Question { 4245 }

purpose of absolute ppressure transmitter
applcation?

Answer

You first need to understand what pressure is. Without a
thorough understanding of pressure you will never understand
some of the basic concepts of instrumentation.

Air have weight. This can be proven by a simple primary
school experiment of taking a balancing beam and attached
two balloons to it. With the two balloons deflated the
balancing beam hangs exactly horizontal and the weight is
obviously equal on both sides of the beam.
If you now inflate one balloon the balancing beam will drop
down on the inflated end proving that air have a definite
and specific weight, since the volume of air on the one side
is now more than on the deflated side and therefore the
weight is more.
Pressure is measured on the surface of the earth as the
weight of the air from space pushing down on the surface of
the earth. The reason air and any other body have a weight
is due to the earth's gravity.
Since the gravity of the earth pull less the further you
move away from the earth's surface, the atmospheric pressure
changes as well and become less since the weight of air from
that point to space is less. Therefore on top of the
Himalayas the atmospheric pressure is much less than at sea
level. It takes a lot of years to get use to the low density
of the air up there and us normal people that are use to
higher density air find it difficult to breath up there. The
reason is the lower atmospheric pressure or the lower
density of the air due to the shorter distance from space to
that height.
We all know that water is always at the same level due to
the earth's gravity being the same all over the world, so
therefore if we look at the level of the sea it will be at
the same level in Canada as it is in Africa and the same at
the coast in China. It is therefore accurate to use the sea
as our reference point when we measure the atmospheric
pressure. So we therefore always refer to world standard
atmospheric pressure as it is at sea level and is therefore
a standard throughout the world. In simple terms the
distance from sea level to space at any place on the surface
of the earth is exactly the same. Therefore the atmospheric
pressure at sea level any place on earth is the same. This
atmospheric air weight is the reason why we have a
measurable amount of air pressure and is therefore referred
to as the atmospheric air pressure pushing down onto the
surface of the earth.
This atmospheric pressure is a specific amount of pressure
and is the same all over the world at sea level. The amount
is 101,3 Kpa (a) or 14,7 PSI.(a) or 760mmHg(a). You should
memorize these values. These values is also refer to as the
ABSOLUTE pressure or 1 atmosphere.
Note the (a) on the end?
If you write atmospheric pressures down you have to add the
(a) to indicate to other people that you are referring to
atmospheric pressure and not gauge pressure.

Gauge pressure is very simply what we call zero pressure at
1 atmospheric pressure at sea level. Just makes life a bit
easier to work in gauge pressure than trying to work in
atmospheric pressure all the time.

So to summarize, zero gauge pressure is equal to 1
atmosphere or 14,7 PSI atmospheric or absolute pressure.

Be aware you will find that some people will also write it
like this: 14,7PSIA or 101,3KpaA.

NB!!
Zero pressure absolute, is a complete vacuum and a complete
vacuum is impossible to achieve on earth. The closest you
will ever get is -0,99999Bar but never -1,013bar.

Where most of the confusion comes in is to find out from
what platform someone is refering to when they talk about
absolute pressures.
What you are suppose to do is to always stay on the platform
of gauge pressure, since this is the world standard we work
on in instrumentation, and then refer to absolute pressure
from there. This means you will refer to absolute pressure
in the negative like - 30Kpa or -200mmHg. In this case it is
in reference to gauge pressure so you do not add the (a).
Should you put yourself on the absolute pressure platform
you would refer to the same pressure as +70Kpa(a) and
+560mmHg(a).
Most of the time people refer to absolute pressure in
mercury but you may use any UOM you feel comfortable with.
In mercury, absolute pressure is -760mmHg standing on the
gauge pressure platform or 0mmHg standing on the absolute
pressure platform.
NB!!!
Be careful with this since there is a big difference to
calibrate a pressure switch for instance to 200mmHg(a) and
-200mmHg(g).
200mmHg(a) is equal to -560mmHg(g) and -200mmHg(g) is equal
to 560mmHg(a), so by calibrating the switch for -200mmhg(g)
and the specs refer to absolute pressure, your pressure
switch will not trip your application on 200mmHg(a) but only
on 560mmHg(a). The same when you work on a absolute pressure
transmitter. Try to stay on the gauge platform where
possible since it makes life a bit easier and less confusion
will occur.

Absolute pressure transmitters are normally used on vacuum
applications or applications where you work in gauge
pressure most of the time, but with a possibility of
negative pressures that could occur.

Referring now only to smart transmitters, we calibrate a
normal smart pressure transmitter for example to something
like this, LRV = 0Bar and URV = 2Bar.
The calibration of a smart absolute pressure transmitter
will look something like this, LRV = -1Bar and URV = 0Bar,
or LRV = -760mmHg to URV = 0mmHg, again saying on the gauge
platform.
It is not recommended but you can, if you need to, calibrate
your tx to absolute pressures as well but then you need to
chance your units of measure on the transmitter to absolute
pressure. That is if your transmitter have the facility to
do so, otherwise the transmitter will work with the measured
pressures as if it is gauge pressures and your transmitter
will not work.
Try and stay on the gauge platform to avoid confusing
yourself and others.

Watch out for the following as well.
With the previous era of pressure transmitters where we had
to use hand pumps and zero and span pots to calibrate the
pressure transmitters we also had to install absolute
pressure transmitters. The golden rule when working with
these transmitters was that you NEVER do a zero calibration
on them since they have already been calibrated by the
manufacturer for as close to absolute zero as possible. So
when you install the transmitter it will immediately
indicate atmospheric pressure and all you need to do is to
pump the transmitter up and adjust the span pot to where the
max pressure should be. Remember span adjustments do not
affect your zero but a zero adjustment will affect your span
calibration.
Now based on this you will find that someone will blow a
casket if you tell them you have done a zero trim on a smart
absolute pressure transmitter. This is a joke since all
smart pressure transmitter can measure up to -1Bar so all
smart pressure transmitters are really all absolute pressure
transmitters as well.
So as a final word from me, don't make things complicated.
Before you start your absolute pressure calibration on a
smart transmitter, open it up to atmosphere and do a zero
trim on it and then just add your -1Bar to +1Bar or whatever
the calibration should be in with the HART and give it back
to production. Simple as that. It will work perfectly, just
make sure the calibration values are the same as on the DCS
faceplate.
Good luck

Question { McDermott, 37299 }

difference b/w single acting and double acting actuator by
using application only?

Answer

Normally a control valve is refer to by it's fail position.
This means "what position will the valve move to should the
supply air or control signal to the valve falls away". This
is important to safe guard the process at various places so
some valves will be fail open and some fail close. In order
to have valve as a fail open or close the valve the actuator
have to be spring loaded. So by having the spring on top or
bottom of the actuator piston, will determine if it will be
a FO or FC valve. This kind of valve is also called single
action since it will only have one output from its
positioner to either the top or bottom of the actuator. The
positioner on the valve is also setup as a single acting
positioner since it will only give a single action to the
actuator, the reverse action will be done by the spring. The
problem with this setup is that it is possible that the
process might be so strong or the pressure so high (during a
blow down or ESD shutdown in the plant) that the spring
might in certain instances be to week to push the valve into
the fail position quick enough, due to the back pressure
from the process and can cause damage to the plant or even a
explosion. To make sure that the valve will go to the fail
position we install a double action positioner with two
outputs. One goes to the top of the actuator and one to the
bottom. This is also very helpful to do very accurate and
stable control on a high flow line since the pressure from
the position do the actual control and not spring control
one way and positioner control the other way as in single
acting control valves. It is also solving the problem that
the valve will now be forced into the fail position by the
spring as well as the positioner supply pressure during a
emergency.
In shutdown valves (open/close ESDV's) the same is true and
sometime at critical and high pressure points we use
hydraulics instead of pneumatics as the double acting agent
to make sure the valve will close during a emergency.
So to summarize the double acting action in ESD and control
valve is just there to make sure the valve will do what it
was designed for. Call it a extra fail safe if you want. In
theory not needed since a single acting valve should do the
trick just as well,but in practice you are at time very glad
you did it especially if you look at the kind of pressures
the valves are working on. With those kind of flows and
pressures you don't want to leave anything to chance.

Question { 7511 }

what is absolute pressure and where use in process?

Answer

You first need to understand what pressure is. Without a
thorough understanding of pressure you will never understand
some of the basic concepts of instrumentation.

Air have weight. This can be proven by a simple primary
school experiment of taking a balancing beam and attached
two balloons to it. With the two balloons deflated the
balancing beam hangs exactly horizontal and the weight is
obviously equal on both sides of the beam.
If you now inflate one balloon the balancing beam will drop
down on the inflated end proving that air have a definite
and specific weight, since the volume of air on the one side
is now more than on the deflated side and therefore the
weight is more.
Pressure is measured on the surface of the earth as the
weight of the air from space pushing down on the surface of
the earth. The reason air and any other body have a weight
is due to the earth's gravity.
Since the gravity of the earth pull less the further you
move away from the earth's surface, the atmospheric pressure
changes as well and become less since the weight of air from
that point to space is less. Therefore on top of the
Himalayas the atmospheric pressure is much less than at sea
level. It takes a lot of years to get use to the low density
of the air up there and us normal people that are use to
higher density air find it difficult to breath up there. The
reason is the lower atmospheric pressure or the lower
density of the air due to the shorter distance from space to
that height.
We all know that water is always at the same level due to
the earth's gravity being the same all over the world, so
therefore if we look at the level of the sea it will be at
the same level in Canada as it is in Africa and the same at
the coast in China. It is therefore accurate to use the sea
as our reference point when we measure the atmospheric
pressure. So we therefore always refer to world standard
atmospheric pressure as it is at sea level and is therefore
a standard throughout the world. In simple terms the
distance from sea level to space at any place on the surface
of the earth is exactly the same. Therefore the atmospheric
pressure at sea level any place on earth is the same. This
atmospheric air weight is the reason why we have a
measurable amount of air pressure and is therefore referred
to as the atmospheric air pressure pushing down onto the
surface of the earth.
This atmospheric pressure is a specific amount of pressure
and is the same all over the world at sea level. The amount
is 101,3 Kpa (a) or 14,7 PSI.(a) or 760mmHg(a). You should
memorize these values. These values is also refer to as the
ABSOLUTE pressure or 1 atmosphere.
Note the (a) on the end?
If you write atmospheric pressures down you have to add the
(a) to indicate to other people that you are referring to
atmospheric pressure and not gauge pressure.

Gauge pressure is very simply what we call zero pressure at
1 atmospheric pressure at sea level. Just makes life a bit
easier to work in gauge pressure than trying to work in
atmospheric pressure all the time.

So to summarize, zero gauge pressure is equal to 1
atmosphere or 14,7 PSI atmospheric or absolute pressure.

Be aware you will find that some people will also write it
like this: 14,7PSIA or 101,3KpaA.

NB!!
Zero pressure absolute, is a complete vacuum and a complete
vacuum is impossible to achieve on earth. The closest you
will ever get is -0,99999Bar but never -1,013bar.

Where most of the confusion comes in is to find out from
what platform someone is refering to when they talk about
absolute pressures.
What you are suppose to do is to always stay on the platform
of gauge pressure, since this is the world standard we work
on in instrumentation, and then refer to absolute pressure
from there. This means you will refer to absolute pressure
in the negative like - 30Kpa or -200mmHg. In this case it is
in reference to gauge pressure so you do not add the (a).
Should you put yourself on the absolute pressure platform
you would refer to the same pressure as +70Kpa(a) and
+560mmHg(a).
Most of the time people refer to absolute pressure in
mercury but you may use any UOM you feel comfortable with.
In mercury, absolute pressure is -760mmHg standing on the
gauge pressure platform or 0mmHg standing on the absolute
pressure platform.
NB!!!
Be careful with this since there is a big difference to
calibrate a pressure switch for instance to 200mmHg(a) and
-200mmHg(g).
200mmHg(a) is equal to -560mmHg(g) and -200mmHg(g) is equal
to 560mmHg(a), so by calibrating the switch for -200mmhg(g)
and the specs refer to absolute pressure, your pressure
switch will not trip your application on 200mmHg(a) but only
on 560mmHg(a). The same when you work on a absolute pressure
transmitter. Try to stay on the gauge platform where
possible since it makes life a bit easier and less confusion
will occur.

Absolute pressure transmitters are normally used on vacuum
applications or applications where you work in gauge
pressure most of the time, but with a possibility of
negative pressures that could occur.

Referring now only to smart transmitters, we calibrate a
normal smart pressure transmitter for example to something
like this, LRV = 0Bar and URV = 2Bar.
The calibration of a smart absolute pressure transmitter
will look something like this, LRV = -1Bar and URV = 0Bar,
or LRV = -760mmHg to URV = 0mmHg, again saying on the gauge
platform.
It is not recommended but you can, if you need to, calibrate
your tx to absolute pressures as well but then you need to
chance your units of measure on the transmitter to absolute
pressure. That is if your transmitter have the facility to
do so, otherwise the transmitter will work with the measured
pressures as if it is gauge pressures and your transmitter
will not work.
Try and stay on the gauge platform to avoid confusing
yourself and others.

Watch out for the following as well.
With the previous era of pressure transmitters where we had
to use hand pumps and zero and span pots to calibrate the
pressure transmitters we also had to install absolute
pressure transmitters. The golden rule when working with
these transmitters was that you NEVER do a zero calibration
on them since they have already been calibrated by the
manufacturer for as close to absolute zero as possible. So
when you install the transmitter it will immediately
indicate atmospheric pressure and all you need to do is to
pump the transmitter up and adjust the span pot to where the
max pressure should be. Remember span adjustments do not
affect your zero but a zero adjustment will affect your span
calibration.
Now based on this you will find that someone will blow a
casket if you tell them you have done a zero trim on a smart
absolute pressure transmitter. This is a joke since all
smart pressure transmitter can measure up to -1Bar so all
smart pressure transmitters are really all absolute pressure
transmitters as well.
So as a final word from me, don't make things complicated.
Before you start your absolute pressure calibration on a
smart transmitter, open it up to atmosphere and do a zero
trim on it and then just add your -1Bar to +1Bar or whatever
the calibration should be in with the HART and give it back
to production. Simple as that. It will work perfectly, just
make sure the calibration values are the same as on the DCS
faceplate.
Good luck

Question { 14536 }

i have one tank of 500mm height, which is in under vacuum of
735 mmHg, I want measure level of the same using DPT , SO WHAT
WILL BE THE URV & LRV, DENSITY OF THE TANK FLUID IS 0.95. is
there necessary to fill the LP side with fluid.

Answer

Thanks Nitin and you are more than welcome to give me a
shout is you get stuck again.

mass44@hotmail.co.uk

Question { 14536 }

i have one tank of 500mm height, which is in under vacuum of
735 mmHg, I want measure level of the same using DPT , SO WHAT
WILL BE THE URV & LRV, DENSITY OF THE TANK FLUID IS 0.95. is
there necessary to fill the LP side with fluid.

Answer

First of all there is no difference between a vessel with a
process pressure of 10 or 50Bar or a vessel with a vacuum.
These pressures cancel each other across the LP and HP legs
on our tx's, and are therefore not taken into account in our
calibration.
To explain in more detail:
If there is 100Bar on the LP side there is also 100Bar on
the HP side so the DP across the tx is still zero.
If there is 750mmHg vacuum on the LP side there is also a
750mmHg vacuum on the HP side so the DP across the tx is
still zero.
So work with the transmitter as if there is no pressure or
vacuum in the vessel.

The best transmitter to use in a vacuum application like
this is a capillary type, but according to your question it
seems you already have a piped transmitter in place. This
makes it a bit more troublesome to do, but by working
carefully and accurately you can achieve accurate and
reliable results.

Take your time, this is one of the MOST DIFFICULT DP level
setup's you will ever came across in any industry.

It is always better to use a wet-leg since condensation will
cause your DP to chance in time. If I look at the sg of the
product you most probably are working with hydrocarbon
condensate, so you would want to install a wet leg in a
application like this.

Process zero with LP leg filled:
1st open both legs to atm and do zero trim. Even better if
you know how to do a factory reset and then do a zero trim.

Connect a 1/2" T-piece to top of LP leg just on the bend
before it goes to the top tap-off point with a needle valve
pointing upwards.
With main process isolation valves still close, fill LP leg
to max and close needle valve.
Try to use glycol since it's density is higher than water's,
and will prevent contamination of the wet leg.
You can also use glycerin or diesel.

Close 5-way manifold equalization valve and open main
process isolation valves and then only open both isolation
valves on manifold.
Open needle valve and fill again LP leg as much as possible,
start closing needle valve slowly and keep filling
to make sure lp leg is filled properly. Playing with the
main LP isolation valve and the needle valve will give best
results to get the LP leg filled to max.
This should put you in the situation that your tx has been
zeroed at atmospheric pressure, so the vacuum pulled
on both sides of diaphragm now and LP leg filled to max,
should now give a accurate process zero to work from.
Write down this displayed value.

What ever this value is is not important you will use this
reading as your process zero reference point to work from
so it can be anything as long as you are sure this is a
accurate and reliable process zero.
To double check if the process zero is good, isolate the tx
again and open it up to atm again not draining the LP leg.
It should still give a zero indication with both sides open
to atm.
Put it back on line and make sure the LP leg is still filled
to max by making use of the needle valve and LP main
isolation valve again.
You should be back at the previously displayed value. Do
this a couple of times to make sure you get to the same
values every time. Only then can you be sure that your
process zero value is reliable and accurate.

To calculate the LRV and URV:
Like I said I am assuming now that the tx had been installed
300mm below the bottom tap-off point.
I am also assuming that the bottom tap-off point is zero
position and the top tap-off point is 100%

You should now have something like (+/-) -750mmH2o on the tx
display.
Let's say the value is exactly -750mmH2o.

Measure from the middle of the tx's diaphragm to the bottom
tap-off point. We make this say 300mm
Measure from the bottom to the top tap-off points. We make
this say 500mm.

Calculation:
LRV is -750 + 300 = -450mm x .95 = -427,5 mmH2o
URV is -750 + 300 + 500 = +50mmH2o x .95 = +47,5 mmH2o

Modify your L/URV's to these new values. There is no need to
use a hand pump when working with smart tx's, just modify
the values with the HART, make sure it's on line and give it
back to production.

Since it is such a small span the level might be to
sensitive so you might want to increase the damping as well
on the Tx.
Good luck.

Question { 14536 }

i have one tank of 500mm height, which is in under vacuum of
735 mmHg, I want measure level of the same using DPT , SO WHAT
WILL BE THE URV & LRV, DENSITY OF THE TANK FLUID IS 0.95. is
there necessary to fill the LP side with fluid.

Answer

Have to agree with Naman that this is not the best way to do
it and there are other devices that will work better if the
current instillation can be modified to accommodate a
different type of instrument like a capillary type of DP Tx
or a level troll.
According to question, the DPT is already in place and needs
to be used so above is the way to do it, and yes it is
possible to do since I have done it myself on a d-aerator
column under constant 0,98Bar vacuum and a span of only
300mm. My situation was the same in that I could not just
modify the installation and had to work with what was
installed.
Keep in mind that once the wet leg is filled the vacuum is
pulling against a closed end. Test this the next time you
drink a Coke out of a can . Suck the straw full and close
the end with your finger and try and suck the Coke
out.(please use a metal tube and not a plastic straw, LOL)
Good luck

Question { 39825 }

WHAT IS THE DIFFERENCE BETWEEN A TRANSMITTER & A SMART
TRANSMITTER.

Answer

We went through various eras or technology time periods in
instrumentation. Believe it or not but measurements of flow
and level had been done by the old Egyptians that build the
pyramids already. They had use for instants a stick with
marks on it to see what the level was in one of their tanks
and various other simple measuring and control devices as well.

We normally are not to concerned with the type of
instrumentation that was used that long ago. We normally say
that instrumentation only started to take off during the
pneumatic era where all measurement and controls were done
by pneumatic and some electrical instrumentation. The world
standard instrumentation signal was also a pneumatic one
then. It was 20 to 100Kpa or 3 to 15Psi. What this means is
that we had to calibrate our pneumatic pressure transmitters
and controllers to give this output of 20 to 100Kpa. You
really had to know instrumentation when you worked with
these pneumatic stuff. We even had pneumatic relays and
pneumatic chart recorders back then.
Later we moved on to electromechanical instruments and some
very simple electronic indicators and chart recorder.

After the transistor and micro processors were developed
things started to move along very fast and we started to see
various electronic instrumentation and transmitters on the
market. I call this the 4-20mA era. These very advanced type
of transmitters had to be pumped up with a small hand pump
as normal, but all you then had to do was make small
adjustments with a small screwdriver to a zero and span pot
to calibrate the transmitter. Very advanced to what we were
use to. These were called the electronic 4 to 20 mA
TRANSMITTERS.
The new electronic world standard signal then started as the
4 to 20 mA signal, but keep in mind that the pneumatic world
standard signal, even today, is still the 20 to 100Kpa signal.
Since then we have moved on to the smart instrumentation era
where we calibrate the transmitters with a HART communicator
and the transmitter itself is actually a small computer that
can even detect a error on itself or tell you if the
calibration you are trying to do is invalid. From there the
technology of SMART TRANSMITTERS. You even get SMART valve
positioners that can accurately detect the erosion of it's
control valve's plug and seat and send a alarm signal to the
CCR that a overhaul on the valve is needed. All just
computer software and programming like anything else in
today's world. Look what you can do with a cellphone today.
Very smart little gadget.
Not to worry they will never be as smart as a instrument tech.
Good luck

Question { 6650 }

how to calibate level transmiter ?range -150 to +150.what
is the zero and span?

Answer

This means that your transmitter is capable of measuring a
differential pressure of 300. Whatever this units of measure
is. You can calibrate the transmitter for any of the
following conditions:
LRV = -150 = 0%
URV = +150 = 100%
Therefore span = 300 UOM (units of measure)
or
LRV = 0 = 0%
URV = +150 = 100%
Therefore span = 150 UOM (units of measure)
or
LRV = -150 = 0%
URV = 0 = 100%
Therefore span = 150 UOM (units of measure)
or
You can also calibrate the tx for any span in between this
300 range.
To find the correct Z/S values for your application measure
the distance from the transmitter to zero position on the
vessel and multiply this with the density of the product you
want to measure (LRV). Do the same with your 100% (URV) value.
Good luck

Question { 4694 }

can any one pl explain me in brief that how we select the
leakage classification in valve,means for liquid or gas
there is any standard for selecting leakage classification
or its depend upon valve size or any any other parameter.

Answer

I cannot remember to much about this but I remember that the
leakage rate is dependent on the valve classification. a
Class 6 valve is a tight shutoff valve with not leakage
tolerance at all. a Class 5 is classified as a modulating
valve and have a tolerances of a small amount of droplets
per minute and so on. The lower the class valve the higher
the acceptable leakage rate. This leakage rate is tested in
a valve shop either by the manufacturer or on your own site
if you have the right equipment to do it with.
We normally had it done by our valve shop after we send a
valve to them for a complete overhaul. Once the control
valve have repaired and overhauled the valve, we had to do
the final testing and acceptance with them. They would
supply us with the specs on the valve so that we can see
what class valve it was and what the acceptable leakage
tolerance was. After this we would witness the testing of
the valve and sign a acceptance certificate.
I suggest you get hold of your valve shop or the original
manufacturer to supply you with a list of the
classifications of the valves you are currently using or
want to use as well as the various acceptable leakage rate
for each class.
Good luck

Question { HPCL, 7101 }

Foe selecting control valve if i have the min,normal & max
data of flow rate,inlet pressure& outlet pressure,inlet
tem.&density than how can i assure that these data are
correct,pl. let me know if therew there is any method to
check it.& also how we assure that the Cv we get from this
data is suitable for valve for controlling require flow.

Answer

Calculate the cv of a LIQUID ONLY control valve for a given
flow, dp and sg

Cv = 11.7 q (SG / dp)1/2

where
q = Expected flow rate (m3/h)
Sg = Specific gravity of the liquid (1 for water in this
example)
dp = Pressure drop across the valve at expected flow rate (kPa)

Example 1.
So if q = 5m3/h
Sg for water = 1
Dp over valve during 5m3/h flow = 10kpa

Do manual calculation:

Cv = 11.7 q (SG / dp)1/2

1) 11,7 x 5 = 58,5
2) 1/10 = 0,1
3) ½ = 1/2 = 0.5
4) 0,1 to the power of 0,5 = 0.31622776
5) So 0.31622776 x 58,5 = 18,49

So a valve with a cv of 18,5 is needed to do a flow of
water(sg=1) at 5m3/h and a dp over the valve of 10kpa

Look at manufacturer valve manual for available valves and
select a valve with a next bigger Cv.

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

Example 2.
So if q = 30m3/h
Sg for liquid = 0,97
Dp over valve during 30m3/h flow = 110kpa

Do manual calculation:

Cv = 11.7 q (SG / dp)1/2

1) 11,7 x 30 = 351
2) 0,97/110 = 0,008818
3) ½ = 1/2 = 0.5
4) 0,008818 to the power of 0,5 = 0.0939042
5) So 0.0939042 x 351 = 32,96

So a valve with a cv of 32,96 is needed to do a flow of
liquid (sg=0,97) at 30m3/h and a dp over the valve of 110kpa

Look at manufacturer valve manual for available valves and
select a valve with a next bigger Cv

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

Example 3.
So if q will be 64m3/h
Sg for liquid is 0,87
Dp over valve during 64m3/h flow is 210kpa

Do manual calculation:

Cv = 11.7 q (SG / dp)1/2

1) 11,7 x 64 = 748,8
2) 0,87/210 = 0,00414285
3) ½ = 1/2 = 0.5
4) 0,00414285 to the power of 0,5 = 0.06436497
5) So 0.06436497 x 748,8 = 48,196


So a valve with a cv of 48,196 is needed to do a flow of
liquid (sg=0,87) at 64m3/h and a dp over the valve of 210kpa

Look at manufacturer valve manual for available valves and
select a valve with a next bigger Cv

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

To do the power off calculation on your windows calculator:
Example:
5 to the power of 4 ------- press 5 then x^y then 4 then =

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

Control Valve selection Flow Coefficient - Cv - for Air and
other Gases

For critical pressure drop the outlet pressure - po - from
the control valve is less than 53% of the inlet pressure -
pi. The flow coefficient can be expressed as:

Cv = q [SG (T + 460)]1/2/ 660 pi
where
q = free gas per hour, standard cubic feet per hour (Cu.ft/h)
SG = Specific gravity of the gas relative to air at 14.7
psia and 60oF
T = flowing air or gas temperature (oF)
pi = inlet gas absolute pressure (psia)

Example:
5000 = Gas flow (Cu. ft./h) (f.a.d)
100 = Inlet Gas Absolute Pressure (psia)
1 = Specific gravity of the
60 = Gas Temperature (oF

Do manual calculation:

Cv = q [SG (T + 460)]1/2/ 660 pi

1) 60 + 460 = 520
2) 520 x 1 = 520
3) ½ = 1/2 = 0.5
4) 520 to the power of 0,5 = 22,8035
5) 22,8035 x 5000 = 114017,5425
6) 660 x 100 = 66000
7) 114017,5425 / 66000 = 1,727

So a valve with a cv of 1,73 is needed to do a air flow
(sg=1) at 5000Cu.f/h, at a air flow temp of 60F and a inlet
gas absolute press of 100PSIA

Look at manufacturer valve manual for available valves and
select a valve with a next bigger Cv

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

For non critical pressure drop the outlet pressure - po -
from the control valve is greater than 53% of the inlet
pressure - pi. The flow coefficient can be expressed as:

Cv = q [SG (T + 460)]1/2/ [1360 (dp po)1/2]
where
dp = (pi - po)
po = outlet gas absolute pressure (psia)

Example:
6000 = Gas flow (Cu. ft./h) (f.a.d)
140 = Inlet Gas Pressure (psia
110 = Outlet Gas Pressure (psia
1 = Specific gravity of the gas
60 = Gas Temperature (oF)

Do manual calculation:

Cv = q [SG (T + 460)]1/2/ [1360 (dp po)1/2]

1) 60 + 460 = 520
2) 520 x 1 = 520
3) ½ = 1/2 = 0.5
4) 520 to the power of 0,5 = 22,8035
5) 22,8035 x 6000 = 136821,051

6) 140 – 110 = 30
7) 30 x 110 = 3300
8) ½ = 1/2 = 0.5
9) 3300 to the power of 0,5 = 57,445
10) 57,445 x 1360 = 78126,05199

11) 136821,051 / 78126,05199 = 1,75

So a valve with a cv of 1,75 is needed to do a air flow
(sg=1) at 6000Cu.f/h, at a air flow temp of 60F and a DP of
30PSIA absolute press

Look at manufacturer valve manual for available valves and
select a valve with a next bigger Cv

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

To do the power off calculation on your windows calculator:
Example:
5 to the power of 4 ------- press 5 then x^y then 4 then =

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

Please note the "1/2" in the formulas is not as it seems, it
is surpose to be a small "1/2" on the top right hand corner
of the bracket, in other words a "to the power of 1/2" and
not " bracket times 1/2 as it is displayed"
Cannot seem to get it displayed like that on this page -
sorry if it is confusing but look at he calculation to see
where the "power of" is used and you will understand what I
mean.

Good Luck

Question { 5870 }

where using the 0-20ma in feiled instrumentation? why its using?

Answer

Please let us know where you have seen this in a instrument
in the field. The normal standard signal is 4 to 20mA.

Question { 10670 }

what is actual difference between pressure Transducer &
Pressure Transmitter.

Answer

Transducer is any device that can converts one type of
energy into another type of energy. In a pressure measuring
instrument the transducer is the actual pressure cell that
converts the measured pressure energy into a proportional
electrical energy.
This pressure cell is connected via a ribbon cable to the
head of the pressure measuring instrument where the
transmitter is situated.
Transmitter is any device that can take a generated signal
and transmit that signal via any means, be it via cable or
radio waves to a receiver.
We normally refer to any measuring instrument's transmitter
section due to it being the most important area to us. The
transmitter's ability to transmit the generated 4 to 20mA
signal is what we work on and not the cell or transducer area.
If you talk about your car do you talk about your 4 cylinder
or do you talk about your Toyota Corolla?
Good luck

Question { CCC, 5821 }

I want to measure level under vaccum by using DP Tx and vaccum vary between -500 to -670 mmHg and process fluid is accetic anhydride if consider process fluid density 1 kg/cm2. how to calibrate Tx ? consider level is 0 to 100 Cm.

Answer

I would not do a installation like this with a piped DP tx
and think that you should reconsider your position before
you do this.

In hazardous or corrosive applications you might need to
make a modification in order to install a instrument that
will contain the process permanently.

If this is not a safe application at the moment you need to
put in a chance proposal with a safety justification and a
proposal for a new instrument that will make the application
safe.

Do not do something just because you we instructed by
someone to do it, you need to satisfy yourself that the
inhalation is safe, reliable and accurate, not someone else.
If it is not, it is your right, duty and responsibility to
do something about it.

In your case the liquid you are working with is corrosive,
hazardous, flammable and will causes violent chemical
reactions with various other liquids. I would recommend you
modify the installation to a capillary DP tx with chemical
seals(exp-Teflon).

Find out from the liquid supplier what type of material is
resistant to the liquid and ask you Tx supplier to supply
you with a capillary type smart transmitter with this type
of material chemical seals on the pad cells.
The supplier will ask you the length capillary length
needed. The capillary lengths should always be as short as
possible to prevent drifting, so look at the installation
and decide where you are going to install the new cap DPT
and double the distance from the transmitter to the top tap
off point on your vessel to get the right capillary lengths.
Do the calibration as normal for a capability type DPT and
forget about the vacuum inside since it is irrelevant in
your calibration.

Just some important calibration information when working
with capillary DPT's.
After the installation, open both pad cell to atmosphere and
have look at the displayed value. When you do a process zero
you might find that the displayed zero have shifted slightly
from the atmospheric zero. It is therefore important to
check the process as well as the atmospheric zero especially
if you are working on a small span. This small shift will
cause big inaccuracies if you do not work from your a
process zero. (yes, similar thing to the old pneumatic DPT
static alignment problem)
To do the process zero you might need to install a piece of
tubing temporary between the HP and LP tap off points in
order to get a equal vacuum on HP and LP side of tx. That is
if you cannot drain your vessel and just keep the vacuum
pressure inside the vessel.
(We normally install 316SS flushing rings for this purpose
but you need to look at the corrosive properties again
before you install these SS flushing rings. You might have
to order them in a similar material as what you used on the
pad cells.)
Use the displayed value as your process zero reference and
just add your measured mm, multiply by 1,08(sg for acetic
anhydride), to get your L/URV's.
If there is a difference between atm zero and process zero
always use process zero for the calibration since the DPT
will measure the level while under vacuum and not at
atmospheric pressure.

Something else you might encounter in a application like
this is that this process zero could fluctuate slightly due
to the variance in vacuum and you might find it difficult to
read. I had similar experiences before on a FPSO where the
whole vessel moves all the time and therefore the process
zero keeps on changing all the time. I have compensated for
that by standing there and watch the reading fluctuating for
about 15 minutes and write done the lowest and highest
readings and then use the average in the middle as my final
process zero.
This is called splitting the error. Your final level
indication after your calibration will fluctuate as well,
but since you have split the process zero error, the reading
will be pretty accurate and you can apply some damping on
the tx to stabilize it some more.

Below is a previous discussion on a similar application but
for non hazardous liquid and no way to modify the piped DPT
installation to a better type of instrument. You can have a
look at it for information but again, in your case you
should not use a piped DP transmitter.

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

First of all there is no difference between a vessel with a
process pressure of 10 or 50Bar or a vessel with a vacuum.
These pressures cancel each other across the LP and HP legs
on our tx's, and are therefore not taken into account in our
calibration.
To explain in more detail:
If there is 100Bar on the LP side there is also 100Bar on
the HP side so the DP across the tx is still zero.
If there is 750mmHg vacuum on the LP side there is also a
750mmHg vacuum on the HP side so the DP across the tx is
still zero.
So work with the transmitter as if there is no pressure or
vacuum in the vessel.

The best transmitter to use in a vacuum application like
this is a capillary type, but according to your question it
seems you already have a piped transmitter in place. This
makes it a bit more troublesome to do, but by working
carefully and accurately you can achieve accurate and
reliable results.

Take your time, this is one of the MOST DIFFICULT DP level
setup's you will ever came across in any industry.

It is always better to use a wet-leg since condensation will
cause your DP to chance in time. If I look at the sg of the
product you most probably are working with hydrocarbon
condensate, so you would want to install a wet leg in a
application like this.

Process zero with LP leg filled:
1st open both legs to atm and do zero trim. Even better if
you know how to do a factory reset and then do a zero trim.

Connect a 1/2" T-piece to top of LP leg just on the bend
before it goes to the top tap-off point with a needle valve
pointing upwards.
With main process isolation valves still close, fill LP leg
to max and close needle valve.
Try to use glycol since it's density is higher than water's,
and will prevent contamination of the wet leg.
You can also use glycerin or diesel.

Close 5-way manifold equalization valve and open main
process isolation valves and then only open both isolation
valves on manifold.
Open needle valve and fill again LP leg as much as possible,
start closing needle valve slowly and keep filling
to make sure lp leg is filled properly. Playing with the
main LP isolation valve and the needle valve will give best
results to get the LP leg filled to max.
This should put you in the situation that your tx has been
zeroed at atmospheric pressure, so the vacuum pulled
on both sides of diaphragm now and LP leg filled to max,
should now give a accurate process zero to work from.
Write down this displayed value.

What ever this value is is not important you will use this
reading as your process zero reference point to work from
so it can be anything as long as you are sure this is a
accurate and reliable process zero.
To double check if the process zero is good, isolate the tx
again and open it up to atm again not draining the LP leg.
It should still give a zero indication with both sides open
to atm.
Put it back on line and make sure the LP leg is still filled
to max by making use of the needle valve and LP main
isolation valve again.
You should be back at the previously displayed value. Do
this a couple of times to make sure you get to the same
values every time. Only then can you be sure that your
process zero value is reliable and accurate.

To calculate the LRV and URV:
Like I said I am assuming now that the tx had been installed
300mm below the bottom tap-off point.
I am also assuming that the bottom tap-off point is zero
position and the top tap-off point is 100%

You should now have something like (+/-) -750mmH2o on the tx
display.
Let's say the value is exactly -750mmH2o.

Measure from the middle of the tx's diaphragm to the bottom
tap-off point. We make this say 300mm
Measure from the bottom to the top tap-off points. We make
this say 500mm.

Calculation:
LRV is -750 + 300 = -450mm x .95 = -427,5 mmH2o
URV is -750 + 300 + 500 = +50mmH2o x .95 = +47,5 mmH2o

Modify your L/URV's to these new values. There is no need to
use a hand pump when working with smart tx's, just modify
the values with the HART, make sure it's on line and give it
back to production.

Since it is such a small span the level might be to
sensitive so you might want to increase the damping as well
on the Tx.
Good luck.

Question { 6103 }

I want to measure flow by using DP Tx and range is 0 to 1200 Kg/cm2 ?

Answer

Your UOM for flow is wrong, it cannot be a pressure UOM.
The 1200Kg/cm2 cannot be the DPT range either since a diff
pressure tx with a range of 1176Bar does not exist.

So unless you ask a sensible question with accurate and
detail information with it, and look at what you write
before you post the question, no one can or would want to
help you.

Question { 10328 }

what is the meaning if the control valve CV is 27,in terms
of selecting procedure?

Answer

Flow coefficient is the proportional constant between
pressure drop and flow rate and it is determined
experimentally by valve manufactures. It is expressed as the
flow rate of water in m3/h for a pressure drop of 1 bar
across a flow passage.

Flow coefficient = Cv or Kv
Cv = Imperial
Kv = Metric

Selecting the correct size control valve:
When the flow coefficient is calculated for the required
flow rate and known pressure drop, the selection of a proper
control valve is normally done by selecting a control valve
with the next bigger flow coefficient(cv)

So in your case here are some info that might help,

With a control valve with a cv of 27 in a clean water line
with a 100mm pipe dia size, a DP across the valve of 1Bar
will give a volumetric flow rate of 23m3/h and a fluid
velocity of 2973m/h

With a control valve with a cv of 27 in a clean water line
with a 200mm pipe dia size, a DP across the valve of 1Bar
will give a volumetric flow rate of 23m3/h and a fluid
velocity of 743m/h

With a control valve with a cv of 27 in a clean water line
with a 300mm pipe dia size, a DP across the valve of 1Bar
will give a volumetric flow rate of 23m3/h and a fluid
velocity of 330m/h.

So the volumetric flow rate stays the same regardless of
pipe dia if the DP stays the same. The only thing that will
change is the flow velocity.

So,

With a control valve with a cv of 27 in a clean water line
with a 100mm pipe dia size, a DP across the valve of 1Bar
will give a volumetric flow rate of 23m3/h and a fluid
velocity of 2973m/h

With a control valve with a cv of 27 in a clean water line
with a 100mm pipe dia size, a DP across the valve of 2Bar
will give a volumetric flow rate of 33m3/h and a fluid
velocity of 4205m/h

With a control valve with a cv of 27 in a clean water line
with a 100mm pipe dia size, a DP across the valve of 3Bar
will give a volumetric flow rate of 40m3/h and a fluid
velocity of 5151m/h

And,

With a control valve with a cv of 27 in a clean water line
with a 200mm pipe dia size, a DP across the valve of 1Bar
will give a volumetric flow rate of 23m3/h and a fluid
velocity of 743m/h

With a control valve with a cv of 27 in a clean water line
with a 200mm pipe dia size, a DP across the valve of 2Bar
will give a volumetric flow rate of 33m3/h and a fluid
velocity of 1051m/h

With a control valve with a cv of 27 in a clean water line
with a 200mm pipe dia size, a DP across the valve of 3Bar
will give a volumetric flow rate of 40m3/h and a fluid
velocity of 1288m/h

And,

With a control valve with a cv of 27 in a clean water line
with a 300mm pipe dia size, a DP across the valve of 1Bar
will give a volumetric flow rate of 23m3/h and a fluid
velocity of 330m/h

With a control valve with a cv of 27 in a clean water line
with a 300mm pipe dia size, a DP across the valve of 2Bar
will give a volumetric flow rate of 33m3/h and a fluid
velocity of 467m/h

With a control valve with a cv of 27 in a clean water line
with a 300mm pipe dia size, a DP across the valve of 3Bar
will give a volumetric flow rate of 40m3/h and a fluid
velocity of 572m/h

So to summarize if you have a control valve with a cv of 27
you need to have a look at the maximum volumetric flow rate
that will be required in the line that this control valve
will be installed in, in order to see if this valve is big
enough to control this flow.

Keep in mind the perfect control position for any control
valve is between 40 to 60% valve opening. You need to select
a valve with a cv that will allow control to be in that area
under normal operating conditions, so it is important to
consider minimum, normal and maximum flow conditions during
the selection process. To be safe select the next bigger cv
than what your calculations says the correct size will be.
Good luck