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.
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Answer Posted / sam
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.
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