In the world of process safety, we have Safe Upper and Lower limits on a lot of common process parameters, such as pressures, levels, temperatures, etc. Flows on the other hand are quite often overlooked as a critical process parameter and when your HHC/EHS is a non-conductive flammable liquid both FLOW and LEVEL must be addressed within the facility’s PSI and SOPs. This article will discuss how a safe upper limit on flow and safe lower limit on level can play a huge roll in improving flammable liquid safety and quite possibly PSM/RMP compliance.
Let’s look at flow first
Having established safe upper and lower limits on flows are not that common. Often times when we come across a safe upper or lower limit for flow we see it in a utility such as cooling water or firewater. This safe lower limit is meant to ensure that the utility is NOT over used and can provide the necessary flow rates based on scenarios developed via the PHA or an engineering review. But in the case of a non-conductive flammable liquid flow rates are CRITICAL as flow is a means by which the flammable liquid will generate static, which we all know is often times the ignition source for these flammable liquids. (I have written over 50 articles on flammable liquids and static – SAFTENG members have access)
In regards to a plant that falls under PSM due to their flammable liquids, this plant must design, construct, operate, and maintain their process to a Recognized and Generally Accepted Good Engineering Practice (RAGAGEP). And the very best RAGAGEPs for controlling static are:
- NFPA 77: Recommended Practice on Static Electricity
- API 2003: Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents
Many facilities have stated in their PSI that either, and sometimes both, are their chosen RAGAGEP for their flammable liquids process. Like I said, these are the two BEST documents for anyone having to manage static electricity as a hazard. And BOTH of these documents cover the concept of controlling flow rates of non-conductive flammable liquids; however, the effort usually ends there, as this is almost NEVER done in these flammable liquid processes.
Both NFPA and API documents state that when flowing a non-conductive flammable liquid that the rate of flow should NEVER exceed 7 m/s (23 ft/sec), but when filling an EMPTY container/tank/vessel the flow rate must not exceed 1 m/s (3 ft/sec) until the fill pipe/dip tube has been submerged to a depth of twice the inlet pipe diameter. This means a 3” fill pipe/dip tube would have to be submerged at least 6” before the flow rate could be increased to 7 m/s (23 ft/sec). These flow rates are the “gospel” in any well managed flammable liquids process but tend to get lost in translation when the flammable liquid is “secondary” to the process such as in businesses that use the flammable liquid(s) in the manufacturer of their finished product(s).
If you are one of the many safety professionals working with flammable liquids and your facility loads or unloads tank trucks or rail cars, or you have a bulk storage tank, or you handle IBCs of flammable liquids, ask yourself the following questions:
- What is the conductivity of my flammable liquid(s)?
- If my flammables are non-conductive (or even semi-conductive) what safety measures are in place to control the hazard of static electricity?
- Do my process controls include controlling flow rates of these non-conductive flammable liquids?
If you are managing a PSM/RMP covered process ask yourself these questions:
- What standards and codes has the facility employed (e.g. adopted) to control the hazard of static electricity in the process? In other words, what RAGAGEP are my grounding and bonding stations built to and maintained to?
- If my facility has adopted either NFPA 77 and/or API 2003, are we really managing flow rates for the non-conductive flammable liquids as called for in these RAGAGEPs?
- Does my PSI (and SOPs) contain a Safe Upper Limit on flow rates for these non-conductive flammable liquids?
Depending on how you answered these questions, you are either smiling and nodding your head or you may be one of the many who are shaking their head and rubbing their eyes and forehead about now! Now I can not promise you that OSHA would bring up these matters in a PSM inspection and I have never in my 20+ years of working with flammable liquids seen OSHA even mention flow rates for non-conductive(s) and attempt to use 1910.106(e)(6)(i) to say that “flow rates” is an “adequate precaution”. But I have seen first hand and heard of many others, where OSHA is looking at Safe Upper and Lower Limits on flow as a RAGAGEP requirement, especially after the Barton Solvent fire in KS a number of years ago. Regardless of what OSHA enforces, using NFPA 77 or API 2003 is an ABSOLUTE necessity in any facility that handles a non-conductive (and semi-conductive) flammable liquid; as the basic bonding and grounding will just NOT provide the level of protection we need. (If you are wondering why, please see my flammable liquid articles)
That about sums up the need to control flow rates of these non-conductive flammable liquids, but don’t be fooled into thinking that “I can just demand we slow down our flow rates for these specific flammable liquids”. These safety projects are often the most difficult because we are “recommending” that the flow of flammable liquids be slowed and it does not take long for the business managers to begin to see this as a “bottleneck” in the production process. Coupled with these pressures, having the ability to control flows from 1 m/s up to 7 m/s is not easy, nor cheap! Just giving you a heads up on your battles.
Now let’s discuss “level”, specifically let’s discuss Safe Lower Level. Yes, that is correct, Safe Lower Level instead of the more common Safe Upper Level. We all have heard the horror stories of a flammable liquid tank being overfilled, a vapor cloud forming, and resulting in a Vapor Cloud Explosion (VCE). These events continue to occur with devastating results, the most recent being the Buncefield explosion and fire in 2005 in Hertfordshire, England. Much smaller, yet as devastating, incidents occur on a more frequent basis. I have written about the safety systems necessary to prevent these common incidents and now I want to address the overlooked safety limit… Safe Lower Level.
Flammable liquid storage tanks and process vessels handling flammable liquids MUST be equipped with a “fill pipe”. This is a pipe that extends down inside the tank and terminates 6” above the bottom of the vessel. This pipe will also have either a 45-degree cut tip or a tee to divert flow horizontally at the bottom of the vessel. If you recall when we were discussing the flow rates, both NFPA 77 and API 2003 used this fill pipe as sort of a gauge as to when flow rates can be increased from 1 m/s up to 7 m/s. The codes state that the flow rate into a tank/vessel (including railcars and tanker trucks) be kept at 1 m/s until the fill pipe/dip tube has been submerged to a depth of twice the inlet pipe diameter. So if we NEVER let our tank level fall below this mark it is MUCH EASIER to meet the flow rates of 7 m/s on a continuing basis vs. trying to have a process that has the means to control flow rates of 1 m/s – 7 m/s. This works great for those businesses that ONLY unload flammables to a bulk tank AND they keep their flows @ 1 m/s from the bulk tank to their processing vessel(s). But keep in mind that transferring from the storage tank to an empty process vessel can introduce many of the same hazards, albeit the process vessel will most likely not have the sediment that a storage tank will have and this sediment can be a large generator of static.
With all that said, we recommend a Safe Lower Limit be set for the level in flammable liquid storage tanks. We do this so that the issues associated with filling an empty tank at the rate of 1 m/s can be avoided and thus the unloading pump can be a single pump with a rate of no more than 7 m/s. We also advise that when this storage tank level deviates to a level less depth of twice the inlet pipe diameter (e., the safe lower limit), the SOP will have specific steps the operator must take to correct this deviation (e.g. using the smaller pump at a rate of 1 m/s until the level is at the safe lower level). We also recommend that the facility have a procedure (sort of like an “initial start-up” procedure) for filling the tank for the first time after it has been emptied for whatever reason (maintenance, inspections, changing product, etc.) PLEASE know this is just refilling the tank with the SAME PRODUCT and not “switch loading”, as switch loading is an entirely different beast. The bottom line, if we can keep the level in this tank above this safe lower limit we can unload transport containers at a consistent rate of up to 7 m/s.
This type of safe lower limit is not actually a hazard when the limit deviates but rather the hazard arises when the “correction” is made to get the level back above the safe lower limit. What occurs inside this tank at this low level is a lot of turbulence and turbulence creates static. When we lower the level below the actual end of the fill pipe (i.e. 6” from the bottom of the tank/vessel) we are now essentially “splash filling” which is a REAL HAZARD with any flammable liquid. In fact, we recommend a nitrogen purge that lowers the Limiting Oxygen Concentration (LOC) below that for the flammable liquid when we are “splash filling”. This is really essential until the level in the vessel is at a safe level such that there is no more splash filling and the turbulence is reduced.
So in closing, when we have these non-conductive flammable liquids we MUST control flow rates to within our prescribed RAGAGEP and if we can maintain a safe level in our storage tank(s) we can maintain a constant flow rate up to 7 m/s which is MUCH EASIER and SAFER than trying to flow at a rate of 1 m/s and then increase the rate up to 7 m/s. Not many plants are capable of managing these varying flow rates and this leads to too high of rates when the tank level(s) fall below their safe lower limit.
Note: In some designs, a “smear line” is used instead of a fill pipe that extends to 6” from the bottom of the vessel. This may acceptable when the fill pipe would interfere with processing (i.e. such as agitators); however, this deviation from using a fill pipe should be reviewed by a qualified engineer to ensure it is designed properly.