Case Study - Optimizing Steam Supply in a Chemical Plant

by Steve Lyons of Premier Control

Background

This project looked at optimising the steam supply system on a chemical plant. Erratic steam supply pressures and temperatures were giving product quality problems, and even occasionally causing the plant to shut down. The main problem encountered was obtaining loop response data, as most of the loops were stand-alone controllers without any trending. Process dynamics were obtained by connecting ExperTune software via an interface directly to the 4 - 20 mA signal. The software analysed the step response data giving accurate process dynamics and some recommended tuning parameters. This enabled the engineers to see the problem, and ultimately resolve it.

Process Description

There are two main sources of steam to the high pressure (HP) main.

3 waste heat boilers that use the excess heat produced when making sulphuric acid to produce steam. These can normally produce all the steam required for the HP main.

2 ICL boilers. Steam is imported into the HP main from these when one or more of the waste heat boilers is shut down

Observed Problems

When the HP main had to import steam from the ICL boilers the import valve modulated open and closed. This caused large fluctuations in mains steam pressure. Trends from the Factory Information Service (FIS) showed large swings in steam flow from all the waste heat boilers.

Investigations

1) Steam Import from ICL Boilers

"As found" operation of steam import valve

Original settings

PB - 75%
Integral - 5 secs/rep

Figure 1

Open loop step tests using ExperTune gave the following system dynamics.

Process Gain - 1.4
Dead Time - 9 seconds
Time Constant - 100 seconds

These dynamics showed that the original settings were far too "aggressive" and that in fact they were the cause of the oscillations. Conservative tuning settings of PB = 100 and Integral = 60 secs. were then put into the controller. A comparison of the two settings was then simulated using the ExperTune package (see fig 2).

Figure 2

2) Operation of Waste Heat Boilers

See appendix for process layout.

Waste heat boilers (WHB) are used to produce steam from unwanted excess hot gases from the production of sulphuric acid. These gases are used to pre-heat the feed water in an economiser, and are also fed directly to the boiler via a sulphur gun to produce steam. The level in the boiler is controlled directly by the feedwater valve with some feedforward action being given by the measured steam flow out. Pressure in the boiler is maintained by an outlet valve which exports steam into the main at around 19 barg (290 psi ).

Trends from the FIS show that steam flows from WHB 4 &5 are oscillatory.

Waste Heat Boilers Steam Flow Trends
Figure 3

What effect this had on the steam pressure control was unknown as these pressure trends are not on the FIS. It was decided to look at the operation and dynamics of the system, again using ExperTune. The initial investigations centered around nos 4 WHB as the steam flow trends showed this to be oscillating more than nos 5 at the time of testing.

Step tests on the level showed that the controller settings were fairly aggressive and that they were set up to maintain a tight level control (analysis gave a process gain of 0.14 and a deadtime of 0.78 mins). The boilers have such a large capacity in comparison with the feedwater valves it was felt the level control parameters could be slowed down in order to reduce the variations in steam flow exiting the boiler (averaging level control). This was done with dramatic effect on both nos 4 & 5 WHB (see Fig 4).

Steam Flow Trends Before and After Tuning
Figure 4

Benefits

Improved quality due to less process variability.
Less wear on hardware, therefore savings.

Less shutdowns due to steam problems therefore increased profitability.

Appendix

Acid Plant Boiler Level Control

About the author

Steve Lyons of Premier Control has a BEng in Instrumentation & Control from Teesside University. He has control engineering and training experience with ICI (petrochemicals plant), Huntsman Trioxide, and offshore oil platforms.