Process Engineering Design Pressure Vessel
First the total volume of gas that Is to be stored needs to be calculated. From the mass balance it is known that 30. 92 smoke of methane can be expected every hour. This correlates to 742. 0 smoke/day. The ideal gas law will be used, with a compressibility factor, Z, to account for departure for UN-dillydally. An operating temperature of 50 will be assumed, as this is the highest feasible ambient temperature that can be expected. Also assume that the pump or compressor used to transport the methane from the flash vessel will not cause the methane to rise above this design temperature.
The expected surge pressure from of the 1 5 ATM outrage pressure is 16. 5 ATM. This will be taken as the operating pressure throughout the design. As the actual pressure Inside the vessel will be 0. 979 from SOURCE. The total volume of the dally quantity of methane at the specified conditions Is calculated below. However the vessels must not fall below O gauge pressure Inside, there must be at least one ATM absolute pressure inside each vessel otherwise they will fail due to the difference in pressure. So total amount of storage needs to account for this.
Assuming a linear relationship exists between pressure and volume at constant enrapture 1 ATM would correlate to one fifteenth of the volume, such that total volume required. Allowances for extra volume in-case of train delays etc will be made later in the design. CHOICE OF END CAP To avoid designing a shell and then having to make adjustments such that It fit a specific end cap the end cap was chosen first and the shell designed around it. Seem- Ellipsoidal end caps were chosen as the operating pressure exceeds 15 bar (pop). Hot pressed caps will be used to save on costs.
Whilst a greater range and larger sizes are available in the hot spun method of manufacturing it is not worth the higher cost. The largest available semi ellipsoidal available by hot pressed manufacturing from Australia Pressure Vessel Heads catalogue has an Internal diameter of 2731 mm. This is approaching the largest size that can be easily transported by conventional means. It will need to be manufactured to order but this is unavoidable as the largest stock size Is mm smaller. SHELL MATERIAL, THICKNESS AND CLASS AS 1548 7-REAR,N,T was selected from table 3. 3. (A) (peg 1 8) as the material to be used for the shell of the tank. It was selected as it is a carbon-manganese steel alloy, has o thickness restrictions, Is part of the AAA steel group which should alloy versatility in vessel classing and exhibits a superior tensile strength at the design temperature. Under design temperature this alloy has a tensile strength, f, of 123 Amp. For economic reasons double butt welds will be used. If the vessel falls into class 3 the joint efficiency, , will be 0. 7, as specified by table 1. 6 AS 1210 (peg 23). Applying the So as a class 3 vessel the minimum thickness of the shell wall is 26. 8 mm. This fails the criteria outlined by table 1. 7,AS 1210 (peg 23) as Note that as well, Justifying choice of an AAA material. An AAA grade material would have provided no benefit. If it is class B, still using a double butt weld, the weld efficiency is increased to 0. 8. Re applying the minimum thickness equation yields For the vessels to fall under class three the diameter would need to be more than halved. This would result in either very long, uneconomic vessels or many smaller vessels. In either scenario it is not worth the savings made from leaner design restrictions and safety checks. Corrosive substance to steel, so a minimal corrosion scenario is assumed. Assume 0. 1 mm of corrosion per year (Collusion and Richardson V). Designing for a tank life of 20 years gives a corrosion allowance of 2 mm. Total shell wall thickness given by mm However steel sheets only available in either 25 mm or 28 mm. 28 mm would be overburdening for no real benefit, so 25 mm sheet will is to be used. This reduces the corrosion allowance to 1. 6 mm, which is still acceptable. This is still below 32 mm, so vessel is still classed as B.
Cap Thickness Firstly the same material as the shell will be used for the end caps, so the tensile strength will be the same, mamma. Table 1. 6 AS 1210 (peg 24) shows the Joint efficiency for double butt welds for circular welds is 0. . Minimum thickness for a semi-ellipsoidal end cap is given by K is a factor dependent on the ratio between the diameter of the cap and its height. The height of the cap is given in Australian Pressure Vessel Heads data sheet and for the selected cap is given as 682 mm. The diameter, D, used is the internal diameter of the shell and end cap.
And applying the minimum end cap thickness equation gives Internal corrosion for the end cap will be the same as that for the shell, so apply corrosion allowance of 1. 6 mm. However from the Australian Pressure Vessel Heads data sheet it can be seen it is only available in 25 mm or 32 mm thickness. The 25 mm thick variant will be used. It will decrease the corrosion factor to 1. 57 mm, however this is still acceptable. And to prevent buckling (peg 35, ASIA 210) strong weld will be easy. Shell Aspect Ratio and Volume The diameter was determined by the end cap as 2731 mm.
Initially let This is the middle of the recommended range for pressurized vessels (page 27) and close to the optimal of 3. This gives a shell length of 9558. 50 mm. However steel sheets only available in mm increments and cutting to size would be wasteful and provide no benefits. Better to Just have slightly larger tanks. So adjust shell length, L, to 9600 mm. Gives a new aspect ratio of This is still within the recommended range. Volume Total volume of pressure vessel given by Volume of madcap is specified in data sheet as 2661 liters and the volume of the shell is easily calculated.
As previously shown of tank space is required to store a days methane. So to calculate the required number of tanks To allow for downtime, maintenance and late trains 20 tanks shall be used. Inlet Nozzle The pressure vessel requires an inlet nozzle, outlet nozzle, pressure relief valve and drain pipe. Table 3 from AS 4343 (peg 9) specifies that extremely flammable gases are rated as Very Harmful Gas (VBG). Methane falls into this category. From table 1. 4 AS 4041 (peg 8) unless design pressure exceeds 2 Amp class 3 piping is acceptable. Class 3 piping acceptable.
To calculate the size of inlet nozzle required the volumetric floodwater of methane is to be calculated. Note that for the following calculations to make literal sense a constant methane pressure of 1. 67 Amp ( must be assumed through the inlet nozzle, el the driving force from the pump. Per day, therefore the volumetric floodwater, Q, is A target gas velocity of is chosen as it sits in the middle of the optimum range. Using this velocity the cross sectional area of the pipe, , can be calculated And the diameter can easily be calculated from the area Note that this is the internal diameter.
PIPE MATERIAL unusually high or low, and this choice of material reflects that. From Table DO AS 4041 (peg 59) it can be seen that at the design temperature this material exhibits a tensile strength, f, of 183 Amp. From table 3. 12. 3 AS 4041 the design factor, M, for class 3 piping is 0. 7. As it is piping seamless welds are to be used which have a weld mint factor, e, of 1. The pressure design minimum pipe thickness, , can be calculated using the internal diameter based equation The contribution of pressure to the necessary design thickness is very small.
The total minimum pipe thickness is a function of the pressure design term and the corrosion allowance can be calculated by As the material is a carbon alloy and methane being relatively UN-corrosive a corrosion allowance of 2 mm will be used. Rearranging the formulae yields And so the theoretical outer diameter So a theoretical inner diameter of 46. 50 mm and outer diameter of 49. 3 mm. DEN 40 schedule 40 (Fluoroscope Pipe catalogue, page 21), with an outer diameter of 48. 3 mm and a thickness of 3. 68 mm, fits the theoretical requirements the closest.
Re checking the gas velocity: This is still in the acceptable velocity range for gases, and so this piping is fine. OUTLET NOZZLE In designing the outlet nozzle the amount of time desired to evacuate all the methane onto the train needs to be considered and 2 scenarios will be evaluated. In the first all of the methane is pumped from the storage vessels through a single pipeline, so tanks are evacuated one at a time. In scenario B there are 2 lines of pipe from the pressurized tanks to the train and 2 pumps, so that 2 tanks can be evacuated at once, effectively halving the volume needed to be pumped.
Scenario A will require less length of pipe of a greater diameter to handle the higher volumetric floodwater. Scenario B will utilize more length of pipe of smaller diameter. Scenario B will also require a second pump. Assume in both scenarios it takes 2. 8 hours to empty the storage tanks into the trains and the gas travels at 7. 5 m/s. Note that pumps are required as, assuming the methane is also transported at 1 5 ATM gauge erasure, the pressure difference across the trains tanks and the storage tanks will not be enough to transfer the gas across.
The pressures would eventually equalize and a sizeable quantity of methane would be left in the storage tanks. Calculations used are same as demonstrated for the inlet nozzle. A corrosion allowance of 2 mm is used and a gas velocity of 7. 5 m/s is targeted. SCENARIO A Best fit is DEN 125 schedule 40. Best fit is DEN 90 schedule 40. So scenario A requires pipe of only 2 sizes higher than B. So whilst B is smaller pipe, and therefore cheaper, the cost of the extra lengths required and the extra pump, it s obvious that scenario A is the cheaper option.
Velocity Check utter diameter of DEN 90 thickness of DEN 90 Schedule 40 This velocity is well inside the acceptable range. LENGTH: The nozzle will be 400 mm in length. This will leave plenty of room to attach the flange without extruding too far. PLACEMENT AND REINFORCEMENT: As seen on the diagram the outlet nozzle will be placed in the middle of the madcap Flange As the design pressure does not exceed 2 Amp a class 150 flange will be acceptable. A bolted full face style is chosen as it is proven and reliable. DEN 90 will be required as to fit the outlet nozzle. Pressure relief and drainage nozzles.
In the case of critical over presentation a pressure relief valve is required to avoid vessel failure. To account for a worst case scenario where over presentation has occurred and methane is still being pumped into the tank at full capacity the pressure relief nozzle will be of the same design as the inlet nozzle so that gas can escape at the same rate as it enters. The relief valve will be spring automated and set at the vessels design pressure of 16. 5 ATM. It will be placed on the top, as seen in the engineering diagram, with the same reinforcing as the inlet nozzle.
In the event of a build-up of liquid within the vessel, such as from the hydrostatic pressure test of water condensation, a drainage valve is required. It will not be operated whilst the vessel is pressurized, so specific design is not of great importance. To simplify the design it will be identical to the design of the inlet and pressure relief nozzles. It will be placed on the underside of the vessel such that liquid build-up will be removed by gravity alone. Details of its placement can be found on diagram A. The will receive the same reinforcing as the other nozzles of the same design.