Air leakage is an issue that impacts all systems that operate under vacuum. The purpose of this article is to explain how air leakage affects a system and how to identify, find, and eliminate an air leak.
Air leaks are deceptively hard to identify. Unlike a positive pressure leak, where liquid or gas is venting to atmosphere, with a vacuum leak you have no external signs to visually identify where the leak is occurring. This means one needs to identify the air leak by other means and then systematically search the system and its piping for the source or sources of the leak.
Why is air leakage into a vacuum system a problem? The air leaking into the system acts as a load. Often a large percentage of the overall load to a vacuum system is condensable in nature; steam, hydrocarbons, or other process loads. When you design an ejector system for a load that is mostly condensable, most of that load will condense in the system’s condensers. This means the last stage ejector in the system is smaller because it doesn’t need to handle as much load, the load has mostly condensed before it reaches that ejector. An air leak might only be a small percentage of the overall load to the first stage ejector, but it will represent a significantly larger percentage of the last stage ejector’s overall load. In this way, an air leak threatens to overload the back end of the ejector system and in doing so it can cause poor performance. It can also have other effects if the process is not compatible with air.
Identifying an air leak is normally done in one of a few ways. On systems that vent directly to atmosphere, the system’s vent can be checked for an excessive flow. An air leakage meter can be used for this purpose, see figure 1 below. If the system is not venting to atmosphere, an off gas flow meter is often utilized to monitor the noncondensable flow. Since air does not condense, an air leak will present itself as an increase in that flow rate. If the system does not vent to atmosphere and the off gas line does not have a flow meter, the suction pressure of the last stage ejector can be measured and compared to the design. If the suction pressure of that ejector is elevated then it could indicate an air leak, although this assumes that the ejector is in good mechanical condition and it’s design motive and discharge conditions are met.