1. PTFE gasket

 

PTFE gasket is based on PTFE as the main material. Because PTFE itself has good corrosion resistance and stable physical properties, it is used in steel power, petrochemical and other industries.

 

industry has been widely used. Combined with the characteristics and working conditions of the petrochemical industry mentioned below, it is required to use PTFE gaskets or other sealing products with good quality and excellent chemical performance, so as to ensure

 

Safe and efficient operation of the equipment.

 

First of all, the price of raw materials for chemical products is relatively high, and any leakage will bring great losses to users. Chemical products are more flammable and explosive, because the leakage is likely to cause explosion, fire, human

 

Incalculable and serious losses such as casualties. In order to be able to safe production, the user's choice of seals is more careful, than quality, than brand, than service.

Secondly, the medium in the petroleum, petrochemical and natural gas industry is mostly a variety of liquids and gases with different corrosiveness, such as aromatics, rust inhibitors, acid gas, drilling fluid, seawater, carbon dioxide, hydrogen sulfide, etc.

 

And these media are often mixed together and have the characteristics of high temperature and high pressure, so the use of PTFE gaskets and other sealing products have excellent quality and good chemical properties, so as to protect

 

Safe and efficient operation of the equipment.

 

Let's look at the working conditions of the chemical industry:

 

Temperature: part of the high temperature, the majority of normal temperature.

 

Medium: concentrated acid, high alkalinity, strong corrosiveness, high slurry concentration, high quality requirements for seals such as PTFE gaskets.

 

Pressure: part of the pressure is higher, the hardness, elasticity, toughness of the seal requirements are different.

 

Line speed: the water pump is around 2900 r/min.

 

PTFE gasket is suitable for sealing parts with severe alternating temperature and pressure. It is a static sealing element for flange connections such as pipes, valves, pumps, heat exchangers, towers, manholes, and hand holes.

 

 

Figure 1: PTFE gasket Source: Dankai

 

Several Precautions for Installation of 2. Sealing Gasket

 

Sealing gasket is to solve the "run, run, drip, leak" static seal parts. Because there are many structural forms of static seals, according to these forms of static seals, there are plane pads, elliptical pads, transparent pads, and other forms of static seals.

 

Mirror pad, conical pad, liquid gasket, O-ring and various self-sealing gasket, etc. The correct installation of the gasket should be checked in the flange connection structure or threaded connection structure, static sealing surface and gasket,

 

other valve parts intact.

1. Before installing the gasket, the sealing surface, gasket, thread and rotating parts of bolts and nuts shall be coated with a layer of graphite powder or weeping agent blending graphite powder with engine oil (or water). The gasket and graphite shall be kept clean.

 

2. The gasket shall be installed on the sealing surface in the middle and correct, and shall not be deflected, and shall not extend into the valve cavity or rest on the shoulder. The inner diameter of the gasket should be larger than the inner hole of the sealing surface, and the outer diameter should be slightly smaller than the outer diameter of the sealing surface.

 

To ensure that the gasket pressure uniform.

 

3. Only one piece of gasket is allowed to be installed, and two or more pieces are not allowed to be installed between the sealing surfaces to eliminate the insufficient gap between the two sealing surfaces.

 

4. The sealing of the elliptical gasket shall make the inner and outer rings of the gasket contact, and the two ends of the gasket shall not contact with the bottom of the groove.

 

5, O-ring installation, in addition to the ring and groove should meet the design requirements, the compression amount should be appropriate, metal hollow O-ring general flattening degree is 10% ~ 40%, rubber O-ring compression deformation rate, cylindrical surface

 

The static seal is 13% ~ 20%; The flat static sealing surface is 15% ~ 25%. If the internal pressure is high, the compression deformation should be higher when used in vacuum. Under the premise of ensuring sealing, the smaller the compression deformation rate, the better,

 

Can extend the life of the O-ring.

 

6, gasket on the cover before the valve should be in the open position, so as not to affect the installation and damage to the valve. When the cover is aligned, it is not allowed to contact the gasket by pushing and pulling to avoid displacement and

 

A bruise. When adjusting the position of the cover, the cover should be slowly lifted, then aligned and gently lowered.

 

7, bolt connection or threaded connection of the gasket installation, the gasket should be in a horizontal position (threaded connection of the gasket cover, a wrench position shall not use pipe tongs). Thread tightening should be symmetrical, wheel

 

Flow, uniform operation method, bolts should be full, neat, no loose.

 

8. Before the gasket is pressed, the pressure, temperature, the nature of the medium and the characteristics of the gasket material should be clearly understood to determine the pre-tightening force. The pre-tightening force shall be reduced as much as possible under the condition that the pressure test does not leak (too large pre-tightening force

 

Tight force is easy to damage the gasket, so that the gasket loses resilience).

 

9. After the gasket is tightened, it should be ensured that the connecting piece has a pre-tightening gap to prepare for pre-tightening when the gasket leaks.

 

10. When working at high temperature, the bolt will produce high temperature creep, resulting in stress relaxation and increased deformation, resulting in leakage at the gasket and requiring thermal tightening. Conversely, at low temperatures, the bolt will shrink and need to be

 

To cold loose. Hot tightening is pressure, cold loosening is pressure relief, hot tightening and cold loosening shall be carried out after maintaining the working temperature for 24h.

 

11, the sealing surface with liquid sealing gasket, the sealing surface should be cleaned or surface treatment. The plane sealing surface shall be consistent after grinding, and the coating adhesive shall be uniform (the adhesive shall be suitable for the working conditions

 

Should be), to try to exhaust air, adhesive layer is generally 0.1~0.2mm. The thread is the same as the plane sealing surface. Both surfaces in contact should be coated. When screwing, it should be in a vertical posture to facilitate the discharge of air. glue liquid

 

Should not be too much, so as not to spill point other valve parts.

 

12. When using PTFE film tape for thread sealing, the starting point of the film should be pulled thin and stuck on the thread surface. Then remove the excess tape at the starting point to make the film stick to the thread

 

A wedge. According to the thread gap, the winding direction should be along the screwing direction, and the end point should be overlapped at the starting point. Slowly pull off the film into a wedge shape, so that the thickness of the film winding is uniform. Screw in

 

Before, press the film at the end of the thread so that the film can be screwed into the internal thread together with the thread. When screwing, it should be slow and uniform. Don't move again after tightening, but also avoid turning, otherwise it is easy to leak.

 

 

Figure 2: PTFE gasket Source: Dankai

 

Reasons and Precautions of 3. Flange Gasket Failure

 

Under normal circumstances, the leakage of flange gaskets has four forms: interface leakage, penetration leakage, diffusion leakage and damage leakage. Combined with many years of engineering construction maintenance and manufacturing work experience, pen

 

The main reasons for the failure of the seal are as follows: uneven distribution of pressing force, stress relaxation, inappropriate surface roughness of the sealing surface and temperature changes.

 

The first: uneven compression force distribution

 

The uneven distribution of pressing force will cause gaps in the flange sealing gasket, resulting in leakage. The uneven pressing force is caused by many factors.

 

1, is a human factor, the construction of asymmetric pre-tightening bolts, easy to cause uneven, this factor can be eliminated in the construction;

 

2, followed by the flange misalignment has an impact on the compression force, theoretically flange compression, the sealing surface is absolutely parallel, in fact, the pipeline center line can not be absolutely concentric, so when tightening the bolts on the flange

 

Generate bending moment, so that the flange force is uneven, this uneven connection, so that the sealing surface more or less deformation, so that the sealing pressure is reduced, under the operating load, easy to produce leakage, solve the right

 

The policy is to master the law of flange deformation, uniform pressing force, and add sealant to the gasket to eliminate the gap and eliminate leakage;

 

3, also, the bolt arrangement density on the pressure distribution is also more obvious, the smaller the bolt spacing, the more uniform the pressure.

 

The second: stress relaxation and torque loss

 

Stress relaxation and torque loss are also the main causes of leakage. After the bolts are tightened on the flange, due to the vibration of the mechanism, the increase or decrease of temperature and other factors, the gasket will be

 

stress relaxation phenomenon, the bolt torque also gradually decreased, resulting in torque loss, resulting in leakage. Generally speaking, the longer the bolt, the greater the residual torque and the smaller the diameter, which is more important to prevent torque loss.

 

Therefore, the use of long and thin bolts is an effective way to prevent torque loss. In addition, the greater the temperature change, the longer the duration, and the more severe the torque loss. Heat the bolt for a certain period of time

 

It is effective to extend it and then maintain a given torque to prevent torque loss. The thinner the shim, the smaller the torque loss. In addition to prevent strong vibration of the machine and pipeline itself, and eliminate vibration of adjacent equipment

 

The influence of, unnecessary impact on the sealing surface, not knocking on the bolts after fastening, etc., can prevent torque loss.

 

The third: sealing surface surface roughness

 

The surface roughness of the sealing surface should be consistent with the form of the sealing surface, such as RF sealing surface form, R a3.2 ~ 6.3 μm, the surface roughness value is too high will affect the sealing effect. (Related reading: The flange situation is right.

 

The influence of gasket selection) must control the degree of defects on the flange surface to achieve successful sealing. Large scratches, grooves, or scratches and other surface defects must be avoided. must be used

 

Greater pressure can make it sealed.

 

Fourth: temperature change

 

The decrease of temperature also has a great influence on the sealing effect. When the pipeline is installed at normal temperature, the pipeline expands and compacts when the temperature rises; when the temperature drops, the pipeline shrinks. Leakage at the flange connection often occurs when the temperature drops

 

(Cooling) process, because the cooling speed of the flange and the bolt is different during cooling, the pressing force of the gasket changes after cooling, resulting in stress relaxation, and the cold shrinkage of the pipeline, resulting in tension toward the bolt.

 

This force will cause leakage, so when selecting gaskets in low temperature media,

 

It should be noted that the flange sealing gasket is flexible at low temperature. The thickness of flange sealing gasket should be as small as possible, and the flange gap should be as small as possible. High-strength bolts are used to reduce strain.

 

This article is originally created by Teflon's little sister. Welcome to pay attention and take you to grow knowledge together!