PVC fitting failures

[chains0763]

hello everyone..crazy situation I need some help with. Irrigation system, 10 hp pump, 2 1/2 inch galv pipe leaving pump..into a 90, then straight down 3 feet to a tee..thats where the nightmare began for me. Im a Master HVAC mechanic for a school district, with 20 yrs of commercial plumbing experience. So irrigation is pretty new to me coming from a big city to a small suburb. Anyway...at the tee...they go from galv pipe..to sch 40 pvc. all 2 1/2 ...out of the tee...its a pvc male adapter, and thats where the blow outs have been occuring. I noticed the poor alignment and figured it was that..corrected it..next day blow out..same NEW fitting now...then I tried a saddle ..brought the pipe up..90's etc..\pvc expansion coupling, fully extended...next day...blam same fitting gone...but now on both sides..pressure runs at discharge of 80-90 psi. So now I switched to sch 80 fittings...nope same problem ..checked the tee threads..all ok..found info saying always use male pvc adapters into female iron pipe threads..funny..so far me using a female pvc adapter leaving a galv nipple from the tee..has corrected one side of the problem...on the other side..I have yet to try it cause im thinking of going ten feet of galv 2 1/2 inch then to a brass flange then to a pvc flange then back to 2 1/ 2 pvc sch 40 which is what their system is..I inherited this job..and need a few hints..any takers..much obliged for anyones input..all have a safe day

[markts30]

1st - always scew plastic into metal... never metal into plastic...
2nd - cut a pvc sched 80 nipple in half and screw it into the galvy 90. use the plain end that is sticking out to glue your pvc onto... I have never had good luck with MIP adapters in pvc if they are under any type of strain...
3rd - You might want to look into a hammer arrester if the pump goes to irrigation valves (the solenoids shut very quickly)...

[valveman]

It is probably not a fitting problem. I would bet you have a check valve after the problem area. This will cause tremendous water hammer when the pump starts. You must have a check valve on top of the pump but, any other check valve in the system will cause water hammer on pump start. This will cause the pipe to see 10 times the pressure on the gauge. If you are running at 90 PSI, the water hammer is causing a shock of 900 PSI on all the pipe system.

[jimbo]

Female plastic fittings are subject to cracking due to the internal pressure exerted by the tapered metal threads. ( hand tight plus 1/2 turn is the rule on plastic.)

But male plastic threads are also vulnerable to breaking at the thin root of the thread. Is this where yours are snapping off?
The only suggestions I have are to make sure there is no mechanical strain on the joint...i.e. misalignment, etc. Sprinkler pipes which make a 90 need to have the pipe braced to prevent movement caused by thrust or hammer.

[chains0763]

Thx for the advice.. No there arent any check valves after the pump, only a double backflow device coming into it from the township supply which is reuqired by code. Second the failure is occuring at male adapters going into a Galvanized tee. I know without a picture its hard to imagine but..out of the pump discharge side..short piece of rigidlok then flanged then a union, short piece of galvanized nipple then a 90, straight down 3 feet to a galvanized tee. I do understand the plastic into metal theory, not metal into plastic. But, since the failure has occured at two..plastic into metal (pvc male adapters) and Ive tried a metal into plastic (female adapter) the side I tried it on now..goes like this...Galvanized tee, the tee at the 3 foot down drop, So its..Galvie tee, galvie nipple then female adpater( i knowI know) then short pvc pipe stub, pvc expanision repair and then to shut off zone valve(hand valve to field), this side has held up with no problems. The other side which I also did two days prior, went other side of galvie tee, pvc sch 80 (now) frompvc sch 40 male adapter etc etc...thats the same failure point twice now, not to loose not too tight where its threaded... Never has the ovc sch 40 pipe itself failed, just the male adapters (not the female adapter) In fact Ive been told this system has never run well in the past before I even started working here, due to control board issues etc. What worried me was last year it all ran fine..or so I thought after I re wired a new control box into the system. None of the pressure switches worked , the micro switch didnt work etc.. All that has been corrected and is now in proper working order. What Im probably gonna wind up doing is going either copper, or galv pipe for at least 10 feet, which would also allow me to align the piping better, the original alignment was so far off that when I cut out the damaged section, it swung away from me the width of the three foot trench. That was how THEY installed the piping. So...yes I understand plastic into metal, and not metal into plastic, but In my opinion, the pvc male adpaters are exposed, inside the galv tee, which when the pump kicks on, the pressure hits the threads which are no way the same taper as IPS pipe and forces it out of the tee. Using a galv nipple then a female adapter, the nipple directs the water thru the female adapter onto the piping etc without blowing out the fitting. So yes In practice and theory I do agree with all of you on the plastic into metal, but here...seems like the other way around is best. Any advise I certainly welcome it, due to the fact that, I am NOT an irrigation tech, nor will the people I work for hire Irrigation techs, so Im stuck with it. Thanks. M

[chains0763]

Hiya Jimbo..your correct, there was misalignment, which I tried correcting unfortunatly by saddling the piping up , then back down to align, due to the inablility to excavate any more of the buried pipe . They wont allow me to dig under the concrete pad, for good reason, and since we are a small school district I really cannot spend time or money doing so , at their command, if you will. Thats why, I have thought it out, and although Id prefer copper, so I could braze each fitting then flange it to the existing pvc pipe, Im gonna have to go with galvanized piping due to the cost. so at this point it will be..tee, at the bottom of the trench, nipple, coupling...5 foot galv 45, nipple 45, 5 foot galv, union ( for future repairs if needed), 3 foot galv pipe, copper female adapter, stub into a brass flange, then pvc sch 80 flange, short pvc stub then pvc ball valve, etc to field... best I can think of doing..any ideas I missed. Oh yeah, all piping fittings are 2 1/2 inch eithe pvc or galv. All fittings have support at grade, and thrust blocks albeit not poured, where I can.

[chains0763]

No sir there no no check valves after the pump. and Ive timed the solenoids on closing.. I have the system set up that when one zone, is done the other zone has already started to open so I dont short cycle the pump etc. I see what your saying and I wish there were check valves out there..but there arent.


Mark

[valveman]

"which when the pump kicks on, the pressure hits the threads which are no way the same taper as IPS pipe and forces it out of the tee."

When the pump "kicks" on, nothing should "hit" the threads. How is this pump being controlled? What "kicks" the pump on when needed? Even PVC fittings will take a lot of pressure but, you must usually still have water hammer to break them.

[chains0763]

Ok, there are three key components as I see in the system which actuate the pump ither on or off. First when a zone calls there is a delay, which allows the heads to , start to pop up, then some pressure from the staic lines is released causing water to come out of the heads..good so far, then on a pressure fall to 75 psi (low pressure cut in switch), the pump kicks on, and the heads come up to full power usually depending on how many heads per zone..running pressure from 80-92 psi. now, each zone when its timie is up, starts to close, BUT built into the toro tower is an adjustment to pre-bring on the next zone..so while say zone one is finished, zone 2 is already opening , which allows the pump to stay running and not cycle short. now after all 24 zones are thru for the day, the pressure builds to say 104-110 psi, but right after the last zone is done, there is a micro switch, a bullet on a shaft which contacts a ..micro switch, part 1 of shutting down the pump, part two is a high pressure cut out switch..the 104-110 psi switch. if either of those fail to do their job, the pump can stay running ..all night or day. I have corrected any flaws there. So for off cycle, its Micro switch, pressure switch, pump...OFF.

[valveman]

OK a 10 HP pump boosting 40 PSI city pressure to 80 PSI will produce more than 200 GPM. When the pump starts into a line with low pressure, it may starting at 300 GPM. This will "hit" the first 90 and cause tremendous water hammer. This happens on all sizes of pump systems, it is just more obvious with larger pumps. The pump needs to start against high pressure or an almost closed valve. This is why a Cycle Stop Valve is an easy fix for water hammer. It would make the pump start at 5 GPM, then smoothly open up to max flow to supply the zone as needed. You would need to do away with the micro switch and the time delay. Just let the zone come on with the timer, the pressure will lower to 75 and the pump will start against a CSV that is in the 5 GPM position. This completely eliminates water hammer on pump start.

Then with the CSV set at 90 PSI, it will hold 90 PSI constant even on very small zones. On large zones, the CSV will just open up completely below 90 PSI, and if the pressure drops below 90 PSI, it is because that is all the pump can do. When the last zone goes off, the CSV will close down to 5 GPM at 90 PSI. Then the 5 GPM will fill a small pressure tank until the high pressure or 110 PSI is reached, and the pump shuts off.

This makes the pump start and stop at 5 GPM, and you will never hear the water hammer "thump" on pump start or stop, which keeps the lines from breaking. I have set up pump systems this way when they were having more than 100 main line leaks per year, and the number of leaks would drop to only 1 or 2 per year.

If you want to keep it complicated, you can do a fairly good job with a master valve on the pump that opens slowly after the pump starts. Starting a pump into a open line causes a water hammer shock wave that blows fitting off.

[chains0763]

sounds like the riht ticket...any place where I can read up on them, cost availability etc.. it will take some doing but I might be able to get them to go for that...I appreciate the info bigtime.

[valveman]

The CSV has a money back guarantee. Should make it easier to get them to try it. Here is a link to get you started. Just go back to Home and you can find lots of other stuff, including videos.

http://www.cyclestopvalves.com/waterhammer_1.html