PG-67.4.3.2.3 A direct-acting overpressure-trip-actuating mechanism, using an independent pressure sensing device, that will stop the flow of fuel and feedwater to the boiler, at a pressure higher than the set pressure of PG-67.4.3.2.2, but less than 20% above the maximum allowable working pressure as shown in the master stamping (PG-106.3).
PG-67.4.3.3 There shall be not less than two pressure relief valves and the total rated relieving capacity of the pressure relief valves shall be not less than 10% of the maximum designed steaming capacity of the boiler as determined by the Manufacturer. These pressure relief valves may be set above the maximum allowable working pressure of the parts to which they are connected but shall be set such that the valves will lift at a pressure no higher than 20% above the maximum allowable working pressure as shown in the master stamping (PG-106.3).
PG-67.4.3.4 At least two of these pressure relief valves shall be equipped with a device that directly transmits the valve stem lift action to controls that will stop the flow of fuel and feedwater to the boiler. The control circuitry to accomplish this shall be arranged in a "fail-safe" manner (see Note).
NOTES: “Fail-safe" shall mean a circuitry arranged as either of the following:
(a) Energize to trip: There shall be at least two separate and independent trip circuits served by two power sources, to initiate and perform the trip action. One power source shall be a continuously charged DC battery. The second source shall be an AC-to-DC converter connected to the DC system to charge the battery and capable of performing the trip action. The trip circuits shall be continuously monitored for availability.
It is not mandatory to duplicate the mechanism that actually stops the flow of fuel and feedwater.
(b) De-energize to trip: If the circuits are arranged in such a way that a continuous supply of power is required to keep the circuits closed and operating and such that any interruption of power supply will actuate the trip mechanism, then a single trip circuit and single power supply will be enough to meet the requirements of this subparagraph.
PG-67.4.3.5 The power supply for all controls and devices required by PG-67.4.3 shall include at least one source contained within the same plant as the boiler and which is arranged to actuate the controls and devices continuously in the event of failure or interruption of any other power sources.
PG-67.4.4 When stop valves are installed in the water-steam flow path between any two sections of a forced-flow steam generator with no fixed steam and waterline:
PG-67.4.4.1 The power-actuated pressure relieving valve(s) required by PG-67.4.1 shall also receive a control impulse to open when the maximum allowable working pressure of the component, having the lowest pressure upstream to the stop valve, is exceeded; and
PG-67.4.4.2 The pressure relief valves shall be located to provide the pressure protection requirements in PG-67.4.2 or PG-67.4.3.
PG-67.4.5 A reliable pressure-recording device shall always be in service and records kept to provide evidence of conformity to the above requirements.
PG-67.5 The coefficient of discharge of pressure relief valves shall be determined by actual steam flow measurements at a pressure not more than 3% above the pressure at which the valve is set to relieve and when adjusted for blowdown in accordance with PG-69.1.4. The valves shall be credited with capacities as determined by the provisions of PG-69.2.
Pressure relief valves may be used that give any opening up to the full discharge capacity of the area of the opening of the inlet of the valve, provided the movement of a steam pressure relief valve is such as not to induce lifting of water in the boiler.
For high-temperature water boilers pressure relief valves shall be used. Such valves shall have a closed bonnet. In addition the pressure relief valves shall be capable of satisfactory operation when relieving water at the saturation temperature corresponding to the pressure at which the valve is set to blow.
PG-68 SUPERHEATER AND REHEATER
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. The location shall be suitable for the service intended and shall provide the overpressure protection required. The pressure drop upstream of each pressure relief valve shall be considered in the determination of set pressure and relieving capacity of that valve. If the superheater outlet header has a full, free steam passage from end to end and is so constructed that steam is supplied to it at practically equal intervals throughout its length so that there is a uniform flow of steam through the superheater tubes and the header, the pressure relief valve, or valves, may be located anywhere in the length of the header.
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 the pressure relief valves for the 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-68.3 Every isolable superheater that may be shut off from the boiler and permit the superheater to become a fired pressure vessel and all nonintegral separately fired superheaters shall have one or more pressure relief valves having a discharge capacity equal to 6 lb/ft2 (29 kg/m2) of steam per hour, using the superheater surface measured on the side exposed to the hot gases. As an alternative the Manufacturer may also calculate the minimum pressure relief valve discharge capacity in lb (kg) of steam per hour from the maximum expected heat absorption (as determined by the Manufacturer) in Btu/hr (W), divided by 1,000 (646). In the case of electrically heated superheaters, the pressure relief valve capacity shall be based upon 3 ½ lb (1.6 kg)/hr/kW input. The number of pressure relief valves installed shall be such that the total capacity is at least equal to that required. Pressure relief valves for separately fired superheaters shall be located in accordance with the rules of PG-68.1 and the mounting rules of PG-71.
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. The capacity of the reheater pressure relief valves shall not be included in the required relieving capacity for the boiler and superheater.
One or more pressure relief valves with a combined relieving capacity not less than 15% of the required total shall be located along the steam flow path between the reheater outlet and the first stop valve. The pressure drop upstream of the pressure relief valves on the outlet side of the reheater shall be considered in determining their set pressure.
PG-68.5 A soot blower connection may be attached to the same outlet from the superheater or reheater that is used for the pressure relief valve connection.
PG-68.6 Every pressure relief valve used on a superheater or reheater discharging superheated steam at a temperature over 450°F (230°C) shall have a casing, including the base, body, and, if applicable, bonnet and spindle, of steel, steel alloy, or equivalent heat-resisting material.
The pressure relief valve shall have a flanged inlet connection, or a weld-end inlet connection. It shall have the seat and disk of suitable heat erosive and corrosive resisting material, and the spring of direct spring-loaded safety valves shall be fully exposed outside of the valve casing so that it shall be protected from contact with the escaping steam.
PG-68.7 The capacity of a pressure relief valve on superheated steam shall be calculated by multiplying the capacity determined in accordance with PG-69.2 by the appropriate superheat correction factor Ksh, from Table PG-68.7.
PG-69.1 Before the Code symbol is applied td any pressure relief valve or power-actuated pressure relieving valve, the valve manufacturer shall have the relieving capacity of his pressure relief valves certified in accordance with the provisions of this paragraph.
PG-69.1.1 Capacity certification tests shall be conducted using dry saturated steam. The limits for test purposes shall be 98% minimum quality and 20°F (l0°C) maximum superheat. Correction from within these limits may be made to the dry saturated condition.
PG-69.1.2 Tests shall be conducted at a place that meets the requirements of A-312.
PG-69.1.3 Capacity test data reports for each pressure relief valve design and size, signed by the manufacturer and Authorized Observer witnessing the tests, together with drawings showing the valve construction, shall be submitted to the ASME designee for review and acceptance.
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PG-69.1.4 Capacity certification tests shall be conducted at a pressure that does not exceed the set pressure by 3% or 2 psi (15 kPa), whichever is greater. Pressure relief valves shall be adjusted so that the blowdown does not exceed 4% of the set pressure. For pressure relief valves set at or below 100 psi (700 kPa), the blowdown shall be adjusted so as not to exceed 4 psi (30 kPa). Pressure relief valves used on forced-flow steam generators with no fixed steam and waterline, and pressure relief valves used on high-temperature water boilers shall be adjusted so that the blow down does not exceed 10% of the set pressure. The res eating pressure shall be noted and recorded.
PG-69.1.5 Capacity certification of pilot operated pressure relief valves may be based on tests without the pilot valves installed, provided prior to capacity tests it has been demonstrated by test to the satisfaction of the Authorized Observer that the pilot valve will cause the main valve to open fully at a pressure which does not exceed the set pressure by more than 3% or 2 psi (15 kPa), whichever is greater, and that the pilot valve in combination with the main valve will meet all of the requirements of this Section.
PG-69.1.6 Pressure relief valves for economizer service shall also be capacity certified using water at a temperature between 40°F and 125°F (4°C and 50°C). The pressure relief valves shall be tested without change to the adjustments established in PG-69.1.1 to PG-69.1.4.
PG-69.2 Relieving capacities shall be determined using one of the following methods.
PG-69.2.1 Three Valve Method. A capacity certification test is required on a set of three pressure relief valves for each combination of size, design, and pressure setting. The capacity of each valve of the set shall fall within a range of ±5% of the average capacity. If one of the three pressure relief valves tested falls outside this range, it shall be replaced by two valves, and a new average shall be calculated based on all four valves, excluding the replaced valve. Failure of any of the four capacities to fall within a range of ±5% of the new average shall be cause to refuse certification of that particular valve design.
The rated relieving capacity for each combination of design, size, and test pressure shall be 90% of the average capacity.
PG-69.2.2 Slope Method. If a Manufacturer wishes to apply the Code Symbol to a design of pressure relief valves, four valves of each combination of pipe size and orifice size shall be tested. These four valves shall be set at pressures that cover the approximate range of pressures for which the valve will be used or covering the range available at the certified test facility that shall conduct the tests. The capacities based on these four tests shall be as follows:
(a) The slope WIP of the actual measured capacity versus the flow pressure for each test point shall be calculated and averaged
For steam
For water
All values derived from the testing must fall within ±5% of the average value:
minimum slope = 0.95 x average slope
maximum slope = 1.05 x average slope
If the values derived from the testing do not fall between the minimum and maximum slope values, the Authorized Observer shall require that additional valves be tested at the rate of two for each valve beyond the maximum and minimum values with a limit of four additional valves.
For steam applications the relieving capacity to be stamped on the valve shall not exceed 90% of the average slope times the absolute accumulation pressure
rated slope = 0.90 x average slope
For water applications the relieving capacity shall not exceed 90% of the average slope multiplied by the square root of the difference between the flow rating pressure and the valve discharge pressure.
(U.S. Customary Units)
stamped capacity ≤ rated slope (1.03 x set pressure + 14.7) or (set pressure + 2 psi + 14.7), whichever is greater
(SI Units)
stamped capacity ≤ rated slope (1.03 x set pressure + 0.101) or (set pressure + 0.015 MPa + 0.101), whichever is greater
PG-69.2.3 Coefficient of Discharge Method. A coefficient of discharge for the design, K, may be established for a specific valve design according to the following procedure:
(a) For each design, the pressure relief valve manufacturer shall submit for test at least three valves for each of three different sizes (a total of nine valves). Each valve of a given size shall be set at a different pressure, covering the range of pressures for which the valve will be used or the range available at the facility where the tests are conducted.
For each valve design where the coefficient of discharge has been determined that is intended to be restricted in lift, the Manufacturer shall have capacity tests conducted on three valves of different sizes. Each size valve shall be tested for capacity at the minimum lift for which certification is required, and at two intermediate lift points between the full rated lift and minimum lift certification points. Each of the three test valves shall be set at a different pressure.
For each valve tested, it shall be verified that actual measured capacity at restricted lift will equal or exceed the rated capacity at full rated lift multiplied by the ratio of measured restricted lift to full rated lift.
(b) Tests shall be made on each pressure relief valve to determine its lift at capacity, popping, and blowdown pressures, and actual relieving capacity. An individual coefficient, K
D, shall be established for each valve as follows:
Where actual flow is determined by test and theoretical flow, WT is calculated by one of the following equations:
For tests with dry saturated steam
For 45 deg seat
(U.S. Customary Units)
WT = 51.5 x pDLP x 0.707
(SI Units)
WT = 5.25 x pDLP x 0.707
For flat seat
(U.S. Customary Units)
WT = 51.5 x pDLP
(SI Units)
WT = 5.25 x pDLP
For nozzle
(U.S. Customary Units)
WT = 51.5 AP
(SI Units)
WT = 5.25 AP
For tests with water
For 45 deg seat
(U.S. Customary Units)
(SI Units)
