Working principle of the rupture disc vacuum bracket

The answers to the questions are as follows: 1. Yes. 2. Since the burst disc has ruptured, the inside of the container is no longer a vacuum; therefore, it is not necessary to maintain a vacuum state. 3. The bracket material is selected based on practical considerations, with 316 SS being commonly used

:Victory: Stainless steel 316 is the most commonly used.

GB 567-1999 Preface: This standard is an improved version of Standard G13 567-1989 (Technical Requirements for Arch-shaped Metal Blast Discs). This version makes significant changes to the following: — The standard name has been changed ； — It expanded the scope of application of the standards. As of the date of implementation, this standard replaces GB 567-1989 simultaneously. Appendix A and Appendix D of this standard are integral parts of the standard, while Appendix B and Appendix C are supplementary appendices. This standard was proposed by the **Bureau of Boiler and Pressure Vessel Supervision under the Quality and Technical Supervision Bureau. This standard is under the jurisdiction of the Boiler and Pressure Vessel Testing and Research Center of the **Quality and Technical Supervision Bureau. The drafter of this standard is the Safety Equipment Factory of Dalian University of Technology. Main drafters of this standard: Ding Xinwei, Li Zhiyi, Wang Shulan, Bi Mingshu, Yu Jianliang, You Hongxin, Wen Dianjiang, Xu Xiaohui, Su Shiyi, Li Yue, Yin Jianzhong. People’s Republic of China **Standard GB 567-1999 Bursting discs and bursting disc devices ft* GB 567-1989 Bursting discs and bursting disc devices 1 Scope This standard specifies the definitions, technical requirements, and performance testing methods for bursting discs and bursting disc devices. This standard applies to rupture discs and rupture disc assemblies for pressure vessels, pipelines, or other enclosed spaces to prevent overpressure or excessive vacuum. The blasting pressure of the blast fragments shall not exceed 500 MPa at its maximum, and shall not be less than a certain value at its minimum. . 001 MPa, 2 Definitions The following definitions are adopted in this standard. 2.1 Blowing disc device: A pressure relief safety device composed of a blowing disc (or blowing disc assembly) and a clamp (or support ring), among other components. When the pressure difference across the rupture disc reaches a predetermined value at a specified temperature, the disc immediately activates (breaks or detaches) to release the pressurized medium. 2.2 Rupture Disc: In a rupture disc device, it is a pressure-sensitive element that can act rapidly in response to overpressure. 2.3 Fragmenting disc assemblies (also known as combined bursting discs) are assemblies composed of two or more components such as a bursting disc, a backpressure bracket, a reinforcing ring, and a protective film. 2.4 Positive arch-shaped rupture disc: The pressure-sensitive element has a positive arch shape. After installation, the concave surface of the rear arch is on the high-pressure side of the pressure system; during operation, this component undergoes tensile failure. 2.4.1 Ordinary positive-dome rupture discs: Pressure-sensitive elements that require no further processing, as they are formed directly from the blank in a positive-dome shape. 2.4.2 Positive arch slotted rupture disc: A pressure-sensitive element that consists of a positively arch-shaped rupture disc made up of an arched sheet with slits (holes) and a sealing membrane. 2.4.3 Positive arch grooved rupture disc – A positively arch-shaped rupture disc with grooves fabricated on the arch surface of the pressure-sensitive element. 2.5 Inverted arch-shaped rupture disc: The pressure-sensitive element has an inverted arch shape. After installation, the convex surface of the rear arch is on the high-pressure side of the pressure system; during operation, this element undergoes compressive instability, leading to rupture or detachment. 2.5.1 Reverse arch type disc with blade (or crocodile tooth) – A reverse arch-shaped rupture disc that ruptures when the pressure-sensitive element loses stability and flips over, coming into contact with the blade (or crocodile tooth). 2.5.2 Reverse-arch shedding type rupture disc: A rupture disc of reverse-arch shape that cracks or sheds along its supporting edge when the pressure-sensitive element becomes unstable and flips over, and is then ejected by the high-pressure medium. 2.5.3 Reverse-arched slotted rupture discs – Rupture discs with grooves fabricated on the arch surface of the pressure-sensitive element. 2.6 Flat-type burst discs: The pressure-sensitive element is in flat shape. **Quality; approved by the Bureau of Technical Supervision on 1999-11-01 and put into effect on 2000-08-01. Gs 567-1999 2.6.1 Plate-type slotted rupture discs: A pressure-sensitive element that consists of a plate-shaped element with slits (holes) and a sealing membrane. 2.6.2 Plate-type slotted rupture discs – Plate-type rupture discs with grooves fabricated on the surface of the pressure-sensitive element. 2.7 Graphite rupture discs: The pressure-sensitive element is made of graphite, and it breaks due to bending or shearing when activated. 2.8 Holders: In rupture disc assemblies, these are components with a predetermined discharge diameter; they are used to fix and support the position of the rupture disc, ensuring its proper functioning. 2.9 Support ring: A ring that fixes and supports the position of the blast disc by mechanical means or welding, ensuring its accurate operation. 2.10 Back pressure: The static pressure present on the discharge side of the blast disc assembly. Back pressure is generated whenever there is another pressure source on the discharge side of the rupture disc device, or when a vacuum condition exists on the inlet side. The difference between the pressure on the discharge side and the pressure on the inlet side is called the backpressure difference. 2.11 Backpressure bracket: In composite blast discs, it is a bracket used to prevent the pressure-sensitive element from being accidentally damaged due to a backpressure difference. The backpressure bracket, located on the concave side of the positive-dome rupture disc, prevents the convex side of the rupture disc from becoming unstable under pressure when a backpressure difference occurs. When the system pressure may experience a vacuum, such a backpressure bracket can be referred to as a vacuum bracket. The backpressure bracket, located on the convex side of the inverse-arch-shaped rupture disc, prevents the concave side of the rupture disc from being damaged under backpressure. 2.11.1 Open-type backpressure bracket: A backpressure bracket that breaks apart when the rupture disc explodes. 2.11.2 Non-rupturing back pressure bracket A back pressure bracket that does not rupture when the burst disc fails. 2.12 Reinforcing ring: In composite blast discs, it is a ring that fits closely around the edge of the pressure-sensitive element, serving to enhance the stiffness of that edge. 2.13 Sealing film: In composite blast discs, the film that serves to seal the pressure-sensitive elements. 2.14 Protective film (layer): A coating film or plating layer used to prevent corrosion of the blast disc elements when they are susceptible to it. 2.15 Discs: Flat metal or graphite sheets that are manufactured from metal strips or sheets, or from graphite rods, prior to being turned into blast discs. 2.16 Blasting pressure: The pressure difference across the blast disc when the blast disc device operates at a given blasting temperature. 216.1 Design burst pressure: The burst pressure value at the design burst temperature, specified by the client when designing a burst disc. 2.16.2 Maximum (minimum) design blasting pressure: The algebraic sum of the design blasting pressure, the manufacturing tolerance, and the blasting pressure tolerance. 2.16.3 Permissible blasting range: The pressure range defined by the maximum and minimum design blasting pressures. This pressure range is determined by the operating conditions of the equipment to be protected as well as the strength of the equipment. When the actual rupture pressure of the burst disc falls within this range, the selected burst disc will not prevent normal operation due to an excessively low rupture pressure, nor will it pose a threat to the safety of the equipment due to an excessively high rupture pressure. 2.16.4 Test burst pressure GB 567-1999: The actual burst pressure value of the blast disc measured at the moment of explosion during a burst test. While measuring this burst pressure, the test burst temperature should also be measured. 2.16.5 Calibrated burst pressure: The arithmetic average of the actual burst pressures obtained from burst tests conducted on bursting discs of the same batch at a certain temperature. 2.17 Detonation temperature: The temperature of the wall of the pressure-sensitive element corresponding to the detonation pressure. This definition can be used as an adjective in combination with the words “design” or “test”. 2.18 Manufacturing range: The range of calibrated burst pressures for a batch of bursting discs, as agreed upon by the supplier and the buyer. 2.19 Blasting pressure tolerance: The maximum allowable deviation of the actual test blasting pressure of the shrapnel from the calibrated blasting pressure. Its value can be an absolute value or a percentage, expressed with a plus or minus sign. When the agreed manufacturing range is zero, this tolerance represents the maximum deviation from the designed burst pressure, and this tolerance range also constitutes the allowable burst range. 2.20 Discharge area: The minimum cross-sectional discharge area of the rupture disc device, taking into account the geometric factors that may affect its discharge capacity (such as residual fragments of the rupture disc after detonation, fragments from backpressure brackets and other accessories). 2.21 Discharge volume (also known as discharge capacity) is the flow rate of the pressurized medium that can be discharged through the discharge area after the fragmentation explodes. 2.22 Batch A group of burst discs that have the same type, specifications, calibrated burst pressure, and burst temperature, as well as identical materials (grade, furnace batch number, properties) and manufacturing processes, constitutes one batch. 3 Requirements 3.1 Design 3.1.1 The discharge volume (discharge capacity) of the explosive fragments can be determined using the method provided in Appendix A (the standard appendix). 3.1.2 The bursting pressure of the ordinary type bursting disc for the positive arch can be estimated using the method provided in Appendix B (the recommended appendix). 3.2 Materials 3.2.1 All metal and non-metal materials used in the manufacture of blast discs, clamps, etc., shall comply with **standards, professional standards (ministerial standards), or relevant technical specifications. 3.2.2 The materials used to manufacture blast discs must come with quality certificates and approval documents, and necessary performance tests must be conducted in accordance with the manufacturing requirements. When selecting materials, the requirements for corrosion resistance against the medium must be taken into account. If necessary, a corrosion-resistant protective film can be applied, or a coating can be applied. 3.2.3 The shrapnel material shall possess uniform and stable mechanical properties as well as thermal stability. The maximum operating temperature of the recommended materials is given in Appendix C (the suggested appendix). 3.2.4 The sealing film, protective film, or coating must be airtight and leak-free. 3.3 Shrapnel 3.3.1 The quality of shrapnel products includes both their appearance and their explosive performance. The appearance, shape, and dimensions shall conform to the design drawings. The blasting performance must pass the blasting test. 3.3.2 The inner and outer surfaces of the blasting fragments shall be free from defects such as cracks, rust, pores, bubbles, inclusions, and pits, and shall not have scratches or other imperfections that could affect their blasting performance. The perimeters of the slits (holes) or grooves in slit-type or grooved rupture discs should be **notched**, and the geometric shape and dimensions of these slits (holes) or grooves must meet the requirements specified in the design drawings. 3.3.3 The allowable tolerance for blasting pressure shall be in accordance with Table 1, or as specified by the design technical requirements. GB 567-1999 Table 1 Allowable tolerance for blasting pressure (According to 3.3.4, blasting tests to evaluate blasting performance shall be conducted at the designed blasting temperature.) The test conditions shall be determined through consultation between the supplier and the user, and should be made as close as possible to the operating conditions of the burst disc. If the test results at room temperature can ensure the blasting performance at the designed blasting temperature, then the testing can be carried out at room temperature. 3.3.5 The scope of fragmentation production shall be determined through consultation between the supplier and the buyer with reference to Appendix D (the suggested appendix). 3.4 Clamps 3.4.1 The basic structural types of clamps include: standard type, enlarged type, threaded type, and other structural types that comply with this standard. The standard type (or insertion type) clamp (see Figure 1) can be installed centrally inside the bolt holes of the flange, with its outer diameter not exceeding the inner diameter of the flange bolts. The rupture disc device must be accurately centered between the flanges in order to ensure the rupture performance of the disc as well as the sealing performance of the flanges. Through consultation between the manufacturer and the user, the following methods can be used to align it: a) The outer circle of the clamp fits precisely inside the bolt holes of the flange, b) Use positioning fittings ； c) Other appropriate methods. The expanding type (or loop flange type) clamp (see Figure 2) generally has the same outer diameter as the mating flange; it is positioned using flange bolts and placed centrally between the flanges. The threaded (or plug-type) clamp (see Figure 3) fixes the blast disc by joining two or more components together with threads. Such clamps are generally suitable for applications where the burst disc discharge diameter is small. Figure 1: Standard type burst disc holder. Figure 2: Enlarged type burst disc holder. Gs 567-1999 I1 14. Figure 3: Threaded type burst disc holder. 3.4.2 The holder must be designed and manufactured in conjunction with the burst disc to ensure proper fit. It should be able to deliver a uniform clamping load, ensuring that the blasting pieces remain under pressure until detonation, so that their edges are not pulled apart, and guaranteeing a tight seal to prevent leakage. 3.4.3 The holder should generally be above the dome of the burst disc, or other measures should be taken to prevent accidental damage to the burst disc. 3.4.4 The holder should generally have a positioning mechanism to ensure proper assembly with the blast disc. 3.4.5 The holder can only be used in conjunction with the blast disc designed for it; it must not be modified or replaced without the manufacturer’s approval. 3.5 Support ring: 3.5.1 The processing of the support ring must ensure that the blast disc can detach promptly. Its height shall comply with the requirements of 3.4.3. 3.5.2 Once the support ring and the blast disc have been assembled together by the manufacturer, they must not be disassembled, reinforced, or have their fixing points altered arbitrarily. 3.6 Backpressure Bracket 3.6.1 The backpressure bracket shall have sufficient stiffness. When combined with a burst disc, it should be able to withstand 1.3 times the maximum back pressure difference, with a pressure retention time of over 1 minute. 3.6.2 The sum of the free cross-sectional areas of the openings in non-extended brackets shall be sufficient to meet the discharge requirements when the rupture disc bursts. 3.6.3 The cracking pressure of extended back-pressure brackets shall be lower than the bursting pressure of the rupture disc. 3.6.4 The edges of holes or seams on the backpressure bracket shall **not have any structures that could puncture or otherwise damage the burst disc or sealing membrane (protective layer). 4 Tests 4.1 Inspection 4.1.1 All materials from the same batch used to manufacture blast discs shall undergo process forming tests to verify the uniformity and overall quality of the materials. The process forming test involves punching and shearing at least 3 test blanks from appropriate sections of the same batch of material for disc rupture disk formation; after the surface quality is verified to be satisfactory, a rupture test is conducted. The maximum deviation of the blasting pressure for each piece should be within the specified allowable tolerance for blasting pressure (see Table 1). 4.1.2 Inspection of finished shrapnel The inspection of finished shrapnel includes the following: a) Conduct a surface quality check on each piece; those that fail the check are discarded, b) Perform blasting tests on samples, which must comply with the requirements specified in 3.3.3 and 3.3.4. 4.1.3 The inspection of the surface quality of the shrapnel shall meet the requirements of 3.3.2. Generally, visual inspection is carried out on each piece under normal lighting conditions; if necessary, a 3.5x magnifying glass can be used. When the material thickness is less than 0.2 mm, in addition to the observations mentioned above, light transmission tests should be conducted on each piece if necessary; the illumination level should be no less than 5,000 lx, and those that transmit light should be discarded. 4.1.4 Density of the sealing film. In addition to the method described in 4.1.3, other methods can also be used to check for leaks. 4.1.5 The corrosion resistance of materials in contact with corrosive media, as well as protective films or coatings, should be determined through corrosion tests or based on existing experience of their use. The uniformity and density of the coating shall be inspected using the methods specified in the relevant standards. 4.2 Blasting Tests The final determination of whether the quality of the finished blast discs is satisfactory depends on the results of the sampling blasting tests. 4.2.1 The number of blast discs to be tested from each batch is specified in Table 2. Test samples shall be randomly selected from the same batch of blasting caps with qualified surface quality in accordance with Gs 567-1999. Table 2 Number of samples for blasting tests: 1 f 10 2 fp*Wft#ARhAfilf 1 0 J; at, } #%tfFh#kMRW*-NIA Z. 4.2.2 The blasting test system should include: a) a pressure medium source, b) a pressure indication and blasting pressure measurement system. ) Temperature measurement system ； d) Heating control system (for testing at blasting temperature). e) Medium discharge channel after blasting (to vent or discharge into a storage container) ； f) Return and venting system for pressure medium ； 9) Safety protection facilities. 4.2.3 The release ports, diameter, and orifice structure of the clamps used for blasting tests shall be identical to those of the clamps used in actual operations. 4.2.4 The pressure medium used for blasting tests should, as much as possible, have the same phase state as the medium actually used by the blast disc. The liquid medium can be oil or water, or some non-corrosive liquid that can operate at high (low) temperatures. The gas medium can be air or nitrogen, or other inert gases. During hydraulic blasting, the pressure chamber of the test system should be filled with liquid. During pneumatic blasting, the test system must first pass a hydraulic pressure test, and effective safety measures must also be put in place. 4.2.5 Pressure measurement for blasting tests can be carried out using digital pressure gauges or Bourdon tube pressure gauges that are within their valid calibration period, or other instruments for measuring pressure. 4.25.1 The entire testing system shall be equipped with at least two pressure measuring instruments. One of these is used to measure the blasting pressure, and it should be placed as close as possible to the test burst disc ； Another is used to indicate system pressure and can be placed in a visible location at the outlet of the pressure source. 42.5.2 The pressure gauge used to measure the blasting pressure, namely the pop-off tube pressure gauge, shall have an accuracy not lower than that specified in Table 3. The maximum range of the pressure gauge shall be 1.5 to 3 times the designed blasting pressure. The pressure gauge used before the test should be calibrated or be within its valid calibration period. For other instruments used to measure pressure, their accuracy must also meet the requirements specified in Table 3. Table 3 Accuracy classes of diaphragm pressure gauges: 0.1–2.5 MPa, class 0.41; 2.5–10 MPa. 4.2.6 For temperature measurement during explosion tests, calibrated glass liquid thermometers or thermocouples can be used, as well as other temperature measuring instruments. Thermometric instruments should be protected from the effects of external heat transfer. 4.2.6.1 To measure the bursting temperature, the assembled rupture disc can be immersed in a liquid heat (cold) medium, or placed in an oven (or freezer) or heating furnace to be heated (or cooled); once the temperature has stabilized, pressure can be increased until bursting occurs. The temperature of the hot (cold) carrier measured at this time can be used as the detonation temperature. When the purchaser (or design unit) is unable to provide the exact bursting temperature of the rupture disc, the bursting test and bursting temperature shall be determined through consultation between the buyer and seller in accordance with GB 567-1999. 4.2.6.2 When measuring the temperature of a pressurized medium, attention should be paid to the uniformity of temperature throughout the medium. 4.2.7 Pressurization rate for blasting tests: The pressure at the inlet of the device shall be increased to 900% of the minimum blasting pressure over a time period of not less than 30 s, and this pressure shall be maintained for at least 5 s. Then, the pressure is increased steadily and continuously until the rupture disc bursts or releases pressure. 4.2.8 High (low) temperature burst tests shall be conducted at a slow rate of temperature increase (decrease). When the designed blasting temperature is reached, sufficient time should be allowed for heat retention to ensure uniform temperature across the bursting disc wall. Thereafter, during the pressure-rise blasting, the amplitude of temperature fluctuations shall not exceed ±10°C. 4.2.9 The results of the blasting test must be accompanied by a formal test report. This report serves as evidence of the quality of the finished rupture discs and should include the following information: a) General details: including the date of the test, the model of the rupture disc, as well as the production batch number and quantity ； b) Information on the test burst disc device: including the basic structure of the burst disc, the discharge diameter of the holder, the material of the burst disc, the designed burst pressure and burst temperature, the manufacturing range, and the tolerance for burst pressure, etc ； c) Test conditions and test methods: including the number of sample rupture discs, the test medium, the temperature of the medium and the ambient temperature, the test apparatus and equipment, as well as the instruments used for testing. d) Test results: including the test burst pressure, test burst temperature, calibrated burst pressure, burst pressure deviation, etc., as well as a conclusion regarding whether the test was successful or not. e) Signature of the tester. When supervision and inspection are required, the signature of the supervisor/inpector is also necessary. 5 Marking, Packaging, Transportation, and Storage 5.1 Marking 5.1.1 Each burst disc shall have at least the following markings: a) Batch number ； b) Model ； . ) Specification (nominal diameter of the discharge port), mm; d) Material ； e) Calibrated burst pressure or designed burst pressure, MPa; f) Burst temperature,℃ ； g) Discharge side direction ； h) Standard code, 1) Manufacturer’s name ； J) Manufacturing license number. 5.1.2 The marker content should be permanent. Metal labels can be used to be fixed to the edge of the blast disc, with the front side facing the discharge side. For blast discs without labels, simple markings or coloring can be applied on the discharge side at the edge of the blast disc; however, a metal label indicating all the contents of 5.1.1 must also be used and fixed near the blast disc. 5.2 Packaging 5.2.1 Fragmentation products shall be equipped with specialized packaging boxes (cases), which can be used for individual packaging or bulk packaging. During packaging, the packaging box (case) and the rupture disc must be dry and clean; it is necessary to prevent the rupture disc from being distorted or its surface from being compressed, as well as any damage that could affect its bursting performance. 5.2.2 The outer surface of each packaging box shall indicate the name, model, specification, quantity, manufacturer, manufacturing date and month, as well as the equipment identification number of the rupture disc. The packaging box must contain a product quality certificate, a certificate of conformity, and an instruction manual for the rupture disc. An information feedback form can also be attached. 5.3 Transportation and Storage 5.3.1 Blast fragment products shall be transported only after they have been properly packaged. During transportation and handling, collisions, impacts, moisture exposure, and contamination should be avoided. 5.3.2 Shrapnel products should be stored in their original packaging boxes, with the front side facing up, kept dry, and protected from environmental corrosion. The storage room should be kept clean and well-ventilated. Gs 567-1999 6 Quality Certificate 6.1 Each batch of blast disc products must be accompanied by a quality certificate and a compliance certificate. 6.2 The quality certificate for blast disc products shall include at least the following information: a) name, batch number, b) model ； c) Manufacturing (batch) quantity ； d) Specification (nominal diameter of the discharge port), mm; e) Material ； f) Applicable medium and temperature; B) Designed burst pressure, manufacturing range, MPa; h) Calibrated burst pressure, MPa; 1) Burst pressure tolerance. j) Blasting temperature, °C, k) Standard code ； 1) Qualification mark, inspector’s seal, m) Manufacturer’s name, manufacturing license number, seal, supervision and inspection mark (when supervision and inspection is required) ； n) Date of manufacture. 6.3 Each burst disc product certificate shall be consistent with the contents of the quality certificate for that batch of burst discs. Gs 567-1999 Appendix A (Appendix to the standard) Detonation disc discharge. (Discharge capacity) A1 Symbol explanation: A – Minimum discharge area of the detonation disc, mm²; W – Rated discharge capacity of the detonation disc (discharge capacity), kg/h; p – Designed burst pressure of the detonation disc (absolute), MPa; Po – Pressure on the discharge side of the detonation disc (absolute), MPa; Pc – Critical pressure of the gas (absolute), see Table A1, MPa; pr – Specific pressure of the gas, pr = p/Pc; Δp – Pressure difference between the inside and outside of the detonation disc during overpressure detonation ； If the discharge side is at atmospheric pressure, this value is taken as the design burst pressure (gauge pressure), in MPa; T is the absolute temperature of the gas discharged from the container or equipment, in K; Tc is the critical temperature of the gas (absolute), as given in Table A1, in K; Tr is the ratio temperature of the gas, given by Tr = T / Tc ； M— the molecular weight of the gas, that is, the molar mass, in kg/kmol; k— the adiabatic index of the gas, as given in Table A1; for air, k=1.40; Z— the compressibility factor of the gas, which is determined from graphs in Table A1 based on T and P. Graph A1(a) is used for p

2.11 Backpressure bracket: In composite blast discs, it is a bracket used to prevent the pressure-sensitive element from being accidentally damaged due to a backpressure difference. 2.11.1 Open-type backpressure bracket: A backpressure bracket that breaks apart when the rupture disc explodes. 2.11.2 Non-rupturing back pressure bracket A back pressure bracket that does not rupture when the burst disc fails. 3.6.2 The sum of the free cross-sectional areas of the openings in the non-expanding brackets should be sufficient to meet the discharge requirements when the burst disc ruptures. —————————————————————————————————— After reading the above explanation, I’m a bit confused: if there are openings in the non-expanding brackets, how can they resist back pressure?

A regular vacuum bracket will of course break when the rupture disc explodes. Some are metal diaphragms with small holes, while rupture disks are plastic diaphragms made of Teflon or similar materials; such vacuum brackets will not break. Since the burst discs have exploded, there must be no vacuum inside anymore; it’s connected to atmospheric pressure. Vacuum brackets are generally made of metal, such as 316

The remarks from upstairs were excellent. Under what conditions is the combination of perforated brackets with plastic diaphragms such as Teflon generally used? I’ve never dealt with this before; I hope you can give me some guidance.

I’ll answer the questions from upstairs. This design is typically in the form of a positive arch with slits; the pressure control is achieved through the metal membrane with those slits and metal components, while Teflon serves as the sealing membrane. It is generally used in applications where the blast pressure is relatively low.