Thread Content
Seven stages of chemical technology from creativity to industrialization (Issue 53/100 in total) - Technology finalization: Calculation of safety valve relief capacity Dear friends: Hello everyone! In the last issue, we talked about SIL classification and determined the safety level of interlocking loops. This issue talks about another important safety protection device - the safety valve. Safety valves are the last physical protection barrier for pressure vessels and pressure pipelines. The interlock has failed and the safety valve is still pressed ； If the safety valve fails again, the consequences will be catastrophic. Therefore, the calculation of the relief amount of the safety valve is a task that cannot be taken carelessly in the safety design of the process package. The discharge amount is too small, and the safety valve diameter is not enough, so it cannot be discharged when overpressure occurs. ； The discharge amount is considered large, the diameter of the safety valve is too large, not to mention the increase in investment, it may have an impact on the downstream system during the discharge. 1. What working conditions should the safety valve prevent? To calculate the discharge amount of the safety valve, we must first understand under what circumstances the protected object will be overpressured. Every working condition that may cause overpressure must be analyzed one by one and the relief amount calculated one by one. Finally, the largest one is taken as the design discharge capacity of the safety valve. There are several common working conditions that require calibration. Fire conditions. When a fire breaks out around the equipment, the external flame transfers heat to the internal medium through the wall, and the liquid vaporizes or the gas expands, causing the pressure to increase. This is the most common safety valve setting condition for storage tanks and towers. Refrigerant interruption condition. The refrigerant in the overhead condenser of the distillation tower is suddenly interrupted - the circulating water pump fails, the refrigerator stops - the steam at the top of the tower cannot be condensed, and a large amount of steam rushes towards the safety valve. Power outage conditions. The entire plant has a power outage, all pumps have stopped, and all refrigerants have been cut off. However, there may still be residual reaction heat in the reactor and residual heat in the reboiler is still being released. How much residual energy can be released after a power outage requires detailed analysis. The control valve is fully open. If the feed control valve is fully opened due to malfunction, the amount of material entering the equipment increases sharply, and the amount of steam generated exceeds normal levels. Other working conditions include: Outlet mis-closure - the operator mistakenly closes the equipment outlet valve, causing the pressure to rise. ； Heat exchange tube rupture - the high-pressure side medium in the heat exchanger flows into the low-pressure side through the ruptured tube, causing overpressure on the low-pressure side ； Thermal Expansion – When a fluid-filled pipe or heat exchanger is cut off, it expands due to heat, causing pressure to rise. The calculation methods for the discharge amount in each working condition are different and cannot be used interchangeably. I encountered a situation in an actual project: Although the calculated discharge amounts are similar for several discharge conditions due to different reasons, because the discharge temperature and medium composition are completely different, the selection of the safety valve and the design conditions of the discharge pipeline are very different. Therefore, we should not just look at the discharge volume number, but also look at the state of the discharge medium. 2. How to calculate the discharge amount under fire conditions? Calculation of the discharge amount under fire conditions can be done in two steps. The first step is to calculate the heat transferred by the fire. The equipment is exposed to an external flame, and the heat of the flame is transmitted into the interior through the wall, causing the liquid to vaporize or the gas to heat up and expand. The amount of heat transferred is proportional to the surface area of the container soaked by the flame. For storage tanks, usually only the area of the tank wall above the ground that the flame can contact is considered. For tanks protected by fire sprinklers, the heat transfer can be multiplied by a reduction factor - fire sprinklers can take away a lot of heat and reduce the wall temperature. For the distillation tower, the discharge amount under fire conditions shall be the greater of the amount of steam generated by the normal heat load of the tower reboiler and the amount of steam generated by the heat transferred from the fire. It is worth noting that when the tower skirt exceeds 7.5 meters, certain standards allow different fire impact ranges to be considered - the skirt is high enough, the distance between the tower and the ground flame is far, and the actual heat transferred is smaller than the direct contact with the flame. This is one of the reasons why the height of the skirt is emphasized when discussing tower design in Issue 43. The second step is to convert the incoming heat into steam production. Divide the incoming heat by the latent heat of vaporization of the liquid to obtain the mass release. Here is an error-prone detail: The latent heat of vaporization must be the value under discharge conditions, not the value under normal boiling point. The relief pressure is higher than the normal operating pressure, the corresponding saturation temperature is higher, and the latent heat of vaporization will decrease. If the latent heat of vaporization at normal boiling point is used, the calculated discharge amount is too small, and the safety valve may not be large enough. 3. How to calculate the refrigerant interruption condition? The refrigerant interruption of the overhead condenser of the distillation tower is one of the discharge conditions that most need to be checked. After the refrigerant is interrupted, the steam at the top of the tower cannot be condensed and all flows to the safety valve or relief system. The discharge amount is the maximum amount of vapor entering the overhead condenser - that is, the total rising vapor produced by the tower reboiler at its maximum heating capacity. It should be noted when calculating that if there is flash steam brought in by the feed, this part must also be added. Also consider the residual heat of the reboiler when the refrigerant is interrupted - the tube wall and shell of the thermosiphon reboiler have a certain heat capacity. Even if the heat supply source is cut off, the residual heat will continue to generate steam for a period of time. This part of the instantaneous additional steam volume needs to consider its superposition effect when selecting the safety valve. 4. How to calculate power outage conditions? A plant-wide power outage is one of the key scenarios that need to be considered in each discharge condition. After a power outage, all pumps and refrigerants stop, and the instrument air may also stop - but the residual reaction heat in the reactor or the residual heat in the reboiler is still being released. The key is judgment: How much leakage can the residual energy in the equipment produce after a power outage. For exothermic reactions, it is necessary to evaluate whether the reaction can continue after a power outage - if the feed pump is stopped, fresh materials will no longer enter, but the materials already in the reactor may still be reacting, and the heat released needs to be evaluated. For distillation columns, the heat capacity of the column kettles and reboilers can result in continued vapor production for some time after a power outage. Accurate assessment requires detailed thermodynamic and heat transfer analysis, including the heat capacity of the equipment material, the insulation effect of the insulation layer, the residual heat release rate after the reaction stops, etc. Power outages are often accompanied by the simultaneous loss of other public works - instrument air may only be maintained for ten minutes, and circulating water may be completely interrupted. This means that there may be no cooling water to cool the discharge medium when the safety valve is discharged. The design of the discharge pipeline needs to fully consider this most unfavorable condition. 5. How to calculate the fully open control valve condition? If the feed control valve of the reactor is fully opened due to failure, the feed volume increases sharply, which may cause the pressure in the reactor to increase. The amount of relief is usually estimated by taking the amount of additional steam produced by the difference between the maximum flow rate and the normal flow rate when the control valve is fully open. The maximum flow rate when the control valve is fully open needs to be estimated based on the valve Cv value and the pressure difference between the front and rear. If there are liquid components in the feed that will flash in the reactor, the amount of flash steam must also be added. This working condition may not be as serious as fire or power outage conditions in some cases, but verification must not be skipped because of this. Especially for systems with relatively high feed pressure and relatively large feed valve caliber, the additional flow caused by the full opening of the control valve may be considerable. 6. Other working conditions that need to be considered. In addition to the above main working conditions, there are several others that are easily overlooked and are worth checking one by one. Heat exchange tube rupture condition. When the high-pressure side medium in the heat exchanger flows into the low-pressure side through the ruptured pipe, it will cause overpressure on the low-pressure side. The discharge amount is equal to the mass flow rate of high-pressure medium flowing from the ruptured pipe into the low-pressure side. Export mis-closing condition. When the equipment outlet valve is closed by mistake while the inlet is still feeding material, the pressure within the equipment may rise rapidly. The discharge amount is taken as the normal flow rate of the inlet. Thermal expansion conditions. After both ends of a liquid-filled pipe or heat exchanger are cut off, thermal expansion may cause a sharp increase in pressure - liquid is incompressible, and even a small temperature rise can cause a large pressure increase. A thermal expansion safety valve needs to be installed to release a small amount of liquid. This kind of relief is usually very small - maybe only a small DN15 or DN20 safety valve - but if it is not set up, the pipe may swell and fail after being exposed to the sun for a few hours. 7. To what extent can the discharge amount be calculated in the process package stage? In the process package stage, the type of safety valve can not be distinguished for the time being, and the discharge area and final diameter can not be selected for the time being - that is a matter of the basic design stage. But in the process package stage, the key data of the safety valve must be determined: What is the protection object, under what working conditions will overpressure occur, what is the discharge volume under each working condition, what is the discharge temperature and the discharge medium. The core of the calculation of the discharge amount lies in the selection of reasonable discharge conditions. When analyzing a system, if all the points with power and energy input in the system are found, the working conditions that need to be considered when setting the safety valve will also be found. If there is steam heating, consider fully opening the steam regulating valve ； If there is a pump feeding materials, consider that the pump outlet valve is accidentally closed. ； If there is reaction heat, consider the reaction to be out of control ； If there is an external fire risk, consider fire conditions. The relief volume is determined at the process package stage, and safety valve selection and relief pipeline design are carried out based on the relief volume during subsequent basic design. With the accurate discharge volume, the caliber of the safety valve, the size and layout of the discharge pipe will be based on it, and it will not be necessary to rework the equipment data sheet and pipeline direction during detailed design. Next Issue Preview Issue 54: Equipment layout and boundary conditions - After calculating the discharge volume of the safety valve from the proposed layout to the formal layout, the design of the safety protection device has a basis. However, the safety of the installation not only relies on interlocks and safety valves, but also the spacing between equipment, the direction of the pipe gallery, and the span of the frame—these spatial arrangements also affect safety and operability. The next issue will talk about the basic principles of equipment layout and the preparation of boundary condition tables.