Sealing gasket installation and use guide

A sealing gasket is a material used to fill or prevent liquid, gas, or dust from passing through a gap between two relatively moving surfaces. Seal gaskets are commonly used in pipelines, valves, pumps and other industrial equipment to ensure sealing performance and prevent leakage. Here is a guide to the installation and use of some gaskets:

1. Gasket installation requirements

• The gasket and flange sealing surface should be cleaned and shall not have any impact on the connection sealing performance if the scratch, spot or other defects exist.
• The gasket’s outer diameter should be smaller than the flange sealing surface, gasket’s inner diameter should be slightly larger than the inner diameter of the pipe; the difference between the two diameters is generally taken as 2 times the thickness of the gasket in order to ensure that after compression, the inner edge of the gasket does not extend into the container or pipeline, so as not to impede the flow of fluids in the container or pipeline.
• Gasket preload should not exceed the design provisions so as to avoid excessive compression of the gasket loss of resilience.
• For gasket compression, it is best to use a torque wrench. For large bolts and high-strength bolts, it is best to use a hydraulic tightener. Tightening torque should be based on the given gasket compression through the calculation; the size of the hydraulic tightener oil pressure should also be determined by calculation.
• Installation of gaskets should be in accordance with the order of the right figure in order to tighten the nut. But should not be screwed once to reach the design value. Generally, at least 2-3 times should be cyclic so that the gasket stress distribution is uniform.
• For flammable, explosive media pressure vessels and pipelines, change gasket should use safety tools so as to avoid tools and flanges or bolts touching, resulting in sparks, leading to fire or explosion.
• If the pipeline leakage must be depressurized and then replaced or adjusted to install the gasket, it is strictly prohibited to operate under pressure.

2. The special installation requirements of the gasket

• When installing the winding gasket matched with the sudden plane flange, attention should be paid to ensure that the seals are concentric with the pipeline, generally using the outer reinforcing ring and the inner circumferential surface of the bolt contact to locate. Do not allow eccentric installation. When installing basic-type wrap-around gaskets in flanges with tongue and groove sealing surfaces, care should be taken to keep the gap consistent with the groove wall surface.
• When installing metal-wrapped gaskets with narrow widths, a steel plate of a certain thickness should be placed on the outside of the gasket and then pressed against the flange so as to avoid the wrapped packing inside the gasket from expanding the interface (or lap) of the metal casing after being pressurized and damaging the gasket.
• When the working temperature of the flange is higher than 200℃, the sealing surface of the flange and the gasket should be coated with a sealant to prevent the gasket from sintering with the sealing surface of the flange at high temperatures, which will increase the trouble of overhauling and replacing the gasket.
• The sealing surface is not flat; it can be coated with sealant on the sealing surface and then installed in the gasket. Sealant generally consists of manganese dioxide, lead, graphite powder and boiled linseed oil (dry oil). According to the sealing medium, working temperature and medium pressure conditions, such as the selection of the corresponding type and grade of liquid sealant.
• For the installation of an octagonal metal ring gasket, when the metal ring gasket and flange sealing groove processing is not ideal, matching research is needed to achieve a close fit. For the large diameter of the metal ring gasket, the research is more difficult; in order to fill the metal surface of the small unevenness, you can use sealant.

3. Before the installation of bolts, flange inspection

Inspection of bolts and nuts:

• The material, type and size of bolts and nuts should be in accordance with the requirements of the drawings;
• The nut on the bolt rotation should be flexible, not shaking;
• The bolts should not be bent;
• The bolt threads shall not be allowed to have disconnection.

Inspection of flange:

The flange type should be checked to see if it meets the requirements, whether the sealing surface is smooth and clean, and whether there is any mechanical damage, radial scratches or corrosion.
Inspection of gasket:

• Gasket material, type and size are in line with the requirements;
• Whether the gasket surface has mechanical damage, radial scoring, serious corrosion, inside and outside the edge of the broken and other defects;
• For oval and octagonal metal ring gaskets, before installation, check whether the flange trapezoidal groove size meets the requirements, and whether the groove is smooth. It may be in the ring gasket contact surface coated with red lead oil to check whether the contact is good. Such as poor contact should be ground.

4. The pipe and flange installation quality requirements

1) Offset
Refers to the pipe and flange is not perpendicular, different center, flange surface is not parallel. The permissible deviation value between two flanges is: when using a metal gasket, it should be less than 2㎜; when using a metal-nonmetal combination gasket and metal gasket, it should be less than 1㎜.
2) Misalignment
It means the pipe and flange are perpendicular, but the two flanges are not concentric in the case that the diameter of the bolt hole and the diameter of the bolt are in accordance with the standard.
No other tools can be used to put the bolt into the bolt hole freely as qualified.
3) Open mouth
Refers to the flange gap being too large. The permissible tensile value between two flanges (excluding the pre-stretched value of the pipe and the thickness of the gasket or blind plate) is as follows: the tensile of the pipe flange should be less than 3㎜; the flange connected with the equipment receiver should be less than 2㎜.
4) Wrong hole
It means that the pipe and flange are concentric, but the chord distance between the bolt holes corresponding to the two flanges deviates greatly. The permissible deviation of the center circle radius of the bolt hole is:

• When the bolt diameter is ≤30㎜, the allowable deviation is ±0.5㎜;
• When the bolt diameter is > 30㎜, the allowable deviation is ±1.0㎜;
• The permissible deviation of the spacing between two adjacent bolts is ±0.5㎜.

The total deviation of the chord distance between any several holes is:

• For flanges with DN≤500, the allowable deviation is ±1.0㎜; for flanges with DN600-1200, the allowable deviation is ±0.5㎜.
• For DN600-1200 flanges, the permissible deviation is ±1.5㎜; for DN1300-1800 flanges, the permissible deviation is ±0.5㎜.
• For DN1300-1800 flanges, the allowable deviation is ±2.0㎜.

5. Method of controlling bolt fastening force

Before installing the bolt flange connection, the tightening force should be estimated according to the new and old degree of the bolt, lubrication status, material and so on.
If the tightening force is too small, the gasket preload stress is insufficient, and the connection cannot realize the seal. If the tightening force is too large, it often causes damage to the gasket or sealing surface. For this reason, the bolt fastening force must be controlled, control the bolt fastening force must use force wrenches, fixed torque wrenches and other special tools to calculate and control the preload.