Protective disadvantages

Extensive proven performance for four or more hours of protection Disadvantages of dense concrete include ... 

Advantages Guiding surfaces protected Disadvantages Prone to leakage (if metal seated)... 

Blends of diene and backbone-saturated mbbers are frequently used in appHcations where discoloration by chemical antiozonants caimot be tolerated, yet where cost is stiH a primary consideration (eg, white sidewalls of tires). Disadvantages are that physical properties have to be compromised and the two mbbers usually differ greatly in their rates of vulcanization. Usually, at least a 25% replacement by the ozone-resistant mbber is needed for an appreciable enhancement in ozone protection (6). 

Wire-bound wood boxes (typical loads, 1 to 2 tons) have limited use for chemical produc ts. The box body, consisting of thin wooden slats held in place by steel wire twisted around each at, is fastened to a solid-deck wood pallet. The top also consists of wire-bound wooden pieces. A PE liner protects the product and prevents it from falling through the slats. Disadvantages of the container are the labor needed for setup and the space required for knocked-down boxes. Since manufacturers are usually near sources of hardwood, shipping costs to users may be high lead times of 3 to 4 weeks are common. 

Protection with impressed current, with galvanic anodes, and a combination of both processes is used for marine structures and offshore pipelines. Their properties, as well as their advantages and disadvantages, are given in Table 16-1. The protective measures must be optimized for every structure. In the impressed current protection of offshore platforms, for example, the difficulties of maintenance and repair will be of major importance, whereas in harbor installations these problems can be... 

Attached anodes provide the particular advantage of proportionately low grounding resistance for the protection current because the actual anode body is set into both sloping sides of the plastic body as long narrow strips. The disadvantage is that the body is exposed on the ship s side and can easily be damaged mechanically. 

Although insulators other than aluminum oxide have been tried, aluminum is still used almost universally because it is easy to evaporate and forms a limiting oxide layer of high uniformity. To be restricted, therefore, to adsorption of molecules on aluminum oxide might seem like a disadvantage of the technique, but aluminum oxide is very important in many technical fields. Many catalysts are supported on alumina in various forms, as are sensors, and in addition the properties of the oxide film on aluminum metal are of the greatest interest in adhesion and protection. 

Respirators are an important part of worker protection. We discussed a variety of pitfalls and disadvantages earlier in this chapter, for the disadvantages are similar to those found in level A and level B protection. Selling respiratory protection is very important. This selling of the program comes through communication, training, and experience. Experienced respirator users know that they work. This faith by workers can... 

Balanced bellows type valves are normally used where the relief valves are piped to a closed flare system and the back-pressure exceeds 10% of the set pressure, where conventional valves can t be used because back-pressure is too high. They are also used in flow lines, multiphase lines, or for ptu affinic or asphaltic crude, where pilot-operated valves can t be used due to possible plugging of the pilot line. An advantage of this type of relief valve is, for corrosive or dirty service, the bellows protects the spring from process fluid. A disadvantage is that the bellows can fatigue, which will allow process fluid to escape through the bonnet. For HjS service, the bonnet vent must be piped to a safe area. 

Used to restrict reversal of flow, check valves should not be considered as positive shut-off valves when flow is reversed, since the seating element is always in the flow stream and subject to erosion (Figures 15-9 to 15-13). A section of a line should not be considered isolated if the only batrier to flow is a check valve. On the other hand, because they do restrict backflow to very low levels, check valves installed in appropriate locations can protect equipment and minimize damage in case of a leak in the upstream line. Some of the advantages and disadvantages of the various check valve configurations are as follows ... 

The disadvantage of this method is that the dichloridites and monochloridites are sensitive to water and thus could not be used readily in automated oligonucleotide synthesis. This problem was overcome by Beaucage and Caruthers, who developed the phosphoramidite approach. In this method, derivatives of the form R 0P(NR2)2 react with one equivalent of an alcohol (catalyzed by species such as l//-tetrazole) to form diesters, R OP(OR")NR2, which usually are stable, easily handled solids. These phosphoroamidites are easily converted to phosphite triesters by reaction with a second alcohol (catalyzed by l//-tetrazole). Here, again, oxidation of the phosphite triester with aqueous iodine affords the phosphate triester. Over the years, numerous protective groups and amines have been examined for use in this approach. Much of the work has been reviewed. ... 

The Operational Characterisics of Platinised-Titanium Anodes Platinised-titanium anodes have the disadvantage that the protective passive him formed when titanium is made anodic in certain solutions can breakdown. This could result in rapid pitting of the titanium substrate, leading ultimately to anode failure. The potential at which breakdown of titanium occurs is dependent upon the solution composition, as is evident from Table 10.16. 

These are iron alloys that contain 14-18% Si and are reported as first being developed in 1912 , although it was not until 1954 that they were first evaluated for use as impressed-current anode material in cathodic protection. Its major disadvantage is that it is a hard brittle material unable to sustain thermal or mechanical shock. 

The use of an impressed-current system or sacrificial anodes will both provide satisfactory cathodic protection, but each has advantages and disadvantages with respect to the other (Table 10.24). 

There is no intermetallic compound formation and the electrodeposit behaves as a simple mixture of the two metals. It can be considered as basically a stable wick of tin through which zinc is fed to be consumed at a rate lower than its consumption from a wholly zinc surface. If the conditions are such that zinc is rapidly consumed, and no protective layer of corrosion products is formed, the coating may break down, but in mildly corrosive conditions some of the benefits of a zinc coating, without some of its disadvantages, are obtained. 

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