oticeable effect in the operation of an ejector unless the increase is significant (>25%).
Besides wasting steam, excess motive pressure tends to choke the venturi with steam, thereby decreasing the suction capacity of the ejector. Note too that the performance of ejectors designed for saturated steam will be adversely affected if operated with super-heated steam.
The specific volume of steam increases with increasing temperature, which may require an increase in pressure to maintain required steam flow. Otherwise, as in the case of low steam pressure, the manufacturer must be consulted for a redesign of the ejectors.
Excess moisture in steam is one of the most common problems found in ejectors. Wet steam causes poor performance and, depending on the degree of wetness, can permanently damage an ejector in a very short period of time. A steam quality of less than 2% moisture is tolerable with most moderate vacuum systems. However, ejectors designed for a vacuum of 5 mm Hg abs or less should have steam that is completely dry or with a few degrees of superheat.
A telltale sign of wet steam is a fluttering needle on a steam pressure gage during operation. But the only sure way to determine quality of steam is to test it with a throttling calorimeter. This is a constant-enthalpy device that measures steam pressure and temperature. When used in combination with a Mollier chart, a reading of steam quality is obtained.
If steam is found to be wet, a steam separator should be installed in the steam line as close to the ejector as possible. Keeping all steam piping, and the steam separator, completely insulated will also help prevent the formation of wet steam.
Water: Multiple-stage ejector systems will normally include condensers between some or all stages. Condenser designs are based on maximum water temperature and available flow. When inlet water temperature increases above design maximum, loads to the following stage increase, resulting in a poorer vacuum at that stage. If the affected stage is the last (Z) stage, then the vacuums of all the preceding stages could also be affected.
The water temperature rise across a surface-contact condenser should be compared with design. A temperature rise larger or smaller than design could be an indication of a flow problem or fouling.
In direct-contact condensers, high or low water flowrates can cause problems in the vacuum system. High water flow could flood the condenser, increasing pressure drop, and therefore, the back pressure on the stage discharging into it. Low water flow may not be distributed properly, allowing condensable load to bypass into the following ejector and resulting in a poor vacuum.
Figure 4. The three basic parts of an ejector are the nozzle, mixing chamber and diffuser. High-pressure motive fluid passes through the nozzle, expands in the mixing chamber (where pressure is converted to fluid velocity), and passes through the venturi throat of the diffuser. Process fluid enters the suction port and is drawn into the mixing chamber. The curves show the changes in velocity (above) and pressure (below) of the motive fluid (blue) and Process fluid (magenta).
Process Loads: A change in process load will have a direct effect on the ejector system. Ejectors operate over a unique capacity curve, and any increase in load will result in a higher absolute pressure. An increase in noncondensables will travel through the system, affecting the following ejector stages. Discharge pressures of each stage will increase to the point of a breakdown in operation. As in the case of a changed steam condition, a change in process load will require redesign by the manufacturer, if design vacuum is to be maintained.
Check The Logs: Manufacturers’ field service reports or process logs may offer clues to present operational problems. the symptoms of a vacuum problem may be similar to a past problem outlined, with solutions, in a service report. Process logs may also indicate problems with steam pressure or changes in process conditions that will have a direct effect on the vacuum system.
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