ing parts. Figure 2 is a diagram of a steam jet vacuum ejector that estab- lishes nomenclature and symbology used throughout this article. The basic operating principle of a steam jet ejector is momentum transfer. The motive nozzle converts steam pressure energy into velocity energy, resulting in a supersonic velocity jet of steam that entrains the vapour and gas mixture from the suction chamber. The resulting mixture of motive steam and suc- tion load enters the diffuser where velocity energy is converted back to pressure. At the diffuser throat, a pressure discontinuity or shock wave occurs which is responsible for an important property of steam jet ejectors – suction pressure is independent of discharge pressure up to a certain limit, and is only influenced by the amount of suction load. Figure 3 presents the perfor- mance curve of a large first-stage ejector, showing the relationship between suction pressure and load. As mentioned above, suction pres- sure is independent of discharge pressure only up to a certain point. When discharge pressure becomes too high, the shock wave in the ejector throat can become unstable, leading to ‘broken’ ejector operation that produces high and sometimes fluctuating suction pressure. When an ejector is operating in a broken state, the suction pressure is unpre- dictable and depends on both suc- tion load and discharge pressure. The discharge pressure that breaks the ejector is known as maximum discharge pressure, or MDP. MDP is commonly represented as a sin- gle number for a given ejector, but it actually varies with suction load. Figure 4 shows a combined plot of suction pressure and MDP curves versus suction load for an actual first-stage ejector. Note that the axes on this figure are reversed from the typical pressure versus load capac- ity curve, with X-axis indicating pressure and Y-axis representing load. This arrangement makes it easier to combine the individual capacity curves of a multi-stage sys- tem in the same plot. Although ejectors can handle a wide variety of suction gasses with varying temperature and molecular
Motive nozzle
Di user throat
Di user
Discharge
Motive steam
P d m=m + m s m
Pm, m m
Suction chamber
P = Absolute pressure m = Mass ow rate
Ps, m s
Suction load
Figure 2 Steam jet vacuum ejector diagram
crudes. Figure 1 shows vacuum residue yields for two different vacuum columns over a range of flash zone pressures. Unit 1 runs a blend of heavy Canadian crudes, while Unit 2 processes a blend of heavy Venezuelan and light US crudes. As pressure increases from 20 to 40 mmHgA, Unit 1 residue yield increases by about 3 LV% on crude and Unit 2 residue yield increases by about 4 LV% on crude. For a crude capacity of 100 MBpd, this corresponds to 3 and 4 MBpd of incremental vacuum residue, respectively. Considering a vacuum gasoil to residue downgrade pen - alty of $15/Bbl, the higher flash zone pressure results in profit losses of $16MM/yr and $21MM/yr. Troubleshooting and diagnosis of crude vacuum systems underper- formance can be difficult and time consuming. The process of deter- mining ejector loads in an operat- ing unit is full of uncertainties. It requires good field data, labora - tory analysis, and flow measure - ments that are often unavailable. Furthermore, it relies heavily on process simulation models that are only as good as the simulation
inputs. For units that have been in operation for more than a few months, additional uncertainties can further complicate efforts to narrow the root cause(s) of high vacuum column operating pressure. These include, but are not limited to, con- denser fouling, ejector motive steam nozzle erosion, and tower internals damage that increases system load. Therefore, every effort should be made to identify and avoid errors in the specification, engineering, and manufacturing of vacuum sys- tem equipment before they manifest themselves at start-up. In vacuum surface condensers, undersized shell nozzles, over-optimistic heat trans- fer coefficients, and vapour internal bypass are common problems. In vacuum ejectors, motive steam noz- zle size and position are common culprits. In addition to generating accurate performance curves, shop testing vacuum ejectors provides a unique and valuable opportunity to prevent such problems.
Steam jet vacuum ejector fundamentals
Steam jet vacuum ejectors are essen - tially compressors with no mov -
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Equivalent water vapour load @ 70˚F (lb/hr)
Figure 3 Suction pressure vs load performance curve
42 PTQQ 2 2022
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