Establishing your safe upper and lower limits for FLOWS

Most businesses have established their safe upper and lower limits for the most obvious process parameters such as pressures, temperatures, and levels; but in a flammable liquid process involving non-conductive flammable liquids establishing a safe upper and lower limit for flow is an ABSOLUTE MUST.  There is not a Recognized and Generally Accepted Good Engineering Practice (RAGAGEP) on flammable liquids published that does not establish flow rates for these types of flammable liquids.

I like to use NFPA 77, Recommended Practice on Static Electricity as my RAGAGEP for establishing flows for my non-conductive flammable liquids.  Let me say this up front, establishing the safe upper and lower limits using NFPA 77 is the EASY PART – designing your process to operate within these critical parameters is MUCH MORE DIFFICULT.  But let’s stay on track and continue discussing how we will establish our safe upper and lower flow limits for our non-conductive flammable liquid(s).

As a general rule of thumb, here are two tables we can use to establish our safe upper and lower limits for NORMAL FLOW.

These two tables provide us reliable data in metrics and US measurements so that we can design our process to ensure that we stay within the established safe operating envelop (SOE).  This data is useful for those parts of the process that see “normal flows”; however, when the non-conductive flammable liquid flows through a strainer, in-line filter, or polishing filter, EVERYTHING changes and these flow rates will NO LONGER be an adequate safeguard to control the accumulation of static electricity in the flammable liquid.  Anytime we have a restriction in the line (e.g. strainer, filter, partially closed valve, orifice, etc.) the issue of RELAXATION TIME comes into play. (SAFTENG Members can read my article on this issue in the Safety Info Post – Flammable Liquids section).

We also have to worry about TOO LOW of FLOW with these non-conductive flammable liquids.  We do not want the flow to be so slow that we can separate out impurities in the flammable liquid and create a two phase flow.  These two different materials flowing in a pipe together will generate a considerable amount of static electricity, thus rendering the above tables an inadequate safeguard.

Unlike many safe upper and lower limits, when dealing with flows of non-conductive flammable liquids we will most likely have MANY DIFFERENT safe upper and lower limits within the different segments and operational phases of the process.  For example, after a filter housing the flow may have to slow to the safe lower limit to allow the static that accumulated as it flowed through the filter to dissipate to the normal flow charge.  An example of an operational phase that may dictate a different flow rate would be when an operator is “flushing the process” with the non-conductive flammable liquid.  Often times this flushing task involves a “polishing filter” which has some inherit risks that have to be considered, but will certainly impact the safe upper flow limit.  Lastly, most RAGAGEPs for controlling static accumulation will establish a safe flow rate for filling tanks.  Often times, the rates have to be slower during the initial filling until the dip leg is fully submerged, at which time the fill rate can be increased – but still, there is a safe upper limit on this increased flow rate.

Bottom line, if your facility has a process that handles non-conductive flammable liquids, EVEN IF the amount does not trigger OSHA’s PSM standard, these flow rates are ABSOLUTELY CRITICAL to the safety of the process and workers operating and maintaining the process.  If you need help, seek professional advice so that your management team can fully understand their process risks.