n 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.
Now Look At The Process: If the preliminary checklist is completed with no obvious problems found, the next step is to determine whether the problem lies in the ejector system or the process. The steps remaining to be considered are: no-load vacuum test (single- or multistage) and an internal inspection.
The best way to determine if the vacuum problem lies in the ejector system or the process is to isolate and test the ejector separate from the process. The standard method of testing an ejector is to attach a blank, or blind flange, on the suction flange of the ejector and measure the vacuum at no-load conditions. A stable no-load vacuum is more difficult for an ejector to reach than a point under load conditions, simply because there is a greater suction-to-discharge pressure differential.
If the no-load vacuum measures the same as the manufacturer’s test, then it is reasonably safe to assume that the ejector is working at its design load point. The problem may lie upstream from the suction of the ejector- possibly an increase in air leakage or process load. It should be noted that unless specified, an ejector may not have a stable vacuum at no-load even though it works at its design load.
Figure 5. This gage, known as a piccolo, has calibrated orifices to check vacuum quality.
To test an ejector at its design load point involves metering the design load into the suction of the ejector. Use a calibrated orifice or a series of orifices (such as can be found on an instrument known as a piccolo — Figure 5), and then compare the measured vacuum to the design vacuum. A curve can be plotted and compared with the manufacturer’s. This procedure should be considered only after all other tests are exhausted. (The manufacturer should be consulted for the proper field testing procedure if a test under design load conditions is desired.)
If the ejector does not obtain the manufacturer’s tested no-load vacuum, the ejector should then be disassembled and internally inspected. No-load testing is easily accomplished on a single-sta