Changing loads will also impact the operation of a vacuum system. The loading will be made up of two components, condensable load that will drop out of the process in the system’s condensers and noncondensable load that will pass through the entire vacuum system. Monitoring the load’s flow rate coming over from the process vessel to the ejector vacuum system is problematic due to the low operating pressure and large line size. The loading is normally monitored where it exits the system, which also allows for the determination as to whether that load is condensable or noncondensable.
The noncondensable materials will pass through the entire system and can be vented to atmosphere when the process allows, otherwise it is directed to a furnace or off-gas recovery system. In order to monitor the noncondensable flow to determine if it has changed over time, a flow meter is normally placed in the off-gas line. The condensable load material will end up accumulating in the system’s condensate receiver. The receiver will either have a level control arrangement or will be controlled by level switches. If the equipment has level switches, the frequency that they are tripped and the volume of the vessel can be used to approximate a flow rate. If the equipment has a level controller, a flow meter can be placed in the discharge line to measure the flow rate. The position of a level control valve can also be trended to determine if the flow rate is changing. Periodic sampling of both the condensable and noncondensable material for analysis can also be useful in determining if the make-up of the load is changing over time. Sometimes stripping steam or nitrogen will be added to a process. If a load is added to the vacuum vessel it is often metered, which can allow for a reading to be recorded or trended.