What is a head

What is a head?

The head refers to the element used to close the end of the container to isolate the internal and external media, also known as the end cover. The head of cylinder descriptors is generally a rotating shell. According to the shape of the head surface, it can be divided into convex shape, conical shape, flat shape and combined shape. Convex head refers to the head whose outer surface is convex, such as semi spherical head, elliptical headdished head and non folded spherical head. Some gas cylinders adopt convex inward combined bottom heads, which can not only ensure strength, but also meet the needs of safe use.

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The head is a component of the container and is connected to the cylinder by welding. According to different geometric shapes, it can be divided into spherical, elliptical, dished, spherical crown, conical shell and flat cover, among which spherical heads, elliptical heads, dished heads and spherical crown heads are also collectively referred to as convex heads. In welding, it is divided into butt welding head and socket welding head. It is used for various container equipment, such as storage tank, heat exchanger, tower, reactor, boiler and separation equipment. Materials include carbon steel (A3, 20#, Q235, Q345B, 16Mn, etc.), stainless steel (304, 321, 304L, 316, 316L, etc.), alloy steel (15Mo3, 15CrMoV, 35crmov, 45crmo), aluminum, titanium, copper, nickel and nickel alloy.

Use of head

Head is an indispensable and important part of pressure vessel equipment in petrochemical, atomic energy, food and pharmaceutical industries.

  • The head is the end cover on the pressure vessel and is a main pressure bearing part of the pressure vessel. The function is sealing. First, the upper and lower bottoms of the tank shaped pressure vessel are made. Second, when the pipe ends and is not ready to extend forward, a head is used to seal the pipe in the form of welding. Similar to the function of the head, there are blind flanges and pipe caps, but those two products can be disassembled. The head can not be disassembled after welding. The supporting pipe fittings include pressure vessel, pipeline, flange, elbow, tee, cross and other products.
  • The quality of the head is directly related to the long-term safe and reliable operation of the pressure vessel.

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Schematic diagram of use of head on pressure vessel

Basic requirements for head

The existing head standards in China are formulated according to different structural types (oval, dished and conical) and forming methods (stamping and spinning), which not only leads to the unreasonable phenomenon that the quality requirements of different standard heads are not completely consistent, but also brings some difficulties to the selection and revision of standard heads.
First, the previous head standards are only matched with GB150 steel pressure vessels, that is, only the manufacturing, inspection and acceptance requirements of heads designed according to the rules are considered. However, China completed the dual track system of basic standards for pressure vessels in GB150 and JB4732 as early as 1995 (with the standard for analysis and design of steel pressure vessels), which is lack of head standards matched with analysis and design, It must be said that it is a major defect in the standardization of pressure vessels in China.
Second, GB150 is a mandatory standard, but the head standard prepared according to GB150 and matched with it is instructive (recommended), which is obviously unreasonable and difficult to ensure the quality of head, an important pressure component.

Material analysis

The inner diameter of the vessel Di = 4000mm, the calculated pressure PC = 0.4MPa, the design temperature T = 50 ℃, the head is a standard elliptical head, the material is 16MnR (the allowable stress of the material is 170Mpa only at the design temperature), the negative deviation of the steel is no more than 0.25mm and no more than 6% of the nominal thickness, the corrosion allowance C2 = 1mm, and the welded joint coefficient of the head welding= 1。 Calculate the calculated thickness, design thickness and nominal thickness of elliptical head.
KpDi
Calculated thickness δ=—————-= 4.73mm

  • 2[σ]tΦ-0.5pc

Calculated thickness: δd=δ + C2=4.73+1=5.73mm
Considering the effective thickness of standard elliptical head δ E shall not be less than 0.15% of the inner diameter Di of the head and the effective thickness δ e=0.15%Di=6mm
δ e> δ d. C1 = 0, C2 = 1, nominal thickness: δn=δe+C1+C2=6+0+1=7mm
Considering the standard specification thickness of steel, the thickness of 1mm is floating upward. The first design rounding value △ 1 = 1, so it is taken as δ n=8mm。
According to the technical data of professional head manufacturer, di = 4000 δ N = 8 head processing thinning C3 = 1.5mm, the second rounding value of thickness △ 2 = 0.5.
If required, the forming thickness of the head shall not be less than the nominal thickness δ N minus the negative deviation C1 of steel plate, the feeding thickness:
δs=δn+C1+C3+△2=8+0+1.5+0.5=10mm, and the minimum thickness after forming is 8.5mm. If the head is used, the forming thickness shall not be less than the design thickness δ D (to be taken) δ E value), then the feeding thickness: δs=δd(δe)+C3+△2=8mm, and the minimum thickness after forming is 6.5mm and greater than the effective thickness δ e. Greater than design thickness δ D and calculated thickness δ。
It can be seen from the above that the two different requirements make the feeding thickness of the head 2mm and the weight difference as much as 300kg.

Thickness definition

The thickness definition of GB150 and relevant head standards is unreasonable, which is mainly reflected in the thickness requirements of the container and head after forming. The forming thickness of convex head and hot drum shall not be less than the nominal thickness minus the negative deviation of steel plate( δ N-c1), which may cause the design and manufacturing to increase the thickness based on the design thickness twice to ensure the forming thickness. Therefore, the concept of minimum forming thickness has been put forward: “the thickness to be guaranteed after processing and forming of hot coil cylinder or convex head shall not be less than the design thickness”. That is, the designer shall mark the nominal thickness and minimum forming thickness (i.e. design thickness) on the drawing δ d) In this way, the manufacturing unit can decide whether to add the manufacturing thinning amount according to the manufacturing process and the design rounding amount of the original design. The definition and marking of this thickness is a popular method in the international pressure vessel industry as of 2013, which is reasonable, but there are the following two problems to be solved in China’s current standards.

Manufacturing process of head

The material is 0Cr18Ni9, φ 1700mm EHA standard elliptical inner diameter head as an example; See Figure 2-1.
Determine the material, internal and external diameter and straight edge height of the head according to the customer’s order, and whether it is made according to the standard head size. If it is a non-standard head, determine the R and R of the head and the special straight edge height; In addition, it is also necessary to determine whether the head customized by the customer needs special technical requirements such as heat treatment, film making (RT) and supervision and inspection. After confirmation, formulate the process flow card and indicate the special process requirements.
φ 1600-φ 1900mm head is the only one that needs three processes: stamping, drum pressing and spinning.

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Figure.2-1 Forming drawing of φ1700mm elliptical head

Compared with other heads, φ 1700mm heads need not only stamping, drum pressing and spinning, but also initial forming through stamping, then overall forming through drum pressing and finally forming through spinning.

Blanking of head

According to the process flow card, The blanking size of φ 1700mm head is φ 2050mm. Generally, the standard width of the blanking plate is only 1500, 2000, 2200mm. In order to save materials, we mostly select the plate with the width of 1500mm for blanking. Due to the insufficient width, it is usually necessary to lower two pieces of material and finally splice them. The small splice plate is called a joint.
Before blanking, the sample needs to be cut from the original plate. The sample needs to be tested for physical properties and chemical element content, because the physical properties determine the thinning rate of the head, and the chemical element content determines the welding quality. See table 2-1 and table 2-2 for the physical properties and chemical element content in 0Cr18Ni9 [2].
In addition, it shall comply with the material P ≤ 0.03%, s ≤ 0.02% for pressure vessels specified in the new volume specification (Supervision Regulation on safety technology of fixed pressure vessels) [3].

Table.2-1 numerical table of mechanical properties

Mechanical properties (not less than)

Hardness value (not greater than)

Specified non proportional extension strength

Tensile strength

Elongation after fracture

HBW

HRB

HV

205

515

40

201

92

210

Table.2-2 chemical element content of blank (no more than)

C

Si

Mn

P

S

Ni

Cr

0.08

0.75

2

0.45

0.03

8-10.5

18-20

Blanking operation of head

Blanking is the first process of head production. Whether the blanking is good or not is the premise of whether the head quality is passed. At the same time of blanking, it is also necessary to consider how to arrange the blanking in order to save the material and achieve the lowest cost. When cutting the wafer, it is necessary to consider the blanking wafer tolerance of – 5 ~ + 5mm.
Shengbo head mainly processes stainless steel head and carbon steel head, and its cutting tools are different. Plasma cutting is used for stainless steel cutting, while acetylene gas cutting is used for carbon steel. If it is relatively thin carbon steel, plasma cutting can also be used, and the compass for cutting round pieces, The cutting radius is determined by adjusting the movable buckle on the compass according to the radius to be cut.

Blanking operation standard of head

  • (1) According to the process card and the physical object, confirm the requirements of the process flow card (including the discharge drawing issued by the official), and confirm whether the material specification is consistent with the physical object.
  • (2) Confirm whether the plate thickness, material and surface quality meet the requirements.
  • (3) During material warehousing inspection, each steel plate shall be equipped with corresponding material quality assurance, which is the inspection data of the steel plate and the reliable basis for the qualified use of the steel plate.
  • (4) Select the required materials according to the process flow card (or discharge drawing).
  • (5) Confirm the blanking diameter according to the process card, the blanking dimension tolerance standard is ± 5, and draw a line after marking the center [1].
  • (6) After blanking, the instruction number, unit number, shape, material and furnace batch number of the product shall be transplanted on the surface of the same plate with a paint pen. If there is a semicircle, the same content shall be transplanted on the semicircle, and finally confirmed by the inspector.
  • (7) According to the requirements of the process card, stamp the steel seal on the outer side of the wafer. The steel seal contents are furnace batch number and material brand. If there are special instructions, stamp the steel seal according to the requirements, and rub the steel seal contents for review. The operator shall sign on the rubbing paper [3].
  • (8) When cutting the disc, if the disc has certain slag and splash, it must be polished thoroughly, otherwise it will easily scratch the surface of the steel plate due to mutual friction in the later disc handling process.
  • (9) If there is a gap around the wafer, if the gap is more than 5mm deep, polish it to the range of 30mm at both ends with the gap as the center. Be sure to polish it smoothly. If the gap is more than 5mm and the groove can be removed, submit it to the welding team for repair welding and grinding. If the groove cannot remove the gap, contact the customer and solve it after discussion.
  • (10) After the foreign matters on the surface of the cut wafer are removed, they shall be stacked neatly according to the single piece number of the wafer (semi-circular wafer), and transferred to the next process together with the process card, which shall be confirmed by the inspector or team leader of the next process.

Welding procedure of head

Welding operation of head

The common welding methods in our factory are: manual welding, argon arc welding and submerged arc welding.

  • (1) Manual welding is mainly used for the welding of 3-6mm carbon steel plates.
  • (2) Submerged arc welding mainly focuses on the welding of carbon steel and stainless steel plates above 8mm.
  • (3) Argon arc welding mainly focuses on the welding of 3-6mm stainless steel.

Common stainless steel welding wires include ER304 and ER316l, and common carbon steel welding wires include J422 and J507 and low-temperature welding materials J426 and J427. HJ431 is commonly used for carbon steel and HJ260 is commonly used for stainless steel,.
Since the welding wire and flux are susceptible to moisture, the welding wire and flux are usually stored in the infrared electronic dryer, and the drying temperature of flux HJ431 and HJ260 is 250-300 ℃. The drying temperatures of welding wires J422 and J350 are 150 ℃ and 300 ℃ respectively
The welding process is carried out in full accordance with the welding process. The thickness of the plate determines that single-sided welding or double-sided welding is required during welding. After welding, RT flaw detection (X-ray) shall be carried out on the weld to judge whether there are pores, cracks and other defects. If the above defects are found, they shall be repaired until the film is taken.

Welding operation standard of head

(1) After the welding process receives the process flow card, confirm whether the material, quantity, single piece number, specification, instruction number of large and small pieces of the process flow card are consistent with the physical object, whether there are sundries around the disc, and whether the spatter on the disc is removed. If it is not treated, grind it with a grinding wheel.
(2) Whether there are welded test plates, whether the groove cutting quality can meet the requirements, whether there are obvious defects on the material surface, and respond in time in case of abnormalities.
(3) Check whether the form and size of welding groove comply with the requirements of welding process card.
(4) The groove surface on each side shall be polished, and the butt misalignment shall not be greater than 10% of the plate thickness and not greater than 1.5mm.
(5) The length of electric welding at the end of the first layer of spot welding seam shall not be less than 50mm, the specification of arc stopping plate shall be 150 * 150mm, and an arc with R shall be on one surface. The discs after formation shall be welded within 8 hours, otherwise the water in the groove must be dried with flame.
(6) Welders shall receive welding materials with welding process and welding record, and register at the secondary warehouse of welding materials. The amount of flux received on site shall not exceed 4 hours, otherwise it shall be put back to the oven for re drying.
(7) Preheat

  • a. For carbon steel and low alloy steel with plate thickness not less than 30mm, the preheating temperature shall be in accordance with the provisions of welding process card and welding technical standard.
  • b. The width of preheating range shall not be less than 4mm and 100mm.
  • c. At any time during welding, the temperature at the groove shall not be lower than the specified preheating temperature.
  • d. When the temperature of the weldment is lower than 0 ℃ (any plate thickness), it must be preheated to more than 15 ℃ before welding.

(8) Welding

  • a. Before welding, the welding equipment must be inspected, and welding can be carried out only after it is normal.
  • b. Weld according to the requirements of welding process, and fill in the welding record at the same time
  • c. According to the welding specification, the number of welding layers must meet the requirements of welding process. Large current and few layers shall not be used during welding.

(9) Root clearing

  • a. Before stainless steel root cleaning, anti spatter coating shall be applied within 300mm on both sides of the groove.
  • b. When carbon arc air gouging is used for root cleaning, the diameter of carbon rod shall be selected as required.
  • c. After root cleaning, grinding wheel shall be used to grind and clean the groove and there are carburized layer, oxide, slag and other sundries on both sides. Grinding width: manual welding, rust, oxide more than 20mm, oil stain more than 30mm, automatic welding, rust, oxide 25mm, oil stain 50mm, grinding requirements on the inner side of groove, carbon steel and low alloy steel δ For the weld with n ≤ 10mm, the carburized layer within 300mm at both ends must be completely removed, and more than 50% of the rest is allowed to be removed. Other non-ferrous metals must be completely removed.
  • d. After root cleaning, composite steel, Cr Mo steel, manual welding and plate thickness δ PT inspection shall be conducted for welds with n ≥ 30mm.

(10) The welder with rich experience in repairing defects during welding shall be instructed by the welding team leader. Stick welding shall be avoided during repair welding, otherwise repair welding shall be carried out after grinding. The following defects must be removed before continuing welding:
① Welding deviation
② Crack
③ Pit
④ Stomata

  • a. The weld shall not be lower than the base metal.
  • b. There shall be no undercut.
  • c. Weld with reinforcement not removed. The surface shall be free of cracks, pores, craters, undercuts, slag inclusions and other defects, and slag and spatter shall not be retained.
  • d. PT inspection of welds with reinforcement removed shall not show any defects.
  • e. If there are defects ①. Repair procedure for carbon steel and low alloy steel: grinding wheel or carbon arc air gouging → PT inspection → grinding welding groove by grinding machine → welding → PT inspection; repair procedure for stainless steel: grinding wheel or carbon arc air gouging → PT inspection( δ N ≥ 2mm (required) → grinding welding groove with grinding machine → welding → PT inspection [4].

(12) Welding internal repair, repair procedure: defect location → defect removal → PT inspection → welding → removal of weld reinforcement → PT inspection [5].
(13) Referring to RT negative film, RT personnel and welders shall determine the defect position on the weld together. When the following requirements are included, ut shall be used to determine the defect position for one-time repair δ When n ≥ 30mm, ut shall be used to determine the defect location and repair for two or three times δ If n ≥ 20mm, ut shall be used to determine the defect position. After ut positioning, RT team shall mark the defect position and depth (the calibration side must be the side with defect depth ≤ 1/2).
(14) After welding is completed and confirmed by the inspector or team leader, sign the name on the process card and transfer it to the next process together with the physical object, which can only be approved by the person in charge of the next process.

Stamping process of head

Stamping operation of head

Stamping operation is an important operation of small and medium-sized head forming. Similarly, stamping is also the process section of head cracking and thinning.

Composition of stamping machine

  • (1) The master cylinder is mainly used to connect the die head and is a necessary part for head forming.
  • (2) The side cylinder is used to press the upper die ring and fix the head disc in the upper and lower die rings. It is an important part to prevent the head from wrinkling and bulging in the pressing process.
  • (3) The overflow valve is used to control the overall pressure of the press to prevent the disc from tearing during pressing.
  • (4) When the pressure of the stamping valve is too large, the stamping valve will relieve the pressure and achieve the effect of pressure averaging.

In addition, oil circuit board, motor, oil circuit board, oil storage tank, console, base, etc. are all components of the press.

Operation steps of stamping

  • (1) First, after undertaking the flow card of the blanking process, find the corresponding wafer.
  • (2) According to the process requirements on the process flow card, replace the corresponding set of molds, and use measuring tools to determine that the size of the selected mold is consistent with the size of the disc to be pressed.
  • (3) Clamp the wafer in the upper and lower die rings, and make sure that the center point of the die head is in line with the center point of the wafer.
  • (4) Operate the relative movement of the upper die ring and die head, conduct pressure test, and judge the pressure according to the rebound strength of the control rod, so as to adjust the pressure.
  • (5) After the pressure test, press the disc for stamping.

Stamping operation standard

  • (1) Check whether the physical object is consistent with the process card according to the task list, and confirm the quantity, material, specification, furnace batch number and other elements of the processed workpieces according to the process card. At the same time, check whether the end of the wafer has cracks, burrs, grinding meets the requirements, whether the wafer has chamfers, whether the front face is upward, etc. if there are special requirements, contact in time if any problems are found.
  • (2) Correctly select the mold according to the process card, check the surface condition of the mold, polish the damage and serious strain found, and clean the rust and other dirt on the surface of the mold.
  • (3) Press according to the size, shape, material and straight edge required by the process card.
  • (4) When warm stamping is required (heating to a certain temperature according to the properties of the plate, and then stamping process), corresponding templates shall be prepared for the head combined with drying gun, gas, oxygen and punching and spinning. For double-sided film coating, the template within the range of about 200-300mm on the front and back sides of the wafer edge shall be removed, and the sundries on the film surface shall be removed [6].
  • (5) Install the upper and lower molds, add mold cushion blocks as appropriate, and apply lubricating oil on the inner and outer surfaces of the disc. The application range is 200-300mm from the top to the inside. The application shall be uniform, and the upper and lower molds shall be coated evenly.
  • (6) When stamping the first head, lift out the head, check the section shape with a sample plate, check whether there is bulge and thinning, and check whether the surface is scratched and hooped. If there are no above abnormalities, stamping can be continued. After the head is formed, if there is scratch on the head surface, polish it in time, confirm the minimum plate thickness, and confirm whether there is scratch, strain, curved skin and orange peel, Bulge, wrinkle, hoop mark and other undesirable phenomena.
  • (7) If the head is bulging or wrinkled, it shall be transferred to the repair process together with the process card.
  • (8) Each head shall be able to see the instruction number, piece number and material, otherwise it shall be transplanted again. After each specification product is inspected, it shall be transferred to the next process together with the process card (name shall be signed) and confirmed by the next process.

Drum pressing process of head

Drum pressing operation

The drum pressing process and stamping process are the same process, and the stamping process scope φ 159 – φ 1900mm and the scope of drum pressing process is φ 1600 – φ 4800mm. Unlike stamping, the drum is mostly formed by the head, and the final forming is completed by spinning. If the stamping does not meet the special requirements of customers, the final forming step is unnecessary. φ 1600 – φ 1900mm head, both processes are required.

The composition of drum press

  • (1) The main cylinder is equipped with upper die head as long as the components of drum are used.
  • (2) The center of the lower die head and the upper die head is fixed by the support base in a straight line.
  • (3) The support frame I is equipped with rolling wheel, which adjusts the front and rear position of the disc.
  • (4) The support frame II is equipped with a rolling wheel and a rotating disc.

In addition, there are hydraulic systems, motors, oil pumps, oil tanks, and control consoles.

Operation procedure of drum press

  • (1) First, after taking on the flow card of the blanking process, find the corresponding next good disc.
  • (2) According to the process requirements on the process flow card, a set of corresponding mould is replaced, and the size of the selected mold is determined by measuring tool and the size of the disc to be pressed is consistent.
  • (3) Unlike press, drum is not pressed from the center of the original sheet, but from the edge of the disc, just align the die head and the disc.
  • (4) During the pressing process, the disc is driven by the rotation of the rotating wheel, and pressed in a certain order.
  • (5) After the pressing is completed, compare with the prepared template. If there is any deviation, further adjustment shall be made.

Drum operation standard

  • (1) The mold is very important to the product quality and whether the drum is pressed smoothly. Therefore, the operator must select the suitable mold and adjust the backing plate timely according to the processing conditions.
  • (2) After receiving the process card, the operator shall carefully see the process card, check the physical object according to the process card, and confirm the instruction number, single piece number and material of the physical object. Quantity, furnace batch number, etc. Select the mold according to the process card.
  • (3) For EHA, the head of EHB shape shall be selected according to the standard of 0.82 * D. for special products such as DHB, PSH and MD, the die shall be selected according to the requirements of process card, but in principle, it shall be selected according to the standards of P * 0.82/1.15 or p * 0.82/1.2 [1].
  • (4) Before pressing, the quality of the wafer shall be carefully checked, and whether the end of the weld disc is smooth, whether there is any notch, and whether the surface has any slag and defects.
  • (5) In addition to carefully checking the defects such as the end of the round plate, the round piece with weld seam shall be inspected whether the grinding of the weld is higher than the base metal, whether there is welding spatter at both ends of the weld, whether the polishing of the weld end is smooth and whether there is any crack gap.
  • (6) When two or more pieces of pressing are required, the joint surface of each piece shall be wiped clean. If necessary, the welding team shall entrust welding team to weld the round plates together for pressing.
  • (7) Check the thickness of the wafer (thickest and thinnest, whether it is consistent with the process card, and whether the dimension of the measuring disc is consistent with the requirements of the process card).
  • (8) Before pressing, all sundries on the internal and external surfaces must be removed to prevent pitting.
  • (9) For general stainless steel materials (plates with more than 5mm), Teflon plate shall be used to bind the upper mold and polish the surface of the lower die smoothly (if necessary, Teflon plate shall be bound) to ensure the surface quality of the workpiece, and the impurities shall be absolutely avoided from entering the mold and semi-finished products during pressing [6].
  • (10) During the pressing process, when the circular piece has a certain R shape, it must be measured with the corresponding r sample plate. During the pressing process, the pressure should be adjusted according to the deformation degree of the workpiece.
  • (11) After pressing, measure the thickness of the plate and measure the arc length. Check the surface quality and check whether the end and the weld end are smooth (polish out if necessary).
  • (12) The semi-finished products inspected shall be transferred to the next process (and the operator’s name shall be signed) with the process card. The next process shall be confirmed and accepted before being approved.

Spinning process of head

Spinning operation

Spinning process is the head forming process after drum pressing, which is mainly aimed at the formation of large head R and straight edge.

Composition of spinning machine

  • (1) The forming wheel is connected with the corresponding die. During spinning, the top of the die is tangent to the tangent between the semi-formed head R and the straight edge at the inner side of the head.
  • (2) The supporting wheel is also connected with the same mold as the forming wheel. During spinning, it is tangent to the forming wheel on the outside of the head.
  • (3) The base is used to fix the head, and the bottom can move back and forth according to the size of the head. When installing the head, make the center of the base and the center of the head in a straight line.
  • (4) The center rod is used to fix the head, and the center and the center of the base are in a straight line.
  • In addition, there are motors, circuit boards, connecting shafts, and consoles.

Operation steps of spinning

  • (1) For the flow card to undertake the drum pressing process, the corresponding head to be spun shall be determined according to the flow card.
  • (2) Select the corresponding upper and lower molds according to the diameter of the head.
  • (3) Clamp the head and ensure that the center point of the head and the center point of the base are in a straight line.
  • (4) First adjust the tangent between the forming wheel and the head, and then adjust the supporting wheel according to the position of the forming wheel.
  • (5) In the process of spinning, observe the relative position of the forming wheel and the supporting wheel, and keep comparing with the template. Until r the requirements are met.

Spinning operation standard

  • (1) The spinning wheel is made of bearing steel or nodular cast iron. The mold is closely related to the product quality and whether it can be spun. Therefore, the operator must select the appropriate mold according to the processing situation. At the same time, because the shape and surface condition of the mold have a great impact on the quality of the workpiece, the mold must be fully spot checked before and during processing.
  • (2) According to the requirements of the process card, confirm whether the instruction number, specification, material and quantity of the wafer, drum pressing or semi-finished products after pre pressing are consistent, check the surface quality and end defects, and timely contact the previous process and deal with them in case of abnormalities.
  • (3) Check whether the ends of semi-finished products are smooth, whether there are gaps and cracks, and whether there are cutting or welding slag on the surface. At the same time, check whether the weld grinding is higher than the base metal, whether there are splashes at both ends of the weld, whether the weld end grinding is smooth, whether there are cracks, etc. [7]
  • (4) The finished product after drum pressing or pre pressing must be checked for defects such as folding, cracking and cracks that are not conducive to spinning processing.
  • (5) When the workpiece is stainless steel, it shall be decontaminated by pre pickling and other methods.
  • (6) The mold is selected according to the process card, and the size requirements of general small circular arc R are used as the basis for the selection of internal wheels.
  • (7) According to different materials, plate thickness and rebound of steel plate, internal wheel slightly smaller than arc r shall be selected.
  • (8) For products with special requirements, the internal wheel shall be designed according to its shape requirements, the surface condition of the die shall be confirmed, and the polishing shall be done well.
  • (9) During spinning, in order to prevent scratches and improve processing performance, lubricant (grease) shall be applied to prevent heat generation during processing, resulting in thermal cracks and scratches on the surface of the head, which can also prolong the service life of the die.
  • (10) In the process of spinning forming, the pressure shall be adjusted in time according to the forming condition of the workpiece.
  • (11) After processing and forming, the size, shape, plate thickness and surface quality shall be checked and confirmed.
  • (12) The formed products shall be transferred to the next process together with the process card (sign your name on the process card) and can be used only after being inspected and confirmed by the inspector or team leader of the next process.

Groove process of head

Key points of groove

  • (1) The beveling process is a key process for all data (including perimeter and total internal height) of a head to meet JB / t4746 standard [1].
  • (2) The groove is to make the welding more thorough when the head and cylinder are spliced.
  • (3) The main cutting process of groove is the same as the blanking process tools, which are cut by plasma.
  • (4) During cutting, determine the type of groove (inner groove, outer slope and x-groove) and the angle of groove according to the process requirements on the flow card.
  • (5) Splash agent shall be applied to the head before groove.
  • (6) The slope test shall be conducted for the groove, and then the angle shall be measured with a protractor to determine that it is within the tolerance range. After adjustment, the slope shall be adjusted.

Groove operation standard

  • (1) Find the real object according to the process card, confirm the instruction number, specification, material, furnace batch number, quantity, etc., and check whether the workpiece has defects such as bulge, crack, delamination and wrinkle, as well as whether it is out of round. If any problem is found, contact and deal with it in time. For the plate thickness of more than 8mm, it is better to repair the out of round groove first, and when it is less than 6mm, press the tooling plate and then the groove.
  • (2) For stainless steel heads, splash agent shall be applied on the inner wall of the head before the groove to prevent molten slag from spraying on the inner wall of the head during the groove.
  • (3) Before lifting the head to the beveling machine, first confirm the weight of the head, select appropriate lifting tools according to the weight, and check whether the lifting tools are safe.
  • (4) after the head is hoisted to the beveling machine, adjust the bevel screw of the groove, adjust the center position with the prying bar, turn the turntable, and aim at the center.
  • (5) Before the groove, draw the groove height line according to the height and height tolerance required by the process card, and confirm the height line with a straight edge. When the slope is inside the slope, the groove shall be leveled first, and then the groove inside the slope.
  • (6) The angle of outer slope or inner groove shall be controlled within ± 2.5 tolerance.
  • (7) After the groove is finished, check the perimeter, height, straight edge, angle, etc. of the head. If it is unqualified, go to the repair process for repair.
  • (8) The qualified head shall polish and remove the slag, etc. and clean the slag inside the head, which shall be transferred to the next process together with the flow card (and sign your name) and confirmed by the inspector of the next process.

Grinding process of head

After the groove, there will be oxide scale, wrinkle and other defects in the groove section. Grinding is to process the groove and make the groove more smooth.
Grinding is composed of pneumatic machine and grinding machine.

Operation steps of air compressor

1. Prepare

  • (1) Check the oil level. The lubricating oil should be between the upper and lower limits of the oil level gauge.
  • (2) Open the oil drain port, screw plug and ball valve to discharge the condensate sunk at the bottom of the oil and gas tank after shutdown until the lubricating oil flows out.
  • (3) Rotate the main engine by hand for several revolutions, confirm that the main engine rotates easily, and check whether the belt tension is appropriate.
  • (4) Turn on the power supply, check whether the parameters displayed on the display are normal, and operate other peripheral equipment of the compressor.

2. Start

  • (1) Open the valve at the gas transmission port, press the “no” button, and the compressor enters the startup program.
  • (2) Check whether the exhaust pressure of the system is normal and whether the exhaust temperature is maintained between 70-105 ℃.

3. Run
Observe whether the compressor has leakage, abnormal sound and vibration. If there is any abnormality, stop the compressor immediately.
4. Shut down

  • (1) Press the “off” button to enter the normal shutdown procedure of the compressor. After a delay of 30 seconds, the compressor stops running.
  • (2) Close the valve at the gas transmission port and cut off the external power supply.
  • (3) In case of non emergency, use the “emergency stop button” to stop when stationary.

Grinding machine operation steps

  • 1. First, adjust the distance of the fixed frame of the grinder to correspond to the diameter of the head.
  • 2. Then clamp the outer surface of the head with the cylinder. Turn the head and check the speed display in the governor.
  • 3. Try grinding first, adjust the speed according to the surface smoothness, and finally achieve the best effect.

Grinding operation standard

  • 1. According to the requirements of the process card, confirm the quantity, specification, material, furnace batch number, etc. of the real object, and check whether the head has bulge, crack, flanging, perimeter, height and other problems. If any problems are found, contact and solve them in time
  • 2. Turn on the automatic grinder, turn on the power supply of the rotary table and call the compressed air valve.
  • 3. Idle the grinder to confirm whether the vibration and sound are normal, and idle the rotary table to confirm whether it is normal.
  • 4. Select appropriate spreader according to the weight of the head, and place the head horizontally on the rotating table.
  • 5. The compressed air pressure is set at 0.6-0.7mpa. When the thickness of the head plate is less than 6mm, the pressure of the working table is set at about 0.1. When the thickness of the plate is less than 8mm, the pressure of the working table is set at 0.3-0.4mpa. In short, the thicker the plate thickness, the larger the working table, and vice versa.
  • 6. Wear labor protection articles before grinding. Irrelevant personnel are not allowed to approach. Hang the grinder and support rod on the head.
  • 7. See the groove form and angle on the process card, then set the grinding angle, turn on the power switch of the rotary table, turn on the grinder switch, and measure the groove angle with an angle gauge after grinding for 5-8 weeks.
  • 8. After grinding, clean the foreign matters in the head, confirm the grinding results and the correct angle, and then measure the circumference, height, straight edge height and section shape of the head.
  • 9. After the work is completed, the product and the process card shall be transferred to the next process (with their own name signed) and can be used only after passing the inspection by the inspector of the next process.

Pickling process of head

Pickling is a process to clean the welding slag, slag, oil stain, oxide scale and other dirt on the surface of formed head.

Pickling operation steps

  • (1) Determine the head to be pickled according to the process flow card.
  • (2) When working, wear labor protection articles, evenly apply acid solution to the outer surface of the head for preliminary cleaning.
  • (3) After completion, rinse with plenty of water. A passive film is formed on the surface.
  • (4) Check whether the dirt is cleaned. The head that does not meet the requirements shall be reworked in time.

Pickling operation standard

  • (1) Confirm the material, quantity, furnace batch number, instruction number and single piece number according to the process card and the physical object, and check whether there are bulges, cracks and other defects.
  • (2) Wear labor protection articles, prepare acid washing solution, and evenly apply the prepared acid washing solution on the head.
  • (3) There shall be no obvious corrosion trace during pickling
  • (4) The oxide scale shall be completely removed, and the heat affected area of the weld and the surface of the hot working head shall not have oxide color.
  • (5) Weld spatter and slag shall be completely removed.
  • (6) The surface shall be free of traces of uneven color, particles of carbon steel and rust color of carbon steel.
  • (7) The passivation film is light and colorless, with a little reverse film thickness.
  • (8) Black oxide layer will be produced on the surface of heat-treated workpiece, so it can be removed only after repeated pickling for several times
  • (9) In the pickling process of composite plate head, effective protective measures shall be taken to prevent the carbon steel layer from being corroded.
  • (10) After pickling. The head shall be air dried quickly, and there shall be no residual acid in the head. After air drying, it shall be transferred to the next process together with the flow card (and sign your name) and can be used only after passing the inspection by the inspector of the next process.

Heat treatment process of head

Heat treatment plays a very important role in head manufacturing. It is a process method to change the internal structure of head by heating and cooling solid metal, so as to improve its performance.
The head mainly adopts stress relief annealing. The equipment includes electric annealing furnace and ordinary annealing furnace.
Stress relief annealing, also known as low-temperature annealing, is a process of slowly cooling steel after heating to a certain temperature (generally 500-600 ℃) for a certain time.
The purpose of stress relief annealing is to eliminate the residual stress caused by deformation processing, machining, casting, heat treatment and welding.

Operation steps of heat treatment

  • (1) First, confirm the head requiring stress relief annealing according to the flow card.
  • (2) The head shall be evenly arranged in the annealing furnace, and the temperature shall be heated to the specified temperature according to the requirements of heat treatment process for insulation (the insulation time is different according to the thickness of head).
  • (3) After the holding time is determined, place the head to cool naturally.

The heat treatment procedure is shown in Figure 2-3 below:

20210726085044 30724 - What is a head

Figure.2-3

  • T1 minimum temperature of furnace inlet (0C), t1 holding time (h);
  • T2 insulation temperature (0C/h);
  • Maximum temperature of T3 discharge (0C), temperature rise rate of S1 (0C/h);
  • S2 cooling rate (0C/h).

Operation standard of heat treatment

  • (1) According to China’s boiler, pressure vessel and pressure pipeline standards, laws and regulations, the company’s heat treatment technical conditions are specified and used as the basis for formulating heat treatment process [8].
  • (2) Find the real object according to the process card, check the single piece number, instruction number, quantity, material and furnace batch number of the real object, and check whether the head has cracks, cracks, bulges, folds and other phenomena.
  • (3) Aluminum and its alloys, copper and its alloys and other non-ferrous metals and their alloys require electric furnace heating.
  • (4) Remove oil rust and blow dry stainless steel and its composite plates before heating, remove oil rust and blow dry non-ferrous metals and their alloys before heating, and remove oil rust and blow dry alloys and their composite plates with high Ni content.
  • (5) Before entering the furnace, the composite layer of the composite plate shall face down.
  • (6) Integral heat treatment shall be adopted for heat treatment, and the temperature in the furnace shall be controlled within 50 ℃.
  • (7) The thermocouple shall be placed to correctly reflect the temperature in the heat treatment furnace.
  • (8) Three or more hooks shall be used when discharging to avoid head deformation.
  • (9) The placement drawing shall be made after charging to prevent the loss of marks caused by the falling off of surface oxide scale after heating.
  • (10) For some changeable thin-walled large-diameter heads, corresponding deformation measures shall be taken to prevent deformation.
  • (11) Select the correct heat treatment type and process parameters according to the process requirements.
  • (12) When the heated head enters or exits the furnace, the temperature in the furnace shall not be higher than 400 ℃ (except austenitic stainless steel).
  • (13) Carbon steel, stainless steel, non-ferrous metals and their alloys shall not be heat treated in the same furnace
  • (14) Check the instruction number, piece number, furnace batch number, material, etc. on the head. If it is lost, transplant it again according to the process card.
  • (15) The heat treatment report and records shall be filled in timely after the furnace is discharged, and can be transferred only after passing the review by the reviewers
  • (16) After the inspection, sign your name on the process card and check with the process card before circulation, which shall be approved by the next process.

Polishing process of head

Operation steps of polishing process

  • (1) Determine the head to be polished according to the process requirements of the flow card.
  • (2) Rotate the head to the table, determine the level, and clamp the head.
  • (3) Polishing from edge to center, or from center to edge; Determine the upper and lower speeds of the beam and the front and rear moving speeds of the workbench according to the diameter of the head.
  • (4) Check polishing and rework as appropriate.

Polishing operation standard

  • (1) Confirm the instruction number, specification, material, furnace batch number, quantity, etc. of the product according to the process card and the physical object, and check whether the workpiece has defects such as bulge, crack and delamination, as well as whether it is out of round. If any problem is found, contact and deal with it in time, check the appearance, determine whether manual treatment is required, and mark the hook on the head before polishing.
  • (2) Select the polishing plate and necessary auxiliary tools according to the polishing requirements.
  • (3) Confirm the weight of the head, select appropriate spreader according to the weight, and check whether the spreader is safe.
  • (4) Lift the head onto the workbench, adjust the center, and ensure that the upper opening of the head is basically horizontal (0-5 deviation).
  • (5) When polishing, people should be on site and pay attention to the polishing situation, especially when polishing the center of the circle, the speed should be fast. When polishing from the center to the outside, the pressure should be increased a little, and when polishing from the side to the center, the pressure should be reduced a little. When polishing the thin-walled head, install the shockproof wheel according to the situation, and pay attention to whether the machine is abnormal in operation.
  • (6) After polishing, check the head for defects and conduct necessary contact treatment.
  • (7) Wrap the polished head with a film, sign your name on the process card, circulate and store it together with the head and process card, and it can only be approved by the next inspection.

Manufacturing defects of head

In the manufacturing process of head, defects often appear, such as pitting, scratch, bulge, deformation, orange peel phenomenon, sudden change of weld edge angle, etc. [9]

  • (1) Pitting: pitting may be caused by pitting on the surface of the plate itself, or by unclean treatment of the die head in various processes of stamping, drum pressing and spinning.
  • Solution: light pitting can be polished. Repair welding shall be carried out if it exceeds 0.2mm. After repair welding, polish it quickly.
  • (2) Scratch: mainly caused by improper operation during handling. For example: hook scratch, forklift scratch.
  • Solution: firstly, pad the protective film on the contact surface between the hook and forklift and the head. If there is a scratch, polish it, and repair welding if it is serious. If the scratched area exceeds 25%, it shall be treated as a whole.
  • (3) Bulge: because the blank holder force of the blank holder die is too small, it may also be caused by uneven material thickness.
  • Solution: using professional drum finishing machine, if it can’t be corrected, it needs to be corrected by spinning, bit by bit.
  • (4) Deformation: due to the inconsistency between the large piece of the disc and the actual thickness of the joint itself, uneven punching force or uneven cold and heat during heat treatment.
  • Solution: the deformed head can be heated by using the principle of thermal expansion and cold contraction, and then cooled quickly with water, or it can be corrected by spinning.
  • (5) Orange peel phenomenon: most of the orange peel phenomenon is due to the delamination of the plate itself, but it can not be observed by the naked eye. The orange peel phenomenon can only be found in the stamping and drum pressing process.
  • Solution: the shallow orange peel can be cleaned by slight grinding. The deeper orange peel needs repair welding and grinding after repair welding.
  • (6) Thinning rate: the thickness of the formed head is uniform, which can avoid stress concentration and increase the service life of the head. The processing allowance [10] is determined according to the manufacturing process to ensure that the thickness of the convex head and hot drum section after forming is not less than the nominal thickness of the part minus the negative deviation of the steel plate.

Incoming processing of head

The significance of the processing of incoming materials

The processing of incoming materials is an important way to expand our business. Many small factories, because of equipment or technical defects, turn over the business at hand to our factory for production, and earn the difference from them. Similarly, the processing of incoming materials is not our own material, but the attention is very high. Because in the head production project, in case of quality accidents such as tearing of the round pieces processed by the incoming materials, the compensation required by our factory is not only a piece of disc, but also the reputation and negative effects.

Working standard of incoming material processing

  • (1) The incoming plate must have a formal material certificate, and the plate shall be marked with obvious code spraying or marked transplantation
  • (2) The thickness of the incoming plate must meet the technological requirements of the product
  • (3) P, S, C, Cr, content in re inspection material certificate.
  • (4) The surface of the incoming round sheet shall be kept clean, and there shall be no splashes, burrs and other dirt.
  • (5) The outer circle of the disc shall be cut and smooth and polished. No defects such as notch, groove, burr, etc.
  • (6) The disc shall be flat. The deformation within one meter range shall not be more than 30mm.
  • (7) The weld on the inner surface of the punch and the outer surface of the edge pressing ring area shall be polished flush with the base metal.
  • (8) The roundness of the upper mouth of the conical head processed by folding edge shall not be greater than 1.25% Di.
  • (9) The unevenness of the upper mouth is greater than 20mm.
  • (10) The conical head of welding part cannot be changed and the angle of edge cannot exceed the standard.
  • (11) The steel stamp mark shall avoid the folded edge or small R transition area.
  • (12) The welding joint has been inspected by the user RT (Ray) to level II.

Summary

The head is made entirely around the process flow card, depending on the size of the head, such as φ 1700mmeha standard elliptical inner diameter head, formulate corresponding process card (see Appendix 2), make corresponding process, and dimension tolerance of forming head (peripheral tolerance, minimum thickness, diameter tolerance, section shape, out of roundness, total height tolerance, etc.). In the process of production, various defects are inevitable (pitting, scratch, drum, deformation, orange peel, etc.), as long as we strictly close the quality and treat the quality problems of each head correctly, I think the door of the head market will be more and more broad.

Source: China Head Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@steeljrv.com

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Reference:

  • [1] JB 4746-2002 head for steel pressure vessels [S]
  • [2] GB/T 4237-1992 hot rolled stainless steel plate [S]
  • [3] TSG R0004-2009 Safety Technology Supervision Regulation for fixed pressure vessels (s) Beijing Yueshi press.2009
  • [4] Wang Ying. Brief introduction to the manufacturing process of pressure vessel in our factory [a], science and technology information [C]. 2009. 568-575
  • [5] JB 4735-1995 “head for steel welding atmospheric pressure vessels” [s]
  • [6] JB 4732-1995 “analysis and design of head for steel pressure vessels” [s]
  • [7] GB 150-1998 head for steel pressure vessels [S]
  • [8] Wang Xingyan and gongjingwen. Quality control of pressure vessel manufacturing [a], Gansu science and technology vertical and horizontal [C]. Volume 38, 2009, issue 2.54-55
  • [9] Du Wenqiang. Prevention of pressure vessel accidents [a], special equipment and operation [C]. 2009, 96-97
  • [10] Yang Li. Necessity of giving minimum thickness in head design [a]. Design and calculation [C]. 2009, issue 2, page 28-30

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