MONOLITHIC ISOLATING JOINTS: The Sealing Systems

Transcription

1 MONOLITHIC ISOLATING JOINTS: The Sealing Systems State of the Art and comparison among the several e systems. s NGPR rev.0 dated Copyright 2005 NuovaGiungas srl ITALY.

2 Introduction The steady increasing demand and use of the monolithic isolating joint, in spite of other systems such as isolating flanges, nowadays considered d OBSOLETE, brought huge improvements to cathodic protection systems of gas and oil pipelines, that translated in financial terms mean huge saving of capitals. This is possible because of the maintenance free characteristics of the monolithic isolating joint and the reduction of the energetic costs, due to the less electric power needed to keep the plants protected. Although there are substantial differences among the several products. The choice of a less efficient product could partially, or fully compromize the original savings. NGPR0501 2

3 For definition, the monolithic isolating joint is a maintenance free non dissembling product and obviously shall guarantee lifetime mechanical and electrical isolating performances duration. On this purpose, the sealing system shall be specifically studied d and developed d for guaranteeing an efficiency duration. NGPR0501 3

4 There are substantially 3 PRIMARY sealing systems nowadays adopted by the manufacturers of monolithic isolating joints O-Rings sealing system Rectangular shaped sealing system U shape sealing system Above systems show vitals differences and service limitations that people operating on our field of business shall be aware of. NGPR0501 4

5 O-Ring system It is a primary sealing system made of 2 or 4 O-Rings counter opposition between metal and isolating only. It comes directly and strictly from the traditional system of the isolating flanges and therefore it hasn t been specifically studied for the monolithic isolating gjoints. NGPR0501 5

6 O-Ring system Strong holds It is Cheap. It doesn t require specific equipments. Seals are of commercial type and of easy procurement. Pressure application circle is limited. It is a symmetric system. Weak points It requires a microscopic precision of the surfaces machining and finishing and very strict tolerances. Sealing is given between metallic and isolating walls only (never between steel-steel) with different related behaviors. It isn t efficient when servicing on pressure cycles and on fast pressure changes plants. The considerable small section, when compressed, it isn t able to guarantee the mechanical seal against structural deformations due to bending, tensile, compression and torsion loads. The considerable small section isn t able to guarantee the mechanical seal on the whole circumference, further to non-symmetric deformations (closure welding) In case of non-symmetric closure, even if partial, it isn t possible to make the typical corrections that could be easily made on flanges instead (like tightening and/or loosing bolts and nuts), this because its monolithic structure. The distance that the fluid shall make before shortening is very short. The isolating spacer is not water proofed by the sealing system with the consequence of eventual gas absorption. It is subject to lacerations for explosive decompression from absorption. With pressure cycles or high pressure, O-Rings are subject to extrusion. NGPR0501 6

7 O-Ring system Among all possible effects that could happen, the worse is certainly the fluid leakage. Let s see why this happen with O-ring gjoints NGPR0501 7

8 O-Ring system The longitudinal loads on the pipeline generate longitudinal deformations that affect the joint too (Hooke s law). Total DEFORMATION is the summary of several deformations. Just think of a 16 NP100 joint with case wall thickness of 15mm in F65, at a test pressure of 150 Bar that sustains a longitudinal deformation on central body of over 0,3mm. Due to the bottom effect. L NGPR0501 8

9 O-Ring system Being the dimension of O-Ring small in purpose for voiding the phenomenon of explosive decompression (see AED lecture), it is sufficient a structural deformation of few decimal of millimeters to cause a leakage (for instance: only 0,2mm per O-ring of 4mm) Critical spread > 0,2mm (Fluid infiltration) MAX bearable detachment without leaking=0,2mm OR=4mm Compression=0,8mm (20%) NGPR0501 9

10 O-Ring system The conditions for the inefficiency of the isolating joint could be already generated during manufacturing, because of the extremely strict t tolerances of this system. Cause: ASSEMBLY/Tensile of welding Effect:detachment NGPR

11 O-Ring system Consequently O-ring of bigger section are used eventhough they are not recommended* if subject to pressure cycles changes, since risk of gas absorption increases and generates e the lacerations at from explosive decompression. * Concerning this matter, private rules exist (AED and similar), that if inquired for, further details shall be reverted later NGPR

12 O-Ring system Then, manufacturers partially had to fix the congenital inconveniences of O-Ring sealing system, adding supporting sets of O-Rings, or adding a secondary, or auxiliary sealing system (also called safety system), but its real purpose is to solve the inefficiency of the primary sealing system instead. Although this solution doesn t void a whole series of troubles that could be of early decrease of isolation, caused by fluid penetration into the internal structure. NGPR

13 What illustrated has been taken for granted by several specifications that forbid or limit use of O-ring in the manufacturing of isolating joints. NGPR

14 O-Ring sealing system NuovaGiungas u fully discharged O-rings from production o of monolithic o isolating joints since the end of the 70s, deciding back then to invest on research and development aimed to more efficient solutions and specifically studied. NGPR

15 RECTANGULAR shape sealing system It is a primary sealing system made of 1 rectangular shaped gasket hardly compressed among the metallic surfaces and the metal and the isolating spacer. It is a sealing system specifically studied for the monolithic isolating joint. NGPR

16 RECTANGULAR shape sealing system Strong holds It is fairly cheap. It is of easy assembly. It doesn t need particular surface finishing and/or particular machining tolerances. It is a system that works at the same time between the opposite metallic surfaces and between metal and isolating spacer. The isolating spacer is fully water proofed by the sealing system. It isn t subject to the extrusion phenomenon. It isn t subject to the phenomenon of explosive decompression being fully surrounded by a proper restraining chamber. Weak points The distance that the fluid shall make before shortening is fairly short. Pressure application circle is very high and bending moments burdening on the flanged components are very important. The pressure pushes on the gasket with an important longitudinal component that reduces the pre-compression effect during assembly. It needs specific equipments and moulds for the realization. It is a non-symmetric sealing system, therefore the pressure pushes in different way on the system, that it isn t compensated in extreme conditions. NGPR

17 RECTANGULAR sealing system The rectangular gasket has been widely used on isolating joints. Although, They happened some inconveniences that won t recommend the use for very high pressures. NGPR

18 RECTANGULAR sealing system In fact the pressure laps the gasket at point A and A1 generating these effects: Amplifies the bending moment pushing on the crown of flanged component. Increases the longitudinal tensile stress caused by the high pressure circle of application. Shrinks the gasket at point A1 reducing the space fluid has to make before shortening. A1 A NGPR

19 RECTANGULAR sealing system NuovaGiungas limited the production of isolating joints, with this sealing system, to pressures up to 200 Bar. Also, for diameters over 20, the isolating spacer has to be made with an innovative method that brings it together with the metallic component to obtain a single piece for voiding the missing i compensation previously mentioned. Although, this method can t be illustrated, because covered by a strict industrial secret. NGPR

20 U-shape sealing system It is a sealing system built around the rigid isolating spacer and it is made of a symmetric U shaped gasket with double profile, tightly compressed in a proper space obtained in the metallic components. It is a sealing system specifically developed for the monolithic isolating gjoint and it is commonly referred to as DSGS (Double Seal Gasket System). NGPR

21 U-shape sealing system Strong holds It doesn t need particular surfaces finishing and/or particular machining tolerances. It is a system that works at the same time between the opposite metallic surfaces and between metal and isolating spacer. Symmetric System The big section is able to oppose serious structural deformations. The distance that the fluid shall make before shortening is very long. The isolating spacer is fully water proofed by the sealing system. It isn t subject to the phenomenon of explosive decompression, being fully surrounded by a proper p restraining chamber. Increasing the pressure the system reacts with a proportional opposing force. Pressure application circle is limited. Weak points Expensive system It needs specific equipments for the realization. Assembly required specific equipments. NGPR

22 U-shape system This system has been originally used for highest pressure services (1000 Bar), but then and after verifying the success obtained, the use has been extended to the whole production of monolithic isolating joints, standard ones included. The very high investment in search of materials, equipments and moulds has been satisfyingly paid back by the worldwide success. NGPR

23 U-shape sealing system The gasket proportionally reacts against the pressure, opposing the fluid by autho-energizing itself. Also, it isn t subject to the explosive decompression phenomenon, being fully surrounded by a proper restraining chamber and it is a matter of fact that can t change its shape. Up today we manufactured isolating joint s models suitable to be taken at test pressure up to 1500Bar and they are hundreds the isolating joints installed allover the world working at pressures up to 1000 bar and are several thousands the isolating joints manufactured for more conventional pressures (up to 100 Bar). NGPR

24 Conclusions Service of O-ring type seal Joints shall be limited to low pressures, small diameters and to pipelines without bending, tensile, torsion and compression loads. Service of Rectangular type seal Joints shall be limited to medium high pressure. Highest pressures (over 300 Bar) to be void. There are no service limitations to U-shape type seal Joints. NGPR

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