Water Line Design -- What material to design for?

[Denob]

I am currently working on a large diameter water transmission line project where PVC, ductile iron, steel, and bar-wrapped steel cylinder concrete pipe will be incuded in the bid sheet and specifications. The idea is to drive down prices due to competetion. My question is: What material do I design for? Should I design for the material that I think will be chosen (lowest bidder)? Do I design for the most conservative material overall? Do I design for the most conservative material for each facet of design (which could mean designing for PVC for joint restraints, designing for steel for pipe deflection, etc.)?

 

[stanier]

You need to not only consider the price of the material but the total life cost. The installation cost will be a factor. It does vary between materials.

You will find that price of materials may not be a simple affair. You will get competition between suppliers for the same product based upon the load on their plant, anticipated prices of raw materials, energy etc and how desparately one company wants the job.

It would be smarter to enter into a long term supply agreements with suppliers to reach a fair price before you start to design. Then you can establish a forward price list that your subcontractors can use when they are awarded a contract. This will also make your estimating more accurate. Also you can keep the shonks off your approved bidders list.

 

[JStephen]

I would not normally expect steel and PVC to be included as alternates to each other. Usually, PVC would be used for smaller lines (18" or so and smaller) while steel would be considered on larger lines (36" or so and larger). Some checking there could help you eliminate one or the other.

In terms of design for the specific types, I would assume that whatever extent of design is done has to be done for each type. Trying to design for one and make the others conform to it could just defeat the purpose of getting competitive bids on the products. You couldn't very well specify the wall thickness of DI pipe to be the same as what you use for concrete or PVC, for example.

Also be careful of how you specify the alternates. If you are willing to pay extra for one over the other, spell that out clearly in the bid documents, and show how bids will be evaluated. Contractors would probably prefer to quote the all-around most competitive material (for them), rather than having to quote EACH material.

 

[Denob]

JStepens, the diameters that I am designing for (24" and 30") seem to be the sizes where there is no clear cut winner. It seems to be the transitional zone, materially speaking.

 

[stanier]

have you factored in all the installed costs of both materials? eg for steel an impressed current cathodfic protection system? Risks such as failures types pinhole, puncture, tear, crack, avalanche (from pipeline Risk-Muhlbauer). Others such as security, repair, damage in construction, surge, sabotage, material reliability,

If Ii could get a steel pipeline for the same life cycle cost (price) as a PVC one there is no comparison from the risk perspective.

 

[BRIS]

Denob

I suggest that you are on a no win situation with this approach. As noted by others there are many other factors to consider other than the cost of the pipe: bedding materials, structures, fittings, service life etc etc. In my opinion the only way you could successfully adopt this approach would be to invite turnkey tenders with the contractor responsible for the material specification, design and construction. You would need to prepare a tight performance specification with a specified service life. Evaluation would need to consider whole life costs including maintenance costs. (different materials also have different internal diameters and different friction losses etc). Your problem then is evaluating the contractor’s proposals to ensure that they comply with the specified performance. Asking the contractor to give a 50 year guarantee is not a real option but surprisingly is not uncommon!

Brian