【 Interpretation of Important Changes in API618 Edition 6 】
Chen Chao
(Sinopec Engineering & Construction Company Limited, Beijing 100101)
[Abstract] : This paper expounds the significance of the API618 specification and its upgrade situation, makes a detailed comparison of the differences between the 5th and 6th versions of API618, and interprets it in combination with engineering practice. After 16 years of summary and improvement, the 6th edition of API618 has put forward many new design concepts and requirements, making the standard specifications more in line with engineering practice and providing better selection and design references for the majority of practitioners. The main update points include exhaust temperature, reverse Angle, pulsation analysis methods, etc. Meanwhile, some requirements in the new version have not yet been applied in actual engineering. In the future, all parties still need to consult with each other, study them in detail and gradually adapt to them.
[Key words] : API618 Reciprocating compressor; API standard
Chinese Library Classification Number: TH457 Document Code: B
Article Number: 1006-2971 (2025) 06-0037-07
API618, "Reciprocating Compressors for Service in the Petroleum, Chemical, and Natural Gas Industries"; (hereinafter referred to as API618), is a specification compiled by the American Petroleum Institute concerning lubrication systems, shaft seals, oil control systems, and auxiliary equipment for the petroleum, petrochemical, and natural gas industries.
Currently, the most widely used is the 5th edition of API618-2008 [1]. The 6th edition of API618-2024 [2] was officially published in 2024, presenting many new design concepts and requirements. Compared with the 5th edition, it has undergone significant changes. This article focuses on comparing the significant changes between the 6th edition and the 5th edition, and conducts analysis and interpretation in combination with engineering practice.
2. Interpretation of Important Changes
2.1 Scope
The scope of application of API618, Edition 6 has been newly added: This standard does not apply to diaphragm compressors.
2.2 Terms, Definitions, Abbreviations
(1) New term: certified point, appropriate It is used at the operating point where performance tolerance is allowed.
（2) Integrate the combined rod load of the 5th edition It has been changed to "cross head pin load";. The two are only name modifications, and the term definitions have not been modified. Both refer to the algebraic sum of the horizontal forces generated by the pneumatic force and the reciprocating inertial force at the cross head pin.
(3) Change the rod reversal in the 5th edition to crosshead pin reversal. The revised definition is more accurate, indicating that the force acting on the piston rod at the crosshead pin has changed from tensile stress to compressive stress or from compressive stress to tensile stress.
(4) New term: gas load, the force caused by the pressure difference of the gas on both sides of the piston.
(5) New term: inertia load, the force exerted on a reciprocating component due to its reciprocating acceleration or deceleration motion.
(6) The maximumallowable continuous combined rodload (maximumallowable continuous combined rodload) and the maximumallowable continuous gas load (maximumallowable) in the original 5th edition have been deleted continuous gas load.
(7) New term: rolled thread, a thread processing method that involves rolling the workpiece into shape with a forming tool. The processing method of rolling threads features high surface finish, high precision and high strength. In engineering, it is generally required that the threads of the piston rod be rolled to enhance the connection reliability between the piston rod and the crosshead.
(1) The 5th edition stipulates that the design and construction life of equipment and auxiliary equipment shall be at least 20 years, and they shall operate continuously for at least 3 years. The 6th edition has removed these regulations and instead suggests that only equipment that has passed on-site certification is acceptable (on-site certification is defined by the buyer), and at the same time recommends that the seller provide documents to prove that the equipment supplied has passed on-site certification.
(2) The 6th edition adds a requirement that if there is no on-site certified equipment, the seller may submit a statement to explain that the provided equipment can be regarded as on-site certified. For example, it can be interpreted as: Although the entire machine has not been on-site certified, all the components that make up the entire machine have been on-site certified.
(3) The 6th edition adds a requirement that the seller should provide the lifespan of all components designed to have a limited lifespan in the quotation. In recent years, the revised API standards have all added the requirement of providing the lifespan of components with a limited lifespan, replacing the previous regulation of continuous operation for three years. However, in recent years' projects, the requirement of continuous operation for 3 to 4 years is still mostly followed.
(4) In Table 2 of the 6th edition - Tripping Speed Table of the Drive Machine, the tripping speed of the constant-speed motor, which was originally 100% of the maximum continuous speed, was modified to 102% of the maximum continuous speed. The main reason for this modification is that it takes into account the 2% frequency fluctuation of the power grid. Since the motor is connected to the power grid and is subject to the frequency limit of the power grid, the motor generally does not exceed the speed limit, but the rotational speed will change with the frequency. However, the piston rod of the reciprocating compressor is designed
The design of moving parts such as the valve, air valve, and crankshaft all need to take into account the variation of rotational speed. Therefore, increasing the tripping speed of the constant-speed motor is equivalent to raising the upper design limit of the unit.
(5) The 5th edition stipulates that after the equipment is installed, the performance of the unit should be the responsibility of both the buyer and the seller. There are many factors that can have adverse effects on on-site performance, including pipeline loads, centering during operation, support structures, handling during transportation, and on-site installation, etc. To minimize the influence of these factors, the seller should review the buyer's pipeline drawings and foundation drawings in accordance with the plan. The seller's review should be limited to the drawings of the anchor bolts or the basic drawings based on the seller's data. The 6th edition deleted these two requirements. In actual engineering projects, some of the engineering drawings issued by the design party based on the manufacturer's information will still be submitted to the manufacturer for review. For instance, the single-line diagram of the pipeline with a preliminary support scheme is used for the pulsation analysis of the manufacturer's computing unit.
(6) The 6th edition adds a requirement that the external threads used for hydraulic stretching must have protective covers on the exposed threads. In engineering, the connection between the piston rod and the crosshead, as well as the connection between the piston rod and the piston, mostly adopts the hydraulic stretching connection method. This is because hydraulic stretching requires intact threads, and to protect the exposed threads, a protective cover is needed.
(7) Regarding the allowable emission temperature of gases, the 6th edition has changed the maximum expected emission temperature under all specified operating conditions in the 5th edition from not exceeding 150 ℃ to not exceeding 135 ℃. The expected emission temperature under hydrogen-rich medium conditions with a molecular weight of ≤12 in the 5th edition was changed from 135 ℃ to 120 ℃. The reduction of the expected emission temperature limit may lead to the need for a four-stage compression process that could previously be completed in three stages, adding an inter-stage cooling separation device, which increases the investment in the unit and the design difficulty.
In the 5th edition, it was suggested that the shutdown set value of the compressor exhaust temperature should not exceed 180 ℃, but in the 6th edition, it was changed to not exceed 175 ℃.
(9) The provisions in the 5th edition regarding the reverse force on the crosshead pin are as follows: When the crosshead pin bushing is verified and should be in a reliable form (such as a bushing with grooves), and the reverse force Angle on the crosshead pin is at least 15 °, and the reverse force is at least 3% or more of the maximum force in the other direction. For some simple crosshead pin bushings (such as those without grooves), the reverse Angle should be at least 45 °, and the reverse force should be 20% or more of the maximum force in the other direction. The manufacturer should provide the actual value to the buyer at the quotation stage. In the 6th edition, the reverse Angle and the maximum reverse force value are no longer specified according to the form of the crosshead pin bushing. It is uniformly stipulated that the reverse Angle of the crosshead pin force should be at least 15 °, and the reverse force should be at least 3% or more of the maximum force value in the other direction. The modification of this clause is equivalent to lowering the requirement for the reverse Angle. In actual engineering design, the failure of the crosshead pin bushing due to insufficient reverse Angle and lack of lubrication occurs occasionally. Therefore, in the design process, the requirement that the reverse Angle should not be less than 45 ° will still be followed.
(10) The 5th edition stipulates that the compressor should be capable of withbearing 110% of the maximum allowable crosshead pin load (combined piston rod load) or gas load within a short period of time. The overload time should be limited to within 30 seconds, and the frequency of simultaneous occurrence should not exceed twice within 24 hours. This requirement was deleted in the 6th edition. In previous engineering designs, for the sake of caution, the maximum continuous allowable load of the crosshead pin during the normal operation of the compressor was required to be 85%. Since the 6th edition removed the requirement for the short-term overload capacity of the crosshead pin, in future engineering designs, the margin of the crosshead pin load during normal operation may be further increased.
(11) The 5th edition suggests that the seller should conduct a dynamic study on the valve to optimize the movement, impact and efficiency of the valve components. This study should take into account the density and force conditions of all operating gases, and the assumptions used should be stated in the research report. The 6th edition changed this requirement to a mandatory one. In engineering practice, the commonly used major brand gas valve manufacturers will all conduct dynamic research on valves.
(12) The 5th edition stipulates that the manufacturer's fastening procedure for connecting the piston rod to the crosshead should ensure that the preload of the fastening bolts is equal to 1.5 times the maximum allowable continuous rod load. The 6th edition was changed to "Manufacturing";
The fastening procedure for connecting the piston rod to the crosshead should ensure that the preload of the fastening bolts is equal to 1.5 times the maximum allowable continuous rod load minus the inertial force of the crosshead. The reason for subtracting the inertial force is that the piston rod and the crosshead move in the same direction, and their accelerations are consistent, so the inertial force does not need to be considered. If inertial force is taken into account, it may cause excessive preload and increase the risk of fatigue failure at the connection.
(13) The 6th edition newly stipulates that the support ring shall not exceed half of the chamfer width of the single-hole valve port or cylinder liner. When the cylinder configuration causes the support ring to exceed half of the valve port width, the valve port design should adopt a multi-hole design to provide sufficient support for the support ring. The purpose of this new requirement is to provide more support for the support ring, increase the contact area between the support ring and the cylinder liner, and prevent the skewing of the support ring.
(14) The 6th edition has uniformly revised the material selection standard for wet hydrogen sulfide environments from NACE MR0175 in the 5th edition to NACEMR0103. NACE MR0175 is applicable to exploration and production operations in the upstream of oil and gas, including downhole and surface facilities in oil and gas fields, as well as plants that desulfurize natural gas in an environment containing H2 S, providing advice and specified requirements for the selection and identification of metals and alloys used in oil and gas production equipment. NACE MR0103 is specifically designed for downstream refinery environments and is suitable for refining acidic petroleum and similar processing conditions that contain liquid or gaseous H2S.
(15) The 6th edition adds a requirement that the spun threads should be spun after heat treatment.
(16) The 5th edition stipulates that if the main bearing uses rolling bearings, the L10 life of the bearing shall not be less than 25,000 hours under the maximum axial and radial loads and rated speed, while the 6th edition has increased the life to 32,000 hours. The L10 life of a bearing refers to the fatigue life that 90% of a group of bearings of the same model can achieve or exceed under standard test conditions.
(17) The 5th edition stipulates that when the power of the compressor exceeds 150 kW, the crosshead should be made of steel; when it is less than 150 kW, the crosshead can be made of ductile iron. The 6th edition has been revised as follows: Regardless of the power of the compressor, the crosshead can be made of steel or ductile iron.
(18) The 6th edition adds a requirement that if the crosshead slipper cannot be replaced or adjusted, the buyer's consent should be obtained. The adjustable thickness crosshead slipper can effectively adjust the alignment between the crosshead and the piston rod. During operation, due to wear or other factors, the alignment deviation between the crosshead slipper and the slide track may occur. By adjusting the thickness of the slipper, the correct position of the crosshead in the slide track can be ensured, reducing wear and vibration.
(19) The 5th edition suggests that the ratio of the throat area of the crankcase pressure relief device to the free volume of the crankcase should not be less than 70 mm ² / dm ³.
In the 6th edition, this area requirement was removed. Instead, if specified, the crankcase should be equipped with one or more relief devices to prevent rapid crankcase pressure rise. The selection of the equipment should take into account the crankcase environment, oil mist or hydrocarbon gases, as well as the possible pressure rise of the crankcase.
In the 5th edition, it is suggested that the cover plate of the middle body can be a solid metal cover plate, a metal mesh safety cover plate, or a metal louver cover plate, all of which are acceptable. In the 6th edition, it is detailed that metal mesh safety covers and metal louver covers are only applicable to working conditions with non-flammable and harmless gases.
(21) The 5th edition stipulates that the metal cover plate of the middle body should be capable of withstanding a cavity pressure of 2 bar or higher. The 6th edition has raised the pressure-bearing capacity of the middle body cavity to 3 bar and stipulates that if the design pressure of the middle body is less than the maximum operating discharge pressure, the seller should provide a solution. In engineering, the venting of the middle body is usually carried out to the highest point of the factory building, and it is generally unlikely that the discharge pressure will exceed the designed pressure
";Sure. However, if the design pressure of the middle body is less than the maximum discharge operating pressure, a safety valve should be installed on the middle body.
(22) The 6th edition newly stipulates that for the double-chamber middle body, the inner middle body (close to the crankshaft side middle body) should be equipped with buffer gas, and the pressure of the buffer gas should be at least 0.5 bar higher than that of the outer cavity to prevent toxic and harmful gases from the outer cavity from entering the inner cavity.
(23) The 6th edition adds a new requirement that the oil leakage rate of the oil scraper ring should not exceed 5 drops per minute.
(24) The 5th edition suggests that when it is necessary to cool the cylinder pressure packing, the seller should determine the minimum cooling flow rate, pressure, pressure drop, temperature, filtration and corrosion protection, etc. The pressure drop of the coolant in the pressure packing should not exceed 1.7 bar, and the temperature of the coolant entering the pressure packing should not exceed 35 ℃. The 6th edition changed this suggestion to a mandatory requirement.
(25) The 5th edition suggests that, if required, for all acidic and harmful gases, the cooling system of the packing should be separated from that of the cylinder jacket. This suggestion was removed in the 6th edition. In actual engineering, regardless of the nature of the compressed gas, the cooling of the packing and the cylinder both adopt the same cooling system.
(26) The 6th edition has newly added requirements for the buffer gassystem The pressure of the air source entering the pressure packing should be at least 1 bar higher than the exhaust port pressure of port A (near the exhaust port of the cylinder body) or port G (the discharge port of the pressure packing) in Figure 1. The pressure of ports A and G should be taken as the larger value. The reason for such a requirement is to ensure that the buffer gas can smoothly enter the pressure packing without causing gas leakage in the cylinder.
(27) The 6th edition has detailed the definition of low-temperature operating conditions. It is recommended that the buyer determine the minimum design metal temperature, which should be lower than the minimum ambient temperature and the minimum intake air temperature. The possible throttling cooling operating conditions also need to be considered.
4. Design of ancillary equipment
4.1 Driver
According to the 6th edition, during the initial design, for non-motor-driven compressors, whether at full load or partial load during step regulation, the unevenness of the peak-to-peak speed of the rotating system should not exceed 1.5 percent of the rated speed. However, in the 5th edition, there is no statement that it is only applicable to non-motor-driven compressors. It is required that regardless of the driving mode, the unevenness of the peak-to-peak speed of the rotating system should not exceed 1.5 percent of the rated speed. In practice, to reduce the speed non-uniformity of a rotating system, it is necessary to prioritize increasing the moment of inertia of the rotating system. Increasing the moment of inertia of the drive motor is the preferred solution to enhance the moment of inertia of the entire rotating system, followed by increasing the moment of inertia of the flywheel. If the driving mode of the compressor is a steam turbine or an internal combustion engine, it is basically impossible to increase the moment of inertia on the steam turbine or internal combustion engine. Only the moment of inertia of the flywheel can be considered to be increased. In engineering, it is generally required that the unevenness of the peak-to-peak speed of motor-driven compressors be less than 1% of the rated speed.
(2) The 5th edition stipulates that the nameplate power of the motor (excluding the service factor) should be 105% of the power under the safety valve tripping condition of the compressor (including losses), and without considering the service factor, it should not be less than 110% of the power under all specified operating conditions (including losses). The 6th edition removed the disconsideration of the service factor. Motors with a service factor greater than 1 can be overloaded for a short period of time. The amendment of this article is equivalent to lowering the power requirement on the motor nameplate, allowing the motor to be overloaded within the service factor range.
(3) It is recommended in the 5th edition that, if required, the starting torque of the motor should meet the requirements of the driven equipment. At 80% of the rated voltage (or the value specified by the buyer), the motor should accelerate from zero speed to full speed within 15 seconds (or within the value agreed upon by both the buyer and the seller). This suggestion was deleted in the 6th edition.
(4) According to the 5th edition, for compressors driven by asynchronous motors, the pulsation value of the motor current should not exceed 40% of the current at full load, and the calculation method should use IEC 60034 or NEMA MG1.
This requirement was removed in the 6th edition. In engineering, excessive current pulsation can lead to thermal stress and harmonic losses, reducing motor efficiency while increasing heat generation and accelerating insulation aging.
(5) According to the 5th edition, when the synchronous motor is connected to the electrical bus of the existing synchronous motor, the buyer should conduct an electrical system analysis and provide it to the compressor manufacturer. This regulation was deleted in the 6th edition. The reason why the electrical system needs to be analyzed when a new synchronous motor is added is mainly to verify whether the parameters of the new synchronous motor are compatible with those of the busbar system and the existing motors, to prevent fault currents and optimize power quality and system performance. It is rare to encounter the situation where new synchronous motors are added in engineering.
(6) The 5th edition stipulates that for compressors driven by synchronous motors, the torsional stiffness and moment of inertia of all rotating parts should ensure that there is at least a 20% difference between any inherent excitation frequency of the compressor and the torsional oscillation frequency of the motor rotor relative to the rotating magnetic field. This requirement was removed in the 6th edition.
(7) According to the 5th edition, motors without thrust bearings should be equipped with permanent position indicators of the rotor relative to the magnetic center line. This requirement was removed in the 6th edition.
(8) The 5th edition suggests that the motor bearings should have insulation measures to prevent leakage current, the bearing housing should be equipped with adjusting pads, and the bearing box should be fitted with oil seals to prevent the entry of dirt and moisture and to prevent oil leakage from both inside and outside. This suggestion was removed in the 6th edition.
4.2 Couplings and coupling guards
(1) The 6th edition adds a requirement that the extended section of the coupling should be long enough to remove the flywheel or the coupling hub without moving the motor. In engineering practice, whether it is a single-bearing or double-bearing motor, rigid couplings without extended sections are still mostly used for couplings. If the flywheel needs to be removed, it is generally achieved through a special structure or by moving the motor backward.
(2) It is recommended in the 5th edition that the seller should provide a shield for each coupling and all exposed moving parts. The shield should be removable without affecting the coupling. The 6th edition changed this suggestion to a mandatory requirement.
4.3 Belt Drive
(1) According to the 5th edition, if the power of the belt-driven reciprocating compressor reaches 225 kW, the consent of the buyer is required. In the 6th edition, it will only be required that reciprocating compressors with a power of less than 150 kW are allowed to use belt drive.
4.4 Lubrication
(1) The 6th edition stipulates that if the power of the compressor is less than 150 kW and rolling bearings are used, The pressurized lubrication system shall comply with the configuration requirements of Class I-P0-R0-H1-BP0-C1, F2-C0-PV0-TV1-BB0 in API614 Table 1. If splash lubrication is used, it shall be permitted by the buyer. After the upgrade of API614 in 2022, general-purpose oil systems and special-purpose oil systems are no longer distinguished by chapters. Therefore, the provisions in the 5th edition that pressurized lubrication systems should follow Part 1 and Part 3 of API614 have been removed Requirements for some (general-purpose oil systems).
(2) Regarding the instrument configuration of gas stations, the 6th edition requires The oil system should be equipped with at least one liquid level indicator (on the crankcase or oil tank), one low oil pressure alarm and pressure transmitter for starting the auxiliary oil pump, one low oil level transmitter for alarm, one filter differential pressure alarm transmitter and one low oil pressure interlock transmitter. The requirement in the 5th edition that two temperature transmitters must be included has been removed.
(3) The 6th edition adds a new requirement that the return oil line of the crankcase should be located above the maximum operating oil level. The main reason for such a regulation is to make the oil return process smoother.
(4) The 5th edition suggests that for all compressors with a power of over 150 kW, the oil system should be equipped with an independently driven, full-volume and full-pressure auxiliary oil pump. The oil pump can start automatically when the oil pressure is low and perform post-cooling after shutdown. The 6th edition has changed this requirement to a mandatory one and added that the auxiliary oil pump must comply with API614 standards.
(5) The 5th Edition contains many requirements regarding oil system coolers, oil system filters, oil system heaters, and oil system temperature control valves. The 6th edition uniformly removed these requirements and only stipulated that the corresponding components of the oil system should comply with API614 standards.
(6) According to the 5th edition, the output of the lubricating oil pump for the cylinder and packing should be adjustable during the operation of the compressor, with the adjustment range being + 100% and -25% of the designed flow rate. The 6th edition changed + 100% to + 25%, reducing the range of traffic adjustment.
(7) It is recommended in the 5th edition that the oil tank heater of the oil injector should be equipped with a thermostat. The heating density is limited to 2 × 3 W/cm ². The size of the heater and the temperature control instrument shall be agreed upon by both the buyer and the seller. When an internal heater is used, the oil tank level of the oil injector can ensure that the heater is fully submerged in the oil even at the lowest level. The 6th edition has changed this requirement to a mandatory one and recommends that the oil tank of the oil injector be equipped with a low liquid level alarm instrument. When the liquid level drops to a certain level, the heater will be interlocked and stopped.
(8) The 5th edition suggests that each oil injection system should be equipped with an oil injector failure alarm. The 6th edition changed this clause to a mandatory requirement. In engineering practice, oil injectors are all equipped with a pressure transmitter to monitor the injection
The oil outlet pressure of the oil system is used to determine whether the oil injection system has failed.
(9) The 6th edition adds a suggestion that, as specified, the oil tank of the oil injector should be equipped with an automatic floating oil filling device for replenishing oil. In engineering, multiple reciprocating compressors can be equipped with a high-level oil tank, and an automatic floating oil filling device can be used to automatically replenish oil to the oil tank of the oil injector, thereby reducing the labor intensity.
4.5 Cylinder and packing cooling system
(1) The 6th edition adds a requirement that the water tank should be equipped with: a heater, an air vent, a liquid level gauge, a purging port, a water filling port, a return water port, a pump suction port, a inspection interface, and a liquid discharge port. The water tank heater should be electric, hot water or steam, and the heat loss of the water tank surface, cylinder, pipelines and pipe fittings at the lowest ambient temperature should be taken into consideration.
(2) The 6th edition adds a suggestion that the electric immersion heater should be able to be removed for maintenance when the cooling water system is in operation. If specified, a pipeline heater can be used. If this provision is implemented, the electric heater shall The indirect type should be adopted.
（3) The 6th edition adds a new requirement that the water system cooler should be located in the shell Both the side and the tube side have exhaust and liquid discharge.
4.6 Installation
In the 5th edition, it is stipulated that if the equipment mass exceeds 450 kg, three directions of top screws should be provided. The 6th edition changed this value to 225 kg.
(2) The 6th edition adds a requirement that the seller should provide the projection drawing of the anchor bolts. The diameter of the bolt holes, the arrangement of the bolts, and the tightening force of each bolt should all be reflected on the layout drawing.
(3) The 6th edition has added requirements for the lifting lugs of the integral base (baseplate). The maximum force on the lifting lugs during lifting should not exceed one third of the material's yield strength. The weld seam of the welded lifting lugs should be in a continuous and fully penetrated form and comply with the AWS D1.1 standard. The weld seams need to undergo 100% non-destructive testing. When all the equipment is lifted together with the overall base, the base must not undergo permanent deformation or cause damage to the equipment.
(4) The 6th edition adds a requirement that the cavity of the overall base should be filled with grouting material and sealed.
4.7 Control and Instrumentation
(1) In the 5th edition, interlocking and alarm of related instruments were mandatory provisions, but in the 6th edition, they were changed to recommended configurations. Moreover, in the alarm interlock table of the 5th edition, high exhaust temperature was an interlock item, while in the 6th edition, high exhaust temperature was changed to an alarm item, and an alarm and interlock for crosshead vibration were newly added. This is the first time that API618 has put forward the requirement of monitoring the vibration of the crosshead. Since the crosshead is a component that moves in a straight line, it is relatively difficult to directly monitor its vibration. The existing vibration measurement method is still to measure the vibration on the external cylinder of the crosshead liner. In order to monitor the vibration of the crosshead more comprehensively, Generally, two vibration probes need to be set at the left and right limit positions of the crosshead's operation.
(2) In the 5th edition, it is suggested that while providing a probe for the piston rod to sink, a non-contact probe for the horizontal direction of the piston rod should also be provided to monitor the horizontal displacement of the piston rod. And it is required that the piston rod probe should be close to the packing. Amplifiers and other equipment should not be installed on the compressor. If the piston rod is coated, the probe of the piston rod should be calibrated. All the above suggestions have been removed in the 6th edition.
(3) In the 5th edition, it is recommended to install temperature monitoring for crosshead pin bearings. This recommendation was removed in the 6th edition. Because the crosshead pin is a moving part, it is very difficult to monitor its temperature. Wireless temperature measurement methods can be used.
4.8 Pipelines
(1) It is recommended in the 5th edition that when the filter screen for the suction pipe of the process gas is provided by the seller, the design, position and orientation of the filter screen should be negotiated by both parties. The 6th edition has changed this clause to a mandatory requirement and stipulates that the temporary filters for the process pipeline shall be provided by the seller.
(2) The 6th edition adds a new suggestion that if required, the seller should provide a final-stage outlet check valve. The form, size and position of the check valve should be determined by both the buyer and the seller. It is not recommended to use swing check valves. This is because the exhaust of reciprocating compressors is intermittent. If a swing check valve is used, it may cause impact damage to the valve disc. In engineering, air valve check valves are commonly used.
4.9 Inter-stage cooler, aftercooler and separator
(1) The 6th edition adds a requirement that the design of water-cooled shell and tube coolers should follow TEMA class C or R. If TEMAclass R is implemented, the API660 standard should also be followed.
(2) The 6th edition adds a suggestion that the seller should provide prefabricated pipes between the compressor and the inter-stage cooler and the aftercooler. In actual engineering, as the layout of the inter-stage cooler or aftercooler is designed by the design party, the pipelines between the compressor and the inter-stage cooler or aftercooler are generally provided by the buyer, but the seller conducts the initial planning.
(3) The 6th edition adds a requirement that if the gas processed by the cylinder is in a saturated state or has become saturated, or if the temperature of the gas in the cylinder is within 10 ℃ of the expected dew point temperature, an independent separator should be set up (the separator should not be integrated with the pulsation suppression device or cooler). If the gas temperature exceeds the expected dew point temperature by more than 10 ℃, an integrated gas-liquid separator can be used. In actual engineering, most multi-stage compressors, regardless of whether the temperature after cooling is within 10 ℃ of the gas dew point, will be equipped with an independent gas-liquid separator. On the one hand, it separates the possible liquid that may precipitate; on the other hand, it can collect the lubricating oil of the oil-injected compressor. Of course, for some oil-free compressors where liquefaction is impossible, a gas-liquid separator may not be set up.
4.1 Pulsation analysis
(1) In the 6th edition, a new note has been added. In variable operating conditions (such as the replacement of compressed gas, changes in operating conditions, or variations in loading steps), if it is unrealistic to analyze the pulsation of all operating condition combinations, the corresponding combinations can be reduced based on experience. In actual engineering, if there are multiple compressors and each is equipped with a gas volume regulation system, the number of all operating condition combinations is extremely large. It is impossible to conduct pulsation analysis one by one. Therefore, it is necessary to appropriately reduce the pulsation analysis operating conditions based on experience.
(2) The 6th edition has removed Appendix O of the original 5th edition: Preliminary Selection Procedure for Low-Pass Filters.
In the 5th edition, there are three methods for pulsation analysis: Method 1 (Empirical formula method), Method 2 (acoustic simulation + pipeline constraint analysis method), and Method 3 (acoustic simulation + pipeline constraint analysis + mechanical analysis). In the 6th edition, Method 1 was deleted, while only Methods 2 and 3 were retained.
(4) The 5th edition requires that for Method 2 or Method 3, The maximum allowable pressure at the flange of the process pipeline side connection pipe of the pulsation suppression device during the preliminary acoustic simulation analysis of the pipeline system model
The pulsation value is specified as follows: When a single pulsation suppression device is used, it is the formula 80% of 8) When there are two or more pulsation suppression devices When connected to a shared pipeline, it is 70% of Formula (8). However, the 6th edition no longer distinguishes between a single pulsation suppression device or multiple pulsation suppression devices. It all requires that the pressure pulsation value at the flange of the pipe opening on the pipeline side of the pulsation suppression device be 70% of Formula (8).
(5) The 6th edition adds a requirement that the compressor seller shall provide the allowable vibration limits for compressor components, such as cylinders, middle bodies and crankcases. If the compressor manufacturer does not provide it, you can refer to the guidelines on unit vibration issued by the European Reciprocating Compressor Association. None of the historical versions of API618 have requirements for the allowable limits of vibration. This is the first time such requirements have been proposed. This vibration limit has been synchronized to GB/T 41850.8-2022 "Mechanical vibration - measurement and evaluation of machine vibration - Part 8: Reciprocating compressor systems".
(6) When calculating the pressure pulsation limit at the connection between the pressure pulsation suppression device and the process pipeline, the 5th Edition stipulates that if the average absolute pressure in the pipeline is less than 3.5 bar, the pressure pulsation limit should be considered based on the average absolute pressure of 3.5 bar. According to the 6th edition, if the average absolute pressure in the pipeline is less than 3.5 bar, the pressure pulsation limit should be considered by multiplying the average absolute pressure of 3.5 bar by a coefficient. For specific formulas, please refer to Formula (9) in API618-2024.
(7) When calculating the pressure drop of a pulsation control device (such as a orifine plate), the 5th Edition stipulates that the pressure drop of the pulsation control device should not exceed 0.25 times the average absolute pressure of the pipeline or the calculated value in Formula (13), and the greater of the two should be taken. The calculation method has been refined in the 6th edition. If the pressure ratio is ≤ 1.4, the maximum pressure drop of the pulsation control device of the suction and discharge system under steady state is 0.5% of the average pipeline absolute pressure. If the pressure ratio is greater than 1.4, it should be calculated through formula (13).
5. Inspection and testing
(1) In the magnetic particle detection section of the 6th edition, a maximum allowable residual magnetic field requirement has been added.
(2) In the 6th edition, it is newly suggested that if required, the medium body should also undergo a hydrostatic test at 1.5 times the maximum allowable working pressure.
(3) According to the 5th edition, the pressure for the hydrostatic test is 1.5 times the maximum allowable working pressure. At the same time, the temperature compensation coefficient should be taken into account. The temperature compensation coefficient is the ratio of the allowable stress value of the material at the hydrostatic test temperature to the allowable stress value of the material at the rated operating temperature. In the 6th edition, the requirement for the temperature coefficient was removed. This is because the operating temperature of reciprocating compressors is often below 150 ℃, and the temperature compensation coefficient below this temperature can be ignored.
(4) The 6th edition adds a requirement that if the temperature measurement elements of the main bearing and the packing are both supplied by the manufacturer, the corresponding temperature values should be recorded during the mechanical operation process, and the oil temperature, oil pressure, rotational speed and piston rod temperature should also be recorded during the mechanical operation test.
(5) The 6th edition adds a suggestion that if required, the vibration of the cylinder and crankcase should be recorded during the operation of the machinery.
(6) In the optional tests of the 6th edition, it is newly suggested that if required, an unloader for the cylinder should be installed and functional tests should be conducted.
Appendix D of the 5th edition: Restoration of gray iron castings or ductile iron has been deleted.
(2) Appendix O of the 5th Edition: Guidelines for the Design of Low-pass Filter Sizes has been deleted.
(3) Appendix P of the 5th Edition: "shaking force Guide for Pipelines and Pulsation Suppression Devices"; has been deleted.
(4) Appendix Q of the 5th edition: Compressor Components conforming to NACE MR0175 standard has been deleted.
(5) The fifth edition Appendix F: Seller's Drawings and Data Requirements has been refined, and the name of the appendix has been changed to Contract Documents and Engineering Design Data.
(6) Due to the changes in the pulsation analysis method, the workflow diagram of the Design method in Appendix M of the 5th edition has been partially refined.
(7) Appendix I of the 5th edition has been refined, covering the exhaust and liquid discharge of the middle body and the buffer system for reducing process gas leakage. The back pressure of the middle body exhaust has been classified into variable back pressure and constant back pressure, and the diagrams of the middle body exhaust and liquid discharge and the buffer system have been refined.
The 5th and 6th editions of API618 were released 16 years apart. After a long period of experience accumulation, the 6th edition has been revised and supplemented in aspects such as exhaust temperature, reverse Angle, materials, current pulsation, and pulsation analysis methods, and some provisions have been deleted. As the 6th edition has only been released for a little over half a year, in practical engineering applications in the future, it is necessary to gradually adapt to the relevant regulations of the new version to better guide engineering practice.
References
[1] Reciprocating Compressors for Petroleum, Chemical, and GasIndustry Services: API 618-2024 [S]. Washington D.C. : APIPublishing Services, 2024.
[2] Reciprocating Compressors for Petroleum, Chemical, and GasIndustry Services: API 618-2008 [S]. Washington D. C. APIPublishing Services, 2008.
Author's Profile: Chen Chao, male, from Jinan, Shandong Province, holds a master's degree and is a senior engineer. He graduated in 2014
Graduated from China University of Petroleum (East China) with a major in Chemical Process Machinery, currently engaged in the management of pumps and machinery in the refining and chemical industry.
E-mail: chenchao. sei@sinopec. com.
