newly
calculated VR.

To calculate the new variable resistors values:
Values used are examples only.
Exp.
Original VR value = 0 to 1000 Ohm increasing with valve opening.
R1 refers to the new VR you will install on the small valve.
R2 refers to the new VR you will install on the big valve.

RT = R1 + R2 = 500 + 500 = 1000 Ohm

Replace the original variable resistors in each FB
positioner with the newly calculated variable resistors (in
exp = 500 Ohm) and connect them in series and then to the
small valve FB positioner electronics.

Suggestion how to do this:
Use other gland entry on the small valve FB positioner and
connect a instrument cable to a small 2 way JB mounted close
by. From this JB connect another cable to the big valve FB
postioner. Connect the two variable resistors in series and
then back to the PCB inside the small valve’s FB positioner.

Another way to do this is to install the VR’s on each valve
and then connect them to a remote mounted SMART temperature
transmitter. The temperature transmitter is then configured
with the HART to take a resistance input instead of “RTD
input” and VOILA you have built your own FB positioner.



About the “only one” control positioner

To use only one positioner and try and do split range
control is NOT possible. I think you are more concerned
about pulling in new cables and it is not a matter of that
you do not have any more positioners available. I am sure
you can get another one from your stores.

Ok so to do the split range control you need to get another
positioner and install one positioner on each valve.
Make use of the second gland opening in the small valve’s
positioner and just daisy-chain( connect in parallel) the
wires to the big valve from the small valve's positioner.

Both valve positioners will now receive the full 4 to 20 mA
signal from the controller in the CCR simultaneously, and
that’s what you want. Calibrate the positioner on the small
valve for 4 to 12mA = 0 to 100% stroke and the big valve 12
to 20 mA = 0 to 100% stroke.

Ok to summarize, your two positioners will be controlled
from a single controller in the CCR. During normal
operations, the small valve will control the process and the
operator will see a feedback in the CCR of below 50%. This
is in relation to both valves and not just one valve, so you
need to explain this to the operator before you sign this
mod off as complete.
If the process changes and the small valve cannot handle the
process, the bigger valve will starts to open up and the
operator will see a feedback of more than 50% in the CCR.
Again explain and train the CCR operator on this control
system and the way it works.

You might find it difficult to find a PID tuning set that
will suit both valves, due to the size difference, but you
might be able to do it if you compromise a little on both.
In other words neither will give perfect control but you
will get them to control good enough with a average PID set.

Above is just one way to do this and it will depend on what
you use out there and what is available.

Some basic rules of thumb about installations and positioner
types:
On all the split range installations I have worked on we
used a pneumatic positioner with internal or a remote
mounted I/P converter. The reason for the pneumatic
positioner instead of a SMART positioner, as well as the
remote mounting of the I/P, was heat. The electronics don’t
like too much heat so in very hot areas, rather use a
pneumatic positioner with a I/P remotely mounted where it is
a bit cooler. You can do this for the feedback as well by
installing the VR’s in small JB’s on the valves and mount
the electronics where it is a bit cooler.
Good luck

Question { 6690 }

in split range control valve,two valve should be in two
different line and that should have some distance,and we
have only one positioner feed back link.so how we connect
one feed back in two control valves?if you have picture
please forward me.my email id kb.syamkumar@gmail.com

Answer

Before we start, something in your question is a bit of a
concern. Split range control is done by installing two
valves in parallel in the SAME line, and NOT two different
lines. The easiest to do this modification is to built a
bypass line beside the original valve and install your
second valve in this bypass line right next to the original
valve. The bypass line only starts only about 1 meter before
and end about 1 meter after the original valve.

Ok back to your question.

If I read correctly between the lines, it seems that you are
doing a modification. The “one positioner” and “one
feedback”, I have to assume must be from the original valve
that was in there. You therefore only have two cables
available to work four signals. One cable from the CCR for
the control and another one for the feedback signal back to
the CCR.
I understand why you would prefer not to pull in any extra
cables. These kind of modifications are big and troublesome
and pulling in new cables will mean documentation updates
and DCS software modifications and all the rest.

Ok so, I will do my write up based on the assumptions that
you only have the two cables, one positioner and one
feedback positioner to work with and that you have already
installed two valves in parallel in the same line. I will
also assume the original valve is one of the valves you have
uses and we will call that the small valve. The second valve
which should be a bigger valve is installed in a bypass line
in parallel with the original valve in the same line, and we
will call that the big valve.

All stand alone FB positioners work with a variable
resistor(VR). Movement of the valve stem will turn this VR
and the resistance will change of this VR. This resistance
is then converted by the electronics of the FB positioner
into the 4 to 20 mA signal back to the CCR controller. You
need to measure this resistance and also see if it increases
or decreases during the upward movement of the valve stem.
Based on this you need to replace this VR with a smaller or
bigger VR in order to get only half or double the resistance
of the original full stroke value.
In other words if the resistance value in the original valve
changed from 0 to 1000 Ohm during a full, 0 to 100%, stroke
of the valve, the resistance will only be 500 Ohm at 50% of
the valve stroke.
Assuming the resistance value increases with upward movement
you need to change to VR with a 50% smaller VR (0 to 500
ohm) and install a identical VR in the big valve as well. If
you now connect them in series they will give you a total of
1000 Ohm which is the same as your original feedback
positioner resistance when you used only one valve. Assuming
the calibration of this FB positioner is still as it was in
the original valve, at 100% stroke the FB positioner will
only see 500 Ohm and the output to the CCR will only be
12Ma. As the big valve starts to open up the resistance of
the VR in the big valve will start to add itself to the now
fully open small valve’s resistance and the mA out to the
CCR will change accordingly until both valves are fully
open. In which case the total resistance will be as per your
original valve’s resistance, 1000 Ohm, and therefore your mA
out will be 20mA.

To do this:
1)
Small original valve:
Install the original FB positioner on the valve and connect
feedback signal cable to CCR as per normal for a one valve
installation. Do a calibration on it as normal. Stroke it up
and down and measure the VR’s resistance change and then
disconnect the variable resistor and remove it completely.
Replace it with the newly calculated VR.
2)
Valve Big:
Either install a new small JB on the valve, with the newly
calculated variable resistor in it or a complete new FB
positioner, if available. If you install a new FB
positioner, disconnect the original variable resistor inside
and remove it completely. Replace it with the newly
calculated VR.

To calculate the new variable resistors values:
Values used are examples only.
Exp.
Original VR value = 0 to 1000 Ohm increasing with valve opening.
R1 refers to the new VR you will install on the small valve.
R2 refers to the new VR you will install on the big valve.

RT = R1 + R2 = 500 + 500 = 1000 Ohm

Replace the original variable resistors in each FB
positioner with the newly calculated variable resistors (in
exp = 500 Ohm) and connect them in series and then to the
small valve FB positioner electronics.

Suggestion how to do this:
Use other gland entry on the small valve FB positioner and
connect a instrument cable to a small 2 way JB mounted close
by. From this JB connect another cable to the big valve FB
postioner. Connect the two variable resistors in series and
then back to the PCB inside the small valve’s FB positioner.

Another way to do this is to install the VR’s on each valve
and then connect them to a remote mounted SMART temperature
transmitter. The temperature transmitter is then configured
with the HART to take a resistance input instead of “RTD
input” and VOILA you have built your own FB positioner.



About the “only one” control positioner

To use only one positioner and try and do split range
control is NOT possible. I think you are more concerned
about pulling in new cables and it is not a matter of that
you do not have any more positioners available. I am sure
you can get another one from your stores.

Ok so to do the split range control you need to get another
positioner and install one positioner on each valve.
Make use of the second gland opening in the small valve’s
positioner and just daisy-chain( connect in parallel) the
wires to the big valve from the small valve's positioner.

Both valve positioners will now receive the full 4 to 20 mA
signal from the controller in the CCR simultaneously, and
that’s what you want. Calibrate the positioner on the small
valve for 4 to 12mA = 0 to 100% stroke and the big valve 12
to 20 mA = 0 to 100% stroke.

Ok to summarize, your two positioners will be controlled
from a single controller in the CCR. During normal
operations, the small valve will control the process and the
operator will see a feedback in the CCR of below 50%. This
is in relation to both valves and not just one valve, so you
need to explain this to the operator before you sign this
mod off as complete.
If the process changes and the small valve cannot handle the
process, the bigger valve will starts to open up and the
operator will see a feedback of more than 50% in the CCR.
Again explain and train the CCR operator on this control
system and the way it works.

You might find it difficult to find a PID tuning set that
will suit both valves, due to the size difference, but you
might be able to do it if you compromise a little on both.
In other words neither will give perfect control but you
will get them to control good enough with a average PID set.

Above is just one way to do this and it will depend on what
you use out there and what is available.

Some basic rules of thumb about installations and positioner
types:
On all the split range installations I have worked on we
used a pneumatic positioner with internal or a remote
mounted I/P converter. The reason for the pneumatic
positioner instead of a SMART positioner, as well as the
remote mounting of the I/P, was heat. The electronics don’t
like too much heat so in very hot areas, rather use a
pneumatic positioner with a I/P remotely mounted where it is
a bit cooler. You can do this for the feedback as well by
installing the VR’s in small JB’s on the valves and mount
the electronics where it is a bit cooler.
Good luck