VRU: Pressure Sensor Placement

Vapor Recover Units must have sensing equipment which enables the applicant to know that the VRU is capturing vapors at peak intervals. An example of this would be a pressure sensor, which senses the pressure of the vapors in the tank and communicates it to the VRU compressor to turn on, meaning to start drawing suction to the tank.
The concern is which location is best to ensure an accurate signal is being transmitted to the VRU compressor, so the most efficient operation is possible to prevent O2 influx or excessive tank emissions.
Placing the inlet pressure sensor on the tank or on the inlet vessel of the VRU compressor will work, but the best place depends on the application. On the most common production facilities, it’s usually best to place the sensor on the vessel at the inlet of the compressor due to the following reasons:

  1. The pressure sensors read in milliamps, so there is usually a small amount of difference due to resistance in the wire/cabling. This will increase or decrease based on length, gauge, wire type etc. This pressure reading loss should be accounted for and sensor scaling in the control system should be adjusted accordingly. This requires a working knowledge of the control system and programming.
  2. The wire/cabling extended between the tank and compressor may be exposed to damage by personnel or weather, which also affects the reading and could result in the VRU either not operating when gas is present or operating when gas is not present and causing O2 inflow to the tank or system.
  3. Should the pipework between the compressor and the tank be altered, such as a valve closed/opened or fittings/couplings leaking, the unit operation could also be effected.

Pressure Sensor Pic
The only real benefit of having the sensor on the tank is to produce an accurate reading of the tank pressure inside to the VRU compressor control system, but this could be affected by all of the various problems mentioned above. If standard building and design practices are employed in the construction of the facility, such as using proper pipe size and alleviating restriction points in the vent lines, there should be no concern, as accurate and equal readings of tank pressure will be possible at the inlet vessel on the VRU Compressor.
If you have any questions or concerns, please contact your local OTA Account Manager.