Using PVC in a chilled water system
I am in the conceptual stages of a new 800 ton chilled water system. One of the mechanical engineers we are working with has said that usage of PVC (6" in this case) for chilled water is an alternative to iron/black pipe.
Has anyone else seen this? Any comments on this?
Saw it in a local jail here. They went PVC underground only. The only problem they had initially was when they started up the secondary pumps, they thought the two way valves were open and they weren't (OOPS!)
Where they joined the black pipe to the PVC it pulled the all thread out of the ground and the joint separated and blew water all in the pump room!
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I saw it on a small 60 ton chiller that I looked after never had any problems while I was there but there sure were a lot of unusual couplings where the pipe had blown apart previously.
i've seen schedule 80 used on chillers... 6" pipe seems small for 800 tons of water flow.
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I've seen mainly rigid black pipe, but I have also seen PVC used, like Cateyes said scheduled 80 & the proper adhesives will work. I wouldn't try it w/ any other PVC piping.
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You may want to check out other engineered thermoplastics like polypropylene or ABS (not sewer ABS) that are designed for the low temperatures of a chilled water system.
Also make sure you look at your plastic/metal threaded transistions. Many manufacturers have special fittings for this. Spears has a nice system where there is an SS or brass threaded insert pressed into the PVC. I don't think they make it in the sizes you are looking at.
I also think 6" is way too small for 800 tons.
I don't know for sure but I've been told that back in the 1970's, when the chilled water system was first installed at this facility that a PVC system was used. I was also told that there were "issues" with failures resulting in the loss of siginificant amounts (all of it) of glycol in the mechanical rooms above patient rooms in a hospital. The PVC system is no longer in place, if it ever really existed.
I'm sure that technology has changed quite a bit in the last 30 years in regards to adhesives and primers.
I just can't help but think about what it would be like to see several thousand gallons of glycol solution pouring down a stairwell. A bad day for the HVAC tech.
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We have installed lots of it, but only use schedule 80, and only use the heavy grey glue. Clear glue tends to develop leaks over time as it is not thick enough to fill the voids on large pipe well. It will require more hangars though...
We also have done a lot of schedule 80 PVC design build process jobs that have worked well. The engineering drawback to PCV is that the fluid velocity must not exceed 5 F.P.S. or blocked thrust can seperate joint/fittings. The expansion is 10 times iron pipe so there needs to be restained expansion joints in straight runs.
Originally Posted by flange
Like what has been mentioned before is that frequent support, beveling of slip ends [to not scrape off the glue during insertion], correct glue and forced insertion tools on large diameters are essential.
The biggest concern is to have a overtemp limit control on the chillwater pumps to shut them down during a chiller outage. We have had the chiller shut down for a plant holiday and the still running chillwater pump water friction melted/distorted the piping in the pump room. This has happened several times at different plants.
Besides the above, it works well, stays clean and does not sweat near as much as metallic pipe.
It doesn't sweat as quickly as the metal pipe because of the insulation factor of the PVC, but it will sweat and sweat a lot. Our guys thought they could get away for a bit without insulating the pipe, didn't hardly sweat after startup, but they left and it began to sweat over the weekend - ruined a bunch of boxed cased goods from the pipe sweating and dripping on the product.
Originally Posted by JRINJAX
The other thing is that you cannot just put on a strap on temperature sensor on PVC pipe and expect to get an accurate measurement. You need to put in a metal well or put the strap on sensor on metal pipe. Again the insulation factor of the PVC just slows down the sensing ability.
Also we used schedule 80 and the gray glue. I would be real concerned about expansion & contraction of the pipe and make sure the installers account for such as mentioned in earlier posts.
Last edited by crab master; 07-02-2008 at 07:13 PM.
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in a pharmcy plant that I know of has 10" pvc. sch40. a maintenance person tried to throttle a 10" butterlfy valve and it slammed shut, he opened it back up in a hurry and they had some bad water hammer and the pipe broke. they had lost several million dollars in drugs and damage.
That is a real problem when you either exceed the engineered strength of the products or have bad installation practices. You must stay less than 5 FPS under all conditions or else....
Originally Posted by freonrick
PVC for chilled water works great, but as noted earlier strapping and hanging is critical. Another contractor installed a system and it broke open over a weekend and flooded the office, the only good thing is it was the buiulding & land developement offices on the second floor of a 12 story building that were wiped out. Also when we repiped the place, we used sch80 pipe, primer and the gray glue also mentioned earlier. The system also has iron pipe in it and again earlier the adpters from iron to PVC have to be considered. A local bank went from copper to pvc and back to penetrate fire walls using copper fips and pvc mips, which gave them nothing but problems till we changed out to flanged adapters. Also I like to add a high temperature limit/alarm to the system.