2 replies to this topic

[Mallon]

HI , 

 

Please can someone offer some advice on the attached purging arrangement. 

 

 

 

We are sweep purging a small mill with nitrogen. Nitrogen is vented out from the mill via a 1.5" Vent line shown on the left. Installed in the vent line will be an oxygen sensor. 

 

Before milling nitrogen is bled into the mill via a 3/4" Tir-Clover Port. There is also a bearing purge bleed into the mill. The bearing purge must be 0.2-0.3 Barg above the pressure inside the mill chamber. 

 

We must get the oxygen level inside the mill from 21% down to 2% . 

 

The volume of the mill and liner system is 0.07727 m3

 

I have used the following calculation to calculate the volume of nitrogen required / number of changes require to get the system down to 2%

 

Number of volume changes: i = ln (ca ⁄ce) (EQ1)

Volume of inert gas required: VN = i x VB (EQ2)

 

Whereby: i = Volume change

Ca = Initial concentration

Ce = Final concentration

VN = Volume of inert gas

VB = Volume of vessel

 

i = ln (21⁄2) = ln 10.5 = 2.35 Air Changes

 

VN = 2.35 x 0.07727 = 0.181 m³ Of Nitrogen

 

So further to the above would like some advice on the below. 

 

1. How do I calculate the time for the system to get down to 2% Oxygen. We want to bleed nitrogen into the system at a lower positive pressure as possible while not taking to long to purge say 5-10 minutes max. How would i calculate the pressure and flow rate for the nitrogen coming into the mill. 
2. Once the system reaches 2% we plan on using the oxygen sensor to monitor the oxygen level and turn the nitrogen flow on and off if needed. Is this recommended or is it better to continue the purge during milling operation. 
3. If were to have a failure on the vent line valve what would be the best fail safe for an overpressure. Bearing in mind the liner system would most likely pop before a valve would. 
4. The vent line is currently going to atmosphere. How would we ensure the pressure drop in the pipework isn't too large to cause a back pressure taking into account the pressure and flow coming into the mill chamber. 

Any help would be appreciated. 

 

Paul

 

 

Attached Files

•  Nitrogen Purge Panel.JPG   44.39KB   1 downloads

Edited by Mallon, 14 July 2023 - 10:11 AM.

[breizh]

Hi,

Consider the paper attached with examples to support your query.

 

On a safety viewpoint make sure your sensor is measuring a representative sample (well mixed), the O2 probe/sensor must be well protected from dust which can affect the reading.

Only experiment at the start up will tell you how long you need to add Nitrogen to reach the target, don't rely on timer only but on measurement with a calibrated O2 meter.

You should have a timer activating opening a solenoid valve to supply N2 in your mill for a given time (not less than?) and O2 measurement to confirm.

 

NB: You are going to use more N2 than calculated. Stay on the safe side.

 

Breizh

[shvet1]

 

1. How do I calculate the time for the system to get down to 2% Oxygen. We want to bleed nitrogen into the system at a lower positive pressure as possible while not taking to long to purge say 5-10 minutes max. How would i calculate the pressure and flow rate for the nitrogen coming into the mill. 
2. Once the system reaches 2% we plan on using the oxygen sensor to monitor the oxygen level and turn the nitrogen flow on and off if needed. Is this recommended or is it better to continue the purge during milling operation. 
3. If were to have a failure on the vent line valve what would be the best fail safe for an overpressure. Bearing in mind the liner system would most likely pop before a valve would. 
4. The vent line is currently going to atmosphere. How would we ensure the pressure drop in the pipework isn't too large to cause a back pressure taking into account the pressure and flow coming into the mill chamber.  

 

1&2/ CEN/TR 15281, also AGA's Purging Principles and Practices and  AIChE CEP magazine feb.2001 may be useful.

3/ No enough info has been provided. See overpressure protection in an applicable local code.

4/ Subject to a hydraulic study, also Crane TP410 or calculator (one, two, three, four, five) may be usefull

 

It is recommended to consult an experienced process engineer for proper design of inerting of this equipment.