Absolute overpressure

Equation 12-18 gives a eonservative estimate of the vent area, and the simple design method represents overpressure (AP) between 10%-30%. Eor a 20% absolute overpressure, a liquid heat eapaeity of 2,510 J/kg K for most organies, and eonsidering that a saturated water relationship exists, the vent size area per 1,000 kg of reaetants is ... 

The area assumes a 20% absolute overpressure. The result can be adjusted for other overpressures by multiplying the area by a ratio of 20/(new absolute percent overpressure). 

Equation 9-24 provides a conservative estimate of the required vent area. By considering the case of 20% absolute overpressure, assuming a typical liquid heat capacity of 2510 J/kg K for most organic materials, and assuming a saturated water relationship, we can obtain the following equation13 ... 

Leung s method (as given in equation (6.5) below) is applicable if all the above assumptions are true. Assumption (d) above, regarding the use of an average rate of heat release, tends to be the most limiting in terms of the maximum difference that can be allowed between the relief pressure and the maximum pressure. The absolute overpressure (often referred to simply as the "overpressure") has been sometimes used to characterise this. This is given by ... 

The absolute overpressure is different from the overpressure of a safety valve which is expressed in terms of gauge pressures. It was originally recommended by Leung131 that the arithmetic mean be used for the heat release rate per unit mass ... 

If.equation (6.2) is used, then comparison with dynamic simulation131 suggested that Leung s method would increasingly oversize at absolute overpressures above 50%. Provided the rate of temperature rise due to the runaway continues to increase at high overpressures, the arithmetic mean (equation 6.2) overestimates the true average q. 

Leung[2] has more recently suggested an alternative average for q to help overcome this problem at high absolute overpressures ... 

Figure 6.2 RELATIONSHIP BETWEEN ABSOLUTE OVERPRESSURE AND RELIEF AREA...

This is normally the gauge pressure increase above the set pressure of a safety valve (see 5.2). The absolute overpressure is the pressure increase above the relief pressure of any relief device (see equation (6.1))... 

Pressure Zero shift, air leaks in signal lines. Variable energy consumption under temperature control. Unpredictable transmitter output. Permanent zero shift. Excessive vibration from positive displacement equipment. Change in atmospheric pressure. Wet instrument air. Overpressure. Use independent transmitter mtg., flexible process connection lines. Use liquid filled gauge. Use absolute pressure transmitter. Mount local dryer. Use regulator with sump, slope air line away from transmitter. Install pressure snubber for spikes. 

The design overpressure of the vessel is 1.92 MPa, and the test pressure is 25% higher. Therefore, the absolute internal pressure p, is... 

Co = velocity of sound in air P = peak overpressure P = absolute aiubieut pressure... 

Pj = upsueain relieving pressure, in pounds per square inch absolute. This is the set pressure plus the allowable overpressure plus the atmospheric pressure. 

From Figure 9-6, Kb = 0.86 for an overpressure of 10%. The effective discharge coefficient is assumed to be 0.975. The gas compressibility factor z is approximately 1 at these pressures. The pressure P is the maximum absolute pressure. Thus P = 69.7 psia. The constant x >s computed from Equation 9-10 ... 

A reactor has a volume of 2 m3. The worst case runaway reaction has been identified and the data from a suitable adiabatic, low thermal inertia test, with a thermal inertia ( ) of 1.05, is given in Figure 6.4. Under these conditions, the reactor would contain 793 kg of reactants. The reacting system is a vapour pressure system. It is desired to relieve the runaway via a safety valve, if possible, with a set pressure of 0.91 barg (relief pressure of 1.0 barg = 2.0 bara). Evaluate the required relief size for an overpressure of 30% of the absolute relief pressure, which gives a maximum pressure of 2.6 bara = 1.6 barg. 

It is recommended that the chosen sizing method be evaluated for a range of overpressures. This will give an indication of the likely consequence of undersizing, in terms of how far the pressure may be expected to rise above the maximum permitted for the vessel. For moderate overpressures (typically 10-30% of the absolute relief pressure), the consequence of undersizing may sometimes be small. 

P1 = Relieving pressure, psi, absolute (set pressure [psig] + overpressure [psi] + atmospheric pressure [psia]). 

Pi Absolute flowing pressure at inlet = set pressure + overpressure + atmospheric pressure Bara... 

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