THIS MONTH'S PUZZLER
An unknown organic film has built up in the tubes of our TEMA Class AEW shell- and-tube heat exchanger. An inexperienced production engineer decided to use dilute sulfuric acid to clean them. Not only didn't this work but we now are concerned about the exchanger's buna-nitrile gaskets. Should we be concerned? And can we remove the fouling more quickly than via hydroblasting for several hours?
SAMPLE AND TREAT
An attempt should be made to obtain a sample of the film for analysis; results may provide a possible approach. In addition, investigate conditions upstream to determine its origin. The use of dilute highly corrosive sulfuric acid should be eliminated; concentrated H2SO4 is often employed as pickle liquor for steel vessels. Consideration should be given to back-flush of the exchanger with an inhibitor with a phosphoric-acid/nitric-acid solution base. This will prevent corrosion and gasket attack. Also, dilute muriatic acid (31.45% HCl) and a degreaser may work. Change the gasket materials to polypropylene or polytetrafluoroethylene in the exchanger to make a maintenance improvement.
Robert F. Drucker, consultant
East Northport, N.Y.
INVESTIGATE THE SOURCE
The organic film probably comes from one of three sources: 1) an upstream contaminant, 2) unexpected product or 3) the breakdown of a gasket, seal or coating. The first questions to ask are: "Has this happened before?" "Has something changed upstream — equipment, raw materials or process?" and "Is the film showing up somewhere else?"
You must follow two paths: a process investigation and a materials' inspection of the exchanger.
Obviously, if the heat exchanger tubing is copper the exchanger must be fully re-tubed; I've seen copper tubes completely destroyed in only an hour of cleaning with muriatic acid. If the tubing is an austenitic stainless steel, it may be severely damaged by stress corrosion cracking (SCC) via the sulfurous ion. SCC damage will be most apparent at weld seams. Buna-nitrile swells when exposed to acid; the gaskets may or may not leak but why take a chance? Given the risk of SCC and the gasket damage, the heat exchanger should be pulled immediately for a shop inspection of the tubes, the sheets and internals.
If the process investigation reveals this organic film will remain an ongoing problem, then you will need to create a new cleaning method. If the film is temporary, then you still need a new method but not as urgently.
Water scale should be easier to remove; carbonates are easier than sulfates. National Association of Corrosion Engineers (NACE) Standard RP 01-70 for austenitic steel could be a useful reference. I suggest a six-step procedure: water-jetting, de-oiling, acid wash, neutralization, passivation and drying.
De-oiling prior to acid washing involves circulating a hot (180°F) solution of 2% trisodium phosphate, 0.1% non-ionic wetting agent and a trace of sodium hydroxide (0.25%). The hydroxide absorbs any H2S evolved from reaction with FeS.
Acid washing involves blinding the piping to be cleaned and then circulating hot, inhibited, dilute phosphoric acid or citric acid under an inert gas. The storage tank containing the acid solution must be inerted, usually with nitrogen.
After cleaning with acid, follow up with a rinse with a 5% sodium carbonate/water solution. The rinse is drained but not flushed; the coating left behind will neutralize acid in the surface crevices. This treatment is sufficient to remove water scale. Flush the sodium carbonate before returning the exchanger to service. Blow dry with inert gas, e.g., nitrogen.
Removing a thick organic layer is a different matter.
Detergents or alkali can be used in conjunction with steam for removing light oily films in a shop. Field washing is more selective. Solvents have often shown success in stripping some organic films. James McCoy's "Industrial Chemical Cleaning," pp. 88–92, Chemical Publishing Co., 1984, recommends avoiding solvents with flash points below 150°F if used in open air, i.e., in the storage tank. McCoy suggests using a naphtha for a paraffin at a temperature above its boiling point. Aromatic solvents such as trimethylbenzene are useful with asphaltenes and tars; typical recirculation temperature is 275°F.
Take care to avoid interactions between oil and acids, if present, as these could saponify. Finalize all cleaning treatment by passivation.
In extreme cases, usually when the exchanger is operated at high temperatures, build-ups may not be easily removed by chemical treatment — re-tubing may be the only good option. As a rule of thumb, if more than 30% carbon by weight is present in a sample of the oil deposit, chemical treatment will not be successful.
Dirk Willard, consultant
Wooster, Ohio
APRIL'S PUZZLER
A safety expert from headquarters surveyed our refinery's pressure relief system and took issue with a number of practices, including use of a check valve for thermal relief, connecting blowdown lines to the discharge of a pressure safety valve, and having several relief devices share a common discharge (Figure 1). Operations dismiss the expert's comments as quibbling to justify the survey and maintain nothing is wrong with the current solutions to these pressure relief problems. Please help me convince them these really are problems.
