Multiple Tank Car Unloading

Unloading multiple tank cars can present challenges for the plant designer and operator. Selecting the correct reciprocating gas compressor and designing a proper piping system can make the daily operations of a plant very efficient. However, if done incorrectly, it can create operational issues that are difficult and expensive to resolve.

Large scale unloading operations can see significant benefits from certain design parameters incorporated at the plant design stage. Many details are specific to the particular location, product being unloaded, owner’s preferences, etc. However, some general guidelines can be applied to nearly all multiple tank car unloading applications.

Two such guidelines are the proper selection of the gas compressor, and the routing method for the vapor piping. Corken has built tank car unloading compressors since the 1940s. Their experienced application engineers can help select the correct compressor for the job. Listed below are some general guidelines for selecting the correct compressor and properly routing the vapor piping.

Issues With Unloading Multiple Tank Cars

Any flowing fluid takes the path of least resistance. When using a single, long vapor header as shown in the diagram below, tank cars 1 and 2 empty first since they have the shortest vapor and liquid lines. Shorter lines have less resistance so the flow rate is higher and unloading times are much faster. However, this is when the issues begin. When tank cars 1 and 2 have emptied the liquid, the high pressure vapor from the compressors now has a direct path to the liquid line. When this happens, vapor flows directly to the liquid header and the unloading process for the remaining tank cars is slowed significantly or even stopped.

Solutions For Unloading Multiple Tank Cars

One model 891 Corken compressor is well suited for unloading two 33,000 gallon tank cars simultaneously and eliminates the issues related to using one long vapor header. For example, when the compressor has emptied the liquid from tank cars 1 and 2, it has no effect on tank cars 3 and 4 since they are unloaded by a different compressor using separate vapor piping.

It is best to locate the compressor as close as possible to the tank car. This minimizes the distance from the compressor to the tank car and the energy loss (heat loss) of the compressor’s discharge gas. Using one model 891 for two tank cars works well with this rule since the compressors can be spaced evenly along the rail spur.

When using a single vapor header, isolation valves should be installed as shown below. This builds redundancy and flexibility into the piping system. For example, if compressor 3 is down for routine maintenance, compressors 2 or 4 can be used to unload tank cars 5 and 6.