Venting Design

The equipment in which the dust is handled or stored should be designed to contain the pressure resulting from an internal explosion. Most dusts show maximum pressures of ca 345—700 kPa (50—100 psi) however, the rate of pressure rise changes from ca 700 to 70,000 kPa/s (100—10,000 psi/s). Equipment-containment design can be coupled with explosive-venting design for the equipment and the building. 

Fixed-roof atmospheric tanks require vents to prevent pressure changes which would othei wise result from temperature changes and withdrawal or addition of liquid. API Standard 2000, Venting Atmospheric and Low Pressure Storage Tanks, gives practical rules for vent design. The principles of this standard can be applied to fluids other than petroleum products. Excessive losses of volatile liquids, particularly those with flash points below 38°C (100°F), may result from the use of open vents on fixed-roof tanks. Sometimes vents are manifolded and led to a vent tank, or the vapor may be extracted by a recov-eiy system. 

Note that the venting design may not necessarily prevent a deflagration, but is intended to relieve the overpressure developed (see reference [72]). 

The vent design must provide at least the area required to satisfy the volume of the enclosure (see NFPA-68) [27]. 

Table 7-31 lists the explosibility index that is a relative measure of the potential damage from a dust explosion. A rating of 2 to 4 requires large vent areas. Above 4, for most cases, the explosion cannot be controlled by venting design and therefore requires the use of protection such as inert gas or explosive suppression systems, some of which are commercially available. 

B. Defining the Reaction Kinetics and Component Physical Properties. The rate expression needed for use in a vent design model should represent the condition that would exist during the emergency. Kinetic data based on the normal reaction rate are only useful in cases when loss of heat transfer can be experienced. 

High-pressure structures are capable of withstanding pressures of more than 1.5 psig (0.1 bar gauge). The vent design is based on the definition of a deflagration index for gases or dusts ... 

The primary application of the VSP is to obtain data necessary to calculate the vent design (size and relief setting) for emergency venting of nonvolatile and reactive runaway systems. The calculated vent design is to limit the maximum pressures at the point of emergency venting to acceptable levels. A secondary application is to provide thermal stability data for reactive systems. 

The data required for the emergency vent design includes [191] (1) the thermokinetic and pressure history monitored under near adiabatic conditions, (2) the character of the type of vented system (vapor, gassy, or hybrid), (3) the phase of the vented material (vapor, liquid, or two-phase), and (4) the degree of two-phase disengagement (turbulent, bubbly, or homogeneous). To determine these characteristics, the VSP defines the system as viscous (100 cp) or nonviscous, and also whether or not it has a foaming tendency. 

Vent designs are to include features to exclude insects, birds, animals, and dust. 

The test is quantitative, but corrections must be made for thermal inertia of the sample container before the data can be applied to process systems. Activation energy, approximate heat of reaction, and approximate reaction order are parameters that can usually be determined. Pressure data obtained during an ARC run can sometimes provide information for vessel vent design. 

Low-pressure models applicable to the protection of process buildings and storage tanks ruled by the API codes. RUST s low-pressure model is usually successful for vent design of internal or external overpressure. 

An open vent that serves as a relief device should be registered for regular inspection to be assured that it is not obstructed. It is also obvious that any open vent designed for overpressure protection should not be designed so that it can be easily blinded. [7]... 

Provided that the necessary amount of physicochemical substance data as well as kinetic and thermodynamic reaction data has thoroughly been obtained experimentally, one can choose today whether to base the vent design for a monoproduction process on either a steady model or on a simulation of the dynamic reactor behaviour with the help of specifically developed computer codes, such as e.g. SAFIRE and RELIEF . 

As escalating process deviations can never be fully excluded, these reaction vessels also have to be equipped with emergency relief devices. The experimental characterization of all processes, even of the known ones, is not possible in the extent necessary for vent design. 

A. Operation Near Atmospheric Pressure. The simplest approach to hydrogen header control at very low pressure uses large water-sealed vents designed to maintain a fairly precise back-pressiure with little or no fluctuation. Figure 11.35 shows such a vent. Section 9.1.10.1 gives design details for this type of system as a pressure-relief device with little heed to gas flow distribution. For more precise control, it is important to distribute the gas as small bubbles over a wider area of the water seal (10-20 nun deep) in order to provide minimum pressure drop and fluctuation in flow. 

A cell vent designed to stop burning discharge from a vent. 

At an early date open baffle systems were realized to result in unwanted reinforcement and cancellation effects due to radiation interference between the front and rear diaphragm surface. To the ensuingbox type enclosure, a vent was quickly added, although it was originally envisioned as a mere pressure equalizer. Later work showed that careful vent design could appreciably improve low-frequency response of the radiating system. Yet, in addition to improved low-frequency response, the vented box affects higher frequencies due to a variety of mechanical and acoustic resonances and diffraction. 

Figure 30 Vented design (2 bar unit). (Courtesy of the Glatt Group.)...

FIGURE 2-13. Conservation Vent Designs, (a) Breather vent mounted on the tank and vented to the atmosphere, (b) Vent mounting for pipe line. (Reproduced with permission of the Protectoseal Company.)... 

Vents should be placed in other locations as well, including the runner system, weld line regions, and other areas of possible gas entrapment. Air vent design is important because its function is to release the air inside the cavity when the mold is closed. Short shot will happen if air is trapped inside the mold. 

The important criteria for vent design and selection are that the vent should open fully at the designated pressure with very little inertia and should be perfectly sealed when closed. The principles and... 

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