# how to determine volume of hidden vessel

• posted

I have an unusual problem staring at me. I'm doing maintenance of an ice sk ating rink, and we have decided to replace the R22 with RS-45 as part of a general upgrade. The system was full of leaks and was constantly losing sig nificant and costly amounts of R22.

This is a liquid overfeed system and as such, the refrigerant runs through the pipes under the ice surface. These pipes are buried in concrete and nob ody knows how large they are. The people who installed this are long gone, and there is zero access to the piping after the large feed and return lin es enter the concrete.

I need to determine how much refrigerant we need to buy. I have heard estim ates ranging from 4000 pounds to 7000 pounds from various experts in the fi eld. That extra 3000 pounds is almost \$30,000, so it would be good to get a better feel for a real number. The refrigerant pipes in a typical rink are about 10 or 11 miles long (seriously), and are (sometimes) 5/8 OD thinwall steel, but that's a variable.

The R22 has been removed from the system - unfortunately, the quantity reco vered really has no bearing on the actual capacity as there had been consid erable leakage.

So, given that I have a system that's shut down and pumped out, and that I can isolate the rink floor pipes from the rest of the system and there are service valves accessible, does anyone know how I might determine the volum e of these pipes?

I was thinking, perhaps, of filling with nitrogen at some regulated flow ra te and watching for a pressure rise and then doing some magic calculations I haven't thought about since high school. Does that make sense? can anyone offer specifics?

I'd appreciate any help any of you can offer.

Thanks.

jpb

• posted

I'd skip the flow measurement and connect a tank(s) of known pressure and volume to the piping and measure the temp & pressure after the system comes to equilibrium.

IIRC, the largest argon tanks hold 330CF of gas at STP. If I did the math right, assuming approx 10 miles of 5/8 tubing, 330 CF of STP gas would settle at a couple or three atmospheres.

• posted

skating rink, and we have decided to replace the R22 with RS-45 as part of a general upgrade. The system was full of leaks and was constantly losing s ignificant and costly amounts of R22.

h the pipes under the ice surface. These pipes are buried in concrete and n obody knows how large they are. The people who installed this are long gon e, and there is zero access to the piping after the large feed and return l ines enter the concrete.

imates ranging from 4000 pounds to 7000 pounds from various experts in the field. That extra 3000 pounds is almost \$30,000, so it would be good to get a better feel for a real number. The refrigerant pipes in a typical rink a re about 10 or 11 miles long (seriously), and are (sometimes) 5/8 OD thinwa ll steel, but that's a variable.

covered really has no bearing on the actual capacity as there had been cons iderable leakage.

I can isolate the rink floor pipes from the rest of the system and there ar e service valves accessible, does anyone know how I might determine the vol ume of these pipes?

rate and watching for a pressure rise and then doing some magic calculation s I haven't thought about since high school. Does that make sense? can anyo ne offer specifics?

Because the system is leaking, you'd first have to determine the leakage ra te, so you can correct for it when you fill it with a measured amount of ga s (is there some reason you can't just use air?). If the rate is low, the f ollowing plan ought to give you an approximate idea of the system's volume.

It suggests a leakdown test that you'd do first: seal off the exhaust end a nd Fill the pipes with air, ignoring the quantity. Pump it to, say, 1.2 atm ospheres of pressure, as a guess. Monitor the leakdown rate by sampling the pressure drop. If the rate is low, you can then pump it up again with a me asure quantity of air, to the same pressure above atmospheric, and use that volume to calculate the volume at one atmosphere with Boyle's Law.

I can't guess what the operating pressure is, but your sample for the leakd own test should be somewhere in the same range, I would guess. Or, if the s ystem runs right around atmospheric, cross your fingers that it will hold u p with a little overpressure.

Good luck! It's an interesting challenge. If the leakdown rate is too high, events will occur too quickly to get an accurate measure this way. I'm not going to do the math to distinguish "high" from "low." That's why you're g etting paid. d8-)

• posted

Nitrogen is cheap and cheerful. CO2 might be another option, not sure if it plays havoc with refrigerants (but you'd be pumping it back out anyway - if you can handle the pressure sit's a nice cheap refrigerant itself, IIRC.)

If you have the underfloor pipes evacuated/isolated and they are not where the leaks are (which would seem like "throw in the towel" time to me, or at least a much more expensive repair) you should be able to carefully weigh a cylinder, connect it to the system, fill to the pressure of your choice (using more cylinders if needed, just be sure to weigh them) and then weigh the depleted cylinders - initial weight minus final weight is the mass of gas you moved (far more precise than a CFM meter, AFAIK) and if you know the temperature of the floor, the pressure, and the mass of gas you should be able to get the volume (metric units on the weight will make that easier.) Obviously (I hope) you can't have leaks in your fill rig and have this work, nor will it work with significant leaks in the pipes.

PV=NRT is the formula that springs to mind from a long time ago, where N is the number of moles of gas (mass will get you to that, temperture is in C, R is the gas constant, and Pressure and Volume.

V=(NRT)/P would be the form you want. Or, y'know, cheat:

Nitrogen's molar mass is 28.014 g/mol

If this part of the system is truly leak free you might get better numbers by waiting a day for the temperature to stabilize, since expanding from the compressed tank will of course cool it somewhat, though the mass of the floor will mostly shrug that off. And you obviously want the most accurate/precise scale you can still weigh a nitrogen tank on.

So long as pressures are not too high, the deviation between the "ideal gas law" and real gasses is quite small - should certainly be enough to get you a good solid estimate.

• posted

e skating rink, and we have decided to replace the R22 with RS-45 as part o f a general upgrade. The system was full of leaks and was constantly losing significant and costly amounts of R22.

ugh the pipes under the ice surface. These pipes are buried in concrete and nobody knows how large they are. The people who installed this are long g one, and there is zero access to the piping after the large feed and return lines enter the concrete.

stimates ranging from 4000 pounds to 7000 pounds from various experts in th e field. That extra 3000 pounds is almost \$30,000, so it would be good to g et a better feel for a real number. The refrigerant pipes in a typical rink are about 10 or 11 miles long (seriously), and are (sometimes) 5/8 OD thin wall steel, but that's a variable.

recovered really has no bearing on the actual capacity as there had been co nsiderable leakage.

t I can isolate the rink floor pipes from the rest of the system and there are service valves accessible, does anyone know how I might determine the v olume of these pipes?

w rate and watching for a pressure rise and then doing some magic calculati ons I haven't thought about since high school. Does that make sense? can an yone offer specifics?

rate, so you can correct for it when you fill it with a measured amount of gas (is there some reason you can't just use air?). If the rate is low, the following plan ought to give you an approximate idea of the system's volum e.

and Fill the pipes with air, ignoring the quantity. Pump it to, say, 1.2 a tmospheres of pressure, as a guess. Monitor the leakdown rate by sampling t he pressure drop. If the rate is low, you can then pump it up again with a measure quantity of air, to the same pressure above atmospheric, and use th at volume to calculate the volume at one atmosphere with Boyle's Law.

kdown test should be somewhere in the same range, I would guess. Or, if the system runs right around atmospheric, cross your fingers that it will hold up with a little overpressure.

h, events will occur too quickly to get an accurate measure this way. I'm n ot going to do the math to distinguish "high" from "low." That's why you're getting paid. d8-)

As Ecnerwal said, a significant leak in the under-ice piping would be prett y much game over. This rink DID have significant leaks there, but 20-someth ing years ago, they installed new pipes and a new slab on top of the old sl ab. I don't see any of the tell-tale signs of an under-ice leak - there's u sually significant discoloration (of the ice) when that happens.

I'm not sure exactly what pressure these pipes run, but for sure, the lowes t they can be is at the suction pressure of the compressor, which usually r uns between 25 & 40psi, so 2 or 3 atm shouldn't be a problem.

As a bonus, once we've got it pressurized, we can watch for pressure drop o ver time to determine whether, in fact, the floor is leak-free.

I surely am hoping that we are on the low side of this range - this stuff i s expensive.

• posted

On 05/06/2017 1:16 PM, rangerssuck wrote: ...

Well, how much _did_ you recover, you know? If _that_ number is big... :)

My only experience w/ rink refrigeration systems were all ammonia-based...I suppose that is telling on their age! :)

• posted

I also don't know what the ramifications of filling the system with CO2 wou ld be. I was planning Nitrogen or Argon - Nitrogen is almost certainly chea per, but Argon would be good to have for other (tig) reasons. [while we're on that subject, here's a guy who is making his own liquid nitrogen.

]

Excellent plan. There BETTER not be significant leaks in the rink piping. I don't believe that there are - the pipes were replaced and a new slab was poured 20-something years ago. There were initial problems with this rink d esign - the R22 would find its way out of tiny leaks and react with the wat er in the concrete, eating the carbon steel pipes to death. - newer rinks u se stainless pipes, or some other material that's run in continuous 200 foo t runs.

I do have a good scale - will weigh down to grams in that range. The temper ature measurement is part of the new control system I'm installing. There's already a sensor in the slab. I'm also installing new pressure transducers , so the bases really are pretty well covered.

I could actually hook up a data logger and watch that the temperature and p ressure keep making sense over time. I'd expect that there will be some cha nge as it settles, and then it would hold steady (assuming no leaks). If th e pressure (vs temperature) slopes down over time, then we've got a problem .

Thanks loads for this. It's been a great help.

Three of my favorite vices. We just yesterday rescued two kittens (maybe 5 weeks old) from a mini flash flood in my back yard.

• posted

skating rink, and we have decided to replace the R22 with RS-45 as part of a general upgrade. The system was full of leaks and was constantly losing significant and costly amounts of R22.

gh the pipes under the ice surface. These pipes are buried in concrete and nobody knows how large they are. The people who installed this are long go ne, and there is zero access to the piping after the large feed and return lines enter the concrete.

timates ranging from 4000 pounds to 7000 pounds from various experts in the field. That extra 3000 pounds is almost \$30,000, so it would be good to ge t a better feel for a real number. The refrigerant pipes in a typical rink are about 10 or 11 miles long (seriously), and are (sometimes) 5/8 OD thinw all steel, but that's a variable.

ecovered really has no bearing on the actual capacity as there had been con siderable leakage.

I can isolate the rink floor pipes from the rest of the system and there a re service valves accessible, does anyone know how I might determine the vo lume of these pipes?

rate and watching for a pressure rise and then doing some magic calculatio ns I haven't thought about since high school. Does that make sense? can any one offer specifics?

Thanks - That's pretty much the same as what ecnerwal recommended, without the attached math. I'll probably just do what he said (which again, is pret ty much what you said).

• posted

ould be. I was planning Nitrogen or Argon - Nitrogen is almost certainly ch eaper, but Argon would be good to have for other (tig) reasons. [while we'r e on that subject, here's a guy who is making his own liquid nitrogen. http ://

]

If you've drained the system, and if you know it has leaks, what gas do you figure is in there now?

If it's drained and if any remaining drips of refrigerant have had time to evaporate in a leaking system, the pressure in there went to atmospheric pr etty quickly. Then I would think that you're beginning to get air into that system, from that moment on.

• posted

How long does it take to pump that thing down? What's the temp of the ice in an ice skating rink?

• posted

We didn't get nearly enough - not enough to re-start the system. As I said, there have been non-insignificant leaks.

This rink was built in 1980. I am familiar with much newer facilities than this that run ammonia systems, though not under the ice. They have heat exchangers and run chilled glycol or brine under the ice.

• posted

would be. I was planning Nitrogen or Argon - Nitrogen is almost certainly cheaper, but Argon would be good to have for other (tig) reasons. [while we 're on that subject, here's a guy who is making his own liquid nitrogen. ht tp://

]

ou figure is in there now?

o evaporate in a leaking system, the pressure in there went to atmospheric pretty quickly. Then I would think that you're beginning to get air into th at system, from that moment on.

Actually, we had it pumped down to -10 psi for three or so hours. That was over a week ago. I went in today and opened the system and unscrewed on of the valves (which is going to be replaced) and just got a little puff of po sitively pressurized gas. NOW it's atmospheric ;)

The vast majority of the leakage is (I'm pretty sure) from the compressor s haft seals. Also, every one of the 50+ valves has leaky packing. There are at least two welded pipe connections that leak. I have a busy summer.

• posted

e skating rink, and we have decided to replace the R22 with RS-45 as part o f a general upgrade. The system was full of leaks and was constantly losing significant and costly amounts of R22.

ugh the pipes under the ice surface. These pipes are buried in concrete and nobody knows how large they are. The people who installed this are long g one, and there is zero access to the piping after the large feed and return lines enter the concrete.

stimates ranging from 4000 pounds to 7000 pounds from various experts in th e field. That extra 3000 pounds is almost \$30,000, so it would be good to g et a better feel for a real number. The refrigerant pipes in a typical rink are about 10 or 11 miles long (seriously), and are (sometimes) 5/8 OD thin wall steel, but that's a variable.

recovered really has no bearing on the actual capacity as there had been co nsiderable leakage.

t I can isolate the rink floor pipes from the rest of the system and there are service valves accessible, does anyone know how I might determine the v olume of these pipes?

We generally run the slab at 18-20F. For the purposes of painting (when the ice first goes down), we get it closer to 15F, so that the paint will free ze in place. If I can find the budget money, I plan to install IR sensors s o we can regulate the temperature of the surface of the ice rather than the slab. Knowing the slab temperature is better than not knowing, but there's close to a hundred thousand pounds of water between the slab and the surfa ce. (10,200 gallons of water per inch of ice in an NHL-sized rink). I can g et MUCH better control of the surface temperature (which is all anyone care s about) if I can actually measure it.

Pumping it down is no small thing. In a normal season (when we're not repla cing the refrigerant), the 75HP compressor (we only use one of the two for this) get the freon out of the floor and into the tank in an hour or less.

This year, we contracted with Rapid Recovery, who also purchased the R22 fr om us. They have truck-mounted pumps (each about 15HP). They used two pumps and it took a whole day, but there were issues with freezing pipes. After they *thought* they had it all, there was vacuum in the system. The next mo rning, there was 10psi of pressure, so they came back and pumped (one truck , this time) again, for another 8 hours and got another hundred pounds of R

22 that must have been hiding behind some ice in the system.

While this has been interesting and (a little) fun, I wouldn't want to do i t again.

• posted

On 05/06/2017 1:50 PM, rangerssuck wrote: ...

Well, I wasn't thinking that; just perhaps sanity-check on volume numbers...

Indeed, yes, these are brine-circulation system--they date back to '60s/70s...

That'd be a lot more economical on refrigerant if so...

• posted

skating rink, and we have decided to replace the R22 with RS-45 as part of a general upgrade. The system was full of leaks and was constantly losing significant and costly amounts of R22.

gh the pipes under the ice surface. These pipes are buried in concrete and nobody knows how large they are. The people who installed this are long go ne, and there is zero access to the piping after the large feed and return lines enter the concrete.

timates ranging from 4000 pounds to 7000 pounds from various experts in the field. That extra 3000 pounds is almost \$30,000, so it would be good to ge t a better feel for a real number. The refrigerant pipes in a typical rink are about 10 or 11 miles long (seriously), and are (sometimes) 5/8 OD thinw all steel, but that's a variable.

ecovered really has no bearing on the actual capacity as there had been con siderable leakage.

I can isolate the rink floor pipes from the rest of the system and there a re service valves accessible, does anyone know how I might determine the vo lume of these pipes?

rate and watching for a pressure rise and then doing some magic calculatio ns I haven't thought about since high school. Does that make sense? can any one offer specifics?

That was my suggestion. Ideal gas law. P*V=N*k*T. I think you could just use compressed air. As long as you know the tank volume on your compressor and have a good pressure gauge. (T is in Kelvin)

George H.

• posted

And if you have the time, you can test for total volume and leaks, too -- fill it, check the pressure, wait a day for temperature to stabilize, then check the pressure once a day for as long as it's reasonable to leave it pressurized -- if the pressure keeps going down and there's no cold snap to account for it, then you've got a leak somewhere.

• posted

Pumped out as in evacuated? Get an industrial-size gas meter in working condition. Connect to the evacuated system with a throttleing valve. Record the gas meter's dial reading. Open the throttling valve slowly until the moter starts turning. As the pressure in the pipes comes up, open the valve more and more to keep the meter chugging along at a safe rate. When the meter stops moving, you have the volume in cubic feet, roughly STP.

If you fill the system with a known pressure of gas, then vent out through the meter, you will expand the gas in the system until it reaches atmospheric pressure. If you filled it to 15 PSIG, then assuming an ideal gas, venting to atmospheric pressure should release as much gas as the system holds at atmospheric pressure, so again the reading = volume. If you pressurize it to only 1 PSIG, then the meter reading would be 1/15th of the actual volume. (All figures for sea level, not Denver.)

Jon

• posted

On 5/6/2017 11:27 AM, Ecnerwal wrote: ...

"K", not "C"

• posted

On 5/6/2017 2:34 PM, rangerssuck wrote: ...

"That range" being 120 lb for a 220 cf tank/bottle/cylinder?

• posted

On 05/06/2017 1:16 PM, rangerssuck wrote: ...

I'm sure they could probably imagine. If I were you, I'd track down the three or four previous contractors/maintenance/service people who left there and get whatever else bad news they found out about the needed repairs.

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