Q&A: What is the typical approach to reduce existing flow/pressure alarms setpoints when trying to run below normal throughputs?
Our expert’ recommendation:
On burner and combustion sides, it will all boil down to whether fuel composition is different or not. If fuel composition at turndown condition is the same as for normal operation, we do not recommend to change the fuel pressure low/high limit settings. In case of a different fuel composition, the consequences have to be studied. Basically the performance curve at the burner will be altered, so the performance curve in the new conditions has to be checked. Based on that, new settings for trips and alarms can be defined. On the other hand, it is necessary to ensure that the already installed instruments can manage these new conditions. To do so, two checks have to be performed: one for fuel pressure elements and one for control valve(s). The instruments may have to be changed to ensure proper operation and accuracy in measurement at low turndown.As far as flow transmitters are concerned, molecular weight setting may have to be corrected in the compensation function at DCS level, as well as the alarm setting if need be, that is to say which should reflect the burner low limit updated with the new fuel composition. Additionally, in case, the heater is equipped with automatic lead-lad control system, necessary changes in manual setting of fuel LHV or wobbe index may also be required to correctly calculate the fuel flow demand and combustion air flow demand in DCS.
Thierry Tison, Furnace Technology Manager, ROC France