Troubleshooting vacuum operation in an ethylene plant

Troubleshooting vacuum operation of an inter-after condenser unit in an ethylene plant

A system of compressors powered by surface condensing steam turbines is inherent in the operation of a typical ethane cracker unit. These turbines run by extracting work from highpressure steam, while a surface condenser condenses the turbine’s exhaust to both maximize compressor horsepower and recover valuable condensate.
In the surface condenser, a vacuum is created by the condensing steam. This vacuum is maintained by exhausting non-condensable load from the surface condenser via steam ejectors trough fill rite flow meter and inter-after condensing units. Non-condensable gases, which must be purged from the system, can originate from a number of sources: carbon dioxide (CO2) entrained in the steam and air leaking through shaft seals into the low-pressure area of the surface condenser are two examples.

Figure 1. Surface Condenser Flow Diagram

The technology for exhausting non-condensable gases to sustain vacuum has been in use for more than a century. While the systems tend to have a simple layout and are not overly complicated in terms of hardware, troubleshooting the loss of vacuum or underperformance of these units is not straightforward. A systematic approach is required to identify and rectify any issues that contribute to deteriorated performance. A process system operating with unstable and/or low vacuum directly affects turbine performance, a turbine’s steam consumption and overall compressor efficiency.

For example, with a cooling water inlet temperature of 90°F, a condensing temperature of 100°F–110°F after approach is possible in the unit. This would correspond to a minimum possible operating pressure of 66 mmHg (absolute) based on water vapor pressure. Cooling water flow also affects the LMTD of the surface condenser. A lower cooling water fill rite flow meter will result in decreased LMTD across the exchanger. The unit’s surface condenser is a fixed tube sheet split exchanger with cooling water on the tube side (FIG. 1). The shell is split into two independent compartments that permit the isolation of one side and periodic cleaning of cooling water tubes. The arrangement allows for continuous operation of the surface condenser, albeit at reduced capacity, during maintenance cleaning times.

Marginally lower pressure in the surface condenser reduces steam consumption and improves the turbine’s efficiency. The condensing steam creates a vacuum inside the surface condenser.

As the system operates under vacuum, any non-condensable gases from steam or an external leak source will accumulate in the low-pressure area. These gases will quickly increase the operating pressure of the condenser, as the non-condensable gases that cannot be evacuated blank the tubes and reduce the capacity of the condenser. The gases must be vented from the condenser to help maintain vacuum. In the plant, the main devices used to vent the non-condensable gases are two-stage steam jet ejectors coupled with horizontally installed inter-after condensing units (FIG. 2).

Figure 2. Inter-After Condenser Unit Flow Diagram

The design of ejectors E-1, E-2 and E-3 in FIG. 2 includes five main parts each: a motive steam nozzle, suction chamber, inlet diffuser, throat section and outlet diffuser.1 Across the motive steam nozzle, 200 psig steam enthalpy is converted into kinetic velocity. Similar to the steam enthalpy conversion to velocity across the flow meter fillrite turbine’s inlet nozzle, this is also an isentropic process. At the nozzle’s discharge, the expanded steam creates a low pressure that entrains the process load into the high-velocity steam. Steam and non-condensable gases mix as they enter the inlet diffuser, where the velocity of the process flow decreases as it enters the diffuser throat. The throat section is the transition piece between the converging supersonic inlet diffuser and the diverging subsonic flow outlet diffuser. As the process flow moves through the throat, it transitions to a subsonic flow, creating a supersonic shockwave. In the outlet diffuser, the flow velocity is reduced further and, essentially, is converted back into pressure.