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Bunker 'C' Concern

Bunker 'C' Concern

Bunker 'C' Concern

(OP)
In our process we receive Bunker 'C' from our heating plant at about 150 degs F. It is further heated through heat exchangers to 275 degs F. before it is atomised in our burner guns in the induration furnace.  Normal shutdown of this system would require light fuel to be added to the lines to flush out the residual bunker.  This year during the mtce shut in June we are looking at a complete shutdown for 24 to 36 hrs without flushing the lines with light fuel.  As the fuel cools in these lines, will they become plugged with cold bunker. What other issues can we expect with startup?

RE: Bunker 'C' Concern

I can only ask a few simple questions here, there will doubtless be some answers from the learned members that will be of use to me as well as you.

I would start by wanting to know something about your system so as to anticipate any vulnerabilities.
For example:
Is this a ring main supply to a number of burners with a feed and return from the ring to the individual burners?

If so, I assume you will bring the ring main temperature back upto to temperature first and then the only problem is whether you can get good enough circulation in the individual burner circuits to quickly get the correct viscosity before ignition.
If not then what is the supply system?

I assume you have independent means to heat the fuel, not just steam raised from the exhaust gas.

Is the pipework simply insulated or is it heat jacketed? I would then want to know if it is safe to heat the fuel in the pipes before circulating or if this risks hot spots and carbonising some of the fuel.
If it is simply insulated, then what temperature do you expect the fuel in the lines to get down to?

Are the pumps OK to circulate cold viscous oil? One would hope so but usually the fuel will be at around 15cSt and here it will range upto what, 380-600cSt? But if it cools down in the pipes much below 30°it will perhaps reach 7-10,000cPs?
In bitumen ring mains cold spots have resulted in such hydraulic pressure being exerted as to stretch flange bolts so that when temperature is reached, the system leaks at every joint. Of course, bitumen can set solid. Your fuel oil should not. But if it is around 20° before you start the pumps and you have pressure relief valves on the pumps, you might have real problems getting any appreciable flow and this may prolong the time of inefficient combustion or the time until you can get the fuel upto temperature.

How do you control the fuel heating?
I imagine that either you are using temperature set point control or a digital viscometer but I have seen a number of plants where they have tried to use other viscometers with not very good success and this will be aggravated if you try and start cold. (At one power plant, the subject of some serious EPA fines in an out of court settlement, they had a number of viscometers per burner - one duty and one standby because the duty viscometer would fail frequently even at operating conditions. Cold flow would have been a real issue - but they were required by the EPA to use viscometers because of the issues with temperature control when using fuel of this uncertain quality).
 
I assume the pumps and meters all have suitable clearances to handle the high viscosities. I don't know the usual here but if the pumps and flow meters (if fitted) are toleranced for 275°F and 15cSt, they may not be too good at 60-80°F and 7-10,000cSt. It is worth checking the pump specifications, refer to the manufacturer if necessary. The usual procedure is to shut-down and start up on distillate so the pumps, and some other equipment, are not expected to handle these duties. It then depends on whether failure mode anticipates some repair and maintenance or that everything should survive the unusual conditions.  

The other worry would be if you have any significant period of time during which you will be firing with the fuel at higher than optimum viscosity until the fuel heaters are back under full control, as this will result in poor combustion and extra pollution. Dependent on where you are, this may prove a problem with the regulators especially if you have CEMS.

PS is there a reason not to shut down on diesel as usual?
Diesel is a good way not just to enable an efficient start up from cold but also to help flush the lines of debris and junk that has built up over time. Bunker C isn't the cleanest of fuels and in burner systems doesn't get much in the way of treatment, unlike engines.

I'd also like to confirm that this really is Bunker C. I remember a refinery I dealt with reported their fuel as bunker C but they were consuming anything and everything including waste streams.

What burners are being used? This may have some bearing on the problem too.

JMW
www.ViscoAnalyser.com

 

RE: Bunker 'C' Concern

As JMW points out with his polite questions, you didn't give us enough information to hazard an answer.

If you are in a cold climate, I can see you out there with a spoon digging out the fuel lines.

Give us a little help so we can try to help you.

rmw

RE: Bunker 'C' Concern

(OP)
In our system there is 1 line coming from the heating plant(no return line). When the bunker enters our plant it is pumped by positive displacment pumps through an HTHW heat exchanger which raises the temp to 275 deg f. then to the indurating furnaces through a loop by which unused fuel is returned back to supply the pumps and maintain pressure in the line between 80 and 100 psi.  There are 6 furnaces that also have their own loops.  There are back pressure valves located on each furnace and in the loop to maintain a contstant pressure through to all of the burners.

The 6" line coming from the heating plant is insulated, however most of the lines in the plant are not. Bunker 'C' temperature setpoint is maintained by a control valve at the HTHW heat exchanger.  We do not use digital viscometers.

The use of light fuel to purge the line coming from the heating plant was deemed to be a risk for people who have to cut and weld new pipe in place.
We are using #6 fuel with KLH burners.  

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