Thermoprobes vs boiler start up curves 3

[miller9]

Thermoprobes vs boiler start up curves
"pressure/temperture".
Are thermoprobes really required?
Startup Temperature: Monitor flue gas temperature ramp from o to 100C RATE CHNAGE 27C HOUR then 100 to 499c RATE CHANGE 55 C HOUR for boiler startup preventing overfiring that could damage superheat tubes, or underfiring that could potentially send wet steam to the steam turbine damaging buckets.
MONITOR TOP AND BOTTOM DRUM TEMPERATURES 101C DELTA MAX.

 

[davefitz]

yes, they are required.

although some operators might think they can simply program the firing rate vs time, this does not correct for the effect of firing the top row of burners vs bottom row of burners.

Large boilers have non -drainable superheater elements that hang over the furnace. Testing in 1960's at a Con Ed plant showed that it takes over an hour at 10% firing rate to dry out these legs. Until those legs are dry, you cannot exceed 1000 F FEGT ( about 10% heat input at 25% MCR air flow).

Another modern on-line method to confirm dry out is to monitor many ( 50 ) tubes at the outlet header for the non drainable superheater. Once all measured temps ware within 30 F of each other, you can conclude the unit has dried out. It would also help a lot if the plant has installed a HP turbine steam bypass system, to allow a large flowrate of steam to help cool and dry the superheaters prior to startup.

If you do not have reliable thermoprobes, then you can characterize the FEGT vs firing rate and which row of burners are in service. Alterantively, you can use acoustic pyrometers, but they are only accurate if yo have the same excess air that they were calibrated ad ( the y work on soundpseeed, which varies with the fluegas molecular weight, which in turn varies with excess air).

 

[rmw]

Well, they are not absolutely required. The old mechanical types based on soot blower technology were not very reliable and often were abandoned or not used at all. Plenty of boilers equipped with them started up many times without them.

But, that said, they are highly recommended as very well explained by davefitz above.

You can also use infrared types that just look across the furnace not penetrate it. They have been shown to work well.

rmw

 

[boilerservice]

davefitz had some really good answers to your question.

Remember that Thermoprobes measure spot temperatures only. The acoustic pyrometer [single channel] will give you average temperature -- and you usually can calibrate for start-up as well as normal load so you can get reasonable accuracy - but it is an average, and the peaks can be well above that average. Acoustic Pyrometer system with multiple channels can give you a fairly accurate map of the furnace temp and can be calibrated for standard start-up conditions.

All methods have some drawback and the size of the boiler [width and heighth of SH inlet plane] has a lot to do with selection of a method. On a small boiler, a thermoprobe and good common sense as to its limitations [remember SH leg temps too] will probably suffice. On a very large utility boiler -- Acoustic pyrometry multi-channel would be best.

HOWEVER -- probably, the best point that davefitz made is that clearing loops is essential -- even with good FEGT measurement - you really need TCs on your outlet legs to see the loops clear. That is EXTREMELY important.

 

[davefitz]

Also, most coal fired US utility boilers are reheater type units without a steam to reheater steam bypas system. There is no steam cooling of the reheater tubes prior to synchronization of the steam turbine, therefore, prior to turbine sync, the reheater tubes must be protected by keeping the FEGT below 1000 F ( indicated 1000F, actual 1100 F).

In Europe. most coal fired uint use drainable superheaters and large steam to reheater steam bypass systems. Thise units can startup musch faster than the types in the US. Also, those usits have therma stress monitoring thermocouples on thick walled components, so there knowledge of the stress limits allow faster startups.