Bolt Specification for Diverter Damper

[ashaari]

Dear all,

I am repairing a diverter damper for my waste heat boiler (producing 13 ton/hr of steam at 44 barg drum pressure). 3 out of 6 bolts which are holding the diverter damper shaft to the diverter damper plate are broken, and I need to replace them. However, we don't have any as-built drawings for the damper, thus I am unable to find the specification for the bolts. (I suspect the bolts which are broken are of the wrong specification).

Appreciate if anyone could suggest a bolt specification for my application. My damper is approximately 3 tonnes. The bolts are exposed to approximately 480degC of exhaust gas, with pressure approximately 100mmH2O. Is A193 B7 sufficient? Or B8?

Thanks!

 

[boilerone]

B7 should be good for the 480 deg. C temperature but I would look at the reason why the bolts have failed because even carbon bolts can still have some strengths at this temperature.

 

[MJCronin]

boilerone....we don't like to practice "whatdoyathink" here.

Do you have a concrete reference on why B7 is "good" for 480 C ?

According to table 6.3 of this NORSK reference, B7 bolts are only good to 400C

http://www.standard.no/PageFiles/1174/M-DP-001r1.pdf

B16 or B8M are better choices. Also note the recommended full nut materials......

Banu, can you come back to this thread and tell us about your final choice and the resons for the bolt failure ?

 

[boilerone]

Make no mistake, MJCronin, according to the question asked this bolt is for the boiler flue gas damper which is a non-pressure part for which application B7 can be used for much higher temperatures plus B7 matches the typical low-alloy damper blade and frame materials used for the 480 deg. C temperature.

 

[rmw]

I want to know if all the original bolts were the same size and grade and if so what grade that was where some broke and some survived.

rmw

 

[MJCronin]

boilerone....

Will a B7 bolt fail.....still seems to be a matter of opinion.

If B7 is "of the same material" as the rest of the existing assembly is simply immaterial

Again, do you have any concrete reference on why B7 is "good" for 480 C (~900F) or higher?

Or is this just a hunch ?

I still maintain that a higher quality fastener material is required ..... otherwise, can you explain why are such fasteners on the market being sold ?

 

[rmw]

Rather than conjecture, I have another question for the OP.

You state "the bolts are exposed to 480degC". Does this mean that they are directly exposed, and operting at 480C, or that they are attaching equipment parts that are exposed to that temp?

The important question is at what temperature do the bolts actually operate? This could have a direct effect on the answer.

rmw

 

[cloa]

and for how long?

 

[ashaari]

Dear all,

Really sorry for the late reply, forgot that I posted this question actually! thanks for your queries.

Just to clarify my statement "I am repairing a diverter damper", it's actually my friend's repairing the diverter damper, and he was the one actually asking the question I've asked. When I came back to him the next day, he already boxed up the boiler and replaced the bolts with B8. I'll try to post some photos tomorrow.

According to my friend, 3 of the bolts are fastened at one end of the damper, while the rest are fastened at the other end. The bolts which experienced failure (apparently frequent) are the ones near the pneumatic actuator.

boilerone,
Yes you are right, we should look into the reasons why the bolts have failed.

MJCronin,
B8 and B8M...are they interchangeable? from ASME II Part D, the max temperature limits, min YTS & min UTS are the same.

rmw,
1. I will get back to you if all the bolts are the same grade, and what grade they are.
2. The bolts are directly exposed to max of 480degC exhaust gas since the bolts are inside the duct. Actually its normal operating is around 420 - 440degC, but the gas turbine exhaust has a trip point of 480degC, so I assume that the equipment directly exposed should be designed to that temperature.

cloa,
the gas turbine is actually a standby and we don't operate it continuously throughout the year. my estimate would be the boiler operates roughly 4 months out of a year, with around 4 startups and shutdowns (due to the standby nature of the gas turbine).

Thanks guys!

 

[r6155]

Are these bolts coated?. Be careful with coated bolts at 480°C.

Regards

r6155