The recycle method is one of the most common, robust control strategies we employ for our Ejector projects. It is primarily used for applications with varying suction flow-rate, or where a fixed pressure is required on the suction side of the Ejector. A good example of this is where an Ejector is used to vent or recover gas from tanks / vessels for applications such as Flare Gas Recovery.
When we design an Ejector, we must ensure it has the capacity to handle the maximum envisaged flow-rate from the tanks / vessels, i.e. its 100% duty point. However, commonly the gas flow-rate reduces far below the 100% duty point. With a reduced suction gas flow-rate and without control, an Ejector will continue to operate, but also reduce the pressure on upstream equipment, including tanks / vessels. This can be highly undesirable, especially if the mechanical design of tanks / vessels does not allow vacuum conditions. This is where the recycle control method offers complete protection.
Recycle Control: How does it work?
The Recycle control method allows the Ejector to comfortably operate with its 100% suction gas flow-rate, even when the primary process does not provide the required gas. It is achieved by recycling gas from downstream of the Ejector, back into the suction side of the Ejector.
The amount of recycle is controlled by a valve and pressure instrument which monitors the suction pressure to the Ejector. If the pressure reduces, the valve opens to recycle ‘make-up’ gas back to the Ejector. When pressure regains, the valve closes again. With the correct instrumentation and valve selection, this method provides a highly accurate control. It has been used on numerous projects where pressure tolerance within tanks / vessels has been very low.
Liquid Jet Compressors (LJC’s) have a gas and liquid stream at the Ejector discharge, so gas must be recycled from a suitable point downstream, such as a separator.
If your process operates with large amounts of recycle over long periods of time, i.e. with reduced demand on the Ejector, it may be more efficient to consider using Multiple Ejectors as shown below.