Chemical cleaning services has fast becoming among the most important process during plant start-up, shutdowns and turnaround activities. The need for such chemical cleaning can be related to the greater understanding of any factors that could deteriorate the smooth operation of a plant. Any unwanted materials those are not removed from the systems could lead to more serious problems later. Contaminant can demote heat transfer, reduces desired flow rates, create pressure drops, induce corrosion probability and reduced the equipment life.
Thus, the removal of unwanted fouling materials and contaminants from tanks, plant-piping systems and other plant equipment is critical for a proper operation at newly constructed systems or existing operational systems. Proper removals of these unwanted materials are essential to ensure the optimum operating efficiency and to reduce the start-up problems of newly constructed facilities and also to rejuvenate the efficiency of existing operating facilities.
This paper on Chemical Cleaning Technology for Petrochemical System will highlight some of the information pertaining to the chemical cleaning services. It will briefly discuss on the general outline of any system or equipment need to be cleaned, advantages of the processes, typical procedures and some case history of past projects.
INTRODUCTION
The recent trends in improved environmental measures, increased energy efficiency, and higher productivity are the primary motives for renewed interest to develop cost-effective chemical cleaning methods in process industries. Today's petrochemical industry requires the flexible and reliable operation of process equipment.
Some of the major problems associated with pipelines, tanks, vessels and other equipment problems are:
• Scale Formation
• Corrosion
• Fouling
• Foaming
• Caustic Embrittlement
Scale is an extremely hard substance created when mineral salts come out of solution as their solubility drops with a rise in water temperature. Scale-forming salts adhere directly to heating surfaces forming layers of insulation on the metal substantially decreasing its heat transfer efficiency.
Caused by the precipitation of calcium and other salts of limited solubility, scale, in addition to its high insulation value, progressively narrow pipe internal diameters and roughens tube surfaces, thereby impending proper flow of water.
In compression refrigeration systems, scale translates into higher head pressure, hence an increase in power requirements and cost. For example, 1/8" of scale in a 100-ton refrigeration unit represents an increase of 22% in electrical energy compared to the same size unit free of scale. Similarly, in absorption systems, scale creates a higher back-pressure on the concentrator, increasing energy expenditure. While scale formation proceeds more rapidly in open recirculating systems owing to the concentration effect on evaporation, once-through systems are not exempt from scaling if high temperatures are combined with silt and iron.
While 1/16th-inch thickness of scale in a fire tube boiler will increase fuel consumption by 12.5% -- any accumulation is a problem. Scale results in metal fatigue/failure causing overheating, energy waste, high maintenance costs and unnecessary safety risks.
Fouling occurs when restriction develops in piping and equipment passages, creating inefficient water flow. The major consequences of fouling to boiler room equipment are energy waste and increased operating/maintenance costs. When fouling is allowed to continue and proliferate in a system, cooling towers, heat exchangers and other critical devices could give rise to the emergence of health-related issues such a Legionnaires Disease.
Dissolved gases, improper pH control or formation of differential aeration cells under deposits causes corrosion to be undoubtedly the single most significant factor leading to the premature deterioration of HVAC equipment and piping. In its most insidious form, piping is only revealed when the equipment fails. Failures of this type can be catastrophic, leading to costly downtime for repairs and equipment replacement, personal discomfort and even total plant shutdown.
Corrosion can be caused for the following reasons:
High oxygen content: As all minerals, such as iron, are found in nature as the oxides, it is normal for iron, for example, to try to find oxygen and change to iron oxide. This type of oxide is red in color. Oxygen is usually dispelled from the systems fairly quickly through the air vents; however, where there are large losses causing a large make-up condition, we then have a continuing supply of oxygen and trouble. Oxygen or air can also get into the system from the cushion tank and also due to a low pressure on the system at the top.
Electrolysis or galvanic action is actually the evolution of oxygen by the action of direct current on water. This current is developed between two different metals, hence is called dissimilar metal corrosion. We can have a current when we have a stress developed in a metal of the same analysis. In these reactions a small amount of metal is removed from the cathode (the cathode is the metal lower on the electromotive scale). The metal, e.g. iron (Fe) then combines with the oxygen formed by electrolysis and a black, magnetic deposit is formed - also a hole in the iron pipe. It should be noted that nothing happens to the other metal, for an example, in a copper-iron situation. The copper is unaffected and the iron is removed into solution. The treatment is to isolate the locations of dissimilar metals with dielectric isolation and treat with cathodic corrosion inhibitors.
Biological growths also known for their role in promoting corrosion, through the formation of acid metabolic products or through the mechanism of concentration cells associated with deposits of all kinds could also cause problems with the uncontrolled multiplication of bacteria, algae and fungi, results in bio-film formation on heat exchange surfaces and has now been recognized as an important contributor to impaired heat transfer efficiency in cooling water systems.
In fact, it has been unequivocally demonstrated that because of the unique surface characteristics of bio-films, their hydrodynamic and insulating properties far exceed those of an equivalent thickness of scale or corrosion deposits.
Of particular concern are slime and spore, former which are difficult to control because of the protection afforded by the polysaccharide sheaths that they secrete and the organisms that metabolize either cellulose or lignin, results in structural weakness and eventual collapse of wooden tanks or towers.
Sludges include dirt, mud, sand, silt, clay, scale salt, and other particulate of airborne origin or entering the system with makeup water. Very often these suspended solids are tightly bound and cemented by corrosion products and organic matter.
Where abrasive, sludge deposits can damage pump seals and in addition to their insulating nature can also promote "under-deposit" corrosion.
Foaming is a condition in which concentrations of soluble salts (aggravated by grease, suspended solids or organic material) create frothy bubbles (resembling the foam in a beer mug) in the steam space of a boiler.
Foaming can cause priming -- in which the bubbles break and create a liquid that combines to form slugs of water that are carried over into the steam system. Pressure from the steam can create velocities as high as 80-100 miles per hour for slugs of water discharged into steam lines. These slugs can wreak havoc with turbine blades, actuating devices and piping downstream of the boiler.
Caustic Embrittlement occurs when hairline cracks appear in highly stressed areas due to high concentrations of alkaline salts that liberate hydrogen, which is then absorbed by the iron in steel, effectively changing its physical properties. This condition is caused largely by boiler water with pH values 11+ and manifests itself in high-temperature areas of the boiler. Unless Embrittlement problems are constantly monitored and controlled, they will take their toll in higher fuel costs, increased safety risks, unnecessary downtime and equipment replacement.
The presence or formation of various types of unwanted materials, rust, slags, scales, sludges and deposits in any parts or systems in an petrochemical operation such as cooling system, boiler and process systems occurs quite fre-quently. The presence of these types of deposits can present some serious problems. Scaling of boiler tubes can lead to tube failure. Deposits in heat exchangers lead to restricted flow and loss of system efficiency. Corrosion and fouling of heavy industrial cooling water heat exchangers result in loss of heat transfer performance and, eventually, failure of these critical equipment assets.
The answer to the aforementioned problems created by scale, corrosion, bio-fouling and sludge is, course, a comprehensive effective chemical cleaning and water treatment program comprising scale and corrosion inhibitors, microbiocides and dispersants coupled with adequate bleed-off and appropriate equipment. Thus an effective chemical cleaning method of corrosion and fouling under actual heat transfer conditions allows the system to operate at maximum capacity or to be optimized or process conditions modified to critical heat exchanger reliability.
