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