Document Oz1z8Do23a4L370ze2p6GXmrX

1 industrial hygienists who u seek refuge from ethical #(; technologs as their lie. But the interpretation of >logy on health is the crucial al hygienist is to he identified ofessional. The industrial just a scientist whose ring a technically competent icr whose skills are applied to otential effect of the work e individual's health. It is, of conscience how the industrial J t this time perhaps the easiest .*mma is to define industrial ng, as a technical branch of the same ethic, the same phy and the,same social Jusirial hygienists may not If they do not, they must c or remain technologists. through an evolutionary 1 i 1 feels of technology on our e, the scientist working with the biologist carrying out ns is agonizing over his role. i I \ trial hygienist could take on guardian of the health ^0prise and leave the health irkers to the physician. 1 al hygienist's future role will and will evolve how he these difficult situations. part and parcel of the larger impact on the health of 1 I ( II I 1 i I i As part of a control technology assessment of the plastics and resins industry, a bulk polymerization process manufacturing polyvinyl chloride is surveyed. Effective control techniques for reducing workplace concentrations of vinyl chloride include a computerized process design, an automatic air monitoring system tor rapid leak detection, a novel local exhaust system, various equipment modifications to prevent leaks, and a PVC stripping operation. Personal and area sampling data are evaluated to determine the effectiveness of the overall control system. The control techniques can be applied to other processes where toxic materials are used. Engineering control assessment of the plastics and resins industry . . . case study: manufacture of PVC by bulk polymerization KENNETH S, SCHOUL77.'b S . JULIUS H. BOCHINSKI. Ph.D. and JAMES A GIDEON, M S. Enviro Control, Inc., One Central Plaza, 11300 Rockville Pike, Rockville, Maryland 20852, National Institute (or Occupational Safety and Health (NIOSH). Robert A Taft Laboratories. 4676 Columbia Parkway, Cincinnati, Ohio 45226 introduction A major portion of the research and development effort of the National Institute for Occupational Safety and Health (NIOSH) is directed toward the publication of criteria documents that present recommended standards for permissible employee exposures to potentially harmful chemical or physical agents in the workplace. The NIOSH documents provide the basis for regulatory standards of the Occupational Safety and Health Administration (OSHA) and are available to assist industrial health and safety programs. In order to demonstrate how' a given safe exposure in the workplace can be achieved, NIOSH is proposing a major, three-phase'v control technology program: (!) a series of industry-wide control technology assessments, designed to document and evaluate control technology options and outline research needs: (2) joint action by NIOSH and industry in research and development towards selected aims, primarily through industrial use of NIOSH demonstration grants; and (3) dissemination of the program results to aid industry in applying the control technology. The plastics and synthetic resin industry, including the manufacture and compounding of thermoplastic and thermosetting resins and synthetic rubber (SIC codes 2821, 2823, 2824), was chosen for the first assessment. The contract was awarded to Enviro Control, Inc., (ECI) in July 1976, and the final report will be completed in November 1977. ECI has completed in-plant studies of the control technology used in 16 polymerization and compounding processes, which provide a good representation of the type and extent of worker protection strategies in the industry as a whole. Particular attention has been given to the polyvinyl chloride (PVC) portion of the industry because of the extensive development and application of controls for vinyl chloride monomer (VCM) during the past several years. In general, the processes used to make different polymers are similar enough across the industry for intra-industry transfer of control technology to be practicable. The following discussion presents the results of one case study of the manufacture of PVC by bulk polymerization, / Assoc J (38) December, 197? American Industrial Hygiene Association JOURNAl (38) 12/17 653 Vfent Figure 1 -- Bulk polymerization process for polyvinyl chloride nuisance dust and has an 8-hot average exposure limit of 15 particulates. It may become | screening, grinding, and baggir | noise ( A potential for employee expos I levels exists in the polymeriz; ' and grinding areas. I control of vinyl chloride me The VCM control system m under several headings: I. Process design, modific I maintenance l 2. Leak detection and pre ( 3. Local exhaust ventilati 4. General ventilation 5. Ongoing improvement: process summary As shown in Figure 1, the major steps in this process are prepolymerization, polymerization, monomer recovery, and resin handling. Prepolymerization involves the production of PVC nuclei from VCM in an agitated, vertical cylindrical tank. The procedure is to manually feed small quantities of additives into the tank through a spout, then pump in the VCM from the tank farm. A catalyst is used to initiate the reaction. When the desired VCM conversion level is reached in the prepolymerizer (usually <10%), the batch is gravity-fed into a horizontal cylindrical autoclave, where the polymerization reaction goes to completion. Agitation is provided by a ribbon blender in the autoclave. The heat generated during the reaction is removed by a reflux condenser located vertically along the autoclave axis, and by the waterjacketed vessel surface. At the completion of the reaction, unreacted VCM is vented to the condensers and compressors located in the monomer recovery area. Further monomer is recovered by stripping the resin of any unreacted VCM. Most of the resin is transferred automatically to the product collector by connecting the air conveyance system directly to the autoclave. The remaining resin is removed manually, usually without anyone's having to enter the vessel. From the product collector, the resin is dropped into a hopper, then passed through several screens to standardize the PV.C particle size. Oversize particles are diverted to a grinder and returned to the screens. The PVC product is pneumatically conveyed to storage or transferred to the compounding or bagging area. toxic chemical agents and harmful physical stresses . vinyl chloride monomer Federal regulations require that employee exposure to vinyl chloride shall not exceed an 8hour time-weighted average of 1 ppm, or 5 ppm for any 15-minute period. Large quantities of this material are present in the prepolymerizers, autoclaves, and recovery system, and it is an extreme engineering challenge to control workplace emissions to such a low level. The plant described here was neither designed nor installed with this degree of control in mind, and a large amount of retrofitting with engineering controls was necessary. Therefore, most of the engineering controls evaluated during this case study are directed towards reducing workplace concentration of vinyl chloride. polyvinyl chloride dust , Fully reacted polyvinyl chloride is considered a I, process design, modification,, I Inherent operating charaeteris. j operating characteristic ' polymerization process permit degree of employee exposu process is totally enclo polymerization autoclaves are | transfer. At this point, residua exceedingly low because of th I the stripping operation. I The most important charact j process is that the VCM doe t suspended or emulsified in a I This reduces the potentia j exposure by (1) obviatin; 1 postpolymerization separa: \ operations, (2) eliminatin j associated with solvent recovi .. contaminated wastewater f treatment, and (3) allowing i temperature recovery condens the potential for leaks and concentration in the off-gas. | Computer control ofprocess. ' is almost totally computer I reducing the possibility of I escape due to operator error ( operator errors do occur, ; analyzed to determine if add I measures can be integrated i 654 Am. Ind. Hyg Assoc. J (38) December. 1977 American Industrial Hygiene Association JOUHt GENC 001710 Vferrl "p-CZD ruiisance dust and has an K-houi time-weighted average exposure limit of 15 mg/m' for total particulates. It may become airborne in the screening, grinding, and bagging operations. I poise A potential for employee exposure to high noise levels exists in the polymerization, screening, and grinding areas. i Homer Bud - Reflux Dandvnsef. Bod Resin Outlet ^ Chute-----, Air Convcyei Fie>, Pump VCM Storage ktlk Shipments xompaundmg '399109 i control of vinyl chloride monomer Figure 2 -- Bulk process reactor The VCM control system may be described under several headings: 1. Process design, modification, and maintenance ' ccrtitrol system. In addition, computer control reduces the number of on-site operators and 2. Leak detection and prevention their time spent in potential exposure areas. The 3. Local exha*Ust ventilation operators spend a large portion of their time in '/ Chloride. 4. General ventilation safe exposure areas, such as screening and 5. Ongoing improvements grinding sites, and nonexposure areas, such as the control room, which is under positive i having to enter the vessel. ! collector, the resin is dropped then passed through several trdize the PVC particle size. are diverted to a grinder and icreens. The PVC product is conveyed to storage or compounding or bagging area. j i ' I process design, modification, and maintenance Inherent operating characteristics. The inherent operating characteristics of the bulk polymerization process permit a relatively high pressure. Bulk process reaction system. The bulk process reaction system shown in Figure 2 was designed to totally enclose the reaction and stripping steps and to reduce maintenance requirements on the degree of employee exposure control. The reactor and the VCM recovery equipment. The j process is totally enclosed until the polymerization autoclaves are opened for resin transfer. At this point, residual VCM levels are main features that result in reduced worker exposure are the following: gents and harmful physical omer j exceedingly low because of the effectiveness of the stripping operation. The most important characteristic of the bulk i process is that the VCM does not have to be 1. Stripping process: VCM stripping is undertaken in the autoclave following the polymerization cycle and is very important to the overall VCM control program. The general steps are as follows (refer to Figure <ns require that employee * suspended or emulsified in a liquid medium. 2): hloride shall not exceed an 8- I This reduces the potential for employee 1 average of 1 ppm, or 5 ppm J exposure by (1) obviating the need for a. At the completion of the polymer period. Large quantities of postpolymerization separation and drying ization cycle, multi-stage recovery esent in the prepolymerizers, operations, (2) eliminating the exposure compressors draw two successive covery system, and it is an j associated with solvent recovery or monomer- vacuums on the autoclave. ing challenge to control * contaminated wastewater disposal and b. During each evacuation, the autoclave is to such a low level. The I treatment, and (3) allowing the use of low^ is heated to facilitate VCM removal e was neither designed nor i temperature recovery condensers, which lessens from the resin. zgree of control in mind, and ( the potential for leaks and decreases VCM c. Nitrogen is used to break the vacuum etrofitting with engineering concentration in the off-gas. after the first evacuation to prevent the ary. Therefore, most of the s evaluated during this case awards reducing workplace ivl chloride. Computer control ofprocess. The process cycle a, is almost totally computer controlled, thus reducing the possibility of significant VCM escape due to operator error or failure. When formation of explosive VCM/air mixtures. d. Air is used to break the vacuum after the second evacuation. ist nyl chloride is considered a operator errors do occur, the reasons are analyzed to determine if additional prevention measures can be integrated into the computer e. The autoclave is opened for resin trans fer after the air pressures inside and outside the autoclave are equalized. l/f Assoc J. (38) December, 1977 American Industrial Hygiene Association JOURNAL (38112/77 655 2. Heat removal; In addition to the jacketed autoclave surface, a reflux vapor condenser is used to remove the heat generated during the polymerization reaction. Condensation of the VCM vapors on the condensers does not result in fouling of exchanger surfaces as would be the case if the exchanger were in direct contact with the liquid reacting mass. The on the prepolymerizcrs and autoclaves to minimize VCM loss to the atmosphere in the event of a rupture disc blowout. The handles of the valves preceding these two rupture discs are interlocked so as to automatically engage the spare rupture disc when the valve ahead of the ruptured disc is closed manually. This design feature counters human errors that could result in significant VCM emissions during an I I J | Se Applicatior Recovery of V Pennsylvania Pump for VCK Dunron Mode' Agitator on Prepolymenze Autoclave-Rib need to enter the reactor vessel for emergency. I maintenance purposes is therefore Prepolymerization entry procedure. In order to ^ substantially reduced. sampling.) Each of the r r "minimize potential exposufe when employees 3. VCM vapor filtering; The VCM vapbr must enter,the prepolymerizer, a vacuum of removed after the completion of the approximately 66 cm (26 in.) is pulled on the ' polymerization step must be free of vessel, after which the vessel is returne'd 'to particulates to avoid damage to compressors and peripheral equipment in the VCM recovery area. The filter assembly is housed (refer to Figure 2) in a dome or manhead above the autoclave. The filter assembly consists of filler atmospheric pressure with nitrogen. Nitrogen is used to avoid potentially explosive gas mixtures. This sequence is repeated once, and the second vacuum break is with air. To increase the effectiveness of this pre-entry procedure, there are plans to provide additional heating capacity medium mounted on a 61 cm (24 in.)- to this vessel to assist VCM removal. diameter coarse screen basket. Potential exposure due to opening and cleaning the filter is reduced by scheduling the cleaning after the autoclave stripping operation. An Vent stack location. The vent stack outlet is located approximately 24.4 m (80 ft.) above the top of the process building. This stack height was air conveyance system for the finished resin permits emptying of the reactor under negative pressure, and the flow of air into the autoclave through the resin transfer manway prevents any residual chosen to prevent vented VCM from entering this or any other process building by putting the outlet above the eddies induced by the building. The vent stack is used to intermittently dispose of small quantities of VCM during normal monomer from escaping into the operations and for venting VCM leaks until w'orkplace. proper maintenance operations can be performed. j l i ( I ( I 1, is sampled every 6 t continuous strip-chart r VCM results in parts individual reading from activates a warning lig! panel and in the process . greater than 5 ppm acti lights. The light syster indicates the need protection and initiates procedure. A computer upon detection of any Vi in excess of 900 ppm. alarm is sounded an evacuated. 2. Portable hydrocarbon d warning light is activa employee goes to the st and determines which recorded a VCM level ii He then takes a port, detector and uses it to lo VCM recovery operations. The VCM recovery operation was designed to minimize the use of compressors and to reduce the load on the compressors. The recovery area is shown in Figure 1. The recovery cycle begins with the VCM at pressures substantially above its con densation pressure. The VCM is passed directly to the condensers, until the system pressure teak detection and prevention Description of system. The process characteristics and modifications already described prevent extensive escape of VCM into the workplace; the major remaining emission Source is leaks from valves, flanges, and compressor, pump, and agitator seals. I leak has not stoppe j* promptly repaired, one ) available flexible exhau | and positioned to captu j escaping VCM unt < personnel can correct t j The circumstances of each and filed. A periodic evaluati drops to the VCM condensation pressure of 2.8 kg/cm2 (40 psi). At this point, the values on the lines bypassing the compressor are closed, the valve on the line to the compressor is opened, and the remaining VCM is compressed above the condensation pressure and then condensed in the chilled condensor and returned to storage. Rapid leak detection is an integral part of the VCM exposure reduction program. The detection system consists of two parts; each is essential to the program's effectiveness; 1. Process monitoring by gas chromato- graph: Air samples are collected in sequence from a number of strategically serves to pinpoint recurrir Important equipment featu; 1 equipment features reduce lc the potential for worker exp I 1. Valves: Various types < ! evaluated for specific J some instances, superio Dual blowout disc interlocks. Dual rupture discs located monitoring points. (The optimum 1 that reduced leaks (one under pressure and one spare) are employed locations are determined by extensive area / requirements. In gener 656 Am Ind. Hyg. Assoc J (38) December American Industrial Hygiene Association JOU zcrs and autoclaves to >ss to the atmosphere in the disc blowout. The handles of ig these two rupture discs are to automatically engage the when the valve ahead of the dosed manually. This design uman errors that could result CM emissions during an TABLE I Seals for Compressors, Pumps, and Agitators Application and Vendor Seat Manufacturer Seat Model No Recovery of VCM Pennsylvania Compressor Pump for VCM Dunron Model Mark II Agitator on Prepolymerizer Auloclave-Ribbon Pennsylvania Compressor Crane Crane Pechiney-Samt Gobain Vendor will supply details type 9T OPICi (316) Type 9B Vendor will supply details i entry procedure. In order to al exposure when employees irepolymerizer,' a vacuum of ' 1 1 cm (26 in.) is pulled on the :h the vessel is returned to ure with nitrogen. Nitrogen is ntially explosive gas mixtures, epeated once, and the second , 1 1 > with air. To increase the tis pre-entry procedure, there de additional heating capacity ssist VCM removal. on. The vent stack outlet is ately 24.4 m (80 ft.) above the ) ! j ' 1 ding. This stack height was nted VCM from entering ess building by putting the , eddies induced by the building,> used to intermittently dispose j ies of VCM during normal or venting VCM leaks until nance operations can be ^ d prevention .stem. The process characterise ions already described prevent >f VCM into the workplace; the emission source is leaks from and compressor, pump, and ( | :ction is an integral part of the reduction program. The consists of two parts; each is rogram's effectiveness: initoring by gas chromato samples are collected in om a number of strategically utoring points. (The optimum e determined by extensive area T < sampling.) Each of the monitoring points is sampled every 6 minutes, and a continuous strip-chart recorder plots the VCM results in parts per million.`Any individual reading from J ppm to 5 ppm activates a warning light in the control panel and in the process areas; a reading of greater than 5 ppm activates flashing red lights. The light system simultaneously indicates the need for respiratory protection and initiates the leak detection procedure. A computer alarm is activated upon detection of any VCM concentration in excess of 900 ppm. If this occurs an alarm is sounded and buildings are evacuated. 2. Portable hydrocarbon detector: When any warning light is activated, a designated employee goes to the strip-chart recorder and determines which monitoring point recorded a VCM level in excess of 1 ppm. He then takes a portable hydrocarbon detector and uses it to locate the leak. If the leak has not stopped or cannot be promptly repaired, one (or more) of the available flexible exhause hoses is opened and positioned to capture and exhaust the escaping VCM until maintenance personnel can correct the problem. The circumstances of each leak are recorded and filed. A periodic evaluation of these records serves to pinpoint recurring problem areas. Important equipment features. The following equipment features reduce leak occurrence and the potential for worker exposure: 1. Valves: Various types of valves have been evaluated for specific applications. In some instances, superior valves were found that reduced leaks and maintenance requirements. In general, most ball valves l Assoc J (38) Decembet, American Industrial Hygiene Association JOURNAL (38) 12/77 were phased out and replaced by butterfly valves. 2. Prepolymerizer; The' agitator shafts have Pfaudler oil seals (refer to Table 1) under nitrogen pressure greater than the maximum vessel pressure. It was reported that no replacements have been required for these seals in over 2 years, and leaks are infrequent. 3. Autoclave: The ribbon-mixer drive shaft seal (refer to Table I) is packed with grease under a pressure of approximately 14.0 kg/cm2 (200 psi), which is greater than the maximum working pressure in the vessel. Preventive maintenance is required twice a week to make adjustments, which generally consist of manually adjusting the piston in the grease cartridge to maintain the required grease pressure. The packing usually lasts 1 year. 4. Compressor seals: The recovery com pressor seals (refer to Table I) are pressurized with nitrogen at a pressure greater than the VCM pressure. Any leaks result in passage of nitrogen into the monomer recovery plumbing and build-up of noncondensable gases (N't, etc.) in the recovery system. The computer system reacts to this situation by alerting the operators to check the compressor seal. However, compressor seal failure has been very rare. 5. Pumps: Pumps used for transporting VCM or process streams containing VCM are located outdoors. Teflon packing material is used in pump seals (refer to Table I) because of its resistance to VCM. Whenever possible, process streams are transported by gravity flow. This is an excellent example of alleviating a potential 657 source of exposure by eliminating the need for leak-prone equipment. local exhaust ventilation The usual purpose of local exhaust ventilation systems in industry is to remove emissions from permanent sources that cannot be controlled by process enclosure or other methods. As the PVC process is fully enclosed and without permanent emission sources, a novel local exhaust strategy is necessary -- one flexible enough to deal effectively with leaks occurring in a variable and unpredictable manner. The system that has been evolved is effective and is efficient from an energy-conservation standpoint. Thirty-four process points were selected on the basis of potential for leakage for installation of permanent exhaust hoods or flexible-hose entry sleeves. Because many of the process points are identical and repeated for each operating line, the total number of distinct types of exhaust takeoffs is reduced to 11, as follows: 1. Autoclave filter manheads -- 11.32 m'/min. (400 cfm) 2. Vacuum break valves--6.23 m'/min. (220 cfm). 3. Autoclave drive end shaft seal -- 11.32 m'/min. (400 cfm) 4. Recovery manifold bleed valve -- 5.66 m'/min. (200 cfm) 5. Low-pressure VCM filters in the recovery area 12.74 m'/min. (450 cfm) 6. High-pressure VCM filters in the recovery area -- 12.74 m'/min. (450 cfm) 7. Prepolymerizer Yarway valves -- 10.76 m3/min. (380 cfm) 8. Additives entry funnel -- 8.5 m3/min. (300 cfm) 9. Autoclave shaft seal hood -- 11.32 m3/min, (400 cfm) 10. All blind flanges --9.1 m3/min. (300cfm) to 11.32 m'/min. (400 cfm) 11. Recovery compressor -- 17.0 m'/min. (600 cfm) The indicated airflow rates are those used in design calculations for estimating required blower capacity. Actual flow rates are equal to or greater than design values. The first seven of these exhaust points consist only of a duct connection sleeve into an enclosure surrounding the individual piece of equipment. The additives entry funnel exhaust (No. 8) consists simply of a flexible exhaust duct dropped into the funnel. The final three points consist of permanently affixed hoods that were designed to fit the geometry of the specific flange, autoclave, and compressor. Each of the hoods and enclosures can be exhausted by connecting it with 1 m (3 ft.) to 8 m (25 ft.) long individually accessible flexible exhaust ducts extending from a common main duct. An important secondary function of this system is the containment of leaks at process points other than those already listed. If there are no leaks in an area served by a given flexible duct, it is "deadheaded" or blocked off with metal plugs or branch dampers. The distinctive feature of the system is that it is used solely as an adjunct to the leak detection and prevention program: i.e., the exhaust is provided to a given hood or enclosure sleeve only when a leak is detected. The logic behind the system is simple and compelling. If the individual hoods or enclosures were exhausted constantly, leaks would be masked and the leak detection and prevention program would fail. This would be unacceptable because the overall VCM containment strategy hinges on engineering modifications that will prevent leaks rather than exhaust them. An additional objection to continual venting is the large expense of installing a system to clean exhausted air to meet the Environmental Protection Agency (EPA) requirements of no more than 10 ppm VCM in vented air. general ventilation The general ventilation system is shown in Figure 3. Under normal conditions the system provides approximately 19 air changes per hour during winter and 37 airchanges per hourduring summer or in emergency conditions. The emergency ventilation system is operated manually when the gas monitoring system detects 900 ppm or more of VCM. The system is designed to induce a relatively consistent airflow pattern from the south to the north end of the building. The exhaust fans are located on two levels to correspond to the two I A.rG^ing` Hr wWjtfK 19 aimmer 37 ertkergency 37 Recovery Compress Room . i . U ; ,__ 1 * f 40 ' EF - Exhaust RV - floo< Ve Fn process areas, separated by an / This assists in reducing the amo \ through the grating so that a le area will not cause high VCM 1 I A solid floor was installs grating between the penthou: process building. This floor has excursions in the penthous 1 segregating it from the process | may occur. The benefits from the syster I primary importance is the hig which dilutes any VCM leakag< j direction of flow is such that lei j* unlikely to exert a large infiuen I, The VCM recovery room at building was originally not pa the main process building. It that VCM leaks in this area wc areas of the main building. Bt 7 infrequently entered by connecting space was almost t i off with a block wall, causing a l airflow of about 0.762 m/s (l i open sections in the parti [ modification, no further e. threshold concentration have , leaks from the recovery j compressor, I ongoing improvements j Major modifications are n 658 Am Ind Hyg, Assoc. J. (38) December 1977 American Industrial Hygiene Association JOUR! f these exhaust points consiM nnection sleeve into Bll p the individual piece of ives entry funnel exhauM ply of a flexible exhaust duct jnncl. The final three point;, I ntly affixed hoods that were I c geometry of the specific i tnd compressor. Each of the urcs can be exhausted be I m (3 ft.) to 8 m (25 ft.) long ible flexible exhaust ducts common main duct. An ry function of this system is leaks at process points oilier listed. If there are no leaks in a given flexible, duct, it is ocked off with metal plugs or EF s Exhaust Fan RV Root Ventilation Figure 3 -- General ventilation system ature of the system is that it is idjunct to the leak detection ogram: i.e., the exhaust is :n hood or enclosure sleeve s detected. d the system is simple and tdividual hoods or enclosures onstantly, leaks would be ak detection and prevention ^Hhis would be unacceptable ^K2M containment strategy ing modifications that will er than exhaust them. An n to continual venting is the nstalling a system to clean meet the Environmental (EPA) requirements of no VCM in vented air. lation system is shown in irmal conditions the system 9tely 19 air changes per hour 7 air changes per hour during nergency conditions. The ation system is operated ie gas monitoring system more of VCM. signed to induce a relatively attern from the south to the ilding. The exhaust fans arc ;ls to correspond to the two process areas, separated by an open grate floor. reducing environmental emissions in com* This assists in reducing the amount of air flowing pliance with the newly promulgated EPA . through the grating so that a leak in one process regulations for vinyl chloride. These area will not cause high VCM levels in the other. improvements are expected to have a beneficial A solid floor was installed over the open effect also on worker exposure rates. Two major grating between the penthouse and the main improvements have been planned: process building. This floor has eliminated VCM 1. The installation of a treatment system for j excursions in the penthouse by effectively reducing VCM concentrations in air segregating it from the process area where leaks exhausted to the atmosphere from the 1 may occur. recovery system to 10 ppm by carbon i The benefits from the system are twofold. Of adsorption or solvent absorption. primary importance is the high air change rate, 2. Better procedures before opening reaction which dilutes any VCM leakage. Also the overall vessels, to meet EPA's requirement of no direction of flow is such that leaks in one area are more than .02 g VCM (in vapor space of 1 unlikely to exert a large influence on other areas. reactor) per kg of resin product. The VCM recovery room at the east end of the Improvements may include an increase in building was originally not partitioned off from the number of successive vacuums pulled the main process building. It was determined on the vessel prior to opening. that VCM leaks in this area were affecting other areas of the main building. Because this area is vinyl chloride monomer monitoring ~~ control * infrequently entered by personnel, the system effectiveness ' connecting space was almost totally partitioned V A full-scale VCM monitoring program was off with a block wall, causing a measured inward initiated in early 1974 and extensive data have > airflow of about 0.762 m/s (150 fpm) through been recorded. Area sampling data are provided open sections in the partition. Since this by the gas chromatograph monitoring system. modification, no further excursions above From 1974 to early 1976, all data points were 4 threshold concentration have been induced by recorded and summarized as weekly averages. leaks from the recovery condenser or To illustrate the effect of implemented control compressor. measures, monthly area sample averages are plotted in Figure 4, The reduction from 1974 to ongoing improvements the first quarter of 1975, when most engineering Major modifications are now planned for controls had been installed, is striking. Hyg Assoc J (38) Decent*', 19^ American Industrial Hygiene Association JOURNAL (38) 12/77 659 GENC 001715 * Reading <1ppm VCM April May Juf>a Juty August Spiemb*r Oelob*f 90 92 94 91 91 95 92 Figure 4 -- Monthly gas chromatograph data averages. Since March of 1976, area sampling data have been summarized in terms of percent readings under 1 ppm, as shown in Figure 4, where it will be seen that 90% or more of all VCM measurements are consistently below 1 ppm. Each employee is also monitored every month with charcoal tube sampling devices, worn over a full shift and thus showing actual 8-hour timeweighted average exposure. Exposures of workers during 1974, 1975, and 1976 are summarized in Table II, where the reduction from 1974 to 1975, after installation of many controls, is very marked. TABLE II Average Distribution of VCM Results Period 1974 1975 1976 <1 2% 43% 73% VCM, ppm 1-3 3-5 5-10 34% 18% 22% 37% 8% 9% 17% 3% 5% >10 24% 3% 2% It is unfortunate that there are no available data to reflect the relative effectiveness of each control technique. However, it is clear that no one control is independently sufficient to meet the 1 ppm VCM standard. The program to reduce VCM exposure is not yet complete. Efforts are continuing to make better use of the currently installed controls, and where needs are pinpointed by the leak detection program additional controls are being implemented. personal protective equipment for vinyl chloride monomer The requirements for respiratory protection from VCM exposure have been integrated into the monitoring alarm-light system. Employees are required to use the full-face supplied-air line system when the amber light (reading of greater than 1 ppm) is activated. However, they may use a short air line and move from one air line connection to another. Respiratory protection is also required when workers are performing tasks that are known to cause exposures (e.g., changing flanges on VCM lines or entering process vessels). When the flashing red light (reading of greater than 5 ppm) appears, the fullface suppjied-air line system again must be used, but a long air line is required and mobility is decreased. If the monitoring system picks up a reading of greater than 900 ppm, an alarm is sounded and the building is evacuated. In this situation, a self-contained air pack is required for reentry. v When employees enter an autoclave for cleaning, a Tyvek suit, a hood, and work gloves are required. Work uniforms are provided daily and showers are recommended but not required. control of other potential hazards polyvinyl chloride dust PVC resin may become airborne from leaks in the screening and grinding system. This is considered nuisance dust with a permissible I time-weighted average exposur I mg/ m\ Dust control is effected I V the enclosed integrity of the scree I by ensuring good mechanical 1 keeping all enclosures under a pressure. The latter is accr providing small ducts from transfer system to the hopper; grinders. Horizontal surfaces e | cleaned of settled resin with i vacuums. Operators rarely hav< I areas, so isolation is considered control. It *' noise j Noise readings in the polymer average from 90 dBA to 92 d sources of this noise are the agi the prepolymerizers and auto steam mixer that has not beer , Figure 5. Due to the fact th? | average exposure time in the a hours per day, hearing protect required. However, plant sa personnel fee! that no exten tolerable and hence require h< for employees who will be in th excess of 1 hour at a stretch. abatement control modificati ' The mixing of cooling water v steam in a pipe induced substar ! and was causing very high i . situation was corrected by rel j mixing jets on the inlet side of T Figure 5). Before-and-aftet levels were not available for |t discussions with various or* | that this modification was ve j The resin transfer blowers 1 were exceptionally noisy. T1 | totally enclosed in a block ho these areas were reduced to v I / 660 Am Jnd, Hyg Assoc. J (38) December, 1977 American Industrial Hygiene Association JO (jnie-weighted average exposure limit of 15 Cooling Dust control is effected by maintaining Water Stunm Supply ilic enclosed integrity of the screens and grindei, ensuring good mechanical fits, and by keeping all enclosures under a slight negative pressure. The latter is accomplished by providing small ducts from the pneumatic i Reading <lppmVCM April 90 **1' June 9.' 94 July 91 ** SAeupgtuesmt ber 9p1(, October g? [ransfer system to the hoppers, screens, and grinders. Horizontal surfaces are periodically cleaned of settled resin with industrial-grade vacuums. Operators rarely have to enter these areas, so isolation is considered a very effective control. * dvergges. noise Noise readings in the polymerization building Figure S -- Steam-water miner. average from 90 dBA to 92 dBA. The major sources of this noise are the agitator motors for live equipment for vinylchloride the prepolymerizers and autoclaves, and one steam mixer that has not been modified as in nls for respiratory protection Figure 5. Due to the fact that the operators' osure have been integrated into average exposure time in the area is less l"han 4 alarm-light system. Employees hours per day, hearing protection is not legally use the full-face supplied-air line required. However, plant safety and health : amber light (reading of greater personnel feel that no extended exposure is ctivated. However, they may use tolerable and hence require hearing protection : and move from one air line for employees who will be in the reaction area in er. Respiratory protection is excess of 1 hour at a stretch. Only two noise- .'orkers are performing tasks abatement control modifications were noted. n to cause exposures (e.g.. The mixing of cooling water with high-pressure s on VCM lines or entering steam in a pipe induced substantial line vibration When the flashing red light and was causing very high noise levels. This er than 5 ppm) appears, the full situation was corrected by relocating the steam line system again must be used, mixing jets on the inlet side of the pump (refer to ine is required and mobility is Figure 5). Before-and-after sound-pressure monitoring system picks up a levels were not available for comparison, but :r than 900 ppm, an alarm is discussions with various operators indicated building is evacuated. In this that this modification was very successful. ;ontained air pack is required The resin transfer blowers (located outside) conclusion On the whole, the engineering challenge of controlling VCM in the workplace to 1 ppm has been met in the PVC bulk polymerization process. The VCM control system consists of a variety of engineering controls, each one of which must function in order to achieve the required conditions. The emphasis is on process enclosure and .good design to minimize the potential for VCM escape to the workplace due to equipment failure or operator error. Ventilation is used to supplement the process design and an ongoing program of improvements has been planned, to reduce both in-plant emissions and environ mental pollution. The principles implemented to control worker exposure in the bulk PVC process are generally applicable to a variety of other chemical operations and specifically applicable to other plastics manufacturing processes. A final report of all 16 case studies, with ees enter an autoclave for suit, a hood, and work gloves were exceptionally noisy. These blowers were's recommendations for technology transfer and totally enclosed in a block house. Noise levels in additional research and development, will be these areas were reduced to well below 90 dBA. available in late 1977. is are provided daily and nmended but not required. Accepted August 8 1977 potential hazards dust rcome airborne from leaks in d grinding system. This is ice dust with a permissible Assoc J (38) December. 1977 American Industrial Hygiene Association JOURNAL (38) 17/77 661 GENC 001717