Poly vs. PVC

[PurpHaze]

I'm noting that some of you guys/gals use poly pipe for your systems. In our area hard pipe is pretty much used exclusively.

1. What are the advantages/disadvantages of poly?

2. I'm assuming that most (if not all) of it is installed by plowing?
/pulling? (I'll relate a poly story of when the city used it on a project.)

3. What's the largest size poly you use? Is there a diameter that you switch over to PVC or is it "all poly, all the time?"

 

[JeffY]

1: Advantages poly are that it can handle a freeze/thaw cycle better than PVC. The poly will expand or contract with minimal risk to shattering. It's a lot more quicker to install since all you do is pull it, dig down to the poly where you want the heads, snap on a blazing saddle, and swing pipe to the heads and it's done. A real good installer can do about 8-10 heads an hour with poly.

2. Plowing the line in minimize turf damage. Plus the cool factor of, How'd you get that pipe in that small slot?

3. Typically use 1" on all our systems. One system we had this year was off a 2" in main and we were watering the shot put area with rotors and were only allowed to install on the premiter as these were "dusters". So we plowed with 1 1/2" and 1 1/4" poly pipe. Just have to make sure the bullet on the plow is large enough and have the proper size chinese finger. Anything over 1 1/2", we use PVC and trench it in.

 

[DanaMac]

Biggest thing with poly is less chance of freeze damage. I know you are in Cali so you don't deal with it. Poly MY freeze and crack once in a while with water left it it. The pipe will give a little before cracking. And when it does, the split is only 2"-4". PVC will literally shatter. By best story on PVC was a job that had been in for dozens of years and drained fine from auto drains. One year they called me out to fix a leak. The leak was shattered PVC, auto drain failed to open. I had to replace 60' (yes 60 feet) that completely shattered. It was 1 1/2" PVC in a trench with 3 other pipes and it was below all the other pipes. The break went thru mulitple glue joints.

So I prefer poly. Plus it is what is was taught with. Stick with what ya know. But most commercial is projects here are PVC. They will usually get the prooper maintenance and get blown out.

 

[Wet_Boots]

Pipe movement from frost and heave forces can also damage PVC pipe, which gets brittle in the cold, and can split for entire lengths. With poly pulled one pipe at a time, you never have a broken pipe underneath other pipes. Poly pipe resin costs more than PVC, so you pay for the poly advantage.

An obvious disadvantage is its lower resistance to mechanical injury. Tree root fertilizing is good for a few service calls every year.

 

[Mdirrigation]

More friction loss thru poly. Pulling poly in rocky soil tends can cause leaks. The inside diamater of a 1 inch poly fitting is smaller than the inside diamater of 3/4 pvc pipe. Poly has a lower pressure rating than pvc . Clamp connections are more likely to leak than solvent welded pvc. Poly shouldn't be used for main lines . Poly is hard to work with when cold.

 

[Broker]

PVC does not do was well when there are small shifts in the soil during the winter. Like I tell any one that is interested in pvc. Every spring I fix broken pvc lines on a regular basis and only once have I seen freeze damage to poly pipe.

 

[Wet_Boots]

More friction loss thru poly

Poly has the same ID and friction coefficient as sch 40 PVC. Of course, with clamped insert fittings, each one is a very slight loss of pressure, but never a significant amount. The 'utility grade' that is used for most sprinkler work is not intended for mainlines under 24/7 continuous pressure, but stronger grades are seeing use for gas lines and water service connections. As for comparing a clamped poly connection with a glued PVC connection, the poly would be more likely to leak, but for a demanding application like service connections, glued (or soldered copper) connections aren't permitted - the the connection must be a mechanical joint, usually a compression fitting.

 

[PurpHaze]

The city (which single handedly is trying to reforest the native valley oak groves that disappeared decades ago) planted trees along a long street median near our shop about 14 years ago. In order to get irrigation to them they set an IBOC and had poly plowed in the length of the median. We were invited to watch the process as this was just getting to our area. Everything went well and I was impressed with the efficiency of the plowing.

However, leaks started occurring due to gophers and ground squirrels and the city had to constantly go back in and make repairs. I was invited to witness one of the repair jobs because they were having a problem. When the tech cut the poly to take out a 6" section that had been gnawed on the pipe on both ends crept significantly like a rubber band tightening up. They were having problems with this creep actually pulling pipe out of fittings also. Two years into the project they abandoned the poly and trencehd for PVC.

Was the poly stretched too much during the plow-in?

How does the poly you guys use hold up to critters?

Poly IS used quite a bit in Ag applications in this area for low-volume watering and there are programs paying/encouraging farmers to switch to this type of irrigation. However, it's run above ground along the row borders. Less gopher/squirrel damage and easiler to repair.

 

[JeffY]

I've not experienced significating stretching of poly pipe. Occassionally after a long pull involving turns, when I pull the pipe out of the ground at the end of the run the pipe will shrink back a few inches. When I see that happen, I don't usually insert any fittings or heads onto the pipe for at least half an hour to let it shrink back as much as possible to it's origional length. I've run into problems with critters chewing through poly. If it's very common, I would say to set those sonic mole things that can drive the critters away with supersonic sound. I'm not familiar with them and don't know how good of a job it really does, but it may help.

 

[Wet_Boots]

I've seen some damage from poly pipe contraction. It kind of depends on factors like the temperature of the pipe at installation, and how much friction was encountered while plowing it in. Using insert tees, instead of saddle tees, for setting heads, gives you a chance to let the pipe contract. Sometimes, one cut of the pipe lets you add the insert tee without having to cut away any pipe to get the fitting in.

I think critter chews may be more of a dry-climate thing, as far as deliberate pipe attacks go (as opposed to random gnawing) - I heard one Arizona man declare they could 'smell the water' inside a poly pipe, but not in a PVC pipe.