Continuing our dive into the world of cryogenics, let's consider how cryogenic bulk storage tanks are filled.
Unlike an atmospheric storage vessel, where you can fill the tank with liquid and any gas above the liquid level can vent away, cryogenic liquids are stored under pressure to prevent excess boil off.
Cryogenic tanks come standard with a single inlet connection for a delivery truck, but valving to direct that inlet into the top and/or the bottom of the tank. Let's consider how these are used.
1. The Traditional Top Fill Method
Filling into the head space of the tank is the fastest and most common method used. However, when fresh liquid comes into contact with the comparably warmer gaseous headspace, pressure in the headspace falls due to the rapid cooling. This can lead to a drop in gas supply pressures during the filling operation.
2. Simultaneous Top and Bottom Fill
The solution to maintaining gas supply pressures and still having an expedient tank fill is simultaneous top and bottom filling. The operator will allow cryogen to flow into the bottom of the tank, which compresses the head space, and to flow into the top of the tank, which reduces pressure in the head space. This is accomplished normally with two manual globe valves at the tank filling connection, which the operator adjusts as needed to maintain tank pressure.
Also, if you ever find yourself needing to fill these tanks, especially if they are empty, make sure you chill in your transfer equipment and the tank itself slowly. Occasionally, the tank supplier will have guidance on the target tank chill down rate. No need to have a very bad day because you were impatient.
#cryogenics #processengineering #mechanicalengineering
