6 replies to this topic

[CMangesh]

Dear Gentlemen,

 

We have boiler(natural gas 90% methane) with 90T/hr capacity. Boiler have long history of high super heater temperature since commissioning (yr 1984). In 2010 we change boiler burner by low nox burner. Then problem of super heater temperature become worst. Earlier high super heater temperature was observed only during start up of boiler, after burner change it coming frequently & last one year it's constant at 490 deg. C. We check with burner mfg. & took some trials by adjusting air flow & flame size, shape but there is no impact on temperature. In normal condition we used to run boiler with 460 deg. C super heater temperature. If anyone have information related this issue please share it.

 

Thanks & Regards,

 

[pradeep4u]

What type of burner u have fixed type  or tilting burner.

 

1.If u have tilting burner  then  may be ur burner is slightly tilted  upwards.

 

2. if u have gas recirculation fans then u can  damper of gas re circulation fans.

 

It also depends  how ur boiler is designed.

[S.AHMAD]

Hi Mangesh

Have you done heat balance?

Or can you share your heat balance calculation since I have never encountered such problem before!

[cea]

Assuming that you have taken aforesaid measures, it is very unlikely to have this problem persist. Are you facing any problem, bottleneck, deviation in your plant operation? Have you noted same steam condition at various location in plant?

[Shankar12]

What about the silica quantity in steam drum

[Ganesh Buddha]

i think that your boiler superheater heat duty is high in terms of normal operation of the boiler. that is why you are getting high superheat. solving this is altogether a complex and difficult issue. 

 

there is a simple solution to this. add a desuperheater downstream of the boiler. if you already have one, there is no issue at all...

[K RAVI VARMA]

Key Note: "Earlier high super heater temperature was observed only during start up of boiler"

 

My approach to a solution: As the superheater tube bank is in radiation zone, it is subject to higher temperature than convection zone. During cold startup, as the boiler surfaces are cooler, part of the heat generated is utilised for heating the furnace and boiler. As a result lower steam generates from convection zone. Lower generated saturated steam from the convection zone absorbs all the radiant energy in superheater, leading to higher superheat during startup. The degrees of superheat lowers once steady state is reached.

 

Key Note: "last one year it's constant at 490 deg. C"

 

My approach to a solution: 

 

• Probably boiler system has been improved. An increased recovery of heat by preheating the air in an air preheater increases air preheat temperature, resulting in higher theoretical flame temperature, radiation heat transfer & higher degrees of superheat. or 
• Lower excess air than designed can result in Higher flame temperature & higher radiation heat transfer in superheater.

One of the following reasons can increase the wall temperature of superheater tubes

• Scaling of the superheater tubes reduces heat transfer, thereby increasing wall temperature of superheater tubes
• Due to improper burner and flame control local hot spots on superheater tubes may arise, this may result in higher temperature in particular coil

regards

Edited by K RAVI VARMA, 29 February 2016 - 09:39 PM.