PIPING HYDRAULICS AND SPECIFICATION BOOK 1 PNEUMATIC CONVEYING
8.5 SAFETY CONSIDERATIONS .1 Introduction
Pneumatic conveying can be used for various applications, including powder handling. The possibility of dust explosion must be a consideration during the design of a pneumatic conveyor, because of the high concentrations of dust present. Typically the minimum concentrations in air for an explosion to occur is in the range of 6x104-0.03 lb/ft3/10-500 g/m3. Although every effort should be made in the design to eliminate ignition sources and prevent electrostatic sparks, some unforeseen ignition sources must be anticipated. The majority of dust explosions are caused by unknown ignition sources.
The conveying pipe can be designed to withstand the explosion pressure.
However, at the delivery point the conveyor will be discharged into a collecting vessel or silo with associated air separation equipment (filters or cyclones), which will require explosion protection. In the event of an explosion the conveyor and feed should be shutdown.
Where the conveying pipe is not strong enough to withstand an explosion, the conveying would have to be carried out using an inert conveying medium because venting would be impracticable.
Pneumatic conveying systems generally offer greater safety to operators than any other type of bulk handling system. They are cleaner, offering less exposure to operators. Typically dust explosions do not occur in operating pneumatic conveying lines. The reason for this observation is two-fold; first, the material loading is usually higher than the upper explosion limit, and second the velocity is usually too high for flame propagation (i.e., it blows itself out). The four requirements for a dust explosion include: 1) dust must collect in the facility, 2) dust must be suspended in air at a concentration above the lower explosion limit, 3) the dust suspension must be ignited, and 4) sufficient dust to sustain combustion must be in close proximity to the ignited dust. Static electricity and motors or switch gear not designed for dusty environments are prime ignition sources. Explosion vents are typically used to protect equipment, but must be vented outside buildings or other enclosures to prevent secondary explosions from ordinary nuisance dusts which may accumulate due to poor housekeeping.
Explosion suppression systems have also been used, but care must be taken to ensure that the devices do not discharge undetected. If this happens, the next explosion may not be suppressed since the device will already have discharged.
Closed loop systems have even lower explosion risk when an inert atmosphere is used. In destination silos, bins, hoppers, and dust collectors it is another story, especially when the system is not operating. During times when the system is not operating, conditions could occur to cause a dust explosion. Equipment and piping must be thoroughly grounded even at sight glasses and hose connections,
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SECTION 8.0 PAGE 110 DATE 8-94 insufficient as the system could be exposed to air during maintenance. One must then remove the fuel, or provide a fire control system (sprinklers) to satisfy the NFPA code. Information from a number of sources on the relative explosiveness of some materials is included in 8.7.26 - Appendix 26. Additionally, a list of materials and their relative explosiveness is included in the reading file from the U.S. Bureau of Mines (1961). This manual gives a detailed method for the sizing of explosion vent areas.
8.5.2 Dust Explosions - General
The UK Institution of Chemical Engineers (IChemE) publishes a three part Guide to Dust Explosion, Prevention and Protection. In addition the Industry and Engineering standards published for the process location should be consulted.
See Appendix 23 for basic data for gases and dusts.
Under certain conditions fine particles of combustible material dispersed in air will ignite and explode. This causes a rapid pressure increase within the containing structure. If the equipment is not designed to withstand the explosion it can result in extensive damage and the possibility of injury.
In enclosed vessels the pressures generated by these explosions can reach 10 barg for organic dusts and higher still for metal dusts such as aluminum. These high pressures cannot be tolerated by most dust handling equipment and therefore protection methods must be considered.
a) Exclusion of Ignition Sources
Without an ignition source there cannot be a dust explosion. All practical measures must be taken to exclude ignition sources. Unfortunately, this step is insufficient on its own and other precautions must be taken because the ignition source for the majority of dust explosions is not known and therefore impossible to design out. Examples of ignition sources include: Flames, Hot Surfaces, Incandescent Material, Spontaneous Heating, Welding or Cutting Operations, Friction Heating or Sparks, Impact Sparks, Electric Sparks and Electro Static Discharge Sparks.
b) Exclusion of Oxygen (Inerting)
Using nitrogen, carbon dioxide or other suitable gases the oxygen content is reduced to below the minimum required to support combustion (typically
< 6 - 15 %). This method is expensive, it requires a closed system to conserve the inert gas and continuous monitoring of the oxygen content.
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The vessels and associated pipework are built to withstand the maximum pressure. This is expensive, except in very small systems.
d) Venting (Bursting Disks)
This method is widely used, it is relatively simple and cheap to install. Vents provided in the walls of the vessel allow dust and combustion products to escape, limiting the pressure rise to an acceptable level.
e) Suppression
In some cases venting is unacceptable (If the dust is toxic or corrosive for example). The start of the explosion is detected by instruments which trigger the release of fire suppressants.
There are a number of conditions that must be satisfied simultaneously for a dust explosion to occur.
The dust must be combustible. Many substances have been involved in serious dust cloud explosions including organic chemicals, resins, metal powders and food products.
The dust must have a particle size distribution which will propagate flame.
In general if the particle size of a combustible dust is reduced the risk of explosion increases. Fine particles stay in suspension more readily than coarse particles, hence the probability of producing an explosible concentration is enhanced. Particles with diameters greater than 500 m are unlikely to cause dust explosions. (However coarse particles can produce fines when handled and this should be considered).
The dust concentration within the suspension must be within the explosible range. Typically the minimum concentrations in air for an explosion to occur is in the range 10 -500 g/m3.
The dust suspension must be in contact with an ignition source of sufficient energy.
When all of the these conditions are satisfied the hazard from a dust explosion is dependent upon the explosibility of the dust, the volume and characteristics of the vessel and the degree of turbulence in the vessel.
The explosibility of the dust can be measured in the laboratory using the standard 201 sphere. The maximum rate of pressure rise (dP/dt) max bar/s and the maximum explosion pressure in an enclosed explosion, P are measured
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SECTION 8.0 PAGE 112 DATE 8-94 over a range of dust concentrations, using a standard technique. The highest value of (dP/dt) max is used to calculate the KST value for the dust.
This equation is referred to as the cube root law. The KST value is defined as the maximum rate of pressure rise measured under standard conditions in 1 m3 vessel, and is used to characterize the explosibility of the dust by reference to four groups. See Table 8-5.
Table 8-5
DUST EXPLOSION DATA
KST EXPLOSION CLASS CHARACTERISTICS
0
The Chilworth Laboratory at Southampton University, UK, carries out this type of work. Other laboratories in the UK can be found in the IChemE Guide to Dust Explosion, Prevention and Protection Part 1 by Dr C. Schofield.
Alternatively the Health and Safety Executive (HSE) can be consulted or their equivalent in Europe. In the USA the Environmental Protection Agency can be consulted.
8.5.3 Sizing of Vents - Basic Methods
The basic principle of venting is that if a dust explosion occurs in a vessel a vent of sufficient area should open rapidly allowing unburnt dust and explosion products to escape, thus limiting the pressure rise to an acceptable level. The acceptable pressure rise is determined by the requirement that the vessel does not rupture and in some cases does not deform.
Venting is a widely used precaution because it is relatively simple and cheap. In
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The dust is a detonating or deflagrating material (will explode in the absence of atmospheric oxygen). When detonation occurs there is insufficient time for the vent to open to reduce pressure.
The vessel has a volume larger than 1,000 m3 (methods not yet confirmed by experiment).
The vent cover will not withstand corrosive and erosive conditions inside the vessel.
It is not possible to vent dust and combustion products to a safe place.
There is no single method for sizing vents to cover all eventualities. The most widely used methods are considered below:
a) Vent Ratio Method MAX RATE OF PRESSURE
RISE (BAR/S)
It will be appreciated that for larger vessels the vent areas can be large and difficult to accommodate. The large areas arise because the vent ratio method is based on rapid flame propagation throughout the whole vessel volume rather than a spherical flame front from a single ignition source. In reality such a high degree of turbulence and fragmentation of the flame front is unlikely to occur throughout the whole vessel volume, resulting in overgenerous vent sizing. For larger vessels the vent ratio is modified as shown in Table 8-7.
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VOLUME RANGE, m3 MODIFICATION
30 - 300
(Full Area for More Explosive Dusts) Full Area of Top
The following list is of specific applications of the vent ratio method and the necessary conditions.
The maximum reduced explosion pressure will not exceed 0.03 barg and the vent cover will not weigh more than 25 kg/m2.
Discharge ducts, if incorporated, are less than 3 m long, (vent ducts are not recommended for very weak vessels).
A high degree of turbulence and flame front fragmentation is allowed for.
Unless detailed information is available to the contrary the volume used in the calculation should be the total volume of the vessel.The vent ratio method was used for many years in the UK and USA to determine vent areas.
b) Nomograph or Cubic Law Method
This method is widely used in Europe. It is based, indirectly, on the cubic relationship for closed vessels which has been shown to apply to vented vessels, within acceptable limits. This law only applies to vessels with L/D ratios less than 5 to 1 and volumes greater than 17 liters.
KST = 1/3
From this equation the relationship between vent area (F) and vessel volume (V) was deduced.
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F1 = vent area on the test vessel necessary to limit the pressure rise to the prescribed value
F2 = vent area on the vessel in question required to limit the pressure rise to the same value
V1 = volume of test vessel V2 = volume of vessel in question
The scale up of test data for different conditions and for the different St classes of dust has been simplified by the provision of a series of nomographs in Figures 8-37A thru 8-37C. A similar series of nomographics is shown in Figures 8-38A thru 8-38C based on the KST values.
To determine the vent area F, m2 for a vessel of volume V, m3 the following information is required:
- PSTAT, barg = vent opening pressure
- PRED, barg =the reduced pressure (maximum pressure in the vented vessel; reduced from Pmax by the presence of the vent)
- The St classification of the dust, from laboratory test
It is assumed the vent cover has low inertia with an area density less than 10 kg/m2. The use of the nomograph is illustrated in Figure 8-39.
Figure 8-40 shows an example of the comparison of vent areas determined from St and KST nomographs.
These nomographs are for strong ignition sources. To avoid making judgments on the likely strength of ignition it is recommended that these nomographs are used for all ignition strengths.
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Figure 8-37A
NOMOGRAPH FOR DETERMINING VENT AREAS BASED ON ST CLASSIFICATION [VDI (1979)]: Pstat = 0.1 barg.
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Figure 8-37B
NOMOGRAPH FOR DETERMINING VENT AREAS BASED ON ST CLASSIFICATION [VDI (1979)]: Pstat = 0.2 barg.
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Figure 8-37C
NOMOGRAPH FOR DETERMINING VENT AREAS BASED ON ST CLASSIFICATION [VDI (1979)]: Pstat = 0.5 barg.
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Figure 8-38A
NOMOGRAPH FOR DETERMINING VENT AREAS BASED ON Kst VALUES [VDI (1979)]: Pstat = 0.1 barg.
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Figure 8-38B
NOMOGRAPH FOR DETERMINING VENT AREAS BASED ON Kst VALUES [VDI (1979)]: Pstat = 0.2 barg.
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SECTION 8.0 PAGE 121 DATE 8-94 Figure 8-38C
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SECTION 8.0 PAGE 122 DATE 8-94 Specific areas for the application of the nomograph method and necessary conditions are summarized as:
KST and St should be measured using the standard laboratory methods.
The lowest pressure that can be designed for without extrapolation is 0.2 barg.
The vent cover inertia must be low and the area density of the cover must be less than 10 kg/m2.
The vessel volume is between 1-1,000 m3
Unless detailed information is available to the contrary the volume used in the nomograph should be the total free volume of the vessel.
The turbulence in the vessel is similar to that in the test method.
The vessel should have an L/D ratio less than 5:1. For weaker vessels use 3:1.
This method takes no account of vent ducts.
c) K Factor Method
It has been shown for compact rectangular vessels the maximum explosion pressure in a vented vessel was related to the vent area.
Pmax
Details of this method are to be found in the IChemE Guide to Dust Explosion Prevention and Protection Part 1, Section 4.3.
The specific areas of application for the K factor method and the necessary conditions are summarized below:
Suitable for St2 in the absence of excessive turbulence.
The vent cover inertia must be low and the weight per unit area of the cover must be less than 10 kg/m2.
Vessel volume between 1 - 1,000 m3.
The area Av used in the determination of K is usually taken as the area of
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SECTION 8.0 PAGE 123 DATE 8-94 Figure 8-39
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The vessel L/D ratio should be less than 5:1 (3:1 for weak vessels).
The method in the IChemE guide makes no allowance for vent ducts.
d) The Rust Method
This is a fourth alternative for calculating vent areas. It is based on the flame front being spherical initially and then determined by the vessel shape. It has been used for plant handling soap and detergent powders. It is particularly useful for weak explosions (St1).
8.5.4 Factors Affecting Estimation of Vent Size a) Weak Vessels
The Vent Ratio Method is recommended.
b) Weak Explosions
KST < 50 bar/s (approx). The St Nomograph will overestimate the required vent size, hence the KST Nomograph or the Rust method should be used.
c) Turbulence
A pneumatic conveyor supplying a solid/air mixture to a silo may create a turbulent dust cloud. In this circumstance the flame front may be fragmented or stretched and centers of ignition may be spread throughout the vessel - increasing the rate of combustion and creating a higher rate of pressure rise.
The vent ratio method should be used, methods based on a spherical flame spread should be avoided.
d) Internal Obstructions
This has a similar effect to turbulence. Internal obstructions can impede the passage of flame towards the vent. Tests may be required to find the optimum location for the vent.
e) Vent Location
The vent should be located so that the flame front is unimpeded and the flame front area must never be reduced so that it is less than the area of the vent. The required vent area can be made up of smaller vents if required,
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SECTION 8.0 PAGE 125 DATE 8-94 The vent location is influenced by requirement to vent the unburnt dust and the combustion products to a safe place.
f) Vessel Shape
In general the methods described are most suited to a compact vessel.
Usually the L/D ratio should not exceed 5:1 although ICI recommend 3:1 for process vessels and silos. There are no standard methods available for vessels with L/D ratios exceeding 5.
The nomograph method is most appropriate for conical vessels and the K factor method for cylindrical vessels.
g) Limitations to Vent Area
In some pieces of equipment it is not possible to have a vent area obtained using the standard methods because of obstructions from ancillary equipment, inlet and outlet pipes for example. In this situation the vessel must be strengthened to allow a higher pressure and consequently a smaller vent size can be calculated.
h) Vessel Operating Pressure
The activation pressure for the vent must be significantly different from the normal operating pressure to avoid the vent opening during normal operation.
i) Interconnected Vessels
Where two vessels are connected by a pipe a dust explosion in one vessel will be communicated to the second vessel, if it cannot be isolated. In the second vessel, precompression and increased turbulence will enhance the rate of pressure rise. The following procedure isrecommended by the IChemE Guide to Dust Explosions, Prevention and Protection Part 1 Section 5.9:
The vent activation pressure should be less than 0.2 barg.
For vessels with similar volumes (within 10 %) both vessels should be sized using the nomograph method.
For vessels of different sizes the vents should be sized using the nomograph method, but both vessels and interconnecting lines should be designed for pressure-shock resistance of at least 2 barg.
Where the smaller vessel cannot be adequately vented it should be designed for a pressure-shock resistance equivalent to the maximum explosion pressure.
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SECTION 8.0 PAGE 126 DATE 8-94 If the vessels are isolated by fast acting valves or extinguisher burners controlled by explosion detection systems then each vessel should be vented separately by the appropriate method.
j) Vent Ducts
1) Fitting a vent has two serious implications:
The flow of dust and gases from the vent is impeded and the pressure reduction in the vessel will be significantly impeded.
The vent duct, initially, may be filled with a mixture of unburnt dust and air which will be ignited by the burning material flowing from the vent.
2) The effect of the duct on the vessel pressure is illustrated in Figure 8-41.
The increase due to the vent duct is large and could cause the vessel to rupture. The IChemE Guide makes the following recommendations:
The increase due to the vent duct is large and could cause the vessel to rupture. The IChemE Guide makes the following recommendations: