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Who can make thick wall HDPE manhole?
- Thread starter boscolee
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This manhole will be buried 60 feet below grade at the bottom of the landfill. Considering the allowable deflection of the structure under loading from landfill compactor, we determine the structure should have a minimum wall thickness of 6 inch. Concrete is out of question because this landfill has very high sulphate content and this structure is expected to last 300 years.
boscolee,
Are you tied to using HDPE? You may want to consider an alternate engineering reinforced thermoplastic and core out the form using strengthening ribs. On cooling a 6 inch section will contract, warp and sink significantly and I would suggest really shouldn't be considered. An FEA of a final revised cored out CAD model will help determine whether the resulting design will withstand the loading.
Regards
Kieran
Are you tied to using HDPE? You may want to consider an alternate engineering reinforced thermoplastic and core out the form using strengthening ribs. On cooling a 6 inch section will contract, warp and sink significantly and I would suggest really shouldn't be considered. An FEA of a final revised cored out CAD model will help determine whether the resulting design will withstand the loading.
Regards
Kieran
patprimmer
New member
I agree with Kieran.
I would also add that there will be considerable creep under load and temperature after 300 years. 300 years is a very long time over which to predict the performance of materials only invented in the middle of the last century, but then again, we won't be around to face the music.
You will never extrude a suitable round section, so you will have to injection mould segments that fit, clip or bolt together. You will need to mould in segmants so that the ribs can be ejected from the mould.
You will also need to be aware of the long term chemical resistance under stress of the material used and stress applied. Once again 300 years is a long time, and reactions that are hardly noticable after 30 years, might progress considerably after 300 years. Most chemical resistance test data is over a year or less.
My first thought is glass filled PET, but it can be difficult to mould with reliable physicals, and it is prone to alkaline hydrolysis.
You might also consider GF nylon, but it is still a bit prone to hydrolysis, and quite prone to attack from acids. It also absorbs water until an equilibrium is reached. I would presume that at 60' down, you might expect to be below the water table in some instances, therefore you would need to consider the effect on it's properties of water absorbtion at equilibrium with 100% relative humidity.
The most critical properties of GF nylon that might be considerably reduced by water absorbtion are it's flexural modulus and creep resistance.
I wonder if thermosets might be considered?
If you insist on HDPE with a 6" wall, the only way that it might even be considered is if it was foamed. You could injection mould cylindrical sections with a thick foamed wall, but cycle times might run into many 10s of minutes, and the cold flow or creep problem will be even more critical.
Regards
pat
I would also add that there will be considerable creep under load and temperature after 300 years. 300 years is a very long time over which to predict the performance of materials only invented in the middle of the last century, but then again, we won't be around to face the music.
You will never extrude a suitable round section, so you will have to injection mould segments that fit, clip or bolt together. You will need to mould in segmants so that the ribs can be ejected from the mould.
You will also need to be aware of the long term chemical resistance under stress of the material used and stress applied. Once again 300 years is a long time, and reactions that are hardly noticable after 30 years, might progress considerably after 300 years. Most chemical resistance test data is over a year or less.
My first thought is glass filled PET, but it can be difficult to mould with reliable physicals, and it is prone to alkaline hydrolysis.
You might also consider GF nylon, but it is still a bit prone to hydrolysis, and quite prone to attack from acids. It also absorbs water until an equilibrium is reached. I would presume that at 60' down, you might expect to be below the water table in some instances, therefore you would need to consider the effect on it's properties of water absorbtion at equilibrium with 100% relative humidity.
The most critical properties of GF nylon that might be considerably reduced by water absorbtion are it's flexural modulus and creep resistance.
I wonder if thermosets might be considered?
If you insist on HDPE with a 6" wall, the only way that it might even be considered is if it was foamed. You could injection mould cylindrical sections with a thick foamed wall, but cycle times might run into many 10s of minutes, and the cold flow or creep problem will be even more critical.
Regards
pat
TheAnswerGuy
Materials
boscolee,
It seems as though you have done your homework and researched a material that will work for your application.
In that case, I will offer this advice:
You should mold or build a core.
The core could be of steel or another filled polymer for strength and then over-mold that core with PE. If you would like to stick with a polymer for the core, I would suggest that you look into a thermoset material (which would be less likly to warp during curing). This scenario is still going to expensive but will allow you to fulfill your need. I would suggest that you find someone that does prototyping for the automotive industry. They would have equipment large enough to meet your needs. Unfortunatelly, I only work with those who do smaller automotive components.
It seems as though you have done your homework and researched a material that will work for your application.
In that case, I will offer this advice:
You should mold or build a core.
The core could be of steel or another filled polymer for strength and then over-mold that core with PE. If you would like to stick with a polymer for the core, I would suggest that you look into a thermoset material (which would be less likly to warp during curing). This scenario is still going to expensive but will allow you to fulfill your need. I would suggest that you find someone that does prototyping for the automotive industry. They would have equipment large enough to meet your needs. Unfortunatelly, I only work with those who do smaller automotive components.
bradphillips
Mechanical
You should be able to fabricate a manhole with a steel frame to support the load and build a thick plastic layer from extruded sheets over the steel frame to protect the steel. Of course there will be plenty of engineering questions to be considered but the "technology" is not insurmountable.
You mention that concrete is out of the question due to the high sulphate content of the landfill soil. Have you considered designing the manhole using concrete but protecting the OD and ID with an embedded plastic lining. The plastic could be specified (HDPE or other) to withstand the various chemical and temperature variables of your project. Embedded plastic liners have been used since the 1940's to protect the interior of santitary sewer manholes in Southern California. If the plastic can be fitted into a concrete form to protect the ID of a manhole, I would assume it could be fitted to protect the OD as well. Precast companies in your area should be using embedded plastic liners. Check to see if any companies are able to pour the structure in place at the site with a monolithic pour. A monolithic pour would eliminate joints in the concrete that would correspond to weld joints in the plastic that could be prone to leak with the hydrostatic pressures being exerted at 60 feet. I believe that the California Green Book has a similar process mentioned in the specification for rehabilitating manholes.
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