nts. If there is a valve in the vacuum line, close it and observe the vacuum line, close it and observe the vacuum gage. If it slowly loses vacuum, air leakage is occurring.
For troubleshooting purposes, overall, an absolute pressure gage is preferred over a common vacuum gage. A common vacuum gage that reads in units of inches of mercury vacuum is food for rough measurements by operators, but this gage does not five the accuracy needed for a system analysis. Besides relative inaccuracy, vacuum gages require a barometric pressure measurement to determine an ejector’s vacuum in units of inches of mercury absolute. There are a variety of suppliers of such absolute-pressure gages.
Replacement mercury U-tubes are also inexpensive, and are available from various suppliers. Mercury-free units are also available, and are typically used for the higher-vacuum ejectors such as W or X stages. There are also many manufacturers of electronic vacuum gages that offer very accurate and portable instruments. The gages are well worth the investment and should be kept in stock only to be used when troubleshooting ejector systems.
Steam-pressure gages should be located on the steam chest of the ejectors or as close to the unit as possible. It is important to know the actual operating steam pressure since an ejector of critical-flow design (suction pressure less than half discharge pressure) will not operate properly when even a few psi below its design motive pressure. Compound steam pressure gages are recommended since they will not be damaged when exposed to vacuum.
Steam: Motive steam plays probably the most important role in the operation of a steam jet ejector. Since internal dimensions are fixed, the ejector is designed for only one steam condition. When the steam condition changes there will be a change in the operation and efficiency of the ejector.
In a critical-flow ejector a decrease in steam pressure of just a few psi will result in a broken or unstable vacuum. An increase in steam pressure above design will not have a noticeable 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, the