Integrating Processes

In article , Curtis wrote:

Clearly you need a copy of my book (still in preparation) 'Design of Averaging Level Controls'. Assuming P+I Control, then in nearly all practical process applications it is possible to calculate liquid level-controller tuning parameters for the case of level control cascaded to flow control, to achieve any desired degree of tuning on what is a continuous spectrum of behaviour between 'tight' level control (limited by noise and stability) and averaging level control (limited by the need to keep worst-case level disturbances within alarm limits), at any pre-defined loop damping ratio. The information required to calculate the tuning settings includes change in fluid volume between 0-100% of level controller range, and between upper and lower level alarm (or operating) limits; together with the volumetric flow rates corresponding with upper and lower ranges of the flow controller. The design principles are simple: First select a target damping ratio for the loop: the choice determines the size of the resonant peak in the system's I/O response to flow disturbances. Next calculate loop gain, Kl from the flow and volume ranges and controller integral-action time, Ti. The chosen damping ratio sets the relationship between K and Ti; e.g. Kl*Ti=4 for critical damping is suitable for >95% of process applications. Controller gain Kc (which is one component of Kl) can then be adjusted for any desired degree of 'tightness' of level control (within the limits described above). Indeed, it is possible to conceive of a level controller having just one tuning knob for adjustment between 'averaging' and 'tight', with the calculation of appropriate Kc and Ti settings taking place behind the panel. When I worked for a major oil company I produced a simple Excel spreadsheet for the calculations, and we still use the same method here for calculating level-controller settings in our process simulation studies.

Unfortunately, it is not so easy to do a numerical design with level control cascaded to a control valve; because, except in the special circumstances of constant-pressure operation and linear installed valve characteristic, there is not the well-defined linear relationship between flow and demand (output) from the level controller that is provided by a slave flow controller. Nevertheless, the design principles are essentially the same; but one needs to check the calcs. for a range of valve-gains under extreme operating conditions - then choose a worst-case.

The single most common fault in on-site level-controller tuning is that engineers do not understand the need to keep the product Kl*Ti (e.g. =4) constant to achieve a defined constant loop damping. Very many level control loops in industry are therefore very poorly damped (underdamped) with low natural frequencies, leading to under-damped oscillation in response to transients, often with very long oscillation periods; sometimes so long that operators don't even realise that the slow ups and downs (sometimes on a time-scale of several hours) are not an inherent feature of the process behaviour.

Kelvin B. Hales Kelvin Hales Associates Limited Consulting Process Control Engineers Web:

Hi Curtis, The easyest way to tune a integrating process is to make a step response experiment, and then tune the controller. Follow this link to se how to do

"Kelvin Hales" skrev i meddelandet news: snipped-for-privacy@khace.com...

Averaging Level Controls'.

is possible to calculate

cascaded to flow control, to

behaviour between 'tight' level

(limited by the need to keep

loop damping ratio. The

fluid volume between 0-100% of

operating) limits; together with the

the loop: the choice determines

disturbances. Next calculate loop

time, Ti. The chosen damping

damping is suitable for >95% of

can then be adjusted for any

described above). Indeed, it is

adjustment between 'averaging'

place behind the panel. When I

the calculations, and we still

process simulation studies.

control cascaded to a control valve;

operation and linear installed valve

flow and demand (output) from

Nevertheless, the design principles are

valve-gains under extreme

engineers do not understand the

constant loop damping. Very many

(underdamped) with low natural

transients, often with very long

that the slow ups and downs

of the process behaviour.

Level Controls'.

possible to calculate

cascaded to flow control, to

behaviour between 'tight' level

by the need to keep

damping ratio. The

volume between 0-100% of

limits; together with the

loop: the choice determines

disturbances. Next calculate loop

Ti. The chosen damping

damping is suitable for >95% of

then be adjusted for any

above). Indeed, it is

adjustment between 'averaging'

place behind the panel. When I

calculations, and we still

process simulation studies.

cascaded to a control valve;

and linear installed valve

and demand (output) from

Nevertheless, the design principles are

valve-gains under extreme

engineers do not understand the

constant loop damping. Very many

with low natural

often with very long

the slow ups and downs

the process behaviour.

Kelvin,

Thanks for your reply. You state in the first paragraph:

Next calculate loop

This is the part I'm having trouble with. To make a move on the valve and watch the level change obviously will not work because the process is integrating. Also, I'm not sure what the controller integral time has to do with the process -- the process gain should be independent of anything in the controller. I have the flow and volume range do I just divide full scale volume over full scale flow?

If so I get 13m3/(500m3/day). But that really doesn't give me much.

When your book is done let me know. It sounds like it would help me out a lot.

Thanks.

Curtis

Assuming that it's truly integrating, i.e. that the head on the valve doesn't change with tank level, then your integrator gain is

(5m3/day/%opening)/(0.131m3/%level) = 38.2 * %level / %opening / day,

which is a perfectly valid integrator gain. It says (for instance) that with 100% opening the tank level should change by 100% in 1/38.2 of a day, or that for a 1% valve opening the tank level should change by

Actually having it follow such a simple model would be a surprise, but that's your problem.

I suspect that your problem with integrator gains is that you want to see something like level vs. opening. Integrator outputs are rates of change, so there needs to be a time factor in the denominator of the integrator gain, or a frequency in the numerator.

Averaging Level Controls'.

is possible to calculate

cascaded to flow control, to

behaviour between 'tight' level

(limited by the need to keep

damping ratio. The

fluid volume between 0-100% of

operating) limits; together with the

controller.

the loop: the choice determines

disturbances. Next calculate loop

time, Ti. The chosen damping

damping is suitable for >95% of

then be adjusted for any

above). Indeed, it is

adjustment between 'averaging'

place behind the panel. When I

calculations, and we still

process simulation studies.

cascaded to a control valve;

and linear installed valve

flow and demand (output) from

Nevertheless, the design principles are

valve-gains under extreme

engineers do not understand the

constant loop damping. Very many

(underdamped) with low natural

often with very long

that the slow ups and downs

the process behaviour.

I started on a reply to your reply (IYSWIM); but I got side-tracked. I'll try and finish it, otherwise maybe an E-mail correspondence might be best.

Kelvin B. Hales Kelvin Hales Associates Limited Consulting Process Control Engineers Web:

Hey there Kelvin,

Actually and email would be great. I have a couple pages of notes I wrote and scanned into pdf that maybe you could look at.

Tim, maybe you'd be interested? They're pretty simple...

Curtis