Piping system open to atmosphere

[borobam182]

Hello everyone,

Was hoping someone would be able to help me with an issue I have.

I have a pipework system which is essentially open to atmosphere. Its an anti ice system. Hot air is passed through a manifold and through a series of risers, then passes out into the airstream of a gas turbine air intake system through an arrangement of nozzles.

Point being, as long as you can guarantee no blockages, the pipework downstream of the final isolation valve would never reach the full nominated design pressure.

Is there a separate code or a section with ASME B31.1 or the BPV code that outlines a more relaxed set of rules to be followed for open to atmosphere systems. I've had a look but cant find anything and was hoping someone may have some experience.

Any help would be welcomed.

Thank you

Craig

 

[LittleInch]

Well normally these are just designated with a design pressure of "atmos" or some sort of nominal pressure. However these are normally for short vent or drain lines of say max 5 m long.

It sounds like you might actually have an operating pressure even if its very low.

I don't follow what you mean by " a relaxed set of rules"??

BPV is only for vessels, not pipes

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

 

[mk3223]

...passed through a manifold and through a series of risers, then passes out into the airstream of...

It looks the "open" pipe is not a straight pipe.

as long as you can guarantee no blockages, the pipework downstream ...

Q: "who" is the person willing to bless the system.

 

[georgeverghese]

Also look at off normal operating conditions such as flashback from the turbine combustion chamber (especially during startup) that could result in deflagration pressures of up to 700kpa in the air intake section; vibration shock from such deflagration events.

Why would you introduce a hot air stream into a gas turbine inlet ? It would reduce turbine thermal efficiency, reduce power output if this hot air rate is say more than 2-3% of design case air intake.

 

[KevinNZ]

What is the source of hot air? How high could the pressure go if the piping become blocked (with ice)?

 

[racookpe1978]

What is the closed-off maximum pressure of the hot air supply? If no relief valve on that supply, then you must provide some escape. (West Texas air pipes froze solid several years ago when a freezing air mass hit. Routine mid-summer temperatures were over 100F, but that didn't matter. The power plants couldn't operate because their pipes froze at 18 degrees F.) You have got to plan against worst-case conditions, plus a specified margin for safety. Not just plan against "best-case, it cannot be blocked ever" conditions."

Now, if the supply fan can operate closed outlet for a while, then that too could be considered "a relief" because you're not going to pressure the pipes.

 

[MJCronin]

ASME B31.1 and B31.3 only apply to systems that operate above 15psig.

Review section 100 of ASME B31.1 for a definition of scope based on operating pressure.

MJCronin
Sr. Process Engineer

 

[borobam182]

Hi all,

Thank you for your comments and apologies for my delayed response.

@LittleInch - yes we will have an operating pressure - a specific back pressure generated form the air distribution nozzles. But this will be nowhere near the full design pressure of the system. And by "more relaxed set of rules" - I'd been told to look into the possibility of a different code or a different rule set to follow based on the fact that the system was 'open to atmosphere', a different rule set to ASME B31.1. I cant see that their is one, but thought I'd ask the question.

@georgeveryhese - we introduce hot air into the air intake for 2 main reasons - emissions control and anti ice.

@KevinNZ - The source of hot air is the gas turbine (typically the compressor stage). The maximum pressure it could get to would be 25 barg if there was a full blockage on every single pipe riser / pipe nozzle, or in the main distribution header.

@racookpe1978 - agree fully with your comments. We must design for worse case and blockages are a real possibility. If one nozzle or 1 riser blocks, there would still be a 'route of escape' through the others, but still, it would have an impact local to the blockage. The system upstream of the anti ice system would provide the relief in the event of a blockage.

@MJCronin - thank you for the advice. I will review that.


The feeling is that regardless of it being an open ended system, ASME B31.1 must apply given that its a direct instruction from the customer to follow this spec.

 

[gwalkerb]

>The feeling is that regardless of it being an open ended system, ASME B31.1 must apply given that its a direct instruction from the customer to follow this spec.

This is pretty much it. If the owner wants a piping system designed and built to a particular code, even if you think the code isn't really necessary or applicable, they get to make that call. The only time you should be able to push back to the owner for code of construction is if they're asking for something that is a clear code violation (e.g. a B31.3 piping system handling sour gas at 300 psig designated as Category D Fluid Service), or if the code clearly excludes the particular service and points to a different code (I'm not too familiar with B31.1, but section 100.1.3 doesn't appear to exclude this particular application)

@MJCronin: the B31.3 15 psig minimum only applies to non-toxic, non-flammable, non-damaging to human tissue fluids operating between -20F-366F (basically Category D at 15 psi or less). While air usually doesn't meet that criteria, depending on the temperatures, it might - if it's hot enough to burn you, I would expect that B31.3 still applies, even if only operating at low pressures.

 

[georgeverghese]

So when the turbine is shutdown, and if water settled in these pipes after a rain event followed by subzero conditions, and the turbine is started up, you could get max air compressor side Pd in this blocked in piping manifold. In any case, avoid low points in this piping system to prevent water pooling.