 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-stage ejector system, but a multistage ejector system must be evaluated by checking the last stage first.
Multistage, No-Load Vacuum: Â Typically, a multistage ejector system is started up backwards from the last (Z) stage to the first. The Y stage cannot operate properly until the Z stage is working, the X stage will not work unless the Y and Z are operating, and so on.
Troubleshooting a multistage ejector system should also proceed in this back-to-front order. Check the vacuum of each stage during process load conditions, and compare it to its design value. When the measured vacuum of a stage is worse than its design, the cause of the problem at that stage must be found and corrected before proceeding any further.
The next step is to performance-test the stage in question by checking its no-load vacuum. For example, if the stage being checked is the Y stage of a three-stage ejector system, a blank should be installed on the Y suction flange. Since the Y stage is designed to discharge to a vacuum created by the Z stage, the Z stage, and intercondenser if present, must operate simultaneously during this test.
With the Y stage steam valved off, first check and record the Z stage no-load vacuum to be sure it is meeting the manufacturer’s tested value. Then, with the vacuum gage still connected to the Z stage, turn the Y stage steam on. If the Z stage vacuum steadily falls off, the intercondenser is most likely the cause of trouble and should be inspected.
As mentioned in the preliminary checklist, a condenser temperature rise much greater or less than design may be an indication of a water-flow problem. A water-flow problem can result in a higher condenser temperature and an overloading of the following stage with uncondensed vapor. In this case, as the Z stage falls off, the Y stage vacuum would follow.
Examples of direct-contact condenser problems include: a blocked tailpipe, air leakage into tailpipe, damaged water distributor, or a plugged water nozzle. Surface condenser problems include: a blocked drain, air leak