So You Think Your Flow Meters are Correct? - Part 2

In Part 1, I wrote about some of the things that I found after doing a verification of the orifice calculations and transmitter configurations in our diesel hydrotreater unit.  I focused on the design phase of the instrument lifecycle.  Two systemic errors in the orifice calculations done by the E&C firm led to errors on about 75% of the meters in the unit.  In this post, I'll discuss some of the things that we found that were the result of events that took place after the unit was started.

Commissioning and Startup

Anyone that has gone through the commissioning and startup of a new unit knows the challenges of dealing with DP flow measurement.  Orifice plates get mixed up, they get installed backwards, transmitters get installed backwards, impulse lines leak and the orifice plates act as strainers for all of the trash left in the lines during construction.  We assume that once we get all of these issues resolved and our meters are finally working, we can begin to relax.  Think again.  There are any number of things can go wrong over the next several decades that the unit will operate.

Ongoing Maintenance and Operation

Today's high quality transmitters last a long time.  We have some Rosemount 1151 pressure transmitters that are more than 25 years old and still going strong.  During the life of a transmitter it may get re-ranged or relocated.  Transmitters fail and get replaced.  The control systems that these transmitters are connected to may get upgraded or replaced.  Every time one of these events take place, something can go wrong that will cause a flow meter to read incorrectly.  Some of the things that we have found are listed below.

Incorrect Re-ranging - We found two transmitters that had their range in the DCS increased by a factor of two.  We assume that everyone knows that the DP increases with the square of the flow.  Not so in this case, as the DP in the transmitter was also increased by a factor of 2 instead of 4.

No Transfer Function - Somewhere in the loop, a square root transfer function has to be applied to the signal.  This can be done in either the transmitter (if it is smart) or the DCS.  Our standard is to do it in the transmitter.  We found two transmitters in this audit than did not have the square root transfer function enabled.

Changes in Service Conditions - Sometimes service conditions change and orifice calculations have to change with them.  In our hydrotreater audit, we found one meter whose service had changed.  The temperature and pressure conditions were similar but the molecular weight changed by a factor of 4.  That makes a big difference in the calculated differential pressure.

Physical Changes - We had one meter in the hydrotreater unit where we were unable to rationalize the orifice calculation and the transmitter configuration.  We finally sent someone to field verify the orifice bore.  It turns out that the bore was different than the documentation.  The transmitter was actually correctly configured.  Sometime in the past the orifice was changed and the transmitter was reconfigured but the documentation was not updated.

We have learned a lot from this and other flow audits.  In Part 3 of this series, I'll talk about what we have done to prevent some of these errors from happening in the future and insure that our flow meters stay as accurate as possible.