Answer / carl_ellis

Here's Mr. Emerson's view on loop checking with instructions
for zero checking a DP transmitter on a 3 valve manifold:

Wiring Checkout and Loop Check

The first step in commissioning a new instrument is the
physical installation. The physical installation steps are
the same for both the traditional and new approach:

1. Review P&ID drawings for location and piping instructions.

2. Review device instructions for proper location, mounting,
and piping.

3. Verify field device is physically located in the correct
location per the P&ID. Verify that a permanent tag label is
securely attached to the device that clearly indicates the
assigned loop tag number.

4. Verify field device is correctly connected by performing
the following inspections:
a. Physical inspection- properly mounted and oriented as
designed, ensure close-coupled connections are securely
fastened; pipe stands are structurally sound

b1. Mechanical - Sensing lines, and other process
connections are properly installed, connected and verified
per instrument design specification

b2. For DP transmitters with manifold valves:
- Always perform a zero trim on the transmitter/manifold
assembly after installation to eliminate
any shift due to mounting effects.
- In normal operation the two block valves between the
process and instrument ports will be open and the equalizing
valve will be closed.
- To zero the DP transmitter (3 valve manifold), close
the block valve to the low pressure (downstream) side of the
transmitter first.
- Open the center (equalize) valve to equalize the
pressure on both sides of the transmitter. The manifold
valves are now in the proper configuration for zeroing the
transmitter.
- After zeroing the transmitter, close the equalizing valve.
- Open the block valve on the low pressure side of the
transmitter to return the transmitter to service.

c. Electrical - Conduit runs are properly connected; seal
drains are installed correctly, etc.

5. Verify the input or output signal is within the device's
normal operating range.

6. Verify that any special/custom device characteristics
required are present (example: custom gas curves, special
construction materials, etc.)

7. Measure and adjust associated air and electrical power
supplies.

8. Measure loop impedance by disconnecting the signal wiring
at both the transmitter and rack room ends, shorting the
signal wiring at the transmitter and measuring the loop
resistance with an ohmmeter at the termination panel end.
This process typically takes two technicians with two-way
radios for communication.

9. With an ohmmeter, verify there are no shorts to ground in
the signal wiring by checking the wiring to ground while the
wiring is still disconnected. This process typically takes
two technicians with two-way radios for communication.

10. Verify proper wiring polarity.

11. Connect the wiring to the transmitter in the field and
in the rack room.

12. Verify that all field wiring is properly labeled per the
instrument loop diagram. This includes
all terminations per the instrument loop diagram.

13. Verification that all instrumentation wiring is
mechanically sound by performing a physical "Tug Test" of
the wires.

14. Verify that the field device enclosure is closed, all
temporary shipping stops removed, and the device is
mechanically fit for service as defined above and in the
device instruction manual.

15. Apply power to device.

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