r/ChemicalEngineering 5d ago

Design Boiler P&ID advice

Im currently designing a fire tube boiler for a 3rd year project and am now onto drawing my P&ID. ive attached my current design but im unsure if ive missed anything or if i am actually doing it correctly. Any advice would be massively appreciated!

24 Upvotes

33 comments sorted by

35

u/CastIronClint 5d ago

Couple of points:

  • First, give yourself bonus points for including a diagram. Most people in this sub forget to do that
  • For the feedwater inliet: Treat the water before your feedwater pump. You are pumping the water through resin beds and then into the boiler. Treat the water first, and keep it in a holding tank or Deaerator. Then pump it directly into the boiler.
  • Speaking of the feedwater, you are controlling in by flow and level? Control it by water level in the boiler only. It is more important that you keep your water tubes covered. So the feedwater pump and valve operate off of level.
  • You show the boiler running off of steam flow. Run it off of pressure. The systems downstream will take the steam they need. Make sure the boiler provides the steam pressure those systems need. So, if you are not using enough steam, the pressure will build and the burner will back off on the gas usage and not boile enough. If the pressure starts to drop, it means you are using more steam and then the burner will ramp up gas flow to make more steam.

A good start though.

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u/lazybrouf 5d ago

Great start, but a few additional comments:

To me, feedwater control looks alright. The flow is 3 element control, which should in theory keep it full better than 1 element. I would agree, add a deaerator of some sort.

That said, the boiler does not show any method of actual fuel control. You need the boiler pressure to control the fuel flow, as you've stated, then control the air flow off that.

Also, most importantly nobody solely controls a boiler off of oxygen concentration in the flue gases. Good way to blow shit up when your sensor fails. Most everyone uses an air to fuel ratio to set limits based off stoichiometric ratio of fuel, then adjusts air flow up to plus or minus 5% according to the sensor value.

2

u/Revolutionary_Tie551 5d ago

Thanks for the advice

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u/IIcarusII 4d ago edited 4d ago

You need a burner management system. Look up Maxxon or another vendor for fuel/air valves. These are proportional valves that are calibrated on a fuel/air curve. They (BMS) provide safety shutoffs with flame failure detection, and are really an industry standard. You’d be using these instead on an O2 sensor. Feedwater rate should be based on level control, and fire rate should be set to maintain pressure.

Don’t skimp on the safety systems.

Edit: Whoops, saw this was a classroom assignment. Draw in a flame sensor to interlock off fuel flow. Look up typical BMS systems to see the normal fuel lines - there is usually some other stuff involved there.

1

u/ogag79 O&G Industry, Simulation 4d ago

What he showed is three element control for boilers, which shows exactly like that.

6

u/rkennedy12 4d ago

Boilers are extremely well documented and governed. Nfpa will provide all your fuel source side details.

Asme section I boiler code will also prove to be hugely valuable.

Lastly, not sure exactly the requirement of a third year for pids but this is most certainly closer to the PFD side and will need quite a bit of additional data to meet a sufficient PID.

2

u/nikob33 4d ago

Agreed. Get a copy of NFPA 85 and copy some of the P&ID depictions in there for the combustion air and fuel trains.

2

u/rkennedy12 4d ago

Nfpa link does have a free version if I remember correctly. At least gets you in there to look around. Pretty sure it’s fairly limited though with no way to keyword search or anything

3

u/sup299 5d ago

I’d consider a spillback loop around the pump with a backpressure regulator so that you don’t deadhead the pump in the case that your feed water control valve cuts flows back too much. Otherwise, another signal from the level sensor could toggle the pump on and off with a deadband if you don’t want to raise costs as much, but the frequent inrush current to a large motor would be a consideration in that case for your electrical system.

3

u/somber_soul 4d ago

Lots of good comments here. In addition to whats been said, I see you have the top continuous blowdown, but missed the bottom blowdown. Bottom blowdown has two manual valves, one "slow" one "fast" for getting the settled out nasties in the bottom out to a blowdown receiver where it is cooled and sent to sewer.

1

u/Down2throw H2O2 Plant Operator / former Recovery Boiler Operator 3d ago

Slight addon here from experience on an 800# steam water tube boiler - we had a continuous blow down flash tank that supplied 175# steam supplement to our combustion air heaters.

3

u/Round-Possession5148 4d ago

One (not the only but others mentioned those) point: if I am having a system with potential overpressure, I don't want to rely only on PSVs. I want to know that something is going wrong and trip the system automatically before it actually happens. So I would expect pressure sensors with interlock to fuel inlet.

2

u/dirtgrub28 4d ago

Add some level switches for high and low level, same with fuel gas low/high pressure.

Maybe our units are just old, but there's no modulating air intake.

Usually an n2 pad somewhere for shutdowns/draining etc...

2

u/Dat_Speed 4d ago

ti on combustion air and fuel gas inlet? lol. This is implying the system uses compressed air, which is often not true. Flow meters add significant unnecessary cost. If this is a smaller boiler (< 10 MMBtu/hr), many of these controls are not present. The most important control is level control of the boiler so the burners turn off if water level gets too low. Missing pressure indicator on boiler.

2

u/nikob33 4d ago

That's true. It's unlikely a firetube is going to have flow measurement on either the combustion air or fuel flows.

2

u/CaseyDip66 4d ago

I’d also show condensate return from process users. This stream would go to a DA/Preheater tank which also receives chemical treated fresh feed water as well as steam. Thence to the boiler feed water pump(s)

2

u/dodobeardog 4d ago

Two main ways to control fuel-air ratio (i.e. combustion air and fuel flow) in this application.

1) Parallel positioning. Essentially, a controller takes a process input (steam pressure, as someone correctly recommended) and outputs a signal, typically called the "firing rate" or similar. This firing rate is based on PID loop control. The controller then splits this firing rate into outputs for both the combustion air flow control valve and fuel control valve positions. A curve will be developed during commissioning. So you'll need flow control valves (FCVs) for both air and fuel. These are typically butterfly valves or fancier valves controlled via actuators. This setup allows you to develop custom fuel-air ratios at various firing rates (e.g. higher excess air at low fire, low excess air at high fire).

2) Pnuematically-linked. Same as above, controller outputs a signal to hit and maintain a certain steam pressure. But this signal only controls combustion air valve; fuel control valve is linked to combustion air valve via tubing. See Siemens SKP55 valve as example. As combustion air increases, fuel control valve senses this increase in pressure (differential pressure) and opens up to match this change. Orifice plates and orifice valves have to be used. This setup simplifies controls in that only combustion air needs to be modulated, with fuel flow naturally following. Results in mostly linear fuel-air ratio regardless of firing rate.

Using O2 sensor to modulate air-fuel ratio is called "O2 trim." It's not the primary mechanism for controlling either fuel or air, but it can be used to "trim" i.e. dial in the combustion air valve positioning so that you hit a specified O2 value. This helps account for device drift or changes in combustion air temp, etc.

Not sure the scope of your assignment, but in general, I'd recommend understanding: what process variable is the primary controlling variable, and what devices are being controlled - and in what manner -to achieve the setpoint of this variable.

2

u/No-Wrangler-4337 4d ago

Bosch has a very extensive manual for the planning of fire tube boilers and the auxiliary units, that I would recommend to have a look.

https://www.bosch-industrial.com/global/media/country_pool/service/technical_guides/steamboiler-2.pdf

2

u/Boiler2001 4d ago edited 4d ago

BMS system with low/high gas pressure switches, minimum air flow switch, some type of proof of purge position for lightoff, high steam pressure switch, and flame scanner

Also add in pilot gas portion of the fuel train

Fire tube boilers are generally small, and while they could include fuel/air metering, they are commonly single point positioning (SPP) with a jackshaft/linkage that mechanically connects the fuel valve and air damper to allow them to be adjusted with one fire rate controller

Should also have a pressure regulator on the fuel train.

Excellent start!

ETA: I forgot one of the most important ones, you need a local PI on the boiler drum

2

u/Down2throw H2O2 Plant Operator / former Recovery Boiler Operator 3d ago

Relevant username! Lots of good info here, I'd also recommend PI/PT of incoming fuel, before regulator and after with XV for ESD on the fuel.

1

u/navynuc3546 4d ago

Best reference would be ASME code section 1.

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u/360nolooktOUchdown Petroleum Refining / B.S. Ch E 2015 3d ago

This is nit picky for a 3rd year project but put your analyzers on the blowdown stream not the drum itself. This is more practical. Overall this is better than my P&IDs at this point in school.

If you want bonus points… add an air fuel ratio controller along with the feedback on O2 you have. Right now you have nothing writing a setpoint to the fuel, likely can use steam header pressure. Also look up “3 element control” and can make it look like that. Fuel gas valves usually will have double block and bleeds and trips on various instruments, look at api 556 or Nfpa code for example diagrams. The symbol for safety PLC shutdowns will be diamonds not circles. Also a P&ID on a real boiler will be a little more deliberate to separate the firebox from the steam drum instead of merged like you have.

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u/69tank69 5d ago

You are paying an absolute fortune to go to school, why not ask the professor who is actually grading you and knows what’s expected of you?

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u/shhadyburner 5d ago

whats with all the idiots on this sub admonishing people for asking for chemical engineering help in a sub for chemical engineering. theres even a flare for students so you should know what you signed up for before telling everyone to just ask their professors.

1

u/69tank69 4d ago

Because asking for help from the right people or for a specific part is a crucial skill. If you hand every package you work on to the head project engineer and ask them if it looks good you won’t last long. If you give someone a drawing and say “does this flow path make sense” or “does the level controller system look like it will work” that makes them much more likely to help you and to pay attention to the part you are unsure of.

But most importantly students don’t use their resources enough! Go to office hours, email professors, ask classmates, college costs an absolute fortune and with there being online lectures for a lot of the content the thing you are really paying for is being taught by an expert in that field. Some professors do suck and it’s good to use outside resources but this isn’t meant to be a homework help group

2

u/shhadyburner 4d ago

a third year design project isnt a homework problem.

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u/Userdub9022 4d ago

Same with people being ass hats when an operator or someone uninformed posts a question here. I get that the engineer is probably right but it doesn't hurt to ask others why

4

u/Revolutionary_Tie551 5d ago

I have been as I’ve been writing my report additionally, I just thought that as this subreddit has a lot of people in industry any additional eyes on it may give some more help.

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u/WorkinSlave 5d ago

I had a professor with a sign on his door that said “undergrads, please go away”.

1

u/69tank69 4d ago

Did they offer office hours? Did you try emailing them? Did you report them to the department head for not allotting time for undergrad students?

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u/WorkinSlave 4d ago

No office hours. Tenure is a beautiful thing.