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1-Element vs. 3-Element Level Control
Continuous process industries, DCS questions. topic
Posted by Ron Wensley on 15 July, 2009 - 6:35 pm
There is an on going debate about the use of 1-E vs 3-E Drum level control in power boilers.

The debate is that 3-E control is only useful in conventional boilers where the heat input is changing. 1-E is all that is needed when the process change is in the steam flow only such as a combined cycle plant where the steam turbine load changes.

The side that states the above claims that when the steam turbine control valves open the drum swells and the steam flow increases, which in 3-E would add more water and trip the unit.

I contend that this first order zero is temporary and that 3-E increases stability IF that zero is taken into account in tuning the controllers.

I would like other to comment on there experiences with using 1-E and 3-E and see where this leads.

Thanks,
Ron


Posted by Bruce Durdle on 15 July, 2009 - 10:59 pm
Ron,
I'm not sure where the version of events " when the steam turbine control valves open the drum swells and the steam flow increases, which in 3-E would add more water and trip the unit" comes from.

In my experience, when the steam turbine control valves open, more steam is drawn from the boiler - the steam flow increases. This reduces the pressure in the drum steam space. As a result, water in the depths of the boiler walls has its pressure reduced and a small amount flashes to steam as a result. This developing of bubbles in the tubes causes a temporary increase in the drum level because of the increased volume.

Now, with a single-element control, the drum level controller will see this short-term increase in level and act to reduce the feedwater flow. The total mass balance on the boiler water side is negative so the total quantity of H2O starts to fall. This is a double-whammy as we are taking more steam off and adding less water - when the level stabilises, the water level will be
low.

By using the 3-element approach, the increased steam flow is used to increase the feedwater flow in proportion. The level controller is set up to act as a trim controller with low gain and should not react significantly immediately on a steam flow increase/level rise.

The only situation where 3-element control could trip the unit is if the drum is very small and stem flow demand disturbances are very large. In this case, it is possible that the increased water flow rate combined with the temporary surge due to swell could take the level above the trip point. However, adding relatively large amounts of cold water to the drum has the effect of "squashing" the level - the drum water temperature reduces and causes bubbles of vapor to collapse, so reducing the total volume and hence the level.

Obviously, the relative magnitudes of each of these effects will depend on boiler details.

You will find (possibly on this list) people who claim that using pressure control on to firing and then linking this to feedwater flow rate is better than 3-element control. However, the pressure controller will work off the same pressure sensor and then has to manipulate the firing which in turn will take a while to feed through to the water side. It would possibly work better if heat input is varying.

My 2c worth...

Bruce.


Posted by Ron Wensley on 17 July, 2009 - 7:43 pm
Bruce,

Thanks for the reply. I agree with your statements and have used this as my stance for a long time. I have been running into situations where a 'professional' has convinced plants that 3-E is never needed on combined cycle plants.

The 1st Order Zero explains the points you have and I feel that this 'professional' does not have a good under standing of the physics behind the process.

Once his bias is created it requires much more 'proof' to convince the plant people that 3-E truly is needed and is their friend.

Thanks for the reply!
Ron


Posted by wboyes on 16 July, 2009 - 1:08 pm
You might refer to Greg Shinskey's "Taming the Shrink and Swell Dragon" in Control a few years back.
http://www.controlglobal.com/articles/2004/136.html

Walt Boyes
Editor in Chief
Control and Controlglobal.com
www.controlglobal.com
Mailto:wboyes [at] putman.net
Read my blog SoundOFF!! At www.controlglobal.com/soundoff


Posted by Ben Janvier on 17 July, 2009 - 5:18 pm
We have tuned and modelled many drum level controllers and it is true that 3-element drum level control will destabilize the drum level when a change in firing rate occurs.

Let's go through a change in firing rate together:
-fuel flow goes up
-steam phase under the drum increases
-steam flow increases
-level increases

In this situation, the level is high and you need to reduce your water flow setpoint. If you have steam feedforward, the steam flow will force the water flow setpoint to go up and your unit could trip - not cool :)

Why is 3-element not advised for boilers that need to change firing rate quickly? The answer is that 3-element assumes that if you keep your water mass constant in the boiler, your level will stay at setpoint. This is incorrect. In fact, as the boiler load increases, the water vapor phase in the boiler rizers increases. Hence, for a constant level, your water volume will DROP as your firing rate is increased. This is the reason why steam flow trips boiler drum levels. We prefer using firing rate (or GT MW for CC plants) as a feedforward signal to drum level control - much better for rapid firing rate changes as it manages your volume balance. I wrote a paper on firing rate feedforward so if you need me to send it to you let me know.

We have implemented the FR feedforward in a few power plants and some of these plants were able to double their MW ramp rate (when drum level was a problem)

Ben Janvier
bjanvier[at] enerosolutions.com


Posted by Ron Wensley on 20 July, 2009 - 7:47 pm
Ben,

Thanks for this reply. I would be very interested in seeing your paper.

If you can send it to me I would appreciate it.

Ron
rwensley [at] controllertuning.com


Posted by Allan Evans on 22 July, 2009 - 10:57 pm
Every once in a while the topic of drum level control crops up. I read the comments with interest. After three years as an I & C boiler commissioning engineer in the UK I moved to New Zealand. Six months later I was commissioning a coal fired boiler that took me a couple of weeks to get the drum level under control. The level would sit at set point, the load would increase, the level would rise, I'd slowly open the feed valve, adding water to the drum and the unit would trip on low water level! We got there in the end but after much research I found out that I had commissioned my first boiler that was a dog!

It was a lesson well learnt. It highlighted the fact that it does not matter how good the control system is, whether it is 1,2 or 3 element, the capability of achieving good level control is directly related to the size of the drum and the magnitude of the load changes. Do not use that as an excuse if you cannot control the level correctly. The vast majority of boilers have correctly sized drums. I'm trying to keep this email short so here are a few bullet points.

Key points to check first if the level is unstable will be the combustion control and water treatment. If the combined cycle plant is tripping when you add water, check the TDS. You may have foaming in the drum. Combustion control also deserves a mention. I have been to many plants that have swinging levels resulting from badly tuned combustion control. Master pressure control should have a fairly high gain and low integral action. I come across so many master pressure units with a gain of 1 and fast integral. The result is like waves coming on to the beach, with the level control responding in sympathy. It can also come from a swinging level. Every time you add water you have to increase the firing rate. Keep that in mind on a load swing upwards. Without repeating my colleagues replies already, an increase in load demand, swells the drum not only because of the pressure drop but also a velocity effect of the increased steam flow. Because the pressure has reduced, combustion demand has increased further contributing to the level rise.

For every kg of steam that comes out we need to put a kg of water in. That's basic. The figures do not quite match because we have blow down and sometimes soot blowing to contend with. So using single element and trying to control a level with a signal that goes the wrong way does not make sense.

Here is my procedure to achieve stable drum control within a couple of hours of operation.

1. Ensure the range of the steam and feed flow transmitters is identical and you have a 4 to 6 seconds dampening on the output.

2. Drum level transmitter installation to have a good sized condensate pot on the high leg, a balance pipe between the high and low connections and about a 4 to 8 second dampening on the transmitter output.

3. The feed water valve to have an equal percentage characteristic sized that at 70% opening the flow through the valve equals the maximum continuous output of the unit. At low loads you will be in single element control. At a value to be determined you will switch bumplessly to 3 element.

4. The logic will be simple. Forget all of the fancy stuff you have read about. All those additions and fancy feed forward signals have to be tuned by experts, and there are not many of them. So you will have two controllers. The first has the steam flow as the set point and the feed water flow as the process value. The second controller compares the drum level with the required set point, the output trimming the steam flow signal. Lets commission the loop.

5. Force the output of the drum level controller to zero, that is no trim effect. At this stage we do not want it to interfere with our steam/feed controller. Set up your preliminary tuning values so that the gain is between 0.7 and 0.8 with repeats of about 3 to 4 per second. Vary the units load. Keep in mind that you are now tuning a flow loop. Simple, that's done, now we move on to the drum level controller that is going to add or subtract the steam flow signal to its controller. Low gain, low integral. Watch the output. When the level has settled down and at set point its output should have no trim effect on the steam flow when the level is at set point. You may find that at set point it always has a bias effect. This could be because of disparity between the steam and feed flow transmitters or there is some blow down. To get around this I add a multiplier on the feed flow signal and adjust it so that when we are at set point the drum level trim is back at mid point.

Done. Combined cycle plants producing power should be easy to set up as the majority of the time the load is steady. Try a co gen plant with fresh air firing and automatically change from co gen to fresh air firing automatically. No problems, but that is another subject.

Allan Evans


Posted by Chandrakant Prajapati on 7 August, 2009 - 7:15 am
Dear Allan Evans

Good and Interesting Points for Commissioning 3 Element Drum Level Controller.

I understood that in Point # 4, you want to Build a cascade Controller where in the First Controller Compares the Steam Flow and Feed Water Flow. Output of this controller would become Set Point for Drum Level controller. Is it what you mean to say?

And I am unable to understand the Point # 5. Could you please elaborate? I understood that FIRST we need to tune Master loop i.e. Flow Loop by Forcing the Slave Controller Output to ZERO? Is it possible to do that in Cascade Mode?

Please revert om my mail ID: chandra_n [at] vsnl.net. It would be great help to me as I am going to commission my first System for Boiler.

Regards
Chandrakant


Posted by srijith.s on 9 October, 2009 - 1:09 am
Dear chandrakanth sir,

i am srijith working as an instrumentation engineer. with the information posted i can conclude that you have a very good knowledge
in three element control.

if possible please forward me some data regarding the calculation of constants in the flow compensation used in conversion of volumetric flow to mass flow.

bye take care


Posted by Ron Wensley on 17 July, 2009 - 7:45 pm
Walt,

Thanks I am looking at it right after this.

Ron


Posted by Romulo Rodriguez on 18 July, 2009 - 8:22 am
The best thing is to implement a dynamic control... 3-element when steady load or small load changes so mass is properly balanced in the boiler, and 2 or 1-element when load variation exceeds certain rate that you may program as a function. I happened to work for many many years for Bailey Controls and that was a standard configuration decades ago when the first Network 90 systems where commissioned.


1 out of 1 members thought this post was helpful...
Posted by Phil Corso on 8 August, 2009 - 12:01 am
Many, many words, but no mention of the role steam and drum-size plays on level control success!

I remember the gOLDen days when the "Integral" portion of PID control, was simply the addition of a capacitor or two.

Search the Control List Archives for the paper I once offered illustrating the impact of drum size.

Regards, Phil Corso (Cepsicon [at] aol.com)



Posted by vishu on 22 May, 2012 - 10:55 pm
what is the calculation of find out the output the controller

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