Allowable overpressure

AP = set pressure plus allowable overpressure minus back pressure, psig... 

The opening pro dded through the rupture disk device after breakage is sufficient to permit a flow equal to the rated capacity of the attached safety or safety relief valve without exceeding the allowable overpressure. 

F2 = coefficient of subcrilical flow, see Figure 7-29 T = relieving temperature of inlet gas or vapor, °R P = upstream relieving pressure, psia, = set pressure + allowable overpressure + atmospheric pressure, usually 14.7 psia), psia P2 = backpressure on valve, psia W = required flow through valve, Ibs/hr V = vapor flow required through valve, standard cu ft/min at 14.7 psia and 60°F... 

P] = upstream reliewng pressure, psiabs. This is the set pressure plus the allowable overpressure plus the atmospheric pressure, psiabs. 

Equation 9-22 was developed by Boyle12 by defining the required vent area as the size that would empty the reactor before the pressure could rise above some allowable overpressure for a given vessel. 

The average dT/dt is typically an arithmetic average between the value at set pressure and the value at peak allowed pressure. The properties Cp, hfg, i, either can be evaluated at the set conditions or can be taken as the average values between the set condition and the peak allowed pressure condition. Alternatively, the term h/g/t)/g in Eq. (23-95) can be replaced by T(dP/dT)tat via the Clapeyron relation. This holds reasonably well for a multicomponent system of near constant volatility. Such an application permits direct use of the experimental pressure-temperature data obtained from a closed-system runaway VSP2 test. This form of Eq. (23-95) has been used to demonstrate the advantageous reduction in both vent rate and vent area with allowable overpressure (Leung, 1986a). 

If the valve is used as a supplemental device for vessels which may be exposed to fire, an overpressure factor of 21% may be used. However, allowable overpressure may vary according to local regulations. Specific application requirements should be referenced for the allowable overpressure... 

Pi = Set pressure - inlet pressure loss + allowable overpressure (21%) + 14.7 psia... 

Have too long a simmer Dual ring control, valves (ASME I) may fail to achieve full-lift at allowable overpressure limit if the nozzle ring is set lower than that recommended, a situation which could lead to a catastrophic event as it could take too long for the valve to react on the overpressure condition. 

The confusion is due to a number of reasons. First is the way ASME does not relate the maximum allowable pressure (MAP) limits to SRV capacity. Throughout the entire document, ASME, Section VIII, Division 1, refers to MAWP when talking about SRV set pressure and allowable overpressure. 

Individual safety relief valve specifications shall be communicated on API 526 specification sheets or equivalent substitute provided by the end user. When the relieving contingency is vapour and liquid, rates and properties for each at the allowable overpressure shall be provided. 

Due to the variety of blocked outlet situations, different methods were used to determine the required relief rates for different types of equipment. The quantity of the material to be relieved was generally determined at the relieving conditions (i.e. the MAWP plus the code-allowable overpressure) based on the capacity of upstream pressure sources or duty of process heaters. 

Allowable Overpressure in Percent or Units 53. Model Number... 

A bellow style pressure relief valve is required to protect a vessel containing an organic liquid. The required relieving capacity is 310 gpm. Inlet temperature is 170°F. Set pressure is 100 psig. Allowable overpressure is 25%. Built-up back pressure is 25 psig. Specific gravity is 1.45 and viscosity is 3,200 cP, Determine the orifice size of the valve. The correction factors are ... 

TOTAL WETTED SURFACE AREA, ft 2. LATENT HEAT OF VAPORIZATION, Btu/lb. RELIEF VALVE SET PRESSURE, psig. ALLOWABLE OVERPRESSURE, (%). RELIEVING TEMPERATURE, oF. COMPRESSIBILITY FACTOR. 

The relieving pressure upstream of the reUef device is the set pressure plus allowable overpressure (9,293). The allowable overpressure is set by the applicable code (e.g., 11), and depends on whether the set pressure is equal to or is lower than the allowable working pressure of the column (9). Often, the relieving pressure is 10 to 20 percent higher than the set pressure a more detailed set of guidelines was presents by Mukeiji (293). 

With the help of the substance property data, which are well known for these common solvents, and the plant data listed above, the mass flow which can be relieved safely by the planned safely valve can be calculated applying the equilibrium model according to Leung. For the sample calculations presented in Table 7-1, a vapour mass fraction of 5 %, an allowable overpressure during relief of 0.2 pset and the simplified model for the determination of the critical mass density have been assumed. 

The blower system incorporates a variable-speed motor, battery, external power-supply cable, housing assembly, control switch, chemical-biological canister, and a means of securing the blower while the crew member is mobile. The mask receives filtered air from the blower unit, which also allows overpressure within the hood which defogs the lens and is vented through an exhaust valve. 

Fusible Plugs A fusible plug is a plug made of a metal that will melt at a selected temperature and relieve pressure in a container. Fusible plug applications include boilers, compressed gas cylinders, and other pressure vessels. They allow overpressure to vent when a container becomes heated from fire or other sources. A plug may consist entirely of the alloy... 

---