Introduction to Rubber Lined Piping:
Corrosion is a menace in operating and maintaining a Chemical Process Plant. Hence given a choice one would be tempted to dispense away with Steel all together as a material of construction for piping used in a Chemical Plant. Unfortunately it is not practical to undermine the usefulness of Steel in sustaining the Pressure and Temperature conditions normally foreseen in any Process Plant. That brings the concept of composite piping wherein an attempt is made to make use of the mechanical strength of Steel, at the same time to impart a high degree of corrosion resistance to the wetted surface of the component made of Steel.
Carbon Steel Piping internally lined with Rubber is a classic example of such composite piping. Due to the intimate bonding between Rubber and Steel during vulcanizing process, it provides one of the most practical solutions in dealing with chemically corrosive fluids within the limitations posed by the endurance of Rubber for the operating temperatures.
Following are the commonly used Rubber varieties in the Chemical Plant in general
- Natural Rubber
- Butyl Rubber
- Nitrile Rubber
The Rubber Lining is a process of application of liner material (normally in the form of rubber sheets or extruded tubes) over the internal surface of the steel piping followed by vulcanizing at the required temperature to attain the desired degree of hardness.
The Lining Process generally consists of following steps
- Inspection of Piping Items for verifying its fitness for the purpose. The aspects which need to be considered with respect to above, are the accessibility of the internal surface for application of lining and suitability of the internal contours for achieving a defect free lining.
Note: In order to provide reasonable access to the internal surface of the Piping for application of liner, necessitates that Piping profile is broken down into flanged spools/ fittings of predetermined dimensions.
- Surface preparation in terms of removal of scale/ rust either by wire brushing or sand/ shot blasting.
- Application of Rubber Solution/ lining material
- Curing of Liner material (i.e. vulcanizing) at the required temperature either by employing hot water or steam. The vulcanizing of Rubber could also be done at ambient temperature by adding certain accelerators along with the filler materials used in the Rubber Stock.
- Inspection of the lined surface for possible defects such as pin holes/ blisters etc.
- Hydro testing of the lined components
RUBBER LINED PIPING SYSTEM:
The limited temperature endurance of the Rubber Lining Material prohibits any welding on steel subsequent to lining. That means that the Rubber Lined Piping System needs to be necessarily prefabricated. At the same time the total profile is also required to be broken down into flanged spools of such configuration that the internal surface requiring lining is accessible from at least one of the open ends for application of lining. It is from the above concern that the line size of Rubber Lined Piping is recommended to be 2” NPS and above.
A Rubber lined Piping System therefore is an assembly of various flanged Spools/ Fittings prefabricated to a predetermined dimensions. The supporting system employed for Rubber Lined Piping is normally a clamp and shoe arrangement. Welding of any support component to steel surface of the lined piping is completely prohibited. The accessories to Rubber lined Piping include soft Rubber gasket and full threaded Fasteners.
- A minimum of 2” NPS line size is recommended for Rubber Lined Piping
- The maximum length of straight Flanged Spools shall be as per Fig 3-A. These dimensions are decided to allow a reasonable accessibility for lining application and to avoid any pipe to pipe welding joint within the spool length based on Single Random Length of Steel pipe. It is logical to use straight Pipe Spools of Standard Dimensions (i.e. the maximum recommended spool length as per Fig 3-A Standard) as far as possible, to minimize the number of flange joints and at the same time to facilitate interchangeability of the spools. However the last spools at the corners or branch locations of the profile (or at any location specific situation) shall be adjusted to suit the dimension of the profile.
The straight Flanged Spools employ fixed flange at 1 end and loose flange (i.e. lap flange with stub end) at the other end to accommodate the bolt hole rotational mismatch of the flanged fittings bolted to the straight spools.
- In order to facilitate interchangeability, all flanged fittings (including Tee) shall be of a Standard Dimension as per Fig 3. Use of flanged fittings of non-standard dimensions is however permitted (under exceptional situations) provided they are procured as non-standard Special Parts.
- For tapping of small size (i.e. Instrument / Drain-Vent Connections etc) a sandwich type Carrier Flange with nozzle of the required size will be employed and procured as a standard Special Part.
- It is likely that due to erection deviations, the prefabricated/lined chain of flanged spools may either leave some gap or may have excess length while connecting the end terminals at site. A design solution (i.e. by employing spacers) for such deviations is impractical and hence the spools requiring adjustment as per site condition shall be fabricated as per actual site condition and sent for lining on a rush basis. During order finalization a commitment on crash delivery of such spools (including material supply) will be obtained from the Vendor. The complete activity shall be coordinated from site in consultation with Project Group.
- As a contingency measure some Solid Spacers (made of Hard Rubber/ Ebonite) of predetermined dimension are procured as standard Special Parts. These spacers can be cut into small rings at site to suit the Gap (encountered during assembly of Rubber Lined Piping) and sandwiched between the flange joint to fill the gap. It may however be noted that the above solid spacer can be used to fill small gaps (of the order of 25 mm) only due to the limited strength of the Rubber. Any gap in excess of 25 mm shall be handled as described in the earlier paragraphs.
- During the design stage it may be necessary to incorporate some spacers to bridge the corner to corner dimensions of the profile. Lined Spacers fabricated from Carbon Steel and subsequently Rubber lined are employed for the above purpose. As far as possible the Lined Spacer of discrete thickness of 50mm, 75mm and 100 mm shall be used.
- The Rubber Lining may obstruct the opening of flap of Sandwich type Butterfly/ Wafer Check Valve into the pipe. In order to address the above issue the Lined Spacer (Type-1) shall be used across the Valve. The thickness of the above spacer shall be decided as per the requirement of the valve under question.
- All the straight flanged spools (including non-standard special parts) shall be assigned part no., in the Isometric Drawings.
- The supporting of Rubber Lined lines is done similar to Carbon Steel Piping with the following exceptions.
- The lines are recommended to be supported, by employing Shoe and Clamp type supports.
- No welding (e.g. Dummy/ Trunnion) is permitted on the Rubber Lined Piping components subsequent to lining.
BILL OF MATERIAL CONSIDERATIONS:
Enquiry Bill of Material:
- The Enquiry BOM shall be prepared on completion of about 90% of the Isometrics for Rubber Lined Piping.
- The Pipe quantity shall be indicated in terms of the total requirement for each size based on the Isometrics
- The quantity of Flanged Fittings shall be counted from the Isometrics and indicated in numbers.
- The Flange quantity shall represent the requirement of Flanges on straight Pipe Spools.
The quantity of each of fixed and loose type Flanges of a particular size will be equal to the number of straight spools of that size.
- The quantity of Solid Spacers of each size is estimated based on the following equation
Q = (30 * No of Lines per size) / 500 (rounded to next number)
The above equation is based on an estimated requirement of 1 spacer (25 mm long) per line per size.
- The total quantity Q of Lined Spacers for each size is estimated to be equal to the number of lines in that size. The above total quantity is split into 3 parts as below
Spacer of 50 mm width 50% of Q
Spacers of 75mm width 25% of Q
Spacers of 100mm width 25% of Q
- The requirement of non-standard Special Parts will be picked up from the Isometrics.
- Unlike other Piping Systems, no spares are included in the Bill of Material of Rubber Lined Piping unless required as per the Contract Agreement.
Order Bill of Material:
- At the ordering stage an updated Bill of Material is issued along with the Construction Isometrics for execution purpose.
- The above Bill of Material will account for only as much material as required for the Isometrics being released for fabrication
- It is expected that nearly 80% of the Isometrics would be included in the above lot.
Final Bill of Material:
The Final Bill of Material will account for the remaining 15% (i.e. 100% requirement of the Project) of the Isometrics which will be issued along with the above BOM.
TECHNICAL SPECIFICATION CONSIDERATIONS:
- The Vendors scope includes supply, fabrication, rubber lining, testing and delivery of rubber lined flanged spools (including non standard special parts) as per Isometrics.
- Items such as Gaskets, Fasteners, On-line Valves, Instruments, Special Parts (with the exception of non-standard Special Parts) are excluded from the Vendor’s scope.
- The Rubber Lining shall be carried out as per BS 6374.
- Prior to Rubber-Lining the internal surface to be lined shall be appropriately prepared by way of grinding excess weld deposits/sharp corners. Besides the surface shall be made free of rust/ scale by sand blasting.
- The Vendor shall confirm the suitability of Rubber Material against the Fluids indicated in the Enquiry document
- In absence of specific requirement Natural Rubber shall be used
- The Finished lining shall have 80 Shore D Hardness and shall be suitable for the temperature range of 45 °C to 95 °C.
- A minimum of 3 mm Thick Rubber Lining shall be achieved on all surfaces.
- The Rubber Lining shall be done by Vulcanizing Process under controlled temperature.
- The Rubber Lined Piping items shall be tested in Accordance with BS 6374 and shall include, but not limited to, following tests.
- Visual Inspection
- Peal Test
- Spark Test
- Hardness Test
- Flatness Test of Lined Sealing Surfaces
- Outside surface of all Rubber Lined items shall be painted with 2 coats of Primer (e.g. Epoxy/ Zinc Phosphate)
- The straight Flanged Spools (including non-standard Special Parts) shall be suitably marked (Whether Proof Marking on the Body and Punching on the Flange rim) the Part Number and the Isometric Number.
- Since the Vendor is required to Quote on a Lump Sum basis it may be pertinent to get the units rates at the time of enquiry to process the order amendment at a later date based on the final status of engineering. Normally unit rates of following items shall be obtained for each size
- Lined Pipe without Flanges
- Lined Loose Flange including welding
- Lined Fixed Flange including welding
- Lined Branch Connection including welding
- Only Lining
- The Final Grouting of the Equipment connected with Rubber Lined Piping is done only after the erection of the connected Rubber-lined Piping.
- The Rubber Lined Pieces are handled carefully to avoid damage to the lined surface.
- The Flange Joints shall be tightened to the specified Torque Value by employing Torque
- The Solid Rubber Spacer (for site adjustment) shall be machine cut as per the required thickness (a maximum of 25mm gap). The above Solid Spacers shall not be provided near the equipment connection.
- In case the gap is more than 25 mm the corrective action shall be taken by re-fabricating the pipe spool as per the required dimension and sending it for lining.