The processing of pressure resistant cabin shells for underwater equipment is just about sealing

The team that makes underwater robot prototypes has paid tuition fees in the pressure chamber in nine out of ten cases. We often encounter this situation when reviewing drawings: after the cabin is processed, there is nothing wrong with the shallow water tank soaking in it. When we put on the pressure cylinder - the end cover position is leaking water,  when we open it, there is a fine mark on the edge of the O-ring groove that is exactly pressed against the sealing ring; Some cabin walls have uneven thickness,  when pressurized, the roundness changes  the sealing surface deviates accordingly. Many buyers thought that the processing of the pressure resistant cabin shell was just about "turning a cylinder, milling an end cover,  sealing it well". However, when it comes to deep-sea structural components, they realize that sealing is only the last test question. If any of the factors such as wall thickness, material, deformation,  chip removal are  controlled, water leakage will occur.

1、 The difficulty of the cabin is never just about being a cylindrical car

The structure of the pressure chamber looks simple: one cylinder with two end caps, a ring of bolts on the flange surface,  a sealing ring in the O-ring groove. But the processing logic is completely different  that of ordinary rotating bodies - ordinary parts exceeding the tolerance is a fitting problem, while cabin exceeding the tolerance comes at the cost of water ingress. The difficulty lies in four aspects: firstly, the flatness  roughness of the sealing surface,  O-ring sealing is extremely sensitive to micrometer level defects; The second is the uniformity of wall thickness. The instability of thin walls under external pressure is first caused by roundness deformation, followed by sealing failure; The third is the corrosion resistance of the material. Point corrosion  crevice corrosion in seawater will gradually destroy the sealing surface; The fourth is deep cavity chip removal  knife clearance. If the inner cavity is deep  the knife rod is suspended  elongated, the chip removal may be hindered, which can strain the inner wall.

2、 The sealing surface wins in the direction of the blade pattern, but loses in a fine mark

The O-ring static seal is filled with micro unevenness of the sealing surface after being compressed by the rubber ring: the smoother  flatter the surface, the more uniform the compression stress distribution. The roughness of such sealing surfaces usually needs to reach the Ra0.8 level,  the flatness is controlled within a few micrometers to a dozen micrometers according to the size of the cabin diameter. Two detailed drawings may  necessarily be written, the workshop must be managed. The first is the direction of the knife pattern - if there are residual patterns on the sealing surface that are perpendicular to the circumference of the O-ring, it is equivalent to leaving a capillary channel for seawater,  the pressure cylinder can still infiltrate with qualified roughness; The final processing follows the circumferential direction of the sealing ring  is formed in one cut to avoid cutting marks. The second is the size matching of the groove - the groove width, groove depth,  groove bottom fillet should match the rubber ring diameter  compression rate,  the groove should be free of burrs; The bottom of the groove is rough, the groove width is narrow,  the rubber ring is twisted  rolling. It works fine in shallow water but fails in deep water. Don't just rely on inflating  soaking water on the detection end - the bubble method can only qualitatively detect obvious leaks,  high-grade water tightness usually requires helium mass spectrometry leak detection. When evaluating outsourcing suppliers, it is possible to directly confirm whether the factory has helium detection capability  whether it can simultaneously issue roughness  flatness detection reports. This condition can screen out a large part of processing plants that are  suitable for deep-sea working conditions.

O-ring static seal

3、 External pressure applies to the entire cabin: uniform wall thickness is more important than nominal wall thickness

The failure mode of external pressure vessels is  rupture, but instability. Once the thickness of the cylinder wall is uneven, the thinnest part will first be partially depressed. As soon as the roundness runs, the flange surface of the end cover will deflect,  the compression of the O-ring will be uneven instantly, causing the seal to break - so this iron rule must be remembered: uniform wall thickness is more important than the nominal wall thickness.

Corresponding to several things in terms of craftsmanship. The precision machining of the cylinder requires controlling the tool: the deep cavity tool holder is suspended  elongated,  the cutting force causes the tool to slightly retreat. The inner cavity is prone to taper  bulge, so shock-absorbing tool holders, rough  fine cutting,  internal cavity support should be added if necessary. The processing stress should be released: leave a natural aging time after rough machining before precision machining to avoid slow dimensional drift during the placement of the finished product. Caution should be exercised when welding structures: the flange of the cylinder body is welded,  the weld seam itself is a weak point of strength  corrosion. High pressure chambers are increasingly using integral material processing to minimize the pressure on the weld seam. Deep cavity chip removal also needs to be monitored - titanium alloy  stainless steel chips accumulate at the bottom of the cavity  are repeatedly cut, causing inner wall scratches  accelerated tool wear, which affects the quality of the sealing surface. In project practice, Huiwen Zhizao will confirm the direction of the cutting pattern, the tolerance range of the wall thickness,  the stress release cycle in advance during the DFM drawing review stage when processing sponge titanium deep-sea cabin orders,  will  remedy them after the start of construction.

4、 Material selection follows the trend: aluminum, 316L,  titanium alloy?

The selection of materials follows the depth, weight,  budget,  each of the three types of materials has its own location:

Selection reminder: Scratches on the anodized layer should be rechecked with emphasis; Don't just compare the unit price of materials, calculate the depth  deployment period first; Titanium alloy parts should be sourced  factories with mature cutting parameters; When mixed, insulation isolation should be done to prevent galvanic corrosion.

In actual selection, galvanic corrosion should also be considered - when titanium, stainless steel,  aluminum are mixed, insulation isolation design should be used,  the expansion coefficients of end caps, penetrations,  cylinders should be matched as much as possible. After the material is determined, whether the processing plant settles the corresponding material complete process plan is more crucial than simply selecting the grade, especially for titanium alloy shells: no matter how high the material procurement cost is, if the thermal deformation  sealing surface quality are  well controlled during the processing stage, it will still cause project losses.

Material selection  selection

5、 5 things to confirm before quoting

1. Working water depth  external pressure level

The test pressure, safety factor,  holding time should be clearly stated,  the cabin wall thickness should be calculated according to this. The words "for deep sea use" are equivalent to no conditions given.

2. Requirements for sealing surface testing

What level of roughness  flatness are required, whether helium mass spectrometry leak detection  reporting can be done,  whether high-grade watertight parts can be quoted before delivery.

3. Material grade  anti-corrosion scheme

6061  7075, 316L  titanium alloy, anodizing  passivation, the processing  testing costs of different materials vary greatly.

4. Overall processing  welded structure

It is recommended to process the entire material of the deep-sea high-pressure chamber to save welding seams; For cost sensitive shallow water tanks, welding can be discussed again, but the flange surface must be fully precision machined after welding.

5. Consistency between sampling  batch production

Do you need a full-size report for the first piece, how to conduct batch inspections,  whether to keep process records for repeat purchases? Records of high-risk parts are more useful than verbal promises.

Common Misconceptions

The first misconception is that "being able to seal" is equivalent to "being able to withstand pressure"

Sealing is just the last test question. The instability caused by uneven wall thickness  the dimensional drift caused by the failure to release rough machining stress all occur before sealing - the water leaking  the pressure cylinder is mostly   the sealing ring of the pot.

The second misconception is that testing only involves soaking in water to see bubbles

The bubble method can only qualitatively detect obvious leaks,  for high-grade water tightness, helium mass spectrometry leak detection is required. The roughness  flatness reports of the sealing surface should also be included with the shipment; Relying on "soaking all night without any leakage" for acceptance is equivalent to leaving the risk behind after deployment.

The third misconception is that materials are selected based on unit price  mixed regardless of galvanic corrosion

Titanium, stainless steel,  aluminum are directly immersed in seawater,  the potential difference will concentrate  bite off the more reactive metals. Mixing requires insulation isolation,  the expansion coefficients of the end caps, cabin penetrating components,  cylinder bodies must also match. It is only when assembly is  possible that it is discovered that they do  match.

Frequently Asked Questions

Q: How to choose the welding structure  overall processing of the pressure resistant cabin shell?

Shallow water, low pressure,  cost sensitive cabin structures are acceptable for welding, but the welds need to be inspected  post weld treated,  the flange sealing surface must be precision machined as a whole after welding. It is recommended to process the deep-sea high-pressure cabin as a whole - milling  shaping the bar  forging as a whole. Without welding seams, there will be one weak point of strength  corrosion,  the long-term reliability of deployment is much higher; The material cost is higher, but it saves the risk of inspection  repair,  this money is worth spending.

Q: Is it enough to make the O-ring groove according to the standard size? Do we still need to verify it?

The standard groove type provides the reference size,  the actual compression rate should be checked together with the sealing form (radial  axial), pressure rating,  rubber ring hardness. It is common for drawings to only indicate the width  depth of the groove, without specifying the specifications  compression rate of the rubber ring. It is necessary for the processing factory to calculate the reverse during the DFM stage: if the compression amount is too large, the rubber ring will permanently deform,  if it is  tightly compressed, deep water leakage will occur. A reliable precision component manufacturer in Shenzhen will confirm the matching relationship between groove type  rubber ring before quoting, instead of blindly processing according to the drawing.

Q: Titanium alloy shell is much more expensive than stainless steel. Under what circumstances is it worth it?

Consider two constraints: depth of penetration  weight. Long term deployment in the deep sea  installation on AUV/UUV are sensitive to endurance  deployment weight. Titanium alloy parts reduce weight by about 40%  are corrosion-resistant  almost maintenance free, making them cost-effective overall; 316L stainless steel is a more practical choice for mid to shallow sea equipment that is fixed on underwater platforms, insensitive to weight,  has limited budget. But regardless of the material, the quality of the sealing surface  the uniformity of the wall thickness cannot be reduced - the money saved on the material cannot compensate for the loss caused by a single seal failure.

Q: Is it necessary to do both helium testing  three coordinate reporting?

Check the risk level. Shallow water short cycle operation parts, the first full-size report  batch sampling inspection are acceptable; For long-term deployment in deep sea, it is recommended to test the key sealing surfaces one by one, cover each piece with helium testing,  keep the report with the goods. The ratio of inspection fee to the value of the entire cabin equipment is zero, don't save it.

Q: What types of underwater equipment parts projects can Huiwen Zhizao undertake?

The cabin, end caps, penetrations,  acoustic cabin components of underwater robots  marine equipment can be assembled  single piece sampling to medium to large-scale production. Mature cutting solutions are available for 316L stainless steel, titanium alloy (including sponge titanium),  aluminum alloy; The company provides hardware component services that integrate design, intelligent manufacturing,  assembly. Software, algorithms,  control are led by customers,  Huiwen Intelligent Manufacturing collaborates to do a good job in hardware. The process review of CNC machining for underwater equipment can be initiated during the quotation stage.

Summary

The difficulty of cabin processing essentially lies in the goal of "continuous  reliable sealing under high pressure", which can be divided  four stages: sealing surface, wall thickness, material,  testing: sealing surface tube holds the direction  roughness of the knife pattern, wall thickness tube holds the uniformity  stress release, material selection follows the depth  weight,  testing is upgraded  water immersion to helium detection  reporting. The suggestion of Shenzhen Huiwen Zhizao for CNC machining of underwater equipment is to clearly explain the working water depth, sealing grade, material grade,  testing requirements during the research  development stage of the drawings, align each item before quoting,  eliminate the risk of water leakage during the review stage. Welcome to provide drawing consultation. We can provide 2D drawings, 3D models,  technical requirements to obtain DFM process evaluation  itemized quotations.