Answer Posted / sam

This procedure can be used to calibrate any type of
instrument in the field on a live plant regardless of it's
type or application. The only variation in the procedure is
what test equipment you will use to calibrate the instrument
with. For instance calibrating a temperature transmitter you
want to take a HART and decade box with you. Calibrating a
pressure , flow or DP transmitter, you want to take a HART
and pressure pump with you like a Druck or hand pump with a
precision gauge on.
The procedure:
Since the transmitter is on line and on a running plant, you
need to determine a couple of things first before you start
working on it. First and most important of all, what does it
do? Is it a control transmitter, a trip transmitter or just
a indication.
Control transmitter means it's output will go to the DCS and
is linked to a PID controller which will give a continues
output, based on this input, to a control element in the
plant plant like a control valve or a heater element. These
are all critical control components that will trip the plant
if you upset their stability.
Trip transmitter means that it is linked to the ESD system
and will start a plant shut down if it is seeing a to high
or low pressure, flow, DP or level depending on the
application you are working on. Just indication means that
it is completely safe to work on and you cannot trip anything.
To make the control transmitter safe to work on you need to
put the DCS PID controller in manual. This will keep the
output of the controller at the current output position and
bypass the output function of the controller. In a fast
moving application the control room operator will send a
field operator with you to control the control valve
manually with it's hand wheel while you work on the control
transmitter.
To make the trip transmitter safe to work on you need to put
a inhibit in the ESD system output trip function of the
transmitter. This is also sometimes refer to as a MOS
(maintenance override switch).
In both cases the control room operators will do this for
you providing that you have all the permit to work and
inhibit permission paperwork in place.
Good luck

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