Why PTFE gasket cannot be used to seal FRP, plastic and other low load flange?

Distributors and manufacturers of gaskets are often asked why PTFE gaskets cannot be used to seal fiberglass reinforced plastics (FRP or FRP), plastics and other low load flanges. Usually, the answer is that there is too little load on the gasket. The answer is simple, but the solution is complex. Unfortunately, in most cases, the gasket is generally considered to be the cause of the failure.

Traditional gasket applications

After all, the application of FRP and plastic pipe in chemical treatment industry is due to its low cost and good chemical resistance. However, none of these materials have the strength of metal pipes.
Therefore, the strength of the flange limits the bolt torque that can be applied, thus limiting the compressive stress applied to the gasket. Generally, rubber gasket is always used to seal this kind of pipeline. Rubber gasket has good resilience and can seal under low load.

When the compression is only 15%, the rubber gasket can also play a sealing effect. However, due to the limited chemical resistance of each rubber elastomer, it is necessary to carefully select the appropriate rubber material. However, rubber of different materials often have the same color, which is easy to be confused. When installing, the wrong gasket often occurs.

20191023233410 46988 - Why PTFE gasket cannot be used to seal FRP, plastic and other low load flange?

Figure 1. Maximum torque provided by nonmetallic flange manufacturer.

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Fig. 2 Cracks of nonmetal flange when the pressure stress is too large.

20191023233448 43260 - Why PTFE gasket cannot be used to seal FRP, plastic and other low load flange?

Fig. 3 When using PTFE gasket, the pressure stress is not enough and easy to leak.

Current requirements for gasket

In today’s chemical treatment plants, a variety of chemicals are used, and many of them are under high temperature and pressure conditions. This requires that the gasket material be changed from rubber elastomer to material with better chemical corrosion resistance.
PTFE is a good sealing material, able to withstand a variety of commonly used strong chemical corrosion products, pH range covers 0-14, except for molten alkali metals and fluorine elements (especially in high temperature and pressure conditions).
Current general standards
Like metal pipes, most FRP and plastic pipe manufacturers are also manufactured in accordance with ASME standards. ASME’s updated standard rtp-1-2011 for reinforced thermosetting plastics (RTP) corrosion-resistant equipment provides the correct guidance for the design of vessels with operating pressure of 15 psig (1 bar).
The non mandatory annex nm-9 “installation of containers made of reinforced thermosetting plastic materials” provides guidance on gasket installation, fasteners, torque, lubrication and other factors, which provides an important reference for the effective connection of flange bolts. Although it is not mandatory, most of these guidelines are used when discussing the use of gaskets for equipment in the industry.

Table 1 nm9-3, flange bolt tightening force

Recommended bolt torque, non-metallic flange (hand lay up)

Pipe size (inches)

Bolt diameter (inches)

Torque (FT lb)

2

5/8

25

3

5/8

25

4

5/8

25

6

3/4

25

8

3/4

25

10

7/8

25

12

7/8

25

14

1

30

16

1

30

18

1(1/8)

35

20

1(1/8)

35

24

1(1/4)

40

Manhole

3/4

30

From ASME RTP-1-2011 (corrosion resistant equipment for reinforced thermosetting plastics), licensed by the American Society of mechanical engineers, all rights reserved worldwide.

20191023233349 66425 - Why PTFE gasket cannot be used to seal FRP, plastic and other low load flange?
Figure 4. Bolt tightening sequence case.

Requirements for gasket load

Although some gasket designs allow them to seal under low loads, most PTFE gasket manufacturers have calibrated their materials for the minimum compression stress (between 2500psi and 3000psi, 17.25mpa-20.7mpa).
However, the typical FRP and plastic pipe joint flange usually can withstand the pressure stress value and temperature less than this calibration value of gasket. For standard FRP flanges and manholes, figure nm9-3 of standard rtp-1-2011 provides recommended torque and bolt tightening procedures.
However, there is no suitable solution to this problem. If PTFE gasket is used to replace rubber gasket for equipment or pipeline connection, the gasket is difficult to play a sealing role unless the recommended guidelines are changed.
This is the main reason that end customers often encounter gasket seal failure. The improved method can be that the equipment manufacturer can improve the flange strength to withstand the greater bolt tightening torque to provide the pressure stress required for gasket sealing, which requires the flange to be changed in the design stage before the equipment is sold.
The reality is that most manufacturers just make some adjustments to ease this demand.
Even when the torque is increased to provide a suitable gasket load, many additional factors may affect the load. Since the load applied by the end customer to the gasket is very low, any factor that may reduce the load applied to the gasket is likely to cause seal failure. Factors that may affect the load include:

  • 1. Flange dislocation, axial and angular
  • 2. Lubrication and a correct lubrication friction coefficient (nut coefficient)
  • 3. Quality and grade of fasteners
  • 4. Use old fasteners
  • 5. Accuracy of torque wrench used.

Any of these factors may determine whether the end customer has a successful bolted flange gasket sealing system. Therefore, both the end customer and their installation company must be aware of the above factors and eliminate the possible impact of these factors on the gasket seal.

Recommendations and Solutions

In the face of leakage, the first reaction of field workers is usually to continue tightening the bolt torque until the leakage is reduced.
Often, this will not cause cracks or destructive cracks in the flange immediately, but it increases the stress at the flange connection, which may lead to cracks in subsequent use.
FRP pipe is like “movable” pipe. When it moves, vibrates, shrinks and expands continuously, overload stress on flange may lead to premature connection failure and shorten equipment life.
If an end customer has a problem, the best advice is to improve the torque applied and the alignment of the two flanges as much as possible so that the gasket can obtain the required stress. New bolts are required for each flange connection.
Lubricate all bearing surfaces, including washers, under bolt heads, and threaded surfaces. Using a precise, calibrated torque wrench, follow the 5-foot LBF progressive torque tightening method recommended in rtp-1-2011.
If the target torque value is very low, gradually tighten to the target torque value using 25%, 50%, 75% and 100% progressive criteria. After the last tightening at 100% torque along the circumferential direction, at least 4 hours later, tighten it in a clockwise direction with 100% torque value.
Note: an equipment manufacturer should consider the matching sealing gasket before completing the equipment design. If the gasket is made of PTFE or the equipment needs PTFE gasket, the manufacturer must consider various factors beyond the recommendation of rtp-1-2011 specification and eliminate the problems caused by these factors.
Otherwise, the terminal customer in question may exert too much torque, resulting in bolt flange connection failure or premature stress fatigue. FSA gasket manufacturers can help solve these or other sealing problems.

Source: China Gaskets 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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why ptfe gasket cannot be used to seal frp plastic and other low load flange - Why PTFE gasket cannot be used to seal FRP, plastic and other low load flange?
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Why PTFE gasket cannot be used to seal FRP, plastic and other low load flange?
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Distributors and manufacturers of gaskets are often asked why PTFE gaskets cannot be used to seal fiberglass reinforced plastics (FRP or FRP), plastics and other low load flanges. Usually, the answer is that there is too little load on the gasket. The answer is simple, but the solution is complex. Unfortunately, in most cases, the gasket is generally considered to be the cause of the failure.
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www.steeljrv.com
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