Calculating Volume of a Mechanical System

Hello all! It has been FOREVER since my last post - work has been so incredibly busy that I just do not have time to post "like I used to" (*tho I do see time freeing up in the future!) . . .

A sign of how busy I am my first post is asking for help with work hahahaha

I am faced with the challenge of calculating the volume of a plumbing system with several thousand feet of piping ranging in size from 4" to 1/2", CPVC, PEX, Cast Iron, Ductile Iron, and copper all used.

I am hoping someone might be able to confirm or disprove my theory/plans/assumptions for going forward and determining a precise system volume.

Background: A water-source heat pump loop system with a water-to-air based evaporative cooling tower, and gas-fired hot water heaters are used for the heating and cooling needs of this occupied building.
The original design intent was to run the external towers year round as they serve the purpose of rejecting excess heat in the winter. The excess heat is calculated to accumulate after ambient temperature is above 35F. If the excess heat is not allowed to be radiated via the cooling tower, the system loop temperature goes above 95 degrees and fails over 1000 heat pumps on "high temp" alarms.
On the converse, during a prolonged deep-freeze, there would be no heat rejection, and there is a possibility that the water in the coils outside could freeze, splitting the coils apart and rendering them useless.

So the need has arisen to inject propylene glycol into the system at a volumetric concentration of 30% for freeze protection. Glycol is around $10/gallon - so obviously we need to be precise with our measurement of system volume.

I was thinking back to a post I remembered from here on 3reef and was hoping it may work for this situation.

In case you do not recall it, the post is here: http://www.3reef.com/forums/water-c...our-whole-reef-system-51209-3.html#post919063

If I use a refractometer and a known formula for determining the system's glycol concentration based on S.G., will this work to determine system volume if we inject, say, 100 gallons or so of glycol into the system to see the resultant change in S.G.?

I know this is a lot and very technical but any and all help is much appreciated!

 

I guess my first question in just how big is the sysmtem? Are we taking a few hundred feet of pipe? a few thousand? more? Do you have access to the original drawings? I understand how your thinking above. Just not sure it will work. You would need to assume that you get complete mixing of the glycol and water throughout the system in order for it to work. For that you would need to run the system for quite a while to get it all mixed. Probably be quicker to get the lengths and pipe sizes of each run and figure it out mathamatically IMO. I also have to do this on a regular basis and would be happy to help if we can get some more information.

 

Wow thanks man.

We are definately talking several thousand feet of pipe.

We do have access to as-built drawings but it would be a lot of time. There are four independent systems (each has a Mechanical equipment room and a cooling tower tied into several thousand feet of pipe.)
All four systems are different in layout so they would have to all be configured individually.

Agreed that the biggest problem with that is going to be mixing the glycol in the system. We may have to drain down the system in order to fill again with a pre-made solution of glycol/water (30% glycol/70% water). We have seen situations where the glycol and water never mixed when pure glycol was added to a system and parts of it froze while others did not.

So we certainly can calculate the volume by developed pipe length, but even with scroll-wheel takeoff counters that is a day or two easy of going through plans in addition to the cost of re-printing all the plans as well (even though we print in-house it's still expensive).

If there is a way it can be done without the drawings - I am all for it.

The other reservation I have with the drawings is that even the as-built can be inaccurate. With thousands of feet of pipe, small mistakes get amplified greatly.

Thanks again for the help!

 

I agree with the as builts, they can be terrible lol. I know we always used 100% glycol and mixed ourself if possible because the premix is pretty expensive in comparison. I would think it would be a couple days of mixing the glycol to get a fairly accurate reading. I guess it's just up to you which route you would take. Personally I would opt to take the measuring/calculating route only because then you know you have it and much less chance of freezing/damage. However I do understand the time involved and so I understand the other route may fit your timeline better. It would be easier to help if I was there to get a better idea of the scope lol.

p.s. you can pick up a cheap glycol guage at autoparts stores if you haven't got one already.

 

I've seen people have to calculate entire systems on oil rigs to figure out how much water is needed to be trucked in just to do a hydrolic test. lol. A lot of work just to dump it out.

 

lol talk about an exercise in futility!

Luckily this system will not need to be flushed or dumped for a long time!

 

I wish you luck lol. Sorry I couldn't be of any more use

 

You were helpful tho!!!!