The 2010 edition of NFPA 13 has section 7.1.2.1, which states 'Relief valves will be required on all wet pipe systems that do not have auxilary air reservoirs. Previously, they were only required on gridded systems. The size of the relief valve was increased to 1/2" and the relief valve now needs to be listed'.
I thought I heard that the reason for the relief valve on all wet systems was because there was something in there that was going to require that the air be expelled from the system when filling. And that therefore due to the lack of air in the system to act as a cushion for the expansion and contracton with temperture flucuation, that the relief valve was now required.
However, I can't see anything in '10 requiring any provisions for removing air from the system. 8.17.4.2.4 allows the inspectors test anywhere after the WFS, so they're not trying to enforce it that way. Anyone see anything more in this regards?

I was unable to open the file FPP1 had linked. Can someone please let me know if they are able to open so I know whether there's a setting that needs fixing on my end or if there was a problem with the posting of the file. Thanks.
I had come across some calculations on this in the past and the verdict derived that while the volume of air increases and decreases vastly greater than water over the range of temperatures that a wet system could be subject to, the fact that the volume of water increases with temperature more than steel, and the fact that water unlike air is uncompressible, that the increases in water volume, though slight in comparison to air, would be more prone to mechanical failure of the piping if there was no air in the system to act as a cushion in this situation.
If I find it I will pass on the info, though the paper only represented one man's option (though seemed very comprehensive), and as the with a lot of scientific theories there would often be others than can present a different view, present different calculations and show the opposite:). With the intent of having a meaningful discussion in this regards, I want to mention some documentation that would seem to indicate that the concern with pressure is more in the absence of air than excess air.
(a) While not in NFPA, Section 15.6 of EN12845 (Fire Protection Association, LPC Rules for Automatic Sprinkler Installation, European Standard), it says 'Pipework which is completely full of water, may be damaged by the increase in pressure due to temperature rises. If complete venting of air in an installation is likely to occur, e.g. in the case of a gridded layout with flushing connections at the extremities, consideration shall be given to the fitting of pressure relief valve'.
(b) In looking at the ROP A2009 (see link below), Section 13-75 log 406, and 13-76 Log 435 (in regards to the Pressure Relief valve section 7.1.2.1 of NFPA 13), also seems to the panel's concern to require relief valve was due to 'growing popularity of venting trapped air from metallic wet pipe systems to inhibit corrosion activity increases the importance of protecting all wet pipe systems from over-pressurization'.
(c) In looking at ROP A2009 Section 13-78 Log 407, the submitter is proposing a means to remove air from wet systems. The panel seemed to agree with the submitter's 'desire' to remove air from the system, but rejected it, suggesting that the submitter resubmit addressing 5 concerns the panel had on the verbiage. However, the desire to remove the air from the system is again stated as to limit corrosion rather than to address the potential for excess pressure build up due to excess air.
(d)The exception to providing a relief valve (Wet and Antifreeze), is to provide an air reservoir. So it would seem the committee feels that air can act as a cushion. It does not mandate (yet, seems pending the revision on ROP 2009 Section 13-78 Log 407 next cycle), the removal of air from the system.
I do not doubt that excess air in the system can drive system pressure to the levels you speak of. I can easily enough visualize the air pockets contracting in the colder temperature, allowing more water past the check valve, and then the system pressure increasing significantly the next day when the air pockets increase in volume when it warms up the next day. I can also see a system with excess air causing false flow alarms. And I can see air in the system increasing corrosion conditions.  So there are lots of good reasons to vent the system, and installing a relief valve is not that big a deal. However it seems at the moment there is no requirement to remove the air from the system.
I know there has been considerable discussion within the industry and on this forum in regards to the location of ITC. In the 2007 edition of the NFPA 13 Handbook there seemed to be a contradiction in that one section is telling you 'ideally this is located at the highest most remote branch line', and another section discouraging it because it allows fresh oxygen into the system whenever an inspectors test was used.
I thought that they were going to be requiring a means to vent the system of air while allowing the ITC to be located anywhere downstream of the WFS. Which would seem to keep everyone happy. I guess if the Section 13-78 Log 407 of  ROP A2009 gets revised with the recommendations of the panel in next cycle will allow everyone to sleep save.

We identified the problem and used a scissor lift to loosen 16 individual sprinklers on the high side of the system branchlines until we could hear the air hissing. The air bled from the system piping for >90 minutes before all 16 sprinklers were dripping (i.e. it required that long to remove the large volume of air from the system). The branchlines for this system are elevated using riser nipples ~24 in. above the near main and ~24 in. above the far main. The low point drain & ITC could not possibly remove the trapped air for this system. When this system is drained and filled, the air removal must be completed manually due to the design of the system.

This is a VERY common problem during Spring & Fall (sometimes summer as well) in Georgia. The weather temperatures during this test in April varied between 38-42F at night to 78-85F during the afternoon. The pressure increases & decreases followed the temperature fluctuations exactly as one would expect; gradual increase and gradual decrease as the temperature changed. You cannot see the pressure gauge change, but if you document the pressure and walk away for 30 minutes, the pressure will be different when you return.

Most systems create a small drip or leak well before the system pressure reaches 500 psi. I have one customer who had 6 sprinklers POP on three different systems over a two month period before they finally let me bleed the air from 5 of the 19 wet sprinkler systems (only one of the 5 systems affected by the "trapped air problem" used a gridded design!), but all had elevation changes which allowed significant trapped air pockets.

I have many more convincing examples, but I am not going to spend the time trying to describe them on this forum. Have an open mind, remember in general what I am saying, look for the problem in the field, verify for yourself whether or not I am right or wrong AND hopefully one day enough people will realize the truth that the NFPA committee will take the appropriate action to resolve the problem by changing the applicable NFPA requirements!

I will go to my grave knowing trapped air creates this problem (as well as SEVERAL other SERIOUS problems) in sprinkler systems..........I have resolved this issue way too many times simply by removing the trapped air.

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