Boiler Pressure 7

[TendaiM]

Good Day

The problem is I have a pressurized boiler and the feed water is pumped into the boiler using a pump which gets its input from a frequency inverter. The frequency inverter gets its input from a controller which monitors the water level in the boiler. The concern however is that the pump is only able to overcome the boiler pressure, hence pump water, into the boiler after a certain frequency, which thus is giving the problem because pump in excessive amounts than is required.

The controller being used is a PI controller. We cannot replace the pump and we need to develop a revised water regulation algorithm. I am working on a model to calculate the expected amount of water to get into the boiler by using a look up table of speed at which pump can pump water versus the boiler pressure. Since I know pump capacity I can then calculate the amount of water which will be pumped in at a frequency at which the pump can overcome the boiler pressure.

Also I can find from a level transmitter the amount of water which has to go into the boiler, hence when water is eventually pumped, I just return the error amount back to the reservoir.

However I feel that they might be a better and cheaper way of control. Maybe introduce a derivative term.

Can you please help me out on this one.

 

[JGard1985]

Hi

Control loops and systems are commonly available. It might be easier to just buy a pre-engineered package such as

http://www.micmod.com/boiler-control

Jeff
Pipe Stress Analysis Engineer

http://www.xceed-eng.com

 

[davefitz]

a variable speed pump is not the correct equipment selection if the boiler pressure is roughly constant over the load reange. Fans and pumps obey the "fan rules" whereby flow is proportional to rpm, pressure is proportional to rpm^2. If you need full max rpm to achieve the required pressure, then you are forced to accept full flow as well.

Thye ideal application for a variable speed pump or fan is a case where the required discharge pressure ncreases by the square of the flow. The worst case is a constant ressure over the load range. In that worst case , you need a control valve at the pump outlet.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[LittleInch]

Can you describe the operation a bit more?

You say the controller "monitors" the water instead of controlling to a level of between levels?

What does the pump curves look like with respect to the required pressure?
Does the boiler pressure change a lot?
what is you current variable range (e.g. 55 to 65 htz?) and consequent flow range compared to the volume of the boiler water?

Can you introduce a fixed speed pump up stream your variable speed pump to do more of the "heavy lifting" and let the VFD be more flexible?

I don't understand why you're not simply controlling on level or a low to high level?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hacksaw]

Tendai.

You are dealing with a boiler safety issue, not just closed loop control. erratic feedwater supply is hazardous to say the least. Get a mechanical engineer involved capable of signing off on the design, legally. You don't want a boiler explosion under any circumstances.

On the controls side, you need to introduce a pressure drop (either fixed orifice or a fail open valve) between the pump discharge and the boiler, and backup feedwater supply pump. You may even require a minimum flow recycle to avoid overheating the pump.

You also require proper pressure and level monitoring and safety relief valving per you Boiler Codes.

 

[KoachCSR]

I agree with hacksaw, any situation which may threaten feedwater supply to the boiler (either over- or under-supplying) is asking for trouble.

What is the operating pressure of your boiler? I note that you mention you're controlling on level only, thus I assume you're utilizing a single element control loop. At higher operating pressures, many boilers are typically controlled utilizing a three-element control loop, which mitigates swings in feedwater demand due to shrink and swell in the steam drum. Three-element control loops I'm familiar with typically utilize a PID controller.

Do you want to develop a new algorithm to regulate water flow because a physical modification (such as a recirculation line or new control valve) has been ruled out?

 

[TendaiM]

Thanks guy for the help. It goes well appreciated.

@ JGard1985: We cannot buy a new Control System, we are in a competetive business so a new control system would be much more expensive for us. But thanks for the suggestion we will keep it in mind
@hacksaw Noted about the safety issues. We have MEchanical Engineers onsite and all our work is signed off by these professional guys. Thanks once again
@Koach we are operating at 11bar.Yes we are using single element control. Physical methods are our last option since our budget is tight , but what where you suggesting on physical methods

 

[racookpe1978]

A boiler explosion - when you knew there was a design cause and a design solution (a solution rejected for being "too expensive") - will be very much more expensive than a proper control system. And will likely put you, and your company, out of work not just for a few weeks, but forever.

 

[KoachCSR]

Thanks, Tendai.

Your initial problem, as I understand it, is that your current pump must operate at a minimum frequency to overcome boiler pressure and supply feedwater to the steam drum. This minimum frequency, at lower demands, causes you to over-supply feedwater to the boiler - which I presume you are compensating for in some way (perhaps utilizing bottom blowdown?).

Are you steadily operating at 11 bar or does your pressure fluctuate regularly over a load range you operate in? If you're steadily operating at 11 bar, I'll have to agree with davefitz and hacksaw in recommending you look at a discharge control valve. If your pressure is fluctuating regularly, I think we'll need some more info which LittleInch described in his post above.

 

[mk3223]

To be helpful for brainstorming the options, is it possible to share a sketch of the existing system, i.e. equipment, piping, and PI control, etc.?

 

[TendaiM]

We are operating in a range of 7-11 bar

 

[LittleInch]

Ok, but what's your volume issue?

" which thus is giving the problem because pump in excessive amounts than is required"

How much does the pump overfill?

How long does it take (is the pump really far too big?)
Is this an operating system or planned system?

Is there some sort of time lag between the level transmitter and turning th e pump off.

I'm having difficulty imagining how the pump keeps going once you've achieved your required level??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TendaiM]

Thanks littleinch We are able to control the water level within a range of 3mm from the set point. However the way we are controlling is that the pump turns on when water level is below setpoint and then we pump water into the boiler till we reach the setpoint. The pump then turns off afterwards and we wait for the water level to go down again. We are looking for a way to keep the pump running not to do this on and off behaviour.

 

[LittleInch]

3mm?

Are you serious? That looks a very tight spec to me. What does 3mm represent as the depth of water in the boiler? Surely there are some fluctuations in a liquid in a boiler.

I think the only way you're going to do this is to make the VFD more sensitive by adding a fixed speed pump U/S or adding a control valve and just running the pump at a high enough speed that it can pump at the highest pressure and then control the level with the Control valve. Or accept what you have - it certainly doesn't sound like an excessive amount to me.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[KoachCSR]

Based on what you're describing, I don't see a way around a physical modification to fix your problem. If the pump must provide excess flow to exceed the pressure of the boiler, no amount of tuning or tweaking of the controller will prevent the on-off operation of the pump.

The correct fix, in my mind, is the addition of a control valve on the discharge line of the pump. Given your operating pressure varies from 7 to 11 bar, an orifice plate sized to provide some back pressure for your low flow condition may provide too much at the high end, depending on your pump's performance. A properly sized control valve with adequate turn-down will allow your pump to run continuously. This valve could be controlled based on a characteristic of the pump discharge flow - I'm guessing you already have a flow transmitter in this line.

It's a shame the initial design of the system was not done correctly - the whole point of the VFD is to not add more energy into the system than required. The necessity now for the control valve essentially eliminates one of the key advantages of the VFD in the first place. That said, constantly cycling your pump will soon be a constant maintenance headache, if it isn't already.

Edit: I agree with LI, 3mm is a tight spec for the set point. I'm guessing you're overfilling to a high setpoint (at which point the controller cuts off the pump) then repeating constantly...if your high setpoint is tight as well, I suppose your pump is cycling frequently?

 

[MFJewell]

3mm?

Are you serious? That looks a very tight spec to me

My thoughts exactly. On our boilers we usually deviate +- 1 inch from NWL during operation. Your boiler may be much larger or smaller than mine, so check the manufacturers specs for acceptable range.

 

[hacksaw]

evidently the posting is being continued in the Control System category, rather impolite to say the least

 

[LittleInch]

And the pump forum....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MJCronin]

Does this boiler have two, independent Low Water Cutouts (LWC) as is commonly requited by state law ?

Many local boiler codes require redundant INDEPENDENT fuel cutouts that is NOT wired through the DCS...

What happens when the FW pump stops ?????

MJCronin
Sr. Process Engineer

 

[LittleInch]

What sort of time step are we talking about from turning pump in to off?

Why does the pump not just slow down and turn off?

How many times an hour does the start stop happen?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.