Aug-2015

Advanced turbomachinery controls improve ethylene plant yield

Advanced integrated turbomachinery controls can significantly alleviate the suction pressure dip of a cracked gas compressor, leading to considerable yield improvements.

Medhat Zaghloul
Compressor Controls Corporation

Article Summary

Ethylene plant yield is a function of pyrolysis furnace back pressure, among other factors. The mechanism to keep the furnace back pressure at a certain value is the suction pressure to speed the cascade control loop on the cracked gas compressor. A schematic arrangement of the gas path from the furnaces to the compressor is shown in Figure 1.

In Figure 1, the design pressures, temperatures and flow rates of the gas path are depicted in a simplified manner for a 300000 t/y ethylene plant. Any particular plant may show slight differences.

In every plant, the selected set-point value for cracked gas compressor suction pressure is determined to ensure that, in the event of single or multiple furnace trip events, the suction pressure does not dip low enough to sub-
atmospheric values, thereby creating the risk of sucking in air with the process gas, which could create an explosive mixture.

The cracked gas compressor is a multi-section centrifugal compressor, with four or five stages of compression required to bring the cracked gas pressure up to the desired value of approximately 40 bara. The cracked gas compressor is usually driven by an extraction type steam turbine using high pressure live steam and extracting steam to the plant’s medium pressure steam network, as well as condensing the remaining steam that is used in the turbine low pressure compartment.

Caustic wash is usually installed between the third and fourth (or fourth and fifth) stages to remove any trace quantities of acid gas (H2S) present in the cracked gas.

As the cracked gas that enters the compressor is saturated with steam, condensate removal is required after each stage’s discharge cooler and prior to introducing the gas to the succeeding stage of compression.

Typically, the first group of compressor stages (upstream of the caustic wash) is protected against surging by a single anti-surge valve, while the stage (or stages) downstream of the caustic wash section is (are) protected by another anti-surge valve.

A suction pressure controller uses the cracked gas compressor’s first stage suction drum pressure as its process variable (PV), compares it to the configured set-point (SP), and the resulting error used to generate a PID response to modulate the speed set-point of the steam turbine driver, between its control range of minimum governor to maximum governor values.

The actual modulation of the live steam valve (V1 valve) is done by a speed governor, cascaded with the cracked gas suction pressure controller. The modulation of the live steam valve, for those plants that have an extraction turbine driver, needs to be closely coordinated with the turbine’s extraction (V2) valve.

In the event of a furnace trip, a significant portion of the cracked gas available from the furnaces drops almost instantaneously. In this example, we will consider 16.7% of the plant’s rated capacity (assuming that normally there are six furnaces in service).

The cracked gas pressure at the compressor’s suction drum will also start to drop steeply after the time delay caused by the process volume of the process equipment between the furnace outlets and the compressor’s suction drum.

Several factors will influence the value of the minimum suction pressure value reached during the furnace trip event. These include:
•    The volume (capacity) of the process equipment between the furnaces outlet and the cracked gas compressor suction drum
•    The tuning of the compressor suction pressure controller, and
•    The speed of response (quality) of turbine speed control.

The volume of the process equipment in question is not a factor that can be influenced by better control strategies. However, the other two factors are influenced by better control strategies. Of those, the quality of the turbine speed control is more important, since it is the ‘inner’ loop of the pressure-to-speed control cascade arrangement.

The turbine speed control could be based on a mechanical-hydraulic governor, or a more modern digital governor. In either case, the extraction control mechanism usually presents control challenges to the turbine speed control loop.

Take the example of an older control system where the speed governor is a mechanical-hydraulic device using the flywheel principle and is linked to the extraction valve actuator with a mechanical linkage. This type of older system would introduce significant oscillations in the speed control whenever there is a change in speed or extraction demand which, in order to produce a smoother response, would require considerable dampening of the control responses, rendering the overall speed control response quite sluggish.

While the most sophisticated dedicated digital control hardware and software algorithms available in the market greatly improve the speed control response, there is still a minimum time constant achievable in terms of the speed control of an extraction type turbine.

One of the most important rules of controls implementation is that the ‘outer’ loop of a cascade control arrangement (in this case the pressure controller) must be tuned at least five times slower than the time constant of the ‘inner’ loop. Thus, it is fair to conclude that even with the best dedicated digital speed control hardware, and the best speed-extraction control algorithms, there is still going to be a somewhat sluggish to very sluggish suction pressure control time response in any ethylene plant.

It was perceived that, in order to deal with the suction pressure ‘collapse’ that would accompany a furnace trip (or multiple furnaces tripping) and prevent the pressure dip from becoming sub-atmospheric, a low suction pressure limiting loop needs to be implemented, in tandem with the normally sluggish suction pressure control loop. This limiting loop would be tuned to be faster than the principle pressure control loop, but would require a set-point value that was offset from the ‘main’ suction pressure controllers.

For many years this was considered state-of-the-art for setting up the cracked gas suction pressure control set-point value. When a time recording is made of the behaviour of such a control arrangement, the result could look something like Figure 2.

With advanced control algorithms that incorporate the full integration of all the compressor’s turbo-machinery controls functions, such as anti-surge, performance, speed and extraction controls, into one dedicated high-speed controls platform, it is possible to improve on the pressure dynamic behaviour time trend shown in Figure 2.