The furnace in our delayed coking unit tripped on a low flow ESD last night; we have had issues with this in the past following a cold night coupled with precipitation.  The DP orifice flow transmitters are installed on each of the 4" charge lines to the furnace with an ESD setpoint of 2.61 MBPD.  At the time of the trip, each pass was set to approximately 4.1 MBPD at the DCS.  Transmitter output is linear with square rooting at the destination.

There are redundant transmitters on each pass: one for shutdown and one for valve control.  Trending the data from both on the pass in question showed the flow appeared erratic before a transient tripped the ESD.  Today we checked the transmitters at a comparable flow rate, and both appear to function correctly after a 150# steam hose was applied with a blanket surrounding the flange taps.  To me this indicates the VTB's are freezing in the transmitter taps causing the erratic readings.

The taps extend around 18" from the flange before mating to the flange of remote seal transmitters.  The taps are 1/2" threaded sch. 40 316SS, and are steam traced with 150# steam and insulated.  An IR temperature gun showed the pipe leading to the remote seal flange at only 120F before we applied the additional steam hose.

The feedstock for the coker comes from a crude unit processing various sour feedstocks; the crude unit has an atmos, gas oil and vacuum tower, so the feedstock is from the very bottom of the barrel.

My question: what is the minimum VTB temperature at the point of flow measurement you would feel comfortable with in this scenario?  I think additional tracing and insulation is in order (short of a flush - that's another issue) but would like a reasonable goal to shoot for.

Any ideas or suggestions would be appreciated.