We want to install a three element control in boiler Drum Level, where steam flow will be feed forward with drum level controller o/p and gives remote set point to boiler feed water flow controller, on which flow control valve operated. We have US1000 of Yokogawa make. Boiler Feed Water to Steam Ratio is 1 to 1.1. Please guide how we configure the controller US1000.

 

A few rules of thumb:

Drum level tuning: Use the Internal Model Control tuning method as this will give you the best result. (don't ne hy on the gain)

Steam Flow FF: Steam flow feedforward often kills you during a large load change. Indeed, when the boiler load goes up, the level swells, the level controller is begging for less water but our friendly steam flow feedforward wants more water and can (and sometimes will) trip your boiler on high level or low level. Quite frankly if you really want to use steam flow as a feedforward signal I would recommend you put a gain of 0.8 and filter the signal using a 30 seconds time constant. The filter is very important as it eliminates the wrong-way response of the steam flow.

Control Scheme: You should not try to add your feedforward signal on the output of the drum level controller. This will limit the drum level response capability (for example,you will not be able to close the Feedwater valve if the steam flow is positive). You can get around this problem by changing the range of your drum level but this will cause you another headache - windup. In other words, try implemeting feedforward as an input to your PID controller (hopefully your DCS has this feature)

Firing rate: Typically I recommend getting rid of the steam flow (or at least filtering it) as a feedforward input and replacing it with the firing rate demand (when firing rate increases feed flow decreases ...)

Good luck,

Ben Janvier, process control consultant
Enero Solutions
[email protected]

 

thanks ben,
sorry, steam flow signal and drum level signal goes to input of the level controller separately and summation of their output goes to feed water controller as set point. Not feed forward with output of level controller.

sanjib
[email protected]

 

I can not find the exact configuration parameters of US1000-11 for Boiler Drum Level Control and I have no options to make simulation so trial and error is not possible, How can I get the these parameters?

service engineer
gizasystems

 

I noticed that when you put US1000 on mode 13 (to apply three elements boiler drum level control) and start to adjust the Level input range you find that US1000 automatically adjusts the steam flow range to the same range of level!

Why? Is it in three element boiler drum level control the steam and level supposed to take the same range?

ahmed safaa
ahmed.safaa at gizasystems. com

 

Benoit, can you please clarify why steam flow feedforward signal needs to be smoothed using filter with time constant at least 30 sec? I saw one application where 100 sec first order filter is used.
Why wrong response is to be expected if signal is not filtered?
What exactly do you mean by wrong response? Do you mean that controller is providing less feedwater when it needs more?

Thanks.