Flame Sensors purge gas flow

[sfacchin]

S!
Folks,
is there any standard, calculation or rule of thumb that specifies the minimum purge gas flow for instrumentation and flame detectors in furnaces? In some applications, like sulphur recovery units, this can be crucial, because if the flow is low, sulphur/ash may deposit in the sensor's tube and in excess it would affect the Claus reaction stoichiometry relation.
Thanks for the answers

 

[Matt17]

Hi sfacchin,

I suggest to ask to flame detectors vendors, or - in alternative - take a look of vendor's specs (if you have).

Regards,

Matt17
Process Engineer

 

[danw2]

There are multiple standards for purging combustion chambers and piping for combustion and flame safeguard control, which might give you an idea for calculating purge times and purge rates for whatever volume is involved.

NFPA 85 (2001) Boilers (pgs 85-27)
[2.5.2.4.1] (single burner)
(8) b. Purge airflow shall reach no less than 70 percent of the airflow required at maximum continuous capacity of the unit.
(9) The purge shall be for at least eight air changes.

[3.4.3.3.4] (multiple burners) (pg 85-38)
Purge rate shall not be less than 25 percent of full load mass airflow. Purge rate shall not be greater then 40 percent of full load mass airflow for coal-fired units.

[3.4.3.3.5]
Components (e.g. precipitators, fired reheaters) containing sources of ignition energy shall be purged for either
(1) a period of not less than 5 minutes or (2) five volume changes of that component, prior to being placed in operation, whichever is longer.

NFPA 86 (date?) (furnaces, ovens and kilns)
[8.4.1.2] requires a minimum of four (4) system volume changes.
"The system volume shall include the heating chambers and all other passages that handle the recirculation and exhaust of products of combustion." (Ref: 8.4.1.2(B))

North American's Combustion Handbook Vol 2 1997 says 5 air changes (pg 79)

National Board of Boiler Inspectors

http://nationalboard.org/index.aspx?pageID=164&ID=204

"at least four air changes of the combustion chamber and gas passes"