Boiler feedwater control

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Thread Starter

madmax

Hi,

I had my viva (oral examination) about Industrial Process Control. I was asked by the examiner (who was bugging the hell out of me) as to why do we need three element control in boiler? I couldn't come up with the answer and instead said that maybe for optimal use of the boiler (which I thought was a crappy answer) and I was blasted by the examiner. Later on it was told to me that it is used for preventing Shrinking and Swelling effect. Does anyone agree with this solution? If it is so, can you please explain it?
 
The boiler drum is operated under high pressure & it contains water steam & steam bubbles - all under high pressure.

In single element control the only process variable used for drum level control is drum level itself. The cotrol system (PI loop)will open the feedwater valve to feed more water into the drum, whenever the level falls and closes the feedwater valve whenever the drum level goes above set point.

Under steady state conditions the controller maintains a steady drum level with feedwater flow (input to drum) and steam flow (drum output)maintaining a steady value.

Now assume a process condition, where the steam flow increases suddenly. This will result in pressure reduction inside the drum. When the pressure comes down the steam bubbles inside water expand and the apparent water level increases inside the drum. This phenomenon is called SWELLING. This condition will misguide a single element control (a control which see only the process varaible - drum level) and it will lead to the closure of feedwater valves (to reduce the drum level) whereas for meeting the actual process condition valve has to open.

No suppose the steam flow goes down abruptly. This will result in drum pressurisation, causing the bubbles to shrink further. The apparent water level comes down (SHRINKING). Under this condition the boiler needs only lesser feedwater flow than earlier as the output steam flow is less. If the control system is in single element mode, ie. it is observing only drum level the feedwater valves will be opened futher to meet the fall in drum level.

So under higher operating conditions drum level control is to be done by considering the following three parameters
1. Drum Level (The only parameter considered in Single element control)
2. Feedwater flow (Input to drum)
3. Steam flow (Drum output).
These are the THREE ELEMENTS used in drum level control.

Feedwater flow and Steam flow are taken into account, for taking care of the abrupt changes in process outputs.

In case of a boiler upto 25% of load (Steam flow) Single element control & Above 25% load three element control is used.
 
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Three position control is advance control required for Boiler feedwater due to the phenomena of "Shrink and Swell".

When steam is generated in the boiler, some of the volume in the boiler tubes is displaced by these steam bubbles. As more steam bubbles are generated, the level indicator does not give correct indication of boiler feed water in the system. As the steam demand increases, this effect becomes more pronounced and the level in the steam drum rises rater than decreases.

In order to overcome this, three position control uses the steam rate and level controller output as setpoint to boiler feedwater controller. With this type of control as the steam demand increases the boiler feed water also increases.
 
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Peter Nachtwey

Boiler water level control is too difficult to explain in detail in one post. In short I agree. Shrinking and swelling is caused by changes in the demand for steam. When the steam demand increases the pressure DROPS which causes the water to boil and in the process the water level INCREASES due to the bubbles. Yes the water level goes up. This would cause a level only controller to reduce the feed water which would be a big mistake. Therefore one must know the feed water flow and the steam flow so one can match the mass of water coming in and going out. The water level control must not let the feed cause the swell too big. There must be a balance between the feed rate and the level. The opposite happens when the steam demand is suddenly reduced. The pressure increases, the boil rate decreases causing the bubbles to collapse and the water level DROPS.
 
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I've presented several articles on this effect in the past three years at Control magazine... http://www.controlglobal.com... they are in the archives and you can search on them. These articles have been written by F. Greg Shinskey, Bela Liptak and David W. Spitzer, all recognized experts in the field.

Here is a list of the articles--you can read them at http://www.controlglobal.com

Taming the Shrink-Swell Dragon 03/23/2004 The shrink-swell phenomenon in the liquid level of boilers and steam generators can dramatically limit control system's ability...

Improved level control of a feed water valve Process control authority Bela Liptak answers a reader's question regarding improved level control for a feed water valve that is too...

Boiling water is easy as one, two, three, four 08/09/2006 There's more to boiler level control than measuring level and adjusting a feedwater valve. Improved measurements and inverse response...

Special rules for tuning level controllers 05/19/2005 Process automation: Loss of level controls can kill a process in a heartbeat, so tuning them properly should be a priority. F.G....

Special rules for tuning level controllers (Part-1) 04/30/2005 Process automation: In Part I of his article on tuning level controllers, process control consultant F.G. Shinskey points out that loss...

Walt Boyes
Editor in Chief
Control magazine www.controlglobal.com
blog:Sound OFF!! http://waltboyes.livejournal.com _________________

Putman Media Inc. 555 W. Pierce Rd. Suite 301 Itasca, IL 60143 630-467-1301 x368 [email protected]
 
It is understood that the Boiler in this case is a steam generator.

It is expected that the Steam drum is with 50 percent water under pressure at operating temperature. As the steam outlet flow is subjected for variations, the drum pressure also effected. If the pressure is decreased due to increased flow of steam outlet, the water will expand while liberating steam. This is swelling condition. In the opposite case, if the steam out let flow is reduced, then the drum pressure will increase. This increased pressure will hold the steam from liberating from the water, alsotend to condense. This is shrinking. The parameters that leads to the vapor pressure variations are important to maintain the drum level. Drum level should be maintained to protect the Boiler tubes from Collapsing and also to protect the equipment using staem as a utility power (turbines etc)
Hope this is making sense!

Later on it was told to me that it is used for preventing Shrinking and Swelling effect. Does anyone agree with this solution? If it is so, can you please explain it?
 
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Hello,
Just a point of clarification - make sure you are not confusing "three-element control" with "three-term control" which can just mean a PID controller (i.e. the three controller parameters). Three element control on a boiler refers to the three separate controllers used to compensate for different effects. This is a good link as an introduction to one-element, two-element and three-element control of a boiler:

http://www.instrumentationguide.com/article/boilerlevelcontrol.htm

Hope this is useful

Andy Clegg
 
K

K Karthikeyan

Check up federal corporation's website (federalcorp.com) hosted by Dave farthing on 1,2&3 element drum lvl ctrl. Easy-to-understand, plain english. good stuff.

- Karthikeyan
 
why compensated steam flow & feed water flow is required for three element control instead of without compensated flow?
 
B
Everybody is saying the same thing: when you have a surge in steam demand, the header pressure will drop making the drum pressure go down: This will cause a swell effect since the steam phase under the drum will expand.

Around the same time, the header pressure will drop and the plant master controller will ask for more BTUs and the boiler will go up in load. Since the steam phase volume is a function of the boiler firing rate, as the firing rate is increased, the boiler drum level increases causing a secondary swell.

So there are two types of swells (same thing for shrink) one caused by the expanstion of the steam phase (drum pressure) and another one caused by steam phase formation.

So guess what the steam feedforward will do when you have a surge in steam demand and the level is swelling like crazy (and the operator is swetting like crazy) : the steam feedforward will ask for more water and will some times trip the drum level.

In order to prevent this, the boiler load rate of change is limitted to a very conservative value.

So what should you do:
1- Filter the steam flow with at least a 45 seconds lag
2- Implement a shrink-swell feedforward so that when the firing rate is increased the feedwater flow goes down as the steam flow goes up.
3- Tune the drum level using tha Lambda tuning techique.

So to conclude, steam feedforward (the thirs element) is detrimental to boiler drum stability during a change in firing rate. Unfortunately, I cannot insert all the examples of improvement I have made on boilers by filtering the steam flow and adding shrink-swell compensation so you will have to either trust me or try it yourself!

Ben Janvier, Senior Process Control Consultant
Enero Solutions
[email protected]
 
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idtmemberdoug

Three element control is used to maintain the boiler drum level at the zero point which is usually 50% full. The level transmitter (dp cell) monitors the level. Another transmitter (dp cell for steam demand out) and another dp cell monitors the feedwater flow into the drum. When there is a call for more steam from the drum the level in the drum appears to rise to the level transmitter (swell) which in most cases it would say to stop adding water, however what needs to happen is for more water to be added. In the same note as more water is added to the drum (which will be full of air bubbles due to boiling) the amount of bubbles will decrease causing the level to fall (shrink) which will cause the transmitter to want to add even more water causing the level to appear even lower to the transmitter. By using a three element control you can monitor the steam demand out and make a compensation to that demand by starting to add more feedwater. The level and feedwater flow are summed and sent as an input to a FIC which also recieves an input from the steam demand flow transmitter (Feedwater flow and drum level are cascaded with the steam demand out).

idtmemberdoug
 
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Benoit Janvier

Steam feedforward is BAD. This control logic is based on the 50's where people though the steam drum level control is a mass balance problem.
It is not.

Matching water with steam will not maintain your level at setpoint. To the contrary, matching the water setpoint to the steam flow might actually trip your steam generator.
Consider this, when you increase your firing rate (increase in steaming rate) and your level goes up and gets closer to the high level trip point why would you ever consider pouring more water in the drum?? This is precisely what steam feedforward will do.

If you want to use steam feedforward, make your life easier - filter the steam fllow with a one minute filter (at least).

Benoit Janvier
Senior Process Control Consultant
[email protected]
 
Wouldn't this deliver the same results as using derivative action in your PID algorithm, with some ratioed lag filtering on the derivative?
 
It's very clear why we are using three element control. But can you please tell me how it is actually/ pratically implemented in plants like saturated steam calulation, etc.?
 
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Brian L. Danielsen

Practicle implementation: You have to build a feedforward/feedback system.

Feedforward. Build an FIC off of the outgoing steam flow. P&T compensate and convert to gpm. The output of the FIC is the SP to the boiler feedwater pumps (BFW) and should be ranged to the same range as the pump PVs. Steam flow is usually expressed in mass flow (MPPH) and pump flow is usually expressed in volumetric flow (GPM).

Feedback. This is your LIC from the steam drum. Its output should be ranged as a trim, where if the level is at the normal water line (NWL), the LIC output is 0. For example, if the BFW are ranged from 0-150 GPM, the LIC output should be ranged as (-150 to +150).

Finally: Add the outputs of the FIC and the LIC together. Don't forget to put a time delay function on the output of the FIC. 45 seconds might be good. This is a tuneable item. When done properly, the feedforward (the FIC) will run the drum control, and the feedback (LIC) will serve as a 'tweaking' type trim.

Final thoughts. Put pressure switchs on your BFW pumps such that they come on automatically. For example, if you have a bank of 4 BFW pumps, one is probably electric and the other three are probably steam driven. This is done for the case where you have a total steam outage. When something happens radically in the plant (ex: the main boiler just died) the system needs a massive amount of water to the remaining boiler in seconds. Many places require the operator to go out to the pumps and turn them on at a local switch. THIS TAKES TOO LONG!!!!! By the time the operator recoginizes the problem, and gets one or more pumps turned on, the steam header has crashed! This often can have terrible production and finacial effects measured in millions of dollars. The 'bringing on' of the next pump needs to be automated, and can be viewed as a split range valve algorithm. When to bring the next pump on can be debated, but one could simplistically say, when the present pump is at, say 95%, bring on the next pump.

In our case of 4 BFW pumps for a makeup boiler sitting at idle, there is one pump running at perhaps 20%, and the other 3 are off. All of a sudden, the main plant boiler dies and the 600# steam header is diving. The makeup boiler jumps to life with the firing rate racing to the maximum. Pump 1 hits the max in 30 seconds and, Bang, pump 2 comes one. 50 seconds later, bang, pump 3 comes on. 50 seconds later, bang, pump 4 comes on but the makeup boiler has caught and overshoot the steam header sp and is settling back down to a lower firing rate. Pump 4 drops off, leaving a steady state pump usage of pumps 1&2 at 100%, and pump 3 throttling based upon the steam users demand.
 
What should be the input to the FIC.. (Whose flow will it compare and control... What would be the SP and PV of the FIC... Is it comparing & controlling Steam Flow or Water flow?)

What would the compensated values of P&T be used??

Where should the added outputs of FIC and LIC be fed?? (Do they operate the Feed Water Flow Valve?)

Regards
 
I could be all wet here. However, I disagree that when load increases that the result of swelling is due to drum depressurisation. What I believe happens is that the surge in load forms more steam bubbles, the pressure stays fairly constant, and it is the formation of these extra bubbles that takes up available volume, hence the rise in drum level. The exact opposite happens with a load decrease. The reduction of steam bubbles decreases drum level because they are taking up less volume which causes a drop in drum level. Again, the pressure stays fairly constant. Good example is on a boiler trip, drum level drops like a rock, even when the feedwater auto opens dramatically.

Again, I could be all wet here.
 
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abhishek Srivastava

Why, in case of a boiler up to 25% of load (Steam flow) Single element control & Above 25% load three element control is used.

Abhishek srivastava
 
Dear Abhishek,
First of all I would like to tell u that, 3 element control is used to counteract the heavy load swings & to stabilize the mass imbalances occur during change in steam demand in full load condition. The mass imbalance causes swell & shrikage in drum level.

To get rid-off swell/shrinkage, 3-element control is neccessary. When load is less than 25%, the effect of load swing is much less as compared to in Full Load condition. So the effect of swell,shrinkage is much less in low loads & therefore single-element(drum level) control is sufficient to maintain the feedwater flow control.

If u need more clarification, dont hesitate to write me.
 
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