More boiler questions

[totvos]

Hello,

I saw a thread by mrcaptainbob that had a bunch of questions relating to boiler operation. I have a boiler with rads in my 50's home, and also have some questions that I hope the experts here can help with.

Basically, I am puzzled by the pressure swings in my system as the temperature goes up and down. When my water is at around 120F, the pressure in my system is about 8 psi (not enough to get to the second floor). When it hits 170F, the pressure is around 25 psi. I read somewhere that the density of water changes about 4% from 4C to just below boiling, so why the tripling of the pressure?

Why this is a problem is because I, too, have a leaking PRV that starts dripping well below its 30 psi set point. Another irritation/complication I have is that my fill valve also does not completely seat so that it will pressurize my system to well above the desirable 15 psi for my two storeys if left open. So, I would like to pressurize my system with the fill, turn off the fill tap, and leave the system be. But with these big swings, I cannot do that -- indeed, even if my PRV did not drip, it would certainly blow if my starting pressure at 120F were 15 psi.

Can anyone shed light on this for me?

Thanks, in advance,

-- Tom Otvos

[LazyPup]

Check the face of your pressure guage carefully. Does it read PSI/A or PSI/G?
A PSI/A guage is reading pounds per square inch-absolute..meaning that it is reading the actual pressure above a perfect vacuum.

Allowing that Standard Atmospheric pressure is 14.69PSI then a PSI/A guage is reading approximate atmospheric pressure at 15 lbs which would be correct when the system is cold.

A PSI/G guage is calibrated to read zero at standard atmospheric pressure, then measures either PSI above stabdard atmosphere or Inches of Mercury (IN/HG) below standard atmosphere. (29.96in/HG is considered a perfect vacuum.)

Water is at maximum density at 39deg F. at standard atmospheric pressure, then it begins to expand if either heated or cooled from that point. We are all aware that water expands when heated, but it also expands when cooled which explains why a water line will burst when it freezes.

While most people believe that water boils at 212 deg.F. in truth, that is an incomplete statement. Water boils at 212 deg.F at standard atmospheric pressure. At zero PSIA or 29.96 in/Hg the boiling temperature of water is 40 deg.F which is why refrigeration techs pull a deep vacuum on an AC system to dehydrate it before charging the refrigerant.

In a boiler as the pressure increases the boiling temperature of the water also increases, I.E:

Pressure/Temperature boiling points for water:
-14.69 PSIG 0 PSIA 40 deg.F
0 PSIG 14.69 PSIA 212 deg.F Standard Atmospheric Pres.
5 PSIG 19.69 PSIA 227 deg F
10 PSIG 24.69 PSIA 239.4 Deg.F
15 PSIG 29.69 PSIA 249.8 Deg.F
20 PSIG 34.69 PSIA 258.8 Deg.F
30 PSIG 44.69 PSIA 274.0 Deg.F

Most mechanical codes limit non attended heating boilers to a maximum of 15PSIG/30PSIA which explains the 30PSI rating on your PRV.

If you have a feedwater valve that is not seating correctly and allowing the vessel pressure to rise above the 30psia set point, that boiler should be shut down and the feedwater valve either repaired or replaced immediately.

[totvos]

Thanks for the input...

My gauge is PSIG and right now, with the temperature at about 100F, it is reading nearly zero.

If I pressurize my system so that it is about 15 PSIG at 120F, what should the pressure be at 180F?

Also, I don't understand why, when I pressurize my system and then turn the fill tap off (creating a completely closed system, I presume), I am slowly losing pressure. I don't hear any drips anywhere. What could be the source of the pressure bleed?

-- Tom Otvos

[LazyPup]

Based upon those temperatures I presume you have a hydronic system (circulating hot water) rather than a saturated steam system.

In theory when the vessel is first filled the pressure on a PSIG guage should read zero, indicating the system is at standard atmospheric pressure.

Given that a hydronic system is a closed loop and should be full of water at all times, once the system is filled the guage would also read the static head of the vertical column of water to the heighest point in the system. The amount of static head will vary from one structure to another simply because not all structures are the same height.

Somewhere near the highest point in the system there should an air purge valve to permit bleeding off any excess air in the system, thus insuring the loop is full. Many systems also have individual automatic air purge valves on the radiators.

I presume you are attempting to manually pressurize the system by open a feedwater bypass valve and using the domestic water line pressure to pressurize the system. Unless you have the complete specifications for your boiler including a field calibrated static head table for your application that can be a very dangerous practice.

Some of the pressure fluctuations you noted are caused by the thermal expansion of the water as it is heated, but given that you noted a pressure drop, sight unseen i would guess that there is an excessive amount of air in the system as well. As you increase the pressure with the bypass the air in the system compresses to maintain equilibrium, then the automatic air purge vents are relieving some of the air which then results in a decrease in internal pressure.

I cannot emphasize enough that you must have that feedwater valve repaired or replaced as it is a very difficult task to safely fire a boiler manually.

[imeduc]

Sounds to me like you have a water logged expansion tank there. If its like a extrol tank the bladder can be shot. For one floor you should have about 15lb psi. Most home boilers work at 160o to 180o

ED

My mistakes dont define me they inform me.

[totvos]

Lazypup,

Yes, it is a closed-loop hydronic system. Each fin-style radiator has a bleed valve to get the air out. I have tried to do that several times, but maybe my process is missing something?

I have a replacement fill valve on order, so that won't be a problem too much longer.

And yes, when I am bringing the system up to pressure, I had been (carefully!) using the fast-fill on the valve. I have since tried to clean out some of the gunk on the valve, and it is operating better so it is only slowly letting extra water in. As a result, I can leave it be, watch the pressure, and when it reaches 15 psig (cold), turn it completely off by the tap. By the way, the gauge reads in BOTH psig and height, and there is an indicator on it that is set to 35 ft or so, 15 psig. That seems more than adequate for my two-storey home.

And to imeduc's comment, my expansion tank is not a newer diaphragm variety, but an old steel tank (about 12 gal capacity) suspended above the boiler. I had partially drained the tank before, and that did not seem to help. I just now (as of this writing) drained the tank completely. That was a slow process because there is only one valve on it (that is accessible), and for the last two hours I have been listening to "glug, glug".

But it is done now, so I am going to repressurize, bleed the rads, and see what happens. If the system is truly closed, then I would expect the pressure to hold. Then, I will turn on the boiler, and see how the pressure/temperature relationship goes.

Will report back...

-- Tom Otvos

[LazyPup]

If you have an old steel expansion tank, once the system is pressurized spray a light solution of dishwashing soap and water on all seams and fittings in the same manner as checking for gas leaks.

Often when those tanks get old they corrode and are slightly porus around the seams. That may be the source of your pressure loss.

[totvos]

Thank you both for your input on this problem. Here is where I stand:

1. After fully emptying the expansion tank, rebleeding all the rads from top to bottom, and repressurizing the system to 15 psig, the needle barely moved 1 psi from 100F to 180F.

2. Having the expansion tank no longer be waterlogged seems to have improved the fill valve problem (watching it for an hour didn't seem to appreciably change the pressure), but I don't believe it has been fixed. I still have a replacement on order and will install that when it comes.

So, right now, I am very happy...it is working the way it should and I understand why it is working the way it should. I guess with the fill valve always filling, the draining of the expansion tank I did a while back seems to have been undone. Also, I had not fully drained the tank which, on deeper thought, was not a smart thing to do. I can also see now why an empty tank will be half full once 15 psi water is pumped in: Charles Law (or Boyle...one of those guys), halving the volume doubles the pressure and balances the 15 psig or 2 atm.

So, steady pressure...hot rads on the top floor...no PRV drips. Yeeha!

-- Tom Otvos

[totvos]

You know, let me ask one more thing. Are there any tricks to rapidly emptying an expansion tank? In my case, it is hanging above the boiler, and has a single valve on the bottom from which water comes out and air gets in. Once the initial pressure shoots out a bunch of water, the rest just comes out "glug, glug", like if you held a Coke bottle upside down.

I thought of maybe sticking some thin plastic tubing up through the valve, but didn't have any handy and wasn't sure if it would get through the internals of the tap.

I also wish I could tell how full the tank is. There is no visual indicator at all...

-- Tom Otvos

[imeduc]

Thats the only way I know . Tap on the tank is how to tell.it should work just fine for you now. With all the "NEW" water in it now . you should check it out and bleed them all again in about 2 to 3 weeks

ED

My mistakes dont define me they inform me.