| Rotogage
Dials
When filling large horizontal
storage containers, the liquid volume is particularly important as
it approaches the 85% full mark. There must always be an adequate
amount of space above the highest liquid level to allow for product
in the container to expand. Overfilled LP-Gas tanks can become
overpressured. This may result in a discharge of product to the
atmosphere or damage to other equipment in the system.
The amount of liquid
expansion depends on two variables, which must be understood.
- Liquid
Temperature: Liquid expands when its temperature rises.
For example: A tank is filled with liquid at 40 oF. The
liquid then warms to 60o F. A certain amount of
expansion of the liquid can be expected. If the temperature
continues to rise to
90 oF, the liquid will expand even more.
During periods of very cold temperatures, the conditions for
filling are most dangerous. A quick temperature rise combined with
an overfilled container could cause the container to become
"hydrostatic" or completely liquid filled with no vapor space.
- Specific
Gravity: In the case of LP-Gases, a lower specific
gravity number indicates that the liquid will expand more readily
upon a change in temperature.
Using the
Rotogage to Indicate Maximum Liquid Levels During Gas Transfer
The RegO®
Rotogage can be used to indicate the maximum liquid level of gas
during transfer into a storage container – provided you know the
liquid temperature and specific gravity of the gas. The maximum
liquid level is where filling should stop to allow for the safe
expansion of liquid.
- Find the
line for the liquid temperature on the Rotogage dial.
- Find the line for
specific gravity of your gas on the Rotogage dial.
- Draw a straight
line from where the temperature line and the specific gravity line
intersect to the center of the Rotogage dial. This line indicates
your maximum liquid level.
- Place the
operating handle on this maximum liquid level and open the vent
valve.
- When liquid
appears through the thumbscrew on the handle, the container is at
the maximum liquid level. The transfer operation should stop.
Example 1
LP-Gas with a specific
gravity of .508 and a temperature of 20 oF is transferred
into a storage container. At what level should the transfer stop to
allow for safe expansion of the liquid?
Draw a straight line from
the center of the Rotogage dial to where the lines intersect for
a temperature of 20 oF and specific gravity of .508.
The maximum liquid level is approximately 85%.
Example 2
At a temperature of 20o
F, what is the maximum level of LP-Gas with a specific gravity of
.584?
Using the method shown
above, the maximum liquid level would be approximately 89%.
For more information, please
reference NFPA 58, 1998 Edition, Appendix F. |
