ASME Section I, Pressure Relief Valves PDF

Code Calculations - ASME Section I • Chapter 2 ^ OBJECTIVE 4 Using ASME BPVC Section I, PG-67 to PG-73, identify code information with respect to pressure relief valves and, using Table A-44, calculate the required pressure relief valve capacity for a given boiler. Of the many devices that a Powe...

Code Calculations - ASME Section I • Chapter 2 ^ OBJECTIVE 4 Using ASME BPVC Section I, PG-67 to PG-73, identify code information with respect to pressure relief valves and, using Table A-44, calculate the required pressure relief valve capacity for a given boiler. Of the many devices that a Power Engineer encounters, pressure relief valves are perhaps the most important. Therefore, it is beneficial to have a good understanding of the codes that apply to these valves, especially the applicable sections of the ASME code. This objective aims to do the following: • Familiarize you with ASME Code, Section I, Paragraphs PG-67 to PG-73, which specifically apply to pressure relief valves (PRVs) • Guide you in locating specific information in PG-67 to PG-73 • Help you use information in the code to calculate PRV capacity Terminology In ASME BPVC, Section I, the main heading for PG-67 to PG-73 is Overpressure Protection Requirements. Throughout the code, the term pressure relief valve replaces what were three separate terms before 2007. They are included here for information only/ since they are not official terms in the current code but may be encountered in other sources. 0 Safety valve: An automatic pressure relieving device actuated by the static pressure upstr'eam of the valve and characterized by full-opening pop action. It is used fo.r gas or vapour service. Relief valve: An automatic pressure relieving device actuated by the static pressure upstream of the valve/ which opens further with the increase in pressure over the opening pressure. It is used primarily for liquid service. Safety relief valve: An automatic pressure-actuated relieving device suitable for use either as a safety valve or relief valve, depending on application. 3rd Class Edition 3 • Part A2 91 ^ Chapter 2 • Code Calculations - ASME Section I CODE PARAGRAPHS FOR PRESSURE RELIEF VALVES This objective cannot restate all information in the code. However, it lists each of the major sections by paragraph number and title. It states a general purpose for each paragraph and gives some direct quotations from each. These selected quotations are only partial, so you are advised to refer to the PanGlobal ASME Academic Extract for complete review of the paragraphs. Only those statements in quotation marks are direct quotes from the paragraphs. Pressure Relief Valve (PRV) Overpressure Protection Requirements PG-67: Boiler Covers the types and numbers ofPRVs required on the various types of boilers (i.e., the boiler proper) PG-67.1: "Each boiler shall have at least one pressure relief valve and, if it has more than ...47 m2 of bare tube water-heating surface, or if an electric boiler has a power input more than 1100 kW, it shall have two or more pressure relief valves ...': PG-67.2: The pressure relief valve capacity for each boiler (except as noted in PG-67.4) shall be such that the pressure relief valve or valves will discharge all the steam that can be generated by the boiler without allowing the pressure to rise more than 6% above the highest pressure at which any valve is set and in no case to more than 6% above the maximum allowable working pressure." PG-67.2.1: "The minimum relieving capacity of the pressure relief valve for all types of boilers shall not be less than the maximum designed steaming capacity at the MAWP of the boiler..." PG-67.3: One or more pressure relief valves on the boiler proper shall be set at or below the maximum allowable working pressure... PG-67.4: This paragraph specifically deals with PRV requirements on forced-flow steam generators. Figure PG-67.4 shows graphically all the locations and specifications for PRVs on these boilers. PG-68: Superheater & Reheater Covers locations and capacities of superheater and reheater PRVs PG-68.1: Except as permitted in PG-58.3.1, every attached superheater shall have one or more pressure relief valves in the steam flow path between the superheater outlet and the first stop valve... PG-68.2: The discharge capacity of the pressure relief valve or valves on an attached superheater may be included in determining the number and size of pressure relief valves on a boiler, provided there are no intervening valves between the superheater pressure relief valve and the boiler, and provided the discharge capacity of the pressure relief valve, or valves, on the boiler, as distinct from the superheater, is at least 75% of the aggregate valve capacity required." PG-68.4: "Every reheater shall have one or more pressure relief valves, such that the total relieving capacity is at least equal to the maximum steam flow for which the heater is designed... PG-68.7: This paragraph stipulates consideration that must be given to the superheater when calculating PRV capacity for superheated steam. Table PG-68.7M is included, with the required correction factors for given pressures and temperatures. 92 3rd Class Edition 3' Part A2 Code Calculations - AS ME Section I • Chapter 2 PG-69: Certification of Capacity ofPRVs Covers rules for the testing of PRV capacities by manufacturers PG-69.1: Before the code symbol is applied to a pressure relief valve, the manufacturer shall have the relieving capacity of their valves certified in accordance with the provisions of this paragraph." PG-69.2: "Relieving capacities shall be determined using one of the methods in PG-69.2. l.': PG-69.2.1 goes on to describe the following three methods: 1. Three valve method 2. Slope method 3. Coefficient of discharge method PG-70: Capacity ofPRVs PG-70.1: "Subject to the minimum number required by PG-67.1, the number of pressure relief valves required shall be determined on the basis of the maximum designed steaming capacity as determined by the manufacturer, and the relieving capacity marked on the valves by the manufacturer. PG-71: Mounting ofPRVs Covers required methods for attaching PRVs to boilers PG-71.1: "When two or more pressure relief valves are mounted on a boiler, they may be mounted either separately or as twin valves... PG-71.2: "The pressure relief valve or valves shall be connected to the boiler independently of any other connection, and attached as closely as possible to the boiler... PG-71.3: "The opening or connection between the boiler and the pressure relief valve shall have at least the area of the valve inlet. No valve of any description shall be placed between the required pressure relief valve or valves and the boiler, nor on the discharge pipe between the pressure relief valve and atmosphere..." PG-72: Operation ofPRVs Covers guidelines for the operating ranges ofPRVs, including popping and blowdown pressures PG-72.1: States maximum pressure relief valve blowdown pressures in relation to various set opening pressures PG-72.2: States allowable tolerances (+/- from set pressure) in relation to set opening pressures PG-73: Minimum Requirements for PRVs This section gives the rules for the design, manufacture, and testing ofPRVs. The major paragraphs include Mechanical Requirements, Material Selections, Inspection of Manufacturing and/or Assembly, Testing by Manufacturers or Assemblers, Design Requirements, and Code Symbol "V Stamp (including certification of individuals qualified to approve and stamp PRVs). 3rd Class Edition 3 • Part A2 93 ^ Chapter 2 • Code Calculations - ASME Section I Calculating Required PRV Capacity With respect to a steam boiler, the term capacity refers to the amount of steam flow that the boiler can generate. For a PRV, capacity means the amount of steam that the valve can pass. Thus, Section I refers to the steaming capacity of a boiler and relieving capacity of a PRV. The main principle for boiler PRV design is that relieving capacity must be sufficient to match steaming capacity. As stated in PG-67.2.1: The minimum required relieving capacity of the pressure relief valves for all types of boilers shall be not less than the maximum designed steaming capacity at the MAWP of the boiler, as determined by the manufacturer, and shall be based on the capacity of all the fuel burning equipment as limited by other boiler functions. Based on this proportionality between relieving capacity and steaming capacity, Section I has appendices with formulas for estimating the amount of steam generated as a function of fuel burned or boiler heating surface. Appendbc A-44 ASME Section I, Appendbc A-44 is titled Pressure Relief Valves for Power Boilers. It contains Table A-44, "Guide for estimating steaming capacity based on heating surface." This table lists the steaming capacity per square metre of heating surface (kg/h-m2) for the boiler proper and for the waterwall heating surfaces. The quantities are given for both firetube and watertube boilers in relation to the type of firing method used (hand fired, stoker fired, oil, gas, or pulverized fuel fired). Using the guidelines in PG-67 to PG-70 and Table A-44 (PG ASME Academic Extract), the capacities of PRVs on a given boiler can be determined, as in the following examples. Example 11: Calculate PRV discharge capacity A stoker-fired firetube boiler has 62 m2 of heating surface. How much steam must the PRVs on this boiler be capable of discharging per hour? Solution 11 From Appenduc A, Table A-44, a stoker-fired firetube boiler must have a pressure relief valve with a minimum capacity of 34 kg/h-m2 of heating surface. Therefore Min required capacity (kg/h) = heating surface m2 x table value kg/h-m2 = 62 m2 x 34 kg/h-m2 = 2108 kg/h (Ans.) Example 12: Calculate minimum required relieving capacity for PRVs A watertube boiler is gas fired and has 65 m2 of boiler heating surface, plus 85 m2 of waterwall surface. What is the minimum required relieving capacity for all the PRVs? Solution 12 From Table A-44, a gas-fired watertube boiler must have a PRV capacity of 49 kg/h for each square metre of boiler surface, plus 78 kg/h for each square metre ofwaterwall surface. Therefore, the minimum required capacity is the capacity for boiler heating surface plus the capacity for waterwall heating surface. Capacity = capacity (boiler) + capacity (waterwalls) = (65 m2 x 49 kg/h-m2) + (85 m2 x 78 kg/h-m2) = 3185kg/hx6630kg/h = 9815kg/h 94 3rd Class Edition 3 • Part A2 Chapter 2 • Code Calculations - ASME Section I Example 14: Calculate steaming capacity Given heat of combustion H = 40 000 kj/m3 and the amount of fuel burned C = 400 m3/hr, find the steaming capacity. Solution 14 w= CxHxO.75 2558 400 m3/h x 40 000 kj/m3 x 0.75 2558 == 4691 kg/h (Ans.) The sum of the PRV capacities marked on the valves shall be equal to or greater than 4692 kg/h steam. Appendbc A-17 lists the heats of combustion for various fuels. 96 3rd Class Edition 3 • Part A2 Code Calculations-ASME Section I • Chapter 2 ^ Example 13: Calculate minimum combined relieving capacity A watertube boiler equipped with a superheater has two PRVs on the steam drum and one PRV on the superheater. The boiler is fired on pulverized coal and has 70 m2 of boiler surface, 95 m2 of waterwall surface, and 1000 kg/h of superheater PRV capacity. What is the minimum combined relieving capacity required for the steam drum PRVs? Solution 13 From Table A-44, a pulverized fuel-fired watertube boiler must have a PRV capacity of 49 kg/h for each square metre of boiler heating surface, plus 78 kg/h for each square metre ofwaterwall heating surface. Therefore, the minimum required capacity is the capacity for boiler heating surface plus the capacity for waterwall heating surface. Capacity = (70 m2 x 49 kg/h-m2) + (95 m2 x 78 kg/h-m2) = 3430 kg/h + 7410 kg/h = 10840kg/h Capacity available in superheater PRV = lOOOkg/h Therefore, the minimum capacity required in the steam drum PRVs = 10 840 - 1000 kg/h = 9840 kg/h But, from PG-68.2, we must show that the pressure relief valves on the boiler (not including the superheater PRVs) have at least 75% of the total required capacity. 9840 10840 x 100% = 90.8%, which is > 75% So, the minimum required relieving capacity for the steam drum PRVs is 9840 kg/h (Ans). Note: Superheaters are included in PRV capacity calculations providing there are no intervening valves between the superheater PRV and the boiler (per PG-68.2). Another method of calculating the PRV capacity is by measuring the maximum amount of fuel that can be burned. Non-mandatory Appendbc A-12 explains how this is done. AppendbcA-12 The maximum quantity of fuel C that can be burned per hour at the time of maximum forcing is determined by a test. w= where CxHxO.75 2558 C = total weight of fuel burned kg/h or m3/h maximum H = heat of combustion of fuel kj/kg/of kj/m3 W = weight of steam generated kg/h 3rd Class Edition 3 • Part A2 95 Code Calculations -ASME Section I • Chapter 2 f£ SUMMARY OF CODE CALCULATIONS The formulas in this chapter are used as engineering rules for power boiler designers. A basic understanding of boiler design helps the Power Engineer understand why specific materials are chosen, how failures occur, and which root causes of failures can be mitigated. Thus, this knowledge also contributes to the overall goal of safe boiler operation and gives a broader understanding of equipment maintenance and inspection. Using the design formulas for boilers and pressure vessels requires a basic understanding of ASTM/ASME specifications and steel grades/types. The specification titles in PG-6 and PG-9 contain important data such as material type (carbon or alloy; ferritic or austenitic) and product type (seamless or welded; pipe, tube, or plate). This information is not always explicitly stated in the question you are trying to solve, so you may have to refer to ASME BPVC, Section I, PG-6 and PG-9. 3rd Class Edition 3 • Part A2 97