In this article we are going to review the phenomenon of backstreaming of motive steam in an ejector system. In addition to a loss of vacuum, backstreaming can result in lost product quality due to contamination in the process vessel. We are going to explain why ejectors backstream and how this backstreaming can be identified and therefore better understood.
Backstreaming is the ejector’s motive steam flowing in reverse and traveling backwards through the suction connection of the ejector. When an ejector is backstreaming motive steam backwards though its suction, that hot motive steam will increase the suction temperature of that ejector. Process vessels and piping upstream of that ejector will also increase in temperature. This will cause any temperature transmitters upstream of that ejector to read hotter than what would be expected during normal operation. This can lead some equipment users to believe that the process vessel is producing more load or a different load make-up. While changes in temperature can be the result of changes in load flow rate or make up, more often than not a sudden change in the suction temperature of an ejector is due to backstreaming. The rapid increase in the ejector suction temperature is the result of the ejector breaking operation and not due to the process load breaking the ejector’s operation.
A critical ejector requires two things to establish and maintain its shockwave, the correct motive steam conditions and the correct discharge pressure. The compression capability of an ejector breaks down when the shockwave breaks down. If the motive steam pressure decreases or the back pressure of the ejector increases beyond the ejector’s design, the internal shockwave breaks down and the critical ejector loses its ability to compress over its design range. This break often happens very quickly and the ejector suction pressure rapidly rises. This results in a scenario where the deepest point of vacuum in the system is upstream of that ejector in the piping and/or process vessel. When the ejector is suddenly unable to pull a vacuum deeper than that upstream pressure, the ejector’s motive steam will flow backwards through the suction towards the point of lowest pressure. Remember, vapors will always flow from a higher pressure to a lower pressure. This backstreaming of flow will continue until the pressure in the upstream piping and/or process vessel equalizes with the poorer suction pressure of the ejector. This is often accompanied by ejector surging. When an ejector is surging, that process flow at the inlet of the ejector will flow backwards and then forwards again, alternating with the surging. This creates a consistently elevated suction temperature at that ejector’s suction.