TEN THINGS TO KNOW FOR VAPOR RECOVERY SYSTEM SELECTION
- Volume of gas to be recovered
The gas to be recovered can be referred to as the suction gas to the VRU. This includes data on the cyclical flowrate of the suction gas (i.e., minimum, maximum and average flowrate). When possible, this is best determined using direct measurement.
- Chemical makeup of the gas to be recovered
This includes the mole percent of methane through hexanes plus (C1 – C6+), molecular weight of the suction gas, and BTU value of the suction gas. Most lab reports for a gas chemical analysis will include this information.
The chemical makeup is also needed to determine the value of the gas recovered. This can be used to determine the return on investment for the VRU.
- Liquids (oil, water) content or drop out from the gas to be recovered
The amount of potential liquids in the gas to be recovered is needed to select the correct VRU. If the gas is very rich (with oil/water), the gas may contain/form slugs of liquids that may limit the choice of suitable compressors that would be operable for the application. In general, we can assume the gas is water saturated and if the gas analysis provided is from the production tanks where the VRU will be pulling gas, we can make the necessary simulations to get the correct selection of equipment.
- Pressure and temperature of the gas to be recovered
This is often referred to as the suction pressure and temperature for the VRU compressor. Determine if there is a minimum or maximum temperature required for the suction gas. Keep liquids above dew point so that liquids do not drop out of solution in the VRU.
Often, we will assume this inlet pressure to be near 0.0 psig and the inlet temperature to be the maximum ambient temperature (i.e., 100°F). These two assumptions generally give the most conservative estimates. Note that if a vapor recovery tower (VRT) is used in the system, we may assume the inlet pressure to the VRU to range between 1.0psig to 3.0psig.
- Discharge pressure and temperature for the gas compressed by the VRU
Always ensure that the discharge pressure is high enough to allow gas to be injected into the system or pipeline receiving the gas. Design VRU discharge pressure to account for fluctuations in the operating pressure of the receiving system/pipeline.
Determine if there are system restrictions on the maximum temperature of the VRU discharge gas.
- Utility availability and requirements
Determine if electricity is available for an electric driven VRU or will a gas fueled driver needed to power the VRU.
- Presence of hydrogen sulfide or carbon dioxide in the suction gas.
This is needed to ensure the correct metallurgy and/or operating conditions are met to minimize corrosion issues with the VRU system, manage any high temperature issues related to the heat of compression and address any safety requirements.
- Potential for oxygen contamination
The goal is to keep air (oxygen) out of the natural gas to be recovered to eliminate the chance of an explosive mixture occurring. If oxygen contamination is of concern when recovering vent gas from atmospheric storage tanks, then the use of a VRT may need to be considered.
- Use the right vapor recovery compressor system for the right application.
Always choose the VRU that fits the specific needs for the facility. Ensure that you can track the runtime of the VRU and the volume of gas recovered. Consider metering the gas collected to document gas recovery volume. Reciprocating compressors specifically designed for dry gases are not the proper selection for a VRU handling wet (liquids laden) gases. The correct compressor selection can be critical to the success of the project
- Proper maintenance and servicing requirements
Ensure that the VRU system has proper routine maintenance and servicing according to the supplier’s recommendations. This will help maximize runtime and the volume of gas recovered. Periodic inspection of the piping and storage tank hatches can determine if there are leaks in the system.