Boiler MAWP derating

DanCampbell

Ok steam guys... We have two Bryan hot water boilers (flexible tube) that were originally low pressure steam and were converted. The original data plates had the MAWP for hot water at 50 psi. When it actually got converted (mid 70's I'm told) they got derated to 30 psi. What is the explanation for this? I can't run enough pressure to get water to the 4th floor and attic (distribution piping) at far end of building without constant air problems. If pump(s) shut off, the head alone blows the relief valves. Boilers are below grade. I'm trying to determine if we can run more pressure, which would mean new relief valves. The existing valves are all sized and installed properly. It's strictly a head problem. Thanks everyone in advance.

nicholas bonham-carter

Is the relief valve popping only when the pump is off?
What size is the expansion tank on this system? Is it pumping away from the tank? Are there automatic air vents on the radiators? 30 psi should be enough pressure for 4 floors. Has the system been doing this since first installed, 40 years ago?--NBC

DanCampbell

The reliefs pop when pump(s) go off, and start weeping if we approach 27 psi. Relief valves are all new. We have two old non-bladder expansion tanks. There are auto bleeders on system. We are actually dealing with about 6 floors total because boilers are below grade and distribution piping is in attic (5th floor). Pumping toward tank, and we have a b&g tangential air separator between boilers and pumps piped to the expansion tank. It's been problematic for years. The contractor they had just kept adding stuff to correct issues. I inherited this system and would like it to work better. Open to any and all suggestions. Thanks Nick.

icesailor

I'm not the official expert on such things, but if the boiler says 50# working pressure, and it didn't start out as a high pressure steam boiler (over 15# steam), it was probably designed as a hot water boiler. When it was converted to HW or whatever it was, using a 30# relief valve won't give you enough elevation (69.3') for the water. 50# would give you 111.5' of elevation from the relief valve to the top of the system.

Many boilers are rated at 50# but come through with 30# relief valves.

Its my understanding that if a boiler is fired and pressurized at over 15# steam pressure, it becomes a High Pressure Boiler and has different rules about who can operate it.

Can you post rating plate photos?

I re-read your post. If it was a 15# steam boiler, and the MAWP is 50#, change the relief valves to 50# valves and raise the pressure.

Just because someone did the conversion wrong, doesn't mean that YOU can't make it right today.

If it was steam back when, and there was some problem, it was probably solvable simply. The converter probably cranked the pressure to 10# and all hell broke loose. So he recommended converting it to FHW. And still didn't understand what he was doing.

JStar

We do this all the time. Like icesailor said, upgrade the relief valve(s) and raise the cold-fill static pressure. Either that, or install a water-to-water heat exchanger for $$$.

neilc

OP also said he is pumping towards the tank, with constant air issues, and has auto vents on rads and an old style tanks system.

Tanks may be water logged, and he should pump away from the tank, and loose the auto venting.

DanCampbell

Thanks for input guys. I will post pics of current rating plates. We are trying to find the original plates. Current plates are Bryan factory plates. Trying to get more background. I am being told that the boilers got derated because they were "low pressure steam" before. Apparently they ran at <15#, around 12 I'm told. Trying to understand why they would derate if original plates said 50#. Don't want to replace the pop-offs if it's not going to be safe/non-code for whatever strange reason. Tanks aren't waterlogged. The air separator is piped directly to the exp. tank which I find odd. How is air going to go into tank if they are both at same pressure?

icesailor

Look on the side of a quality ball valve, It will say 150# Steam. 300# WOG (Water, Oil or Gas). That means you aren't supposed to apply more than 150# of steam or 300# of water, oil of gas pressure to the valve. No de-rating.

You can have a boiler rated at 50# MAWP and install a 30# PRV. You just can't do it the other way around. Put a 50# PRV on a boiler that is rated at 30# MAWP.

HB Smith residential boilers are rated at 30#. H B Smith commercial boilers are rated at 15# steam or 80# water.

DanCampbell

The current tags are Bryan factory data plates. I just emailed them (Bryan). I think we should be able to run over 30#, but I'm not an engineer. I want to make sure I'm not missing something before I switch the reliefs. Supposedly the old data plates were 15#/50#. We just found the original install certificate, says 15# steam, doesn't have the water press. on it.