Tech Tip - Over/Under Compression in a Rotary Screw Compressor

By May 16, 2018 Enalysis No Comments

Tech Tip

Over/Under Compression in a Rotary Screw Compressor


Screw compressors are a crucial part to the natural gas industry, and typically operate under wide variants of operating conditions in both gathering and process applications. Whether the compressor is experiencing pressure and volume fluctuations, temperature swings or changing gas composition, the screw compressor is a reliable piece of equipment that powers through operational swings without issue. However, this doesn’t mean that the compressor is operating as efficiently as it could be!

Optimization can result in significant power savings and/or reduce the potential for catastrophic failure. It is beneficial to analyse the performance at frequent intervals to ensure your screw compressors continues to operate without issue, at low cost. The most common method to optimize a screw compressor is by adjusting the Internal Volumetric Ratio (Vi). Adjustments to the Vi are recommended when the unit is experiencing over or under compression, or the internal discharge pressure exceeds the max allowable.

The following graphic allows you to visualize the relationship between pressures, Vi and gas composition to better understand the compression process, and the effects those variables have on power requirement. Note that the process highlighted in the graph is as follows:

  1. A volume of gas is captured in the rotors: (At Suction Pressure)
  2. The volume of gas is compressed in the rotors: (Increasing Pressure)
  3. The volume of gas is exposed to the discharge line and expelled: (At Discharge Pressure)


*Note that the graphic is conceptual, and does not account for partial loading due to the slide valve.

For questions regarding the performance of your screw compressors, please contact your Detechtion representative.