Survey (inspection?) of sulphuric acid delivery lines 4

[andyenergy]

A query has been raised over a 'survey' that is required on our sulphuric acid delivery lines. These lines, 2" and 3" OD are carbon steel (I think) and are used to deliver 90 - 96% sulphuric acid to bulk storage tanks.

The person requesting the survey has not specified exactly what they require and how extensive this 'survey' should be. I have volunteeered to come up with a proposal. I think this should include a visual survey of the entire line to identify areas where the painted exterior may be broken down or where rust might be showing in the paint. I also will propose thickness surveys at vulnerable locations. I am aware that air traps and the like are potentially damaging on such lines and would naturally try to identify high points or other such features.

Is there anything else I should be proposing or considering?

Any feedback from those that might have experience with similar plant would be gratefully recieved.

 

[unclesyd]

We survey all our 93% H2SO4 lines with Radiography once a year. We have detail maps of each pipeline. We do very little actual ultrasonic thickness measurements instead we use the RT for thickness. As you state we look at the branches, elbows, welds, and mid points of each run of pipe. The RT is contract, all other inspection is done by onsite personal.
We rotate or replace the pipe based on the results of RT thickness measurements.

 

[EdStainless]

I have seen sites that use UT for thickness monitoring. If you do this it is important that you mark the locations for testing so that you can re-test the exact same spots.
The big advantage fo RT is the coverage.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[InspEngr]

Our sulphuric acid varies from ~98% down to ~89% and we employ both RT and UT (along with visual). My recommendation is much like Unclesyd's. Since you have relatively small bore piping I would suggest RT, especially at areas of turbulence (bends, reducers, tees, etc). I would also get a shot or two (RT or UT) on long, straight runs to check for hygdrogen grooving along the top-center of the pipe.

-InspEngr

 

[andyenergy]

Thanks guys. I've been delving back through operating experience and have yet to find anything to sugegst the pipework itself is ar risk of failure. However in the absence of specific areas to inspectand perhaps monitor, I think I need to go and look at this pipework in ordre to come up with a strategy.

Your input has been very useful.

Regards

Andy

 

[unclesyd]

Let us know what the you find. If you don't find the aforementioned grooving or loss of wall in the turbulent areas we will all be surprised.

PS: If you don’t find anything we will want to know all about your piping and pumping arrangement.

 

[andyenergy]

Am going to the plant this week to look at the arrangement.In the meantime have done a bit more research especially on hydrogen grooving - a term I'd not come across.

http://www.dupont.co/sulfurproducts/techdata/equipment.html

contained a load of really useful information confirming what you guys are suggesting.

Also, there was a recent failure of an elbow which had general wastage and internal grooves and this appears to be this phenomenon. Like yourselves, I will now be surprised if we don't find anything that we are concerned about. I will be sure to keep you informed.

Thanks again for the tips.

Andy

 

[NickelMet]

I know I'm not going to say anything too different than that above, but thought I might throw in my two cents. For piping surveys in our plant, we do the following:

1) UT monitoring of select locations (TMLs), all documented in order to return to them periodically. Definite TML locations are in areas of turbulence and/or change in direction.
2) RT shots on a recurring basis, depending on the findings (fouled or thinned areas get more frequent checks).
3) PMI (positive metal identification) is done on each circuit, making sure the piping, fittings, and welds are of the correct metallurgy.
4) CMLs (corrosion monitoring locations) are UT grids in areas of noted high corrosion/thinning. Also, there may be a CML around an injection point.
5) External visual inspections include looking for breaks in the coating, insulation, and/or paint barriers as well as leaking areas (or areas of potential leaks).
6) All dead legs and stagnant areas are identified on the piping isometric drawings and may require a higher degree of UT scanning and/or RT work.

For sulfuric acid service, you may want to check out the following documents:

a) NiDI #10057 - SELECTION AND PERFORMANCE OF STAINLESS STEELS AND OTHER NICKEL-BEARING ALLOYS IN SULPHURIC ACID (1990) - Nickel Development Institute (

http://www.nidi.org)

- free
b) NiDI #1318 - THE CORROSION RESISTANCE OF NICKEL-CONTAINING ALLOYS IN SULPHURIC ACID AND RELATED COMPOUNDS (1983) - Nickel Development Institute (

http://www.nidi.org)

- free
c) NACE RP0391-2001 - Materials for the Handling and Storage of Commercial Concentrated (90 to 100%) Sulfuric Acid at Ambient Temperatures - not free
d) NACE RP0294-94 - Design, Fabrication, and Inspection of Tanks for the Storage of Concentrated Sulfuric Acid and Oleum at Ambient Temperatures - not free

And lastly, this is straight from Dupont Q/A on their website:

Inspection Procedures
--------------------------------------------------------------------------------

Q. How do you inspect piping? How often?
A. In general piping is formally externally inspected visually every five years and ultrasonic thickness tested biennially. Again, actual plant experience may dictate an increase or decrease in this schedule. Extra attention should be paid to elbows, tees, valves and any other places in the piping where flow disturbances (and erosion/corrosion) could occur. Use API 570, Class II piping standards for guidance.

--------------------------------------------------------------------------------

Q. How do you inspect an acid storage tank?
A. There are three types of inspections normally done on a sulfuric acid tank. The first is an external "walk-around" inspection.
Every year someone should "walk-around" the tank, looking for signs of sulfate leakage or other metal deterioration, making sure the insulation (if so equipped) is weather-tight, observing the overflow/vent line to make sure it is not plugged, looking at the tank foundation/supports.
A more thorough external tank inspection would include an ultrasonic thickness test. Areas around welds and nozzles should be carefully reviewed
An internal tank inspection is the most thorough. It involves emptying and cleaning the tank. Someone (preferably an API-certified inspector) then enters the tank and visually inspect the internal welds and tank surfaces. Ultrasonic thickness testing should be done on the floor and any other questionable areas at this time. Be sure to follow all OSHA guidelines when entering the tank.

--------------------------------------------------------------------------------

Q. How often do you inspect an acid storage tank?
A. General guidance for tank inspections is as follows:
An annual external "walk-around" inspection
A biennial ultrasonic thickness test.
Every 5-6 years empty and clean the tank, and internally inspect the tank.
These inspection frequencies can be increased or decreased, based on actual findings when the tanks are inspected. Be sure to document all inspections and keep copies in the tank files. Use API 653 for guidance on vertical tank inspections

=====================

I included the tank items because you will be around tankage. Hope this helps.

~NiM

 

[andyenergy]

Thanks NickelMet -

I'm putting togther the proposals as we speak and this was helpful. I'll let everyone know what the outcome of the inspections were when they're done (it may be a week or so).

Andy

 

[Metalguy]

Andyenergy,
When the sulf. acid % drops below ~95% it gets pretty corrosive to CS. Fluid velocity is also important--this is one situation where dead legs suffer less corrosion! Do not ignore the central areas of horizontal runs--the iron sulfide junk that builds up along the bottom will raise the velocity along the mid-wall.

Temp. is a BIG factor also--esp. if over ~90 deg. F.

 

[andyenergy]

I appreciate the feedback - just to put you all in the picture. This particular pipework is the delivery lines to the bulk sotorage tanks on a power plant. I looked at the lines the other day and it consists mainly of vertical pipes a the delivery point then long straight sections with 90 degree bends with very generous radii. I'm recommending that fully circmferential UT thickness surveys are carried at three equidistant positions on straight sections, bith horiziontal and vertical. Similar surveys immediately after bends and on the extrados of bends.

In addition I'm asking for radiography after bends and on the top half of horizontal straight sections. I'm asking, but it might be difficult to get the operators to do this.

This also raised the concern over other pipework in the acid plant - in condensate polishiong plant. My seaches of operation experience found some failures in this section of pipe, but it is pumped and flow rates are higher. On some of our plant the material was rubber lined carbon steel and this is being replaced with plastic pipe, whree platic cant be used stainless is being used. Cncenetrations can be much lower (~5%) but I know of one plant where a tight radius carbonsteel tee failed due to flow grooving (hydrogen grooving?) I feel a bit more comfortable that this pipework is being addressed.

I truly am grateful for the input. Most times I can muddle along in my own field and from time to time help out when I can on Eng-Tips. I'm overhwelmed by the generosity of the contributors.

Best wishes and I'll let you all know what I find.

Andy

 

[Metalguy]

In my previous post I meant to write "mid-pipe", not mid-wall. I think many cases of so-called H grooving are really just wastage caused by higher acid flow rates--because of the iron sulfide corrosion buildup along the bottom of horiz. pipe runs. Storage tanks usually don't suffer like piping does. Velocities have to be very low-~3 ft/sec or so, and you can't easily tell where the vel. is higher when the pipe is half full of junk.

I've seen thick CS piping that was almost unaffected except for a narrow band along the mid-point, where it was paper-thin.