Q & A > Question Details
Crude heater is twin cabin type which is operated at 367degC for heating crude from preheat-2 to crude distillation column. There are total 8 passes, 4 passes in each cabin.Pass 1-4 in one cabin and rest 5-8 in another cabin.From past 8 months, To minimize the deviation in all passes coil outlet temperature,when flow is increased in pass 5 & 8,its coil outlet temperature increased rather than decreasing and pass 6 & 7 coil outlet temperature reduced when its flow reduced.Also, this effect is observed in skin temperature.
However, it is confirmed by checking temperature of radiation outlet with temperature gun and ensured the deviation.
When flow in passes 5-8 is made equal, deviation in its coil outlet temperature reduced for the same heater box temperature.
However no such issue is found in rest passes i.e. pass 1-4. What can be the possible reason?
14/08/2016 A: Milorad Popovic, Retaired, milorad.popovic@gmx.com
Commonly we see the coil outlet temperature (Tout) variation in 8-pass heater and do balancing of coil flows and Touts within few percent. If variation up toward ~10% a corrective action including firing side, firebox heat distribution, heat-flux profile should be taken. Even though each pass is designed for equal flow and equal avg heat-flux some tubes run at higher heat-flux, vaporization and oil film temp.
Most probably some heater upset (slop, water, low flow trip, local overheating, tube impingement.,) happened 8 months ago. These coils got coated with inorganics and coke, impeded heat transfer and slightly hydraulics (check coil dp). I assume that in a problematic Cell 2; two coils down-flow and two up-flow what makes difference in two-phase flow regime.Check problematic coils for discolorated spots,vibration and bow tendency, measure dp.
A few Tube skin temperatures to be accompanied by IR thermography to cover all tube length and hangers much hotter than tubes. Try to adjust burners for uniform heat distribution, flatter heat-flux profile without distinct max. flux, indicated by glowing yellow hangers and near refractory. Measured Firebox floor and bridge wall temperature are not enough so include also IR thermography for heat flux profile. When a heater draft excess air and burner firing adjusted to generate proper as designed heater firebox flux and temperature profile then equal coil flows could indicate actual Touts problem. Balancing flows should never be compensation for poor firing, firebox heat distribution,.
Calculate and compare the coil mass velocities in relation to calc'd heat flux. Generally, higher mass velocity increases thermal conductivity, reduce oil film dT so that tube is less heated for desired Tout.
18/05/2016 A: Eric Hennings, Technip Stone & Webster, EHennings@technip.com
Verify that the actual FT is on the physical pass. The FT may be wired to another.