One of the most misunderstood operating procedures in OSHA’s PSM and EPA’s RMP standards is the “Initial Start-Up procedure”. OSHA, nor EPA has offered a lot of written guidance, on this requirement and this has led to a lot of confusion as to why it is called out and how this SOP differs from a “Normal Start-Up” SOP. But anyone who has a PSM (or maybe an RMP) process that handles a non-conductive flammable liquid knows all too well how the initial start-up SOP differs from their normal start-up, but I will explain…
For those who may not be familiar with “non-conductive” flammable liquids should first read the dozen or so posts in my Flammable Liquids Section so that they have a better understanding of how these flammables pose significant static risks. With that said, these flammable liquids require SPECIAL ATTENTION from both the engineers who design the process and the operators who will be transferring these flammable liquids. For the sake of time and to stay on course with this posting, I will cut to the chase. The speed at which we transfer these flammables is CRITICAL to process safety. And these speeds vary depending on the process conditions – hence “initial start-up” when the receiving storage tank is EMPTY.
Now I do have to mention a RAGAGEP that in the USA is sort of the go-to RAGAGEP for controlling static electricity flammable liquid processing:
NFPA 77: Recommended Practice on Static Electricity
In the newest edition of NFPA 77, it simply states:
(b) Employ reduced fill rates at the start of filling through downspouts, until the end of the spout is submerged. Some consider 3 ft/sec (0.9 m/sec) to be a suitable precaution.
In older versions of NFPA 77, it was a bit more descriptive:
8.5.2 Conductive Fixed-Roof Storage Tanks. Charge accumulation in the liquid in a tank can lead to static electric discharge between the liquid surface and the tank shell, roof supports, or tank appurtenances. The charge generation rate is influenced by turbulence in the liquid and by the settling of particulate matter, such as water droplets, iron scale, and sediment.
8.5.2.1 Precautions. If the vapor space in the tank is likely to contain an ignitible mixture (e.g., in cases where intermediate vapor pressure products or low vapor pressure products contaminated with high vapor pressure liquids are stored) or where switch loading is practiced, the following protective measures should be taken:
(1) Splash filling and upward spraying should be avoided.
(2) The fill pipe should discharge near the bottom of the tank, with minimum agitation of water and sediment on the tank bottom.
(3) If possible, the inlet flow velocity should be limited during the initial stage of tank filling to reduce agitation and turbulence, and the following also should be applied:
(a) The flow velocity of the incoming liquid should be no greater than 1 m/sec until the fill pipe is submerged either two pipe diameters or 0.6 m, whichever is less.
So as we can see our initial filling of our “NEW Storage Tank” (could also apply during start-up after maintenance) with a non-conductive flammable liquid we need to restrict our fill rates to 1 m/sec (~3ft/sec) UNTIL the fill pipe is submerged either two (2) pipe diameters or 2ft, whichever is less. This means we need to have the means to regulate our transfer speeds during our “initial filling” (e.g. Initial Start-Up). Once we have the level in our tank so that the fill pipe is submerged in either two (2) pipe diameters or 2ft, then we can increase our transfer speeds in certain process conditions.
(4)The following applies to storage tanks greater than 50m3 (50,000 gallons) containing liquids that are either nonconductive or for which conductivity is unknown:
(a) The inlet flow velocity can be increased to 7 m/sec after the fill pipe is submerged.
(b) Where operating experience has shown that the practice is acceptable, such as in the petroleum industry, the inlet flow velocity can be increased above 7 m/sec, but in no case to a velocity greater than 10 m/sec.
So as we can see, our “Initial Start-Up” of our new flammable liquid storage tank will require us to regulate our transfer speeds. Once we have gotten the level in the storage tank within its “Safe Operating Envelop” we can operate at normal process speeds. This is what I meant when I said this would also come into play when we empty the tank for any reason, most often for its internal inspection. When we return the tank to service from an empty state OR we take the level to a point that exceeds its “Safe Lower Limit” (e.g. usually less than 6” deep since that is how far the fill pipe should extend to the bottom of the tank) we need to abide by these reduced flow rates.