Solids in Gas Stream, Above auto-ignition Temp

[ProcessRookie]

Hi All,

I have an inerted vessel which normally contains a dry solid powder at above its auto-ignition temperature. The current design is a bursting disc only, vent goes to atmosphere at ground level. This will obviously catch fire as soon as it relieves so we want to change the system to remove the open plant fire hazard. The idea going around the plant is to use a dump tank open to atmosphere which will be designed to withstand a fire for a short period, keeping the bursting discs as is. I don't like this idea as the fire could spread back to the vessel due to the BDs remaining open. Putting a PSV after the discs is also risky because the solid could block the PSV.

Any ideas/help/thoughts would be much appreciated. its a real headache of a problem as cost will be a major factor as we all know.

The scientist describes what is; the engineer creates what never was.

 

[georgeverghese]

In the oil / gas industry where we deal with flammable gas and liquids, we purge the vent collection headers and the final vent knockout drum with some inert gas continously (N2 preferably) so we keep O2 out of the vent headers and the connected vessels.
Another option that is not so awkward as a BD is a buckling pin operated relief valve - these are now approved by the ASME as being equivalent to conventional RVs' in terms of relieving integrity. But these are not spring loaded reclosing devices. But you can at least manually close the valve from remote when you believe the relief event is over.

 

[ProcessRookie]

Hi georgeverghese,

Thanks for that. I've had a look at rupture pin technology on line and it looks good. Are they full orifice devices? The advertise as being good for slurry & solid service so could be a winner. You say they can be re-seated remotely, I assume this means the operator has to physically go to the valve and stick a new pin in place or is there a way to do this from SCADA?

In terms of the N2 purging, do you then just have a small flow of Nitrogen continuously venting to Atmosphere or Flare? I won't be able to put this line to flare, but there may some guidance on safe distances for N2 venting I can use for locating the vent line I suppose.

The scientist describes what is; the engineer creates what never was.

 

[georgeverghese]

My recollection on this rupture pin is that an pneumatic actuator is fitted to this valve to enable closing it if required. You could wire this up to enable remote lose from the CR or local close. The rupture pin cartridge assembly can only be replaced at local, but you can do this without breaking any flanges.
Yes, we always keep this N2 flow going on all flare and vent headers - there are recommendations on what the min flow should be to keep out O2 from getting into the vent headers. A low flow alarm at CR is configured to alert the operator of purge gas flow. You could set this up as a PALL or FALL an RO and a final globe valve for fine flow adjustment - you'd typically want to set up the RO in critical-choked flow, so purge gas flow wouldnt be affected by the expected range of downstream pressures.
All flare and vent headers (and connected equipment upstream) should be set with a mechanical design pressure of at least 700kpag to account for vent header internal deflagration pressures generated as a result of total loss of N2 purge.

 

[ProcessRookie]

Thanks again, that's really helpful.

The set pressure of the RPRVs will be 1barg, so it will be a fine balance for me in terms of meeting minimum N2 requirements and not exceeding back pressure limits of the safety valves.

Do you know what guidance the deflagration pressure rating information is in?

The scientist describes what is; the engineer creates what never was.

 

[georgeverghese]

Since N2 purge flow is low, this should not affect the permissible backpressure limit on these RVs' even if the RV set point is 1barg.
Deflagration pressure values are those I've applied from oil/gas OpCo design guidelines. In some cases, a minimum design pressure of 350kpag may be acceptable in low pressure vent headers, while 700kpag is applied in high pressure vent and flare collection headers. Then again, these are values for hydrocarbons seen in oil/gas installations, and may not be applicable to these powders in your case. I'd speak to a process safety engineer in your industry for what values are applicable in your industry. If no values are forthcoming and the consequences to people and plant capital assets of an internal deflagration in your plant are high, a safe approach may be to auto shutdown the plant on loss of purge.