Management of flammable fluids / gasses in a plant room 2

[11sas11]

I am a mechanical engineer that normally works in the thermodyanics space.

Our company is looking to change fluids away from a non flammable fluid to pentane because it has low Global Warming Potential (GWP).

This fluid boils at 35.7°C in atmospheric conditions, and is of course extremely flammable.

We are building a small plant that fits inside a container, but may have between 0.5 and 1.0 tonnes of material in it. I am trying to work out a safety system in case of a leak. In our system the fluid is likely to be warm (40°C) to hot (145°C). Hence if we get a small leak in the hot area it will be vapour (that may condense out), but if we get a big leak in the cooler parts it will come out as liquid.

I expect I will need to bund the plant, and have active ventilation fired by gas detectors mounted near the floor (heavier than air gas). However, I am struggling to find guidlines suitable to my application. Can someone point me to relevant standards or design guidelines for this sort of equipment?

We expect we are Zone 2 since occassional leak only. Under normal circumstances no gas / liquid should leak out. We would need to fail a flange, pipework etc. Even our pumps are mag drive so there is no possiblitly of a seal leak.

Of course I have to design for the catestrophic failure that could potentially occur.

 

[Snickster]

it's been a while since I have engineered for explosive environments but what I recall is standards to go by is NEC/NFPA 70 Part 500 for hazardous areas, for petroleum industries API 14C is followed for safety system, for wastewater industry NFPA 820 is followed. NFPA 486 is usually followed for design of purging and pressurization systems whick gives specific requirements for design of enclosures containing electrical equipment..

Basically electrical installations in locations where flammable gases, etc, may be present will have to be designed for a specific hazardous class depending on if the flammable item has a potential to be present if failure occurs, or will be present under normal operating conditions. This is defined in NEC/NFPA 70-500. Spaces can be rated to a lesser classification or may even be declassified with proper ventilation and gas detection.

I have attached a file which discusses basics of hazardous areas. Most is for overseas Europe applications but there is a section on US requirements.

 

[Snickster]

The attached file.

 

[11sas11]

Thanks, I'll have a read!

 

[Snickster]

Also I was involved in the design of a prototype system which had propane as the process fluid. It was a system that used propane as the circulation process fluid whose purpose was to extract oil out of rape seed and tar sands. We did not have any special ventilation systems but we had roll up doors that opened the building during operation. We also had two gas detectors - one set at 20% LEL and one set at 40% LEL. Upon detection of gas leak the safety system interlocks shut down all electrical items and motors ect. and closed all isolation valves. I beleive we had all electrical equipment designed for NEC/NFPA Class 1 Div. 2 Group D per that standard. We had all controls and motor control centers in a separate isolated room separated by vapor impermeable fire walls to limit required classified electrical equipment.

 

[georgeverghese]

I'm not a tech or process safety engineer, but moving out from some CFC to pentane in an enclosure with forced ventilation does bring up a whole host of new tasks. At the least instrumentation and machinery all need to be good for Zone 2 (at the least) application, which is going to bump up equipment and material costs significantly. See AS 60079 series of publications for Aus; there may be some other supplementary AS publications that may be relevant also. You may most likely have to get a competent engineer in hazardous area classification to do up your HAZ drawings, and this person is usually sourced from a consultant engineering company. My advice is not to try to go solo on this yourself.

 

[Snickster]

All equipment must be properly grounded too. I agree that you should not proceed on your own. We had a team of mechanical (me), process, electical and instrumentation engineers, a HAZOP and proper documentation for the prototype plant.

 

[11sas11]

Thanks all. Yes @georgeverghese we have put an external design audit by an expert into our process. We are similar moving from a CFC to pentane. @snickster, sounds like good strategy and our elec eng loves grounding everything everywhere

I am getting close with all the info. We have ATEX compliant parts everywhere and we are definately zone 2, we will spill into a bunded area, just sorting out the ventilation system atm.

 

[MortenA]

ATEX (in EU) does not help much with regard to the "how" but will help you with regard to the "when/where".

If your skid is placed inside a "box" such as a container active ventilation with LEL detection or as Snickster says, open doors might be the way to go (might also be needed for cooling?

Remember that a lot of ATEX equipment only have valid certificated for room temperature up to 40ºC (and ATEX as such only applies up to 60ºC but much equipment has only been tested up to 40ºC ambient temperature).

--- Best regards, Morten Andersen

 

[georgeverghese]

Considering all these additional complications, increased cost and risks with shifting out to to pentane, is there some reason for not choosing non flammable R245FA or R410A ?

 

[11sas11]

It is a good question. We are actually moving from R245fa to pentane. The reasons are cost and availability. We have used R245fa for many years and it was selected on the non toxic, non flammable basis. Clients are also pushing for lower GWP fluids in the machines.

 

[georgeverghese]

R245FA has GWP index of 1030, R410 >2000, well above the EPA acceptance upper limit of 150. And isopentane GWP < 5, while that for n-pentane is apparently <10

https://www.dcceew.gov.au/environme...bal-warming-potential-values-hfc-refrigerants

 

[georgeverghese]

From that list, there are:
at least 10 fluorocarbons with GWP < 150
3 of them with GWP < 50 - all HFC blends

 

[ProSafPlant1]

There is an NFPA standard 36 Solvent Extraction Plants that provides useful related guidance, although typically for a plant handling hexane as solvent.
Good process Safety Management practice should include a PHA such as HazOp along with various release, fire and explosion scenario modelling and fire hazard assessment.