BOILER INCIDENT -LOSS OF 24 V CONTROL SUPPLY

To avoid a failure repetition put a second supply for redundancy connected in such a way that if 1st supply fails second supply takes over with a time delay small enough to avoid such problems.

Do you know why the power supply failed?

My Valuable Suggestion is - make damned sure it can't happen again! Didn't it occur to anyone to think what might happen if the entire control system died because of a power supply failure?

My Other Valuable Suggestion is - turn off your caps lock. Posting in upper case is interpreted as shouting, and generally frowned upon.

Seriously, I'm sorry you guys got into this spot, but I can't suggest anything more than the above. Its out of my field.

Our control supply was backed up with two large banks of batteries, do you have back up? Also auxiliaries main supply should have been running off the alternator not outside system, so that when the turbine tripped supply would be lost to all the auxiliaries such as the feed pump.

Regards JD.

First, its hard to read when all are in caps........

I had a controller that was giving inconsistant problems......The issue I had was a grounding problem. Check your ground

phoenix911

In line with what has already been posted, I would suggest a system redesign so that on control failure everything shuts down safely. This might be as easy as a zero volt coil on the main system breaker which will trip that breaker when 24Vdc is absent.

Chas

Sorry for posting the problem in caps, i would like to explain the control supply system in the plant. The plant is designed with 2 rectifers, supplying 26 v dc to two half busbars. It has a bus-coupler . When one rectifier is under maintenance , bus-coupler is closed and the 26 v supply will be fed by one rectifier. On the day of incident, one rectifier was under maintenance. the other rectifier's out put DC feeder breaker to the busbar tripped. 26 v dc bus became dead and caused loss of 26 v to the unit. With each rectifier, we have a set of battary bank. But it is connected up steam of the load side breaker. If the load side breaker trip, battary supply can not fed the bus. Due to the oss of 26v control supply, The DCS lost power supply. We have one protection that, if 26 v power supply is lost to boiler burner management system, all burners will trip and gas master trip valve will close and turbine control valves will close. But, the all the HV& LV motros will remain in service with out any command control. Turbine can be tripped from desk with 220 v dc control supply. Since feed pump was in service and could not be stopped immediately, boiler flooded with water.

I would suggest the lines are becoming clogged with some scale or impurities from the water and it is now affecting the controls. Inspection time for the pipes .

I think the system design is poor and not analyzed properly for the risk factors and its solutions.

Power supply can fail for various reasons and worst of it may be fault leading to overloading condition. It is easier to place multiple power supplies to let one good one take over. However, it will not be able to solve the problem of overloading. You need to have an automatic change over circuit also to over come this type of fault. The circuit which is drawing more current need to get disconnected quickly and if possible need to get automatically replaced by another similar function circuit to continue the job. This technique requires redundant monitoring or automatic fault correction mechanism.

You can not expect a simple answer to a serious problem of a design flaw for a process control plant.

Why are you shouting

Apparently the main 24V breaker was overloaded, maybe it was designed for just one section of the bus, you should check the thermal protection setting of the breaker. If that's not the case, maybe there was earthing faults. No matter what, the turbine should be checked carefully, water in steam will cause the rotor to bend.

I think you should set up 3 action plans in parallel:

1st: Inspection and maintenance of the plant sections which have been or might have been affected by the water inrush. assuming that you have not designed or build the plant, I would contact the companies which have designed and build the plant. Describe what has happened and ask for their advise as which inspections and / or maintenance actions have to be done. Please make sure that also potential consecutive fault will be considered! Then follow their advise.

2nd: Further to your description of the incident I assume that the control system was not adequately designed / built. As any malfunctions in the system might potentially result in harzardous conditions a "failure mode effect and consequence analysis" (FMECA) should have been made to prove the control system design. Is such an analysis available, has it been done properly, have necessary risk mitigation actions been carried out? - If not, you might be able to hold the company which has designed / built the control system responsible for carrying the cost of all corrective actions you have to take now. Check your contracts!

3rd: Review the control concept, identify risk potentials and define actions to prevent re-occurence of control system failures. I am pretty sure that you will identify other hazard potentials which have not been covered adequately .

Good luck!

nudnik

Please stop SHOUTING.

To prevent recurrence, arrange for a level of redundancy to be designed into the 24V systems so that, in the event of one system tripping out, another takes over while an alarm is raised on the first system.