Dimensioning of Safety Valves Auditorium Tecnimont 21.09.2016
Objective of the presentation Design of Safety Valves ASME VIII / API 520 The objective of the presentation is to show the design of safety valves in compliance with API 520 Standard specifications for the design of safety valves Formulas for the design of safety valves s Influencing the stability in operation 2/ 16
Relieving cases according to API 520 for the calculation of the required mass flow of safety valves API 521 is designed to aid in the selection of the system that is most appropriate for the risks and circumstances involved in various installations. This standard provides guidelines for: Examining the principal causes of overpressure Determining individual relieving rates Including fire vapor generation and fire gas expansion Selecting and designing disposal systems 3/ 16
National and international standards for the calculation specifications of safety valves Calculation levels for safety valves AD 2000 ISO 4126-1 API 520 Merkblatt A2 ASME VIII Calculation levels of inlet pressure loss and back pressure ISO 4126-9 Chapter 7 + 9 AD 2000 Merkblatt A2 Chapter 6 4/ 16
What parameters are important for the design and how are they related? High Performance Coefficient of discharge α w : the rated coefficient of discharge from component testing (often also referred to as α d ) Orifice area A 0 : actual orifice area Substance information medium-dependent substance data Operating data: state parameters like pressure and temperature A 0 5/ 16
Coefficient of discharge and rated coefficient of discharge German Code American Code VdTÜV Merkblatt SV 100, 3.3.1 ASME-Code Sec.VIII, Div. 1, UG-131 (e) Coefficient of discharge = q measured q theoretical K d = q measured q theoretical Rated coefficient of discharge w = 0.9 x K = 0.9 x K d q measured q theoretical = actual measured q m = calculated q m or K d = coefficient of discharge d or K = rated coefficient of discharge 0.9 = correction factor 6/ 16
Differentiation of media Medium Steams/ gasses Liquids Steam Subcritical Supercritical Low viscosity High viscosity Saturated steam Superheated steam Twophase flow Liquid phase Gaseous phase 7/ 16
Design for gasses/steam as per API 520 API A = W C K d P 1 K b K c T Z M Rated coefficient of discharge Back pressure correction factor Bursting disc correction factor How to calculate? 8/ 16
Correction Safety Valve Bursting Disc Combination Remarks to the sizing of a spring-loaded safety valve in combination with upstream bursting disc Unaffected are: Performance Coefficient of discharge Opening characteristics If the choosen bursting disc is non fragmenting, non restricting of effective flow area and standards EN ISO 4126-1 is fulfilled There is no influence of the opened bursting disc on the proper function of the safety valve. Pressure loss by bursting disc must be checked (< 3%) Acc. to European Standards: Application of the combination is like a single safety valve Documented in the VdTÜV data sheets of the safety valves in combination with REMBE Capacity reducing factor of 0.9 is required if the combination is not tested Acc. to American Standards: Capacity reducing factor of 0.9 is always required (K c ) Best Availability Safety Valves and Bursting Discs in Combination LESER GmbH & Co. KG 03.02.2012 Rev 00 9/ 16
Back pressure impact on the user? Back pressure Effect on the capacity for steam and gasses taking the p ao /p o curve into consideration This ratio is observed for absolute pressures. rated coefficient of discharge Kdr /αw Correction factor for back pressure Kb Capacity minimisation must also be taken into consideration for low set pressures. p = 0.3 bar g (set pressure) p ao = 1.013 bar a (ambient pressure) p o = (0.3 barg + 0.1 barg + 1.013 bar a) (pressure in the system to be secured) p ao / p 0 = 1.013 bar a / (0.3 barg + 0.1 bar g + 1.013 bar a) = 0.72 >> K b = 0.81 Ratio back pressure / set pressure p ao / p o 10 / 16
Back pressure Definition Back pressure Built-up back pressure External back pressure Constant Variable Back pressure Exists only in the outlet while the safety valve blows off. It is dependent on the flow loss in the discharge line. Exists permanently in the outlet system The external back pressure is dependent on the blow-off of the safety valve Back pressure = built-up back pressure external pressure 11 / 16
Back pressure stability Setting The following measures prevent malfunctions resulting from the back pressure: Constant back pressure settings to differential set pressure (CDTP) use of stainless steel bellows POSV Variable back pressure use of stainless steel bellows POSV 12 / 16
Inlet pressure loss Influencing factors p 2 = p 1 p = 0 p 2 = p 1 - p p > 0 Valve is closed Valve is open p2 p2 p p p1 p1 13 / 16
Inlet pressure loss Standards and bodes A maximum pressure loss of 3% between the vessel and the safety valve is permissible for the most common international standards and codes. API 520 Part II (2015), 7.3.5 When a pressure relief valve is installed an a line directly connected to a vessel, the total non-recoverable pressure loss between the protected equipment and the pressure relief valve should not exceed 3 percent except as permitted in 4.2.3 for pilot-operated pressure relief valve. 14 / 16
Inlet pressure loss measurements The following measures prevent malfunctions that are caused by an inadmissible inlet pressure loss: Reduction of the flow rate through increasing the pipe diameter reducing the mass flow through a smaller valve reducing the mass flow through a lift stopper reducing the mass flow through an O-ring-damper Reduction of the flow rate through shorter inlet pipeline low-resistance connection to the vessel Incorrect Correct Measuremtents on the the SV - Bigger blowdown POSV - Remote Sensing POSV 15 / 16
Input from the EPC or the resposnsible application engineering company about the required mass flow of the governing relieving case or cases What is the defined standard of the application which leads to the relevant sizing standard Operating data, Medium data and SV data (for final sizing) to define the required safety valve Installation of the inlet and outlet in order to check the influence of the back pressure and the inlet pressure drop on the sizing and the actual safety valve 16 / 16