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Savings through fine-tuning of process control loops

March 26, 2014

In process control loop optimization, there are some common questions:
1) Is there any savings potential we can talk about if we fine-tune our process control loops (e.g.) fuel flow control loop for boiler or incinerator.
2) What are those applications in which savings can be thought of if fine-tuning is carried out on their associated control loops.
3) Procedure of calculating savings after fine-tuning of specific control.
How to calculate savings from pre and post tuning cases? Taking example air to fuel ratio in boilers to control steam pressure. Steam pressure SP = 60 bar and PV varies from 50 bar to 70 bar (10 bar up and down from SP or in other words amplitude is 20 bar) due to poor loop tuning. Now after performing fine-tuning this whole control scheme we are able to reduce PV variations to 57 bar to 63 bar (3 bar up and down from SP or in other words amplitude is 6 bar). Now controlling is very tight and optimized but how can I calculate savings from such tuning?

Setting the Optimal combustion ratio is in a direct result of the effect on combustion efficiency = how much fuel you need to produce the required energy / heat. Then there is the element of compliance with emission limits. Their failure is expensive.
It is also necessary to evaluate the transport of steam. Steam traps work best under constant conditions = extended service life. On the side of steam consumers can be observed higher products quality and lower costs for control and maintenance. In case of heating and transport should be assessed loss of heat, which are naturally higher at the higher temperature (at steam connected with the pressure). Any efficiency of energy transfer is obviously better at steady-state conditions.

For the steam pressure example you have two possible cases, apart from less mechanical stress to your equipment and improved reproducibility of your operations thanks to steadier utilities:
1) before smoothing your control loop your effective minimum pressure in the dips had negative impact to your operation, such as too much time lost / capacity constraint in a heating operation. This negative impact would be gone or reduced now, hence your benefit. You can now heat more/faster and more consistently, you gain productivity.

2) Even with your old minimum pressure heating operations were still fine. In this case you can lower your pressure setpoint (in your example from 60 to 53 bar) to get to the same effective minimum pressure with lower swing amplitude and save energy by reducing heat losses in steam distribution and increase efficiency of the burner/boiler.
Reality could also be a mixture of both cases.

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