Pipe heating question 1

[giuman]

Hello everyone!
This is my first topic! I have a problem regarding heat transfer.
I'm working on a power plant project fueled by biomass. As you probably know, boilers have blowers in order to remove dusts from the exchanging banks, and this is done by spraying superheated steam on the interested surfaces. It takes a while for heating the main pipe (which brings the steam to the blowers) from the initial ambient temperature. During this time steam is not conveyed to the blowers and passes through the main pipe. Indeed, condensation may occur and it is mandatory to not send liquid water to the blowers, due to the detrimental effects that would occur. Only when the pipe is entirely heated to steam temperature it is allowed to send it for cleaning the banks.

I'm required to calculate the time needed for heating the pipe. Could you help me please? How do I do it? Could you please write me the process for having the solution?

I know everything about the pipe (diameters, material, termophysical properties...), as the conditions of the entering overheated steam and the ambient conditions. The time required for heating the pipe should be around 10-20 mins I think.

Thank you in advance to everyone for the help!!!

 

[ione]

giuman,

Try the link below

http://www.spiraxsarco.com/resource...team-consumption-of-pipes-and-air-heaters.asp

and please avoid using bold characters without measure in future posts.

 

[giuman]

Dear ione,
thanks for the useful link!
Do you know how can I estimate the mean rate of condensation (called "ms" in the link) in order to have the time required for heating the pipe?
Can it be, let's say, a percentage of the inlet steam flow?

 

[ione]

I don’t know which temperature and pressure are you working with, but since you’ve mentioned it's superheated steam I warmly suggest you to handle with extreme care. Go to and ask your supervisor or mentor or more skilled and experienced colleagues, familiar with these issues, and check with him/them all procedures relevant to start-up (your company should or better must have them). If you overstressed your pipes with a too fast start-up, this could lead to potential catastrophic failure. So pipes properties aren’t definitely a trivial issue in this case.
Frankly I would never take such sensitive information on a web forum and use it blindly on the field without any double-check: it isn't the right way of doing engineering.

Keeping in mind what above if you want a number and just as a pure rule of thumb (so take it for what it is) you could start making your considerations with a ramp of 50 °C/hr.

 

[racookpe1978]

And continue bleeding steam until you get no more water from ALL of the steam traps at ALL points of the line.

 

[LittleInch]

giuman,

Your post appears to be simply about the mechanical heating properties / time to heat up. This is only one aspect you need to consider and always bear in mind that what you are looking at is a transient event, which is very difficult to condense (no pun intended) into a single equation.

Your biggest issue is almost certainly getting all condensation out of the system and also differential expansion / stress analysis in your pipe network on a transient basis. That is what breaks pipes, not the time taken to heat up the pipe. It is clear from above and also any thing you find written that says the slower you can do this the better, within a reasonable time frame.

As ever we have no real visibility of your system so don't know things like size, flow rate, mass of piping system, frequency of operation, layout of piping etc

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[crshears]

Post about sootblowers...talk about generating [no pun intended!] nostaglic memories...

I haven't worked in a coal-fired steam plant for a few years now, but I do recall that sootblowing was not normally required until the boiler / steam generator had been in service for several hours.

The way our plants were piped, all of the steam supply valves to the sootblower piping were normally open, and if I recall correctly there were orifice plates installed in series with the sootblower system drain valves to allow for removal of condensate without inordinate loss of steam while the system warmed up.

The sootblowers normally used extraction steam from the turbine, thus there was a gradual increase in the temperaure of the steam supplied to the sootblower lines as the turbine came up on load. For the foregoing reasons, when the delta tees showed it was time to undertake sootblowing operations, all the lines would already be fully warmed up, and there would be live steam right up to each individual sootblower's cam valve.

Thermal shock was thus not an issue, or at the very least was precluded by the design of the system; if the boiler is only one of a multiple of them, I'd hazard a guess that as that boiler was brought up to pressure your sootblower lines could be warmed up along with it in the same way using the normal supply line from the superheater outlet, which would have the added benefit of creating a flow of steam through the superheater while steam is being raised, helping to avoid superheater overtemperature issues...however, your facility might be differently built for any number of reasons, so you'll have to do the research to determine what will work best in your application.

Hope this helps.

CR

Carl

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[FredRosse]

There is $$ value in NOT keeping all the piping always warmed up and drained, standby losses can have a real cost factor. Depending on the sootblowing schedule, it may be economic to only warm up the piping (and completely drain condensate) just before use, however, if the operational instruction is not always properly followed, there is a real risk of waterhammer, thermal distortions, piping overstress, etc. The analysis of the transient can be performed with the GOTHIC Code, available from EPRI, but that is really just a small part of the overall problem.