ENGINEERING SPECIFICATION
ES 30-99-00-0103
PIPELINES LAUNCHER & RECEIVER SYSTEMS
2 25.03.2007 SPrE (HH) SSpE (KG) HE (JD/FG) Promoted to be ADCO Standard 1 3.11.2004 ILF SME (KG)/
SPrE (AW) HE (JD/FG) Project Specific
REV. DATE ORIGINATOR REVIEWED
(Custodian)
APPROVED (Forum Facilitator)
DESCRIPTION
TABLE OF CONTENTS
1. INTRODUCTION 3
2. TECHNICAL DESIGN SPECIFICATION 9 3. LAY-OUT, ANCILLIARY FACILITIES AND OTHER CONSIDERATIONS 46 4. DESIGN CONSIDERATIONS FOR SOUR SERVICE 50 5. OPERATING PROCEDURES 53 6. SAFEGUARDING SYSTEM AND PHILOSOPHY 53 7. SUGGESTED PIGGING SCHEDULING 58 8. SAFETY PRECAUTIONS 58 9. EMERGENCY SHUTDOWN (OPTIONAL) 59 10. SHARED (TEMPORARY) PIG TRAPS 59 11. PIG TRAP DATA SHEET 60 12. TYPICAL PIG LANUCHER/RECEIVER SYSTEM ARRANGEMENT 60 13. MECHNICAL INTERLOCK SEQUENCE – A CASE EXAMPLE 61
ATTACHMENTS
APPENDIX - 1 : DATA SHEETS – LAUNCHER & RECEIVER
APPENDIX - 2 : TEMPLATE PID AND OPERATING SEQUENCE FOR OIL PIG TRAPS
APPENDIX - 3 : TEMPLATE P& ID AND OPERATING SEQUENCE FOR GAS PIG TRAPS
APPENDIX - 4 : TEMPLATE P& ID AND OPERANTING SEQUENCE FOR MULTI PHASE FLOW PIG TRAPS
APPENDIX - 5 : PIPELINE PIGGING REQUIREMENTS
APPENDIX - 6 : INTRUSIVE AND NON-INTRUSIVE PIG SIGNALLER VENDOR SKETCHES
APPENDIX - 7 : FLOW RAE VS VELOCITY GRAPH – LIQUID & GAS APPENDIX - 8 : BARRED TEE
1 INTRODUCTION 1.1 Scope
This Standard Design Specification (SDS) specifies requirements and gives recommendations for the design of pig trap systems across all ADCO Fields having a diameter of 200 mm to 1400 mm (8 to 56 inch). It is provided for use by the project teams and design contractors. It is written in the context of onshore horizontal pig trap systems for liquid, gas and multi-phase hydrocarbon fluids either permanent or temporary installations and shall not cover Automatic and Bi-directional Pigging. This SDS consists of specific design directives, recommended practices and typical operational requirements. Exact direction has been provided where possible. In addition, it incorporates all the requirements of Shell Design Engineering Practice (DEP) titled “Design of Pipeline Pig Trap Systems” DEP 31.40.10.13-Gen (December 1998 issue including DEP circular 18/02).
1.2 Intended Use and Regulatory Considerations
This SDS is intended for use while designing pigging facilities for new pipeline. This specification shall standardise the minimum safety measures and configurations for launcher / receiver systems. It will serve as a basis for ongoing and/ or new projects and shall be used to scrutinize the existing systems to determine their deficiencies. If UAE local regulations exist in which some of the requirements may be more stringent than SDS, the Contractor shall determine by careful scrutiny which of the requirements are more stringent and which combination of requirements will be acceptable as regards safety, environmental, economic, and legal aspects. In all cases the Contractor shall inform ADCO of any deviation from the requirements of SDS which is considered to be necessary in order to comply with UAE local regulations.
1.3 General Definitions
The Contractor is the party that carries out all or part of the design, engineering, procurement, construction, commissioning or management of a project, or operation or maintenance of a facility. ADCO may undertake all or part of the Contractor duties. The Manufacturer/Supplier is the party that manufactures or supplies equipment and services to perform the duties specified by the Contractor.
ADCO (Abu Dhabi Company for Onshore Oil Operations) is the party that initiates the project and ultimately pays for its design and construction. ADCO will generally specify the technical requirements or may also include an agent or consultant authorised to act for, and on its behalf.
The word “must/shall” indicates a mandatory requirement. The word “should” indicates a preferred course of action. The word “may” indicates one acceptable course of action.
1.4 Specific Definitions
Balance Line A small-bore line that allows pressurisation of the barrel on both sides of a pig at the same time.
Barred Tee A tee-piece provided with bars across the internal bore of the side branch to prevent entry of a pig.
Bypass Line Piping between the pipeline and associated plant or facility through which fluid flows under normal operational conditions. Cup/Disc Flexible membrane forming a seal between the pig and the
pipe wall.
Drain Line A small-bore line used to drain fluid from the barrel.
End Closure A fitting, including a removable part or assembly that provides quick and easy access to the major barrel when open and seals the bore when closed.
Kicker Line Piping connection from pig trap to incoming/outgoing pipeline used to promote movement of pig into/out of a pig trap.
Main Line The major portion of a pipeline, between pig traps.
Major Barrel Enlarged pipe section used for used for loading or retrieval of pigs.
Minor Barrel Pipe segment between reducer and pig trap isolation valve, of the same diameter as the pipeline.
Pig A mechanical device driven through pipeline-by-pipeline fluids to prepare, gauge, inspect or maintain a pipeline in a suitable condition.
Pig Launcher A pig trap for launching pigs into the pipeline and comprises major and minor barrel with reducer, end closure and instruments.
Pig Receiver A pig trap for receiving pigs from the pipeline and comprises major and minor barrel with reducer, end closure and instruments.
Pig Signaller A device installed on a pipeline or pig trap to give an indication of the passage of a pig.
Pig Trap System
A pig trap together with all associated piping, valves, supports and instruments.
Piping Pipework associated with the pipeline but not part of the main line.
Pressurising Line
A small-bore line with valves to allow equalisation of pressure across a larger valve, avoiding damage to the seats of the larger valve.
Catch Pit or Drip Tray
Draining facility underneath an end closure. Tell-tale vent
or Safety bleeder
A safety device as part of the end closure door locking mechanism to safeguard personnel during door opening.
1.5 Applicable Codes and Standards
All design, engineering and materials shall generally conform to the latest issue of Shell DEP’s except where ADCO standards, philosophies, specifications, local statutory codes and regulations and the Technical provisions of this section indicate otherwise.
The order of precedence of requirements shall generally be as follows, unless otherwise directed in writing by ADCO:
UAE Local Statutory Codes and Regulations,
Project Specific Features and Design Requirements.
Technical Specific Features and Design Requirements under this SDS,
Shell DEP’s and all referenced Codes and Standards therein,
ADCO HSE Manual and ADCO HSE Risk Management
International Codes and Standards
1.5.1 SHELL DEP Standards
[Ref 1] Design of Pipeline Pig Trap Systems : 31.40.10.13-Gen [Ref 2] Pipeline Fittings
: 31.40.21.30-Gen [Ref. 3] Pig signallers : Intrusive type : 31.40.21.33-Gen Pressure vessels (Amendments/Supplements
to ASME VIII, Div. 1 and Div. 2)
Data/requisition sheet for design of a pig trap system for a pipeline
: 31.40.10.93-Gen
Data/requisition Sheet for pig trap end closure : 31.40.21.94-Gen Pressure relief, emergency depressurising,
flare and vent systems.
: 80.45.10.10-Gen
EP HSE Manual (Shell Standard) : EP-95000
1.5.2 International Standards (Latest Applicable Revision)
Process Piping : ASME B 31.3 Liquid transportation systems for hydrocarbons, LPG,
anhydrous ammonia, and alcohols
: ASME B 31.4 Gas Transmission and distribution piping systems : ASME B 31.8 ASME Boiler and Pressure Vessel Unfired pressure
vessels
: ASME Section VIII Sizing, Selection & Installation of Pressure Relieving
Devices in Refineries. Isolation stop valves in pressure relief piping.
: API RP 520 part II, section 4
IP Model code of Safe Practice: Area Classification code for Installation & Handling of Flammable fluids
: IP 15
Pipeline Safety Regulation : BS8010 Parts 2 & 3
Sulphide stress cracking resistant metallic material for oil field equipment
: NACE MR0175 Evaluation of Pipeline and Pressure Vessel Steels for
Resistance to Hydrogen-Induced Cracking
: NACE TM0284
1.5.3 ADCO Standards for HSE
Health, Safety and Environment Management Policy Guidelines
:
Guidelines for Preparation of the Health, Safety and Environment (HSE) Philosophies Document
:
Environmental Protection and Occupational Health Management – Policy and Guidelines
:
Health, Safety and Environmental Risk Management Guidelines
1.6 Climatic Conditions
¾ Wind
i) Prevailing Direction From NNW ii) Mean Speed 8.0 m/s iii) Maximum Design Speed 44.7 m/s
iv) Airborne Particles Saliferous dust and sand with frequent sandstorms ¾ Temperature
i) Maximum Solar 85 ° C ii) Maximum Shade 58 ° C iii) Average Shade Summer 36 ° C iv) Average Shade Winter 22 ° C v) Average Shade Yearly 28 ° C vi) Average Minimum Shade 4 ° C ¾ Humidity
i) Relative Maximum at 43 °C 95% ii) Relative Maximum at 54 °C 60% ¾ Solar Radiation => 946 W/m2 ¾ Rainfall
¾ Dew => Heavy
¾ Mists => Early morning mists causing evaporative cooling loss i) Frequency Infrequent
ii) Maximum 51mm/year iii) Minimum Trace
1.7 Abbreviations
ADCO : Abu Dhabi Company for On shore Oil Operations ADNOC : Abu Dhabi National Oil Company
API : American Petroleum Institute
ASME : American Society for Mechanical Engineers CAPEX : Capital Expenditure
DEP : Design Engineering Practises ESD : Emergency Shutdown
H2S : Hydrogen Sulphide
HAZOP : Hazard and Operability Study HC : Hydrocarbon
HIC : Hydrogen Induced Cracking HSE : Health, Safety and Environment ID : Inside Diameter
IPS : Intelligent Pigging System
NACE : National Association for Corrosion Engineers NPS : Nominal Pipe Size
P & ID : Piping and Instrument Diagram PPM : Parts per Million
QOEC : Quick Opening End Closure
SDS : Standard Design Specification for Pig Trap system (Engineering Specification for Pig Trap System)
UAE : United Arab Emirates
This SDS specifies requirements and recommendations for the design of horizontal pig trap systems for onshore pipelines liquid, gas and multi-phase hydrocarbon fluids, permanent or temporary installations. This Standard Design Specification shall be used for design purposes only and not for material procurement.
2.1 Description
A simple definition of Pig Trap is – a piece of pipeline equipment that allows easy loading or unloading of a pig into, or out of, the pipeline. Its purpose is to provide in a safe manner and without flow interruption the means to either insert and launch a pig into the pipeline or receive and retrieve a pig from a pipeline.
A pig trap comprises the following basic components that are illustrated in attached template P & ID’s, Appendix 2, 3 and 4:
A quick opening closure or blanked flanged end, which provides quick and easy access to the major barrel when open and seals the bore when closed.
A major diameter section, referred to as the Major Barrel.
An eccentric reducer for connecting the major and minor barrels.
A minor diameter section corresponding to the line pipe size and referred to as the minor barrel.
Various nozzles such as vent, drain, pressure indicator, kicker, bypass, relief, equalizing and pig signaller.
Lifting lugs, supports and earthing lugs
The boundaries of a pig trap system are depicted in Figure 1 and are defined as:
A point on the incoming/outgoing pipeline, on the pipeline side of the main tee but including the main line pig Signaller.
The pipeline side of the isolation valve of connecting facilities.
Kicker line and valves.
Pig trap isolation valves.
Instruments like pressure indicator, thermal relief valve
The sections below describe each pig trap and pig trap system component, in order to give a clear understanding of its purpose, and give detailed minimum requirements. The main components and configuration of typical pig trap are shown in template P & IDs. The requirements for additional components not shown in template P & IDs shall be determined in accordance with the Design Codes and standards specified in this document, based on actual service conditions.
In designing a pig trap system special attention shall be paid to the following features:
Safe operations with proper positive isolation this is achieved by fitting a blank, insertion of a spade or by rotating a spectacle blind;
Operational flexibility to facilitate commissioning and decommissioning operations;
Adequate venting, draining and purging facilities;
Facilities for possible chemical injection, sampling, water dousing, etc.;
Types of pigging operations (e.g. cleaning, intelligent, etc.) and frequencies.
General environmental criteria (maximum & minimum ambient temperatures, etc);
Pipeline fluid properties, including any corrosive or erosive materials;
Pig trap layout and orientation;
Operating conditions, design & test pressures and temperatures;
Mechanical design codes, material of construction.
The design shall be based on the most onerous type of pigging operation, which is envisaged for the pipeline. This shall, in most cases, mean designing for intelligent pigs. Where intelligent pigging is not required, all aspects of this SDS remain except pig trap lengths and access area.
Pig traps system are designed and fabricated based on:
Basic design parameters
Functional design parameters
The following sections describe the design parameters in detail.
2.3 Basic Design Parameters
The basic design parameters cover the following aspects of pig trap design.
2.3.1 Design Code and Code break
The pipeline design shall be based on ASME B31.4 or ASME B31.8, depending on the product.
For the purpose of code break locations it is also assumed that the piping of the facilities to which the pipeline/pig trap system is connected is designed in accordance with ASME B31.3. Where this is not the case e.g. at intermediate pig trap stations (e.g. Mile Stations) or where the pig trap ties into a slug catcher designed to ASME B31.8, the code break is not applicable.
The entire pig trap system should be designed, constructed and tested according to the same code as the pipeline. There are 4 options of design code break between ASME B31.8/B31.4 and ASME B31.3 [Ref 1]. The design code break between ASME B31.8/B31.4 and ASME B31.3 shall be as depicted in Figure –1 when pig traps are procured as prefabricated item.
Wall thickness transitions shall meet the welding configuration requirements as specified in the design codes ASME B31.4 (clause 434.8.6) and ASME B31.8 (Appendix I, Figure I5).
The distance between welds for fittings and branch connections shall be at least one pipeline diameter, but not less than 300 mm.
The maximum thickness for design pressures shall not be greater than 1.5 t, where t is the nominal thickness of the thinner pipe.
2.3.2 Design Factor
For pipelines designed with a hoop stress design factor higher than 0.6, the pig trap systems shall be designed with a factor of 0.6, to increase safety margins.
For pipelines designed with a hoop stress design factor equal to 0.6, the pig trap systems shall be designed with a factor of 0.5, to increase safety margins.
For pipelines designed with a factor of less than 0.6, the pig trap systems shall be designed with a factor equal to that of the pipeline.
This design factor shall be applied to the pipe work within the boundaries of the pig trap system plot, including any corrosion monitoring pits associated with the pig traps. The above approach has the advantage that conversion of an oil pipeline to a gas pipeline can in practice be done without any modifications to the pig trap systems.
2.3.3 Design Pressure
The entire pig trap system shall be designed, constructed and tested according to the same code as the pipeline. The pig trap design pressure shall be equal to that of the associated pipeline.
2.3.4 Design Temperature
The maximum design temperature shall not be less than the maximum temperature, which the pig trap system could attain or to which it could be exposed during operation, start-up or shutdown.
The minimum design temperature shall be based on minimum ambient temperature and on the conditions (e.g. blowdown), which could occur during operations.
See Climatic Conditions para 1.6.
2.3.5 Design Velocities for Piping Diameter
The recommended maximum velocities for the purpose of piping diameter selection are:
For piping in intermittent service:
In case of liquid 8 m/s. In case of gas 40 m/s.
For piping in continuous service:
In case of liquid 4 m/s. In case of gas 20 m/s.
Based on these velocities and the assumption that parts of the pig trap are in intermittent service, piping diameters are suggested in (Table 3). For every design, however, it should be checked that the velocities do not exceed designated maximum and that piping pressure drops are not excessive. For pigging speeds in the pipeline refer paragraph 2.4.1.3.
2.3.6 Test Pressure
The pig trap shall be hydrostatically tested in shop based on code requirements as indicated in the relevant data sheet.
2.3.7 Corrosion Allowance
In absence of other guidelines a minimum corrosion allowance of 3 mm shall be considered for minor barrel, reducer and major barrel.
Caution: Due to 3mm corrosion allowance, the minor barrel ID would be less than the pipeline ID. This transition in thickness shall be smooth and verified as part of pipeline pigging studies to avoid pig getting stuck in the minor barrel.
2.3.8 Materials
The materials of construction shall be as indicated in the Pig Trap data sheet.
To aid the selection process the line product shall be specified i.e. sour, toxic, corrosive along with impurities etc. This will influence the selection of metallic as well as elastomeric materials typically the sealing elements.
Metallic Materials:
All components in sour service shall be resistant to HIC (Hydrogen Induced Cracking) and conform to NACE specifications.
All main line items shall be compatible with the line pipe with respect to weld ability, wall thickness/ material grade transitions and dimensions.
Dimensional considerations include actual internal diameter, ovality and wall thickness transition taper angles.
All items shall be designed and manufactured to avoid brittle fracture at possible low service temperatures. Although transmission pipelines are excluded from its scope it may be relevant for many of the pig trap components especially the vent / blowdown line in high pressure gas systems.
For selection of metallic materials refer DEP 30.10.02.11 - Gen.
Non-metallic Materials (especially the elastomer seals):
The properties of elastomeric seals are strongly influenced by the choice of compounding ingredients for the base polymer, fillers, additives, curing system, and by the compounding process. For the selection of a seal, it is therefore important to specify the desired performance properties, which the seal must meet instead of selecting on the basis of an elastomer type.
Elastomeric seals have to meet three basic requirements:
1) the material must be chemical resistant to the environment with which it will be in contact;
2) the material must be able to withstand the temperature in service and during storage;
3) the material must be able to withstand mechanical loads as pressure, stress or abrasion.
When selecting a suitable material for seal application the following mechanical properties shall be specified:
1) The required hardness: quoted in shore A or IRHD, the normal hardness for sealing application lies between 60 - 95 IRHD (according to ASTM D 2240 or ISO 48);
2) Compression set: this is the ability of an elastomer to recover from a deformed state to original dimensions; a low compression set is usually preferred in seal applications, testing according to ASTM D 395 or ISO 815;
3) Tensile strength, elongation, modulus at 100 % elongation: characterises the material strength and stiffness, according to ASTM D 412.
4) Attention shall be paid to some special seal problem areas as:
a. Explosive decompression (ED): sudden decompression from a high pressure gas environment can lead to the production of blisters and surface splits in most elastomers. For further information, see DEP 30.10.02.13-Gen.
b. High pressure extrusion: a seal must have sufficient strength to resist extrusion out of its containment groove when subjected to high pressure and temperature. High pressure performance is mainly determined by the seal hardness and the seal stiffness at high temperature. Techniques to avoid extrusion in high pressure application include decreasing the clearance gap of the seal groove, increasing modulus and hardness and the use of back-up rings. c. Effect of corrosion inhibitors: in some cases corrosion inhibitors attack
elastomeric seals. High pH amines are the most hostile chemical. Three main degradation mechanisms are known when up to a few per cent of such chemicals are added to a fluid.
d. Methanol resistance: the swelling effect of methanol is greater than that of other alcohols.
For selection of non-metallic materials refer DEP 30.10.02.13.
2.4 Functional Design Parameters
The functional design parameters are discussed below.
2.4.1 Pigging Operation and PIG
Pigging operations are required in order to prepare, operate and maintain pipelines. These operations can be summarised as follows however requirements will vary for individual pipelines:
Dewatering after hydrotest;
Cleaning the pipeline both during commissioning and after it is in operation;
Gauging internally;
Inspection of the pipeline both internally and externally (geometric verification);
Structural integrity evaluation (investigation on metal loss and material defect)
Controlling of liquid hold-up in gas pipelines; and
A Pig can be defined as a mechanical device that is inserted in the pipeline to carry out pre-defined tasks along the length of the pipeline depending on its construction.
2.4.1.1 Pig Selection
Pigs must first be selected to carry out the intended function (task). The pig can generally be described by its function like cleaning pig, gauging pig, de-watering pig, intelligent pig etc. These pre-defined functions all rely on one or combination of the following aspects of the pig’s design characteristics:
Ability to seal
Ability to clean
Ability to gauge
Ability to withstand the operating conditions
Compatibility with service or pigging medium
Ability to last the entire pig run
Some of the characteristics may need to be enhanced in order to provide the best pig for the task. This will be influenced by the pipeline development stage at which the pig must carry out its task. These stages are - construction, pre-commissioning, commissioning, Inspection, maintenance, repair, decommissioning etc. Pigs designed to carry out tasks at one stage of pipeline may not possess features that are suitable to enable them to carry out the same task at other stages. This is due to a combination of the following factors:
Pigging distance
Frictional resistance of the materials in contact
Lubrication
Pigging speeds and
Pipeline design factors like:
Æ Pipeline construction feature - bend radius, tees, wyes, diverters, multiple pipeline diameters, material, internal coating/ lining etc
Æ Service – liquid, gas, multiphase Æ Temperature
Æ Pressures Æ Flow
2.4.1.2 Types of Pig
Based on their function, specific features and operating mode, pigs can be classified into one of the following categories:
¾ Conventional Pigs
These are passed through a pipeline to perform a specific function related to construction, operation or maintenance (e.g. cleaning, dewatering, batching). They do not generate or collect any data related to the pipeline condition. Those typically used in pipeline construction and operation are:
• Foam pigs - are used for liquid sweeping and are cylindrical in shape with a conical or parabolic nose and made of resilient polyurethane foam. The diameter is normally 0.5 inch – 2 inch larger than the internal diameter and the length is usually twice the diameter. They can be bare with a back disc or coated to achieve different duties.
• Cleaning pigs equipped with polyurethane driving cups - usually consist of a disc pig or conventional cup pig provided with polyurethane blades or hardened steel brush units, which are attached to a steel body with springs to maintain pressure against the pipe wall. In some cases cleaning blades are shaped to induce rotation of the pig to provide uniform wear of the blades and pig cups during operation.
• Gauging pigs equipped with polyurethane driving cups and aluminium gauging plate(s) - designed for gauging new pipelines to prove the pipe is circular and that there is no excessive weld penetration or construction debris remaining in the pipe. One pass will assure passage of other units. Constructed of a tubular steel body fitted with easily replaceable neoprene or polyurethane cups and a gauging plate made from steel or aluminium alloy. Gauge plates may be installed on either the front or rear of the pig or at both locations.
¾ Specialty Pigs
These are designed to perform ‘special’ functions as internal coating application, line plugging for maintenance and repair, wet buckle prevention, etc. They do not generate or collect any data.
¾ Intelligent Pigs
These are used to gather information on the pipeline geometry, configuration and wall conditions. Information obtainable from intelligent inspection tool runs includes:
geometry measurement, e.g. wall thickness and mechanical defects
external/internal metal loss, including general and pitting corrosion
mill defects, i.e. laminations and inclusion clusters
hydrogen induced cracking
crack detection
metal detection in padding material close to the pipeline.
Various criteria are important for the selection of the tool to be used. Briefly, the tool to be used should:
not-interfere with pipeline operation
detect all significant defects
discern spurious indications from actual defects
determine defect sizes, position and location
distinguish internal from external metal loss
give an accurate view of the overall pipeline integrity
2.4.1.3 Pigging Speeds
The pigging speeds are dependant on the product flow and failure to operate within the specified optimum range will affect the performance and may require additional runs. The velocity can also affect the efficiency with respect to the sealing capability of cups, discs and cleaning capability of brushes. Excessive velocities may damage both pig and pipeline. Below are listed some criteria’s and pigging velocity however, it is advisable to take Vendor feedback for pig velocity on case-to-case basis.
¾ General
The pressure required to move a pig depends upon a number of factors like:
The pressure in front of the pig
Condition of the pipe wall surface
Pressure exerted by the seals on the pipe wall (the ‘fit’ of the pig in the pipe)
The presence of a lubricant and
for Intelligent Pigs on the working principle and on the capabilities of devices used for data logging.
Pigs move in the pipeline under the influence of the ‘differential ‘pressure across them i.e. the pressure behind the pig (P1) minus the pressure in front of the pig (P2). This differential pressure (‘delta P) gives the pig a velocity. (i.e. It gives the pig both speed and direction). If P1 is less than P2 the pig will move backwards.
The differential pressure contributes to the efficiency of pigging operations by improving the sealing efficiency i.e. it forces the seals against the pipe wall, making them act like non-return valves.
¾ Pigging with Liquid
Liquid flows improve cleaning efficiency by allowing the pig to maintain a constant velocity. Velocities can be maintained, within limits. They can be lowered or controlled at the pump or globe valve. Seal wear rates are also reduced as the product or pigging medium acts as a lubricant.
The recommended pig velocities irrespective of pipe size are: Cleaning Pigs => 0.5 – 2.5 m/sec
Intelligent Pigs => 1.0 – 2.5 m/sec ¾ Pigging with Gas
Compressed air operations impose several conditions on the pig – no lubrication is available and pigging occurs in a series of high speed ‘excursions’ between localized restrictions. The pig stops and the pressure builds until there is sufficient energy to launch the pig past the obstacle. As the pig moves forward rapidly, the pressure is dissipated until the pig reaches the next restriction, causing the process to be repeated.
The velocity profile is erratic when pigging with gasses, due to their ability to be compressed. Once the pig begins to move this energy is released very rapidly. Aside from the safety aspects, pigging in this manner is inefficient; increases wear on the pig seals due to friction heat build-up and are generally more destructive to the pig. Increased speed also causes a decreased pressure differential across the pig, which, in turn, results in decreased sealing efficiency.
The recommended pig velocities irrespective of pipe size are: Cleaning Pigs => 0.5 – 2.0 m/sec
Intelligent Pigs => 1.0 – 2.0 m/sec
¾ Pigs with BY-PASS facility
It is possible to design pigs with a by-pass facility to run pigs at lower speeds than the product flow speed. ‘By-Pass’ is the controlled flow of pigging medium, gas or liquid, through and/or past the pig. It has two effects:
It reduces the pressure (P1) behind the pig and hence the differential pressure (delta P) across the pig – variations in the differential pressure affects the speed of the pig. The pig slows down as the differential pressure decreases.
The by-pass facility can also be equipped with Jetting head which provides the necessary jetting action to increase the efficiency of cleaning pigs by preventing brushes from becoming blocked with loosened deposits and holding these deposits in suspension in front of the pig rather than allowing them to form into a solid ‘slug’. These slugs are difficult to deal with at the receiver, also they may cause the pig to stuck.
With a by-pass facility pigging can be done at product flow speeds upto 10 m/sec, while the pig moves at a lesser speed of 3 to 5 m/ sec depending on design.
2.4.1.4 Pigging Pressure and Flow
It is not possible to recommend minimum pressures while pigging because it is dependant on many factors like function and type of pig, seal material and type, pipeline condition, types of deposits, lubricant, liquid or gas pigging, temperature, viscosity etc.
Refer Appendix- 7 for flowrate Vs Velocity graph for liquid and gas. The Graph -1 shows the flowrate in a pipe against the velocity for liquid.
For gas it is not possible to provide a single graph showing the flowrate in pipe Vs velocity. This is due to the compressibility nature of the gas. The gas velocity in the pipe is a function of the actual system pressure. The pressure will change along the gas pipeline also the gas velocity will increase with lower pressures. The Graph –2, 3 and 4 shows the flowrate in a pipe against the velocity for gas at pressure 10 bara, 50 bara and 100 bara respectively for different pipe sizes. The graphs are valid for following gas condition:
Model Gas => Methane (100%) Gas Density at Standard condition of 1 atm/ 60°F => 0.68 Kg/m^3
Gas Density at 10 bara @ 20°C => 6.71 Kg/m^3 Gas Density at 50 bara @ 20°C => 36.24 Kg/m^3 Gas Density at 100 bara @ 20°C => 78.29 Kg/m^3
Note that these diagrams should only be used as a reference for the worst-case conditions i.e. low pressure (high speed) section at the end of a pipeline.
2.4.1.5 Pigging Frequency
During normal operation of pipeline the pigging frequency is determined by quantum risk of internal corrosion to the pipeline. This shall be determined by ADCO’s Corrosion specialist and is outside the scope of this document. Nevertheless some guidelines for pigging frequencies are shown in Appendix 5.
2.4.2 Barrels
The barrel is the section of the pig trap, from the pig trap valve up to and including the end closure, which is required to launch and receive pigs. It consists of four parts:
¾ End Closure. As defined under 1.4. ¾ Major Barrel. As defined under 1.4.
To facilitate the insertion and removal of pigs, for pipelines smaller than 20-inch diameter the major barrel (for launcher and/or receiver) should be 2 inches more than the pipeline diameter. However if the barrel wall thickness is greater than 15mm then the barrel becomes tight to push the inspection tool in. This case shall be discussed with Vendor before finalizing the major barrel diameter.
For pipelines with diameter of 20 inch and larger the major barrel oversize should be 4 inches. Typical sizes for major barrels are given in (Table 1). ¾ Reducer. As defined under 1.4.
The transition connection between major and minor barrels shall be an eccentric reducer with the bottom of the entire barrel at the same level. The length of the reducer section shall be in accordance with ASME B16.9.
Internal diameter of the minor barrel shall match the minimum inside diameter of the pipeline to which it is attached. This may be particularly relevant if thick wall low-grade pipe is used for the major barrel and the pipeline is thin wall high-grade material. The internal diameter of the minor barrel should be the same as that of the main line. However, transitions in the internal diameter due to wall thickness variations greater than 2.4 mm shall be tapered to a maximum angle of 14° to the pipe axis to allow for the smooth passage of a pig.
The internal surfaces shall be flush and designed to allow free passage of pigs. All nozzle connections shall be flanged and 2” NPS minimum. Outlet connection 3” NPS and larger shall be internally contoured. Outlet connections 4” NPS and larger shall be fitted with scraper bars. All nozzles except drain connections shall be located on the side or top of the pig trap.
Table 1 Typical Diameter of Major Barrel and Pipe work [Ref 1] Pipeline Diameter, mm (inches) Major Barrel, mm (inches)
200 (8) 250 (10) 250 (10) 300 (12) 300 (12) 400 (16) 350 (14) 400 (16) 400 (16) 450 (18) 450 (18) 500 (20) 500 (20) 600 (24) 600 (24) 700 (28) 650 (26) 750 (30) 700 (28) 800 (32) 750 (30) 900 (36) 800 (32) 900 (36) 900 (36) 1000 (40) 950 (38) 1 050 (42) 1 000 (40) 1 100 (44) 1 050 (42) 1 150 (46) 1 200 (48) 1 300 (52) 1 400 (56) 1 500 (60)
2.4.3 Launcher
For launchers, the minor barrel provides “head” space for the pig cups to engage and prevents the nose of the pig coming into the contact with pig launcher isolation valve providing that the balance line is open. This section is relatively short compared to major barrel.
For launchers the barrel shall be sloped (typically 1:100) down towards the drain outlet.
Pig traps should be designed for the longest pig that will be used (usually an intelligent pig) plus a margin of 10%. Refer Table 2 and Figure 2A [Ref 1] for dimensions of pig launchers designed for intelligent pigs.
2.4.4 Receiver
For receivers the minor barrel must be long enough for the pig to trigger the pig signaller and the length shall accommodate intelligent pigs so that the isolation valve(s) can be closed without damage to the pig or valve.
The length of the pig receiver unit shall be designed as a minimum to receive one intelligent pig and one cleaning pig plus 10% margin.
For receivers the barrel shall be sloped (typically 1:100) down towards the end closure to improve draining of liquids from the barrel. If there is a chance of solids getting collected ahead of the pig, then additional lengths should be considered. Refer Table 2 and figure 2B [Ref 1] for dimensions of receivers designed for intelligent pigs.
2.4.5 Bi-directional Traps
The application of bi-directional traps is not foreseeable within ADCO installations.
2.4.6 Barrel Reducer
A standard eccentric-type reducer (in accordance with ANSI B16.9) or a fabricated reducer (with a minimum length of no less than that of the standard reducer) shall be used. The reducing section shall be oriented with the bottom flat.
Table 2 Barrel Lengths for Intelligent Pigs [Ref 1]
Approx. Minimum Barrel Length, (m) Launcher Receiver Pipeline Diameter, mm (inches) Approximate Maximum Tool Length, m Approximate Maximum Tool Weight, kg AL BL AR BR 200 (8) 3.9 170 4.1 1.5 3.9 3.9 250 (10) 4.3 300 4.3 1.5 4.3 4.3 300 (12) 4.3 365 4.3 1.5 4.3 4.3 350 (14) 4.8 380 4.8 1.5 4.8 4.8 400 (16) 5.1 700 5.1 1.5 5.1 5.1 450 (18) 5.1 810 5.1 1.5 5.1 5.1 500 (20) 5.1 840 5.1 1.5 5.1 5.1 600 (24) 5.7 1 600 5.7 1.5 5.7 5.7 650 (26) 5.8 2 000 5.8 1.5 5.8 5.8 700 (28) 5.8 2 000 5.8 1.5 5.8 5.8 750 (30) 6.0 2 000 6.0 1.5 6.0 6.0 800 (32) 6.6 2 270 6.6 1.5 6.6 6.6 900 (36) 6.6 3 560 6.6 1.5 6.6 6.6 950 (38) 6.6 3 600 6.6 1.5 6.6 6.6 1 000 (40) 6.6 4 090 6.6 1.5 6.6 6.6 1 050 (42) 6.6 4 550 6.6 1.5 6.6 6.6 1 200 (48) 6.6 Vendor to confirm 6.6 1.5 6.6 6.6 1 400 (56) 6.6 Vendor to confirm 6.6 1.5 6.6 6.6 2.4.7 End Closures 2.4.7.1 General
The barrel closure shall be a quick opening door with side hinges or a swivel davit, provided with lubrication facilities. The quick acting design should allow opening and closing by one man in a period of approximately one minute, without the use of additional devices. Closures 18” and larger shall be hand wheel operated. Large size heavy closures shall be fitted with mechanical means to break seal when opening door and compress seal when closing.
The end closure shall be installed in the vertical plane.
Closure door shall be equipped with an interlocking device to prevent it being opened while the barrel is still under pressure. The end closure shall conform to the general requirements of ASME VIII, Division 1, Section UG-35 (b) (Quick Actuating Closures). In particular a fail-safe design of the opening mechanism shall ensure that the failure of any part of the opening mechanism shall leave the closure closed rather than open.
End closures with exposed screw expanders or captive ratchet braces are not recommended because of the high maintenance requirements and the non-fail-safe aspects of some opening mechanism designs.
For certain services, end closure leak through the seal ring grooves. In such cases it is advisable to overlay the seal ring grooves with Inconel 625. However this shall be decided during detail design of the project based on Operations feedback.
2.4.7.2 End Closure Components
The end closure shall consist of the following components:
A removable door, which provides full-bore access when open, and terminates and seals the bore when closed.
A welding end hub, for joining to the major barrel of a pig trap. The material used for the welding end hub shall be compatible with the major barrel material.
A closure-handling device, suitable to lift, hinge or swing the door. When the handling device is attached to the closure, it shall be attached to the welding hub, not to the major barrel of the pig trap.
Ring seals for pressure containment. The activation of the seals shall be such that the fluid within the trap is contained at any pressure between 1 bar (abs) and the pig trap design pressure. Elastomeric materials for ring seals shall resist explosive decompression and shall be suitable for long-term exposure to the transported fluid at the design pressure and temperature conditions. The cross-section of the seals shall not exceed 7 mm in diameter for design pressures of 150 bar and above. The ring seal material shall be compatible with transported fluid.
Two safety devices to prevent inadvertent opening of the closure before the pig trap is depressurised as detailed under Safety Devices below.
Note:
An end closure with a flanged-end for joining to the major barrel has recently become available. This may have application in certain circumstances, e.g. where the use of a temporary barrel extension piece is proposed to allow intelligent pigging. This specification does not recommend using such end closures due safety reasons. This type of barrel extension adds one more flange joint and can be compared to portable pig traps. Also the barrel extension is possible only on launcher. On receivers extending the neck pipe would be difficult and would mean moving the entire installation.
2.4.7.3 Safety Devices
The end closure shall have the following safety devices:
A pressure-locking device to prevent opening of the door when the pig trap is pressurized.
A safety bleeder/ tell tale vent that when released will alert the operator to a possible hazard unless pressure in the pig trap is relieved completely. Opening of the door shall not be possible unless the bleeder is released. Engaging the bleeder shall only be possible when the closure is closed. The bleeder shall be designed such that there is no risk of blockage.
The devices shall be constructed and located so that they cannot readily be rendered inoperative. The devices shall be easily accessible for inspection.
The interlock system shall be through mechanical trapped key systems. Refer ’Safety and Interlock Systems paragraph 6.0 for further details.
2.4.8 Pipe work 2.4.8.1 Bypass Line
A bypass line is required to connect the pipeline with related facilities such as a booster station, tank farm, etc. Typical sizes for the bypass line are given in (Table 3), based on fluid velocities for continuous service. However the bypass line is outside the scope of this standard document.
2.4.8.2 Kicker Line / Receiving line
A kicker line is required to connect the major barrel with the bypass line to enable diversion of the fluid through the barrel to launch or receive a pig. For a launcher the kicker line shall be connected to the major barrel as close as possible to the end closure and for a receiver as close as possible to the reducer. Typical sizes for the kicker line are given in (Table 3), based on fluid velocities for intermittent service. Install barred tees on kicker/ receiving line to avoid broken pigs passing through.
2.4.8.3 Balance Line
A balance line shall be provided on launchers to enable filling and pressurising or depressurising (equalizing) of the barrel on both sides of the pig at the same time. This is to prevent a pig, which is ready to be launched from moving either forwards (and thereby hitting and possibly damaging the pig trap valve) or backwards (and losing the seal in the reducer). To ensure this, the balance line, branching off from the kicker line, shall be connected to the minor barrel as close as possible to the pig trap valve. Consideration should also be given to the provision of a balance line on receivers to prevent any possible pressure differential across a received pig.
The drain line configuration as depicted in P & ID, Appendix-2, 3 and 4 can also be used as balance line. However it is recommended that the drain lines shall not be used as balance line for new installation.
2.4.8.4 Pressurising Line
The pressure equalising line across the kicker valve shall be provided for operating pressure higher than 1000 psig in oil service. For gas and multiphase service the pressurising line across kicker valve shall be provided for line size 6” @ 600# rating and above. This smaller diameter pressurising line around kicker valves serves to control speed of operation, control of barrel pressurisation and/or to avoid damage to the kicker valve seats or other internals. The pressurising line should be at least 2-inch diameter.
2.4.8.5 Safety Relief Line
A pig trap system shall be protected against overpressure by providing relief valves. The valve shall be provided at the highest point on the pig trap major barrel and piped to flare/vent system.
Where flare/vent system is not available, the valve shall be piped to a safe location (atmosphere). Relief valves discharging to atmosphere should be located at the maximum practical elevation to keep discharge piping (to safe location) as short as possible. Dispersion analysis shall be performed to determine the safe distances. To keep this discharge pipe free from liquid a small (8 mm dia) weep hole shall be drilled at the lowest point.
2.4.8.6 Drain Line
Drain points shall be provided near end closure and near the pig trap valve at lowest point of the barrel to allow maximum drainage of liquid accumulated in the barrel. Connection of drain lines to pressurized systems should be avoided to enable the contents of the barrel to be emptied as necessary. In addition, before opening the drain valve(s) the barrel shall be depressurised and vented to minimise vapour release or over pressurisation of the drain system.
Diameters of drain lines are as shown in (Table 3). The barrel drain lines shall be sloped (at least 1:300) towards a closed drain system or a designated open drain. To provide positive isolation of the barrel from the closed drain system during opening of the pig trap end closure, a double block and bleed valve arrangement shall be installed on the barrel drain line with spectacle blind. The first drain valve shall be located as close as possible to the barrel to avoid accumulation of dirt/debris in the branch that may render the drain valve inoperative.
A back flushing connection on the drain line underneath the vessel should be installed if there is a possibility of blockage from deposits of sludge, sand, etc. in the barrel.
2.4.8.7 Vent/flare/blowdown Lines
A vent line shall be provided near the end closure to vent/purge the barrel and near the pig trap isolation valves to ensure depressurisation behind a pig in the event of it being stuck in the minor barrel. The vent line shall be sized based on liquid content, blow down calculations etc. However the diameter of the vent line(s) shall be at least 50 mm (2 inch).
The vent line shall be equipped with isolation valve with interlock to ensure that it cannot be opened when the end closure is also open. For high-pressure gas and multi phase flow systems consideration should be given to the provision of a blowdown line, incorporating a globe valve or restriction orifice sized (such that over
pressurisation of the vent/flare system is avoided) for controlled depressurisation. During depressurisation, pig traps in gas service and associated piping may become cold. Protection of personnel should be by means of operating procedure rather than insulation.
Table 3 Typical Diameters of Pipe work [Ref 1] Pipeline Diameter, mm (inches) Bypass Line, mm (inches) Kicker Line, mm (inches) Balance Line, mm (inches) Drain Line, mm (inches) 200 (8) 100-150 (4-6) 100 (4) 50 (2) 50 (2) 250 (10) 150 (6) 100 (4) 50 (2) 50 (2) 300 (12) 150-200 (6-8) 100 (4) 50 (2) 50 (2) 350 (14) 150-250 (6-10) 100 (4) 50 (2) 50 (2) 400 (16) 200-300 (8-12) 150 (6) 100 (4) 100 (4) 450 (18) 250-300 (10-12) 200 (8) 100 (4) 100 (4) 500 (20) 250-400 (10-16) 200 (8) 100 (4) 100 (4) 600 (24) 300-450 (12-18) 200 (8) 100 (4) 100 (4) 650 (26) 400-500 (16-20) 250 (10) 100 (4) 100 (4) 700 (28) 400-500 (16-20) 250 (10) 100 (4) 100 (4) 750 (30) 400-550 (16-24) 250 (10) 100 (4) 100 (4) 800 (32) 400-600 (16-24) 250 (10) 100 (4) 100 (4) 900 (36) 450-650 (18-28) 300 (12) 100 (4) 100 (4) 950 (38) 500-650 (20-28) 300 (12) 100 (4) 100 (4) 1 000 (40) 500-800 (20-32) 300 (12) 100 (4) 100 (4) 1 050 (42) 500-900 (20-36) 400 (16) 100 (4) 100 (4) 1 200 (48) 600-900 (24-36) 450 (18) 100 (4) 100 (4) 1 400 (56) 800-1 000 (32-40) 500 (20) 100 (4) 100 (4) 2.4.8.8 Orientation
The orientation of branches on the Pig Trap System shall be as follows: ¾ Drains: bottom of pipe (at 180° position)
¾ Vents, pressure gauges, purge connection, thermal relief and pig signaller: top quadrant of the pipe (at 0° position)
¾ Kicker line, balance line and by-pass line: side of pipe (at 90° position) The diameters of the branch connections are specified in Table 3.
2.4.8.9 Pressure Indicator Connections
Pressure indicator connections shall be installed at the following locations: ¾ On the major barrel near the end closure.
¾ On the minor barrel near the pig trap valve.
¾ On the bypass line on the pipeline side of the bypass valve.
¾ On the main line downstream of the ESD, if an ESD valve is installed in the main line, or
¾ On the bypass line upstream of the ESD valve, if an ESD valve is installed in the bypass line.
The diameter of these connections shall be 2-inch, with 2-inch double block and bleed valve with ½” NPT outlet for instrument connection. All pressure indicators shall be visible to those operating the closure mechanism.
2.4.8.10 Pressure Transmitter
A Pressure Transmitter shall be installed on the Pig trap for the Control Room to know the status of pig trap during stand-by condition, wherever facility to transfer instrumentation signals exists.
2.4.8.11 Audible Local High Pressure Alarm
A local audible high pressure alarm for receivers in gas service including the trunk lines shall be provided so that the Operator is alerted well in advance of the pressure rise to take necessary action. This requirement applies to stations where power or pneumatic supply is available.
2.4.8.12 Purge Connections
A 2” flanged purge connection with two isolation valves and check valve shall be provided for all systems. It should be located near to the pig trap valve or near the end closure to allow purging and/or flushing the full length of the barrel with nitrogen before opening the end closure. The adjacent vent valve should remain closed during purging.
2.4.8.13 Chemical Injection Connection
A flanged off, 2-inch connection with two isolation valves and check valve shall be provided for launchers on the bypass line.
2.4.8.14 Spectacle Blinds
A provision of positive isolation facility like spectacle blinds is not normally required in pig trap systems where the work duration is considerably small. Hence the risk of swinging and re-swinging the spectacle would be greater and time consuming than the actual work with only valved isolation. Secondly for hazardous service if a spectacle blind is fitted with the spade in position, then the flange bolts shall not be loosened for swinging the spade unless it is certain that the valves are not passing. As per SHELL guidelines the spectacle blind may be provided for following systems:
¾ For all flashing liquids and toxic systems in ASME class 600 and below. ¾ For all systems in service of ASME class 900 and above.
It is recommended to provide spectacle blind on Kicker line and drain / vent lines when connected with to other facilities with the spacer in place during normal operations. Since the spectacle blinds are required for isolation of the trap from the pipeline, these shall be located on the pig trap side of the valves.
The spectacle blind on the main line is not recommended as it may interfere with pigging.
2.4.8.15 Sample Connection
A suitable sample connection shall be installed in the pig receiver area on the drain line, primarily for sampling sludge for any bacterial growth & effectiveness of chemical injection. The orientation of the sample tapping shall be from bottom of pipeline or horizontal for liquid service, horizontal for multi phase flow and from top of pipeline or horizontal in gas service. The sample points shall be designed for safe operations. There shall be no spill-over or sour gas venting during sampling. The venting and draining philosophy as outlined for vent and drain shall be applicable for sample point drain and vent if any. The minimum size of sample tap shall be 2”, reducing further to suit sample connection.
2.4.8.16 Water Inlet Connection
Hydrogen Sulphide (H2S) reacts with iron and forms iron sulphide (pyrophoric scale/ dust). Pyrophoric scale may accumulate in the debris collected by the pig. In a dry condition this material may smoulder i.e. when exposed to atmosphere. The debris must be kept wet and disposed off in a suitable area by burning. Hence a water inlet connection shall be provided on launchers and receivers in gas service containing H2S in order to soak the pyrophoric dust and douse the pig in the traps before opening the end closure.
2.4.8.17 Branch Connections
The configuration of the branch connections between the various lines and ancillary items should be as shown in (Table 4), based on the typical pipe diameters listed in (Table 3).
The diameter of all branch connections shall be at least 50 mm (2 inch).
Table 4 Branch connection configurations [Ref 1] Branch Connection
Location
Sizes mm (inches)
Configurations
Bypass line on main line
75 on 100 (3 on 4) Tee Size 4” & larger Barred tee Kicker line/ receiving
line on major barrel
Size 4” & larger Welded branch
connection (see note 1) Balance line on
minor barrel, Balance line on Kicker line/ receiving line
50-100 on 150 (2-4 on 6) and above
Welded branch
connection (see note 1)
Drain on minor and major barrel
Kicker line/ receiving line on bypass line
Size 4” and above Tee Pressuring line to
Kicker line/ receiving line
50 (2) and larger Welded branch
connection (see note 1) Small items
(e.g. vents and gauges)
50 (2) Weldolet
Notes:
1) "Welded branch connections" include tees, extruded outlets/sweepolets and weldolets as well as fabricated items. In all cases they shall conform to the design codes (ASME B31.4 and B31.8).
2) The distance between branch connections should be addressed to ensure that it does not coincide with pig cup/disc separation as this may result in pig stoppage. Barred tees shall be installed on all branches larger than 50% of the pipeline diameter. However it is advisable to install barred tees on kicker/ receiving line to avoid broken pigs passing through the kicker/ receiving line. The reducing barred tees shall minimum satisfy the requirements of Figure-3 (Refer Appendix – 8)
2.4.9 Pig Signaller 2.4.9.1 General
Pig signallers are used to provide confirmation that the pig:
¾ Has been successfully launched from the launcher or arrived at the receiver; and
¾ Has successfully passed through the pig trap valve, barred tee etc. There are two types of pig signallers:
1. Intrusive Pig Signallers and 2. Non-intrusive Pig Signallers
2.4.9.2 Intrusive Pig Signallers
The intrusive type pig signaller has a trigger penetrating into the bore of the pipeline and into which the transported fluid will enter and pressurise the housing. Intrusive pig signallers are, by definition, static, mechanically actuated and provide a momentary indication of a pig’s presence at a specific point in the pipeline. Because the actuating mechanism is a mechanical trigger that intrudes into the pipeline, a signaller must also incorporate a satisfactory means of retaining the pressure within the pipeline.
The main features of intrusive pig signaller are: ¾ Reliable and provides visual signal. ¾ Proven track record.
¾ Active type of sensor.
¾ Debris entering the signaller clogs up and prevents the trigger movement. This trips the external flag / switch.
2.4.9.3 Non-Intrusive Pig Signaller
This type of pig signaller is externally mounted on the pipeline and therefore is not exposed to the transported fluid or the internal pressure in the pipeline. These signallers are easy to maintain.
There are number of non-intrusive systems on the market:
a. Monitoring change in magnetic field by the movement of the metallic body of the pig - This approach is very difficult to set because there should not be any other moving metallic parts in the vicinity, i.e. steel capped boots, tool boxes etc, this can change the field and inadvertently trip the signaller.
b. Ultrasonic systems - This approach again can be fooled by pressure surges and therefore give a false reading.
c. Monitoring movement of a magnet fitted into the pig body - This devise relies on a small coin sized magnet being fitted on to the pig body, which the external non-intrusive signaller can pick-up. This gives a more reliable signal. The non-intrusive signallers system can be fitted with a number of accessories, including LCD read outs to give time and date and number of pigs passed. This can be linked to either mobile phones, or pagers to send SMS style messages.
The salient features of non-intrusive pig signaller are: ¾ Acoustic, passive type intelligent sensor
¾ Detects the passage of pig at speeds up to 0.5 m/sec. Data on detection of pig for pig speed below 0.5 m/sec is not available.
¾ The pipeline wrapping and coating has to be removed to provide bare metal-to-metal contact between the line pipe and instrument.
¾ Requirement of power plus computer and hence expensive. ¾ Practically with no maintenance.
2.4.9.4 Pig Signaller Assembly
Pig signallers shall be robust and designed to be installed for long periods. In addition, they are installed at points along the pipeline where the course of the pig must be confirmed (e.g. wyes, tees) and for providing a warning of approach at receiver.
The pig signallers shall be installed at the following locations as a minimum: ¾ On the minor barrel of launcher and receiver. For launcher it shall be
towards the pig trap valve while for receiver it shall be near the small diameter side of the reducer,
¾ On the mainline, one intelligent pig length downstream and beyond the barred tee on each trap to confirm that the pig has successfully passed both the pig trap valve and the barred tee,
¾ At approximately 1.0 km upstream of pig launcher (Note 1)
Note:
(1) Optional requirement for new facilities. The requirement shall be finalized in consultation with Operations during detail design stage.
Considering the salient features mentioned in paragraph 2.4.9.2 and 2.4.9.3 and the vendor feedback that the non-intrusive pig signallers are still in the developmental / field trial stage, it is concluded to use Intrusive type Pig Signallers and requirements for this type are given in DEP 31.40.21.33-Gen. The proposed assembly shall be as indicated in Figure-4 below.
Pig Signallers shall be bi-directional, shall give positive indication of pig passage and shall not solely dependent on a superficial or painted marking or other method that can become obliterated. The indicators shall be designed for manual reset and allow replacement of all moving parts with pressure in the line
Every signaller must incorporate a mechanism that will provide a positive indication that a pig has passed. This is normally done in one of two ways:
¾ Mechanical/Visual (a local flagged indicator)
¾ Electrical (magnetically linked proximity switch provides an electrical signal to a controller)
The pig signallers shall always be provided with full-bore ball valves and a system to allow removal and/or maintain under full pressure. Proprietary signallers designed for removal under pressure normally have small-size flange and bolt assemblies and consideration should be given to the use of a standard 50 mm (2-inch) flange assembly for such items.
It is recommended to use Pig Signallers from the below mentioned vendors or equivalent:
¾ GD Engineering ¾ Pipeline Engineering ¾ INPIPE Products
Refer Appendix –6 for vendor sketches of Intrusive and Non-intrusive pig signallers and pig signaller removal tool consisting mainly of jacking bracket.
2.4.10 Valves
2.4.10.1 General and Valve Operations
Most of the valves are required for isolation purposes i.e. on/off use. Depending on the intended frequency and operations requirement the pig trap and kicker valves may require power-operation (hydraulic or motor actuated). Irrespective of frequency of use, large valves may require power operation (i.e. motor operated valve – MOV). The motor operated valve especially on the trap isolation, bypass line and kicker line shall be able to stop immediately and reverse the stroke if needed. Actuated valves shall have provision for a hand wheel or hand pump on the actuator.
Wherever MOV’s are provided and SCADA exists the valves shall be provided with position indicators for display in central control room / DCS.
The pig trap valve and Kicker valve are the main isolating valves between the pipeline and pig trap. Also these valves are subjected to pigging debris and hence it is advisable to have these valves as tight shut-off valves.
The main pipeline valves (downstream of launcher and upstream of receivers) for 16” and above for all ratings shall be motorized if power is available. The kicker valves need not be motorized. In the absence of motorization the valve shall be provided with gear operation inline with ADCO specifications. All gear operated valve shall have the option to fit portable pneumatic tool to facilitate valve operation. A set of Portable Pneumatic Tool shall be provided for each location.
The locations where power is available, valves on mainline below 16” size could be motorized depending on project specific decision.
Refer paragraph 6.0 for ’Safety and Interlock Systems’ to be used for valves.
2.4.10.2 Valve type Selection
The pig trap valve needs to be piggable. They shall be full-bore ball valves or through-conduit type gate valves. The kicker valve do not need to be full-bore but since they may be subject to pipeline debris a ball valve or through-conduit gate valve should be used.
A comparison between ball valves and through-conduit gate valves are listed below. A through-conduit gate valve may be slab-gate or expanding-gate type.
There are reverse slab gate valves. These are used to reduce the actuator size in pneumatic or hydraulic actuated valves. The reverse slab design helps in reducing the torque requirement for close/ open operation. Since the present design for pig trap valves is with electric motor operated valves, the reverse slab type gate valves are not considered.
Table 5 : Comparison of Ball Valves & Gate Valves
Ball Valves Gate Valves
Advantages ¾ Compact size ¾ Lighter weight
¾ Lower cost for high pressure (Class 600 and above)
¾ Faster operation
Advantages
¾ Lower cost for low pressure service (Class 300 and below)
¾ Better resistance to dirt in the open position (expanding-gate type)
¾ Maintainable in-situ
¾ Upstream and downstream sealing ¾ Better fire resistance
Disadvantages
¾ Soft seats may be damaged in dirty service
¾ Only top entry type maintainable in-situ
¾ More sensitive to foreign hard particles
Disadvantages ¾ Slower operation ¾ larger height ¾ larger weight
Based on the above criteria it is concluded that for main oil lines and oil transfer lines gate valves shall be preferred. For the gas service including the trunk lines, the use of gate or ball valve shall be project specific.
2.4.10.3 Additional Consideration in Valve Design
It is reported that ADCO is facing severe valve-passing problem at most of their pig trap installations. Following are some of the recommendations ADCO may consider to overcome the problem. It is to be emphasized that the suggestions below are only general and do not replace the ADCO valve specifications in any way. However the suggestions might help to evaluate the existing valve specifications. It is to be noted that the suggestions below are based on our experience & in no way attempts to prepare valve specifications, which is outside the scope of this document.
General
¾ In corrosive services, consideration should be given to overlaying critical areas in the valve (e.g. seal pockets).
¾ Where the service is erosive (e.g. presence of solids), harder seating areas (i.e. seat rings and closure elements) and/ or hardfacing overlay should be used.
¾ Where CO2 is present, elastomer selection should consider the possibility of
explosive decompression.
¾ Electroless- nickel plated coatings often fail after continuous long immersion in salt water. Their uses should be limited to the less severe services (neutral PH’s and temperatures < 93.3 degree C) or to locations/ services where successful use has been documented.
Suggestions
¾ Ball valves in corrosive service (i.e. on all piping material classes having a minimum corrosion allowance of 3mm) shall receive a 3 mm thick weld overlay in stainless steel grade 316L on all seal pockets and related contact faces. This requirement only applies to dynamic seals (i.e. seat-to-body and also upper stem seals). It does not apply to static seals such as body seals or to parts already made of stainless steel having minimum nominal chromium content of 17%.
¾ The mainline pig trap valve shall have the following additional features:
Metal to metal contact (no soft seat seal)
Tungsten carbide coating of all surfaces subject to metal-to-metal contact (seats and ball). This coating shall be of 400 microns minimum thickness, in complement of the 75 microns ENP coating, which has to be applied on the remaining parts of the trim (i.e. ball, stem, seats).
3 mm thick weld overlay in Inconel 625 of all the seal pockets and related contact faces (whatever the corrosion allowance of the relevant piping material class). This requirement only applies to dynamic seals (i.e. seat-to-body and also upper stem seals). It does not apply to static seals such as body seals or to parts already made of stainless steel having minimum nominal chromium content of 17%.
¾ In case of solid particles in the fluid the valve design shall incorporate the following:
No lip seals are permitted.
No soft seals are permitted for the seat-to-ball seals: valve must be metal-to-metal seating.
Seats and ball contact faces must be Tungsten Carbide coated in order to achieve a minimum surface hardness of 1100 Vickers. The thickness of the Tungsten Carbide coating must be a minimum of 400 microns (thickness of the finished, machined surface).
¾ Operating procedures are to be followed to prevent the valve seats from getting damage i.e. the ball valves shall not be opened against differential pressure. For large bore valves a 2” pressure equalizing line shall be provided and upon equalization of pressure the main ball valve shall be opened.
2.4.10.4 Double block and Bleed Philosophy
A double block and bleed system shall be installed at the location of the pig trap isolation valve as follows:
1. All Main Oil Lines with 600# and above. 2. Transfer lines with 300# and above. The bleed requirements shell be as follows:
1. Wherever double block valves are provided for isolation, provide a bleed connection with a valve for lines of sixes 6” and above.
2. For hydrocarbon systems having H2S concentration exceeding 500 ppm, the bleed should preferably be connected to a closed system (flare/drain). In instances where this can not be applied, proper dispersion analysis be carried out for routing bleed o a safe location.
2.4.10.5 Pig trap isolation valve
The pig trap isolation valve(s) shall be full-bore tight shut-off ball valve or gate valve, installed to isolate the barrel from the rest of the pipeline. The minimum internal diameter of the valves shall be consistent with that of the pipeline to avoid difficulties in pigging activities.
2.4.10.6 Bypass valve
The bypass valve shall be provided with “Inching” facility.
2.4.10.7 Kicker valve
The kicker valve(s) shall be a tight shut-off ball valve or gate valve installed to isolate the bypass line from the barrel.
2.4.10.8 Pressurizing valves
A pressurizing line shall be installed and it shall include an isolating valve (ball valve) and should preferably include a throttling valve (globe valve). The isolating valve shall be installed on the bypass line side for tight shut-off of the pressurizing line and the throttling valve shall be installed on the balance line side to control the flow in the pressurizing line.
2.4.10.9 Balance valve
To cater for possible low flow conditions and to ensure that pigs can always be launched, a balance valve (ball valve) shall be provided in the balance line, so that all flow may be diverted behind the pig by closing the balance valve during launching. This valve should be normally open.
2.4.10.10 Drain valves
The drain valves shall be tight shut-off ball valve for isolation and globe valve for throttling as depicted on P & IDs.
2.4.10.11 Vent valves
The vent valves shall be tight shut-off ball valve for isolation and globe valve for throttling as depicted on P & IDs.