×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Dilution air to prevent explosion

Dilution air to prevent explosion

Dilution air to prevent explosion

(OP)
Hope this is not off topic but hopefully a Chem Eng input may help.

I have been working with the commissioning of burners on asphalt plants for many years in Australia on both gas and oil fuels. I am due to commission one on propane in the very near future and have put in all submission documentation for the burner (eg fuel rates, start gas, purge rates and time etc) to the gas authority for approval of commissioning gas. This has up to now sufficed.

This particular inspector was surprised to hear we intended introducing bitumen into the rotary drum as this is a drum mixer. Obviously he has only seen batch plants which only dry the aggregate and mix bitumen externally. He is worried about volatiles in the heated bitumen or asphalt mix exceeding the limit on LEL values. and requires a dilution air calc submitted. This is of the type found in NFPA 86A and copied into our Australian codes. I personally have never done this before but started working through it. I feel however that it isn't applicable to bitumen for the following reason. Bitumen has a flash point of >250degC. Our normal product temp will be 180deg to 200degC max and an overtemp cutout set at 220degC. Hence there should be no combustable vapours given off of any significance. So a dilution air requirement to keep this from reaching say 50% or even 25% of the LEL is unnecessary. Sometimes asphalt plants use RAP a recycled pavement which could contain contaminants of higher volatile compounds but this will not be the case at our plant where only virgin bitumen is used.

Does any knowledgeable person on explosive atmospheres agree or have any comments?

As an aside to this. Other asphalt plants make a product called cold mix. This is made at a low temperature of about 80degC and definitely adds diesel to the aggregate. If it was required to do a calc on minimum dilution air what values would you put in for the equation for dilution air flow in NFPA 86A.Years ago when involved with emission tests on a specific stack they had to calibrate their instrument to check for hydrocarbons from a diesel fired burner. Hexane was used as the calibrating fluid. So would you use values for hexane shown in the tables we have(ie for vapour generated by 1 litre of liquid etc) to insert in the equation. If so there is one equation quantity designated Q which is the max instantaneous evaporation rate in litres/sec at our operating temp. Where would you find that for diesel.
Hope this isn't too long to respond but the first issue on straight bitumen is the most inportant.
Thanks in advance
Rod.

Rod Nissen.
Combustion & Engineering Diagnostics
nissenr@iprimus.com.au
www.canded.com.au
 
 

RE: Dilution air to prevent explosion

I dont know anything about asphalt plants but flash points are measured under standardised laboratory condtions, eg  ASTM  D93, D56, D1310. The resulting measurement refers only to those standardised conditons and should be only used for comparison of different chemicals or hydrocarbons. The absolute values really dont tell the whole story. You can ignite at less than the quoted flash point if the conditions are right. Also, although your bitumen has a flash point of 250C it will almost certainly be generating significant voulmes of vapour (any light ends for instance) long before you get to 250C.

Regards,

RE: Dilution air to prevent explosion

(OP)
Thanks Geordie. Where would I find information on distillation temps of these light fractions and the % given off as temperature is raised in a typical bitumen?

Rod Nissen.
Combustion & Engineering Diagnostics
 
 

RE: Dilution air to prevent explosion

I don't know if the data for your bitumen is available per say but these numbers can be readily obtained from a Thermal Analysis Lab. This type analysis at one time was done routinely in refineries to classify different feed stocks.  

We used this type analysis on several occasions over the years when we tried different schemes to get rid of solid process waste.  The last time we used a lab was when we were installing Pyrolysis Furnaces to clean polymer equipment.  In our case the polymer melted at 285°C and we didn't see much gas evolved until we got close to the Pyrolysis temperature.  

A lab that does Pyrolysis/GC, Thermogravimetric/FT IR,or Evolved Gas Analysis could do the job in minutes.  Even a Differential Scanning Calorimeter would tell if you have components evolving at what temperatures.    

RE: Dilution air to prevent explosion


At the mentioned operating temperatures the practical LFL may be zero. Thus no air dilution would make the operation safe from the flammability viewpoint.

RE: Dilution air to prevent explosion

What is the heated air temperature coming out of the burner?  What is the distance between the burner and the bitumen inside the drum?

Bill

RE: Dilution air to prevent explosion

(OP)
Hi Bill,
It is a direct flame into the rotary drum (so flame temp at the most) but separated by about 7 mtrs of a dense veil of aggregate and sand. There is no possibility of direct flame onto the bitumen as the burner cuts out if the cold stone feeders stop.The outlet temperature after the area of Bitumen addition as it goes into the ducts is about 230deg. I actually have NATA test reports showing the Bitumen flash point id 344degC.

Rod Nissen.
Combustion & Engineering Diagnostics
 
 

RE: Dilution air to prevent explosion

Rod,

It does not sound like you have a credible risk for explosion.  To test this hypothesis ask yourself some what ifs.  For example:  What if you lost power to the drum?  What if you lost in-feed of cold product and could not get the bitumen out of the drum during a power interruption?  What if the burner malfunctioned and was inputing un-burned gas and carbon monoxide in high concentrations?  

If any of the what-ifs that you can dream up could lead to a explosive atmosphere then you will know what to do.

Regards,

Bill

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources