Regulate Incoming Steam Pressure 2

[JessCalizar]

We have steam from coal-fired boiler and steam from waste heat boiler of sulfuric acid plant, all connected on a common header. The problem is, the pressure of steam coming from the coal-fired boiler is fluctuating and is almost always < 42ksc, while that coming from the sulfuric acid waste heat boiler has a pressure of 42 ksc. how can we regulate the pressure of steam coming from coal-fired boiler given the ff conditions:

-boiler feed cannot be incresed
-pressure regulation is to be done on the piping system only

This is my proposal, attach a pressure regulator valve at the steam line of coal-fired boiler that if pressure is < 42, then steam will be redirected to the existing desuperheater and be converted to low pressure steam. This is just an idea, I don't have details yet as to what % oopening of the valve should be normally operated, etc.

Any help is welcomed..

 

[TD2K]

Numerous users connecting into a common header isn't uncommon. The swing producers will be set up to adjust their firing to maintain steam header pressure. The pressure at the various users will vary somewhat based on the piping and system hydraulics. If the pressure swings above header pressure, excess steam is frequently let down to a lower pressure steam header to avoid lifting your safety valves on the steam producers.

I'm not really clear what your problem is. It sounds like you think diverting part of the boiler's steam production to the LP steam header will raise the supply pressure?

 

[JessCalizar]

the problem is the steam pressure coming from the coal-fired boiler is often lower than the steam header pressure. How can i ensure that the pressure at the header is maintained?

 

[TD2K]

I'm still not following you. If the steam from the coil fired boiler is going to the steam header today, how can it be a lower pressure? I'd start looking at my pressure transmitters, looking at how my steam distribution system and the system pressures and see if you have an issue.

 

[JessCalizar]

there are two producers attached on the steam header, one from the coal-fired boiler, the other from the waste heat boiler of sulfuric acid plant. there is no problem with the steam coming from the sulfuric plant, as it can maintain a pressure of 42ksc, the problem is with the coal-fired boiler, as pressure of produced steam fluctuates from 38 - 42 ksc. This header ultimately leads to a turbine-generator which requires HP steam at 42ksc. what alterations should be done on the header piping system so that steam with a pressure of 42 ksc will only be allowed to go to the turbine-generator?

 

[TD2K]

Thanks, I think I understand the system better. The pressures are for the steam drums? Are you saying at some times the steam pressure in the coil fired boiler and the waste heat boiler are both 42 ksc (more or less) and at other times the pressure in the coal fired boiler drops to 38 ksc while the steam pressure in the waste heat boiler is still at 42 ksc?

 

[JessCalizar]

yes precisely sir.

what should be done?

 

[TD2K]

Okay, you have a waste steam boiler that produces steam at a constant 42 ksc. There's a steam flow down a header to a point where it picks up steam from your coal fired boiler. The pressure in this boiler varies from 32 to 42 ksc. The combined steam flow then goes to your turbo generator.

I'd start by trying to understand the system's hydraulics to identify why there is a 4 bar pressure drop between the sulfur plant's waste heat boiler and your turbo generator under some scenarios. It may be that you are just trying to move too much steam between those two locations for the size of the line you have (I had asked if there was a letdown valve at the waste heat boiler which you agreed with, if steam is going through the letdown valve it's not flowing down the main steam header). If the pressure drop is due to the amount of steam you are trying to move through the existing pipe, you would need to install a larger or a parallel steam header to reduce the piping pressure drop over the section or sections in question.

Try to get some data to see what the pressure profile is and does that match what you would expect based on your calculations.

 

[GHartmann]

I would also look at the coal boilers firing / pressure controls to see if they are interfering with the presssure from the boiler.

In theory if the coal fired boiler steam drum is connected to the 42 kg/cm2 header, then it always has to operate at slightly above 42 kg/cm2 pressure. Unless of course there is a back-pressure controller in the system.

I would look more at why the coal boiler cannot produce steam at the required pressure than adding another layer of control.

Secondly, I would look into if it's absolutely necessary for the turbine generator set to require exactly 42 kg/cm2 steam and not 38 kg/cm2 steam (that is a reasonable operating range).

I would assume that from the combined 42 kg/cm2 header steam is letdown as needed to the other plant users and that the excess 42 kg/cm2 steam is fed to the turbine generator set.

To me it doesn't make sense to have the coal boiler let down into the low pressure header system, unless the coal boiler rate matches the low pressure requirements. It might be you want to fire the coal boiler to maximum rate to generate as much electricity as possible.

GOOD LUCK!

 

[JessCalizar]

Thank you sirs, i will definitely need to conduct a thorough study on this problem. will do what you had suggested,

thank you very much.

 

[TD2K]

Good luck. Let us know what you find.

 

[GHartmann]

Yes, please let us know what you find and the solution you come up with.

Good luck.

 

[rmw]

Can you share any kind of crude diagram of your plant layout? Which boiler is nearer or farther from the turbine?

I'm with Mr. Hartman. There is something wrong with the firing rate controller or the combustion control system that allows the pressure to fluctuate.

Does the turbine load fluctuate? Maybe the controls are set up so that the waste heat boiler is allowed to produce uncontrolled and the coal fired boiler is taking the swings that varying turbine loads create, if its load is variable. You may need to add some feed forward controllers to anticipate load swings and give a rate kick to the coal fired boiler.

Sounds like a situation where the tail is wagging the dog.

rmw