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butagirl  
#1 Posted : 01 December 2017 14:33:04(UTC)
Rank: New forum user
butagirl

Hi all,

I am in the process of carrying out a hazardous area assessment on a solvent recovery plant and I have come across an area where an air-operated diaphragm pump is being used to pump a flammable solvent at atmospheric temperature (well below its flash point of 59C). In the past, this has generally been perceived in-house as suitable because the pump has no sources of ignition, but since the flammability risks in this case pertain to the formation of flammable mists, a diaphragm failure could cause a solvent/air mist to be emitted from the air exhaust. There is no mention of zone extents in IP15/EI15 for diaphragm pumps in the case of diaphragm leakage (other than for double diaphragm units with barrier fluids), but my instinct says it's likely to be more hazardous than using a sealless electric pump for this duty. Obviously, an electric pump would still need to be suitable for the zone, but can anyone tell me whether my instincts are correct and what likely zone extents I could assume for a diaphragm failure (air supply approx 5barg)? I have no means to carry out detailed dispersion calculations.

Many thanks.

Ian Bell2  
#2 Posted : 01 December 2017 19:46:51(UTC)
Rank: Super forum user
Ian Bell2

Which edition of IP 15 are you using for HAC?

I have access to the 3rd Edition - but think 4th Edition has been issued, so obviously mine is possible not the most current.

I assume you decided the fluid you have is 'C' category.

The introduction to Appendix C3.2 says (my bold & italics)

Hazard radii appropriate to the release hole sizes quoted for items of equipment in section 5.4 may be directly read off Tables C9(a) and (b). Where the equipment is not covered by section 5.4, hole sizes may be determined by historical data or synthesis techniques

I would use table(s) C9(a) or C9(b) in the 3rd Ed of IP 15. If you assume your pump develops a pin prick leak across the diaphragm to the air side - assuming a 1mm or 2mm holes would give a hazardous area of between 2-4m. Based on a 5bar pressure of release.

However, what is the length the exhaust? Does it have 'no length' and the potential air/vapor mix is released immediatley?  

What is your ventilation classed as ? Grade of release?

butagirl  
#3 Posted : 04 December 2017 08:49:31(UTC)
Rank: New forum user
butagirl

Hi Ian,

I'm using the 4th edition, but the principles are in essence the same. In answer to your questions, the fluid is Category C due to the possibility of mist formation, and the ventilation is good. The exhaust is local to the pump.

I'm not convinced that the hazard radii for a pressurised liquid as given in the tables can be considered to be the same for an air/solvent aerosol mixture, but I'm not really sure how it would vary. Although the mass flow is likely to be lower, resistance to flow for an equivalent hole size would also be lower, which leads me to suspect that the hazard radius would end up being larger.

Ian Bell2  
#4 Posted : 04 December 2017 09:36:51(UTC)
Rank: Super forum user
Ian Bell2

If you have access to BSEN60079 pt 10, no doubt you are aware this is an alternative standard for HAC. There are various formula in that standard for calculating the extent of the HAC by 'hand. Results are usually similar to IP15. If you have no means of doing further dispersion calcs I.e. using PHAST software modelling, then you are left with the standard. Assuming your ventilation is high, then I wouldn't be too worried, there is already quite a larger conservative factor already built into the results. I don't think you will end up with a bigger HAC zone, as droplets rapidly loose momentum after release. Remember the bigger your HAC zone, potentially the more expensive as you need possibly ATEX rated equipment over a larger area.
John Elder  
#5 Posted : 04 December 2017 15:52:28(UTC)
Rank: Forum user
John Elder

Is the pump located inside or outside and how good is the ventilation.

EI15 Edition 4 now refers to Table C13 in Annex C part 2 but will not be of much help in your case.

BSEN 60079-10-1 gives a good example (Page 71) of a normal industrial pump with mechanical (Diaphragm) seal, mounted at ground level , located indoors, pumping flammable liquid. Unfortunatly you will have to refer to the standard to see the example. and seeing the workings could then probably calculate the outcome for your substance.

butagirl  
#6 Posted : 07 December 2017 10:35:30(UTC)
Rank: New forum user
butagirl

Thanks for your assistance. Looking at the relevant standards, it sounds as though the hazardous area probably isn't as large as I initially assumed.

That said, because the solvent is well below its flash point, I could probably mitigate against the formation of flammable mist by putting a demister type muffler arrangement on the air exhaust.

Ian Bell2  
#7 Posted : 07 December 2017 11:26:58(UTC)
Rank: Super forum user
Ian Bell2

No problem. Sent you a private message.
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