Very often you can hear that “the gasket is leaking.” This statement is not always true. In fact, the connection is always leaking, and the gasket is only one of its components. The gasket is often expected to compensate for imperfections in flange surface finish and flange movement resulting from changes in operating temperature and pressure, vibration, etc. In many cases, gaskets can do this, but only if the correct type and material are selected and the correct installation procedure is followed.
A) What to do and what not to do when installing gaskets
- The main flange and counter flange must be the same type and correctly aligned. The total misalignment of the flanges should not exceed 0.4 mm.
- It is unacceptable to try to tighten flanges that are far apart from each other using fasteners. In such cases, it is necessary to use spacers using spacers on both sides of the spacer.
- Fasteners must be selected so that their elastic limit is not exceeded when the required load is applied.
- Additional tightening of bolts after a connection to a flat non-metallic gasket has been exposed to elevated temperatures is not permitted. (The gasket may harden and additional force will cause it to break.)
- It is necessary to ensure that there is no corrosion on the fasteners, as its presence will reduce the load-bearing capacity of the fasteners.
- You must ensure that the gasket material meets the specifications for the connection.
- It is necessary to make sure that there are no burrs or scratches on the working surfaces of the gasket, especially in the radial direction.
- The material should be selected so that the load capacity of the nuts is 20% higher than the load capacity of the studs or bolts. Always use washers of the same material as the nuts.
- If necessary, lubricant should be applied to the threads, but only in an even, thin layer. When using stainless steel fasteners, you should ensure that the specific type of lubricant can be used.
- Reuse of fasteners and gaskets is prohibited.
- Always use gaskets of the minimum permissible thickness.
- When cutting gaskets for flat flanges, the bolt holes must be cut before cutting the outer and inner diameter of the gasket. If the bolt holes are located close to the outer diameter of the gasket, cutting them out after cutting out the gasket can lead to disruption of its shape.
- Gaskets should be stored in a cool, dry place away from heat, moisture, oils and chemicals. They should also be stored flat and horizontal (i.e. not hung on hooks).
- Avoid applying grease to gaskets and flange faces.
B) Tightening the flange connection bolts.
Connections should be tightened evenly in three or even four passes, in a crisscross pattern, as shown in the figure. Please note that in this sequence, tightening one bolt may loosen the other(s), so it is recommended that all bolts be re-tightened in a circle as a final step. Some connections may need to be re-tightened immediately prior to commissioning to compensate for the relaxation of the gaskets and fasteners. Expected relaxation is 10% momentarily during the first day. Also, in some cases, when using certain types of gaskets in conjunction with flanges of certain types of connecting surface on heat exchangers, it is necessary to additionally tighten the connection during the initial heating of the heat exchanger.
A reasonable requirement is to first tighten no more than 80% of the maximum indicated in the table, tighten if necessary, and do not exceed the maximum in any case. In this case, the strength class of bolts or studs is usually used at least 5.8
B) Troubleshooting
| FAULT | POSSIBLE REASON | SOLUTION METHOD |
| The leak occurred immediately when the medium was supplied to the pipeline | Insufficient or excessive load in the connection or the load is applied unevenly | Carefully insert the new gasket. Check flange alignment, flange faces, and tighten bolts as described. |
| The leak occurred after a short period of use. |
|
|
| The leak occurred after several hours or days of use. | Chemical impact on the gasket from the environment or its mechanical destruction. | Check the chemical compatibility of the gasket material with the medium at a given concentration under operating conditions. Check the correct choice of gasket type. |
Preload (tightening) necessary to ensure tightness sealing flange connection in working conditions.
For sealing high-pressure pipeline components, they are mainly used , manufactured according to .
Widespread use of shutters with these fasteners contributed to the following: simplicity and manufacturability in manufacturing; reliable calculation and design methods; long-term traditions of designing and manufacturing SVD. The disadvantages of these valves are the high labor intensity of the bulkheads associated with the length of time it takes to screw in the connected threaded parts, as well as the difficulty of mechanizing and automating the process of assembling and disassembling the valve due to the large number of pins. The desire to reduce the labor intensity of the bulkhead process and its mechanization has led to the creation of a wide variety of designs of special devices for preloading (tightening) studs or bolts and nuts.
Tightening fasteners by applying torque
The main advantages of the torque tightening method are its versatility, simplicity and high performance. Disadvantages - rather low efficiency (only 10% of the total work spent on tightening a threaded connection is to create axial force) and the occurrence of torsional stresses in the stud during tightening, which reduce.
When tightening the connection, the torque M kr applied to the nut is spent to overcome the friction of the nut end against a stationary supporting surface and friction of the contacting surfaces of the thread of the nut and stud:
M cr = M t + M p, (1)
Where M t is the moment of friction of the end of the nut on the stationary supporting surface of the parts being connected; M p - torque in the thread;
M t = f T Q 3 R T, (2)
Where f T is the coefficient of friction at the end of the nut; Q 3 - tightening force; R T - conditional friction radius of the nut;
R T = (1/3)(D G 3 - d shb 3) / (D G 2 - d shb 2), (3)
where DT is the diameter of the outer supporting surface of the nut; d shb - internal diameter . Torque in thread
M p = Q 3 (P/ 2π + f p d 2 / 2), (4)
Where R— thread pitch; f p is the coefficient of friction in the thread; d 2 - average thread diameter. For threaded connections when the contacting surfaces are lubricated with industrial oil and there are no electrolytic coatings on them f T = 0.12, f p = 0.20.
Tightening fasteners by applying axial forces to the shank of a bolt or stud
The method of tightening threaded connections by applying axial forces to the stud rod is free from the disadvantages of the considered method. The method consists of stretching the stud rod with a special device (hydraulic jack), followed by loosely screwing the nut to fix the stud rod in a stretched state.
The peculiarity of the method is that after tightening the nut without applying torque, the connection elements remain unloaded: the connection thread stud - nut and micro-irregularities of interfaces nut - washer And . As a result, after removing the tensile load on the stud, these elements are loaded and deformed, as a result of which the residual tightening force decreases.
Measuring the degree of force reduction in a stud using the unloading factor
Force reduction degree in high heels appreciate unloading factor. The stud unloading coefficient takes into account the reduction in force in the studs when the load is transferred to the main nut after the load of the loading device is removed and is equal to the ratio of the force stretching the stud to the residual force in it.
Sequence of tightening fasteners in a flange connection
Due to the fact that when tightening practically only one or several studs (group of studs) are loaded at the same time, then it is necessary to observe a certain sequence when tightening each stud or individual groups of simultaneously tightened studs. Compliance with a certain sequence when tightening the studs is due to the peculiarities of tightening a group threaded connection, which are as follows. Tightening on high pressure pipelines leads to axial displacement of the sealing surface of the flange or plug due to a decrease in the linear dimensions of the sealing ring in the axial-radial direction, deformation of microroughnesses of the contacting surfaces, compression of the materials of the flange of the vessel body and lid in the area of the sealing surfaces and other deformations. As a result of these deformations, an axial movement of the cover plane occurs, on which the nuts of the main fasteners rest.
Consistently reducing the tightening force of flange fasteners
Loading modes of flange connection studs
The loading modes of flange connection studs are divided into
- one-time and
- group.
One-time tightening mode for flange fasteners
The fastest, most reliable and ideal from the point of view of ensuring accuracy and uniformity of loading is method of tightening all studs at once connections. In this case, all connection studs are loaded simultaneously with forces of equal current values.
Group methods for tightening studs or bolts of flange connections
If it is impossible to create a one-time loading mode, group modes are used. In group tightening mode, all valve studs are divided into groups of simultaneously tightened studs. Groups of studs must be evenly distributed along the perimeter of the bolt circle. Number of studs in a group there must be multiple of the total number of studs flange connection.
Group tightening mode can be
- single-bypass and
- multi-bypass.
Group single-pass mode for tightening fasteners of a flange connection
At single-bypass mode the load is applied sequentially to each group of simultaneously tightened studs only once. In this case, the load on the studs of each group changes from the maximum (for the first group) to the design tightening force (for the last group). The advantage of this tightening mode: relatively short duration the process of tightening the studs, as well as more high accuracy loading (compared to multi-bypass mode), due to the large number of bypasses and associated loading errors. The main disadvantage is the relatively high loading force of the studs of the first group compared to the loading force of the last group (often differing by 8-10 times).
In connection with these disadvantages, obstacles to using the single-bypass tightening mode may be:
- insufficient loading device power;
- insufficient stud mounting shank strength, which must correspond to the loading force of the studs of the first group.
Group multi-pass mode for tightening flange studs with nuts
In this case, use multi-pass group tightening mode. This mode consists of carrying out several loading rounds that follow one after another studs of all connection groups. The loading force of the studs during these bypasses depends on the adopted version of the multi-bypass tightening mode. The most common variant of the multi-bypass tightening mode is bypass-equalization.
Calculation of tightening modes for flange studs and nuts
Calculation of stud tightening modes. The one-time stud tightening mode is a special case of the single-round group tightening mode, in which the number of stud groups n=1, i.e. All flange studs are loaded simultaneously. In the single-pass mode of tightening the studs, the current loading force of the next group of studs (RD26-01-122-89)
Where K z 1 - unloading coefficient of studs of the corresponding group; Q n is the final tightening force of the studs of the last group; n = m/i—number of groups of pins in the gate; m— number of pins in the gate; i— number of simultaneously operating loading devices (hydraulic jacks); z— serial number of the loaded group of shutter plates. Ultimate Power Q n per group of studs at the end of the tightening process,
Q n = Q 3 / n,(6)
Where Q 3 - total tightening force of all bolt studs.
Relative compliance coefficient of the sealing gasket
α =λ 0 / λ Ш ( Q), (7)
λ 0 and λ Ш ( Q) - axial compliance of the sealing gasket and group of studs. Current value of the loading force of one stud of the corresponding group
Q z = Q z/ i. (8)
Current value of the loading force of one stud of the first group Q" z=1 is compared with the permissible load on one stud [ Q"]; the condition must be met
Q" z=1 ≤ [ Q"] (9)
Permissible load on one stud [ Q"] is taken equal to the smaller of two values:
1. from the condition of ensuring the strength of the mounting area of the stud thread
[Q"] ≤ 0,8 σ 20 Tsh F Sh, (10)
Where σ 20 ТШ - yield strength of the stud material at a temperature of 20°C; FШ - cross-sectional area of the mounting section of the stud;
2. or by the working force of the loading device (hydraulic jack)
[Q"] ≤ Q Well. . (eleven)
If condition (9) is not met, then it is necessary to calculate the bypass-equalizing mode of tightening the studs, and the current value of the loading force of the next group of studs with the corresponding bypass
, (12)
[Q] = i[Q"]. (13)
Required number of rounds
(14)
Where K z2 is the unloading coefficient of the studs in the bypass-equalizing tightening mode.
Stud relief factor for flange connections
Difference in the unloading coefficient of flange fasteners for sealing gaskets of different sections
Maximum coefficient values TO n unloading of studs in a single-pass tightening mode (the first group of fasteners) for an O-ring of the corresponding type are given in the table below.
| Section view of the steel gasket | Maximum value K n | |
| double cone gasket | 1,4 | |
| triangular gasket | 1,45 | |
| Rice. 1. Dependence of coefficient ψ
z from WITH increasing load axial compliance flange parts decrease, and therefore the coefficient of unloading of the studs also decreases. In this regard, the unloading coefficients of the studs of different connection groups are different. For the first group of studs, which is loaded with maximum load, the unloading coefficient is minimal; for the last group of studs the unloading coefficient is maximum. Unloading coefficient for a group of studs of the corresponding serial number K z = ψ z TO n, (15) Where ψ z is a coefficient depending on the type of sealing ring, the number of groups of studs in the flange connection and the serial number of the group (Fig. 6.35, 6.36).
Rice. 1. Dependence of coefficient ψ
z from For valves with an octagonal sealing ring and a flat metal gasket, accept ψ z = 1, since the difference in loading forces between groups of studs is small and, therefore, the unloading coefficient is almost constant and equal to the maximum value TO n. The unloading coefficient of the studs for the first bypass in the bypass-equalizing tightening mode is determined as for the single-bypass tightening mode. During subsequent rounds, the unloading coefficient for each group of studs is taken equal to the unloading coefficient for the last group of studs of the first round. If the loading device (hydraulic jack) is equipped with a mechanism for screwing in nuts with torque control, then with a stretched stud this moment is determined by the empirical formula M Kpz = 7.7.10 6 F w d p , (16) Where M Kpz - torque, N m; F w - cross-sectional area of the stud, m2; d p - thread diameter of the fastener, m. In this case, the unloading coefficient of the studs (bolts) K zM = 0.85 ( K z - 1) + 1. (17) ConclusionThe use of the considered methods of sequential tightening of flange fasteners ensures uniform compression of the sealing gasket, and, consequently, reliability and tightness of the flange connection. Bibliography
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The reliability of any system depends on the reliability of the weakest link in the system. Welded joints of steel pipes are reliable and are used in most cases. But situations arise in which the use of a welded joint is impossible. Connecting various fittings, providing a collapsible connection, the possibility of preventive maintenance and repair of pipe fittings as well as working units of units, connecting dissimilar pipes: cast iron-plastic, cast iron-steel, steel-plastic, steel-asbestos-cement, plastic-asbestos-cement and solving many more technological problems. The flange connection must ensure the reliability and durability of the operation of such connections. In general, the flange design involves a pair of flanges and a gasket and rings connected by bolts or studs.
Flanges - general characteristics
To unify products and allow these products to be used in different countries of the world without additional processing, a clear classification of flange connections has been introduced. Sometimes the same flange will have different designations in different classifications.
Main classifications used in the world:
- GOST is a standard adopted in the USSR and valid in the post-Soviet space;
- DIN - German standard valid in Europe;
- ANSI/ASME is an American standard valid in the USA, Japan and Australia.
There are standards conversion tables that indicate which standard a particular flange meets.
Various materials are used to make flanges:
- cast iron;
- malleable iron;
- carbon steels;
- stainless steels;
- alloy steels;
- polypropylene.
Polypropylene flanges have become widespread in the last decade. They are mainly used for installing non-pressure systems, connecting PE pipes to metal pipes, connecting pipe fittings on which a flange mount is installed. These flanges, like metal flanges, are made by casting or stamping.
Flanges are also divided into types:
- flat (GOST 12820-81);
- collar (GOST 12821-81);
- loose flanges on a welded ring (GOST 12822-80);
- flanges for vessels and apparatus (GOST 28759.2-90);
- ring plug (GOST 12836-80).
It is allowed to manufacture square flanges that have at least 4 holes for bolts or studs. Such flanges can be used on systems with a maximum pressure of no more than 4.0 MPa.
According to the nomenclature and, accordingly, GOST 12815-80, the flanges of fittings and connecting parts of pipelines have nine main versions of the sealing surface:
- Spanish 1 - with a connecting lip, the most common design of flanges, has a special connecting lip in the form of a chamfer at an angle of 45°
- Spanish 2 - similar in design to the previous model, only the connecting protrusion is at an angle of 90°;
- Spanish 3 - with a depression on the inside and a protrusion on the outside at an angle of 45°;
- Spanish 4 - with a spike;
- Spanish 5 - with a groove in the form of an annular recess;
- Spanish 6 - under the lens gasket, a chamfer is selected on the inside;
- Spanish 7 - for an oval-section gasket, an annular recess in the mold on the end side;
- Spanish 8 — with a spike for a fluoroplastic gasket;
- Spanish 9 - with a groove for a fluoroplastic gasket.
For flanges of vessels and apparatus there are specific requirements for execution, specified in GOST 28759.2-90, and for flat welded flanges - in GOST 28759.390
Design features of flanges
Flanges, like any pipe or shut-off valves, have several design features. When choosing and deciphering flange designations, you must know these features.
Conditional pass
The nominal diameter of the flange is the internal diameter of the pipe, fitting or shut-off valve to which the flange is welded. It is accepted based only on the nominal diameter of the pipe.
For flat welded flanges with a nominal diameter of 100, 125, 150, depending on the design, the letter (A, B, C) is indicated - the outer diameter of the pipe depends on it; if the letter is not specified, the letter A is considered by default.
Rows
All geometric dimensions of the flange will depend on the nominal diameter. The same flange with the same nominal bore can be manufactured in two ways - row1 and row2. They differ in different center-to-center distances between the connecting holes, and also, in some cases, in different diameters of the connecting holes. By default, flanges are manufactured in row 2.
Pressure
An important property of a flange connection is the ability to maintain system pressure without leaks or destruction of the system. This indicator is designated as conditional pressure. The nominal pressure indicator depends on the geometric dimensions of the flange, material of manufacture, design, and sealing gasket.
Important: When ordering flanges, you should remember that there are different pressure dimensions: in kgf/cm2, Pa (MPa), atm., bar. Therefore, it is necessary to indicate exactly what pressure a given product should be designed for.
Temperature
The operating temperature of the liquid will become the flange temperature; it should be noted that the pressure and temperature parameters are interdependent. As the temperature increases, the maximum pressure at which the flange connection operates will drop. The dependence can be expressed by linear interpolation. The relationships between operating temperature and pressure for each flange are given in special tables and GOST standards.
Flange designation
Each type of flange has its own specific designation; let’s look at each of them.
Flat weld flanges
Let's take an example of the designation of flat welded flanges:
Flange 1-65-25 09G2S GOST 12821-80
Flat welded flange version 1 with nominal bore (DN) - 65 mm, designed for nominal pressure of 25 kgf/cm2, made of steel 09G2S in accordance with GOST 12821-80.
When choosing a flange for a fluoroplastic gasket, after the number DN, indicate the letter F.
Collar flanges
Flange 1-1000-100 st. 12x18n10t GOST 12821-80
Designates a flange of version 1, with a nominal bore of 1000, designed for a pressure of 100 kgf/cm2, made of steel 12x18n10t, which is structural stainless steel.
For square flanges, additionally indicate in the name - square flange.
Just like in flat flanges, when using a fluoroplastic gasket, the letter F is indicated.
Loose flanges on weld ring
The designation for loose flanges and flat flanges is slightly different. Since this product uses a welded ring, the designation of the flange is also accompanied by the designation of the ring, for example:
Flange 50-6 ST20 GOST 12822-80
Ring 1-50-6 ST 35 GOST 12822-80
Here: 50 - nominal diameter, nominal pressure 6 kgf/cm2, flange made of steel st20, ring made of steel st35.
For conditional passage 100, 125, 150, you must also specify the letter (A, B, C), by default - A.
Gaskets for flange connections
Sealing an assembly or connection under excess pressure, often in aggressive environments, plays an important role in the design of a flange connection.
Depending on the type of flange or needle design used, pressure, temperature, chemical properties of the medium, the following are used as sealing gaskets:
- KShch (7338-77) - technical acid-base rubber;
- MB(7338-77) - oil and petrol resistant rubber;
- T(7338-77) - technical heat-resistant rubber;
- PON(481-80) - general purpose paronite;
- PMB(481-80) - oil and petrol resistant paronite;
- Asbestos cardboard;
- Fluoroplastic-4.
Tightening flange connections
Tightening flange connections is a key point in flange installation. To achieve maximum sealing, all parts must be precise.
Preparing elements
Clean and degrease the flange surfaces, check for scratches, depressions and dents. Inspect the flange itself and fastening elements - bolts and nuts - for corrosion. Remove burrs from the threads; first, you can also “drive” each bolt and nut along the threads. Lubricate the threads of the bolt or stud. Prepare and install the gasket. Make sure it is installed correctly, it should lie in the center.
Important: Do not use old gaskets; if it is not possible to replace the gasket, several old gaskets can be installed.
Tightening sequence
Reliable and correct fixation of the flange will ensure the correct tightening order of the bolts. To do this, lightly tighten the first bolt, select the next bolt from the opposite side, and tighten it lightly. The third bolt that you tighten is behind the first by a quarter turn (90°) or close to this angle. The fourth is opposite the third. Continue the sequence until all bolts are tightened. When tightening 4-bolt flanges, use the criss-cross technique.
Torque
To obtain the most airtight connection, the bolts must have the required tightening torque. The tension from tightening should be evenly distributed across the flange. During tightening, the bolt is subjected to a tensile force opposite to the tightening force of the connection. If you tighten too much, you can strip the threads on the bolt or break the bolt itself.
To adjust the tightening force, different tightening techniques are used:
- hydraulic tensioning mechanism;
- hydraulic torque wrench;
- pneumatic impact wrench;
- manual torque wrench.
As a last resort, you can use hand-tightening, but this method is best done by a professional.
Regardless of the chosen tightening method, the force with which the nuts are tightened must meet the product specifications.
After installing the flange and starting the system, a loss of torque of up to 10% is possible in the first 24 hours of operation. This is inherent in any bolted connection due to vibration, shrinkage of the gasket, and temperature changes.
After a day or two, additionally tighten the threaded connections to the specified torque, according to the specification.
ment indicated in the table below.
a The table below applies to the bolts shown in figure. A.
2. Table of tightening torques for flange connection bolts
a Unless otherwise specified, use the specified standard when tightening flange connection bolts.
the reasons given below.
3. Table of tightening torques for pipe connection bushings with O-ring
a Unless otherwise instructed, when tightening the O-ring pipe connector bushings,
ring, use the standards given below.
4. Tightening torque table for O-ring plugs
a Unless otherwise instructed, when tightening O-ring plugs, use
standards given below.
5. Tightening torque table for hoses (with conical and mechanical seals)
a Unless otherwise specified, when tightening hoses (conical and mechanical seals)
use the standards given below
a The following points apply when applying engine oil to the threads.
6. Tightening torque table for connections with mechanical seal
a Tighten the mechanical seal connections (sleeve nuts) on low-pressure pipes.
pressure clad steel used on engines, up to moments representing"
listed in the following table.
a Apply the following tightening torques to mechanical seal connections,
first applying a layer of motor oil to their threaded areas.
For reference: Depending on the specific technical characteristics, connections with
mechanical seal, the dimensions of which are indicated in brackets ().
7. Torque table for 102, 107 and 114 series engines (bolts and nuts)
a Unless otherwise specified, when tightening bolts and nuts with metric threads to
8. Torque table for 102, 107 and 114 series engines (pivot joints)
a Unless otherwise specified, when tightening swivel joints with metric threads to
For 102, 107 and 114 series engines, use the specifications below.
9. Torque table for 102, 107 and 114 series motors (Bevel screws)
thread)
a Unless otherwise specified, when tightening screws with tapered threads (unit: inch)
For 102, 107 and 114 series engines, use the specifications below.
The flange connection is the most vulnerable and weak point of the pipeline.
Assembling pipes with flanges is one of the most common and critical operations in the manufacture and installation of pipelines, since failure of the flange connection necessitates shutting down the pipeline.
Leaks of medium through leaks in flange connections during testing and operation of pipelines occur due to weak tightening of the flanges, distortions between the flange planes, poor cleaning of the sealing surfaces of the flanges before installing a new gasket, incorrect installation of the gasket between the flanges, the use of low-quality gasket material or material that does not comply environmental parameters, defects on the sealing surfaces (mirrors) of the flanges.
The process of assembling a flange connection consists of installing (fitting), aligning and fastening the flanges at the ends of the pipes, installing the gasket and connecting the two flanges with bolts or studs. Before assembling the flange connection, the connected sections of pipes are verified for the straightness of their axes.
When fitting flanges to pipes in accordance with SNiP ShT.9-62, the following requirements must be met.
Flange perpendicularity deviation P to the pipe axis (distortion), measured along the outer diameter of the flange (Fig. 99, a) should not exceed 0.2 mm for every 100 mm diameter of the pipeline designed to work under pressure up to 16 kgf/cm 2, 0,1 mm- under pressure from 16 kgf/cm 2 up to 64 kgf/cm 2 and 0.05 mm under pressure above 64 kgf/cm2.
The flanges must be installed so that the holes for the bolts and studs are located symmetrically to the main axes (vertical and horizontal), but do not coincide with them (Fig. 99.6). Displacements of the axes of bolt holes in flanges T relative to the axis of symmetry should not exceed ± 1 mm with hole diameter 18-25 mm,±1.5 mm- at 30-34 mm and ±2 mm- at 41 mm.
The displacement of the axes of the flange holes along the circumference of the pipe is checked using a plumb line or level, from which the vertical or horizontal axis is found, and then the displacement of the holes is controlled with a ruler.
The perpendicularity of the flange is checked with a test square (Fig. 100) and a feeler gauge. Flange gap 2 and a square 1 measured at points diametrically opposite to the points of contact.
For fitting onto pipes with nominal bore up to 200 mm For flat and butt-welded flanges with their centering along the inner diameter of the pipe, use the device shown in Fig. 101. The device consists of a lever device 1 mounted on the rod 3, and disk 5 . To install the flange 6 the lever mechanism is inserted inside the pipe 2. When the rod rotates 3 clockwise the levers diverge, pressing the bars 4 to the pipe wall, while the disk is installed strictly perpendicular to the pipe axis. Flat flanges are installed on the device disk (position 1 ), and butt welded ones - along the end of the pipe and the fixture strips (position II). After checking the position of the flange, it is secured by electric arc welding.
Rice. 99. Flange position when installed on a pipe:
a - deviation from the perpendicularity of the flange to the base. pipes,
b - displacement of the axes of bolt holes in the flanges relative to the axis of symmetry
Rice. 100. Control square:
I- square, 2 - flange, 3 - pipe
Rice. 101. Device for fitting flanges with alignment along the inner diameter of the pipe:
1 - lever device, 2 - pipe, 3 - rod with knob, 4 - bar, 5 - disk, 6 - flange
When assembling pipeline elements and assemblies on assembly stands, special mobile devices are used for fitting flanges.
For fitting butt weld flanges with nominal bore up to 5O0 mm the most rational device is shown in Fig. 102, a. The welded flange is installed on replaceable control pins 1 , manufactured according to the flange bolt hole diameter. These pins are using a double thread screw 2 and handles 3 move and fix the position of the flange bolt holes symmetrically to the vertical axis. The perpendicularity of the flange to the longitudinal axis of the pipe is achieved by pressing its mirror to the plane of the installation carriage 4. The coincidence of the flange axis with the pipe axis is achieved by moving the carriage with the flange vertically using screw 5 and a handle 6. The device is mounted on guide rollers 7, and after assembly and tack the element is easily rolled away.
When assembling a flat flange on such a device, an installation ring is inserted inside it so that the pipe does not reach the end of the carriage (flange plane) by the required amount. The disadvantage of this design is the need for individual alignment of the internal hole of the flange and pipe during assembly.
In Fig. 102.6 shows a device for fitting flat flanges with nominal bore up to 500 mm. It differs from the one described above in that a mandrel is attached to the installation carriage together with the control pins 8, having a series of cylindrical projections, the diameters of which correspond to the internal diameters of the assembled flanges. The width of the protrusions is taken taking into account the value to which the flange is not adjusted. The end surfaces of the protrusions are processed strictly perpendicular to the longitudinal axis. The flange is put on the pipe and pressed with a mirror to the end surface of the mandrel. The installation carriage is moved using screw 5 so that its height is on the same axis with the pipe.
Rice. 102. Devices for fitting flanges:
A- butt welded, b- flat welded; 1
- control pin, 2
- two-thread screw,
3, 6
- handles, 4
- installation carriage, 5
- screw, 7
- guide rollers, 8
- mandrel
If the flange is not skewed or the amount of skew is acceptable, final assembly of the connection is carried out with the installation of gaskets. Before installation, soft gaskets (made of paronite, cardboard, asbestos) are moistened with water and rubbed on both sides with dry graphite. It is impossible to lubricate gaskets with mastics or graphite diluted in oil, since mastic and oil burn to the flange mirrors and damage their surface.
The tightness of a flange connection largely depends not only on the cleanliness of the surface of the flange mirrors, the quality and size of the gasket, but also on the careful and skillful assembly and tightening of the nuts. Before assembling flange connections with a projection and a recess, you should make sure that the projection of one flange fits freely into the recess of the flange mating to it, and that the gasket is not displaced in one direction or another.
The assembly of pipes with loose flanges on a welded ring or flanged pipe is no different from the above and comes down mainly to preparing the end of the pipe.
Correcting misalignment of flanges during their assembly by tightening bolts or studs, as well as eliminating gaps by installing wedge gaskets is not allowed. Such interference causes one-sided compression of the gasket and unacceptable stretching of bolts or studs, as a result of which the connection becomes loose. Overtightened bolts or studs may break during operation.
The nuts of flange connections with paronite gaskets are tightened using the crosswise method. First, tighten one pair of opposite bolts, then the second pair, located at an angle of 90° to the first. Gradually tighten all the bolts by turning the nuts transversely. With this sequence of tightening the nuts, there is no distortion in the flange connections.
Nuts with metal spacers are tightened in a circular manner, i.e., with a three- or four-fold circular circuit, all nuts are tightened evenly. Flange connection nuts are tightened using hand and power ratchet wrenches. Power tools include wrenches with electric or pneumatic drive. The uniformity of tightening and the amount of cold tension of flange connection studs and valve covers on high-pressure pipelines are controlled with torque wrenches by measuring the elongation of the stud during tightening. The permissible size of the cold tension of the studs is in the range from 0.03 to 0.15 mm for every 100 mm stud length.