Safety valve calculation regulations

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1. Scope of application 1.1 These regulations apply only to the installation and calculation of pressure relief valves used to prevent overpressure in pressure vessels within chemical production facilities where the pressure is greater than 0.2 MPa; they do not cover ultra-high pressure systems with pressures exceeding 100 MPa. Safety valves suitable for pressure vessels and pipelines within the aforementioned range in chemical production facilities ； It is not applicable to safety valves used on pressure vessels in other industries, such as various tank trucks, various gas cylinders, boiler systems, containers made of non-metallic materials, as well as the nuclear industry and power industry. 1.2 The calculation methods are taken from the Process Design Manual; when applying these regulations, safety valves should generally be calculated in accordance with them, while for complex operating conditions, calculations shall still be carried out according to the relevant sections of the Process Design Manual. 1.3 These regulations provide an analysis of the causes of overpressure; it is necessary to read this chapter carefully when using them. 2. General instructions for calculation 2.1 Safety valves are suitable for clean, particle-free, low-viscosity fluids. In applications where a pressure relief device is necessary but safety valves are not appropriate, rupture disks or a combination of a safety valve and a rupture disk should be used. 2.2 During the Process Package Design phase (PDP), each operating condition listed in these regulations shall be analyzed in accordance with the operating specifications of the process unit and these provisions (see Section 5.0). Based on the material flow tables from the PDP, the emissions for each operating condition shall be determined and entered into Valve Data Sheet 1. 2.3 During the basic design phase (BDP) and the detailed design phase (DDP), in accordance with the provisions of the discharge capacity calculation documents, Safety Valve Data Table 2 (data summary table) and Safety Valve Data Table 3 are prepared based on Safety Valve Data Table 1. Safety valve Data Sheet 3 is submitted to the relevant department as a condition. 3. Terminology Definitions 3.1 Accumulation: The pressure that exceeds the maximum allowable operating pressure of a container during the discharge process of a safety valve, expressed in pressure units or as a percentage. The maximum allowable accumulation is determined by the operating specifications of the application and fire safety regulations. 3.2 Back pressure: It is the pressure present at the outlet of the safety valve due to the pressure in the relief system. Back pressure can be either constant or variable. Back pressure is the sum of additional back pressure and accumulated back pressure. 3.3 Superimposed back pressure: The static pressure present at the outlet of a safety valve when it operates; it is the pressure resulting from the discharge by other valves. This pressure can take two forms: fixed or variable. 3.4 Built-up back pressure: The pressure increase in the discharge pipe due to flow once the pressure relief valve opens. 3.5 Maximum allowable accumulated pressure: It is the sum of the maximum allowable operating pressure and the maximum allowable accumulation. 3.6 Maximum allowable working pressure: refers to the maximum pressure that the top of the container is permitted to withstand at the design temperature. This pressure is based on the normal thickness in equipment calculations, metal corrosion allowance, load, and pressure. The maximum allowable operating pressure is the basis for setting the pressure protection device of the safety valve. 3.7 Overpressure: Pressure that exceeds the set pressure of the safety valve, expressed in pressure units or as a percentage. It is the same as the accumulation at the maximum allowable operating pressure set by the container, assuming no piping losses at the safety valve inlet. 3.8 Set pressure of the safety valve: The safety valve will activate when the static pressure at its inlet reaches this value. 3.9 Operation pressure: The pressure at which the vessel is typically operated. The design of pressure vessels typically includes a maximum allowable operating pressure, which provides a sufficient margin over the operating pressure to prevent the safety valve from opening unnecessarily. 3.10 Relieving conditions: Used to indicate the inlet pressure and temperature when the safety valve is overpressured. The discharge pressure is equal to the set pressure of the safety valve plus the overpressure, while the discharge temperature is the temperature of the fluid under discharge conditions; it may be higher or lower than the operating temperature. 3.11 Blowdown pressure difference: The difference between the set pressure and the closing pressure of the safety valve, expressed as a percentage of the set pressure or in pressure units. 4. Causes of overpressure Overpressure is caused by an imbalance in materials or energy within a certain part of the system, or by an imbalance in both materials and energy. Therefore, analyzing the causes and quantities of overpressure is a comprehensive study of the special and complex conditions in the material and energy balance during the process. The setting of the safety valve ensures that the pressure in a process system or under any operating condition within such a system does not exceed the maximum allowable cumulative pressure. System pressure includes pressure vessels, heat exchangers, and other equipment and pipelines. Its design is based on (a) the normal operating pressure at normal operating temperatures, (b) the effect of any mechanical loads, which can cause deviations from the operating load, and (c) the set pressure of the safety valves. The process system design must define a minimum relief value to prevent any device from exceeding its maximum allowable accumulated pressure. 4.1 Sources of overpressure: Discharge devices are created as a result of the release of liquid or gas due to energy input. The two most common sources of energy are, first, energy input that indirectly causes pressure to increase through vaporization or thermal expansion ； The second is entry under direct high pressure. Overpressure can be caused by one or both of the above factors. The discharge capacity of the safety valve is the maximum discharge capacity; this maximum capacity allows for pressure relief to protect the equipment from overpressure caused by any individual factor. The probability of two unrelated failures occurring simultaneously is very low, so it is usually not necessary to consider it. 4.2 Effects of pressure, temperature, and composition Since temperature and pressure affect the flow rate and composition of liquids and gases, temperature and pressure must be taken into account when determining each leakage rate. When a liquid is heated, it turns into a gas. Due to the increase in pressure inside the sealed container and the influx of heat, the equilibrium is disrupted, resulting in the formation of gas. In most cases, the container contains a mixture of different components with varying boiling points. The components with a lower boiling point evaporate first; as more heat is supplied, the heavier components begin to evaporate as well. Eventually, if enough heat is provided, even the heaviest components will evaporate. During the pressure relief process, it is necessary to study the gas release volume and molar mass at different times in order to determine the maximum gas release volume and composition. The released pressure can sometimes exceed the critical pressure (or subcritical pressure) of the system components. In this case, it is necessary to refer to the relationship between the compression coefficient and density-temperature-enthalpy. If the overpressure is caused by an influx of excess material, this excess material must be discharged at a temperature calculated under the condition that the inlet and outlet enthalpies are equal. There are no other materials entering or leaving the system; any overpressure is caused by external heat. The heat enthalpy input from the outside equals the sum of the material still present in the container and that which has evaporated. By calculating or plotting the relationship between the cumulative discharge volume and time, the instantaneous maximum discharge volume can be determined. This maximum leakage rate usually occurs near the critical temperature. 5. Determination of the discharge capacity of a single safety valve: This section lists various causes of overpressure, which serve as the basis for determining the discharge capacity. The following sections detail some common accidents that require overpressure protection. 5.1 Operator influence: Determining the maximum discharge condition also requires considering the operator’s reactions and understanding of a series of incorrect actions. The typical acceptable response time ranges from 10 to 30 minutes, depending on the complexity of the device. The effectiveness of this reaction also depends on process kinetics. 5.2 Closing outlet valves: When all the outlet valves of the equipment or system are closed, in order to protect the equipment or prevent overpressure in the system, the capacity of the relief device must be greater than or equal to that of the source of overpressure. If not all outlet valves are closed, the leakage rate from the open outlet valves also needs to be taken into consideration appropriately. The overpressure source comes from pumps, compressors, high-pressure supply lines, and volatile gases. This situation occurs in heat exchangers; closing the outlet valve can cause thermal expansion or the generation of gas. The leakage rate is the leakage rate at the set pressure plus overpressure, rather than the leakage rate under normal operating conditions. When this difference is not taken into account, the leakage rate often **decreases**. When determining the discharge volume, the friction loss between the overpressure pipeline and the discharge pipeline must also be taken into account. 5.3 Cooling or reflux failure 5.3.1 General principles The required discharge volume depends on the heat and mass balance under the system’s discharge pressure. In a distillation system, the vent volume calculation depends on whether there is reflux or not. When the refrigerant flow stops, the remaining cooling capacity is usually not taken into account, as the time during which this capacity is effective is very limited and depends on the actual layout of the piping. If the process piping system is unusually large and not insulated, heat loss must be taken into account. Due to the difficulties in performing detailed heat and mass balance calculations, sections 5.3.2 to 5.3.9 outline simple principles for determining acceptable discharge amounts. 5.3.2 Total condensation leakage refers to the total amount of gas that enters the condenser. The temperature used in the calculation corresponds to the set pressure plus the overpressure, as well as the new gas phase composition resulting from the heat entering minus the heat released. The amount of pulse accumulation at the top of the normal liquid level is usually limited to within 10 minutes. If the refrigerant fails for more than this period of time with no backflow, the components, temperature, and gas phase volume at the top will experience significant changes. 5.3.3 Partial condensation leakage rate is the difference between the volume of inlet and outlet gases under leakage conditions. The amount of gas entering should be calculated based on the criteria in 5.3.2; if the composition or flow rate of the reflux changes, the amount of gas entering the condenser should be determined by the new conditions. 5.3.4 Fan failure: Due to natural convection, even if the fan fails, as long as the discharge conditions remain unchanged, there is usually still a condensation capacity of 20%–30% of the normal level; the discharge volume is 70%–80% of that in the cases of full condensation or partial condensation. However, the actual amount of condensation usually depends on the design performance of the air cooler; the presence of fans and mechanical hot spots reduces its cooling capacity. 5.3.5 Blinds Closed: When the blinds of the air cooler are closed, it indicates a fault with the entire refrigerant system; the calculation for the leakage rate is the same as that in 5.3.2 and 5.3.3. The closure of the blinds may be caused by a failure in the automatic control system, a malfunction in the mechanical connections, or structural vibrations affecting the blinds when they are in the manual position. 5.3.6 Top reflux: In most cases, for example, when the pump is turned off or a valve is closed, reflux failures occur, and the flow rate into the condenser equals the amount of refrigerant lost as described in 5.3.2 or 5.3.3. Stopping the reflux results in different gas phase compositions, which affects the release rate. In this case, the size of the safety valve is determined based on the condition where the refrigerant flow comes to a complete stop, but each condition must be established through testing of the relevant components and the system. 5.3.7 Pump cycle: The leakage volume is equal to the vaporization volume, which is caused by the heat obtained during the pump cycle. Vaporization latent heat is the latent heat under the release temperature and release conditions. 5.3.8 Top reflux plus pump circulation: Top reflux and pump circulation usually do not fail simultaneously, but a partial failure and a total failure are still possible. The discharge volume is the same as in 5.3.6 and 5.3.7. 5.3.9 Stop of side-line reflux: Same as 5.3.6 and 5.3.7. The discharge volume is large enough to discharge the amount of vaporization generated by the heat obtained from the system. 5.4 Failure of the absorption stream: A lean condition in terms of hydrocarbons; generally, a lean condition does not cause leakage ； In the acid gas removal unit, a large amount of gas (about 25% or more) is absorbed by the absorbent; if the absorbent is depleted, the pressure will rise to the discharge pressure because the downstream system cannot handle the increased flow rate. It becomes even more difficult to analyze the downstream gas entering methanation in the syngas carbon dioxide absorption unit. Any amount of carbon dioxide that exceeds the design capacity entering the methanation process, as well as any partial failure of the absorbent, can lead to a rapid rise in temperature, causing the methanation feed valve to close and the vent valve to open. If the vent valve is closed, it can cause overpressure. For each operating condition, its process and instrumentation characteristics must be studied, with the scope of investigation including the impact on downstream process units as well as the piping and instrumentation related to the absorbent. 5.5 Accumulation of non-condensable gases: Under normal conditions, non-condensable gases do not accumulate as the process stream is released. However, certain piping arrangements can cause these gases to accumulate at a particular point, leading to the rupture of the condenser; this effect is equivalent to a loss of refrigerant. 5.6 Volatile materials entering the system 5.6.1 Water entering hot oil Although water entering hot oil can cause overpressure, there is no established method for calculating the amount of leakage. If the amount of water and heat entering can be determined, the diameter of the relief valve can be calculated in the same way as for material valves; unfortunately, the amount of water entering can never be known. At the same time, due to the extremely large volume expansion from liquid to gas (at standard pressure, it is approximately 1:1,400), the gas is generated instantaneously, and whether the valve can open quickly becomes an issue. Usually, no pressure relief device is provided for this accidental situation. Every effort should be made to eliminate this possibility in process design and operation. To prevent water from accumulating and forming pockets, steam trap hydrostatic valves should be installed, and double isolation valves and vent valves should be installed on the water-to-hot process pipelines. 5.6.2 Entry of light hydrocarbons into hot oil: The explanation is the same as in 5.6.1; when light hydrocarbons enter hot oil, the rate at which they change from a liquid to a gas state is not less than 1:1,400. 5.7 Failures of automatic control valves in the process flow 5.7.1 General provisions Automatic control valves on equipment or systems are directly controlled by the process or indirectly controlled by process changes (such as pressure, flow rate, level, or temperature). When a control valve fails, it should ensure that it is in either the fully closed or fully open position, as required by the basic design. 5.7.2 Determination of leakage rate: The leakage rate under any operating condition is calculated without taking into account leakage caused by control valves; that is, the location of the control valves should ensure that the normal process flow can pass through. The normal valve position is first determined based on design capacity and system shutdown conditions, rather than considering accidents. Therefore, if the conditions flowing through the control valves do not change (see 5.7.5), it is necessary to adjust the normal flow rates of these control valves and modify the drainage conditions to ensure that the downstream system can handle the increased flow rate. 5.7.3 Control valve inlet The control valve inlet may have one or more inlet pipelines. For control valves with only one inlet pipeline, only the fully open position is considered, without taking into account the position where the control valve may fail. The control valve opening could also be caused by a malfunction in the instruments or improper operation. For control valve systems with multiple inlet pipelines, it must be ensured that any control valve on these pipelines is in its normal operating position. Therefore, the discharge volume is the difference between the expected maximum inlet flow rate and the normal outlet flow rate, under the assumption that one control valve in the system is closed while the other control valves remain at their normal operating points (i.e., fully open, fully closed, or throttled). If one or more outlets of the control valve are closed, or if multiple inlets of the control valve are opened thereby causing its first inlet to open, the discharge volume is the difference between the expected maximum inlet flow rate and the flow rate at the open outlet. 5.7.4 Control valve outlets: When determining the discharge volume for each control valve outlet, the fully open and fully closed positions should be taken into consideration; positions of the control valve resulting from failures in the instrumentation system or improper operation are not considered. If one or more inlet valves open due to the incorrect closure of the outlet valve, the pressure relief device must be capable of preventing overpressure; the amount of pressure that can be relieved is the difference between the maximum inlet flow rate and the maximum outlet flow rate. It is necessary to calculate all flow rates under relief conditions, as well as taking into account closures caused by carelessness on the part of the control valve operator. 5.7.5 Considerations for special discharge rates: Although the specifications and dimensions of control valves, such as diaphragm valves, are determined based on normal operating conditions, these valves can still function under abnormal conditions, such as when safety valves discharge. The design and operating positions of the valve should be in line with the control signals under abnormal operating conditions. Since the amount of fluid released under pressure relief conditions differs from that under normal operating conditions, the amount of fluid to be considered when calculating the capacity of the control valve must be determined based on the temperature and pressure during those pressure relief conditions. In extreme cases, the state of the fluid changes (for example, from liquid to gas, or from gas to liquid); for instance, there is a significant difference between using fully open control valves for handling liquids and those for handling gases. Vaporization occurs where the liquid level is absent, allowing high-pressure gas to pass through the control valve into a system whose pipe diameter is calculated based solely on the vaporization of liquid under normal conditions. 5.8 Unnormal process heat ingress flow rate is the maximum flow rate of gas generated under discharge conditions (including non-condensable gases resulting from overheating), which is less than the normal amount of condensate or gas. For each operating condition, the designer should consider the system’s potential operating conditions and every one of its components. 5.9 Internal explosions (excluding deflagration): When an internal explosion is caused by a mixture of gas and air, pressure relief should be provided by rupture disks rather than safety valves, as the rapid increase in pressure resulting from the spread of internal combustion would be too slow to be managed effectively using safety valves to protect the container. The discharge area depends on the following factors: a. Initial conditions (temperature, pressure, composition). b. Clear physical properties of the combustion-diffusing vapor or gas. c. Volume of the container d. Trigger pressure of the relief device e. Maximum pressure in the event of an explosion 5.10 Chemical reactions: Determine the diameter of the emergency relief pipe for chemical reactions using the DIERS (Design Institute of Emergency Relief Systems) method. The specific methods are as follows: a. Define the design basis for abnormal operating conditions in chemical reactions; b. Calculate the system characteristics under abnormal conditions through experimental testing; c. Determine the pipe diameter using two-phase flow calculation formulas. Uncontrolled chemical reactions are usually associated with the following factors: a. External fires; b. Loss of mixing; c. Loss of cooling; d. Incorrect loading of reagents. 5.11 Power failures: To determine the amount of leakage that occurs during power failures, it is necessary to carefully examine which equipment is affected by such failures and how they impact the operation of the facility. 5.12 Expansion of Liquids 5.12.1 Causes Expansion of liquids refers to the increase in volume that occurs as a result of rising temperature; there are usually the following reasons for this phenomenon ； a. When a pipe or container filled with cold liquid is severed, it gains heat from steam heating, coiled tubes, ambient heat, or a fire. b. The cold side of the heat exchanger is isolated, and the liquid on the cold side is heated by the hot side. c. A pipe or container filled with liquid at room temperature is severed, and the liquid inside is heated by radiant heat. Expansion coefficients of hydrocarbons and water at 60°F: Liquid specific gravity (oAPI), value in °F – 3–34.9: 0.0004; 35–50.9: 0.0005; 51–63.9: 0.0006; 64–78.9: 0.0007; 79–88.9: 0.0008; 89–93.9: 0.00085; 94–100 and lighter: 0.0009. For water, the value is 0.0001. [Hubei Weihua] invites you to attend the 5th Symposium on Catalytic Hydrogenation Processes and Continuous Flow Technology – Nanjing, May 12, 2022. Pipe diameter and set pressure: It’s not easy to determine the leakage rate, as the leakage amount for liquids under actual conditions is very small; therefore, it’s necessary to select appropriate values for these parameters, rather than using safety valves with an excessively high safety factor. 3/4”x1” safety valves are commonly used. If there is reason to believe that this size is insufficient, the method in 3.14.3 should be used. To select the appropriate set pressure, all design flow rates when the system is shut down should be considered; the set thermal expansion pressure must never exceed the maximum pressure of the system being protected. If there is only one operating condition involving liquid expansion, the set pressure of the safety valve should be set relatively high ； If the outlet of the safety valve leads into a closed system, the effect of back pressure should be considered. 5.12.3 Special conditions: When installing safety valves on large-diameter, uninsulated pipes on the ground, as well as on large containers or heat exchangers filled with liquid, the diameter is usually greater than 3/4” ×1”. When the inlet and outlet valves at both ends of a long pipe at or below room temperature are closed, the temperature rise caused by solar radiation can be calculated; if the total heat transfer rate and the thermal expansion coefficient of the liquid are known, the leakage rate can be determined. The formula for calculating the volume of liquid that expands and is released is as follows: W = BH/Cp. Where W represents the mass of liquid released, in kg/h; B is the coefficient of volume expansion, in 1/°C; H is the maximum heat transfer rate under normal operating conditions, in kj/h; and Cp is the specific heat at constant pressure, in kj/kg°C. 5.13 External Fires 5.13.1 The impact of external fires on containers with wet surfaces… Liquid vaporization. The area below the liquid level inside a container is referred to as the wetted area. In the event of a fire, the heat from the external fire causes the material inside the container to vaporize through the wetted area; only the wetted area of containers with a length of 7.5 meters or less is taken into account, as the area above 7.5 meters is generally not considered. The wetted area includes the external surface area of the pipes within the range affected by the fire. a. For a container filled with liquid, the wetted area is the surface area within a height of 7.5 meters. b. For buffer tanks, separation tanks, and process vessels, the wetted surface refers to the surface at a normal liquid level not higher than 7.5 meters. c. The wetted surface of the distillation column is the sum of the surface area of the liquid layer on the trays at the normal height of the column bottom and at a height of 7.5 meters. d. The wetted area of the storage tank is the surface area within a height of 7.5 meters. e. The wetted area of a spherical vessel is the greater of the surface area of a hemisphere or the surface area within a height of 7.5 meters above the ground. 5.13.2 Correction factor for the outer wall of the container (F) – Facilities located outside the container wall can prevent the heat from the flame from reaching the container, and the correction factor for the outer wall of the container (F) is used to reflect their impact on heat transfer. According to the \"Regulations on Safety Inspection of Pressure Vessels\" issued by the Ministry of Labor: a. When the vessel has no insulation on the ground: F = 1.0; b. When the vessel is covered with sand underground: F = 0.3; c. When the vessel is equipped with a water spray system with a flow rate of more than 10 l/(m²×min): F = 0.6; d. When the vessel has good insulation on the ground, the value is calculated using Equation (2). According to the American Petroleum Institute standard API-520: a. When the vessel has no insulation on the ground: F = 1.0; b. When the vessel is equipped with a water spray system: F = 1.0; c. When the vessel has good insulation on the ground, the value is calculated using Equation (1): F = 4.2×10^-6λ/do(904.4–t) (1), where λ is the thermal conductivity of the insulating material, in kj/(m×h×°C); do is the thickness of the insulating material, in meters; t is the discharge temperature, in °C. d. For vessels that are underground or covered with sand, the calculation is carried out using Equation (3), with the thermal conductivity and thickness of the insulating material replaced by those corresponding to soil or sand. Furthermore, insulation materials are not fire-resistant; therefore, the outer wall correction factor (F) for insulated containers is 1.0. 5.13.3 Safe discharge volume 5.13.3.1 According to the “Regulations on Safety Inspection of Pressure Vessels” issued by the Ministry of Labor: a. Without insulation layer: W = 2.55×10^5 FA0.82/HL (2) Where: W: Mass discharge rate, kg/h; A: Total wetted area, m²; HL: Heat of vaporization under discharge conditions, kJ/kg; F: Correction factor for the outer wall of the vessel, taken as F = 1.0. b. With insulation layer: W = 2.61×(650–t)×λ×A0.82/(HL×do) (3) Where: W: Mass discharge rate, kg/h; A: Total wetted area, m²; HL: Heat of vaporization under discharge conditions, kJ/kg; λ: Thermal conductivity of the insulation material, kJ/(m×h×°C); do: Thickness of the insulation material, m; t: Discharge temperature, °C. 5.13.3.2 According to the American Petroleum Institute standard API 520: When there are adequate fire protection measures and mechanisms in place to promptly remove any materials that leak onto the ground, the discharge volume of the vessel is given by: W = 1.555×10^5 FA0.82/HL (4) Otherwise, equation (5) should be used: W = 2.55×10^5 FA0.82/HL (5) The symbols in these equations are the same as those in equation (2). 5.13.4 Effect of external fire on containers with unmoistened surfaces… Gas expansion. Containers with unmoistened surfaces refer to those in which steam, gas, or supercritical fluid is present; under normal conditions these containers are in a gas-liquid two-phase state, but under release conditions they become entirely gaseous. In the event of an external fire, a container without a wet surface will be damaged within a short time due to the softening of the metal material. Installing a safety valve does not protect such containers from damage; therefore, fire-fighting equipment and drainage systems must be installed to keep the containers away from flammable materials. The calculation formula is as follows: W = 8.764 × (MP1)0.5 × A1 × (Tw – T)1.25 / T1^1.1506.
W: Mass flow rate of release, in kg/h
M: Molecular weight
P1: Release pressure, in MPa
A1: External surface area of the container below 7.5 m above the ground, in m2
Tw: Temperature of the metal wall, in K; 866 K for carbon steel
T1: Temperature of the gas, in K
T1 = P1/Pn × Tn
Pn: Normal operating pressure of the gas, in MPa
Tn: Normal operating temperature of the gas, in K

5.14 Rupture of heat exchanger tubes
5.14.1 Necessity of installing pressure relief devices
According to ASME standards, heat exchangers and similar containers should be equipped with safety valves to prevent overpressure in the event of internal failures. This is reflected in the following four issues: a. The type and extent of potential internal failures; b. Determining the amount of fluid that will leak out; c. Selecting safety valves with rapid response times to prevent overpressure; d. Choosing an appropriate installation location to enable timely detection of overpressure. A rupture in the heat exchanger tubes, allowing large amounts of fluid from the high-pressure side to flow into the low-pressure side of the heat exchanger, can lead to accidents, while even minor leaks can cause overpressure in the heat exchanger. The standard hydrostatic test pressure is 1.5 times the design pressure of the equipment; therefore, the design pressure on the low-pressure side of the heat exchanger should be no less than 2/3 of the design pressure on the high-pressure side. If the actual test pressure on the low-pressure side is lower than 1.5 times the design pressure, it is necessary to determine whether a safety valve is required on that side. If the design pressure on the low-pressure side is greater than or equal to 2/3 of the design pressure on the high-pressure side, then there is no need to install a safety valve on the low-pressure side. For newly designed heat exchangers, the design pressure on the low-pressure side can be increased to reduce risks. Click to view -- Summary of Chemical Engineering Skill Training Courses for 2023. If the design pressure on the low-pressure side of a heat exchanger is less than 2/3 of the design pressure on the high-pressure side, it should be considered as an emergency condition, and a safety valve should be installed on the low-pressure side. 5.14.2 Determination of discharge rate
5.14.2.1 Discharge rate when the fluid on the high-pressure side is in liquid phase
W = 72000 × Ct × At × √(2g(PH – PL)/ρ)
W: Mass discharge rate, kg/h
Ct: Flow coefficient; default value = 1.0
At: Cross-sectional area on the pipe side, m²
At = (Dtube²/4)π
Dtube: Inner diameter of the pipe, mm
g: Acceleration due to gravity (g = 9.80665)
PH: Operating pressure on the high-pressure side, kPaA
PL: Discharge pressure on the low-pressure side, kPaA
ρ: Density of the fluid on the high-pressure side, kg/m³

5.14.2.2 Discharge rate when the fluid on the high-pressure side is in gaseous phase
W = 20 × At × √[2g × (kH/(kH–1)) × (MPH²/(ZH × RT_H)) × ((PL/PH)^(2/kH) – (PL/PH)^((kH+1)/kH))]
W: Mass discharge rate, kg/h
At: Cross-sectional area on the pipe side, m²
At = (Dtube²/4)π
Dtube: Inner diameter of the pipe, m
g: Acceleration due to gravity (g = 9.80665)
kH: Adiabatic index of the fluid on the high-pressure side (k = Cp/Cv)
M: Molecular weight
ZH: Compressibility factor of the fluid on the high-pressure side
R: Gas constant; R = 8.314 kJ/kmol·K
TH: Operating temperature on the high-pressure side, K
PH: Operating pressure on the high-pressure side, kPaA
PL: Discharge pressure on the low-pressure side, kPaA

5.15 Summary table for determining the discharge rate of a single safety valve
Table 1: Summary table for discharge rates of a single safety valve
| No. | Operating conditions | Pressure relief device (liquid) | Pressure relief device (gas) |
|-----|----------------------|--------------------------------|-----------------------------|
| 1 | Outlet valve on vessel closed | Maximum pump inlet flow rate | Amount of water vapor and gas entering plus amount generated under discharge conditions |
| 2 | Cooling water failure in condenser | – | Total amount of gas entering the condenser under discharge conditions |
| 3 | Reflux failure at column top | – | Total amount of gas entering the condenser under discharge conditions minus amount condensed from side-stream reflux |
| 4 | Side-stream reflux failure | – | Difference between amounts of gas entering and exiting under discharge conditions |
| 5 | Desorption failure in absorber | – | Normally no discharge occurs |
| 6 | Accumulation of non-condensable gases | – | Same as #2 for columns; same as #1 for vessels |
| 7 | Entry of highly volatile substances | Water enters hot oil; light hydrocarbons enter hot oil | – | – | – Usually difficult to predict for columns; for heat exchangers, use a pipe with cross-sectional area twice that of a normal pipe to accommodate gas generated by leakage of volatile fluids |
| 8 | Overflow from tank or vessel | Maximum liquid flow rate into pump | – |
| 9 | Automation system failure | – | Each operating condition must be analyzed individually |
| 10 | Abnormal heat or gas entry | – | Estimate maximum gas generation, including non-condensable gases produced due to overheating |
| 11 | Heat exchanger tube rupture | – | Steam or vapor enters through an area twice the cross-section of a single pipe; same result as in #7 for heat exchangers |
| 12 | Internal explosion | – | Not controlled by conventional pressure relief devices; aim is to prevent external effects |
| 13 | Chemical reactions | Estimate gas generation under both normal and runaway conditions |
| 14 | Thermal expansion: Cold source shut down, pipelines outside the unit severed | – | – |
| 15 | External fire | – | – |
| 16 | Power failure (steam, electricity, etc.) | – | Analyze entire unit to determine impact of power loss; size safety valves based on worst-case scenarios |

Distillation columns: All pumps stop, resulting in no reflux or cooling water.
Reactors: Consider effects of agitation, rapid cooling, or steam cut-off; size safety valves based on gas generation during runaway reactions.
Air coolers: Fans stop rotating; size safety valves based on difference between normal and emergency thermal loads.
Buffer tanks: Maximum liquid inflow rate.

6. Specifications for calculation of discharge rates
6.1 Calculation table for liquid expansion
6.2 Calculation table for external fires
6.2.1 External fire – Calculation table for liquid vaporization
6.2.2 External fire – Calculation table for gas expansion
6.3 Heat exchanger tube rupture
6.3.1 Heat exchanger tube rupture – Fluid on high-pressure side is liquid
6.3.2 Heat exchanger tube rupture – Fluid on high-pressure side is gas