PTQ Q1 2023 Issue

design the system with razor-thin margins to reduce steam and CW consumption. They do this in part because selec- tion criteria will often include utility consumption compari- sons. In many cases, the MDP is so low that even small changes in design conditions or underperformance of the first-stage inter-condenser will lead to performance break. Even worse, when the first-stage inter-condenser fails to sub-cool there is a guarantee that the system will experi- ence performance break. First-stage inter-condenser design evolution Modern large first-stage inter-condensers are supplied as X-type shells that include a sub-cooling zone. This design is an evolution from older and smaller designs that used E-shells. The advantage of the X-shell is that it decreases pressure drop compared to E-shell, lowering steam con- sumption. The X-type first-stage inter-condenser is designed to sub-cool the outlet vapour below the bulk out- let condensate (see Figure 3 ). 1 This is accomplished with the ‘long air baffle’. The long air baffle’s function is to force the outlet vapour across exchanger tubes with the coldest CW, thereby sub-cooling the gas below its condensate. Roughly 20-25% of the exchanger tubes are located under the long air baffle and are dedicated to sub-cooling. The sub-cooling zone is critical. When working as intended, the sub-cooled zone minimises second-stage ejector gas load. The selection of the ejector MDP is based on the load to the second-stage ejector commensurate with the sub-cooled zone working as intended. In practice, when the first-stage inter-condenser does not sub-cool the outlet vapour, the outlet vapour to the second-stage ejector increases above its design point. The higher load increases the suction pressure of the second-stage ejec- tor, which ultimately leads to higher first-stage ejector discharge pressure. When the first-stage ejector discharge pressure exceeds its MDP, performance break will occur. Mechanical design flaws that fail to seal the long air baffle or damage, corrosion, or improper installation of the seal strips are the root cause of the inter-condenser’s inability to sub-cool outlet vapour and overload the second-stage ejector. A performance break is guaranteed if the ejector system was designed with minimal or inadequate pressure drop margin. Therefore, the first-stage inter-condenser must perform as intended and sub-cool outlet vapour to avoid performance break. Easy to diagnose, hard to troubleshoot Long air baffle bypass is easy to diagnose because the vapour leaving the first-stage inter-condenser should be colder than the condensate draining from the bottom of the shell. When the temperatures are inverted, that is an indication of a bypass around the sub-cooling zone (see Figure 4 ). Troubleshooting vacuum systems is inherently difficult because much of the information required is not readily available. It is more the norm than the exception for vacuum systems to be poorly instrumented. Flow meters for key streams are often missing. Conclusions made with incomplete data or data based on design first-stage gas loads or using original vacuum system design information

1st stage ejector vapour inlet

Cooling water outlet (typically 10˚F higher than inlet)

Tube bundles

‘Long air bae’

Vapour outlet

Tube support

Condensate outlet

Cooling water inlet (coldest)

Figure 3 X-shell long air baffle

vacuum system design focuses on first-stage design con- ditions because the first stage costs more and consumes more steam and CW than the rest of the system. For that reason, they represent the biggest opportunity for optimi- sation and savings. An ejector uses motive steam at a much higher pressure than the suction pressure to entrain the process stream into the steam chest (see Figure 2 ), where it is boosted from suc- tion pressure P s to discharge P d at the ejector outlet. Ejector operating discharge pressure P d is set by the downstream ejector process load and system pressure drop. An ejec- tor’s design and performance can be related by a set of key parameter ratios, compression, entrainment, and expan- sion, as defined in Figure 2. These ratios are used to specify the required size, shape, and motive steam requirement. For a given suction load and pressure, two parameters that impact first-stage ejector sizing are motive steam pres- sure and MDP. Higher pressure motive steam will generally require less steam than lower pressure steam. However, in most refineries, steam header pressure levels are set, and the ability to optimise is limited. In rare cases, the selection of steam conditions can be used to optimise ejector design. Once the motive steam pressure is set, the first-stage ejector discharge pressure is optimised to minimise the size of the ejector and the amount of motive steam it will use. This relationship is reflected in the compression ratio P d /P s , where P d is MDP and P s is the suction pressure. As the compression ratio increases, so does the required motive steam. Designing for higher MDP increases motive steam consumption. With end users not providing adequate or any design margin guidance, vacuum system suppliers

1st stage ejector outlet vapour

106 105 78

CW outlet

87 87 72

Gas outlet

109 113 94

CW intlet

101 104 78

Temperature, ˚F

Condensate to hotwell

Figure 4 Long air baffle bypass


PTQ Q1 2023

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