Retention Time

Retention time for gas-liquid separation (2 phase) is a term used to describe the average time a separator allows flowing fluid to remain in the liquid section of the separator at the designed flow rate. It is important that sufficient retention time (also known as residence time or detention time) is allowed for liquid to fall out of the gas phase and gas bubbles to escape from the liquid. By specifying the required retention time based on the specific mixtures pressure, temperature and properties, the required liquid section can be sized to ensure the drop out. Based on API Specification 12J recommendations, liquid retention time in a 2 phase separator that doesn’t not experience foaming, wax deposition and slug flows is 1 to 4 minutes.

For a 3 phase separator, the retention time is described as the total time fluid remains in the separation section at the designed flow rate. It is important to note, there are 3 key ways to separate oil and water including: chemical, heat and time. Retention time utilizes time to aid separation. When a mixture of oil and water are allowed to stand without agitation, the mixture begins to separate into horizontal sections of oil, free water and emulsification. This time it takes for the mixture to separate into these horizontal layers is called the retention time. It is recommended to have a field study completed to get the most accurate retention time. If a field test is unreasonable, the following “typical” liquid retention times can be assumed with the understanding they will not yield the most exact results.

API Gravity Above 35: 3 to 5 minutes

API Gravity Below 35, Temperature 100+F: 5 to 10 minutes

API Gravity Below 35, Temperature 80-100F: 10 to 20 minutes

API Gravity Below 35, Temperature 60-80: 20 to 30 minutes