What controls the Steam Pressure inside a boiler?

[rollingstein]

I've seen various types of boilers in industries e.g. 7 bar, 10 bar, 30 bar etc. What exactly in the operations / design of these boilers sets the steam pressure that will be produced?

For simplicity let us restrict ourselves to fire tube coal fired boilers that produce saturated steam. Something like in the sketch below.

https://commons.wikimedia.org/wiki/...media/File:Steam_Boiler_2_English_version.png

Is the pressure set by just the balance between the heat input rate & the steam removal rate? i.e. If you stoke a boiler fast enough you raise the Pressure so long as you have enough area & the shell can take the Pressure? Conversely if you consume steam fast enough every boiler becomes a low pressure boiler?

Is my understanding correct? Or is there another mechanism?

 

[Hesch]

What exactly in the operations / design of these boilers sets the steam pressure that will be produced?

As for saturated steam, it's the temperature of water/steam that matters.

You can look up saturated steam pressure as a function of temperature in a "steam-table".

If you suddenly use a huge amount of steam in a boiler, thereby lowering the pressure, the boiling water will boil explosively, thereby lowering the temperature accordingly.

 

[rollingstein]

You can look up saturated steam pressure as a function of temperature in a "steam-table".

Indeed but that wasn't what I was asking.

Ok, let me rephrase: How is a 30 bar boiler different in design than a 10 bar boiler. Other than thicker walls. Assume both are rated for producing 2 tons / hr of steam.

 

[Nidum]

The 30 bar boiler needs more heat input than the 10 bar boiler . Therefore bigger fire and greater heating surfaces .

Also as pressure gets higher so does the steam temperature so better materials become necessary for construction .

 

[SteamKing]

I've seen various types of boilers in industries e.g. 7 bar, 10 bar, 30 bar etc. What exactly in the operations / design of these boilers sets the steam pressure that will be produced?

For simplicity let us restrict ourselves to fire tube coal fired boilers that produce saturated steam. Something like in the sketch below.

https://commons.wikimedia.org/wiki/...media/File:Steam_Boiler_2_English_version.png

Is the pressure set by just the balance between the heat input rate & the steam removal rate? i.e. If you stoke a boiler fast enough you raise the Pressure so long as you have enough area & the shell can take the Pressure? Conversely if you consume steam fast enough every boiler becomes a low pressure boiler?

Is my understanding correct? Or is there another mechanism?

For old boilers, it's a guy reading a pressure gauge attached to the boiler. When the needle on the gauge shows the pressure is too high, then the guy (who is usually called the 'engineer'), cuts down on the rate of firing of the boiler, let's off steam (get it?), or both.

Newer boilers are fitted with automatic controls, where a set pressure is entered into the system, and it does the monitoring of the boiler's operation to make sure this set pressure is maintained.

http://www.nationalboard.org/Index.aspx?pageID=134

Now, they got codes and stuff to make sure you have some way to control the boiler so it doesn't, you know, go BOOM!

In any event, the boiler should be equipped with a safety valve which is designed to open if the boiler approaches its max. allowable working pressure. Ironically, the safety valve should never open, or a lot of people get into trouble, automatic controls or no.

 

[rollingstein]

Ironically, the safety valve should never open, or a lot of people get into trouble, automatic controls or no.

Trouble with their bosses or trouble with the law?

 

[SteamKing]

Trouble with their bosses or trouble with the law?

Depending on the type of boiler and the situation, possibly both.

Things like ASME codes tend to get written into or referenced by local laws and operating regulations. You don't keep your equipment operating according to code, you often get a citation and a fine (or worse) if an inspector catches you, or unfortunately, an accident of some sort. Things like exploding boilers (admittedly an extremely unlikely occurrence) tend to bring unwanted publicity, even today.

 

[rollingstein]

Things like exploding boilers (admittedly an extremely unlikely occurrence) tend to bring unwanted publicity, even today.

It's a mystery to me how they explode even today. With the number of redundant protections I see even on a small sized boiler it ought to be really hard to make one explode even if you wanted to.

 

[SteamKing]

It's a mystery to me how they explode even today. With the number of redundant protections I see even on a small sized boiler it ought to be really hard to make one explode even if you wanted to.

Tying down the safety valve will do the trick. Fortunately, such casualties are rare.

Which is why boilers and other pressure vessels are stamped with things like ASME markings during manufacture and inspected while in service.