Shell Side Air-Removal Equipment Performance
In accordance with the HEI Standard, steam jet air ejectors are tested to meet the “70°F Air
Equivalent” (also known as dry air equivalent (DAE)). A temperature entrainment curve is used
to make this conversion (See Figure 15 [8]) for both air and steam.
With mixtures of air and steam, the DAE flow rate of each constituent is determined and they are
then added together to determine the total flow rate. The example given in the HEI Standard [8]
is for 660 lb/h of mixture consisting of 200 lb/h air and 460 lb/h of steam at 400°F. Using the
temperature entrainment curve of Figure 15 [8], at 400°F the entrainment ratio for air is 0.921
and that for steam is 0.892. The molecular weight entrainment ratio for steam is 0.81. Thus:
DAE of air component = 200/0.921 = 217 lb/h
DAE of steam component = 460/(0.892 * 0.81) = 637 lb/h
DAE of mixture = 854 lb/h
An alternative method of establishing the DAE of an air/steam mixture is to refer to Figure 17
[8]. In the above example, the percent air in the mixture is 200/660 = 30.3%. From Figure 17, at
a temperature of 400°F and 30.3% air in the mixture, the entrainment ratio is given as 0.773.
Thus the DAE of 660 lb/h of this mixture is:
660/0.773 = 854 lb/h
3.1.4.3 Compression Ratio
The compression ratio (see Equation 2-8) for an SJAE system depends on the number of stages.
The design details are again determined according to the experience and research of the
manufacturer and are difficult to determine from theoretical considerations only. The
compression ratio will vary with the motive steam flow rate and the stability of operation must
be taken into account (See Section 2.2.2 and Equation 2-8).
3.1.4.4 Ejector Efficiency
If the pressure and temperature of the steam are known at the motive steam inlet and ejector
outlet, together with the pressure at the air/vapor inlet, the efficiency can be calculated using the
Mollier diagram as discussed above.
3.2 Troubleshooting
For each steam jet air ejector, the maximum discharge pressure at the vent of the aftercondenser,
must be specified, as well as any vent temperature limits. It is important that the steam be dry
and contain no moisture to avoid ejector erosion. The minimum pressure and temperature of the
steam supply to the steam chest inlets on the ejector nozzles must also be specified. If the steam
pressure drops below this minimum, the vacuum system can become unstable.
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Shell Side Air-Removal Equipment Performance
It is recommended that the criteria specified in the following subsections be used when
troubleshooting these systems [12].
3.2.1 Poor Vacuum
Poor condenser vacuum can be the result of deviations in one or more operating parameters:
• Low steam pressure
• Superheated steam
• Nozzle orifice area different from design
• Total condenser air in-leakage
• Loop seal drain too short
• Excessive discharge pressure on atmospheric stage
• Poor main condenser operation
• Leaking air inlet isolation valves
3.2.1.1 Low Steam Pressure
Each ejector nozzle is specially designed for the steam pressure specified for the application. If
the pressure is less than design, the system cannot achieve the desired vacuum, and the following
should be checked:
• Compare the steam pressure at the inlet to the ejector steam chest with the rated pressure. If it
is not possible to increase the supply steam pressure, check with the manufacturer for
possible nozzle changes to allow for the lower steam pressure.
• Check whether there are any obstructions in the steam supply system that might be causing
the low pressure.
• Check whether any pressure-reducing valve in the system is functioning correctly.
3.2.1.2 Superheated Steam
Mass flow through a given nozzle is less for superheated than for saturated steam. Note that
saturated steam passing through a pressure-reducing valve will become superheated. Steam
supplied to a steam jet air ejector should never contain moisture because this can cause erosion
as well as performance problems. If the motive steam is not dry saturated but is superheated, the
ejector manufacturer should be alerted so that the design of the SJAE can be adjusted to meet
this steam condition.
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Shell Side Air-Removal Equipment Performance
3.2.1.3 Clogged Nozzle Orifices
Small nozzles designed for high steam pressures are more apt to become clogged than those
designed for lower pressures. Properly designed steam ejectors will allow the steam nozzle to be
cleaned in place. An alternative method is to remove the entire steam chest assembly. Then
remove the plug located on the steam chest and blow out any chips or scale from the nozzle end.
3.2.1.4 Total Condenser Air In-Leakage
Check the main condenser air in-leakage with the instrument provided on the discharge of the
after-condenser. If air leakage is excessive, check the vacuum system for tightness (see Section
11).
3.2.1.5 Loop Seal Drain Too Short
Condensate drain lines and loops seals must be properly designed to prevent short circuiting of
the air between the main turbine condenser and the intercondenser.
3.2.1.6 Excessive Discharge Pressure on Atmospheric Stage
Excessive discharge pressure on any ejector stage can cause unstable operation. Starting at the
final ejector stage, discharge pressures should be checked and compared with design.
3.2.1.7 Poor Main Condenser Operation
When condenser equipment has been in operation for extended periods of time, deterioration in
performance is often attributed to the ejector vacuum system. However, the main turbine
condenser may itself be the source of the problem. Some of the possible causes include high
cooling water temperature, insufficient cooling water flow, or excessive fouling of the condenser
tubes.
3.2.1.8 Leaking Air Inlet Isolation Valves
In a twin-element SJAE, poor condenser vacuum can result when SJAE performance is degraded
because of leakage through a seemingly closed first stage air inlet valve. This leakage causes a
recirculation flow to occur between the two elements and so reduces the overall efficiency of the
SJAE. If the other and previously open air inlet valve is found to be leaktight when closed, SJAE
performance and condenser vacuum may be improved by switching elements.
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Shell Side Air-Removal Equipment Performance
3.2.2 Gradual Loss of Vacuum
Some of the causes for a gradual falling off in vacuum could be attributed to:
• Nozzle or diffuser eroded or corroded
• Improper operation of condensate trap
• Clogged loop seal drain pipe
• Leaking SJAE system condenser tube
• Wet steam
3.2.2.1 Nozzle or Diffuser Eroded or Corroded
If the unit is operated in the shut off condition and the pressure is greater than 0.25 in. HgA (0.85
kPa), it is possible that the nozzle or diffuser is eroded or corroded. It is recommended that the
parts be inspected periodically and a record made of the wear found. If replacement of these
parts occurs too frequently, the cause of failure must be determined. Usually, it is found to be
wet steam.
3.2.2.2 Improper Operation of Condensate Trap
To correct this problem, the trap should be disassembled and cleaned, proper drainage also being
ensured.
3.2.2.3 Clogged Loop Seal Drain Pipe Tube
To correct this problem, clean or replace the loop seal piping.
3.2.2.4 Leaking SJAE System Condenser Tube
Check for any leaks by applying a hydrostatic test on the vapor side of the inter- and
aftercondensers. In order to locate the tube that is leaking, it will be necessary to remove the
waterbox cover and close the inter- and aftercondenser drain valves. Replace or plug any
damaged SJAE condenser tubes.
3.2.2.5 Wet Steam
A fluctuating steam pressure gage may indicate the presence of wet steam. The steam piping
should be examined to ensure that there are no low points for condensate to accumulate and that
the piping is properly insulated.
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Shell Side Air-Removal Equipment Performance
3.2.3 Poor Vacuum and/or High Outlet Water Temperature
Typically, the cooling water supply to the ejector system is the condensate from the main turbine
condenser. At low turbine loads, the condensate flow may be insufficient to sustain proper
cooling within the ejector system. If no alternative source of fresh water supply is available to
replace the condensate flow, a loss of vacuum may result, along with high discharge
temperatures on the outlet of the SJAE condensers.
3.3 Field Testing
It is difficult to check the operation of an ejector in the field, but some testing can be
accomplished by checking the shut-off performance of each ejector. It is recommended that tests
be performed when the unit is first placed in operation and that these readings be kept on file for
future reference.
If an ejector is operating satisfactorily but suddenly loses vacuum and then reestablishes its
performance immediately, the probable causes are among the following:
• Momentary drop in steam pressure
• Slugs of water in the motive steam
• Momentary increase in back pressure
• Momentary increase in air leakage
• Temporary increase in condensing water temperature
• Temporary decrease in condensing water flow
If an ejector operates satisfactorily over an extended period of time and then gradually loses
vacuum, it may be an indication of internal wear. Ejectors should be inspected periodically and
components replaced as needed.
3.4 Checking the Operation of a Two-Stage Vacuum System
The following procedure can be used to check the operation of a two-stage vacuum system:
1. Check the blanked off suction pressure in the first stage with both ejector stages operating.
This pressure should be 0.25–0.30 in. HgA (0.85–1.0 kPa) or less. If this reading is obtained,
then the problem lies elsewhere. If this reading is not obtained, then there is a problem with
the ejector system. Note that the steam pressure should be maintained at no more than 15-
20% above the rated ejector motive pressure. If this reading is erratic, refer to Section 3.2.
2. Shut off the steam supply to the first stage and check the shutoff reading for the atmospheric
stage, which should be 2.5 in. HgA (8.5 kPa) or less. If this reading checks, then the problem
resides in the first stage.
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Shell Side Air-Removal Equipment Performance
3. If the vacuum system cannot be taken off line, check the first stage and second stage inlet
pressures and compare with Table 3-1. If the pressure is not within the specified range, the
problem may be due to:
• Condenser water flow
• Steam pressure
• Condenser drains
• Cleanliness of SJAE condenser tubes
Generally, the inlet pressures should correlate with each other as shown in Table 3-1.
Table 3-1
Expected Correlation Between First- and Second-Stage Inlet Pressures
First-Stage Inlet Pressure Second-Stage Inlet Pressure
in. HgA (kPa) in. HgA (kPa)
1.0 (3.4) 5.0 to 5.5 (17.0 to 18.6)
1.5 (5.1) 6.0 to 6.5 (20.3 to 22.0)
2.0 (6.8) 6.5 to 7.0 (22.0 to 23.7)
2.5 (8.5) 7.0 to 7.5 (23.7 to 25.4)
3.0 (10.2) 7.5 to 8.0 (25.4 to 27.1)
4.0 (13.6) 9.0 to 10.0 (30.5 to 33.9)
4. The air side pressure drop across the intercondenser should also be checked and compared
with design.
3.5 Liquid Ring Vacuum Pump System Performance and Diagnostics
Assuming that the air-removal system was correctly designed for the specified conditions, many
of the operational problems experienced with liquid ring vacuum pumps (LRVPs) are usually
associated with fouling of the seal water cooler. This raises the seal water temperatures, which
can result in reduced pump capacity and limit the ultimate vacuum that the pump can achieve.
The increased seal water temperature may lead to cavitation damage for pump impellers.
The fundamental LRVP design criteria include:
• The absolute pressure to be maintained at the pump suction
• The total weight (lb/h or kg/h) or the volume flow (ACFM) of gas to be handled
• The temperature of the gas to be handled
• The composition of the gas to be handled, each constituent being given in lb/h (kg/h)
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