Rated relief rate

ASME code stamp relief rate is based on actual PRV orifice area and rated discharge coefficient (=0.9 elective discharge coefficicnc). 

Most equipment is hydrotested at 150% or 130% design pressure or MAWP. 130% hydrotest pressure is new code requirement, but some companies still use 150% hydrotest pressure for conservative reason. These hydrotest pressures show that ASME code required system ovopressure (10%, 16%, or 21%) is on the conservative side. 

ASME/ANSI B31.3, Process Piping Guide, states that piping pressure can exceed 20% design pressure or allowable stress for 50 houi at any one time and for less than 500 hours per year or it can exceed 33% design pressure or allowable stress for 10 hours at any one time and for less dian 100 hours per year. 

(7a) and q. (8a), if real gas compressibility fector (Z) is used, calculated orifice size will be under 

For real gas, its isentrople expansion coefficient (n) is tfifferent from its ideal gas isentropic expansloii coefficient (k). Most hydrocarbon vapor at high pressure, its heat capacity (Cp) is equal to its ideal gas Cp plus some correction. 

---

VII]. 1 Rated relief rate In order to size the PRV inlet and outlet headers, a rnaximum relief rate or the rated relief rate has to be calculated. For vapor, liquid, and two phase PRV, the rated vapor and liquid relief rate are calculated as follows ... 

Aspinn possesses a number of properties that make it an often recommended drug It is an analgesic effective m relieving headache pain It is also an antiinflammatory agent providing some relief from the swelling associated with arthritis and minor injuries Aspinn IS an antipyretic compound that is it reduces fever How aspmn does all this was once a mystery but is now better understood and will be discussed m Section 26 6 Each year more than 40 million lb of aspirin is produced m the United States a rate equal to 300 tablets per year for every man woman and child... 

Radiation sensitive cast polymers from DADC are also used in resists for microelectronic circuitry. Relief images result from differential rates of solution in alkali induced by exposure to high energy radiations. 

For most processes, the optimum operating point is determined by a constraint. The constraint might be a product specification (a product stream can contain no more than 2 percent ethane) violation of this constraint causes off-specification product. The constraint might be an equipment hmit (vessel pressure rating is 300 psig) violation of this constraint causes the equipment protection mechanism (pressure relief device) to activate. As the penalties are serious, violation of such constraints must be very infrequent. 

Tlie safety valve is similar to the relief valve except it is designed to open fiillv, or pop, with onlv a small amount of pressure over the rated limit. Conventional safety valves are sensitive to dovvmstream pressure and niav have iinsatisfactorv operating characteristics in variable back pressure applications. The balanced safety relief valve is available and minimizes the effect of dovvmstream pressure on performance. 

In some instances, plant-specific information relating to frequencies of subevents (e.g., a release from a relief device) can be compared against results derived from the quantitative fault tree analysis, starting with basic component failure rate data. 

Required Relief Rate The required relief rate is the venting rate required to remove the volume being generated within the protected equipment when the equipment is at its highest allowed pressure ... 

W = required mass flow rate, Ib/min F = inlet mass flow rate, Ib/min = stream density entering the relief system, Ib/ft ... 

Transient Material and Energy Ralances The relief rate requirement at any instant during any event is developed on the basis that the total volume of vapor plus liquid is just equal to the vessel vol-... 

The time-dependent nature of the emergency pressure relieving event is obtained by the simultaneous solution of Eqs. (26-27) and (26-28). Generally, the only unknown parameters in these two equations are the venting rate W and the vent stream quahty (Xo). The vent rate W at any instant is a func tion of the upstream conditions and the relief system geometry. 

The couphng equation is a vapor mass balance written at the vent system entrance and provides a relationship between the vent rate W and the vent system inlet quahty Xq. The relief system flow models described in the following section provide a second relationship between W and Xo to be solved simultaneously with the coupling equation. Once W andXo are known, the simultaneous solution of the material and energy balances can be accomplished. For all the preceding vessel flow models and the coupling equations, the reader is referred to the DIERS Project Manual for a more complete and detailed review. 

Accelerating Rate Calorimeter (ARC) The ARC can provide extremely useful and valuable data. This equipment determines the self-heating rate of a chemical under near-adiabatic conditions. It usu-aUy gives a conservative estimate of the conditions for and consequences of a runaway reaction. Pressure and rate data from the ARC may sometimes be used for pressure vessel emergency relief design. Activation energy, heat of reaction, and approximate reaction order can usually be determined. For multiphase reactions, agitation can be provided. 

Reactive System Screening Tool (RSST) The RSST is a calorimeter that quickly and safely determines reactive chemical hazards. It approaches the ease of use of the DSC with the accuracy of the VSP. The apparatus measures sample temperature and pressure within a sample containment vessel. Tne RSST determines the potential for runaway reactions and measures the rate of temperature and pressure rise (for gassy reactions) to allow determinations of the energy and gas release rates. This information can be combined with simplified methods to assess reac tor safety system relief vent reqiiire-ments. It is especially useful when there is a need to screen a large number of different chemicals and processes. 

Maintenance and testing. It is not a good idea to apply vacuum on a vessel during maintenance or testing without full knowledge of the external pressure rating, unless a suitable vacuum relief device is in place and operable. 

Insufficient diluent due Provide automatic control of diluent to under feed or exces- addition sive evaporation result-, Select diluent less susceptible to evaporation ing in insufficient heat sink. Possibility of run- automatic/manual isolation based on away reaction due to detection of unexpected reaction rate high temperature excur- Provide emergency cooling Sion or high concentra-. adequately designed relief device tion of reacting species Monitor liquid level CCPS G-11 CCPS G-23... 

Available equip- Procure equipment that can be used in other ment determines processes (current or future) without operating the process chem- close to its operating envelope, istry selected., Provide equipment with comparable pressure Operating close to rating for the entire system the safe operating envelope of the batch sizes to equipment capabilities equipment and the relief capability. ... 

Install emergency relief/device/system on utility service set at or below vessel pressure rating... 

Failure of compo- Ensure all system components, including flexible nents in connectors are rated for maximum feasible subatmospheric vacuum conditions pressure convey-, Ensure adequate pressure control system and ing operations. back-up (e.g., vacuum relief devices) API 2000 CCPS G-3 CCPS G-11 CCPS G-22 CCPS G-29 CCPS G-3 9... 

Rapid eycling can occur when the pressure at the valve inlet decreases at the start of relief valve flow beeause of excessive pressure loss in the piping to the valve. Under these conditions, the valve will cycle at a rapid rate which is referred to as chattering. The valve responds to the pressure at its inlet. If the pressure decreases during flow... 

Defective pressure gauge. 10. Plugged oil sump strainer. 11. Defective oil relief valve. PACKING OVERHEATING 1. Lubrication failure. 2. Improper lube oil and/or insufficient lube rate. 3. Insufficient cooling. 

Maximum Pressure and Rate of Pressure Rise and KJ These explosibility parameters are used in assessing whether equipment will contain the maximum pressure developed during deflagration, or to design deflagration relief vents and other explosion prevention systems (see NEPA 68 and 69). The test method is given in ASTM E 1226. 

Deflagration venting (see NFPA 68) is highly applicable to powder operations. This is because most powder operations are at atmospheric pressure, the rate of deflagration pressure rise is usually small enough for vent relief panels to be of a practical size, and the subsequent fire is limited because most powders cannot bum in bulk. 

If casing limitations are fixed by user-supplied relief valves, this information should be conveyed to keep the vendor from rating the compressors on other data. Evaluations can be more of a problem if the same design basis isn t universal with all vendors. Startup and shutdown consideration influence various components, shaft end seals, seal system pressures, and even thrust bearings in some instances. The use of an alternate startup gas, or the desire to operate a gas compressor on air to aid in plant piping dryout should be covered. 

Steiic effects of another kind become important in highly branched substrates, in which ionization is facilitated by relief of steric crowding in going from the tetrahedral groimd state to the transition state for ionization. The ratio of the hydrolysis rates in 8OV0 aqueous acetone of t-butyl /F-nitrobenzoate and 2,3,3-trimethyl-2-butyl p-nitrobenzoate is 1 4.4. 

Also, the vapor load at the time of relief may be reduced below the normal design rate, due to the higher pressure, which may suppress vaporization at the time of the overpressure. Pinchout of a reboiler is such a situation. In such a case, steam pressure design conditions may be used, rather than the maximum steam pressure which could exist under pressure relieving conditions of the steam system. These changes can be taken into account, where appropriate, both for the equipment involved and for downstream equipment. 

Partial Condensing - The relief requirement is the difference between the incoming and outgoing vapor rates at relieving conditions. The incoming vapor rate should be calculated on the same basis as stated earlier. If the reflux is changed in composition or rate, the incoming vapor rate to the condenser should be determined for the new conditions. 

Pumparound Flow Failure - The relief requirement is the vaporization rate caused by an amount of heat equal to that removed in the pumparound circuit. The latent heat of vaporization would correspond to the temperature and pressure at PR valve relieving conditions. "Pinchout" of steam heaters may be considered. 

Conventional PR valves and discharge systems should be designed such that built-up back pressure does not exceed 10% of set pressure (both measured in psig), to avoid chattering problems. In the case where a pressure relief valve system is sized for fire conditions, with 21 % overpressure, built-up back pressure up to 21 % of set pressure is permissible. However, the lower rates resulting from other contingencies still must meet the 10% limitation. 

The required relieving rate for a pressure relief valve is determined from consideration of the contingencies which can cause overpressure. Basic... 

---