Persistent Issues with Tank Container PTFE Seals? Deconstructing the Root Causes of Leakage and Deformation Failures

Solid, dense PTFE gaskets offer excellent chemical corrosion resistance and perform reliably under conditions of flat flange surfaces, adequate bolt preload, and static stability. Sealing points on tank containers range from external flanges and manholes to internal valve seat seals (e.g., in butterfly, ball, and main valves).

As mobile storage and transport equipment, tank containers are subjected to transport vibrations, fluid loading/unloading, temperature and pressure fluctuations, and repeated assembly/disassembly and cycling; consequently, various seal failures frequently occur in the field. This article analyzes the underlying causes of different types of leakage—including typical cases of extreme valve deformation failure—by addressing common application‑related questions.

Ⅰ. Sealing Issues Related to Mechanical Operating Conditions

PTFE possesses a non‑crosslinked molecular structure; under pressure, it undergoes creep (cold flow), resulting primarily in permanent plastic deformation. Transport vibrations and fluid impact forces exacerbate this inherent weakness. Furthermore, valve seat seals—unlike static flange seals—must withstand not only static compression but also the squeezing forces of opening/closing and the effects of friction and erosion.

Question 1: Why do some tank containers leak while others function normally when using the same type of PTFE seal?

This involves two distinct scenarios: flanges/manholes and internal valve components.

1. Flanges and Manholes:

   Repeated handling and assembly of tanks can lead to scratches, dents, slight warping, or corrosion on sealing surfaces. Due to its hardness and limited compressibility, PTFE cannot fill these microscopic defects, creating leakage paths. While seals perform reliably on pristine flanges, damaged flanges are prone to leakage. Gaskets cannot rectify flange warping, severe corrosion, or misalignment; hardware damage must be repaired first. For flanges with poor surface conditions, expanded PTFE (ePTFE) gaskets may be considered as an alternative.

2. Valve Seats (Butterfly Valves, Ball Valves, etc.):

   Inconsistent performance of identical PTFE valve seats stems from factors such as valve machining precision, wear on the valve core (plug/ball), and the presence of solid particles in the medium. Particles become embedded in the PTFE surface, and the opening/closing action of the valve plug scratches the sealing surface; misalignment of the valve plug causes localized stress overload and creep‑induced crushing of the seat, resulting in internal leakage.

Question 2: Why does leakage occur after storage or long‑distance transport, even though pressure testing was successful?

This is a failure caused by creep/cold flow, which can occur in both the flange assembly and the valve internals.

• Flange: Under continuous bolt compression, the PTFE slowly flows laterally and thins; this causes a loss of bolt preload and sealing contact pressure, leading to external leakage.
• Valve internals: The seat is subjected to long‑term preload from the valve plug; transport vibrations accelerate stress relaxation and cold‑flow deformation of the seat, creating a gap between the plug and the seat that results in internal leakage.

Creep is the result of a coupled temperature‑compressive stress effect: the higher the temperature and the greater the contact pressure, the faster creep progresses.
Once gaskets and seats have been compressed in service, they undergo permanent deformation; reuse is prohibited, and new parts must be installed during every maintenance disassembly.

Common misconception: Simply increasing bolt torque or valve preload will exacerbate cold flow and accelerate leakage.

Typical tank container scenario: No leakage during factory pressure testing, but external or internal valve leakage occurs after sea/land transport or prolonged static storage.

Improvement strategy: For applications requiring creep resistance, prioritize the use of filled/modified PTFE or PTFE‑encapsulated gaskets.

Question 3: Why are PTFE seals prone to failure under conditions involving vibration and thermal cycling?

PTFE has extremely poor resilience; its deformation is primarily plastic and permanent.

• External flange: Transport vibrations combined with thermal expansion and contraction cause minute changes in the flange gap; the gasket cannot compensate for this displacement, leading to a loss of sealing stress and external leakage.
• Valve internals: Vibration causes the valve plug to grind against the PTFE seat; mismatched coefficients of thermal expansion among the valve body, plug, and PTFE lead to a loss of sealing contact pressure and internal valve leakage.

Common tank container scenario: All static pressure tests pass, but internal valve leakage occurs after delivery to the customer's site.

Question 4: Insufficient bolt clamping force prevents a proper seal; why does internal leakage occur after the valve is opened and closed a few times?

1. Flange side:

   Pure PTFE gaskets require high specific sealing pressure to establish a seal. Some tank container flanges have a limited number of bolts, resulting in insufficient clamping load to achieve effective contact; ePTFE (expanded PTFE) gaskets could be evaluated as an alternative.

   SunPass ePTFE sheet conforms to irregular flanges and shows improved creep resistance versus solid PTFE. It provides robust chemical & thermal stability, unlimited shelflife without ageing or embrittlement, and performs reliably under thermal cycling and aggressive fluids.

2. Valve side:

   In addition to static compression, PTFE valve seats are subjected to frictional shear forces during opening and closing. Pure PTFE valves have poor wear resistance; repeated cycling causes the seat surface to fuzz and wear down, compromising the sealing line and leading to internal leakage. This type of wear‑related failure is specific to valve sealing interfaces and does not occur with flange gaskets.

Question 5: Why does sealing performance deteriorate over time for frequently disassembled manholes and frequently cycled ball or butterfly valves?

PTFE undergoes irreversible permanent deformation when subjected to pressure or compression.

• Manhole flanges: Opening the cover compresses the gasket, causing deformation and failure; the gasket must be replaced during every maintenance cycle.
• Valve internals: Opening and closing the valve compresses and rubs the seat against the valve core (or disc). After repeated operation, the seat develops indentations and wear, leading to gradual internal leakage even without disassembling the valve.

Typical extreme failure case: Repeated loading/unloading of high‑temperature media via a butterfly valve leads to seat deformation, preventing the valve from closing tightly.

Tank containers use butterfly valves for repeated loading and unloading of high‑temperature media; the medium remains at a high temperature, and the valve undergoes frequent opening and closing cycles. High temperatures significantly accelerate PTFE seat creep. Each closing action exerts compressive stress on the PTFE primary sealing seat. Under the combined effect of high temperature and repeated compressive cycling, the PTFE seat undergoes irreversible plastic flow and localized bulging deformation. Eventually, the deformed bulge physically obstructs the valve core, preventing it from reaching the fully closed position. This results in a failure to close completely—a structural deformation failure rather than just simple internal leakage.

Root Cause: High temperatures amplify the creep effect, compounded by cyclic compressive loads; standard virgin PTFE seats are unsuitable for these operating conditions and should be replaced with creep‑resistant, filled/modified PTFE seats.

Note: "Creep‑resistant modification" is not a single formula; for high‑temperature, high‑frequency butterfly valves on tank containers, chopped carbon fiber‑filled PTFE is the preferred choice. Graphite, glass fiber, and glass microspheres each have limitations; the material must be matched to the specific medium, and one cannot simply specify "modified PTFE" in general terms.

• Carbon fiber filling: Offers superior creep resistance; suitable for high‑temperature and high‑frequency cycling; avoid use with strong oxidizing media.
• Graphite filling: Good lubricity but weaker creep resistance; prone to generating black particles that contaminate the medium.
• Glass fiber/glass microspheres: Not resistant to hydrofluoric acid or high‑temperature, concentrated strong alkalis; glass fibers can also cause abrasive damage to the valve core sealing surface.

SunPass offer professional application‑condition analysis service. Share your operating temperature, pressure, cyclic load, media & concentration, equipment type, actuation frequency and existing pain points such as creep, contamination, leakage or short service‑life.

Our engineers evaluate virgin PTFE, ePTFE and multiple filled‑PTFE grades to deliver custom recommendations. Assess risks upfront to prevent wrong‑specification‑caused leakage, cargo loss and downtime.

Ⅱ. Questions Regarding Temperature and Chemical Compatibility

Question 6: Product literature states PTFE has a temperature rating of -200 to 260°C; why do gaskets and seats still deform and leak under high‑temperature conditions?

Important Distinction: The -200 to 260°C range refers to PTFE's chemical and thermal stability (meaning the material does not decompose), not its ability to function reliably as a seal over the long term; chemical stability does not guarantee a leak‑free seal.

When virgin (unfilled) PTFE is subjected to continuous pressure, creep worsens with the temperature gradient:

• 80–120°C: Creep begins; tank containers cannot be retightened on‑site, and valves lack in‑line preload compensation; combined with vibration, the risk of leakage increases.
• 150–180°C: Significant creep range; tests show preload loss exceeding 60% after just a few hours at 150°C; while stationary equipment allowing for retightening might barely manage, long‑term operation of tank containers in this range is not recommended.
• Near 200°C: A mechanical "tipping point" where creep worsens severely; permanent deformation of gaskets and seats accelerates rapidly, leading to a loss of sealing contact pressure; leakage occurs even if the medium is chemically inert to the material, and vibration or thermal cycling further increases the probability of failure.

• 260°C: The upper limit for chemical resistance, but mechanical sealing fails; permissible only for very short durations and very low pressures; long‑term use in tank containers is prohibited.
• 327°C: PTFE reaches its melting point; the material softens and collapses, completely losing its sealing capability.

Note: The 230–240°C range represents a short‑term peak limit for stationary equipment where periodic re‑tightening is possible. Tank containers are mobile equipment; flanges cannot be re‑tightened, and valves do not allow for in‑service preload compensation. Therefore, this long‑term temperature rating does not apply. While momentary peaks of 230–240°C are permissible, continuous operation at these temperatures is strictly prohibited.

Creep is influenced by the combination of temperature and compressive stress; at a given temperature, higher compressive stress leads to more severe cold flow. Vibration accelerates stress relaxation, making the risk for tank containers significantly higher than for static equipment.

Question regarding low temperatures: PTFE is rated for -200°C; why do leaks occur during low‑temperature transport in winter?

While the base PTFE material performs adequately at low temperatures, the hardness of gaskets and valve seats increases, reducing their ability to compress and conform. Leakage becomes more likely if there are minor flange defects or clearances in the valve core fit.

Question 7: The medium is non‑corrosive to PTFE and the flange is not leaking, yet the valve exhibits slow internal leakage. What is the cause?

Two failure mechanisms:

1. Physical permeation: Medium molecules penetrate the PTFE matrix; a pressure differential across the valve causes slow permeation leakage. This is not a result of material corrosion.
2. Physical wear/damage: Solid particles carried by the medium scour and scratch the valve seat sealing surface, damaging the seal and causing internal leakage. Flange gaskets are not subjected to this type of shear friction from the medium and do not suffer similar damage.

Incompatible media: Molten alkali metals, high‑temperature active fluorine gas, chlorine trifluoride, and other strong fluorinating agents directly destroy PTFE molecular chains; use of PTFE gaskets or valve seats is strictly prohibited in these applications. Material selection cannot be based solely on chemical compatibility.

Ⅲ. General Points on Tank Container Operation and Maintenance

• During maintenance, clean impurities from sealing surfaces; prioritize the repair of warped flanges or scratched valve cores, and remove solid particles from the medium. For butterfly valves subjected to high temperatures or frequent cycling, inspect the PTFE seat closely during disassembly for bulging or localized deformation; if deformation is found, replace the seat immediately—do not attempt to repair or reuse it.
• When tightening flanges, use a torque wrench and a cross‑pattern, incremental tightening sequence; avoid rapid tightening on one side or over‑torquing. During valve assembly, strictly control the valve seat pre‑compression; for high‑temperature and high‑frequency cycling applications, do not rely on increasing preload force to improve sealing performance.
• Replace flange PTFE gaskets during every maintenance overhaul; it is recommended to replace PTFE valve seats simultaneously when dismantling valves for maintenance—do not reuse old parts.
• Tank containers are mobile equipment; since on‑site retightening to compensate for preload loss is often impractical, risks regarding creep and vibration must be assessed during the selection phase rather than relying on later remediation. For butterfly valves subject to high temperatures and frequent cycling, prioritize filled/modified PTFE valve seats to avoid structural failures—such as creep‑induced bulging or the inability to fully close the valve—associated with pure PTFE seats.
• For long‑distance transport or long‑term storage of tank containers, perform internal valve leakage tests in addition to checking for external flange leaks. During high‑temperature loading and unloading operations, verify that the valve fully reaches its closed position to ensure the valve core seats correctly.

IV. Guidelines for Supplier Selection

Select products made from virgin PTFE resin and reject recycled materials; recycled materials often have high porosity, high permeation risks, and inconsistent batch quality.

When sourcing critical PTFE seals for ISO tank containers, poor‑grade recycled material, vague material specs and sample‑to‑bulk quality gaps create major operational risks. SUNPASS aligns with rigorous global procurement requirements.

SUNPASS use 100 % virgin PTFE resin only and reject recycled feedstock that causes high porosity, permeation and inconsistent batches. Complete documentation including COC and MSDS is available per order.

SUNPASS clearly separate static flange gasket grades from dynamic valve‑seat materials. For high‑temperature, high‑frequency butterfly valves, SUNPASS specify exact filler type and ratio instead of generic “creep‑resistant modified PTFE”. Application limits for carbon‑filled and glass‑filled formulations are well‑defined against aggressive media.

For critical projects, creep‑wear‑compression test data and pre‑production sample validation are supported.

As a direct manufacturer since 1979 (not a trading company), SUNPASS control full‑process quality to avoid sample‑bulk deviation. Our engineering team also offers professional application‑condition analysis for your sealing challenges.

Reach out for material traceability documents and technical support.