I am going to bet that if you’re reading this article that your facility has a flammable gas that gets transferred from one container to another. And I will also bet that these transfers are done under the safety blanket of “bonding and grounding” as we do for our flammable liquids. But did you know, most flammable GAS transfers do not call for bonding and grounding – although I still recommend it. But why don’t flammable gas standards require bonding and grounding?
For example, the most widely used flammable gas standard is NFPA 58/1910.110, albeit it is specific to LPG/Propane. But since OSHA nor NFPA have an all-encompassing “flammable gas” standard we make do by applying NFPA 58 to our other flammable gases that do not have a specific standard. NFPA 58 specifically states: (emphasis by me)
6.23.1.3* Grounding and bonding shall NOT be required on LP-Gas systems.
NFPA explains why this is…
A.6.23.1.3 Because LP-Gas is contained in a CLOSED SYSTEM of piping and equipment, the system need not be electrically conductive or electrically bonded for protection against static electricity. For information on grounding and bonding for protection against static electricity, see NFPA 77, Recommended Practice on Static Electricity.
To expand on this more…
Almost all flammable gases have minimum ignition energies (MIE) low enough for static electricity to be an “ignition source”. However, before static can ignite a flammable gas or vapor, that gas/vapor MUST FIRST be in its flammable range inside the vessel or piping. And this is where we have the fundamental reason why the flammable gas codes/standards do not, normally, require the traditional bonding and grounding.
The “liquified pressurized gas” does INDEED generate static electricity as it flows through the piping AND it is capable of generating a large enough charge to exceed their Minimum Ignition Energies (MIE). However, with all of that said we still are not required to bond or ground the containers.
Flammable gases are defined by OSHA, using vapor pressure and LEL/LFL criteria, as:
A flammable gas shall be classified in one of the two categories for this class in accordance with Table B.2.1:
| Category | Criteria |
| 1 | Gases, which at 20°C (68°F) and a standard pressure of 101.3 kPa (14.7 psi): (a) are ignitable when in a mixture of 13% or less by volume in air; or (b) have a flammable range with air of at least 12 percentage points regardless of the lower flammable limit. |
| 2 | Gases, other than those of Category 1, which, at 20°C (68°F) and a standard pressure of 101.3 kPa (14.7 psi), have a flammable range while mixed in air. |
So we can see that a flammable gas is NATURALLY a gas at standard temperature and pressure (68°F and 14.7 psi). We then look at their Lower Explosive Limit(s) (LEL/LFL) and we see they will be less than 13% or they gas will have a large flammable range, greater than 12 percentage points.
To be specific, LPG/Propane has a flammable range of 1.8% to 8.4% (using AirGas’s SDS) and here lies the heart of not needing bonding and grounding.
When these flammable gases have such a high vapor pressure, and thus very low boiling points, inside a closed system the vapor spaces are almost ALWAYS above the Upper Explosive Limit (UEL). For example, the vapor/gas spaces will be over 8.4% LPG. Remember, LPG has a boiling point of -44°F. This would be considered very “volitale” by most folks.
So on a day with the outside temperatures at “freezing” (e.g. 32°F) the liquid LPG is well above its Boiling Point inside the container and thus, so much propane has bolied off the liquid, the head pressure in the container is such that it keeps the liquid from boiling (e.g. a state of equilbrium). This head space is 100% propane gas – well above the UEL of 8.4%. Remember, this is all taking place inside a CLOSED SYSTEM so their is no, or very small amounts of air/oxygen in the system.
BUT HERE is why so many industrial users of LPG will have bonding and grounding system in place for the transfer of LPG/Flammable gases. Imagine we just completed our 10-year internal inspection of our LPG tank. The tank sat open to the atmopshere for days during the inspection and during this time, the atmopshere inside the tank was 20.8% oxygen. When we close the tank, we will still have 20.8% oxygen inside the CLOSED SYSTEM and when the first truck arrives we will, for a period of time, have all the elements of a flammable atmosphere, as well as ignition sources (i.e. static) inside the tank. Hence the need to control static accumulation.
SAFTENG members can also see my articles about “initial start-up procedures” and I discuss this very set up. In those articles I discuss, purging the space with N2 as part of the start-up procedure AND using bonding and grounding as a means to prevent an explosion during the filling of the tank. Some clients will pull a full vacuum on their tanks (MAKE CERTAIN the vessel is rated for a full vac FIRST!) and this will remove the Oxygen/air thus removing the potential hazard of a flammable atmosphere.
So in closing, NFPA 58 does not require bonding and grounding because the vast majority of times, there will not be a flammable atmosphere in the containers and piping due to the low BP of propane, the low and narrow flammable range of propane, and all of this takes place in a CLOSED SYSTEM. We just need to be aware of those times when we introduce air/oxygen into the CLOSED SYSTEM and thus we will pass through a flammable atmopshere during the filling of the tank and piping. But now you know why NFPA does not require bonding and grounding on propane (e.g. flammable gas).
NOTE: the same rationale applies to OSHA’s 1910.110, Storage and handling of liquefied petroleum gases…
1910.110(b)(17)(iv) Since liquefied petroleum gas is contained in a closed system of piping and equipment, the system need not be electrically conductive or electrically bonded for protection against static electricity.
BUT we need to be VERY CAREFUL with less volitale flammable gases that may have a higher LEL and higher Boiling point, such as some of the newer flammable refrigerants, which we will discuss in a later post.