API STD 521 Guide For Pressure-relieving And De...

API Standard 521, Pressure-Relieving and Depressurizing Systems, provides guidance, recommendations, and alternatives for the design of pressure-relieving and vapor de-pressuring systems at liquefied natural gas terminals, petrochemical facilities, gas plants, and other petroleum production facilities.

API STD 521 Guide for Pressure-relieving and De...

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"id":124,"name":"PSVs Discharging to Atmosphere: What to Consider","user":"33","published":true,"content":"Pressure safety valves (PSV) are used extensively in oil and gas applications to protect vessels, pipes, and other equipment against overpressure. More commonly, the PSV discharge is sent to a flare via a flare header which collects several relief sources through a flare network of headers and sub-headers. Alternatively, the PSV discharge can be directly sent to the atmosphere on some occasions. The setup required to connect the relief sources to a flare system could be too expensive or complex for remote gathering systems and hence the PSVs might be directed to the atmosphere in such cases. Relieving PSVs directly to the atmosphere, however, requires special safety attention and considerations. This article highlights key areas to look for in the design when a PSV is directly routed to the atmosphere.Safety Guidelines of PSV ReliefThe first precaution is the regulatory and safety requirements of the PSV relief, ensuring you can discharge the gas safely and the gas will not return to the grade level. Here are some guidelines:Air, nitrogen, steam, water vapour, and other fluids considered non-hazardous could be more safely discharged to the atmosphere if the relief does not create safety issues (damaging equipment, injuring personnel, or creating oxygen-deficient environments). One must exercise caution for vents of water vapour as the water might condense and form small water droplets which will fall toward grade and collect on surfaces. A small amount of finely dispersed liquids (particle diameter < 600 microns) can be discharged safely if the vapour pressure of the liquid is high at ambient temperature (Molecular weight of less than 80 is required for most hydrocarbons). The volatile liquid will evaporate quickly after discharge. Vapours containing mist could ignite well below the lower flammability limit. Determining what materials and at which locations in the facility the relief can be safely discharged to the atmosphere is a complicated process requiring detailed evaluation of risks and mitigation options. A comprehensive assessment of the process for determining the safety of discharge to the atmosphere is discussed elsewhere [Refer to References].Sizing and Evaluation of PSVsAssuming the above study has been performed and discharge to the atmosphere is deemed to be safe, sizing and evaluation of the PSV would include the following steps just like any other PSV:Determine the scenarios for the pressure build-up and calculate the corresponding relief rate as outlined in API 521. The scenarios include blocked discharge, fire, power failure, control valve failure, tube rupture, etc.Size the orifice for the scenario requiring the highest relief rate as outlined in API 520.Size the inlet and outlet lines for the PSVs as per following: Inlet and Outlet LinesThe inlet line is checked for an inlet line pressure drop of a maximum of 3% of the set pressure for rated flow. Outlet lines discharging to flare headers are sized to limit the Mach number to 0.7. For PSVs discharging directly to the atmosphere, limiting the Mach number to 0.7 is not practical and a sonic condition could be allowed in the discharge piping. The sonic conditions at the outlet will lead to a significant amount of backpressure. The pressure drop in the discharge line should be calculated using the “Isothermal” or “Adiabatic” method as outlined in API 521. It is important to make sure the discharge line pressure drop calculations consider the acceleration term as the gas is compressible. Regular pipe operation in most process simulators ignores this term which leads to inaccurate prediction of the backpressure in the system. Keep in mind that when existing PSVs are evaluated, a very large relief load might be sent to a discharge pipe with a relatively small diameter. This could lead to excessive backpressure in the system. The high flow rates in the small pipe would lead to pressures higher than the atmosphere right at the discharge of the pipe. Evaluation of Existing SystemsWhen evaluating an existing system discharging to the atmosphere, the key design considerations that must be checked for this PSV application:Is the installed PSV orifice adequate for the new conditions?Compare the required relief rate with the installed relief rate. If the required relief rate is less than 25% of the rated capacity of the PSV, the PSV is too large and would chatter if kept in this application. The PSV should be replaced with a smaller PSV. If the required flow rate is greater than 95% of the rated capacity of the PSV, the PSV should be replaced with a larger size PSV.Backpressure PSV limitation Using the rated flow of the PSV along with the existing discharge line size, backpressure should be calculated and checked against the allowable back pressure depending on the PSV type. The maximum allowable backpressure for the conventional PSV is 10% of set pressure, for balance bellows is 30% of the set pressure. For pilot-operated valves, the allowable backpressure is 50% of set pressure, but depending on the valve manufacturer, the backpressure could be as high as 100% of the set pressure. It is important to note that in this evaluation only built-up backpressure is considered.Outlet flange class rating: the maximum backpressure should also be checked against the pressure rating of the existing. For example, 150# flange class is limited to 257 PSIG at 100 F.Minimum outlet pipe exit velocityFor PSVs discharging to the atmosphere, a calculation for 25% of the rated capacity needs to be conducted to ensure the velocity at the discharge is sufficiently high to allow for dispersion of the flow and avoiding the formation of flammable cloud at grade level as per API (6th edition) section 5.8.2.2.Outlet pipe momentumChoke conditions at the discharge point would lead to high forces and hence momentum calculations must be performed for the outlet pipe to determine if reinforcements are needed for the outlet pipe.Do you have any questions or comments regarding this article? Click here to contact us.ReferencesAPI Standard 521 Pressure-relieving and Depressuring SystemGuidelines for Pressure Relief and Effluent Handling Systems, 2nd Edition, CCPS (Center for Chemical Process Safety)Understanding Atmospheric Dispersion of Accidental Releases, CCPS (1995a)Guidelines for Use if Vapor Cloud Dispersion and Source Emissions for Accidental Releases, CCPS (1996)Guidelines for Consequence Analysis of Chemical Releases, CCPS (1999b)","category_id":6,"keywords":"safety; psv; relief","published_date":"2021-08-30","created_at":"2021-08-30 12:19:50","updated_at":"2021-11-13 14:31:17","slug":"psvs-discharging-to-atmosphere-what-to-consider","downloads":null,"description":"This article highlights key areas to look for in the design when a pressure safety valve (PSV) is directly routed to the atmosphere.","tagtitle":"PSVs Discharging to Atmosphere: What to Consider","image":null,"category":"id":6,"name":"Relief \/ flare studies","for":"Article","image":"\/img\/uploads\/d163422a4a027e9e2c011f7dcf7213703a941c98.png","created_at":"2016-12-16 00:47:27","updated_at":"2017-04-05 09:08:05","published":true,"editable":false,"slug":"relief-flare-studies"

"id":140,"name":"The step-by-step guide: Double-Pipe Heat Exchanger design","user":"4","published":true,"content":"Heat Exchangers are an essential unit operation in the design of chemical processes, usually linked directly with energy efficiency aspects of a facility. Wildly used in a plethora of industrial applications such as oil and gas, pharmaceutical, food and drink, and HVAC. In a heat exchanger unit design, key fundamental principles like the zeroth, first, and second laws of thermodynamics are applied.Heat exchanger design revolves around understanding a basic equation:Equation 1Where,Q is the heat exchanger dutyA is the area for heat transferU is the overall heat transfer coefficientLMTD is the log mean temperature differenceThis equation relates the duty of the exchanger (Q) with the surface area available to exchange the heat between fluids at different temperatures (A), the driving force of the exchange (LMTD), and the specific fluid and material properties (U). Each of these can be manipulated in various ways depending on the heat exchanger design. A classic example of a heat exchanger is the double-pipe type shown in Figure 1.Figure 1 Double-pipe exchanger simple schematics [2]A double-pipe heat exchanger known also as a hairpin-type exchanger (due to its U-shape bend resembling a hairpin) is made up of two concentric pipes [1]. The outer pipe (shell-side) is usually a bare pipe, and the inner pipe (tube-side) is either a bare pipe or more commonly a pipe with longitudinal fins, providing additional surface area for heat exchange (A). Double-pipe exchangers may also be operated with multiple inner pipes to increase the surface area (A), at which point it would be considered a multitube hairpin heat exchanger.Double-pipe exchangers may be operated either co-currently or counter-currently, changing the heat exchange driving force (LMTD). More commonly they operate counter-currently to maximize the heat exchange.The compact design with longitudinal finned tubes creates ample surface area for heat transfer, benefiting fluids with low heat transfer coefficients (U) (like most gases) [1]. The design of double-pipe heat exchangers offers further benefits due to its construction since the U-shape bend in the hairpin design creates an allowance for thermal expansion of the fluid. The modular construction of the exchanger offers the ability to increase capacity of heat exchange by the introduction of more sections [2].Hairpin exchangers are versatile but their application to process design depends on the UA requirements of the system. GPSA provides a quick guide in section 9 about when hairpin designs should be considered. See Table 1.[1]. Table 1: GPSA Information on When to Consider Different Hairpin Designs Based on the Required UA of the System [1]UA (W\/\u2103)   Exchanger Design>79,000   Uneconomical hairpin design53,000 – 79,000   One or more 300-400mm tubes26,000 – 53,000   One or more 100-300mm tubes<26,000   Both double-pipe and multi-tubes should be evaluated.Calculations for Double-pipe ExchangerIn Process Ecology’s engineering design practice, we have sized several double-pipe heat exchangers, and below we share a step-by-step guide for the design of these units.Commonly in these types of calculations, you have a known fluid that is being cooled or heated. Using the equation below you can calculate the duty required for the heat exchanger to achieve the desired temperature change (assuming the known fluid is the hot side of the exchanger).Equation 2Where,Q is the duty of the heat exchangermhot is the mass flowrate of the hot fluidCphot is the specific heat capacity of the hot fluidThot in is the temperature of the hot fluid coming inThot out is the temperature of the hot fluid going out With the known required duty, Equation 1 can be used to determine the remaining parameters.Due to the longitudinal fins providing additional surface area, double-pipe exchangers have a more complex equation to determine the total area available for heat transfer. The following equations may be used:Equation 3Where,Ah is the area for heat transferdo is the tube's outer diameterHf is the fin heightNf is the number of fins Perry’s Chemical Engineering Handbook provides an alternative method for the calculation of surface area for heat transfer based on typical standards for double-pipe exchangers as shown in Figure 2 [2].Figure 2: Perry’s Chemical Engineering Handbook Table 11-15 Double-Pipe Hairpin Section Data [2]The overall heat transfer coefficient U may be calculated as per the equation below:Equation 4Where,Uo is the overall heat transfer coefficientho is the outer (shell) heat transfer coefficienthi is the inner (tube) heat transfer coefficientrw is the wall resistancerf is the fouling resistance  The wall and fouling resistances depend on your pipe material and fluids properties. The inner heat transfer coefficient hi can be calculated using standard equations such as Nusselt’s number. Calculations can then be made using either the Sieder-Tate correlation for laminar flow or the Petukhov and Kirillov equation for turbulent flow.The outer heat transfer coefficient ho, however, must be carefully analyzed as the longitudinal fins create different hydraulic diameters, which in turn change both Reynold’s number and Nusselt’s number. The hydraulic diameter can be calculated as follows.Equation 5Where,Dh is the hydraulic diameterAc is the net cross-sectional area in the annulusPw is the total wetted perimeter of the annulusDi is the shell-side pipe's inner diameterdo is the tube-side pipe's outer diameterδ is the width of the finHf is the height of finNf is the number of finsWith the hydraulic diameter the Reynolds number can be calculated as:As noted earlier, Nusselt’s number may be calculated with either the Sieder-Tate correlation for laminar flow or the Petukhov and Kirillov equation for turbulent flow similar to h­i.With the overall heat transfer calculated and the LMTD of the exchanger defined, the required area can be calculated using Equation 1. This can then be compared with Equation 2 or Perry’s Chemical Handbook calculations to determine the length requirements. An iterative approach to these steps leads to the size of a double-pipe heat exchanger. Sources:[1] GPSA Midstream Suppliers, Engineering Data Book, 14 ed., vol. SI Volumes 1 & 2, Tulsa, Oklahoma: GPSA, 2016. [2] D. W. G. Robert H. Perry, Perry's Chemical Engineers' Handbook, Kansas: McGraw-Hill Handbooks, 1999. [3] M. I. Stewart, Surface Production Operations - Design of Gas-Handling Systems and Facilities, Third Edition ed., vol. 2, Kidlington, Oxford: Gulf Professional Publishing, 2014.   ","category_id":3,"keywords":"Heat Exchanger Unit design Gas Oil and Gas Energy Efficiency","published_date":"2023-03-12","created_at":"2023-03-14 04:09:39","updated_at":"2023-03-23 04:10:16","slug":"the-step-by-step-guide-double-pipe-heat-exchanger-design","downloads":null,"description":"Heat Exchangers are an essential unit operation in the design of chemical processes. Know all about this unit in this article.","tagtitle":"The step-by-step guide: Double-Pipe Heat Exchanger Design","image":null 041b061a72