I have a job interview coming up on September 2. To make a long story short, I have had this interview before and failed because my answers were not elaborate enough. One of the questions was "explain the difference between schedule 40 and schedule 80 pipe. My answer was “Schedule 40 is the most commonly used pipe in commercial applications. Schedule 80 has a thicker wall diameter and though I have not seen it used I would think that it is for high- pressure applications or in a more corrosive enviroment.”
I have researched and found better answers to the other questions but I can not find a better answer to this one. So. I come to you to fight my ignorance and to help me get this job. Any better answer will be greatly appreciated.
Pipe schedule number = 1000 x (psi internal pressure ÷ psi allowable fiber stress)
You’re correct about greater wall thickness, but corrosion resistance will be dictated by the material used to make the pipe. If product x will eat through Acrylonitrile Butadiene Styrene in y amount of time, a thicker pipe just means it will take longer for failure to occur. The most commonly available types are PVC, CPVC, ABS, PE, PB, PVDF, FRP Epoxy, and polypropylene. Some are NSF rated for potable water systems, all are manufactured to meet various ASTM standards, depending upon the intended application.
Excerpts from the Machinery’s Handbook, 26th edition:
“The wall thickness designations “Standard” “Extra-Strong” and “Double Extra Strong” have been commercially used designations for many years. The schedule numbers were subsequently added as a convenient designation for use in ordering pipe. “Standard” and Schedule 40 are identical for nominal pipe sizes up to 10 inches, inclusive. All larger sizes of “Standard” have 3/8” wall thickness. “Extra Strong” and Schedule 80 are identical for nominal pipe sizes up to 8 inch, inclusive. All larger sizes of “Extra-Strong” have 1/2 inch wall thickness."
“Plastic pipe can be specified my means of Schedules 40, 80 and 120, which conform dimensionally to metal pipe, or through a Standard Dimension Ratio (SDR). The SDR is a rounded value obtained by dividing the average outside diameter of the pipe by the wall thickness. Within an individual SDR series of pipe, pressure ratings are uniform, regardless of pipe diameter.”
Specific to PVC:
Properties: rigid, fire self-extinguishing, high impact and tensile strength
Common uses: water, gas, sewage, industrial process, irrigation
Operating temperature: With Pressure 100F (38C), Without Pressure 180F (82C)
Joining methods: solvent cement, elastomeric seal, mechanical coupling, transition fitting
Your meaning would be clearer if you omit the word “diameter”.
An example of the uses of the two different types of PVC is seen in the standards for installing underground electrical service (which I’ve been considering for my house). The buried conduit must be 3" schedule 40 PVC. The risers (e.g. the conduit that goes up the power pole at the street) must be 3" schedule 80.
Danceswithcats & Fuji Kitakyusho
Excellent answers but a bit over my head. The job title is " Building Mantainance Mechanic" a glorified janitor. Though I would not be mopping floors I would be doing minor repairs to bathroom facilities, replacing light bulbs (at seventy feet above the floor,Yikes!) and general maintainance on conveyor belt systems. Why they asked me about piping I do not know because anything involving that they will call in their plumbers. They are looking for very basic knowledge so if you don’t mind could you dumb your answers down a bit?
Xema Thanks for the observation. That was basically a typo but as Danceswithcats posted “a greater wall thickness” is a much better reply. I would not want to make the same blunder in my interview.
Hmmm. If I knew what they wanted you to know, I could better answer the question. In adult evening education for residential remodeling, I stress the difference between potable water NSF rated PVC and dielectric UL listed PVC as they get mixed by homeowners who think if it fits, it has to be OK.
For a Building Mantainance man, I don’t think you really messed up your original answer, because the difference between the two is wall thickness. Being able to explain the difference in application might have been what they were looking for.
Of course, the questions this time might be different, so I’ll add a couple of items that might come in handy (sorry if you know this stuff already).
The difference between PVC and CPVC pipe. PVC is good for cold water service. CPVC is for hot water service. I’m not sure what the high temperature limit is for PVC, but it’s fairly low, and domestic hot water is too hot for PCV. CPVC is cream colored, PVC is white (for water use).
Pipe can also be rated as DWV (for drain, waste, and vent). This is used for drain and vent piping where there is no pressure other than gravity to make the fluid flow. I’ve seen both PVC and copper DWV pipes.
In steam pipe systems, Schedule 40 steel pipe is often used for the steam pipe and Schedule 80 steel pipe used for the condensate pipe. The reason for this that condensate is corrosive, and the thicker steel is used to provide a longer service life of the condensate pipe.
Gate valves are used for on/off. Ball valves do the same thing but can be operated very quickly (1/4 turn). Globe valves are used for throttling flow (gate and ball valves don’t do this very well). Butterfly valves also throttle flow and are used on larger pipes.
The job is certainly more than a glorified janitor. The more you know, the better you can do your job.
Think about it from your potential boss’s perspective: using Schedule 40 when Schedule 80 is called for could damage the building, be extremely inconvenient, and expensive. So you need to know when schedule 40 won’t do. Likewise, you’re expected to be frugal with your budget, which ultimately comes from your boss’s profits. Using Schedule 80 willy-nilly might keep the building from leaking, but you might run out of annual money in July - check the prices.
I would phrase my answer like this: “Schedules 40 and 80 both deal with the ratio of wall thickness to inner diameter of a pipe, and basically differentiate between a light duty pipe and medium duty pipe. You can use 40 for just about anything, but there are a few occasions where I wouldn’t use anything but 80, and those are…(list). Since 40 is generally cheaper, you can save a lot of money by using 40 – or even schedule 30! – on a light-duty job. Obviously it depends on your budget priorities and the local building code, but I generally go into a job assuming I’ll be using (name your favorite type of pipe).”
Are you sure that this was not an acceptable answer? I’m an engineer who does a lot of piping work and that answer would seem fine to me.
As to jurph’s proposed answer I would be astounded if the interviewer wanted that level of detail from a maintenance mechanic. I certainly would not want a maintenance mechanic designing any high pressure piping systems, and there are almost no typical domestic uses for fluid carrying sch 80 pvc. So any use of sch 80 would be for repairs of existing systems, in which case all you really need to know is that… Sch 80 is thicker.
Only change I would make would be to drop out the supposition of what uses sch 80 is put to.