|
To comply with AMCA spark-rsistant construction requirements, the ventilator must: 1) No bearings, drive components, or electrical devices shall be placed in the air or gas stream unless they are constructed or enclosed in such a manner that failure of that component cannot ignite the surrounding gas stream. 2) The user shall electrically ground all fan parts. 3) For this Standard, nonferrous material shall be any material with less than 5% iron or any other material with demonstrated ability to be spark resistant. 4) The use of aluminum or aluminum alloys in the presence of steel which has been allowed to rust requires special consideration. Research by the U.S. Bureau of Mines and others has shown that aluminum impellers rubbing on rusty steel may cause high-intensity sparking. AMCA Type A Spark Resistant Construction requires all parts of the fan in contact with the air or gas being handled shall be made of nonferrous material. Steps must also be taken to assure that the impeller, bearings, and shaft are adequately attached and/or restrained to prevent a lateral or axial shift in these components. Fan applications with airstreams of explosive or flammable particles or gases require spark-resistant system components for the safe handling of such airstreams. This includes components such as ductwork, dampers, filter devices, heating or cooling coils, and fans. This article presents practical considerations and methods of providing fans with varying types of Spark-Resistant Construction (SRC). The Air Movement and Control Association (AMCA) established a standard set of Classifications for Spark-Resistant Construction. In AMCA Type B Spark Resistant Construction the fan shall have a nonferrous impeller and nonferrous ring about the opening through which the shaft passes. Ferrous hubs, shafts, and hardware are allowed, provided construction is such that a shift of impeller or shaft will not permit two ferrous parts of the fan to rub or strike. Steps must also be taken to assure that the impeller, bearings, and shaft are adequately attached and/or restrained to prevent a lateral or axial shift in these components. AMCA Type C Spark Resistant Construction requires the fan shall be so constructed that a shift of the impeller or shaft will not permit two ferrous parts of the fan to rub or strike. The use of the above Standard in no way implies a guarantee of safety for any level of spark resistance. “Spark-resistant construction also does not protect against ignition of explosive gases caused by catastrophic failure or from any airstream material that may be present in a system.” The AMCA standard provides the system designer with a uniform way to specify the system requirements and provides fan manufacturers with general guidelines. The fan manufacturer must still develop unique designs to deal with the physical and practical limitations of fan equipment when developing construction methods to comply with AMCA. A major limitation is the practical availability of truly “nonferrous” alloys that really can be used in fan construction. There are certain alloys or noble metals than are truly nonferrous, alloys that contain no iron, but for the most part they are extremely expensive and/or difficult to obtain in forms and strengths necessary for fan construction. For most purposes, the fan manufacturer uses more readily available alloys that are considered nominally nonferrous and which have strength and work properties suited to fan construction. Aluminum is the most frequently used alloy due to its low cost. However, as pointed out in the AMCA Standard, when aluminum is in close proximity to steel, careful maintenance programs are necessary to prevent rust, because aluminum rubbing against rusty steel can cause high-intensity sparking. In applications where such maintenance is not possible, an SRC method that places steel in the airstream is not recommended. Regardless of which classification is chosen, airborne foreign or “tramp” particles could either strike each other, or strike one of the components of the fan, causing a spark. Protection against SRC does not eliminate the potential for spark generation. Fans with any type of SRC are only intended to minimize the potential that any two or more fan components might generate sparks within the airstream by rubbing or striking during operation. No type of SRC can be guaranteed to eliminate the possibility of generating a spark, nor would any SRC type preclude sparks resulting from any foreign influence such as airborne materials striking each other. The AMCA Standard requires construction that will not permit a wheel and/or shaft to shift due to some malfunction during operation. If two components are allowed to shift and rub against each other for any length of time, either sparks or frictional heat could become a hazard in an explosive or flammable gas stream. Normally, standard procedures of fastening the wheel to the shaft and locking the shaft in the bearings are sufficient. However, the degree of hazard in these situations dictates that extraordinary precautions to more securely prevent such shifting are in order, so further methods of attachment or restraint are required. A fan furnished with AIRSTREAM TYPE SRC should provide the greatest degree of spark resistance. In the event that two or more fan components in the airstream rub or strike together, a properly maintained fan should be able to continue in operation for some reasonable period of time, without producing a spark. However, the severity of a hazard that calls for AIRSTREAM TYPE SRC dictates that the fan should be closely monitored and shut down immediately upon such an occurrence. If allowed to operate, the rubbing or striking of these fan components will generate frictional heat, quickly deteriorate, and eventually catastrophically fail. Good safety practice cannot be ignored ! A fan furnished with WHEEL TYPE SRC differs from AIRSTREAM-TYPE SRC in that only the wheel itself is constructed of a spark-resistant alloy. A spark-resistant buffer is added around the housing opening through which the shaft passes as shown in Figure 2. The remainder of the fan components are furnished in their standard material, usually mild steel. Fans furnished with WHEEL TYPE SRC should not continue in operation for any length of time with the wheel rubbing any component or with the shaft striking the buffer. Practically speaking, it is not possible to predict a “safe” length of time, because there may be other ferrous components within the fan airstream which could be torn or jarred loose by the rubbing or striking of the wheel or shaft, and such loose ferrous objects could create a spark. Also, the buffer cannot support the weight or withstand the forces of the rotating shaft for any prolonged period of time. The AIRSTREAM TYPE and WHEEL TYPE SRC specifications go further to minimize the potential for sparking by taking extraordinary precautions to minimize the potential for abnormal movement or shift of the fan’s airstream components. While the standard bearing mounting bolts will resist vertical or axial movement, the addition of bearing stop blocks will resist horizontal movement and effectively secure the bearings in place. The addition of shaft set collars will further resist shaft movement through the bearings. These combined features virtually eliminate the possibility of any movement in the shaft and bearing assembly. There are many ways to secure the fan wheel to the shaft, but standard setscrews and keys are not enough for the more severe applications. Figure 4 details one alternative which includes a bolted aluminum wheel retaining plate on the end of the shaft. Other methods might include countersinking the shaft to accept a setscrew, sweat-fitting, or tapered bores to prevent the wheel from slipping on the shaft axially. The precise method will vary by fan size and type. The BUFFER TYPE SRC specifications utilize standard, usually mild steel, airstream component parts and employ spark-resistant plates or buffers to stop the wheel or shaft from coming into direct contact with other airstream components. A fan design which requires an inlet cone is usually furnished with an aluminum cone to act as the buffer on one side. The BUFFER TYPE SRC is intended to provide a low cost alternative for non-critical applications. The user or specifier must exercise caution in selecting this type so that the safety of the installation is not compromised for the sake of initial cost. Generally, aluminum wheel construction is utilized for AIRSTREAM TYPE AND WHEEL TYPE SRC. Because the material strength characteristics of aluminum decrease sharply at elevated temperatures, it is not recommended for handling anything other than nonabrasive airstreams at less than 200°F. In cases beyond these limits, BUFFER-TYPE SRC may be the only readily available alternative. As with the WHEEL TYPE SRC, fans furnished with BUFFER TYPE SRC should not continue to operate for any length of time with the wheel or shaft rubbing the buffers. High speed fans will tend to wear away buffers more rapidly than slower speed fans, and thus BUFFER TYPE SRC should be used with caution on high speed fans. The greater wheel tip speeds and shaft surface speeds, combined with their corresponding weights and forces, educe the amount of time available to react. Periodic inspection of the fan, and particularly the airstream, is recommended. The build-up of foreign material or rust, the potential deterioration due to abrasion or corrosion, or the accidental shifting of any fan part could lead to further hazards of potential ignition or explosion. Moving explosive or flammable gas streams through fans requires the utmost care in system design and equipment selection. The system designer must weigh the total system from all angles to minimize risk, particularly when the system components and/or fans are in environments that are located in areas where people are likely to be working or passing. The explosiveness of the gas mixture, the people factor, and the potential for foreign or “tramp” elements to enter the system, are all necessary concerns in determining to what degree special-material construction should be used. Vibration detectors to warn of impending malfunction of bearings or rotating assemblies are a good preventive measure to forestall the actual rubbing or impact of two parts in any mechanical equipment, and should certainly be considered in “severe risk” situations. The extraordinary measures to pre-vent wheel and shaft movement offered in AIRSTREAM TYPE SRC and WHEEL TYPE SRC are features to help minimize the potential of allowing two parts to strike. In particularly hazardous applications, the location of the fan and perhaps the entire system should be a major consideration. In some cases, protective enclosures around the fan or other mechanical parts in the system may be another protective step to lessen the danger in the event that a spark might occur in spite of the precautions taken. The system designer is in the best position to weigh the alternatives and specify the required fan equipment. For additional information please refer to http://www.chicagoblowercanada.com/oemfans.html Oleg Tchetchel Industrial Process Ventilating Designers fan@cogeco.ca Chicago Blower Canada Co. http://www.chicagoblowercanada.com/airfoilblower.html http://chicagoblowercanada.com
Related Articles -
fan, blower, ventilator, explosion proof, spark resistant, centrifugal, axial, propeller, wall, roof, exhaust, supply,
| 
