304L vs. 316L Economics

[CW3]

I'm putting together a pump skid that will be used to circulate chemical cleaning solutions for boiler/HRSG chemical cleaning operations. Some of the chemistry is acidic (Citric, EDTA..) some is alkaline (Ammonia, Soda Ash...). Pumping pressures never exceed 150-160 psig and temps never exceed 180F. I'm going to use Sch10 stainless pipe with carbon steel slip-on flanges (so that wetted surface only is stainless). In selecting the stainless material, I was looking at 304L or 316L. I don't want corrosion problems around the weld areas, and I need to make an economical selection while selecting the properly suited material. Any thoughts.....? (gittings such as ells, tees, and reducers will also come into play).

Thanks,

CW3

 

[StoneCold]

CW3
I would go with 316L but I am not a metalurgist. You may want to ask the Corrosion Engineering people about that. As far as slip on flanges go, to get stainless on all wetted parts you have to get lap joint flanges. They are about three times the cost of just getting weld neck flanges. Sch 10 vs Sch 40. I would go with sch 40, you can price it out but the cost savings is not really there.

Hope this helps.

StoneCold

 

[unclesyd]

StoneCold has given you some very sage advice about the piping and materials for not being a materials person.

Another problem with CS on a multipurpose cleaning skid is that the whole works will become a rust ball after short time so if you can swing it weld neck flanges are the way to. Another point is that you should watch you pipe and supports if the unit is any size. A cleaning unit will see a lot of abuse. Use a totally enclosed motor for the pump if possible.
Watch the design as there is nothing more embarrassing that to breakdown on critical job.

 

[Montemayor]

You imply you seek the best economic solution. The answer will probably be 316 ss if stainless is a process requirement. (Of course, you’re the only one who can determine or decide what alloy is needed) Manufacturers have almost standardized on 316ss as a replacement for 304ss in order to eliminate having to deal with two similar alloys. Up to 5 years ago this had transpired in the valve market and in the heat exchanger tubing field. This fabrication move has made 316 more available and cheaper than the 304 alloy. This is a paradox where market demand once again determines the price and not the material or the properties of the alloy.

If your process requires stainless and your pressures are as low as you state, then 316ss, schedule 10s is the way to pipe it up. If you require flanges for maintenance, I always employ carbon steel backup flanges on van stone lap joints. This is done where the external carbon steel backup flange is not subject to external corrosion. The same criteria applies to the flange bolts and nuts. I would employ a minimum of welding on the piping. The excellent features and installation flexibility of the van stone flanges make them ideal for this application.



Art Montemayor
Spring, TX

 

[TrevorP]

As usual, Montemayor provides sound advice. You need to check each chemical you intend to use against the material of construction. Phosphoric Acid is quite often used for cleaning applications, and I'm pretty sure that this requires 316L. 304L is still slightly cheaper for pipework than 316L , but there's not much in it. As Montemayor says 316L is standard for exchanger tubing so is actually cheaper than 304L. Sched 10 should be satisfactory unless you expect high corrosion rates. As for flanges, I would stick to stainless falnges for smaller pipes - it will look so much better than rusty flanges, and not much cost difference to painted flanges. Only if you have large bore pipe does the cost difference start to have a significant impact.

 

[CW3]

I knew I'd get good advice here. No pipe larger than 6-inch. I think I will specify SS lap-joint stub ends. Less welding with the butt joint as opposed to two welds per slip-on. All of our motors are explosion proof, outdoor, severe service, etc. Pumps are all stainless Durcos. I have to spec valves for the 6-inch. We incorporate reverse-flow manifolds, dual in-line basket strainers, and several 6-inch branch connections. The valves are only used to direct flow. Space on a skid-mounted unit comes at a premium, especially when flow meters are incorporated, so every inch counts. I was considering the use of 6-inch SS knifegate valves. I am concerned, though, about leakage at the knife packing gland to the outside world. I have to have zero-leaks. DeZurik seems to have a good valve of this type. I solicit the wisdom and experience of the panel here.

Thanks again,

CW3

 

[TrevorP]

This is not really my area, but if leakage at the stem is important, I would personally steer clear of knife gate valves. My preferred option for this is generally ball valves, but as space is limited, I would tend to go for butterfly valves.