Answer / sam

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

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