Maximum venting pressure

APg = allowable venting overpressure (psi), that is, the maximum venting pressure minus the relief device set pressure Pp = maximum venting pressure (psig)... 

Constant Flow into Protected Equipment For the steady-state design scenario with a constant, steady flow of fluid from a pressure source that is above the maximum aUowed pressure in the protected equipment, volume is being generated within the equipment at a rate RV = F/ f. Substituting into Eq. (26-21) and noting that the specific volume of the vent stream is l/p, gives the required mass flow rate ... 

It is the rate of temperature inerease (i.e., power output) between the set pressure and the maximum allowable pressure, whieh determines the vent size and not the peak rate. Boiling is attained before potential gaseous deeomposition (i.e., the heat of reaetion is removed by the latent heat of vaporization). The reaetion is tempered, and the total pressure in the reaetor is equal to the vapor pressure. The prineipal parameter determining the vent size is the rate of the temperature rise at the relief set pressure. 

Figure 7-65G. Alternate venting nomograph for dusts of ciass St.-I whose maximum deflagration pressure does not exceed 9 bar ga. Reprinted with permission, NFPA 68-1988, Deflagration Venting, (1988) National Fire Protection Association, Quincy, MA 02269. See note Figure 7-63A.

Most state laws and safe practice require a safety relief valve ahead of the first stop valve in every positive displacement compressed air system. It is set to release at 1.25 times the normal discharge pressure of the compressor or at the maximum working pressure of the system, whichever is lower. The relief valve piping system sometimes includes a manual vent valve and/or a bypass valve to the suction to facilitate startup and shutdown operations. Quick line sizing equations are (1) line connection, (i/1.75 (2) bypass, ii/4.5 (3) vent, dl63 and (4) relief valve port, cU9. 

L2 is the second smallest dimension of the enclosure to be vented, and P is the maximum internal pressure that can be withstood by the weakest member of the enclosure. 

Vent ducts with a length of LD > LDS have no additional effect upon the pressure increase. Therefore, LDS will be the maximum vent duct length that has to be considered. The above-mentioned equation (23-18) is not valid for metal dusts. 

Relief Valve A relief valve is used in pumps, reactors, and equipment which create positive pressure. These valves are usually connected between the pump outlet and a vent or a large-volume accumulation vessel. The valve is normally closed, but will open whenever the system pressure exceeds the desired maximum operating pressure. 

Fauske s method14,51 (given below) is based on emptying the reactor (or achieving vapour/ liquid disengagement) before the pressure has risen from the relief pressure to the maximum accumulated pressure in a vented system. The method incorporates the simplified equilibrium rate model, ERM, for saturated liquid inlet (see 9.4.2) together with the Clausius-Clapeyron relationship (discussed in 6.3.3). The method makes use of adiabatic experimental rate data for the runaway, whose measurement is described in Annex 2. 

LNG tanks are designed for fairly low pressures, typically less than 1.72 MPa (250 psi), otherwise their advantage over storing natural gas as CNG would be lost. As LNG warms, vapor is produced that builds up pressure in the tank. To protect the tank from rupture due to over-pressure, vent valves are included that will release some vapor when the maximum set pressure is reached. Vent valves come in two types those that reseat when the pressure in the tank falls below the maximum, and those that don t. The second type is used only to protect the tank from over-pressure rupture, while the first type is used to control the pressure in the tank. Often they are used together in the same tank. Both types of vent valves may be made from stainless steel, brass, and aluminum materials compatibility problems are rare because of the cleanliness of LNG. 

NFPA 57 recommends that LNG dispensers be protected from collisions with vehicles and that they incorporate an emergency shut-down system. The maximum delivery pressure of the dispenser shall not exceed the operating pressure of the LNG tanks on the vehicle being refueled, and the LNG delivery hoses must have a shut-off valve at the end and a breakaway valve in the event a vehicle drives off with the refueling hoses attached. Bleed and vent valve must be incorporated into the dispensing lines to allow them to be drained and depressurized prior to disconnection if necessary. 

Pp = maximum allowable venting pressure, psia Ms = sample mass used in a DIERS test or equivalent test, g dP/dt = pressure rise in test, psi/sec Tt = maximum temperature in test, K... 

Pred = maximum explosion pressure during venting, bar g Pstat = vent closure release pressure, bar g V = vessel volume, m ... 

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