Heat Exchanger Cleaning

Heat Exchanger Cleaning

Heat Exchanger Cleaning is going on BY DIFFERENT METHODS AS ACID CLEANING in plant seeing the real approach about Life Cycle assessment of the AROUND 70 +- Heat Exchangers in both the plant THE MINIMUM COST OF EACH HEAT EXCHANGER IS ABOVE 20 LAKHS RUPEES based on cleaning of deposit material & Corrosion & Condition Monitoring this information could be good for improvement as well as DISCUSSION FOR BEST QUALITY & FUTURE SUGEESTIONS BY CONTRACTORS.

To improve the performance of fouled heat exchangers requires that the tubes be cleaned

Periodically. Each time the tube deposits, sedimentation, biofouling and obstructions are removed, the tube surfaces are returned almost to bare metal, providing the tube itself with a new life cycle, the protective oxide coatings quickly rebuilding themselves to re-passivate the cleaned tube. Tube cleaning procedures for shell and tube heat exchangers are performed off-line, the most frequently chosen and fastest method being mechanical cleaning. shows a mechanical tube cleaner in action.

Among other off-line methods is the use of very high-pressure water but, since the jet can only be moved along the tube slowly, the time taken to clean a heat exchanger can become extended. Great care must be taken to avoid damaging any tubesheet or tube coatings which may be present; otherwise the successful removal of fouling deposits may become associated with new tube leaks or increased tube sheet corrosion, which are only revealed after the unit has been brought back on-line. Chemicals are also used for the off-line cleaning of heat exchanger tubes. Several mildly acidic products are available and will remove more deposit than most other methods; but it is expensive,takes longer for the operation to be completed, and the subsequent disposal of the chemicals, an environmental hazard, creates its own set of problems. It has also been found quite frequently that some residual material will still need to be removed by mechanical cleaning methods.For off-line mechanical cleaning, the tool selected has to be the most appropriate for removing a particular type of deposit. Moulded plastic cleaners (pigs) are quite popular for some light silt applications. Brushes

 can also be used to remove these soft deposits as well as some types of microbiological deposit. Brushes are also useful for cleaning tubes with enhanced surfaces (e.g. spirally indented or finned); or those tubes with thin wall metal inserts or epoxy type coatings.

With harder types of deposit, metal cleaners of various designs have been developed, often with a particular deposit in mind. shows a current version of a cleaner consisting of several U-shaped tempered steel strips arranged to form pairs of spring-loaded blades. These strips are mounted on a spindle and placed at 90 degrees rotation to one another. Mounted at one end of the spindle is a serrated rubber or plastic disk that allows a jet of water to propel the cleaners through a tube with greater hydraulic efficiency. A stainless steel version of this cleaner is also available.

 

Metal Tube Cleaners

Calcite Cleaners

Water Gun

Portable

Booster Pump

HydroDril

UREA STRIPPER

The substance urea is employed for, among other purposes, artificial fertilizer and raw material for the plastics industry. Urea is synthesized from carbon dioxide (CO2) and ammonia (NH3). This chemical reaction does not provide immediately a 100% yield. In order to complete the reaction a so-called “urea stripper” is employed. Urea strippers are manufactured from stainless steel alloy and consist of vertical tubes with the reaction progressing on the interior of the tubes while steam is introduced on the exterior of the tubes. After a period of time a deposit develops in the urea stripper. A typical deposit in a urea stripper consists of ferric oxide (hematite) with admixtures of nickel, chromium and molybdenum oxides and is generally grey to black in colour. This deposit causes reduced heat transfer resulting in a reduction of production process efficiency.

Iron oxide deposits can generally be removed readily from a stainless steel surface by acid pickling. This however also results in a very small portion of the base material being dissolved. This in turn increases the roughness of the stainless steel surface. This so-called acid corrosion can be slowed down by addition of inhibitors (pickling inhibitors), but this in itself will be insufficient to protect the stripper completely from material loss. It is not acceptable for part of the base material to be exposed to any corrosion. 

Research into an alternative cleaning method for the removal of the iron oxides involving the possibility of minimum corrosion were carried out in collaboration with DSM and Stamicarbon. A cleaning solution based on EDTA appears to dissolve this deposit completely at high temperature without corroding the base material. In a pH neutral medium the iron oxide will dissolve forming an iron -EDTA complex. The cleaning temperature is selected to maximize the reaction of the EDTA with the iron oxides. During cleaning the free EDTA and dissolved iron levels are continuously monitored in order to provide insight into the progress of the cleaning process. Prior to cleaning the total quantity of EDTA and the number of batch treatments required are determined on the basis of the quantity of deposit and the size of the Stripper. Cleaning is carried out in batches employing a “Fill & Soak” method, in which nitrogen gas is used as agitation agent. Pump circulation is not possible due to the high temperature at which cleaning is conducted.

 By

DR. AMAR NATH GIRI