This parallel ejector arrangement will result in steam savings at lower loads, when all of the ejectors are not required. In a system with one train of 100% capacity ejectors, all of the ejectors must be run at all times regardless of the process load. Ejector motive steam can’t be throttled to control vacuum in any system designed with critical compression ejectors (compression ratio greater than 1.8). While turning parallel ejectors on and off changes the capacity of the ejector system, it’s not an effective means of precise vacuum control. Ejector turndown with parallel ejectors results in step changes in capacity. Methods of precise vacuum control have been discussed in a previous article, which can be provided upon request.
There is also the potential to use parallel ejectors to increase the redundancy of a system or to allow for maintenance to be performed on a specific ejector while the remainder of the system is still being operated. An example of this would be a 200% configuration, where the system has two parallel 100% ejectors. Another common arrangement would be a 150% configuration, where the system has three parallel 50% ejectors. In either of these instances, any one specific ejector can be isolated and removed from service for inspection or maintenance work without disrupting the process. This will depend on the system having isolation valves installed and contingent upon those valves seating correctly. These configurations are generally selected in processes that have extended periods of continual operation, like those in many refining and petrochemical processes.
When utilizing multiple parallel ejectors of varying capacity, the corresponding downstream condenser capacity also needs to be taken into account. In a system designed with three 50% capacity parallel ejectors what does one size the downstream condenser for, 100% capacity or 150% capacity? With a 100% capacity design for the downstream condenser, the system has the required capacity to operate at full load but the condenser will be overloaded when all three of the upstream 50% ejectors are placed into service. This can happen when switching over from one ejector to another or if all three 50% ejector’s are accidentally placed into service at the same time. This is sometimes mistakenly done to try and gain capacity. While putting all of the ejectors into service in this scenario might give you more ejector capacity, it can overwhelm the downstream condenser if it’s not designed to handle that additional motive steam load. To avoid this potential issue, the condensers are often sized for the same amount of load as the corresponding upstream ejectors. This prevents the system from being operated in a way that would result in a vacuum upset and also provides additional condensing capacity making the system more resilient to fouling.
One of the biggest issues with utilizing a system with parallel ejectors is properly isolating the ejectors that are not in service. When an ejector is taken out of service, the proper isolation valves need to be closed in the right order to ensure that load does not recycle back through the dormant ejector. Normally this is accomplished by isolating the ejector suction line. Once a suction line is isolated, the corresponding motive steam to the ejector can then be isolated. It is the opposite when starting an ejector up, the motive steam is placed into service prior to opening up the suction valve. At no point in time should the motive steam to the ejector ever be placed into service while any discharge valves remains closed. When the ejectors are operated in the correct manner, they can be placed into service or removed from service without causing vacuum instability. If there are concerns with how to turn on or off parallel ejectors, a work instruction can be created to aid operators with those process changes. Graham is always willing to review these procedures upon request.