This article is going to cover the troubleshooting process and outline some of the steps that can be taken to successfully troubleshoot an ejector vacuum system. While this article is not a full comprehensive troubleshooting guide, it should provide some high level thought processes to follow when attempting to troubleshoot a vacuum system. Graham Corporation also has engineers available to assist with any troubleshooting needs.
When approaching a vacuum system that’s in need of troubleshooting, the first question you should be asking yourself is, Do I know of any conditions that have recently changed? Have we brought another unit online? Did we switch boilers? Have we changed the unit’s feed rate or feed stock? Has something been opened and worked on? Have we turned off a cooling tower fan? Often vacuum problems can be associated with some change or action that has impacted the operating condition of the vacuum system.
One other useful observation when troubleshooting a system is, Did the problem gradually get worse or did the problem occurred suddenly? If the problem occurred suddenly then the ejector system has most likely broken performance and the internal shockwave in one or more of the ejectors have dissipated. A slow gradual loss of performance is more often associated with a gradual increase in loading, a worsening motive steam leak, corrosion, fouling, or wear. If your equipment had experienced a step change in performance, the cause is more likely to be related to a specific event or change in process conditions.
One of the most common causes of vacuum system problems is an issue with the system’s utilities. Ejectors require a motive steam supply that is -0% / +20% of the design pressure and not wet or overly superheated beyond the design. Ejector systems also require a cooling water supply flow rate that is at or higher than design and no hotter than the design supply temperature. Many vacuum system problems can be traced back to issues with a system’s utilities. Verifying that your ejector system has the correct utility conditions is one of the first things that should be done when a system is not operating per the design.
Another common cause of vacuum issues is a change in the system’s loading. Since changes in loading result in a direct impact on a system’s vacuum, verifying that you are not overloading a system is important. An increase in condensable loading can directly impact the capacity of system in accordance with the first stage ejector’s performance curve. An increase in noncondensable loading or an air leak can overload the downstream equipment causing the system’s performance to break operation. The type of load coming over into a vacuum system can be differentiated because the condensable load will have a larger impact on the front end of the system but will mostly have been condensed by the time the flow reaches the tail end of the system. The tail end of the system will mostly be seeing noncondensable loading.
In an ejector system, the front end of a system is supported by the downstream equipment. An ejector does not set its discharge pressure, the downstream piping and equipment sets that pressure. If an ejector’s discharge pressure is higher than design, the ejector is being asked to compress out to a higher back pressure which requires more work from the ejector. If this can’t be achieved, the vacuum in the system breaks down. This means that the tail end of the vacuum system supports the upstream ejectors. If the last stage ejector is not able to operate at the right suction pressure, then the rest of the system upstream of that ejector will not be working correctly because they don’t have the right discharge pressure. If a problem develops with a specific ejector or condenser, the operating pressures in the system will be poorer upstream of that point. This can be used to help identify which component in the vacuum system is the source of a specific problem.
In order to fully troubleshoot a vacuum system, a survey of the vacuum system is required. This includes measurements on the system’s cooling water and motive steam. It also requires pressure and temperature readings to be taken on the process at the inlet and outlet of each ejector and condenser. This provides one with a full understanding of the pressure profile of the system from the process vessel through to the discharge of the system. When these numbers are compared to the system’s design, it allows for conclusions to be drawn with regards to the cause of the system’s performance issues. However, caution is required when diagnosing the “cause” of a vacuum problem as sometimes the vacuum issues are being caused by more than one issue. Multiple compounding issues are not uncommon. An example of the survey data that can be gathered is shown below.