When I worked at a Coker for several years we evolved from orifices to wedge meters. Until we had the wedge meeters installed, we would run with the individual pass controllers bypass valves cracked open to help avoid low flow trips. Also, you really need to check your upstream temperatures and be sure that you are not too hot or have built up too much coke as the hard little balls of coke that get pumped to the heater have got to go through the orifice. If they are too big, they will result in low flow heater pass tripping. After the wedge meters were installed, they just about did away with the low flow tripping except when a big enough ball of coke would plug the actual control valve. Under no circumstances do you want to run the heater with the pass flow blocked. You’ll ruin that pass flow and pipes of chrome metalurgies are very expensive and the down time would be astronomical in cost. Better to bypass the heater and clean it out, then resume. We had a procedure for not having to come all the way down and would run the coker like a crude unit while we cleaned out the heater. It can be done, but when you put hot charge back to the heater and into the system you have to be darn sure the pipes are dry. Hot charge hitting a condensate build up will blow things apart, it’s not pretty. Hope this helps you.