Viscoelastic Behaviour of PE Pipe - In a Sprinkler System

[hgordon]

Hi all,

I am hoping someone has experience with underground polyethylene pipe.
We have a sprinkler system which is dropping pressure consistently, probably from a leak, but I thought maybe the PE pipe could be creeping causing the pressure to drop.
The sprinkler pipework consists of U/G PE pipe and above ground steel pipe with sprinklers. The system is kept at 13 bar but after 8 hours the pressure drops to 11 bar (when it should remain constant).
An automatic jockey pump was installed to help mitigate this issue - it starts at 10 bar to raise the pressure back to 13 bar.

I am wondering if the PE pipe could be creeping causing the pipe to expand and the pressure to drop - this was happening in the commissioning pressure test where the pipe was charged to 16 bar then it dropped to 10 bar over 24 hours. But does the PE pipe keep this creep behavior over time?
Or does the PE pipe undergo stress relaxation and stay permanently expanded over time and creep stops happening?

Thanks,

 

[LittleInch]

What is the pipe rating? SDR? PE grade?

What is your ground temperature?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

PE has something like 10 x the thermal expansion of steel. Turn the jocky pump off. Close inlet and outlet valves to/from the irrigation pipe. Does the pressure drop from 13 to 8 between morning to late afternoon and does it increase overnight from 8 to 13B the next mornings? If the pe is not buried deep, you may be experiencing nearly unrestrained conditions with significant thermal longitudinal movement and diameter change. What temperature change do you have during the same 8 hours? If you have to turn the pump on to return to the initial 13B pressure, its probably a leak, Depending on how much if any pressure is recovered when not using the pump, it could be both. A pipe that is not leaking should recover nearly all pressure. Any air in the line may also show up as pressure loss and gains with temperature change, but without a real leak, it should return to similar pressures when temperatures are also similarly equal.

It may also be a backflow through the pump when it is off. You could try installing a check valve at the pump discharge to prevent that.

It will expand with pressure both in the short term and eventually become permanent, but this sounds like temperature sensitivity to me. Temperature expansion will always occur until it becomes longitudinally restrained under the soil, but even then, diameter will still continue to change with temperatures.

 

[hgordon]

It will be PN16 pipe, not sure on grade and SDR but it will be standard.

The ground temperature I am not sure - 5-10 deg C. The above ground ambient is typically 15-20 deg C.

 

[1503-44]

What diameter and how long a total pipe length are we talking about here? LittleInch will want to know that too

 

[hgordon]

It is about 80 m of 125OD HDPE PN16 pipe.

Probably there is another 5x65mm tee-offs to 5 classrooms at the school before it turns back to steel above ground but yet to confirm- totaling <20m length.

 

[Compositepro]

Are we discussing agricultural sprinklers or fire sprinklers?

 

[hgordon]

I didn't see your response -thirtynine.
The sprinklers are for fire.
I tend to think it is a leak but was interested in the viscoelastic and temperature effects of the PE pipe.

The installing plumber talked about needing to backfill the black PE before pressure testing - otherwise the sun causes too much heating for accurate pressure testing.
I assume once the pipe is backfilled the temperatures are not much under the soil - although some of the pipework is under a large concrete pad.
We have not done overnight testing with the pump off as we need to keep the system live for fire.
Dec 2019 (summer here) the leaking was worse where the pressure was dropping quicker, so maybe the temperature is having an effect.
What we normally see is thermal expansion of the water within steel pipe within the roof spaces below a sun heated 'tin' roof - and pressure can increase dramatically, which must cause leaks or trapped air to escape, as the pressures can drop at night.
However this system does not show these symptoms - more of a immediate but gradual drop - like a weeping leak.

If PE pipe looses its creep behavior over time and with constant pressure, at least I can rule this out as the system has been installed for 2 years.

There is no leaks around check valves or drains.

 

[1503-44]

Yeah. We seem to be in different seasonal cycles as well as different time zones.

Steel EXP = 0.000012 mm/mm-C°
HDPE EXP = 0.000120 mm/mm-C° exactly 10x steel
E= 150000 psi

Very Roughly I get around 5 bar change in pressure for a 5°C temp change from diameter expansion alone. If the pipe is not totaly restrained and also expanding a little lengthwise in the ground, that will be more. Say 7 Bar change. I think you are talking thermal expansion. Plus you cant find any leaks, so ...

 

[LittleInch]

It sounds like you don't actually know what this pipe is. "It will be" is different to "It is"....

You need to find out and be sure.

It seems like a very high pressure for a sprinkler system and if the hydrotestate was 16 bar it implies 10 bar rated pipe.


Can you plot pressure loss vs time? A straight line is a leak, a curve is possibly creep.

Are thet any pressu6e relief valves? They often weep at high pressures then seal as the pressure falls.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

As long as you're going to plot P vs time, write down the temperature too.

Leak rates have been known to slow as pressure decreases; making a curved plot as well.

 

[hgordon]

The pipe is PN16 as stated on the plans.
The pressure decay over time for the commissioning test was due to creep.
The recent test we did does follow a similar curve, 1 bar drop in 1 hour then 1 bar drop after another 7 hours.
No pressure relief.
I will find some literature on PE pipe to see how it behaves under pressure.

Thanks,

 

[1503-44]

Engineering properties of PE discusses deformation and creep

https://www.awwa.org/portals/0/files/publications/documents/samples/30055_excerpt.pdf

 

[curtis2004]

hgordon (Chemical)(OP)28 Sep 20 22:40
The pipe is PN16 as stated on the plans.
The pressure decay over time for the commissioning test was due to creep.
The recent test we did does follow a similar curve, 1 bar drop in 1 hour then 1 bar drop after another 7 hours.
No pressure relief.
I will find some literature on PE pipe to see how it behaves under pressure.

Thanks,

Hi HGordon,

Have you measured water amount required to restore pressure?
If I remember correctly, there is water allowance amount can be added during hydrostatic test per NFPA 24. It depends on internal volume of plastic pipe.

Curtis

 

[cvg]

water allowance is for leakage, not creep

 

[hgordon]

NFPA is probably not very detailed for PE commissioning pressure tests. There are standards for PE installation.
We as a fire company pressure test sprinkler pipework all the time, but we leave underground stuff, especially PE visco-elastic pipe to the civil contractor as the pressure testing is very different - mainly because of creep and stress relaxation.
The PE pipe will creep when pressurized and the pressure will drop again, but if the pressure is continually re-charged and held constant the pipe stretch will reach an equilibrium with the pipe strength and the soil around it.
So I do not think creep will cause a pressure drop - like a leak would.