PE Rising Main Expansion

[swazimatt]

I was recently supervising a rising main that has straight lengths of about 200m either side of 90 degree segmented bends. this is a PE100 pipe Od of 280mm and PN16 for a new residential development and the rising main will ultimately pump at about 60 l/s


The designing engineer had a scour vlave-t connected to the downstream side (not the pumpstation side) of the bend where a stub flange has been welded to the pipe. the pipe is butt welded with the exception of one EF-joint on the upstream side of the bend.
The engineer's design had a 0.5x0.8m thrust block on the bend and also a 1mx1mx1m reinforced concrete anchor block on the DS side of the scour-T. I thought both of these were a bit of an overkill on butt-welded pipe but partly agreed with having some sort of anchor at the scour as thrust induced movement could cause an issue with the branch of the scour valve (80mm diameter SS pipe) being a weak point. Because of space constraints I was hoping to snooker the design engineer into allowing the contractor to cast the anchor block and the thrust block on the bend as a single item. I felt that if the bend was anchored and was close to teh scour-T there would be almost no movement of the T. The further away from the anchor the more elastic movement in the pipe being my reasoning.

Anyway the designer came back and said the reason he wanted the anchor block on the downstream side was for thermal expansion/contraction caused by the wastewater (see quote below):
"Thanks for your email. I did consider that however we need the anchor block on the southern side of the Tee to counter longitudinal forces as result of thermal expansion/contraction of the PE pipe. The warmer wastewater flow as you know can create quite a temperature difference on the PE pipe.

If the line was trenched there would be bit more slack for ‘snaking’ of the pipe however given it’s drilled that is also reduced. Hence protection of the flanged connection is required. Also, if the DI flange was ever to be replaced, we wouldn’t need to worry about the replacement not fitting as the anchor block should hold the PE pipe in place"

I see this more as bit of BS with some science sprinkled in, but thought i would ask the experts. I find it hard to believe that there will be a huge difference in wastewater temperature. It will sit for maximum of a few hours at the really early stages of the development but when fully occupied only a few minutes sitting in the wetwell, but all of this is a few meters below ground (the scour valve is almost 5m deep, average depth is probably about 3m. the pump station is about 6m deep. The pipe was drilled in, but has had a few weeks to settle so doubt there will be any issues there (snaking is normally to allow for contraction after the pipe has been sitting in the sun above the trench)
Tenperature is mild - NZ north island so only a few days of mild frost per year

What are your thoughts? I know PE pipe has a relatively high coefficient of expansion but i have no idea how "hot" wastewater is, fresh or even a day old (in which case the council normally doses it against septicity)

 

[bimr]

Not sure of the manufacturer of the PE pipe for the project in question, but have seen a different manufacturer recommend an anchor on the end of the PE pipe. While it is true that if the PE pipe has fluid flowing, the temperature would be relatively constant. However, the pipe may be installed at a different temperature depending on the climate and may be out of service at other times.

Wastewater is not "hot", the wastewater temperature is probably the same as the soil that the pipeline is buried in.

You don't mention what your role is in the project. The engineer of record is generally responsible for the design of the project. Unless there is some kind of major mistake that you are bringing to the attention of the design engineer, one wouldn't think there is any reason to request a change since the design engineer is responsible.

Are you willing to be responsible for making a change?

 

[swazimatt]

My role is supervising the contractor and signing off the installation. So i can see when something is impractical and in my opinion without real basis. I understand that they take the design responsibility which is why i asked if it could be changed. I have no intention of changing their decision and have decided that it is not worth continuing the argument.
The main reason for requesting the change is that it will be adding time (and cost) to the project. As i said the pipe is 5m deep at this point and the contractor can only source a single trench shield for that depth, meaning they will now have to do this operation in at least 2 stages to satisfy the designers requirement for the anchor block downstream of the T

The pipe was installed weeks ago so any stretch caused by the drilling operation will have settled. It will have also adjusted to the ground temperature by now. It would be safe to say it is stable

But i want to satisfy myself that it is not correct, or is correct.

 

[bimr]

Refer to Chapter 6, page 245:

Plastics Pipe Institute® Handbook of Polyethylene Pipe

 

[coloeng]

Was the contractor aware of the requirement to place the concrete anchor block where the design engineer wants it before he bid the project? If so, he should have budgeted the time and money for it.

 

[swazimatt]

Was the contractor aware of the requirement to place the concrete anchor block where the design engineer wants it before he bid the project? If so, he should have budgeted the time and money for it.

I do not think this is relevant. As technical specialists we have a duty to our client to ensure they get what is required. if it was the other way round and i saw that the design was under designed i would also question it. At this stage i have left it because i can see the designer is not interested in adressing it and if i carry on arguing it my fees will end up costing the client more than the cost of the additional work and if anything the end product will be Possibly) overdesigned so not a risk

Refer to Chapter 6, page 245:

thanks. i have only skimmed through this, but it appears to confirm what i thought