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 compressors, and the method of vapor pipe routing. At Corken, we have been building tank car unloading compressors since the 1940s. Our experienced application engineers can help you select the correct compressor for the job. The following pages show some general guidelines for selecting the correct compressor and properly routing the vapor piping.

Issues With Unloading Multiple Tank Cars

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

Solutions For Unloading Multiple Tank Cars

The capacity of one Corken model 891 compressor is well suited for unloading two typical 33,000 gallon tank cars simultaneously. Using a dedicated model 891 compressor for each pair of tank cars eliminates the issues related to using a single long vapor header. For example, when tank cars 1 and 2 empty the liquid, it has no effect on tank cars 3 and 4 since they are unloaded by a different compressor using separate vapor piping.

In any tank car unloading application, 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 reduces the energy loss (heat loss) of the compressor’s discharge gas. Using one model 891 per two tank cars works well with this rule since the compressors can be spaced evenly along the rail spur.

Using a single vapor header with isolation valves (as shown in the diagram) allows redundancy in the system. For example, if compressor 3 goes down for routine maintenance, either compressor 2 or 4 could be used to unload tank cars 5 and 6.