1 reply to this topic

[mykid]

Hi I am a process engineer, currently working on a project and come across a problem while trying to identify the hazardous area.
Hope the expert here can help to throw some light in clearing my doubt.

The project is a gas compression system drive with gas turbine and air cooled aftercooler. The process gas is classified as Group 2A/2B, and having a specific gravity of 0.9, which can be lighter or heavier than air according to API 505. The area around air cooler is classified as Zone 2 due to abnormal HC leakage through flange. The question is do we consider the turbine exhaust as a source of ignition? The exhaust is about 500C, and currently the stack is within the 3m radius for Zone 2.

Argument: The gas is a mixture of light and heavy components, taking n-octane as the heaviest component, the autoignition of this component is about 200C. Do we consider the flue gas temperature as a source of ignition for the flammable gas and therefore has to ensure that the stack is away from the Zone 2 classification? or can we argue that the gas will only ignite if the metal temperature is exceeding the autoignition temperature, not the gas temperature? How to calculate the LEL level if the flammable gas leaks from pipe? Can we take into account of the dilution from the flue gas?

The air cooler is force draft which will encourage air flow through cooler which will help the gas to disperse more rapidly in case of leakage.

Hope my question is clear.

[KR]

Hi

We need to classify the hazardous zones especially for the electrical equipments [e.g. British Standard, BSEN60079-10 2003] and for that auto ignition temperature [AIT]of the flammable gas/vapour must be known to classify the electrical equipments to ensure that they are protected such that they do not heat up [heat due to electrical current] to level of the AIT.

1)for calculation of Zone: you need to consider various credible scenarios for the leakage based on component failure rate e.g. pinholes from flanges

With zones, you need to mention temperature class, gas group etc.

2)Temperature class : For For that you need to find out autoignition temperature of the flammable gas mixture [Hint: First find out the mole fraction of each gas component and then the AIT for the gas mixture]. Once you know the AIT of the gas mixture, you can designate temperature class e.g. T1, T2,....T6 [where T6 = 85 deg C and T1 = 450 deg C] for the zone.

3)Gas group : You know the gas group i.e. IIA/IIB. They are based on Minimum Ignition Energy [MIG] of the material.

In your case, let's say, AIT is 200 deg C. Therefore, let's assume all the electrical equipments are classified as Zone 2 IIA T4. But,as you mentioned, stack is within the zone radius of 3m and stack gas temperature is 500 deg C which is higher than the AIT of the gas mixture [i.e. 200 deg C]. Therefore,as per my opinion, stack would be considered as a ignition source [provided AIT of the gas mixture is less than 500 deg C]

Hope,it helps.