Identifying the root cause of underperforming ejector systems
Underperforming ejector systems illustrate the importance of the long air baffle, its proper operation, and how to quickly identify problems when they arise.
Scott Golden, Tony Barletta, Steve White and Darrell Campbell
Process Consulting Services
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Vacuum column steam ejector systems are typically engineered with very small design margins. The incentives to design with low margin are lower installed initial cost and/or lower utility consumption. Systems designed with low margin must operate as designed. Otherwise, loss in performance can lead to millions of dollars in lost opportunity. A common problem which results in breaking of the first-stage ejector operation, and as a consequence significantly higher operating pressure, is a failure to seal the first-stage inter-condenser ‘long air baffle’.
First-stage inter-condenser design
Figure 1 is an elevation view of a first-stage inter-condenser using a typical X-shell exchanger with a long air baffle. Vapour from the first-stage ejector is partially condensed in the inter-condenser to reduce the load on downstream ejectors. The vapour mixture from the first-stage ejector is non-condensable gas, condensable oil, and water vapour.
The non-condensable vapour is cracked hydrocarbon gas created in the heater and air leakage. The condensed oil is primarily unstripped naphtha/kerosene/diesel boiling range material that leaves the bottom of the crude tower. The water vapour is process steam leaving the vacuum tower (stripping and heater coil) and first-stage ejector motive steam. There is a small amount of saturated water leaving with crude tower bottoms product.
Figure 2 shows the flow pattern across the tube bundle. The coldest cooling water contacts the vapour from the fluid mixture exiting the bundle area above the long air baffle, ensuring that the gas leaving is colder than the condensate. The purpose of the long air baffle is to direct the vapour flow through the coldest tube section to allow sub-cooling of the gas compared to the liquid out. This minimises vapour to the second-stage ejector.
First-stage ejector outlet vapour enters the top of the exchanger and ideally is distributed uniformly along the length of the exchanger bundle top side. The inlet vapour partially condenses as it moves from the top of the bundle to the bottom. Condensate falls to the bottom of the exchanger shell, and the uncondensed vapour enters the area under the long air baffle for further cooling.
The first-stage inter-condenser X-shell design with a long air baffle is conceptually two exchangers in series, as illustrated in Figure 3. The long air baffle is a seal plate, either slanted or ‘L’ shaped and typically contains 20-25% of the tubes under the projection of the baffle. This essentially creates two exchangers in series. When modelling performance of an X-type first-stage inter-condenser, a two series exchanger arrangement is a better representation of true performance.
In the example, 90ºF CW has a 3ºF rise across the area below the long air baffle, with the remaining 7ºF rise across the area above the long air baffle. Condensate leaving the first-stage inter-condenser is created from the area outside the long air baffle and the remaining area under the baffle. If the long air baffle is properly sealed, the vapour leaving the first-stage inter-condenser must be colder than the condensate since it has been further cooled by the surface area located under the baffle.
Maximum discharge pressure and ejector break operation
If the discharge pressure of the first-stage ejector operates above the maximum discharge pressure (MDP), then the operation of the ejector will be unstable due to a lack of the necessary shockwave in the ejector. This operating condition is called ‘break’. Broken operation is characterised by an increase in suction pressure. Depending on the ejector design, the suction pressure can be 20-40 mmHG higher than design.
The second-stage ejector load and the first-stage ejector system pressure drop determine the first-stage ejector discharge pressure. Figure 4 shows three major components that make up the first-stage ejector system pressure drop. They are:
• The pressure drop in the piping from the first-stage ejector to the inter-condenser (DP1)
• The inter-condenser pressure drop (DP2) and the piping from the inter-condenser to the second-stage ejector (DP3).
The second-stage ejector suction pressure is a function of the load to that ejector. If the load goes up, so does the pressure. The first-stage ejector discharge pressure is calculated by adding the system pressure drop to the second- stage ejector suction pressure. If the process load or the inter-condenser pressure drop is higher than design, then depending on how much margin was applied to the design, the ejector discharge pressure may exceed the MDP and result in broken operation.
It is not uncommon for the ejector MDP to be only 3-8 mmHG above the normal design operating pressure. Calculated first-stage inter-condenser pressure drops are about the same magnitude, about 3 to 5 mmHG. This leaves very little margin for inter-condenser fouling or underperformance of the long air baffle.
A small leak around the baffle can easily increase the load to the second-stage ejector and increase the suction pressure by 5-10 mmHG. Therefore, the performance of the long air baffle and its proper sealing are critical to first-stage ejector performance.
Quickly identify long air baffle problems
If an ejector is breaking, it is quick and easy to determine if the long air baffle may be the problem. The temperature of the vapour outlet should be, at a minimum, the same temperature or a few degrees lower than the temperature of the condensate outlet. The condensate and vapour outlet streams are rarely instrumented, so a trip to the unit is required to measure these temperatures.
Since the temperature difference between the two streams is small, accurate measurement is required. If the vapour outlet temperature is higher than the condensate temperature, then the air baffle is not performing properly. Data taken from operating units (see Figure 5) show the first-stage inter-condenser outlet gas temperature 8-15⁰F higher than the condensate. The root cause is the hot gas bypassing the long air baffle.
Long air baffle hot gas bypass
Low motive steam pressure, high cooling water temperatures, low cooling water flow, and exchanger fouling can all lead to underperformance and ultimately broken operation. The conditions do not cause the vapour outlet temperature to be higher than the liquid condensate.
Figure 6 shows how second-stage inlet gas load increases with increasing condensing temperature at a constant pressure. Note the very sharp break in vapour load, which occurs as the condensing temperature causes the water vapour pressure to approach the condensers operating pressure. This is a result of condenser underperformance, possibly due to the conditions listed previously, and can also cause ejector break. It will not, however, result in the vapour outlet temperature exceeding the condensate outlet temperature.
The X-shell first-stage inter-condenser long air baffle design principle is to minimise the gas outlet temperature, thereby reducing second-stage gas load for a given non-condensable gas load. First-stage inter-condensers hot vapour bypass of the long air baffle is a major cause of ejector system break.
A key feature of the long baffle design is the baffle edge sealing. Seal strips are a critical component of TEMA X-type condensers with removable bundles. In removable bundles, the baffle cannot be welded to the shell. To seal the long baffle and prevent vapour bypassing, sealing strips are attached at the baffle to shell junctions. These strips are made from thin corrosion-resistant material. These can be easily damaged during bundle installation.
Seals should be replaced, not reused, every time the bundle is removed. The installer should never allow the bundle to twist or rotate during installation. If the bundle is not sealed properly, some fraction of the vapour entering the exchanger will bypass the condensing area and flow directly to the outlet nozzle. This adds additional load to the next stage, increasing suction pressure and back pressure on the first stage.
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