MAWP

Accumulation The rise of pressure above the MAWP of the pro-tec ted system, usually expressed as a percentage of the gauge MAWP. Note The MAWP and accumulation terms are not included in the ANSI definitions since they relate to the protec ted system instead of the relief device. 

Some liquids might create an ignitable froth layer (5-1.3.1), although mist accumulation will be offset under conditions where there is periodic suction of air through the hose, it is possible to specify a vacuum tmck tank with MAWP > 50 psig, sufficient to contain a deflagration beginning at up to... 

The code stipulates that when multiple valves are used, only one of them needs to be set at the maximum allowable working pressure (MAWP). The additional valves can be set at up to 105% of the MAWP. For design purposes the maximum allowable working pressure is assumed to be the same as the design pressure. 

In addition, 3% tolerance on set pressure is permitted for valves nominally set at design pressure or MAWP. Thus, careful adjustment of the set point in the... 

Pg = relief device set pressure (psig). Note that the relief device set pressure can range from the vessel s MAWP to significantly below the MAWP. 

An 800-gal reaetor eontaining a styrene mixture with a speeifie heat of 0.6 eal/gm °C has a 10-in. rupture disk and a vent line with equivalent length = 400. The vessel MAWP is 100 psig and the rupture disk set pressure is 20 psig. The styrene mixture had a self-heat rate of 60°C/min at 170°C as it is tempered in a DIERS venting test. Determine the allowable reaetor mixture eharge to limit the overpressure to 10% over the set pressure. 

The reaetor eharge is quite small for an 800-gal reaetor. If the pressure is allowed to rise to 10% above MAWP, then... 

A 3,500-gal reaetor with styrene monomer undergoes adiabatie polymerization after being heated inadvertently to 70°C. The maximum allowable working pressure (MAWP) of the reaetor is 5 bar absolute. Determine the relief vent diameter required. Assume a set pressure of 4.5 bara and a maximum pressure of 5.4 bara. Other data and physieal properties are given as follows [12] ... 

Maximum Allowable Working Pressure (MAWP) The maximum allowed pressure at the top of the vessel in its normal operating position at the operating temperature specified for that pressure. 

Like all pressure vessels, the cylinder has a maximum allowable working pressure (MAWP). The maximum allowable working pressure of the cylinder determines the setting of the relief valve that is downstream of the cylinder. The MAWP of the cylinder should be a minimum of 10% or 50 psi greater than its operating pressure. 

The design pressure for a vessel is called its Maximum Allowable Working Pressure (MAWP). In conversation this is sometimes referred to simply as the vessel s working pressure, The MAWP determines the setting of the relief valve and must be higher than the normal pressure of... 

Often, especially for small vessels, it is advantageous to use a higher MAWP than is recommended in Table 12-1. It may be possible to increa the MAWP at little or no cost and thus have greater future bility if process changes (e.g., greater throughput) require an incre operating pressure. 

The MAWP of the vessel cannot exceed the MAWP of the no valves, and pipe connected to the vessel. As discussed in Volui Chapter 9, pipe flanges, fittings and valves are manufactured in dan dustry standard pressure rating classes. Table 12-2 is a... 

If the minimum MAWP calculated from Table 12-1 is close t ... 

Often, a slightly higher MAWP than that calculated from Tabl possible at almost no additional cost. Once a preliminary MAWP is cted from Table 12-1, it is necessary to calculate a wall thicl r shell and heads of the pressure vessel. The procedure for doii s... 

This concept can be especially significant for a low-pressure vessel wl minimum wall thickness is desired. For example, assume the caiv .,.i i.ons for a 50-psig MAWP vessel indicate a wall thickness of 0.20 in., and it is decided to use K-in. plate. This same plate might be used if a MAWP of 83.3 psig were specified. Thus, by specifying the hi MAWP (83.3 psig), additional operating flexibility is available at e dally no increase in cost. Many operators specify the MAWP based on... 

A vessel can only be overpressured if the upstream vessel has a higher pressure than the vessel in question. A compressor scrubber with a MAWP of 285 that gets flow from a 285 MAWP separator does not need to have a relief valve sized for blocked discharge. The upstream relief valve will keep the upstream separator pressure from going higher than 285, so there is no way it can oveipressure the downstream scrubber. The scrubber PSV only needs to be sized for fire. 

Figure 13-1 shows the various relationships between MAWP and the relief valve set pressure. The primary relief valve should be set to open at no more than 100% of MAWP and to relieve the worst case flow lates,... 

Figure 13-1. Various relationships between MAWPs and relief valve set pressure. [Reprinted with permission from API RP 521.)...

Rupture discs may be used alone, but they are normally used as a backup to a relief valve set to relieve at approximately 115% MAWR This ensures that the disc ruptures only if the relief valve fails or in the unlikely event that the pressure rises above 110% MAWP and the relief valve does not have enough capacity. 

When a spec break is taken from a higher to a lower MAWP, there must be a relief valve on the lower pressure side to protect the piping from overpressure. The relief valve can be either on the piping or. more commonly, on a downstream vessel. Spec break problems most commonly occur where a block valve exists on a vessel inlet, or where a bypass is installed from a high pressure system, around the pressure vessel which has a relief valve, to a lower pressure system. 

Maximum allowable working pressure MAWP) Highest pressure to which the system can be subjected during operation. Thus, pressure is established by a relief device set pressure and must be less than or equal to the material strength limitations of equipment. This pressure e.stabli.sh-es piping class for fittings and pipe wall thickness requirements, both of which are discussed in Volume 1. 

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