Feed Swings

[Freddy Martinez]

Since our last T/A, one of our passes have been swinging in increments of +/- 1000 BPD. A bad orifice reading did cause us to plug the pass last month during a rainstorm forcing us to pig the furnace. Since coming back up on rate the pass still swings, but with the added insulation and work done on the orifice taps I tend to believe the flow as evident by the inlet pressure tracking with the flow. I have already checked for any light material possibly making it to the furnace. The flow rate, inlet pressure, and valve output  profile points me to a sticking CV, but when checking the movement of the valve it does not appear to be sticking. Our plan is to pull the valve anyway for an inspection, but I am still working this in parallel incase the CV is not the culprit. Our charge pumps puts up a constant 800+ PSIG discharge pressure.

What I mean by the profile resembling a sticking valve is the flow and pressure begins to fall and the CV opens, but takes a movement of 5% on TDC for the flow to finally increase at which time it is increased by 1500 BPD. One thing to note is when we had the valve on by-pass, the flow still moved and at one time the inlet pressure dropped by 70 PSIG with the valve on by-pass. I have trended this pass action with the other 7 passes and did not see an indication of them stealing flow from the trouble pass during this time. We currently have the BFW injection on manual and isolated the sweeping BFW with no improvement. The only thing helping us not to trip the furnace is that we are 3000 BPD above the low flow trip point. This pass is at the end of the header. We have already checked the tuning of all the other passes and these were deemed good.

Any suggestions would be greatly appreciated.

[antonio saura Cebrian]

according that you said this header going to the heaetr is no symetrical , and that that pass is in the end of the header. what type of control valve do you have installed? long time ago in one of the our unit we have something  like that (swing the flow and the pressure)  we have install rotary or rotative valve, the problem  was with the solid coming from the bottom of the fracc, because this solid  was accumulating in this valve (the solid preffer go straight a head to the end of the header)  until this valve
the problem was reduce when this valve was open for the operator , for that reason we took how rutine open this valve 100% and return to the initial position than fast as the operator can do it , one time for week
 
also if I understood you said if you put in bypass this valev it had the same behaivor, this swing can be for the velocity medium ? or some leak for the emergency steam?

[Claus Graf]

I agree with espana2005, you might have solids accumulation at the end of the header. If the header is the same diameter all the way, the last pass will have a lower flow (speed). The coke will give you problems with the orifice. Does your coker have a strainer inside the fractionator? Or does it have strainers on the suction line of the heater charge pumps? What is the mesh size?(What is the biggest size of coke that can go through?)
 
Regards
 
CG

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