ASME Section I Pressure Relief Valves and Devices PDF

Objective 1 Discuss the code requirements, construction, and operation of ASME Section I Pressure Relief Valves and Devices. DEFINITIONS Pressure relief valves automatically open to relieve pressure, and then automatically reclose. They exist in many different varieties. ASME PTC-25 Pressure Relief...

Objective 1 Discuss the code requirements, construction, and operation of ASME Section I Pressure Relief Valves and Devices. DEFINITIONS Pressure relief valves automatically open to relieve pressure, and then automatically reclose. They exist in many different varieties. ASME PTC-25 Pressure Relief Devices defines 11 different types. The many terms used to categorize pressure relief valves can be quite confusing. The words “safety valve,” “relief valve,” “safety relief valve” and others are used indiscriminately. This is likely because all pressure relief valves have great similarity in construction, operation, and purpose. They are all actuated by, and relieve, pressure from pressurized equipment. They all have pressure set points and capacities. They even look similar. However, each valve has distinct operating characteristics and applications. Therefore, it is important that the term used to describe each type is accurate and does not create confusion. ASME BPVC I, Part PG-73.1.4 states: Unless otherwise defined, the definitions relating to pressure relief devices in ASME PTC25 shall apply. ASME PTC-25 Pressure Relief Devices defines pressure relief valves as follows: Pressure Relief Valve: a pressure relief device designed to actuate on inlet static pressure and reclose after normal conditions have been restored. Temperature and Pressure (T&P) Relief Valve: a pressure relief valve that may be actuated by pressure at the valve inlet or by temperature at the valve inlet. Relief Valve: a pressure relief valve characterized by gradual opening that is generally proportional to the increase in pressure. It is normally used for incompressible fluids. Safety Relief Valve: a pressure relief valve characterized by rapid opening or by gradual opening that is generally proportional to the increase in pressure. It can be used for compressible or incompressible fluids. Safety Valve: a pressure relief valve characterized by rapid opening and normally used to relieve compressible fluids. Figure 1 summarizes the main types of pressure relief valves Power Engineers encounter. Note that the term “pressure relief valve” refers to all forms of reclosing pressure relief devices. Figure 1 Pressure Relief Valves According to ASME PTC-25 On Track The acronym PRV may stand for pressure relief valve or for pressure reducing valve. In this chapter, the acronym PRV will only be used for pressure relief valve. ASME BOILER AND PRESSURE VESSEL CODE SECTION I (BPVC I) REQUIREMENTS ASME BPVC I, Part PG-67.1 states: Each boiler shall have at least one pressure relief valve and if it has more than 500 ft2 (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 states: 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. ASME BPVC I, Part PG-68 addresses the rules that govern superheater and reheater PRVs. PG-68.1 states: “the pressure drop upstream of each pressure relief valve shall be considered in the determination of set pressure and relieving capacity of that valve.” This means that the set pressure of a superheater safety valve must take into consideration that the steam pressure will drop as it flows from the steam drum to the superheater outlet. If set to the steam drum pressure, safety valves on superheaters would likely never open. In an overpressure situation, steam flow through the superheater would cease. The superheater tubes would overheat due to the loss of the cooling steam. Therefore, a superheater PRV must open before the drum PRVs do. They will also close after the drum PRVs. This ensures that the superheater will always have steam flow. SAFETY VALVE CONSTRUCTION SAFETY VALVE CONSTRUCTION A safety valve is a pressure relief valve characterized by rapid opening, and normally used to relieve compressible fluids. These valves are commonly found on steam boilers, steam lines, compressed air vessels, and refrigerant receivers. The focus here will be on safety valves used in steam service. Figure 2 – Cutaway View of Cast Steel Safety Valve Safety valves are made of cast iron, steel, alloy steel, or bronze. Cast iron and bronze safety valves are used for lower pressures and lower temperatures. ASME BPVC Section I, Part PG-73 prohibits the use of valves with superheaters bronze parts operating for over 230°C. ASME BPVC Section I, Part PG-68 requires all superheater and reheater pressure relief valves to be constructed of steel or alloy steel. A safety valve is held shut with a spring that holds a valve disc tightly against a seat. When the steam pressure reaches the set point pressure (popping pressure) of the valve, the disc will rise slightly from the seat, and steam will begin to escape. Figure 2 shows a cutaway of a safety valve approved for saturated and superheated steam service. It has a cast steel body, and flange connections at its inlet and outlet. It also has an exposed spring to prevent the high temperature of superheated steam from weakening the spring. The safety valve shown in Figure 2 has an exposed spring to prevent the high temperature steam from affecting the mechanical properties of the spring. Figure 3 shows the construction of a safety valve. The safety valve is attached to the drum at the top of the steam space. The valve disc “D” is held firmly on its seat by the pressure of the heavy coil spring “J”. Screwing the nut “L” up or down adjusts the point at which the valve will lift and relieve the pressure. This will decrease or Figure 3 – Safety Valve Construction increase the compression of the spring “J”. Tightening lock nut “N” prevents the nut “L” from shifting. When the valve has been set by nut “L”, the cap “B” is put in place. An authorized inspector passes a wire through hole “O”, and attaches a seal. This prevents access to the adjusting nut “L”. The try lever can manually lift the valve. The lever raises the valve spindle, which is connected to the valve disc. This must only be done if the valve is subjected to a pressure of at least 75% of boiler operating pressure. If the safety valve is opened without pressure in the boiler, the try lever or valve spindle can be damaged. As well, any debris in the safety valve discharge line could fall into the valve, and prevent it from closing (ASME BPVC I, Part PG-73.2.4). In order to ensure consistent operation, tightness, and proper seating of the disc, the valve disc is equipped with guides at the bottom and top. This allows the valve disc to move only in a vertical direction, with no horizontal shifting. OPERATION OF A SAFETY VALVE Refer to Figure 4. The safety valve disc has a lip (skirt) that forms a small compartment called a huddling chamber. This compartment fills with steam when the valve starts to open, and increases the effective area of the disc. As soon as the valve lifts, the pressure of the steam acts on this increased disc area, which results in a greater force against the compression spring. This increase in force causes the valve to pop wide open. The valve will remain open until the pressure drops to below the popping pressure. Figure 4 – Safety Valve – Disc and Seat Detail Figure 5 – Pop Action of a Safety Valve Refer to Figure 5. As the valve begins to lift, steam rushes into the huddling chamber and acts on an increased area (as indicated by the shaded ring). This causes the valve to suddenly lift and “pop” to its full opening. The lifting force exerted on the disc by the boiler pressure is dependent on: • The area of the disc exposed to the pressure. • How freely the steam can escape from under the lip. When the valve is opening, the lip causes the safety valve to open wide. This action is reversed when the valve is closing. The pressure under the lip decreases when the boiler pressure drops sufficiently to allow the spring to begin closing the valve. This allows the spring to close the valve quickly. ADJUSTING the BLOWDOWN ASME PTC-25, Pressure Relief Devices defines blowdown as: The difference between actual popping pressure of a pressure relief valve and actual reseating pressure, expressed as a percentage of set pressure or in pressure units. ASME BPVC I, Part PG-72.1 states: Pressure relief valves shall be designed and constructed to operate … with a minimum blowdown of 2 psi (15 kPa) or 2% of the set pressure. Figures 4 and 6 show threaded adjustable annular rings called “blowdown adjusting rings.” These rings may be moved up or down to vary the size of the huddling chamber escape port opening. Once adjusted to a particular position, these rings can be locked into place with a set screw, and then sealed in position. The huddling chamber has ports that allow steam to escape from under the lip. The blowdown adjusting ring can completely block off, partially obstruct, or fully open the huddling chamber escape ports. If the adjustment ring blocks the huddling chamber escape ports, the steam under the lip cannot exit from the chamber. This creates a higher pressure under the lip, because the valve is open. The boiler pressure must drop significantly for the valve to reclose. Figure 6 – Detail of Blowdown Adjusting Ring When the huddling chamber escape ports are fully open, the steam under the lip can exit from the chamber. This creates a lower pressure under the lip, because the valve is open. In this case, the valve recloses with far less drop in boiler pressure. If the adjustment ring takes an intermediate position (it only partially obstructs the escape ports), the valve will reclose at a pressure in-between the reclosing pressures described above. To change the position of the blowdown adjusting ring, remove the locking screw, and turn the adjusting ring in the desired direction with a screwdriver. After the adjusting ring is set, lock it into place with the locking screw. To prevent unauthorized persons from tampering with the blowdown adjustment, the locking screw is secured in place by a wire seal, installed by the jurisdictional Boiler Inspector (see Figure 6). ADJUSTING the POPPING PRESSURE The PRV spring is designed for a certain range of opening (or popping) pressure. The PRV set pressure may be increased or decreased by a maximum of five percent (5%) from the setting marked on the valve. This can be done by changing the compression of the spring. Caution ASME BPVC I, Part PG-72.3 cautions that when a pressure relief valve’s set pressure needs adjustment, it must be done by the PRV manufacturer, its authorized representative, or an assembler. Power Engineers and maintenance personal must not tamper with PRV set pressure adjustments. The content in this section is for information purposes only. Power Engineers and maintenance personnel, unless specially trained and certified, must never tamper with the set pressure of a PRV. Such an action is considered illegal in all Canadian jurisdictions. Side Track ASME BPVC I, Part PG-72.4 permits set pressure adjustments outside of the ± 5% range. This involves the installation of a new spring acceptable to the PRV manufacturer. The spring installation and valve adjustment shall be performed by the manufacturer, his authorized representative, or an assembler. When a PRV set pressure is changed, a new valve data tag must be furnished and installed on the valve, and the valve must be resealed. The new tag must identify the new set pressure, capacity, and date the valve was reset. Before adjusting the pressure at which the safety valve opens, the jam nut on the adjusting nut, located on the spindle above the spring, must be loosened. To increase the popping pressure, screw the adjusting nut down to further compress the spring. To decrease the blowoff pressure, screw the adjusting nut upwards to decrease the spring compression. More information governing safety valve design, material selection, capacity, testing, adjustments and sealing are covered in ASME BPVC I. Figure 7 shows a cast-iron safety valve. It is suitable for boilers operating at pressures up to 1720 kPa, and steam temperatures up to 230°C. Figure 7 – Safety Valve with Adjustable Popping Pressure PRESSURE RELIEF VALVE MOUNTING ASME BPVC I, Part PG-71 covers the installation of PRVs. It states that safety valves mounted on the drum, superheater, reheater inlet and outlet headers must be placed in an upright position with the spindle vertical. They must be connected to the boiler independent of any other steam connection, and be located as close as possible to the boiler without any unnecessary intervening pipe or fitting. A discharge pipe, if used, should be separately supported, leaving ample room for expansion so that no force is exerted on the valve. Figure 8 shows the discharge piping arrangement for a typical safety valve. This arrangement is similar to that shown in ASME BPVC, Section VII. Figure 8 – Safety Valve Discharge Piping Details