Did NFPA make “30 seconds” the norm for “relaxation time”?

9.2 Combustion Characteristics of Liquids, Vapors, and Mists

Where filters are employed, provide relaxation time in the piping downstream from the filters. A relaxation time of 30 seconds is considered by some to be a suitable precaution.

10.5 Filtration

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10.5.1.2  To prevent the charges described in 10.5.1.1 from entering the receiving vessel, the filter should be placed far enough upstream so that the charge can decay to the magnitude it would be in the pipe flow. Common industry practice is to provide 30 seconds of residence time in the pipe or conductive hose downstream of the microfilter, especially if the conductivity of the liquid is not known. For nonconductive liquids that have both very low conductivity (i.e., less than 2 pS/m) and high viscosity (i.e., greater than 30 centistokes) at the lowest intended operating temperature, longer residence times might be appropriate. In those cases, a residence time of up to three times the relaxation time constant of the liquid should be considered.

11.1 Portable Tanks, Intermediate Bulk Containers (IBCs), and Non-Bulk Containers

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11.1.1.4  Where used for nonconductive flammable liquids(Class I liquids, as designated by NFPA 30), filters should be placed such that there is at least 30 seconds of residence time in the pipe between the filter and the receiving vessel in accordance with 10.5.1.2.

12.1.5 Medium-Sized Fixed-Roof Conductive Tanks

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12.1.5.3.1
To prevent dangerous accumulation of static electric charge, flow velocity must be limited through a relaxation region upstream of the tank. This region consists of a run of piping through which the liquid has a residence time equal to the lesser of 30 seconds or 3 relaxation times. Relaxation time should be based on that liquid having the lowest possible conductivity that might be handled. The 30 seconds criterion should be adopted where the lowest value of conductivity is not known.

12.2 Loading of Tank Vehicles

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Table 12.2 Summary of Precautions for Loading Tank Vehicles

Charge Relaxation. A residence time of at least 30 seconds should be provided between any microfilter or strainer and the tank truck inlet. A waiting period of at least 1 minute should be allowed before the loaded tank compartment is gauged or sampled through the dome or hatch. [However, sampling and gauging via a sample well (gauge well) can be done at any time.]