Pressure vessel basics for all engineers

Transcription

1 Pressure vessel basics for all engineers by Sharjeel Aslam Faiz

2 1. Contents 2. Preliminary Design Loads Detailed design Information in-flow to a fabrication drawing Information out-flow from a fabrication drawing Materials... 5 Carbon steel... 5 Stainless steel Corrosion Allowance Parts of a pressure vessel Shell... 6 Cylinder... 6 Sphere... 7 Conical Heads... 7 Torispherical... 7 Ellipsoidal... 7 Hemispherical Nozzles Flanges... 8 Flange types... 8 Flange rating Supports Gaskets Fabrication Joining Inspection Final testing Sharjeel Aslam Faiz Page 2 of 11

3 2. Preliminary Design A pressure vessel is a closed container which holds the fluid under pressure. Some of the common pressure vessels are reactors, distillation columns, separators, knock out drums, heat exchangers etc. A process engineer handles the preliminary design and sizing. The process data sheet may contain a brief sketch with major dimensions. It may specify generic material of construction like carbon steel or stainless steel. It will indicate the design conditions like maximum, minimum temperature. It will specify the maximum internal or external operating pressure at which the vessel operates. A margin is added over it to get the design pressure. The mechanical department has two types of engineers, rotary and static. The static equipment engineers work on the pressure vessel design. They use the information present on the process data sheet and make a mechanical datasheet. The mechanical data sheet contains additional information like codes and standards, exact material of construction, loading, inspection criteria etc. There are two options to proceed further. The first one is that the engineering company makes the detailed fabrication drawing and send it to the pressure vessel fabricator. The second option is to send the mechanical data sheet to the fabricator and let him do the detailing. Process data sheet Mechanical Data sheet Calculations Fabrication drawings bill of material Figure 1 Workflow 3. Loads The mechanical engineer calculates the thickness of pressure parts by applying the design conditions. Typical loads are internal pressure, external pressure, seismic loads, wind etc. Some loads could occur at different operating conditions. 4. Detailed design The designer checks the vessel as per various cases and uses the most stringent case. A factor of safety is used to reduce the stress to an allowable value. This stress is known as the allowable stress. The most important part is to get the correct thickness that will resist the pressure. The codes will specify rules for design of pressure parts while it is the designer who will design the non-pressure parts using good engineering practice and proven rules of mechanical design Sharjeel Aslam Faiz Page 3 of 11

4 The thickness calculated is the minimum thickness. A corrosion allowance is added to it. For example, the minimum thickness is 8.3 mm, corrosion allowance is 1.5 mm. Hence, the thickness chosen will be 10 mm since it will be commercially available. The thickness of a vessel increases as the design pressure increase. The thickness will also depend on the material. A stronger material will reduce the thickness. A mechanical engineer makes detailed calculations of all parts. The engineer shows the results on a fabrication drawing. The fabrication drawing shows the detailed information like front view, top view, orientation of nozzles. It also shows welding details, nozzle schedule, vessel weight and bill of material. Useful information is provided in the notes section. The strength of steel reduces when it is exposed to fire. A vessel which is situated in a hazardous area needs to be protected by using fireproofing. This is a type of passive protection. The process consists of applying a certain thickness of material so that it can resist the fire over a period of time. The fire needs to be controlled within this period. Monitors and spray systems provide active fire protection. Generally fireproofing is done on the supports and not on the vessel. 5. Information in-flow to a fabrication drawing Piping The orientation shows the top view of the pressure vessel along with the nozzles. At the initial stages, this information is kept under HOLD. It is cleared when the piping department routes the piping up to the vessel. Instrumentation Instruments are proprietary items which undergo continuous research and development. The instrument vendor drawings provide the exact type of end connection. The mechanical engineer selects the mating instrument flange as per this detail. Mechanical Rotary A pressure vessel could have a mixer mounted on it. In such a case, the static equipment engineer needs to ensure that it will mate with the pressure vessel flange. Note: Recommendations of proprietary vendors like instruments, mixers, agitators need to be taken in consideration Sharjeel Aslam Faiz Page 4 of 11

5 6. Information out-flow from a fabrication drawing Fabrication drawing Civil foundation drawing Equipment layout The vessel weights serve as an input to the civil department for making the foundation drawings. Also, if the vessel requires fireproofing, then the mechanical engineer needs to inform the civil department. The civil department will make a sketch or a specification indicating the fireproofing details. The vessel dimensions serve as an input to make the equipment layout. 7. Materials Carbon steel The most common material is carbon steel. It is the workhorse of the construction industry since it is cheap. It cannot be used in corrosive applications. It is ductile and can be easily welded. It can be used up to 400 deg C. Stainless steel Stainless Steel is used to combat corrosion. SS 304 is the most common stainless steel with 18% Chromium and 8% nickel. Chromium-makes steels stainless beyond 10.5% and increases corrosion resistance. Nickel-makes SS tough and it is good for low temperature applications. Molybdenum gives pitting resistance and strength at high temperature. SS 304 reduce carbon to get 304L add Molebdenum to get 316 Figure 2 Simplified derivation considering SS 304 as base steel reduce carbon to get 316L Sharjeel Aslam Faiz Page 5 of 11

6 Stainless steel like SS 304 can be used at low temperature. They have excellent notch toughness, a property useful in low-temperature applications. Stainless steels are excellent for hygienic conditions. They resist corrosion by forming a thin layer of chromium oxide. Note: All properties of the material are mentioned in the mill test certificate (MTC). MTC shows the chemical composition of major elements, physical properties like tensile strength, yield strength and so on. 8. Corrosion Allowance All vessels suffer from corrosion in one form or the other. A certain amount of corrosion allowance may be added to the minimum thickness depending on the material used, the process conditions etc. The designer may select a steel that corrodes little or does not corrode like stainless steel. If stainless steel is used, corrosion allowance may not be needed. But economics may require the use of carbon steels. Carbon steels are more prone to corrosion hence a corrosion allowance of 1.5 to 4 mm is considered usually. This is done for the inside of shells, heads, nozzles etc. For internal parts, the corrosion allowance is added on both sides. Corrosion can take place on the outside from the atmosphere. This can be reduced by painting. 9. Parts of a pressure vessel Pressure parts like shell, head, nozzles, flanges. Non-pressure parts like supports, lifting lugs etc. 10. Shell A shell is the primary component of a pressure vessel. It can be of the following shape. Cylinder Sphere Conical Cylinder It is the most common form because it can be easily manufactured. The fabricator makes cylindrical shells by rolling plates. If the size is small and standard, then the fabricator can use pipes. Steel mill will make the pipes under controlled conditions. Hence, the quality is better. Various diameters and lengths can be obtained by rolling and joining the plates Sharjeel Aslam Faiz Page 6 of 11

7 Figure 3 Types of shell cylinder sphere cone Sphere The sphere is the natural shape an object takes when pressurized. The thickness of a sphere will be generally half than that of the cylinder. A sphere made with exotic materials will be cheaper. A sphere is difficult to manufacture compared to a cylindrical shell. Conical A conical section joins shells of different diameter. 11. Heads Heads are closures attached to the shell. The common types of heads are explained below. Torispherical These heads are suitable for low pressure application. These heads take the least space and have the lowest volume. hemispherical ellipsoidal torispherical Figure 4 Types of heads Ellipsoidal It is also known as 2:1 semi-ellipsoidal head. 2:1 denotes the ratio of major to the minor axis. The depth of this head is D/4, where D is the diameter. Hemispherical For a given pressure, hemispherical heads will have lesser thickness than that of torispherical or ellipsoidal heads. The depth of head is D/ Sharjeel Aslam Faiz Page 7 of 11

8 Notes: 1. If the diameter of the vessel is less, pipe caps can also be used as closures. 2. Flat heads can also be used. But a flat shape is not good at resisting pressure. Hence, the thickness will increase and it will not be economical. 12. Nozzles Nozzles are attachments to the vessel shell or head. They act as connections. The inlet and outlet nozzle provides entry and exit to the fluid. A manhole is needed to inspect the vessel. It can also be used to insert internals like filters. If the vessel is small, then hand holes are provided. A vessel can have a sight glass to see the reaction inside. Instrument nozzles are needed to provide indication and control. Each pressure vessel needs a pressure safety valve to relieve the pressure. Drains are provided to empty the vessel. A vessel generally has spare nozzles. The process engineer lists all the nozzles in the process data sheet. It is given in a detailed form in the mechanical data sheet. It contains information like service, size, flange class etc. 13. Flanges The common end connection to a nozzle is a flange. Flange types Slip-on flanges are used for when the pressure and temperature is not severe. The flange slides on the pipe and joined by two fillet welds. Hence, it is known as slip-on flange. Welding neck flanges are joined to the pipe by a butt weld. A butt weld penetrates completely throughout the thickness of the pipe. Hence, they are strong and can be used for severe service. Blind flanges serve as closure to openings. Flange rating Flanges are classified based on their rating. For example, 150 Cl, 300 Cl. Cl stands for class. Class denotes a series of pressure temperature ratings defined in ASME B 16.5 The most common type of flange facing is the raised face. 14. Supports Supports are not pressure parts, but they are important in ensuring that the vessel is supported properly Sharjeel Aslam Faiz Page 8 of 11

9 Leg supports are used for small vessels which are generally not subjected to vibrations. Legs are made from common structural shapes like pipes, angles, channels etc. Legs are known as columns also, as in the case of spheres. Saddles are used for horizontal vessels. Two saddles are used. While one of the saddles will be fixed, the second one can be made slotted for absorbing thermal expansion. Skirt supports are continuous and are used for supporting tall vertical vessels. Lugs are another inexpensive form of supporting vertical vessels. 15. Gaskets Gaskets prevent leakage of fluid under pressure. In order to do that, they need to resist the fluid pressure, temperature and need to be compatible with the process fluid. Gaskets are needed because two mating flange faces do not mate perfectly. They are rough and have some irregularities. Gaskets can be nonmetallic, semi-metallic or metallic. Nonmetallic gaskets are used for low pressure applications. Semi metallic gaskets like spiral wound gaskets consist of metallic and nonmetallic part. The metal provides the strength while the nonmetallic part helps in sealing. Spiral wound gaskets can resist higher temperature and pressure than the nonmetallic gaskets. Gaskets can be metallic like metal ring gasket used with ring-type flanges. These can resist high pressure and temperature. These gaskets need to be softer than the flange and need more bolt torque to tighten. 16. Fabrication A fabricator is the name given to the manufacturer of pressure vessels. The fabricator receives the material requisition (MR) from the static equipment engineer. The MR is a method of transmitting the user requirements to the fabricator. The MR consists of fabrication drawings, specifications and quality assurance plan (QAP). The MR also specifies the list of documents which the fabricator will submit at the time of dispatch. Upon receipt of the MR, the fabricator precisely estimates the cost of the vessel. The cost includes the material cost, electrode cost, labor, transportation, permits, taxes, profit and site visits (if any). Care should be taken to compile the MR. If any document is not included, it may lead to a price implication and disruption later Sharjeel Aslam Faiz Page 9 of 11

10 The fabricator compiles all the requirements in a bid. The static equipment engineer reviews the bid. This is known as technical bid evaluation. The satisfactory bidder is called for commercial discussions. Upon receipt of the purchase order, the fabricator proceeds with manufacturing and procures the material. Forming followed by welding is the most common method of manufacturing a pressure vessel. Fabrication is done in fabrication shops. Here it is easy to control the manufacturing process. Some vessels are so long that part of the welding and joining will be done at the site. Some vessels may be subjected to an additional heat treatment method known as post weld heat treatment (PWHT). Welding hardens the weld and the area near the weld (known as heat affected zone). Also, some stresses remain locked up in the material. The material is restored back to its ductile nature by PWHT. The code determines which conditions require a PWHT to be done. The process is carried out by inserting the steel in a furnace and heating and cooling it under controlled conditions. 17. Joining Welding is the most common method of joining steels. Steels are joined by electrodes. Every manufacturing shop needs to carry out welding with qualified welding procedures and welders. 18. Inspection A vessel needs to be checked for quality during and after fabrication. This can be done by non-destructive testing methods. Even though there are many ways to inspect the vessel, the importance of visual inspection can be overemphasized. Dye penetrant inspection. It helps locating cracks, porosity etc. present on the surface. It is cheap and easy to use but can only defect defects on the surface. Radiographic inspection provides a permanent record and can be used for inspection of internal defects. Magnetic particle inspection. This method is suitable for magnetic material and can detect surface and near surface defects. The magnetic lines run through the material. The magnetic lines will be disturbed near the crack. But the crack needs to be perpendicular or at an angle to the magnetic field. Ultrasonic inspection requires the transmission and reception of sound waves. Cavities will reflect part of the sound waves and will arrive earlier than the rest of the sound waves Sharjeel Aslam Faiz Page 10 of 11

11 Inspection visits are scheduled as per the requirements of QAP. During inspection, the inspector makes a visit report. If there are any comments, the manufacturer makes rectification or clarifies. 19. Final testing Final hydrostatic testing ensures the integrity of the vessel. The test pressure is more than the service pressure. But it should not cause permanent deformation, hence inspection is crucial and cannot be waived off. Vessels will be tested with water. In case of austenitic materials, the water should contain less than 30 ppm of chloride ions. Pneumatic tests are rarely used unless water cannot be used to fill a vessel safely. Pneumatic tests are dangerous because air is compressible. Once the vessel is acceptable, it is dispatched from the fabrication shop Sharjeel Aslam Faiz Page 11 of 11