# Data Entry - Relief Valve and Bursting Disk

The data entry requirement for a relief valve element is shown in Figure 1 below: Figure 1: Data Entry for a Relief Valve and Bursting Disk. Unique Name: Allows you to enter a descriptive name for this element. It is not necessary to add any data to this field. If you enter data here, you can use this later to sort via the Lists. Status: There are 3 possible status values for relief valves: 'On', 'Off', or 'Ignore Pressure Loss'. •'On' is the normal position and tells the application that this node is open and to calculate the flow out of the element. •'Off' means that this component is closed to flow. You can close resistances to isolate part of a network. •'Ignore Pressure Loss' means that the node is Open, but the pressure loss calculation is skipped. Elevation: This is the height above a selected datum. You can select any datum (baseline with elevation = 0) that is convenient for you. When you construct any network, you need to decide on a datum. This can typically be ground level or the centerline of a pump. In flow calculations, absolute elevations are not important, only the relative elevations of the nodes are important. The elevation that you enter in this row is the elevation of the connector measured from the datum. Elevation Unit: The units of measurement used to specify the component elevation. Role: Allows you to configure relief protection configuration, whether 'Safety Valve Only', 'Bursting Disk Only', or 'Safety Valve and Bursting Disk' combination. Automatically Size: This option is set to 'On' by default, meaning the software will automatically calculate the relief valve size based on the set pressure, design flow rate, discharge coefficient (Kd), back pressure factor (Kb), and combination correction factor (Kc). FluidFlow also suggests the closest matching standard orifice size based on the calculated system operating conditions. Note, that this is only applicable to the API RP520 pressure loss model as the ISO 4126 methodology doesn't utilize standard orifice sizes. If the 'Automatically Size' option is set to 'Off', additional rows will appear: Figure 2: Additional data entry requirements for a Relief Valve when 'Automatically Size' is set to 'Off'. When the 'Automatically Size' option is set to 'Off', the software will calculate the flow rate, fluid velocity, pressure loss, etc., through the relief valve (relief valve and bursting disk) based on the relief valve size manually selected by the user in the row titled 'Standard Orifice Size'. Use Standard Size: FluidFlow contains a set of standard orifice sizes which are designated by orifice letter as defined in the API standards. Each letter refers to a specific effective orifice area, for example, the effective orifice area of a P orifice is 6.38in2 / 4120mm2. Selecting 'Yes' allows you to select a standard orifice area from a dropdown list. Selecting 'No' gives the option of entering your own non-standard orifice area. Non-Standard Size Area: This row is available if you select not to use a standard valve orifice size. Enter the valve orifice area in this row. Size Unit: These are the units used to specify the non-standard valve orifice area. Relief Valve Name: You cannot add a name directly into this row by typing an entry. Instead, when you focus on this row a button appears at the right edge. Clicking the button pops up the Set Component dialog, which allows you to make a relief valve model selection. Note, care should be taken to ensure that the selected relief valve is suitable for the given modelling application, for example, a JBS type relief valve is applicable to gas/steam systems whereas a WB200 is more suited to liquid systems. Further information on the relief valve usage may be obtained by selecting the Database | Relief Devices menu item, expanding the 'Relief Valve' field to reveal all safety valves in the database, and then selecting the relevant valve from the list. Pressure Loss Model: This refers to the calculation method you wish to use. Users can choose from the methodology applied in 'API (American Petroleum Institute) RP520 Part I' or 'ISO 4126-1'. •'API RP520 Part 1' allows calculation for all fluid phases (Non-Newtonian flow may not be accurate). •'ISO 4126' is for liquids and gases/vapors only. Overpressure: The pressure increase over the set pressure of the relieving device. The overpressure which actuates the pressure relief devices is caused by an inadvertent energy input. The most common causes of overpressure scenarios are external fire, blocked outlet, inadvertent opening of control valve, check valve malfunction, utility failure (power, cooling water, instrument air, steam, fuel gas, fuel oil, inert gas, etc.), liquid overfilling, thermal expansion, abnormal process conditions such as runaway chemical reactions, and equipment failure (pumps, fans, blowers, compressors, etc.). The overpressure is generally expressed as a percentage of the set pressure at which the relief valve is designed to operate. The options for this entry are: 'None (0%)', 'Sole Device (10%)', 'Multiple Devices (16%)', and 'External Fire (21%)'. Accumulation is the pressure increase over the Maximum Allowable Working Pressure (MAWP) of the vessel. It is expressed in pressure units or as a percentage of MAWP or design pressure. The overpressure is based on the set pressure of the relieving device while the accumulation is based on the MAWP of the vessel. Overpressure is the same as accumulation only when the relieving device is set to open at the MAWP of the vessel. The difference between overpressure and accumulation is illustrated in Figure 3. Figure 3: Difference between Overpressure and Accumulation. Note, ASME Code Section VIII and API RP520 Part 1 set out details of permissible pressure accumulation levels for a variety of relief valve installation configurations. FluidFlow allows users to choose between each. The following is a synopsis of the various options available. - None (0%) This option allows the user to set the overpressure/accumulation during abnormal operating conditions to 0% over the set pressure/MAWP. - Sole Device (10%) This option may be selected when the source of overpressure is NOT being generated by an external fire condition and the system is protected by a single relief device. In these circumstances, the system is allowed to experience an overpressure/accumulation during abnormal operating conditions of up to 10% over the set pressure/MAWP. The single valve must therefore be capable of relieving the entire system's overpressure. Design Note: The valve relieving capacity must equal or exceed the capacity capable of being generated by the protected system at its overpressure condition. - Multiple Devices (16%) There is no requirement in ASME Code Section VIII that requires the use of multiple relief devices nor is there a requirement that multiple valves be of the same size. However, some applications may be such that the required relieving capacity is too high for a single device. In this scenario, the overpressure/accumulation for a non-fire generated overpressure condition shall not exceed 16% above the set pressure/MAWP. This additional overpressure/accumulation allows the various relief devices to be set at different pressures. This staggered set point approach helps eliminate interaction between the multiple safety valves. Note, that the primary safety relief valve must be set no higher than the design pressure/MAWP but may also be set lower than the design pressure/MAWP. Any additional or supplementary safety valves can be set higher than the design pressure/MAWP but at a level not higher than 5% above the design pressure/MAWP. Design Note: The sum of the individual valve relieving capacities must equal or exceed the capacity capable of being generated by the protected system at its overpressure condition. Although there is no requirement to ensure each relief valve is the same size, it is a common occurrence and is useful in terms of utilizing the supply of spare parts. - External Fire (21%) This option may be selected as an emergency situation where the protected system may experience an overpressure condition as a result of exposure to an external fire. This option applies to both single and multiple safety valve installations. The allowable overpressure/accumulation under this condition is 21% above the set pressure/MAWP. To minimize the quantity of valves required to protect the system, it is important to firstly select valves for overpressure operating conditions. As fire sizing should be considered as an emergency situation, consideration should be given to the valves already selected for an overpressure operating conditions and any fire sized valve relieving requirements should only be additive to those already selected. Design Note: The sum of the individual valve relieving capacities must equal or exceed the capacity capable of being generated by the protected system at its 21% overpressure/accumulation condition. Set Pressure: The inlet gauge pressure at which the pressure relief device is set to open under service conditions. The Maximum Allowable Working Pressure (MAWP) is the maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature specified for that pressure. The MAWP is the basis for the pressure setting of the pressure relief devices that protect the vessel. The MAWP is normally greater than the design pressure but can be equal to the design pressure when the design rules are used only to calculate the minimum thickness for each element and calculations are not made to determine the value of MAWP. The Design Pressure (DP) is the pressure, together with the Design Temperature (DT), used to determine the minimum permissible thickness or physical characteristic for each vessel component as determined by the vessel design rules. The design pressure is selected by the user to provide a suitable margin above the most severe pressure expected during the normal operation at a coincident temperature. It is the pressure specified on the purchase order. This pressure may be used in place of the MAWP in all cases where the MAWP has not been established. The design pressure is equal to or less than the MAWP. Design Note: During the pressure relief valve design, the engineer uses the design pressure as the set pressure of the relief valve since MAWP is not available. Moreover, the design pressure is equal to or less than the MAWP, (DP <= MAWP). Thus, it is safe to use the design pressure as our set pressure since the relief valve will open at this pressure (If the design pressure is lower than the MAWP). If the design pressure is equal to the MAWP, it is still safe since the hydrotest pressure of the equipment is 130% of the MAWP per ASME Section VIII Div. 1 or 150% of the MAWP for other codes/standards. For the 'Safety Valve and Bursting Disk' role, the bursting disk is installed at the inlet of the safety valve, the devices are considered to be close-coupled, and the specified burst pressure and set pressure should be the same nominal value. Pressure Unit: The pressure units used to specify the Set Pressure. Design Flow: This represents the required capacity, relieving capacity, or relieving load of the safety relief valve. The relieving load/capacity is the calculated mass/volumetric vent rate which is required to maintain the system pressure within the allowable limits. The relieving load due to a particular overpressure scenario should be calculated based on the heat and material balance under failure conditions. Note, that care should be taken when modelling gas systems to ensure the correct Reference Gas Volume Flow Units have been applied, i.e., NTP (0oC and 101325 Pa) or STP (15oC and 101325 Pa). To set the reference gas condition, select the 'Options | Calculation...' menu item, followed by the 'Gas' tab. You can then select NTP or STP from the drop-down menu. Flow Unit: The flow units used to specify the valve relieving flow capacity. Discharge Coefficient (Kd): For relief system design purposes, the ideal flow calculated is adjusted to establish the design/actual capacity of the relief valve. The discharge coefficient is the ratio of measured relieving capacity to the theoretical relieving capacity, i.e., Actual Flow Capacity/Theoretical (Ideal) Flow Capacity. The actual mass flow rate through the relief valve is diminished from the theoretical value by the existence of a fluid boundary layer. Therefore, a "discharge coefficient" (Kd) is applied to the theoretical value required to obtain the actual flow rate. ASME code requires a "safety factor" of 0.9 or less to account for uncertainties in the calculated flow and to obtain the "certified flow". Certified coefficients of discharge "0.9Kd" and actual relief device nozzle areas "ADevice" are available from the National Board Pressure Relief Device Certifications, aka the "Red Book". Note that certified coefficients of discharge provided in the NB "Red Book" include the "0.9" ASME safety factor. The NB "Red Book" uses the symbol "K" to represent the certified coefficient of discharge. The value of "K" is 0.9 times the experimental "best estimate" coefficient of discharge "Kd". Note, that users are required to enter a value for Kd when the 'Automatically Size' option is set to 'On'. If you are not using the 'Automatically Size' function and intend to manually select a relief valve from the Input Editor, please note that the value for Kd is defined for each relief valve in the component database. Typical values for Kd = 0.975 for many nozzle-type valves however, it is recommended that accurate values for Kd be obtained from the device manufacturer. Note also, that users have the option of defining additional relief valves in the components database via the Database | Relief Devices menu item. It is recommended that when adding new valves the value for Kd should be obtained from the manufacturer. Back Pressure Correction Factor (Kb): With a balanced valve, high back pressure will tend to produce a closing force on the unbalanced portion of the valve disc. This force may result in a reduction in lift and an associated reduction in flow capacity. Capacity correction factors called back pressure correction factors, are provided by manufacturers to compensate for this reduction in flow. Note, Kb = 1.0 if back pressure conditions do not apply. Combination Correction Factor (Kc): This input field appears when the selected role is 'Safety Valve and Bursting Disk' also known as combination systems (Safety Valve and Bursting Disk in series). Combination systems experience extra turbulence as a result of the bursting of the disk and are therefore modelled as a safety valve nozzle modified by a certified 'combination factor' Kc. The correction factor 'Kc' is applied to account for pressure loss across a bursting disk upstream of a relief valve during overpressure conditions. Typically, Kc = 0.9 when a bursting disk is installed in combination with a relief valve. Nonetheless, this value can be superseded by a certified manufacturer Kc value for the relief device combination. Use Kr from Database: This field is displayed when the selected role is 'Bursting Disk Only' and enables users to manually enter a 'Kr' value by selecting the 'No' option, or to apply a certified 'Kr' value (via the 'Kr Component Name' field) from manufacturers as defined and stored in the Relief Devices database by selecting 'Yes'. Kr Component Name: Applicable only to the 'Bursting Disk Only' role, when 'Use Kr from Database' is set to 'Yes'. Selecting this field will display the Set Component dialog for the Bursting Disks database. Certified Resistance Coefficient (Kr): This input field appears when the selected role is 'Bursting Disk Only' and 'Use Kr from Database' is set to 'No'. It allows you to numerically define a 'Kr' value. The resistance of the bursting disk is modelled using a certified resistance coefficient "Kr". The flow resistance for bursting disk devices can be obtained from the manufacturer. The flow resistance is expressed in terms of velocity head loss "Kr". ASME Code certified values should be used where available. Loss Basis: This input field appears when the selected role is 'Bursting Disk Only'. It is used to configure the friction loss calculation basis involving bursting disk Kr values using Equation 6-26 of Crane Technical Paper 410. The user has the option to select either 'Upstream Velocity' or 'Downstream Velocity' to calculate the frictional loss. Heat Loss Model: For a relief valve there are 3 possible entries: 'Ignore Heat Loss/Gain', 'Fixed Temperature Change', and 'Fixed Transfer Rate': •Selecting 'Ignore Heat Loss/Gain' means that flow across the valve is assumed to be adiabatic. •Selecting 'Fixed Temperature Change' allows you to specify a temperature change for the fluid as it flows through the valve. •'Fixed Transfer Rate' allows you to specify a fixed amount of heat to be either taken out of or added to the fluid as it flows through the valve. Selecting 'Fixed Temperature Change' adds additional rows to the Input Editor: Figure 4: Additional input data required for Heat Loss Model set to Fixed Temperature Change - Heat Transfer Direction: This value can either be: 'Out of Network' or 'Into Network'. •'Out of Network' means that heat is lost from the fluid. This has the effect of a temperature drop in the fluid as it flows through the relief valve. •'Into Network' means that the temperature of the fluid is increased as the fluid flows through the valve. - Temperature Change: Use this row to specify the temperature change that occurs across the relief valve. The 'Heat Transfer Direction' setting determines if the temperature is increased or decreased. FluidFlow adjusts fluid physical properties across the valve as the temperature changes and also calculates the total amount of heat that needs to be transferred for the specified temperature change to occur. NOTE: You should be aware that NO check is made to determine if this temperature change is possible, i.e., can this temperature change actually occur? You can think of this mathematically as FluidFlow makes the calculation Heat Transferred = Mass Flow * Specific Heat * Temperature Change but does not make the calculation Heat Transferred = Heat Transfer Coefficient * Heat Transfer Area * Log Mean Temperature Driving Force If you specify a temperature that means a change of phase state you will NOT obtain accurate results unless you have the two-phase module activated; instead you will get a warning and the friction loss calculation will be made according to the upstream phase state. - Temperature Unit: The unit of temperature used to specify the temperature change. Selecting 'Fixed Heat Transfer' can be conceptualized as the converse of selecting 'Fixed Temperature Change'. As before, additional rows appear in the Input Editor: Figure 5: Additional input data required for Heat Loss Model set to Fixed Transfer Rate - Heat Transferred: Allows you to specify the quantity of heat transferred to (or from) a relief valve. This row works in relation to the 'Heat Transfer Direction' setting to determine the direction of heat flow. FluidFlow adjusts fluid physical properties across the valve as the heat transfer occurs and also calculates the total temperature change that will occur. NOTE: You should be aware that NO check is made to determine if this heat transfer is possible. You can think of this mathematically as FluidFlow makes the calculation Heat Transferred = Mass Flow * Specific Heat * Temperature Change but does not make the calculation Heat Transferred = Heat Transfer Coefficient * Heat Transfer Area * Log Mean Temperature Driving Force If you specify a heat transferred quantity that means a change of phase state occurs, you will NOT obtain accurate results unless you have the two-phase module activated; instead, you will get a warning and the friction loss calculation will be made according to the upstream phase state. - Heat Transfer Unit: The unit of heat transferred quantity that is used to specify the heat transferred to or from the fluid as it flows across the element. User-Defined Fields: Allows you to add extra "fields" to the element, for use in, for example, scripting or reporting. Clicking on the input will display the User-Defined Fields dialog to add/edit/delete fields. Properties On Flowsheet: This value can either be: 'Show' or 'Hide'. This row is available for all components. This allows you to decide what properties you wish to show on the flowsheet and for which components. If 'Show' is selected then additional rows appear in the Input Editor, from which you can set text font, color, size, text position, and the properties you wish to display: Figure 6: Additional input data required for showing properties on the flowsheet - Alignment: This row controls the alignment of the component properties on-screen. Possible values are 'Top', 'Bottom', 'Left', or 'Right'. The alignment of the displayed text is relative to the center of the component. - Font: Text font properties can be set from the standard windows font dialog. - Properties: Shows/hides Input and Result fields for the display of properties on the flowsheet. This allows for the visibility of any input or calculated/result data to be controlled for each component. Individual property setting is made via the Select Element Properties dialog.