There is a potential for fire and/or explosion problems associated with drying biosolids. In a continuing effort to make the indirectly heated dryer the safest and most reliable biosolids dryer in the market place, we employed an independent company, Packer Engineering, to conduct a safety audit of the indirectly heated dryer. As a result, several opportunities for improving the overall safety features of the indirectly heated dryer were identified.
The following items have been implemented or improved in the indirectly heated dryer’s design and are now standard features.
- Designed per applicable guidelines of the National Fire Protection Association (NFPA)
- Relief Management System (NFPA 68)
In the event a dust explosion does occur, the dryer unit and the dry product storage vessel are equipped with explosion relief vents that will relieve a sudden over-pressurization. When a deflagration event occurs, the sudden release of energy and the associated increase in temperature causes an over-pressurization. In order to mitigate the effects of the over- pressurization, rupture discs and panels are installed to provide a means for the pressure and air volume to be vented in a safe manner. Using the methods dictated in NFPA 68 guidelines the vents are sized and rated based on the volume of the vessel they are protecting and the dust characteristics present.
- Inert Blanket (NFPA 69)
- All combustion and explosive reactions require a minimum oxygen concentration. The MOC for dried biosolids combustion is in the range of 6 -10% by volume. Maintaining the oxygen concentration below the MOC will significantly reduce the probability of combustion or explosion. During normal operation steam is produced from the water in the sludge. As a result, an inert steam blanket is provided over the entire dryer volume, reducing the oxygen concentration. However, at startup and shutdown, supplemental water is injected into the drying chamber to produce steam in sufficient quantity to minimize the oxygen concentration per NFPA 69 guidelines.
- For a dust explosion to occur, a flammable atmosphere must be present. Test have shown that the minimum explosive concentration (MEC) of dried biosolids range from 53 to 320 g/m3. Through the indirectly heated dryer's safeguards the water spray and steam minimize dust concentrations in the dryer system.
- Components Designed and Supplied (NFPA 654)
NFPA 654 set standards for the prevention of fire and dust explosions from the manufacturing processing and handling of combustible particulate solids. NFPA 654 is used in the design of the indirectly heated dryer’s material handling equipment and electrical components.
- Electrical and Building Classification (NFPA 499 and NFPA 820)
NFPA 499 guideline and NFPA 820 guidelines are used to determine the electrical and building classification of the dryer equipment and the dryer location. The determination of the classification of the dryer facility falls under the “authority having jurisdiction”, typically the local fire authority. The dryer facility is typically classified three ways;
- Class II, Division 1- “in which combustible dust is in the air under normal operating conditions in quantities sufficient to produce explosive or ignitable mixture or where mechanical failure or abnormal operation of machinery or equipment might cause such explosive or ignitable mixtures to be produced.”
- Class II, Division 2-“ where combustible dust is not normally in the air in quantities sufficient to produce explosive or ignitable mixtures, and dust accumulations are normally insufficient to interfere with the normal operation of electrical equipment or other apparatus, but combustible dust may be in suspension in the air as a result of infrequent malfunctioning of handling or processing equipment.”
- Unclassified- “Release of ignitable dust suspensions from some operation and apparatus is so infrequent that area classification is not necessary.
- Spark Detection System
Working in conjunction with the product temperature safeguards, a Factory Mutual approved spark detection system monitors and responds to prevent any sparks or embers from entering the storage vessel through the product conveyors. In the event the system detects a spark or ember an alarm will sound, the product will be doused with water and diverted away from the storage vessel until a preset time after no more sparks are detected.
- Safety Interlocks with Visual and Audible Alarms
- PLC Security Screens (pass code protected)
- The different operation screens of the indirectly heated dryer’s HMI are pass code protected as a security and safety feature. The indirectly heated dryer’s Electrical Engineers work with the municipal management team to establish the security levels of their employees with relation to the HMI’s operational controls (screens).
- The control system is housed within a UL labeled NEMA 4 enclosure.
- Hot Oil Heater and Hollow Flight Auger
- ASME design, built, tested and code stamped.
- Registered with the National Board.
- All new systems will have the Oil Heat Exchanger isolated as per NFPA 221.
- Indirectly Heated Dryer Feed Hopper
Feed hoppers on new installations will be supplied with a weigh scale sensor for level determination and will be monitored by the systems PLC. This will provide valuable information to the PLC and aid in the safe operation of the dryer and its components.
- Closed System
The indirectly heated dryer dehydration chamber operates under a negative pressure, which helps to maintain a clean and safe work environment within the dryer building.
From process philosophy to safety of operation, the indirectly heated dryer stands alone among indirect biosolids dryers. The philosophy behind the development of the indirectly heated dryer has always been to create a safe and effective sludge drying system that will minimize the consumption of energy, maximize the reduction of biosolids volume, protect the environment from unwanted emissions, and offer a high degree of reliability. This has not changed, and we guarantee it!
MOC - Minimum Oxygen Concentration
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Deflagration
A deflagration is an explosion in which the reaction front moves at a speed less than the speed of sound in the medium driven by the rate of heat transfer.
Any material that will burn in air when in a solid form may explode when it is in the form of a finely divided suspended powder. Biosolids are composed primarily of carbohydrates, proteins and fats, and will burn readily in the solid form. In order for a deflagration to occur, there must be (1) dust present in an explosive concentration range, (2) oxygen present and (3) a source of ignition.
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MEC- Minimum Explosion Concentration (g/m3)
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PLC - Programmable Logic Computer
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HMI - Human - Machine Interface
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ASME - American Society of Mechanical Engineers
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The National Board- The National Board of Boiler and Pressure Vessel Inspectors was created in 1919 to promote greater safety to life and property through uniformity in the construction, installation, repair, maintenance, and inspection of boilers and pressure vessels. The National Board membership oversees adherence to codes involving the construction and repair of boilers and pressure vessles.
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