Powell Railcar Padding System

for Chlorine and Sulfur Dioxide

Pressurized railcars of chlorine and sulfur dioxide frequently require additional pressure to facilitate the railcar unloading operation at user facilities. This practice - often called pressure padding - introduces air or nitrogen into the vapor space above the liquid in the railcar to force the liquid out of the liquid angle valve at the top of the car. Typically, dry air, as opposed to nitrogen, is used as the padding gas.

Powell Padding System

The air used in any padding system must be of high quality to protect the integrity of the system.

In addition, the air must meet several other criteria:

  • The air must be clean to eliminate debris and contamination.
  • The air must be oil free to eliminate the possibility of a heat-producing reaction between the chlorine and any oil that could be present in the stream.
  • The air must be dried to an atmospheric dew point of -40°F or below at operating pressure to ensure moisture does not enter the system, since even a minimal amount of moisture can result in corrosion and potential failure of the tank car, piping, transfer hoses, chlorine vaporizers and valves. Also, the air should be provided from an independent source other than the plant supply to ensure that during the padding operation, the overall system pressure is maintained at a level higher than the car pressure.  This minimizes the possibility that the chlorine or sulfur dioxide could backflow into the air supply system, causing extensive damage to the air dryer, compressor, and other components.

The system should also include a backflow protection mechanism to prevent the backflow of chlorine or sulfur dioxide which could also occur if a component should fail.  This would cause extensive damage to the air dryer, compressor, and other components.


The easiest, most reliable way to comply with all of these critical requirements for railcar padding is with the Powell Railcar Padding System.

The Powell Railcar Padding System provides an independent source of clean, oil free, dry air in accordance with The Chlorine Institute, LLC Pamphlet 66 guidelines for chlorine tank car pressure padding. This stand alone padding system is completely isolated from other air supplies to prevent vapors from accidentally mixing into the instruments and equipment. Built in redundant features protect equipment in the event of power loss, compressor failure, or failure of individual components.

The standard Powell Railcar Air Padding System incorporates an air padding compressor, air drying system, and backflow prevention sub-assembly mounted on a skid along with a control system housed in a NEMA 4X box.

The unit provides clean, oil free, dry air with a dew point of -40°F or below at line pressure. The flow rate of the system is typically 30 SCFM to 35 SCFM.  A Powell Engineer can provide assistance for systems requiring greater than 35 SCFM. The system can be custom designed with purchaser's choice of flow rates. The typical maximum outlet pressure is 180 psi to 190 psi. The unit will automatically fill tank cars in an on/off mode with a minimum of 5 psi differential (ie, 145#-150#).

An oil and water separator is standard on the Powell Railcar Padding System. After separation, water can be drained to a waste treatment system and waste oil can be reclaimed for recycling.

Railcar Padding System

Control System

The control system includes a display screen to indicate line pressures, high moisture, low differential pressure, and low compressor oil pressure. The display also indicates power on for automatic valves. An alarm horn is included and a common alarm contact is provided.

The Powell Railcar Padding System is shipped completely assembled requiring only on-site connection to power and external air line piping.

Backflow Prevention Unit Available as Stand-Alone Component

The backflow prevention component of the Powell Railcar Padding System is available as a skid-mounted, stand alone component. The unit is designed to prevent the back flow of chlorine from the tank car to the air drying system and to automatically fill the tank car. Built in redundant features protect the equipment in the event of power loss, compressor failure, or failure of individual components.

The Backflow Prevention Unit includes all appropriate instrumentation as well as a control system mounted on the skid.

Chlorine Line Equipment

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