The main desired outcome, when using a vacuum system, is going to be achieving the design vacuum at the suction of the system. One of the primary ways to monitor a vacuum system’s performance is by using an absolute pressure transmitter and trending that pressure over time. In multiple stage ejector systems, additional transmitters can be placed at the suction of each downstream ejector to allow for additional readings to be monitored and trended. Verifying that the pressure transmitters are on a regular calibration cycle is also very important to maintaining good accurate readings.

Monitoring the supply conditions of the system’s motive steam and cooling water are also important. Placing transmitters close to the system on these utilities can also help identify the cause of performance problems and highlight when the system’s design limits are being exceeded.

-         Cooling Water Supply & Return Temperatures

-         Cooling Water Flow

-         Motive Steam Pressure

-         Motive Steam Temperature

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.

Completing a detailed performance survey of the vacuum system on a yearly basis is also an important part of understanding how the equipment is operating. It allows for a detailed review of the equipment’s performance, which is accomplished by gathering a full set of pressures and temperatures with calibrated instruments. The data can then be used to mitigate performance problems and provides valuable information for turnaround planning. Graham has a group of engineers dedicated to providing these services. An annual review of the equipment performance also helps a site dedicate resources to the right maintenance activities and allows for the acquisition of the correct replacements and spares.

Involvement of the original equipment manufacturer can prove to be beneficial when monitoring a vacuum system’s performance. Regular conversation between the equipment manufacture and the equipment user can help identify issues before they arise or allow for a rapid response to any sudden performance problems. This is often achieved through an equipment service agreement or via Graham’s performance monitoring program. If you have a system that would benefit from these services, please don’t hesitate to contact Graham for more information.