Rated relieving capacity

The stamped capacity of a spring loaded safety or safety relief valve (nozzle type) when installed with a rupture disk device between the inlet of the valve and the vessel shall be multiplied by a factor of 0.80 of the rated relieving capacity of the valve alone, or alternatively, the capacity of such a combination shall be established in accordance with Par. 3 below ... 

When the rupture disk is installed on the inlet side of the pressure relief valve (see Figures 7-10, 7-11 and 7-12), the ASME code requires that for untested disk-valve combinations that the relieving capacity of the combination be reduced to 80% of the rated relieving capacity of the pressure relief valve [ 1 ]. 

Rated relieving capacity The measured relieving capacity approved by the applicable code or regulation, to be used as a basis for the application of an SRV on a system which requires compliance with the code. 

When a specific valve design is selected for the application, the rated capacity of that valve can be determined using the actual orifice area, the rated coefficient of discharge and the equations presented in this book. This rated relieving capacity is then used to verify that the selected valve has sufficient capacity to satisfy the application. 

When combining two relief devices, they can be used after each other or in parallel. When used in parallel, for whatever reason, there is no need for a derating. However, the rated relieving capacity of an SRV in combination with a rupture disc is equal to the capacity of the PRV multiplied by a combination capacity factor to account for any flow losses attributed to the rupture disc which might depend on the type of installation. 

Vacuum (a) Removal of liquid or vapor at greater rate than entering a vessel, capacity determined by volume displaced. (b) Injecting cold liquid into hot (steamed out) vessel, the condensing steam will create vacuum, and must be relieved. Capacity is equivalent to vapor condensed. 

Once the total pressure loss is calculated, we can determine the de-rating factor to be multiplied by the effective relieving capacity of the SRV, which can be found on its tag plate. 

Nominal relieving capacity per valve, ASME flow rate (gpm) Nominal set pressure (psig)... 

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... 

When a PR valve is relieving at rated capacity, the total frictional pressure drop between a vessel and the inlet of the valve should be less than 3% of the set pressure (kPa). In this calculation, the effect on static pressure of fluid acceleration is ignored rather, only friction loss is considered. 

Back-pressure can affect either the set pressure or the capacity of a relief valve. The set pressure is the pressure at which the relief valve begins to open. Capacity is the maximum flow rate that the relief valve will relieve. The set pressure for a conventional relief valve increases directly with back-pressure. Conventional valves can be compensated for constant back-pressure by lowering the set pressure. For self-imposed back-pressure—back-pressure due to the valve itself relieving—-there is no way to compensate. In production facility design, the back-pressure is usually not constant. It is due to the relief valve or other relief valves relieving into the header. Conventional relief valves should be limited to 10% back-pressure due to the effect of back-pressure on the set point. 

Safety Valve this is an automatic pressure-relieving detice actuated by the static pressure upstream of the valve and characterized by rapid full opening or pop action upon opening [1,10], but does not reseat. It is used for steam or air service (Figure 7-2). Rated capacity is reached at 3%, 10% or 20% overpressure, depending upon applicable code. 

Safety-Relief Valve this is an automatic pressure-relieving device actuated by the static pressure upstream of the valve and characterized by an adjustment to allow reclosure, either a pop or a non-pop action, and a nozzle type entrance and it reseats as pressure drops. It is used on steam, gas, vapor and liquid (with adjustments), and is probably the most general tyqDe of valve in petrochemical and chemical plants (Figures 7-3, 7-3A, and 7-4). Rated capacity is reached at 3% or 10% overpressure, depending upon code and/or process conditions. It is suitable for use either as a safety or a relief valve [1,10]. It opens in proportion to increase in internal pressure. 

Relief Yalve normally selected for liquid relief service such as hydraulic systems, fire and liquid pumps, marine services, liquefied gases, and other total liquid applications. The valve characteristically opens on overpressure to relieve its rated capacity, and then reseats. 

Given the rate of fluid flow to be relieved, the usual procedure is to first calculate the minimum area required in the valve orifice for the conditions contained in one of the followng equations. In the case of steam, air or water, the selection of an orifice may be made directly from die capacity tables if so desired. 

Relief Valve a relief valve is an automatic spring loaded pressure-relieving device actuated by the static pressure upstream of the valve, and which opens further with increase in pressure over the opening pressure. It is used primarily for liquid service [1,10] (Figure 7-1A and 7-1B). Rated capacity is usually attained at 25 percent overpressure. 

Another area of confusion might involve the definition of capacity and how the term is used in ASME and API. Relieving rates are determined from what can go wrong scenarios and, if allowed to go unchecked, would overpressure the vessel. 

Like other pressure vessel codes, the PED recognizes that most SRVs need some increase in pressure above their set pressure to be fully opened and relieve their full rated capacity. This is what we call overpressure. 

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. 

The rate of cholesterol esterification in plasma is not correlated with HDL concentration (A12, R17, S45, S58, Wl, W2) but is correlated with the concentration of VLDL or triglyceride (A12, P8, R17, S58, T7, Wl, W2). Although HDL is the major substrate for LCAT, VLDL and indirectly LDL are the major recipients of the esterfied cholesterol, transferred (it is thought) by lipid transfer protein. Accumulation of esterified cholesterol in the recipient lipoproteins is associated with a decrease in LCAT activity (C7, Fll, F13) that can be relieved by the addition of recipient lipoproteins but not by addition of LCAT substrate (Fll). Hopkins and Barter (H32, H33) have explained these observations by showing that the depletion of HDL esterified cholesterol by transfer to VLDL enhances the capacity of HDL to act as a substrate for LCAT. 

The vapor to be relieved is the vapor in equilibrium with the liquid (that is, saturated vapor at the conditions existing when the valve is relieving at the rated capacity). Table 5-3 shows environmental factors for bare and insulated vessels. 

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