LIQUID TO STORAGE

Figure 4-2 A typical tank car unloading system for unloading compounds with low boiling points. Courtesy Dow Chemical Company, U.S.A.

Regulations that have occurred since it was last issued and tells where in the

Federal these changes can be found.

One of the most effective means of transferring powder from one location to another is a pneumatic conveying system. In these systems the powder is intro- duced into a moving air stream. The air carries it to its destination, where the solids are separated out. The designer must determine how to introduce the powder to the air stream, which is under pressure. He must design it so that the air cannot escape into the feed tank and blow the feed out the top. Not all the powder will be removed at its desired destination, so a filter must be installed before the air can be dis- charged from the system. Care must also be taken so that the powder does not get into the blower, where a spark could ignite it. A series of flow diagrams for a pneumatic conveying system are shown in Figures 4-3 through

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Figure 4-3 A vacuum system for transferring solids from storage silos to one receiver.

Courtesy Kraus, M.N.: “Pneumatic Conveyors, ” Chemical Engineering, Oct. 13,1969, p. 60.

SAFETY

It is during the construction of the detailed flow sheet that safety begins to affect the design. In the scope some concerns about safety were expressed. These and other general principles are now put to use.

Whenever powders are transported, they should be tested to determine whether there is a strong possibility of a fire or explosion. If the potential exists and there is a high probability of extensive damage, then a preventive device must be able to detect and snuff out the explosion in less than 0.1 sec after the initial blast occurs.

Figure 4-4 A positive pressure system which can deliver product to many receivers.

Courtesy Kraus, M.N.: “Pneumatic Conveyors, ” Chemical Engineering, Oct. 13,1969, p. 61.

Figure 4-5 A closed loop system which can operate under either vacuum or pressure.

Courtesy Kraus, M.N.: “Pneumatic Conveyors,” Chemical Engineering, Oct. 13, 1969, p. 61.

Figure 4-6 A combination vacuum-pressure system. A vacuum is used to withdraw material from a hopper car and positive pressure is used to transport it to storage silos.

Courtesy Kraus, M.N.: “Pneumatic Conveyors, ” Chemical Engineering, Oct. 13,1969, p . 6 1 .

Fenwal Incorporated makes such a system. It consists of a series of pressure or radiation detectors and suppressors. A detector, after receiving the signal that an explosion has begun, fires a suppressor (often water and bromo-chloromethane) at speeds up to 600 ft/sec (200 m/sec), which snuffs out the explosion or fire. Thus a damaging explosion is prevented by an explosion. The equipment usually must be cleaned out if a suppressor discharges, and often the batch of material being processed must be discarded because of the resulting contamination. It does, however, prevent damage to the equipment and nearby personnel. The disadvan- tage is that the full system may cost over $100,000.

As another example, consider vessels that are under a positive pressure. For all pressure vessels, including storage tanks, a vent system must be installed to protect the vessel from rupturing. The vent goes to either a flare or a blowdown tank. These

Safety

Figure 4-7 A parallel dust return system which permits a vacuum system to distribute material to more than one receiver.

Courtesy Kraus, M.N.: “Pneumatic Conveyors, ” Chemical Engineering, Oct. 13,1969,

p. 60.

are located a distance from the other pieces of equipment and in an unfrequented area. The vent system is designed so that at a given pressure a pressure-relief device will release. This opens the vessel to a specially sized vent line. To design the vent line it first must be determined from chemical reaction rates and heat transfer rates how much gas must be removed per unit time in order to prevent the pressure in the vessel from exceeding the design pressure .In doing this one should assume that the gas is leaving at a velocity of half the speed of sound. This gives a safety factor of two to the design. The pressure drop in the line at these conditions is assumed to be the design pressure minus atmospheric pressure.

Unless the precautions given in the previous two examples are taken, some employees could be injured or killed. Even if this did not occur the financial loss could be large, since if critical pieces of equipment were damaged it might take months before they could be repaired or replaced and before production could be resumed. For each day the plant does not run a number of expenses continue (salaries, depreciation, taxes, insurance). There is also the problem of supplying customers. If a customer goes to another supplier it may be difftcult to lure him back. As a result, the company may furnish him with product made at a distant plant and not charge him the extra transportation expenses, or may buy a competitor’s product and sell it to the customer at less than the purchase price.

Insurance costs also depend on the safety precautions taken. When a homeowner buys fire insurance the cost depends on the type of construction and the nearness and effectiveness of fire protection equipment, as well as the value of house and its furnishings. To determine the effect of these factors a statistical analysis is made of the factors contributing to fire losses. Insurance companies are noted for not losing money, and so rates also depend on the past record of those insured. In determining automobile insurance rates the age and the number of previous accidents and traffic

violations are considered. In fact, people who have had a number of accidents find it very difficult to obtain insurance at any price. The same is true with industries.

But still, the most compelling reason for having a safe plant should be to protect the employees. Industrial corporations have not always felt this way, but they have now been forced to accept the Williams Steiger Occupational Safety and Health Act of 1970 The Congress of the United States declared the purpose of the act was “to assure as far as possible every workingman and woman in the nation safe and healthful working conditions and to preserve our human resources.” It requires that “each employer shall furnish to each of his employees employment and a place of employment which are free from recognized hazards that are causing or are likely to cause death or serious physical harm to his employees.”

Under this act the government is charged with setting up standards and checking to see they are followed. Anyone can request that a plant be inspected to see if it is in violation of the rules. If violations are found the company must make whatever changes are requested and may also be fined. Failure to correct possible injurious conditions can result in the plant being closed.

This act places all responsibility for safety on the employer. If the law requires that a hard hat or safety goggles be worn when a given task is performed, and a worker who has been issued these devices refuses to wear them, the company can be fined and given a citation.

Under this act the designer is charged with building an inherently safe plant. He is charged with building it in accordance with the best safety standards available. No plant should be designed that requires employees to wear earmuffs or ear plugs or requires that temporary barriers should be erected for safety purposes when safety can be achieved by some other means.

To assist the engineer in this effort a large number of organizations have de- veloped safety standards and suggestions. In a series of three articles lists these organizations and discusses their functions. He lists the subject areas in which they may be of assistance and lists a number of their publications that might be useful to the process engineer. Many of the codes developed by these organizations have been adopted as federal standards.

On May 29, 1971, the standards of the OSHA act were published in the Federal Register. A revised version of these was published on October 18, This includes regulations on the exposure of employees to hazardous chemicals. For some chemicals an absolute maximum concentration that may be present in the atmosphere is given. For others the standard merely says the 8-hr weighted average may not exceed a given level. For some substances both limits are given, plus a peak limit for a given period of time. For instance, benzene has an 8-hr weighted average maximum of 10 ppm and an accepted ceiling concentration of 25 ppm. However, for 10 min of an 8-hr shift it is permissible for the concentration to reach 50 ppm.

The code also sets maximum noise and radiation exposure levels. It requires that all stored liquified petroleum gases have an agent added that will give a distinct odor as a warning against leaks. It gives codes for the storage of flammable or combusti- ble liquids. For these substances it prescribes the minimum distances between storage vessels, between the vessels and the property line, and between the vessels

Safety 93

and buildings. These distances depend on whether the material has boilover charac- teristics, whether the liquid is unstable, the type and size of tank it is to be stored in, the type of protection provided for the tank, and whether the tank is above ground or buried. Standards are also given for compressed gases such as nitrous oxide, hydrogen, oxygen, and acetylene. Specific regulations for such industries as tex- tiles and pulp and paper are promulgated.

These regulations must be compared with those of the Environmental Protection Agency. Since the goals of the two federal agencies are different it cannot be predicted which standards will be more stringent. All that can be said is that both must be met.

The degree of detail present throughout the OSHA regulations can be illustrated by the following items from the section on sanitary standards. There is a require- ment that toilet facilities should be located within 200 ft (70 m) of an employee’s normal work area and that he should not be required to climb more than one flight of stairs to reach them. The toilet facilities must provide hot and cold water, hand soap, and towels. Not only does it state the ratio of water closets to persons but it states that the walls must be at least 6 ft high and must be a maximum of 1 ft off the ground. Further, each cubicle must have a door latch and a clothes hanger.

When toxic materials or injurious dusts may be present, it specifies that a separate lunchroom must be provided. If the maximum number of people using the lunchroom is less than 25,13 (1.4 per person must be provided. The required square footage gradually decreases to 10 (1 when the number of people exceeds 150.

Just as with the Environmental Protection Agency (EPA) standards, these will also be added to and revised. They are given in the Code of Federal Regulations, under Title 29 (Labor), Chapter 17, Part 1910. l3 To keep up to date one should follow the same procedure given previously for EPA standards.

To enable the government to determine where hazardous areas may be, each company is required to keep track of all injuries and fully document their causes and what is being done to prevent a recurrence. For comparative purposes the company usually also determines the disabling injury frequency rate. This is the number of lost-time injuries per million man-hours worked. A lost-time injury occurs whenever an employee is unable to report to work at his next scheduled time as a result of an accident that occurred while he was working. A lost-time injury could occur if an employee carrying hot coffee tripped and burned himself. Another lost-time injury would result if a person were killed in a boiler explosion. This method of accounting does not take into account the severity of the injury. One graphic way of looking at this statistic is to consider that the average person works 40 hours a week or approximately 2,000 hours a year. Then a lost-time injury rate of 5 would set the probability of any employee having a lost-time injury at 1% per year. In an attempt to measure the gravity of the injuries, a severity rate is calculated in terms of days an employee is unable to report for work due to injuries per million man-hours worked. To compute this, fatalities and permanent total disabilities are arbitrarily assessed at 6,000 days per case. Also, when permanent impairment of some employees’ facility occurs, in addition to the days missed a small allowance is

added to account for any loss in the employees’ efficiency that may be due to the impairment. The record for a selected group of industries is given in Table 4-5.

According to the Bureau of Labor Statistics, the injury frequency rate for all manufacturing companies rose from 11.8 in 1960 to 15.3 in 1970. The National Safety Council estimates that there are around 15,000 job-related deaths each year and another 2,300,OOO workers suffer disabling injuries. The total cost associated with these accidents is nearly $9,500,000,000/yr. These figures are conservative,

since they include only those companies that belong to the National Safety Council, and these companies are considered the most safety-conscious ones.

Table 4-5

1970 Safety Record for Various Industries

Industry

Federal civilian employees Electric, gas, and sanitary

services

Electrical machinery equipment and supplies

Chemicals and allied products Textile-mill products

Petroleum refining and related industries

Paper and allied products Machinery (except electrical) Manufacturing (U.S. average) Primary metal industries Rubber and miscellaneous

plastics products Fabricated metal products Metal mining and milling Stone, clay, and glass products Nonmetal mining and milling Contract construction Food and kindred products Coal mining and preparation

Frequency Rate Disabling Injuries per 1.000.000 Man-Hours 6.6 6.6 8.1 333 8.5 562 10.4 579 11.3 1116 13.9 937 14.0 583 15.2 759 16.9 1128 18.6 795 22.4 1003 23.7 3238 23.8 1540 24.1 2624 28.0 2100 28.8 1156 41.6 7792 Severity Rate Time Charges per

1 ,OOO,OOO Man-Hours

554

813

Source: Statistical Abstract of the United States 1973, US. Department of Commerce, U.S. Government Printing Office, Washington, D.C., 1973.

For their own benefit, companies record not only injuries but near misses. These are accidents that could have, but did not, result in a lost-time injury; Upon analyzing these, problem areas can be discovered and improvements made before a major disaster occurs. It can also be determined which men are accident-prone.

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These are people whose psychological makeup causes them to attract trouble. When such a man is detected, he must be placed in a position where the probability of his being injured or causing an injury is very low.

In order to promote safety, companies have contests, conduct periodic safety meetings, give prizes, award plaques, and conduct big advertising campaigns. Managers feel that making the employee aware of safety will help improve the company’s over-all performance. Experience has shown them to be correct.

Not only does the government have standards but, as noted previously, before a plant is insured the insuring company requires certain safeguards. Insurers will also suggest many others that, if adopted, may result in a lower rate. Since it is impossible to design a plant in which no accidents can occur, the engineer must always weigh the cost of the unrequired safeguards against the probability of an accident. If the cost is high and the probability is low, he may accept the risk. The insurance companies will naturally try to get as many of these measures installed as possible, since this reduces their risks.

Even with all this help the engineer must still scrutinize his plant and try to anticipate what type of losses can occur. This study usually begins by noting that nearly all losses are due to explosion, fire, and/or mechanical failure. Then the possibility of each type of loss is evaluated.

In determining the probability of fire and explosions it is important to be able to classify chemicals by risk. According to these classifications, various appropriate safeguards can be installed. The Dow Chemical Company has developed a process safety in which a Fire and Explosion Index is calculated for each unit of the plant. This unit may be a processing area such as the reaction area or a physical region such as a storage area or a finishing building. The index is based on the

material factor of the most hazardous material present in significant quantities.

“Significant” means that the substance is present in a high enough concentration to represent a true hazard. For instance, when a hazardous material is a reactant it may be present in such small concentrations after the reaction-vessel stage that it poses no threat to man or equipment. It then would be the most hazardous sub- stance only in the storage, feed, and reactor areas, and another material would be the most hazardous in the other areas.

Thematerialfactor is a number between 1 and 20 that indicates the susceptibility

of a compound or mixture to tire or explosion. A list of these factors for specific compounds is given in reference 15 along with their flash points, autoignition temperatures, and explosive limits. This factor is then adjusted for special material hazards such as the presence of oxidizing materials or spontaneous heating, general process hazards such as reactions or physical changes, and special processing hazards such as high or low pressures or temperatures. The result is the Fire and Explosion Index.

The protective features recommended depend on this index and are given in reference 15. For instance, for an area subject only to fire an explosion or blast wall is not required unless the Fire and Explosion Index exceeds 40. For an index below 20 a ventilation rate that would result in total change of the air in the building every

30 min is adequate. If the index is between 20 and 40 the air should be completely changed every 6 min. When the index exceeds 40 this should be done every 4 min.

The National Fire Protection Association classifies liquids by their explosion and flame-propagation These ratings are then used to specify the type of electrical equipment required. These standards have been adopted by OSHA. gives a procedure for obtaining the material classifications of individual compounds and mixtures.

The Manufacturing Chemists Association uses a different It deter- mines an explosion and a fire-hazard classification for each unit.

presents a method for calculating the possible losses to a plant based on the MCA code and the determined probability of a mechanical failure. He calls this Systems Safety Analysis. It will determine whether more or less protection against possible losses is desirable. The first step is to determine all the possible events that could contribute to a given loss and assign a relative possibility to each. A loss analysis diagram is then constructed that indicates the relationship between these events. Sometimes two or three events must all occur before a loss occurs. Sometimes only one. The probability of the loss is then calculated and compared with a previously chosen maximum tolerable probability. If it is greater, more safeguards must be included. If it is less, the system is acceptable. The problem is determining the