By Mr Tan Kia Tang, Senior Assistant Director, Department of Industrial Health, Ministry of Manpower

Introduction

Fugitive emissions are “leaks” or “releases” that occur whenever there are discontinuities in the solid barrier that maintains containment. Sources of fugitive emissions include pumps and compressors, storage and processing vessels, loading facilities, flow control and pressure relief valves, and leakage from pipelines carrying materials from one process to another. Fugitive emissions are usually small in quantities but are the origin of the continuous background exposure of workers. This paper provides some examples of engineering measures and technological innovations to control and abate the common sources of fugitive emissions from chemical processing plants, petrochemical complex and refineries. Due to the extent and complexity of the chemical and petroleum industries, enumeration of sources of fugitive emissions would require detailed analysis of individual process unit operations.  It is beyond the intent of this paper to discuss in detail all sources of emissions and potential engineering control applicable to these emissions.

Pump Emissions

Pump and compressor seals are a large source of emission leakage contributing to workplace exposures. In centrifugal pumps, the main source of leakage is the drive shaft that passes through the impeller casing, while in reciprocating pumps, the emissions are through the openings in the cylinder or fluid end, which actuates the piston. Proper sealing of the annular clearances between shafts and casings can reduce pump leakage. The packed seals can be replaced with more effective mechanical seals. The mechanical seals use springs to press the rotating element and stationary seal member in the stuffing box to reduce fugitive emissions. Several types of mechanical seals are available viz single, dual and double mechanical seals in the increasing order of superiority. Highly toxic or hazardous liquids can be transferred by sealless or canned pumps, which are magnetically driven. These encapsulated pumps have no shaft entry and thus eliminate seal leakage.

Loading Emissions

Loading of petrochemical or petroleum products is another major source of emissions. In conventional loading-by-loading racks or loading arm assemblies, the hydrocarbon vapors in the filling tank are displaced by the incoming liquid thus causing vapors emissions in the vicinity of the loading operators. Splash or top filling generates turbulence and thus agitates the liquid being loaded and produces more hydrocarbon vapors. A better solution is through bottom loading which introduces the incoming liquid under the surface of the liquid in the tank. Quick coupling valves and lines have facilitated the conversion from splash filling method to bottom loading techniques. A further method of control fugitive emissions in loading operation is to provide a vapors return line to duct the displaced vapors to a suitable collection device such as vapors absorption or recovery system, or pollution control system such as an incinerator or a flare stack.

Simplified Sketch of Typical Sealless Pump

Pressure Relief Valve Emissions

In petrochemical and refinery operations, pressure relief or safety valves are used to protect process vessels from over-pressurization. These valves are usually spring loaded to effect closure through a disk, which is held against process pressure. When the spring set pressure is exceeded by a high process pressure, the disk moves, releasing process fluid, thereby relieves the process pressure. The disk is reseated when the process pressure falls below the set pressure. However, disks do not always reseat properly, and corrosion of the valves often reduces the efficiency of re-seating which results in emission leakage. A rupture disk can be installed upstream of the pressure relief valve to minimise fugitive emission. The disk is a thin metal dish and is designed to burst at a specified pressure. It separates the process fluid from the safety relief valve and thereby prevents leakage through the valve. To check against backpressure on the rupture disk resulting from a “pin-hole” leak, a pressure gauge could be provided between the disk and the relief valve. For highly toxic materials (e.g., substances with acute effects or chemical carcinogens) relief valves should discharge to a closed vent system either to flaring or through emission control equipment. Where this is not practicable, the discharge should be vented through an elevated vent.

Pressure Relief Valve with Upstream Rupture Disk

Release from Sampling Lines

A source of potential exposure to toxic hydrocarbons in petrochemical plants and refineries is during manual batch sampling operations. Conventional method of taking samples of streams for laboratory analysis requires draining sufficient liquid through a line before a representative sample is collected in a container.    A better method to control fugitive emission of the material being sampled is by using a closed loop sampling system. The loop is first purged or flushed with the material being conveyed. The valve drain to the container is then opened and the sample is collected. The drain valve connection to the sampling loop is short and the amount of ‘dead” space is small, thereby reducing sample contamination.      A much better way of collecting toxic stream is by using a sample “bomb” where the flushing takes place through the loop and the “bomb” itself. The “bomb” with the sample collected is disconnected from the loop for laboratory analysis. The best sampling control is using automatic in-line automatic analyzer sampling system. Although this automatic sampling is not entirely free of fugitive emissions, it greatly reduces potential toxic exposure problems posed by manual sampling.

Emissions from Bulk Storage Tanks

Fugitive emissions from bulk storage tanks depend on the types and conditions of storage. The following are various types of storage tanks in increasing order of fugitive emission control.

Fixed Roof Tank with Atmosphere Vent This type of storage tank is used for storing low toxicity liquid. It provides the least control over fugitive emissions. When the tank is being filled, the displaced vapour passes through the vent directly to the atmosphere. During emptying of liquid from the tank, air from the atmosphere passes through the vent into the tank. Fixed Roof Tank with pressure-vacuum Vent  A pressure-vacuum vent or “breather” is used to compensate for diurnal effects, with pressure building during the heat of the day, and evening coolness or rain resulting in a reduced pressure which usually is slightly below atmospheric pressure. The latter condition opens the vacuum vent, admitting atmospheric air to balance the pressure. External Floating Roof Tank  A roof is constructed to float on the surface of the liquid. The roof is sealed against the walls of the tank to significantly reduce evaporative losses. However, bad weather conditions often create problems for this tankage control system. Internal Floating Roof Tank An improvement on the external floating roof tanks is the installation of a fixed roof that provides protection against the sun and rain. Top and open side vents are provided on the fixed roof to allow dilution venting of evaporated vapour that may accumulate in the vapour space. Closed Floating Roof Tank In a closed floating roof tank, the vents on the fixed roof are closed and a pressure-vacuum vent provides relief for any pressure variation in the vapour space. Injection of an inert gas e.g. nitrogen provides blanketing in the vapour space to achieve an essentially zero emission tank.

Closed Floating Roof Tank

Valves, Flanges and Open-ended Pipes

Valves are the major contributors to fugitive emission losses. A variety of valves are available for use as block, manual and automatic control valves. Packing glands are used in these valves to reduce stem emissions. These include asbestos, graphite or carbon, glass fiber, plastic and metal packings. The performance of these packings varies and the choice depends on the process temperature, pressure and other factors. Where available, emission data on the control effectiveness of packing should be obtained from manufacturers before purchase for installation. Innovative types of valves have been developed to contain toxic or corrosive fluids and to eliminate fugitive emissions. These valves do not have packing glands or use packings as secondary protection. They are of the hermetically sealed types such as bellow, diaphragm and pinch valves. Flange emissions are a relatively large fraction of total uncontrolled emissions in the process industry. The emission rates depend upon the gasket materials, surface roughness and the flange-bolting system. The standard gasket for many years in chemical and petroleum plants has been the asbestos gasket. A wide variety of non-asbestos gaskets are available either in sheet, spiral wound, envelope or jacketed forms. These are made of Teflon, flexible graphite, glass and ceramic fiber. They are selected in accordance with their physical characteristics to meet process pressure, temperature and chemical resistance requirements. However, limited information on emission data of these gaskets is available. Welded pipes represent an improvement over flanged pipes. Welded connections eliminate leaks and should be considered where toxic leakage is a continuing problem in existing plants or may be a potential exposure problem in new plants. All open-ended valves or lines should be equipped with a cap, plug or blind flange, or a second valve to effectively seal the line and thus prevent fugitive emission leakage to the atmosphere. Where toxic liquids or vapours may potentially leak from a bleed valve, the bleeder outlet should be connected to a closed drain with leakage indication using a level glass or rotameter.

Closed Container

Summary

Valves, pressure relief devices, and flanges are the major common sources of fugitive emissions. Newer and improved packing and gasket materials should be investigated for installation in new plants or as replacements during maintenance on existing process units. Pumps and compressors are a large source of fugitive emissions. Other sources of fugitive emissions include bulk loading facilities, storage tanks and manual sampling operations. Engineering innovations are the best solutions of eliminating or minimizing these fugitive emissions. Engineering controls should be considered at the plant design stage, as these will be more economically installed than after the plant is operating. The largest contributor to in-plant emissions is lack of maintenance of plant equipment. Regular monitoring and maintenance is imperative for detecting and controlling fugitive emissions and leaks.