Relieving conditions

When determining pressure rehef requirements, one should calculate capacities of control valves for the relieving conditions of temperature and pressure, since these are in many cases significantly different from capacities at normal operating conditions. Downstream equipment must be analyzed imder relieving conditions. 

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. 

Abnormal Process Heat Input - The required capacity is the maximum vapor generation rate at PR valve relieving conditions, including any noncondensibles produced from overheating, less the normal condensation or vapor outflow rate. In every case, one should consider the potential behavior of a system and each of its components. For example, the fuel or heating medium control valve or the mbe heat transfer may be the limiting consideration. Consistent with the practice... 

It is also important to note that the lowest design pressure of any section of the casing must be specified to be no lower than the pressure which it may be subjected to under the PR valve relieving conditions. This is necessary to recognize pressure drop within the casing. The PR valve should be sized to pass the normal steam flow to the turbine, but credit may be taken for steam flow which is withdrawn from an intermediate turbine stage if it would not be blocked by the same contingency as closure of the exhaust. 

P, = Flange inlet pressure at relieving conditions (including accumulation),... 

T, = Inlet temperahire at relieving conditions, K. Note For the special case where a PR valve is being sized for fire, T, may be estimated by raising the midboiling point of the liquid to the inlet pressure P, using vapor pressure charts. 

In production facility design, the most common relieving conditions are (1) blocked discharge, (2) gas blowby, (3) regulator failure, (4) fire. (5) thermal, and (6) heat exchanger tube rupture. Relief valve design How rates are commonly determined as follows. 

It is essential that the ASME code requirements be understood by the designer and individual rating and specifying the installation details of the safety device. It is not sufficient to merely establish an orifice diameter, since process considerations which might cause overpressure must be thoroughly explored in order to establish the maximum relieving conditions. 

There have been at least six different formulas proposed and used to determine the proper and adequate size of rupture disk openings for a specific relieving condition. The earlier studies of Sylvander and Katz [25] led to the development of the ASME and API recommendations. This approach assumes that a fire exists under or around the various vessels in a process. This fire may have... 

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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. 

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Failure of feed controller. A common practice (10) is to set the relief requirement equal to the difference between the maximum feed flow and the normal column outlet flow at relieving conditions, assuming that other valves in the system stay at their normal operating positions. 

Alpha-titanium alloys. These alloys range in yield tensile strength from 173 to 483 MPa. Variations are generally achieved by alloy selection and not heat treatment. They usually contain alpha stabilizers and have the lowest strengths. However, they are formable and weldable. Some contain beta stabilizers to improve strength. Alpha-titanium alloys are generally in the annealed or stress-relieved condition. They are considered fully annealed after... 

Product T1-3A1-2.5V is a near-alpha alpha-beta alloy Condition/ that is generally used in the cold-worked and Microstructure stress-relieved condition. Ti-3A1-2.5V can be heat... 

For corrosion-resistant applications, Ti-3A1-2.5V usually is used in the annealed condition (as opposed to the cold-worked and stress-relieved condition tjrpically used in aerospace applications). Weldments of Ti-3A1-2.5V also exhibit corrosion resistance similar to the base metal. This alloy contains so little alloy content and second phase that metallurgical instability and thermal response are not significant. Therefore, weldments and associated heat-affected zones (HAZ) generally do not experience corrosion limitations in welded components when normal passive condi-... 

Ti-3A1-2.5V is usually used in the annealed or in the cold worked plus stress relieved conditions. However, due to the small amount of P stabilizer present, a small age hardening response is possible from the solution heat treated condition. Only a small increase in strength is possible in thin sections by solution treating and aging. For example,... 

Zirconium alloys are generally used in the annealed or stress-relieved condition. They can be fully annealed at a temperature range of 675 to 800 C for 2 to 4 h at temperature. When R69705 is heat treated at temperatures in excess of 675°C, the subsequent cooling rate should be controlled. The cooling rate should not exceed 110°C/h until the temperature of the material is less than 480°C. Stress relieving of zirconium alloys is done at 540 to 595°C for 0.5 to 1 h at temperature. 

Used to indicate the inlet pressure and temperature of a pressure relief device at a specific overpressure. The relieving pressure is equal to the valve set pressure (or rupture disk burst pressure) plus the overpressure. The temperature of the flowing fluid at relieving conditions may be higher or lower than the operating temperature. 

Loss of reflux on the column should be considered as a total loss of cooling. It is assmned that with loss of reflux the liquid accumulates and floods the reflux drum, and then the cooler. The cooler loses the cooling duty and the column pressure will increase to its relieving condition. No credit should be taken for the normal outflow of uncondensed vapor. The stahc head of the flooded system should be allowed for in design pressure consideration as explained previously. 

Xo = vapor mass fraction at the PRV inlet. This is estimated at the PRV relieving conditions. 

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