1st stage ejector outlet
1st stage gas outlet
Area above ba e
Area below ba e
CW inlet
CW outlet
1st stage discharge piping pressure drop
90˚F
93˚F
100˚F
DP1
Condensate leaving bundle area above ‘Long air ba e’
Condensate leaving area under ‘Long air ba e’
2nd stage suction pressure drop
DP3
Condensate leaving 1st stage intercondenser
Figure 3 Model for X-shell simulation
DP2
Intercondenser pressure drop
If the long air baffle is properly sealed, the vapour leaving the first-stage inter-condenser must be colder than the con- densate since it has been further cooled by the surface area located under the baffle. Maximum discharge pressure and ejector break operation If the discharge pressure of the first-stage ejector operates above the maximum discharge pressure (MDP), then the operation of the ejector will be unstable due to a lack of the necessary shockwave in the ejector. This operating condi- tion is called ‘break’. Broken operation is characterised by an increase in suction pressure. Depending on the ejector design, the suction pressure can be 20-40 mmHG higher than design. The second-stage ejector load and the first-stage ejector system pressure drop determine the first-stage ejector dis- charge pressure. Figure 4 shows three major components that make up the first-stage ejector system pressure drop. They are: • The pressure drop in the piping from the first-stage ejector to the inter-condenser (DP1) • The inter-condenser pressure drop (DP2) and the piping from the inter-condenser to the second-stage ejector (DP3). The second-stage ejector suction pressure is a function of the load to that ejector. If the load goes up, so does the pressure. The first-stage ejector discharge pressure is cal- culated by adding the system pressure drop to the second- stage ejector suction pressure. If the process load or the
Figure 4 Pressure drop across ejector stage
inter-condenser pressure drop is higher than design, then depending on how much margin was applied to the design, the ejector discharge pressure may exceed the MDP and result in broken operation. It is not uncommon for the ejector MDP to be only 3-8 mmHG above the normal design operating pressure. Calculated first-stage inter-condenser pressure drops are about the same magnitude, about 3 to 5 mmHG. This leaves very little margin for inter-condenser fouling or underperfor- mance of the long air baffle. A small leak around the baffle can easily increase the load to the second-stage ejector and increase the suction pres- sure by 5-10 mmHG. Therefore, the performance of the long air baffle and its proper sealing are critical to first-stage ejec- tor performance. Quickly identify long air baffle problems If an ejector is breaking, it is quick and easy to determine if the long air baffle may be the problem. The temperature of the vapour outlet should be, at a minimum, the same temperature or a few degrees lower than the temperature of the condensate outlet. The condensate and vapour out- let streams are rarely instrumented, so a trip to the unit is required to measure these temperatures. Since the temperature difference between the two streams is small, accurate measurement is required. If the vapour out- let temperature is higher than the condensate temperature, then the air baffle is not performing properly. Data taken from operating units (see Figure 5 ) show the first-stage inter-condenser outlet gas temperature 8-15⁰F higher than the condensate. The root cause is the hot gas bypassing the long air baffle. Long air baffle hot gas bypass Low motive steam pressure, high cooling water tempera- tures, low cooling water flow, and exchanger fouling can all lead to underperformance and ultimately broken operation. The conditions do not cause the vapour outlet temperature to be higher than the liquid condensate. Figure 6 shows how second-stage inlet gas load increases with increasing condensing temperature at a constant
1st stage ejector outlet vapour
106˚F 105˚F 78˚F
CW outlet
87˚F 87˚F 72˚F
Gas outlet
109˚F 113˚F 94˚F
CW intlet
106˚F 106˚F 78˚F
Condensate to hotwell
Figure 5 Field temperatures
12
Revamps 2022
www.digitalrefining.com
Powered by FlippingBook