One of the most important things that can be done during a turnaround is cleaning the condensers. The condensers in a distillation ejector system should be cleaned on both the shell and tube sides. In most instances the bundles are a removable type, which allows the bundles to be pulled and water washed on both sides. Fixed tube sheet units are not recommended for this service but have been specified in some instances. In those installations the shell side of the unit can only be chemically cleaned. Making sure the bundles are cleaned helps ensure optimal performance of the condensers.

Upstream of most condensers in a distillation service is an ejector. That ejector is designed for a maximum discharge pressure, which is set by the downstream condenser. When that maximum allowable discharge pressure is exceeded the ejector does not slowly lose performance, it undergoes a sudden step change in performance as the ejector breaks operation and loses its internal shockwave. Even if the system’s condensers are working well going into a turnaround, the condensers likely have some amount of fouling. That fouling may not be enough to cause a performance problem at that point in time but that doesn’t mean they will be able to run without issue until the next turnaround. The fouling will eventually reach a point where the operating pressure of the condenser exceeds the maximum allowable discharge pressure for the upstream ejector, resulting in significantly poorer performance. If this point is reached shortly after a turnaround, then the system may perform poorly for years before the fouling issue can be properly addressed.

                                  

While cleaning the condensers is important, inspecting them for damage is also important. Any cooling water tube leaks will add additional water load to the system’s hotwell. This increases both waste water treatment costs and can eventually overcome the hotwell pumping capacity if leaks grow large enough. Turnarounds provide an opportunity to plug tubes and review a bundle’s condition. Monitoring a bundles condition over time allows a replacement OEM condenser bundle to be purchased in advance of a planned outage. Make sure that replacement bundles are identical to the originals and that they are manufactured by a company that specializes in vacuum condensers. Identifying that a condenser bundle needs replaced in a turnaround and procuring a properly designed bundle of the right materials within a turnaround is difficult at best. Often a compromise in materials or design needs to be made to meet the turnaround schedule, which can have negative consequences for the tower’s operating pressure and ultimately this has an negative economic impact.

Condensers with a removable x-type bundle are built with internal sealing strips. These sealing strips seal the internal baffle to the shell to prevent the vapors from bypassing the condenser bundle. Whenever a condenser bundle is removed from the shell these sealing strips need to be replaced. If these sealing strips are damage or removed and not replaced, the vapors can bypass the sealing strips and load up the downstream equipment. This can directly affect the operating pressure of the tower and cause poor performance. It’s imperative that the sealing strips are replaced each time the bundle is removed and that the new sealing strips are installed in the correct manner and not damaged prior to the bundles being re-installed.

Turnarounds also allow for an inspection of the system’s ejectors. An ejector inspection is even more important for installations that do not have parallel trains of ejectors or isolation valves that would allow ejectors to be inspected while the system is operating. When inspecting an ejector the ejector motive steam nozzle should be checked for wear, debris, and steam leaks. The diffuser internals should also be checked for wear and damage. The motive nozzle and diffuser throats should be measured and compared to design. If worn by more than 7% of the cross sectional area, they should be replaced. In some larger ejectors with fabricated diffusers, the diffuser cone thicknesses can be checked in advance of a turnaround via UT thickness measurements. This allows for corrosion or wear to be identified prior to a turnaround so that the proper replacements can be ordered.

Motive Steam Leak

A motive steam leak that occurs upstream of the motive nozzle will introduce a steam load to the ejector. This will cause a poorer suction pressure which can negatively impact the entire system's vacuum.

An often overlooked aspect of a distillation ejector system is the condensate drain legs and hotwell. If the condenser drain leg or the system’s hotwell become fouled with process build up or debris, it can lead to a back-up of condensate. This can flood the system’s condensers and upset the tower’s operating pressure. These condensate drain legs are also under vacuum the entire way down to the liquid level in the hotwell. If one were to develop a leak, air would be pulled into the drain leg and upwards towards the system’s ejectors. This too can cause condenser flooding. This is why cleaning out these down legs and verifying their mechanically integrity is important for the long term reliability of the system. A turnaround also provides an excellent opportunity to clean process build up and debris out of the hotwell.

In order to understand how an ejector system is operating, multiple test points are needed to take various pressure readings. In order for these readings to be taken while a system is in operation, an isolation valve is needed at each test point. A turnaround provides an opportunity to install isolation valves at existing plugged connections or to weld in new connections where none exist. Doing so allows for a more comprehensive assessment of the vacuum system operation.

It’s understood that turnarounds by their very nature are filled with many needed maintenance activities, limited resources, and the discovery of unplanned issues, all of which can distract from the needs of the vacuum distillation ejector system.  Care needs to be taken when looking at turnaround scope to make sure that the vacuum system’s needs are addressed during each turnaround so that the ejector system is able to operate to its design and maintain its reliability.