Orifice area

Section 10 of this Handbook describes the use of orifice meters for flow measurement. In addition, orifices are commonly found within pipelines as flow-restric ting devices, in perforated pipe distributing and return manifolds, and in perforated plates. Incompressible flow through an orifice in a pipehne as shown in Fig. 6-18, is commonly described by the following equation for flow rate Q in terms of pressure drop across the orifice Ap, the orifice area A, the pipe cross-sectional area A, and the density p. 

A Spouted-bed inlet orifice area cm in 1 Wear displacement of indentor cm in... 

Figure 13. Safety valve orifice area required for hydrocarbon vapor release.

S = Specific gravity at flowing temperature versus water at 15°C fi= Viscosity of fluid at flowing temperature, centipoises or mPa-s A = Effective orifice area, mm from the manufacturer s literature). 

Calculate individually the orifice area required to pass the flashed vapor component, using Equation (5a), (3b), (4), (5), or (6), as appropriate, according to service, type of valve and whether the back pressure is greater or less than the critical flow pressure. 

Calculate individually the orifice area required to pass the unflashed hquid component, using Equation (8). The pressure drop term Pj should be made equal to the set pressure minus the total back pressure developed by the vapor portion at critical flow pressure, except when the critical flow pressure is less than the calculated total back pressure (superimposed plus built-up), considering the combined liquid and vapor flow. In the latter case, P should be made equal to set pressure minus the calculated total back pressure. 

Add the areas calculated for the vapor and liquid components to obtain the total orifice area required. This may be somewhat conservative, since flashing does not take place instantaneously across the PR valve orifice. 

Sizing for Mixed-Phase Vapor and Liquid Service - PR valve sizing for a mixture of hquid and vapor at inlet conditions may be calculated using the sum of the orifice areas required for each phase individually, in the same way as described above for flashing liquid service. 

Sizing methods for pilot-operated pressure relief valves are in accordance with the accepted formulas described above, utilizing the appropriate discharge coefficients and effective orifice areas as recommended by the valve manufacturer. The following points should be noted ... 

Number of Valves Required - Normally a manufacturer s standard PR valve with Drifdce area equal to or larger than the calculated requirement is specified. In some cases, e.g., large reheving rates or to prevent chattering, two or more valves are necessary. Likewise, if there is an appreciable difference between the calculated orifice size and the available size, multiple PR valves are desirable to more nearly match the available area to the required orifice area. The column for "spares" indicates the requirement, if any, for spare PR valves installed on the equipment. Normally, this applies only in the case of refinery preference or local regulations, but is required in many European countries. 

Equivalent leakage area (ELA) The specified design pressure difference that allows a given air quantity to pass through a set orifice area. 

In addition to valve velocity, the manufacturer can furnish the effective flow area of the valve. This area is determined by measuring the pressure drop across the valve with a known flow rate and then calculating an equivalent orifice area that provides the same pressure drop. Valves with larger effective flow areas have less pressure drop and better efficiencies. The effects of the seat area, the lift area, and the flow paths are automati-... 

The flow rate for gas through a given orifice area or the area required for a given flow rate is obtained by ... 

There are no precise formulas for calculating orifice area for two-phase flow. The common convention is to calculate the area required for the gas flow as if there were no liquid present and the area required for the liquid flow as if there were no gas present. The two areas are then added to approximate the area required for two-phase flow. 

In a later paper, Brasie (1976) gives more concrete recommendations for determining the quantity of fuel released. A leak potential can be based on the flashing potential of the full amount of liquid (gas) stored or in process. For a continuous release, a cloud size can be determined by estimating the leak rate. For a combined liquid-vapor flow through holes of very short nozzles, the leak rate (mass flow per leak orifice area) is approximately related to the operating overpressure according to ... 

It is important to understand how the over-pressure can develop (source) and what might be the eventual results. The mere sohdng of a formula to obtain an orifice area is secondary to an analysis and understanding of the pressure system. Excess pressure can develop from explosion, chemical reaction, reciprocating pumps or compressors, external fire around equipment, and an endless list of related and unrelated situations. In addition to the... 

The orifice area of these devices (see illustrations) is at the outlet end of the SRV nozzle through which the discharging vapor/gases/liquids must pass. These values are identified in industry as (valve body inlet size in.) X (orifice letter) X (valve body outlet size, in.). For example, a valve would be designated 3E4. 

The standard orifice area designations are (also refer to mechanical illustrations of valves, previously shown this chapter) ... 

Note These letters and orifice areas are not consistent for these large orifices between various manufacturers. Some sizes go to 185 sq in., which is a very large valve. When two values are shown, they represent two different published values by manufacturers. 

For final calculation use from Figures 7-23 or 7-24 and substitute in the above equation. Determine the needed Reynolds number, Re, using the next size larger orifice. Area is determined from that made in the first trial calculation [33]. 

Calculations of Orifice Flow Area using Pressure Relieving Balanced Bellows Valves, with Variable or Constant Back Pressure. Must be used when backpressure variation exceeds 10% of the set pressure of the valve. Flow may be critical or non-critical for balanced valves. All orifice areas. A, in sq in. [68]. The sizing procedure is the same as for conventional valves listed above (Equations 7-10 ff), but uses equations given below incorporating the correction factors K, and K,, . With variable backpressure, use maximum value for P9 [33a, 68]. 

A = required orifice area in square inches. This is as defined in ihe ASME Code and ANSI/APl Std 526. 

Since the viscosity correction factor is dependent upon the actual orifice area, direct solution is not possible and a trial orifice size must be found before Ihe Ku can be determined accurately. 

Step 1 — Calculate IMsI Orifice Calculate the trial required orifice area from the liquid equation on page 40 (Alternate used in this example). 

Select Ihe next larger orifice size, or an M" orifice with 360 sq in. orifice area. (This should be about 20% greater than the calculated area to allow lor reduction of capacity due to Ihe viscosity correction factor "Ku")... 

A= effective discharge area, in square inches (from manufacturers standard orifice areas). 

After the value of R is determined, the factor Kvt is obtained from the graph. Factor Kv is applied to correct the "preliminary required discharge area. If the corrected area exceeds the chosen standard orifice area", the above calculations should be repeated using the next larger standard orifice size. 

This relation can be converted to solve for the required orifice area at 25% overpressure for non-viscous liquids discharging to atmosphere [24]... 

A = required orifice area, sq in. P] =. set pressure of valve, psig... 

Pressure Relief Valve Orifice Areas on Vessels Containing Only Gas, Unwetted Surface... 

Due to gas expansion from external fire, the API code [10] provides for calculation of the pressure relief valve orifice area for a gas containing vessel exposed to external fire on die unwetted surface ... 

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