Threaded seals

Seals for threaded connections.

Unfortunately, not one of the threaded connections has absolute tightness, this is especially true for gas and high-pressure pipelines. To solve the problems of sealing threaded connections, various seals are used.

1. Linen strand

Linen strand with red lead on drying oil is still a very common method of sealing threaded connections. Practically on this method, the entire installation of water pipes, heating systems and gas pipelines is based, since this is indicated in the SNiP for plumbing systems and for gas supply. There are other methods of using the flax span, for example, in combination with various sealants and pastes, the main task of which is to prevent corrosion processes in the threaded joint and to extend the life of the seal. Iron red is often used, which, unlike red lead, does not protect the thread surface of steel fittings from corrosion. It is also known to replace this type of seal with a "dry strand", that is, without any paint, which causes significant corrosion of the thread, especially when used on heating and water supply systems.

The disadvantages of using this method of sealing are obvious - the complexity and the need for careful adherence to technology, large tightening efforts of fittings during assembly, low manufacturability. At high temperatures, for example when sealing heating systems, thermal destruction of flax fibers is possible. Also, in some cases, the disadvantage may be the difficult dismantling of the connection.

However, the linen strand is still used mainly due to its low cost and availability.

2. Sealants

Tightness is achieved by filling the turn-to-turn space of the threaded connection with a special substance selected in accordance with the operating conditions of the system. In general, today, sealants are the optimal sealing method, combining ease of installation, corrosion protection and efficiency. Can be divided into three main classes: non-hardening sealants, hardening sealants, and anaerobic sealants.

2.1. Non-hardening sealants

These are viscous pastes based on synthetic resins and polymers with fillers. They effectively protect metal surfaces from corrosion, and work well not only in threaded connections. Can be used in vibration conditions. Can be used in combination with various gasket materials. The obvious advantage is the ease of assembly and disassembly of the connection. The use of non-hardening sealants is not recommended for joints with high pressure, differential pressure and temperature. Not suitable for use in aggressive and explosive environments.

Fig. 2 Multipak non-hardening sealant

 

Table 1. Examples of non-hardening sealants

Name	Manufacturer	Characteristics
Loctite 5922	Henkel	Alcohol solution of modified rubber with fillers. Formation of gaskets in place. Sealing flanges, fittings and other connections. Working temperature: from -50° to +200°С.
Permatex 80633	Permatex	Maintains plasticity at both high and low temperatures. Resistant to most industrial fluids. Working temperature: from -54° to +149°С.
MULTIPAK	Unipak	Can be used at a maximum pressure (max pressure 4.4 bar and a temperature of no more than +70°C), drinking water (max pressure 16 bar and a temperature no more than +85°C), heating (max pressure 7 bar, T max 130 ° C).
ROCOL FOLIAC GRAPHITE PJC.	Ragosine Oil	Vegetable oil based sealant enriched with graphite for metal threaded and flanged connections. It is used in cases where it is necessary to supply air, steam, water and gas at high and low pressures. Primarily used for sealing joints under steam, air and water pressures up to 97 bar. It is used in a wide range of operating temperatures from -20°C to +450°C.
Permabond P2	Permabond	Water based sealant for pretreatment of threaded connections. Most commonly used for sealing threaded pipe connections and plugs, fittings for hydraulic and air brake systems, and valve blocks. Working temperature: up to +150°С

 

2.2. Hardening sealants

Typically, these are water-based or solvent-based pastes or gels that gradually harden after being applied to the threads. The hardening time depends on the composition of the sealant and can range from a few seconds to several hours. They protect surfaces well, allow easy and quick assembly of the joint, after drying, they reliably seal the joint due to resistance to extrusion. Depending on adhesion to surfaces and strength characteristics (shear loads), they are subdivided into sealants and adhesives. Adhesives are used in cases where it is necessary not only to ensure tightness, but also to fix the connection.

The disadvantages of this method are the inability to disassemble the connection without destroying the sealant, shrinkage during drying and during operation, low chemical resistance.

Fig. 3 OilSEAL hardening sealant

Table 2. Examples of hardening sealants

Name	Manufacturer	Characteristics
Loctite 5772	Henkel	Threaded hardening sealant for metals, especially for sealing threaded connections in nuclear power plants Working temperature: from -50 ° С to + 150 ° С.
Loctite 577	Henkel	Universal hardening sealant for coarse metal threads. Suitable for quick applications in low temperature environments, for example, for maintenance outside the factory. Working temperature: from -50°С to +150°С.
Permatex 56501	Permatex	Sealant for heavy duty threaded connections. Designed specifically for metal pipe threaded connections. The product replaces Teflon® type pipe seals. It cures quickly, after which it can withstand pressure up to 700 kg / cm2. Prevents abrasion and protects mating threaded surfaces from corrosion. Contains Teflon (Polytetrafluoroethylene). Working temperature: from -50 ° to 170 ° С;
UNITEC GS 20	Unipak	Strong locking compound for nuts, bolts, etc., non-removable joint. Working temperature: from -55 ° С to + 150 ° С.
ROCOL OILSEAL	Ragosine Oil	Sealant, in the form of a white thixotropic liquid based on organic resin and solvent, enriched with clay for a durable seal of metal threaded and flanged connections. Designed for sealing joints under pressure up to 138 bar of gasoline, diesel fuel, mineral and vegetable oils, paraffin and "white spirit". It is used in a wide range of operating temperatures from -50 ° C to + 200 ° C

 

2.3. Anaerobic sealants

ANAEROBIC is a word that characterizes the absence of oxygen or air, as well as the absence of dependence on oxygen or air.

A relatively new type of seal that is rapidly gaining popularity due to its unique set of performance properties.

Anaerobic sealants are acrylic polymers, which in their initial state are liquid compositions of various viscosities, consisting of acrylic monomers and oligomers, initiators, catalysts, stabilizers, dyes and other fillers. These compositions are able to be stored for a long time in an oxygen-permeable container without changing their properties and to form a homogeneous strong polymer mass in narrow metal gaps, such as the inter-turn space of the thread, the flange gaps, etc.

When the sealant enters the space between the metal surfaces, the initiators contained in the material begin to interact with the metal, starting the process of polymerization of acrylic monomers, with the formation of a polymer with a cross-linked structure. The intensity of the polymerization process depends on the intensity of interaction with the metal, and the speed can be increased by treating the surface with an activator. When using anaerobic sealants on products made of non-ferrous metals or alloy steels, as well as on surfaces with galvanized and other protective coatings, the polymerization rate is the lowest. Temperature also significantly affects the rate of polymerization. At low temperatures, polymerization slows down, and at high temperatures, it accelerates.

From the foregoing, we can conclude that it is most effective to use anaerobic compounds for sealing metal products, but thanks to activators, the scope is expanding, and modern compositions are used on ceramic, glass, polymer and other surfaces.

The advantages of anaerobic sealants are their high physical and mechanical properties, a wide operating temperature range (from –90º to +200ºС), good chemical resistance, manufacturability and ease of use. The big advantage is that the composition hardens only in the gap and the excess sealant can be easily removed. In some cases, anaerobic compounds have become truly indispensable. Unfortunately, it is not always possible to provide conditions for the effective use of these sealants.

Fig. 4 Anaerobic sealant Loctite 262

 

Table 3. Examples of anaerobic sealants

Name	Manufacturer	Characteristics
Loctite 245	Henkel	Sealing and bonding agent for medium-strength screw connections. Working temperature: from -50 ° to + 200 ° С.
Loctite 262	Henkel	Sealing and preventing self-unscrewing. Permanent fixation and sealing of large studs and bolts (up to M25). Working temperature: from -50 ° to + 150 ° С.
Permatex 51531	Permatex	Elastic sealing material with a permissible gap of less than 0.38 mm. It is allowed to use in cases where the installation of "anaerobic" seals (curing without air access) is stipulated. Working temperature: from -54 ° to + 149 ° С.
UNITEC GT18	Unipak	Universal polytetrafluoroethylene sealant, easily demountable joint. Working temperature: from -55 ° to + 150 ° С.
Permabond А126	Permabond	High strength anaerobic adhesive for long term assembly of coaxial connections or metal threaded parts. Used on tight fit parts. Working temperature: from -55 ° to + 150 ° С.
Permabond А129	Permabond	It is a thixotropic (non-dripping) medium strength anaerobic sealant. It is specially designed for coarse threads. Working temperature: from -55 ° to + 150 ° С.
AN 302-25	WEICON	Sealing of threaded joints of pipes and fittings, vibration resistant. Operating temperature: -60 ° C ... + 150 ° C
AN 305-86	WEICON	The sealing of threaded pipe connections is dismantled using conventional tools. Operating temperature: -60 ° C ... + 150 ° C

 3. Thread sealing tapes, cords and threads.

3.1. FUM (ФУМ) tape and FUM (ФУМ) harness

In Russia, FUM tape (fluoroplastic sealing material) produced according to TU 6-05-1388 is mainly widespread. It is intended for sealing threaded connections of technological pipelines through which corrosive liquids and gases are transported in the temperature range from minus 60 ° С to plus 200 ° С. It can be used in technological equipment of the pharmaceutical, food and medical industries, in pipelines for hot and cold water supply, including drinking. FUM tape ensures tightness of threaded joints operating at pressures up to 9.8 MPa (100 kgf / cm2). Depending on the application, the FUM tape is produced in three grades: grade 1 - the highest and first grade - for sealing systems with mediums of general industrial type, as well as systems with strong aggressive media (concentrated and dilute solutions of acids and alkalis); contains lubricant - medical vaseline oil according to GOST 3164-78 - from 17 to 20%; grade 2 - for sealing systems operating on oxygen and other strong oxidants; does not contain grease; grade 3 - for sealing systems with general industrial media, as well as systems with strong, aggressive media (concentrated and dilute solutions of acids and alkalis); represents the edge parts of the FUM tape of grades 1 and 2, as well as the SKL film, manufactured according to TU 301-05-49-90. manufacturing.

 

Table 4 FUM Tapes, TU 6-05-1388

Brand, grade	Appearance	Tensile strength MPa (kgf/cm2), not less than	Elongation at break	Mass fraction of grease
Brand 1, Superior grade	The color of the tape can be white with various shades. The presence of dark spots and individual inclusions is allowed. The surface of the tape must be flat without breaks. Waviness is allowed.	6,86 (70)	95	Not determined
Brand 1, First grade		4,9 (50)	80
Brand 2		for thickness: up to 0.105 mm inclusive - 6.86 (70); over 0.105 mm - 4.90 (50)	70	0,15
Brand 3		2,94 (30)	40	Not determined

 

FUM tape is an unsintered PTFE-4 film, due to which the material has a high chemical resistance and can be used in the food industry. Does not allow re-sealing and loosening-tightening, since it does not have elastic properties.

Advantages: ease of use. The connection is easy to assemble and disassemble. Excellent chemical resistance. Can be used for sealing in oxygen atmosphere. Disadvantages: low reliability under mechanical stress and vibration. It is not effective with frequent temperature changes, due to depressurization as a result of temperature deformations.

In addition to the domestic tape produced according to TU, there are similar products of European and Chinese production. As a rule, these belts are produced without lubrication and are analogs of FUM grade 2. Some Western manufacturers subject the belt to additional orientation (drawing) in one or more directions, which gives the material some elasticity.

Fig. 5 Imported FUM tape

FUM harness manufactured according to TU 6-05-1570 is a profiled product made of unsintered fluoroplastic-4D. Bundles are available in round or square sections and are used primarily as stuffing boxes. For thread seals, bundles are practically not used, therefore we will not consider them in detail in this section.

3.2. Thread-sealing cords and threads

Polymer cord or thread impregnated with a special sealing compound. Such products are produced by almost all major manufacturers of seals and fittings, specializing in sealing threads. Bundles or threads can be made from nylon, fluoroplastic and other polymeric materials. Products are "sharpened" for domestic use, pipelines for hot and cold water supply or gas pipelines of small diameters. The most famous threads and cords are SanTehnik, Loctite, Tangit Uni-Lock, Record. The materials are easy to use, you just need to wind the thread on the thread and tighten the connection. Moreover, the seal is quite reliable and cheap. Works well at low pressures and temperatures from -50 to + 140 ° C. The materials have been developed for use in high humidity environments and are therefore well suited for repairs on working pipelines. Not suitable for large diameter pipelines, high pressures and temperatures, as well as aggressive media and oil products.

Fig. 6 WEICON screw-thread sealing thread 

Table 5. Examples of sealing cords and threads

Name	Manufacturer	Characteristics
Tangit UNI-LOCK	Henkel	Universal thread for sealing threaded joints of pipes of cold and hot water, drinking water, gas up to 6 inches in diameter. Within 72 hours after assembly, it allows, without losing tightness, to adjust the pipe connections. Can be used for metal and plastic pipes.
Loctite 55	Henkel	A non-curing thread sealant that immediately provides maximum pressure resistance. Approved for gas and drinking water. Operating temperature range: -55 ° C - + 149 ° C
UNIFLON	Unipak	The UNIFLON thread is used to seal all types of threaded connections, on pipes made of any materials in heat supply systems, water supply, ventilation, in refrigeration units, for installation and repair of oil and gas pipelines, etc. Suitable for sealing threaded pipe connections of all sizes. Operating temperature range from –200°C to +240°C, pressure 30 bars to 100°C.
Rekord ("Рекорд")	INTERMO (ИНТЕРМО)	"Record" is an untwisted nylon cord consisting of 280 ultra-thin polyamide fibers impregnated with organosilicon paste. The working temperature of the connection is from -55°C to +120°C.
WEICON DF 175	WEICON	It is used for sealing all types of threaded connections. Suitable for sealing threaded pipe connections of all sizes. Operating temperature range from –200°C to 240°C.