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We have two boilers working in paralel. However, due to high steam generation from the waste heat boiler in CCR unit (about 25%) being higher than design, both boilers are maintained operating at about 50% capacity of design. Still steam generation is higher than our requirements. We are maintaining both boilers running due to reliability concerns as stopping one boiler will lead to a delay in startup in case of emergency?
So could you please share if you have similar experiences and is there any hot stand-by procedure for boilers or any procedure that can help us to immediately startup a boiler in case of a running boiler trip as now there are huge losses of energy due to unused steam venting and condensing.
 
Answers
04/03/2020 A: Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
Your desire to keep both boilers operating is well-founded. Leaving one boiler off-line on standby has the issue you describe about responding to a trip, but also the off-line boiler is likely to suffer accelerated corrosion. Keeping both boilers on-line at turndown is a better situation. I’m not aware of a hot standby procedure. Instead, I’d approach the problem in three stages:
Firstly, why has the waste heat boiler steam generation increased so much? Is this due to fouling of upstream heat exchangers that would usually recover heat within the CCR unit? If so, a plan should be developed to clean the offending exchanger at the next opportunity.
Secondly, what stops you operating below 50% capacity? If it’s a multi-burner boiler, can you shutdown a burner? If not, your burner supplier might be able to provide guns that can operate at lower duty (though this might reduce maximum capacity too). If it’s a controllability issue on the water supply or steam pressure control, can this be addressed by retrimming a control valve or pinching in an isolation valve somewhere? (Bear in mind using an isolation valve in the steam system like this will inevitably damage the valve so if you do take this approach you should plan to replace it at the next shutdown, and your risk assessment / management of change should consider the fact that you may not be able to use this valve for isolation until it is replaced.)
Finally, if you cannot find a way to reduce the steam production from the waste heat boiler or the main boilers, can you find a constructive use for the additional steam? Do you have pump pairs with one motor driven and one steam turbine driven? If the steam system is balanced, using the electric drive is usually more efficient. If the system is out of balance, switching to turbine drives may be more efficient. If you have an FCC, does the power recovery train include a steam turbine and a motor/generator? If so, can you put more steam in the turbine and generate more electricity? Can you increase reboiler duty anywhere and gain some fractionation benefit?
04/03/2020 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
I would look at adding additional steam consumers. Replacing a large motor with a condensing turbine is one option. Normally that’s not a desired option but if you have excess steam it becomes a effectively free energy. You would need to think about impact on relief scenarios and the cooling water system. If you are not using adequate stripping steam in your crude column you could up the steam to help your steam balance while also improving yields.

If you go to one boiler on line, you run the risk of shutting the refinery down if the boiler trips. You might be able to develop a load shedding plan to shut down selected units to avoid losing the whole refinery with a sudden boiler shutdown.
04/03/2020 A: keith bowers, B and B Consulting, kebowers47@gmail.com
A first step is a thorough analysis of the Minimum Firing duty of each boiler. It is vital the dynamic response of the boilers and system are evaluated to ensure the desired 'security' is real. A 'steady state' analysis is not sufficient to evaluate the system. In many such systems, a complete DYNAMIC analysis will find the Hp header and 'mud drums' will 'flash to empty' causing boiler trip on low water level when a large load suddenly hits--such as a major power failure causing 'steam driven spares' to start-up.

Of course, this is precisely when the need for Hp steam is most critical. Over the years, several complex refiners have experienced sudden complete power failures, even with two or more 'independent' HV feeds.

Several refineries have suffered major vessel overpressure damage causing months of outage for repairs and recertification, with a primary cause being a pressure relieving system 'inadequate for the complete power failure design case. ' The presumption of 'complete power failure' had been discarded as the design case for the flare system because of the multiple HV feeds to the plant, and those 'multiple HV feeds were NOT truly 'independent,' but merely multiple local connections to the single power grid.
Boilers running at minimum firing rate usually cannot ramp up fast enough to avoid 'loss of level' in the Hp steam drum when numerous 'steam spares' automatically start on loss of electrical power. A good system DYNAMIC analysis will help develop a 'spare start-up sequence and plan ' that will avoid a Boiler Trip. There are usually several significant steam spare drivers whose start can be delayed several minutes on power failure without risk of equipment damage or product loss. Only a Dynamic Analysis of the SYSTEM can identify and show a workable strategy for dealing with a major step increase in steam demand that does not cause boiler trips.