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Design Codes - Pipework

This Technical Measures Document covers the use of piping standards and the design and maintenance of piping systems. Reference is made to relevant codes of practice and standards.

Related Technical Measures Documents are:

The relevant Level 2 Criteria are 5.2.1.5(35)a, b, c and 5.2.1.6(38)g.

Introduction

Piping systems are the most commonly used method of conveying fluids in the process industries. The integrity of piping systems is dependent on various considerations and principles that should be observed when designing, constructing and maintaining process plant piping. Pipework is made up of many components including pipes, flanges, supports, gaskets, bolts, valves, strainers, flexibles and expansion joints. Such components are available in a variety of materials, types and sizes and may be manufactured to a national standard or maybe a manufacturers proprietary item. Transmission pipelines are not included within the scope of this document.

General principles

The operator should demonstrate that it designs piping systems based on the requirements of nationally accepted standards, and uses competent persons to implement good design practices with respect to mechanical and process design.

Some companies publish their own internal piping standards based upon the relevant information from national and industry sector standards. These internal standards often include materials selection and other items required to specify pipes and piping system components for the specific fluids handled by the company. Where this is the case the company should demonstrate compliance with the standard, that the standard has been produced by competent persons and that the standard is subject to periodic review.

Design - Implementation of Pipework standards

Pipes and piping components are normally manufactured to meet the requirements of national standards such as ASME B31 Code for Pressure Piping or BS 1560 Circular Flanges for Pipes, Valves and Fittings. Manufacture to such standards ensures that the items are suitable for use under specific operating conditions. Normally a standard defines the allowable stresses, and temperature and pressure ranges under which the piping component may be used. Additionally some industry sector groups publish standards for handling specific substances. As examples Euro Chlor publishes standards for chlorine piping systems and the LPG Gas Association publishes standards for LPG piping systems.

Material selection

Whilst carbon and stainless steels are commonly used materials of construction, increasing use is being made of non- metallic and lined or plastic piping systems. The selection of the material of construction should taken into account variation in process conditions that may occur under foreseeable upset conditions. The strength of some materials changes considerably at elevated temperatures. Typically the mechanical strength of plastic pipework and bellows reduces considerably at elevated temperatures. Steels may suffer from brittle fracture at low temperatures. The operator should demonstrate that procedures are in place to ensure that deviations in process conditions such as fluid temperature, pressure and composition are identified and assessed in relation to the design of the pipework.

Corrosion

Corrosion / erosion due to fluid flow occurs in pipework. In practice for pipes it is usual to select materials of construction which corrode slowly at a known rate and to make provision for the material loss due to corrosion / erosion. All piping components such as gaskets and bellows must be compatible with the fluid. The operator should demonstrate that it has procedures in place to ensure the correct selection and use of materials of construction in piping systems.

Exterior surface corrosion of pipework components and supports can appear as pitting or crevice corrosion. Painting to an appropriate specification will significantly extend the period to the onset of corrosion but the durability of the paint finish is largely dependent on the quality of the surface preparation. Improperly installed insulation can provide ideal conditions for corrosion and should be weatherproofed or otherwise protected from moisture and spills to avoid contact of the wet material on equipment surfaces. Application of an impervious coating such as bitumen to the exterior of the pipework is beneficial in some circumstances. Wrapping or taping pipework to provide protection is also a common practice. Cathodic protection is an electrochemical method of corrosion control which has found widespread application in the protection of carbon steel underground structures such as pipework and tanks from soil corrosion. The process equipment metal surface is made the cathode in an electrolytic circuit to prevent metal wastage.

The operator should demonstrate that it has inspection and maintenance programmes in place for pipework systems carrying hazardous fluids and in particular for lagged pipework.

Pipe joints

Pipe joints are often the point of leakage on pipework systems and the number of joints in piping systems should be minimised where practicable. Joints can be permanently welded for high integrity systems or reformable types such as flanged, screwed or compression fittings may be used. Welded joints should meet the requirements of a standard such as BS 2971 'Specification for Class II welding of carbon steel pipework for carrying fluids'. Welds should be inspected by an appropriate method depending on the application requirements. Radiography and ultrasonics are widely used where high integrity welds are required. The operator should demonstrate that procedures are in place to ensure the appropriate level of integrity is provided by the jointing method employed.

Evaluation of stresses, reactions and movement

Lack of consideration of the stresses, reactions, and movement of the piping and connected equipment at the design phase can result in failure of supports, leakage at flanged joints, distortion of valve bodies, and failure of in line items such as bellows. Notably the Flixborough disaster occurred due to a bellows failure. The operator should demonstrate that competent persons carry out the mechanical design of piping systems.

Routing and supporting

Piping containing hazardous fluids should be protected from damage by external mechanical impacts, leaks from adjacent pipework and external sources of heat. Pipework should be routed to take into account the requirement for safe access for operation, inspection and maintenance. Pipe supports and bridges should be designed with sufficient mechanical strength for the loads exerted on them and for traffic impact resistance where appropriate. Armco barriers or the equivalent should be used to protect pipe routes close to roads.

For hazardous fluids, the piping should avoid 'dead legs' and be designed to facilitate drainage to prevent trapping of fluid. Pockets should be avoided in piping carrying slurries, fluids that can create blockages or form corrosive condensate. The design of sampling systems should be appropriate to the hazard of the fluid and have due regard for problems such as freezing or solids / hydrate formation causing blockages. Double valving or the use of high integrity proprietary sampling systems are likely to be appropriate for the sampling of hazardous substances. The operator should demonstrate that procedures are in place to ensure that the orientation and routing of pipework minimises both the likelihood of loss of containment and any subsequent loss of inventory of toxic or flammable substances, and that measures are installed to protect pipework where necessary.

Operational demands

During design, the operation of each piping system needs to be clearly understood not only under normal conditions but also those conditions arising during start up, shutdown and as a result of process upsets. These items should also be addressed during project HAZOP studies. The operator should demonstrate procedures are in place to ensure that phenomena known to cause problems in piping systems are considered and allowed for in the mechanical design or designed out where practicable. These should include :

Isolation

Valving is the primary means of isolating process equipment and is provided for both maintenance and emergency purposes. For maintenance it is also necessary to consider the requirements for meeting isolation standards (spectacle blinds, etc). Where isolation is provided it is possible to trap liquid between closed valves and in some circumstances temperature changes in the fluid can result in thermal expansion of the trapped liquid and loss of containment. The location of isolation valves should be a prime consideration during HAZOP studies. The operator should demonstrate that procedures are in place to identify the requirements for the safe isolation of process plant for emergency and maintenance purposes. Where thermal expansion is a problem, the operator should be able to demonstrate that training and awareness programmes have taken place and that pressure relief has been installed where necessary.

Static

Where fluid flows through a pipe static electricity is generated. The conductive properties of the fluid and the pipework system affect the charging process and in some cases it is necessary to restrict flow rates to control static generation. There is a basic requirement for the earthing of process equipment to prevent ignition of flammable vapours by static discharge. The operator should demonstrate that procedures are in place to ensure that pipework design meets the requirements of standard codes of practice such as BS 5958 : 1991 for the control of static electricity. Additionally procedures should be in place for the periodic testing of the continuity to earth of pipework where necessary.

Construction and fabrication

Fabrication specification should specify welding and jointing procedures, alignment tolerances, defect limits, extent of visual and non-destructive testing. Pipework should be constructed in accordance with isometrics signed off by competent persons. Pressure tests should be carried out to written procedures to confirm adequate containment at process conditions. Commissioning procedures should be in place to ensure that installed pipework is inspected before use to identify any design faults that may have been introduced at the construction stage and to confirm suitability for use.

Materials control

Stores control systems should be in place to ensure that only items suitable for the particular process duty can be drawn for repair / replacement work. Similarly purchasing controls should ensure that purchased materials are suitable for the particular process duty. The operator should demonstrate that robust systems are in place for controlling the purchase, storage and issue of materials and items for use on hazardous process plant. Changes to specification, etc should be considered under Change Control Systems.

Industry applications

Chlorine

Special steels are required for handling chlorine at low temperature to avoid embrittlement and at high temperatures chlorine burns mild steel. The flow rate of liquid chlorine through pipework is restricted to avoid removing the ferric chloride coating on the pipe surface which protects against erosion / corrosion. Wet chlorine gas corrodes mild steel and ebonite lined steel is used for this duty. Where ROSOVs can trap liquid chlorine pressure relief is installed on the pipeline.

LPG

For above ground liquid duty carbon steel seamless pipe is used and the joints can be either flanged or welded. Screwed fittings are only permissible on pipe diameters up to 50 mm and compression fittings can not be used. Copper and polyethylene pipework are not permitted on liquid duty but copper can be used on vapour duty for pipes of up to 15 mm diameter in conjunction with compression fittings.

Ethylene Oxide

Mild steel or stainless steel maybe used with ethylene oxide, however mild steel must be rust free to prevent initiation of exothermic polymerisation. Flanged joints should have stainless steel spiral wound PTFE joint gaskets or trapped Fluon gaskets. CAF is not suitable for use above 25°C and natural rubber is not permissible. Joints should be kept to a minimum.

Codes of Practice relating to Pipework Systems

There are a large number of British Standards covering the components that may be present in a piping system, such as:

Some of the more general standards and codes of practice of interest are given below.

Further reading material

Perry’s Chemical Engineers’ Handbook, Section 6, Transport and Storage of Fluids, McGraw Hill.

Lees, F.P., 'Loss Prevention in the Process Industries: Hazard Identification, Assessment and Control', Second Edition, 1996.

Case studies illustrating the Importance of Design Codes - Pipework

2016-07-22