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Safety Engineering Group
Standard Relief/Leak Rates
Commercial dewars built to DOT standards size pressure relief devices in compliance with CGA Pamphlet S-1.1.
Below are flow rates provided by the manufacture for some specific dewars used at BNL.
For flowrates of relief valves not listed or for flowrates from other sources, such as pipe failure, contact the SME.
1Loss of insulating vacuum flow rates as required by CGA S-1.1
Note: Most regulators and relief valves flow rates are given in standard cubic feet per minute (SCFM) of air.
To convert flow rate to the gas used, use the following:
To determine the maximum flow of helium through a regulator that has a maximum air flow capability of 300 SCFM.
Helium flow = 300 SCFMair x 2.69 (helium multiplier) = 807 SCFMHe
Typical 180l He dewar:
Circle Seal 532B-4M-15MV popoff relief valves
@ 10% overpressure – flow rate 1.6 SCFMair = 4.3 SCFMHe x 2 (typically found on dewar) = 8.6 SCFMHe
Relief Rates Upon Loss of Insulating Vacuum on Cryogenic Vessels
The loss of the insulating vacuum vessel (vacuum insulation to air) for a liquid nitrogen dewar typically provides a heat load between 250 to 1,000 W/m2. The heat load is significantly reduced if the vessel uses super-insulation (typically Mylar, used in liquid helium vessels) to approximately 6000 W/m2. If not super-insulation is used, the provides a heat load into the helium dewar can be 38,000 W/m2.
If the surface area of the cryogen is know or can be estimated, than the relief rate used can be calculated by using the heat of vaporization of the cryogen (Helium = 20.3 J/g; Nitrogen = 198.3 J/g; Argon = 160.81 J/g), density and gas expansion rate.
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