Failures of Sealing of Flanges on the Top of the Cryogenic Tank's Pump Well (Part Two)

2. Reasons for the failure of the sealing surface of the top flange of the pump well
Under normal circumstances, there are two forms of leakages on the flange's sealing surface: penetration leakages and interface leakages. When permeation leakage occurs, the fluid leaks through the capillary tube of the material of the gasket. In addition to the fluid state properties such as medium pressure, temperature, viscosity, and molecular structure, the influencing factors are mainly related to the structure and material of the gasket, while the interface leakage is the leakage from the contact surface between gaskets and flanges; the leakage is mainly related to the size of the contacting gap. Interface leakage is a common form of flange sealing failure that must be paid attention to. The causes are uneven distribution of compression force, stress relaxation, inappropriate roughness of the sealing surface, and temperature changes.
 
Stress relaxation and torque loss are the main causes of interface leakages. After the bolts are tightened on the flange, the gasket is forced to produce compression deformation, so that the unevenness on the sealing surface of the flange is filled with the help of the deformation of the gasket. The bolt torque also gradually decreases due to the factors such as pressure, mechanism vibration, temperature changes, etc., resulting in torque loss and decreases in the compression force. When the specific pressure of the pre-tightened sealing drops to less than that of the working seal, there is a gap in the flange port, causing a leak. The greater the temperature changes, the longer the duration is. The stronger the vibration is, the more serious the torque loss becomes.
 
Ordinary stainless steel bolts with low strength cannot be matched with spiral wound gaskets. Bolts with high strength that have undergone strain strengthening treatment should be selected in low-temperature conditions. The tensile force of the bolt is greater than that of the flat gasket due to the great pressing force and torque required by the sealing of the spiral wound gasket. Table 3.3.10 in the Chinese Standard HG/T 20635-2009 Selective Regulations of Steel Pipe Flanges, Gaskets and Fasteners stipulates that spiral wound gaskets should match bolts with medium strength and high strength. In addition, when the pump is started, the top matched flange bears great pressure, and the bolt generates a force in the stretch direction. Ordinary stainless steel bolts with low strength have a small yield ratio, and stainless steel has ductility. Once it enters the yielding state, it will slowly elongate under the action of force. Under the action of the pre-tightening force, a certain stretch and deformation have occurred, and under the operating conditions, it will be stretched again due to the pressure, which reduces the sealing pressing force. When the specific pressure of the pre-tightened seal of the flange's sealing surface drops to less than that of the working seal, especially when the pump starts suddenly, the working temperature of the flange changes from normal temperature to low temperature rapidly. Because the cooling speeds of the flange and bolts are different during cooling, the pressing force of the gasket changes again after cooling and stress relaxation occurs for the flange bolts. Leakage is particularly prone to happen in the cooling process when the temperature drops. American standard studs include A320 GR8, CL1 and CI2. CL1 is a bolt that has not been strain-strengthened (see ASME B16.5s Table 1B), which is a bolt with low strength. Table B-1 I Group a in ASME B16.55.4.2 Appendix B clearly stipulates that bolts with low strength cannot be matched with spiral wound gaskets. Therefore, special attention should be paid to the selection of studs manufactured by foreign standards. The strain-strengthened A320 GR8 CL2 should be selected.
 
The surface roughness of the flange should be consistent with the form of the sealing surface and the selection of gaskets. The specific values are shown in Table 1. For example, soft gaskets need a certain nick to seal well, but the designed nicks of the wound gasket are not conducive to the sealing; the raised face sealing surface should be adopted. Ra is between 3.2 to 6.3 um, and too high roughness value of surfaces will affect the sealing effect.
 
Table 1 Matching values of flanges and their sealing surfaces with Ra required by the specification

At the same time, the degree of defects on the flange surface that matches the gasket should be controlled to achieve the purpose of successful sealing. Surface defects such as large nicks, grooves, and scratches should be avoided. Otherwise, a greater pressing force is required to seal perfectly. The quality of the spiral wound gasket and the protection before installation are also very important, especially the spiral wound gasket with large diameters. They are easily damaged during transportation. If the gaskets are not inspected carefully, the collapse will easily occur during installation.