Design Codes

This Technical Measures Document covers the design codes for plant.

This Technical Measures Document covers the design codes for plant. Reference is made to relevant codes of practice and standards.

The relevant Level 2 Criteria are:

This Technical Measures Document includes the following sections:

Reference is made to relevant codes of practice and standards where applicable.

Related Technical Measures Documents are Corrosion / Selection of Materials, Design Codes - Pipework, Explosion Relief, Relief Systems / Vent Systems, Training, Plant Modification / Change Procedures, Reaction / Product Testing.

The design of a process plant is a complex activity that will usually involve many different disciplines over a considerable period of time. The design may also go through many stages from the original research and development phases, through conceptual design, detailed process design and onto detailed engineering design and equipment selection. Many varied and complex factors including safety, health, the environment, economic and technical issues may have to be considered before the design is finalised - See Technical Measures Document - Training.

At each stage it is important that the personnel involved have the correct combination of technical competencies and experience in order to ensure that all aspects of the design process are being adequately addressed. Evidence of the qualifications, experience and training of people involved in design activities should be presented in the Safety Report to demonstrate that the complex issues associated with design have been considered and a rigorous approach has been adopted.

The process design will often be an iterative process with many different options being investigated and tested before a process is selected. In many occasions a number of different options may be available and final selection may depend upon a range of factors.

The process design should identify the various operational deviations that may occur and any impurities that may be present. In the mechanical design, the materials of construction chosen need to be compatible with the process materials at the standard operating conditions and under excursion conditions. The materials of construction also need to be compatible with each other in terms of corrosion properties. Impurities which may cause corrosion, and the possibility of erosion also need to be considered so that the detailed mechanical design can ensure that sufficient strength is available and suitable materials of construction are selected for fabrication - See Technical Measures Document - Corrosion / Selection of Materials.

Detailed mechanical, structural, civil and electrical design of equipment comes after the initial process design which covers the steps from the initial selection of the process to be used, through to the issuing of process flow sheets. Such flowsheets will include the selection, specification and chemical engineering design of the equipment. These are then used as the basis for the further detailed design.

This Technical Measures Document primarily considers the latter stages of the detailed design processes and identifies the detailed design issues, codes and applicable standards for the mechanical design of equipment.

Design factors are an essential component in order to give a margin of safety in the design. Design factors may be appropriate in either the mechanical engineering design or in the process design where factors are often added to allow some flexibility in process operation. For mechanical and structural design the magnitude of design factors should allow for uncertainties in material properties, design methods, fabrication and operating loads.

Plant design should take account of the relevant codes and standards. Conformity between projects can be achieved if standard designs are used whenever practicable.

Modern engineering codes and standards cover a wide range of areas including:

Many companies have their own in-house standards which are primarily based on the published codes, such as BS5500, with added extras which cover either technical or contractual matters. In the safety report the base document for the in house codes should be clearly stated and the key safety related deviations or enhancements demonstrated so that the assessor can determine their adequacy.

A Safety Report should demonstrate that consideration has been given to the appropriate standards and codes of practice developed by legislators, regulators, professional institutions and trade associations. It should also demonstrate that for any equipment that is installed, the operating procedures, testing regimes and maintenance strategies that are in place meet or exceed these requirements in terms of safety performance.

The principles of inherently safer design are particularly important for major hazard plants and should be considered during the design stage. The Safety Report should adequately demonstrate that consideration has been given to the concepts. Some companies now have design procedures that require a review of designs and seek to ensure that inherently safer concepts have been addressed.

Inherently safe design should be considered during the design stage in an effort to reduce the hazard potential of the plant. Protective equipment installed onto standard equipment to control accidents and protect people from their consequences is often complex, expensive and requires regular testing and maintenance. Attempts should be made to reduce the requirement for such protective equipment by designing simpler and safer processes in the first instance. A number of approaches can be considered but basically an inherently safer plant can be achieved by minimising the inventories of hazardous substances in storage and in process and hence the risk of a major accident can be significantly reduced.

Some of the techniques that can be considered are:

Further guidance on inherently safer design can be found in `Cheaper, Safer Plants' - Kletz, T.A., 1984, IChemE, ISBN 0 8529 5167 1.

A design should be subject to a number of detailed assessments throughout its development. Evidence that some system of assessment has taken place should be provided in the Safety Report. A number of different features can be examined and assessed. Examples are given below:

These assessments all have a specific individual focus but in the context of COMAH it needs to be demonstrated that major accident hazards are not introduced as a result of the assessments that are undertaken. For example any decisions taken as a result of a value engineering assessment that result in standby equipment not be installed, or equipment of a lesser specification being chosen should also demonstrate that the major accident hazard implications of such decisions have also been considered.

A number of companies have developed detailed procedures for design studies that incorporate many of these assessments into a formalised structure.

Evidence that Hazard identification and/or HAZOP studies have been carried out should be provided as evidence that a design has been evaluated and carefully considered before being installed on the plant. See Technical Measures Document - Plant Modification / Change Procedures.

There are several general topics that are common to the detailed mechanical design of many types of equipment and these are discussed in greater detail below:

A number of potential hazards can be introduced if these are not given adequate consideration. Loss of containment may occur due to leaks, equipment failure, fire or explosion and result in a major accident.

Temperature and pressure are two basic design parameters. Any equipment that is to be installed should be designed to withstand the foreseeable temperature and pressure over the whole life of the plant. The combination of temperature and pressure should be considered since this affects the mechanical integrity of any equipment that is installed.

In determining design temperatures a number of factors should be considered including:

Consideration needs to be given to the temperature of the fluids that are to be handled and any excursions in temperature that could occur as a result of the failure of temperature control systems. Account should be taken of foreseeable reactions that may occur that are likely to increase or reduce the heat input to the system.

The extremes of ambient temperature should be taken into account for plant situated outside buildings. Solar radiation on the exposed surface area of large storage tanks can significantly increase surface temperatures for storage vessels leading to significant thermal expansion of vessel contents. Likewise the low temperatures that can be achieved under conditions of snow, ice and wind, which can cause solidification of contents in vessels and pipelines, should also be considered. External facilities should be designed to accommodate the cycling of temperatures between extreme weather conditions.

If secondary heating and cooling systems are employed then the maximum and minimum temperatures that can be achieved by these secondary systems should be assessed assuming failure of any control systems associated with these systems. Care should be taken to ensure that the maximum temperature that can be achieved by heating oil systems or the minimum temperature that can be achieved by cryogenic cooling systems does not compromise the design of the equipment. It should not adversely affect the mechanical strength and hence integrity, or result in additional process hazards as a result of overheating, decomposition or runaway reactions.

The strength of materials decreases with increasing temperature and therefore the maximum design temperature should take into account the strength of material used for fabrication.

Evidence should be provided in the safety report that the process conditions and environment in which the equipment is to be utilised have been assessed and that an appropriate design temperature has been selected.

A vessel should be designed to withstand the maximum pressure to which it is likely to be subjected in operation.

For vessels under internal pressure the design pressure is usually taken at that which the relief valve is set. This is normally 5-10% above the normal working pressure to avoid inadvertent operation during minor process upsets. Vessels subjected to external pressure should be designed to resist the maximum differential pressure that is likely to occur. Vessels likely to be subjected to vacuum should be designed for full negative pressure of 1 bar unless fitted with an effective and reliable vacuum breaker device.

Account should also be taken of foreseeable reactions which may occur that are likely to increase the heat input to a system, or gas evolution and hence result in increased or decreased temperatures and pressures. Where strongly exothermic reactions or runaway reactions are possible it may not be possible to adequately design the equipment to withstand the maximum predicted temperature and pressure. Under such circumstances some form of pressure relief system may be appropriate in order to protect the equipment and prevent catastrophic failure of the equipment from occurring. See Technical Measures Document - Reaction / Product Testing.

Pressure vessels should be fitted with some form of pressure relief device set at the design pressure of the equipment to relieve over-pressure in a controlled manner - see Technical Measures Documents - Relief Systems / Vent Systems, and Explosion Relief. The set pressure of a relief valve should be such that the valve opens when the pressure rise threatens the integrity of the vessel but not when normal minor operating pressure deviations occur. It is necessary to balance a number of factors in the selection of relief valve set pressures since if the potential cause of pressure rise is runaway reaction then setting the relief pressure at a high level above the normal operating pressure may allow the reaction to reach a higher temperature and to proceed more rapidly before venting starts.

During the operation of the relief valve the pressure at the inlet to the relief valve (the overpressure - this is usually taken to be no more than 10% for design purposes) can be expected to increase above the set point for the relief device. The accumulation in the vessel is the permitted increase in the system pressure above the design pressure in an emergency overpressure situation. The maximum allowable accumulated pressure (MAAP) is specified within the various codes and this should be taken into account when the relief valve set point is selected. Normally the relief valve set point is set below or up to the maximum design pressure which allowing for the overpressure during a relief event ensures that the overall pressure is below the MAPP. Specific guidance on the recommendations for pressure relief protective devices is given in Appendix J of BS 5500 : 1997. Other codes permit higher MAAPs in certain circumstances.

Discharge of hazardous substances from relief systems under emergency conditions should be routed to secondary containment vessels or to safe locations so that additional hazards to personnel or equipment and the possible escalation of an incident does not occur. This should be considered as part of the mechanical design of the equipment if such systems are to be employed.

Evidence should be provided in the Safety Report that the relief systems have been suitably designed and consideration has been given to the discharge locations. Secondary containment facilities may be appropriate for discharge of relief streams. Documentation for relief streams should be available for inspection.

Consideration should be given to the possibility of pressure cycling in equipment and subsequent failure of the equipment due to metal fatigue

Another important consideration in mechanical design is the selection of the material of construction.

In some cases the available materials of construction may constrain the design temperatures and pressures that can be achieved and limit the design of the equipment.

The most important characteristics that should be considered when selecting a material of construction are summarised below:

The selection of a suitable material of construction is often carried out by disciplines such as process engineers. The advice of specialist materials engineers should be sought in the event of difficult applications being identified.

The Safety report should contain evidence that the materials of construction that have been selected are compatible with the process fluids to be handled and the design conditions that have been chosen.

If materials to be used in the process are corrosive then this should be taken into account in the plant design and layout. Materials of construction should be carefully selected, protected where possible and regularly inspected if the presence of corrosive materials or a corrosive environment is anticipated.

The layout of plant and equipment for corrosive materials is discussed in `Safety and Management - A Guide for the Chemical Industry' - the Association of British Chemical Manufacturers, 1964. Printed by W.Heffer & Sons.

This topic is covered fully in the Technical Measures Document - Corrosion / Selection of Materials. See also Causes of Plant Failure.

General guidance on corrosion allowances for pressure vessels is given in BS 5500. The standard recommends that all possible forms of corrosion such as chemical attack, rusting, erosion and high temperature oxidation are reviewed, that particular attention be paid to impurities and to fluid velocities, and that where doubt exists corrosion tests should be carried out.

The life of equipment subjected to corrosive environments can be increased by proper consideration of design details. Equipment should be allowed to drain freely and completely and the internal surfaces should be smooth and free from locations where corrosion products can accumulate. Fluid velocities should be high enough to prevent deposition but not so high as to cause erosion.

The corrosion allowance is the additional thickness of metal added to allow for material lost by corrosion and erosion or scaling. For carbon and low-alloy steels where severe corrosion is not expected a minimum allowance of 2mm is often used, where more severe corrosion is anticipated an allowance of 4 mm is often used. Most design codes and standards specify a minimum allowance of 1mm.

A large proportion of failures in process plant and vessels are due to corrosion. It is often the prime cause of deterioration and may occur on any part of a vessel. The severity of the deterioration is strongly influenced by the concentration, temperature, and nature of the corrosive agents in the fluids and the corrosion resistance of the construction materials. Corrosion may be of a general nature with fairly uniform deterioration, or may be very localised with severe local attack. Erosion is often localised especially at areas of high velocity or impact.. Occasionally corrosion and erosion combine to increase rates of deterioration.

Erosion is a particular problem for solids handling in pipework, ducts and dryers. It occurs primarily at sites where there is a flow restriction or change in direction including valves, elbows, tees and baffles. Erosion is promoted by the presence of solid particles, by drops in vapours, bubbles in liquids or two-phase flow. Conditions that can cause severe erosion include pneumatic conveying, wet steam flow, flashing flow and pump cavitation. If erosion is likely to occur then more resistant materials should be specified or the material surface protected in some way. For example plastic inserts can be used to protect erosion-corrosion at the inlet to heat exchanger tubes.

See also BS 5493: 1977 - Code of Practice for protective coating of iron and steel structures against corrosion.

Design issues, codes and standards applicable to several general categories of equipment have been identified and are discussed below in further detail:

There are numerous texts available on the details of pressure vessel design however the basis of the design of pressure vessels is the use of appropriate formulae for vessel dimensions in conjunction with suitable values of design strength.

Pressure vessels can be divided into `simple vessels' and those that have more complex features. The relevant standards and codes provide comprehensive information about the design and manufacture of vessels and vessel design and fabrication is an area well covered by standards and codes. In general terms outright failure of a properly designed, constructed, operated and maintained pressure vessel is rare.

Design and manufacture is normally carried out to meet the requirements of national and international standards with one of the earliest being the AOTC 1939/48/58 `Rules for the construction, testing and scantlings of metal arc welded steel boilers and other pressure vessels'. The other principal standards in the UK were BS 1500 and BS 1515, both of which are now withdrawn and superseded by BS 5500. The other most commonly used design code is ASME VIII. However it is unusual, though not unknown, for companies and operators to employ their own design codes.

Generally pressure vessel design codes covers equipment such as reactors, distillation columns, storage drums, heaters, reboilers, vaporisers, condensers, heat exchangers, bullets, spheres etc. Basically any equipment with a "shell" that may experience some internal pressure is covered. This section does not cover piping systems (see separate Technical Measures Document on Design Codes Pipework), atmospheric storage tanks and rotary machines. These are considered in further detail later.

A simple pressure vessel does not have any complicated supports or sections and the ends are dished. The main code for simple vessels is BS EN 286-1:1991. `Simple unfired pressure vessels designed to contain air or nitrogen'. All aspects of designing and manufacturing the vessel are covered in this code.

Traditionally the two principal codes and standards BS 5500 and ASME VIII, are employed in the design and manufacture of pressure vessels within the United Kingdom. Importantly both of these demand adherence to satisfaction in the design and manufacturing process of an independent inspection authority. This authority is responsible for adherence during both the design and construction phases in accordance with the standard or code.

Factors that should be taken into account in the design process for pressure vessels include:

Pressure vessels are subject to a variety of loads and other conditions that cause stress and can result in failure and there are a number of design features associated with pressure vessels that need to be carefully considered.

Consideration should also be given to other parts of the vessel not directly within the pressure envelope, but critical to vessel integrity ie any failure which could lead to breach of the pressure boundary e.g. vessel skirt or support legs. Other factors which require careful consideration include; a means of in-service periodic examination ie a means of determining the internal condition of the vessel by the provision of access openings; a means of draining and venting the vessel; and means by which the vessel can be safely filled and discharged.

Pressure vessels are subject to a variety of loads and other conditions that cause stress and in certain cases may cause serious failure. Any design should take into account the most likely failure modes and causes of deterioration. Deterioration is possible on all vessel surfaces in contact with any range of organic or inorganic compounds, with contaminants, or fresh water, with steam or with the atmosphere. The form of deterioration may be electrochemical, chemical, mechanical or combinations of all.

For more information see Technical Measures Document - Corrosion / Selection of Materials.

Two principal codes and standards are employed in the design and manufacture of pressure vessels - the American ASME VIII system and BS 5500 in the UK. Importantly both of these demand adherence to satisfaction in the design and manufacturing process of an independent inspection authority. This authority is responsible for adherence during both the design and construction phases in accordance with the standard code. The codes and standards cover design, materials of construction, fabrication (manufacture and workmanship), inspection and testing, and form the basis of agreement between the manufacturer and customer and the appointed independent inspection authority. These codes relate to vessels fabricated in carbon and alloy steels and aluminium.

Computer programmes to aid the design of vessels to BS 5500 and the ASME VIII codes are commercially available.

Although the majority of pressure vessels are constructed from metallic compounds pressure vessels can also be constructed from materials such as glass reinforced plastic (GRP), or fibre reinforced plastic (FRP). The main relevant standard is BS 4994:1987 - Specification for Design and Construction of Vessels and Tanks in Reinforced Plastics.

Some vessels that are used are not designated as pressure vessels. The description atmospheric storage is applied to any tank that is designed to be used within a limited range of atmospheric pressure, either open to the atmosphere or enclosed.

Vertical storage tanks with flat bases and conical roofs are often used for the storage of liquids at atmospheric pressure and may vary in size considerably. The main load to be considered in the design of such tanks is the hydrostatic pressure of the liquid contained within the tank. However consideration should also be given to other parameters and the wind loading and any likely snow loading should also be considered.

The design of atmospheric storage tanks in general is governed by API Std 620 Design and construction of large, welded, low-pressure storage tanks and API Std 650 Welded steel tanks for oil storage.

Tanks should be suitable for their operational duty and all reasonably expected forces such as tank contents, ground settlement, frost, wind and snow loadings, earthquake and others as appropriate. The selection of the type of tank to be used for a particular duty will be influenced by considerations of safety, technical suitability and economy. The safety considerations are usually related to fire hazards which in turn are dependent on the physical properties of the stored material e.g. flash point, vapour pressure, electrical conductivity etc.

API Standard 2000 gives guidance on the design of vents to prevent pressure changes that would otherwise occur as a result of temperature changes or the transfer in and out of liquids. Excessive loss of vapours from vent systems may result from outbreathing and may present a hazard.

Reactors are often the centre of most processes and their design is of utmost importance when considering the safety hazards of a plant. Reactors are most often considered as pressure vessels and the mechanical design should be in accordance with the codes and standards described earlier.

Reactor design should minimise the possibility of a hazardous situation developing and provide the means for dealing with a hazardous situation should it develop. Arrangements for venting, pressure relief and blowdown need to be adequately addressed in the design. For relief systems consideration should be given to the implications of the release of reactor contents and containment and control systems may be necessary to prevent a hazardous situation from developing as a result of the discharge of a relief system.

The design of the reactor may affect the efficiency of the reaction process and hence the generation of by-products and impurities. The effectiveness of the reaction step will often determine the requirement for and complexity of downstream separation processes. In addition, low conversions may result in large recycles being required.

Many different types of reactor system are available and some of the important criteria to consider are given below:

Addition of reactants - the order and rate of addition of the reactants may affect the rate of reaction and the generation of by-products. The generation of unstable by-products or excessive reaction rates may increase the potential for a hazardous situation to develop. The position of addition of reactants may also be important - sub-surface and directly into an intimate mixing zone within the reactor may result in the minimisation of the generation of reaction by-products;

The safety report should describe how the reactor system has been designed with the principles of safe design in mind and how the selection of the mixing, chemical addition systems and relief systems have been selected in order to minimise the potential for a major accident.

The transfer of heat between two process streams is a common activity and requirement on a chemical plant. A number of direct or indirect techniques can be employed. The most common form of equipment used to transfer heat is a heat exchanger which can be designed in many different shapes, sizes and configurations necessary to obtain the required heat transfer between one stream and another. A number of different heat transfer operations are possible with some involving a change of phase of one or more component. Heating, cooling, evaporation or condensation may all need to be considered and the equipment designed accordingly to account for the differing requirements.

The basic design is commenced by an approximate sizing of the unit based on assumptions made concerning the heat transfer characteristics of the substances involved and the anticipated materials of construction. More detailed calculations are then required to confirm and refine the original design and to identify an optimum layout. Once the process design has been completed the mechanical design of the unit can then be carried out.

The design of heat exchangers is covered in many texts. A common reference for design engineers however is `Process Heat Transfer - D.Q.Kern, International Student Edition, McGraw Hill, ISBN 0070341907.

The mechanical design features, fabrication, materials of construction and testing of shell and tube heat exchangers is covered by `BS 3274: 1960- Tubular Heat Exchangers for General Purposes'.

The standards of the American Tubular Heat Exchanger Manufacturers Association (TEMA standards) are also widely used. Many companies also have their own standards to supplement these various requirements.

The TEMA standards give the preferred shell and tube dimensions, the design and manufacturing tolerances, corrosion allowances and the recommended design stresses for materials of construction.

Design temperatures and pressures for exchangers are usually specified with a margin of safety beyond the conditions normally anticipated. Typically the design pressure may be 170 kPa greater than the maximum anticipated during operation or at pump shutoff, and the temperature is commonly 14°C greater than the maximum anticipated service temperature.

Major problems associated with heat exchanger design that may affect safety include fouling, polymerization, solidification, overheating, leakage, tube vibration and tube rupture. The shell of an exchanger is normally a pressure vessel and should be designed in accordance with the relevant pressure vessel design code - BS 5500 or ASME VIII (Rules for construction of pressure vessels, Division 1). More specific guidance is given in API RP 520:1990.

Special consideration needs to be given to the preventing overheating within heat exchanger equipment especially if sensitive materials are involved, for example materials which may undergo exothermic decomposition.

The safety report should demonstrate that heat exchange equipment has been designed and maintained in accordance with the relevant codes and standards and that consideration has been given to the various failure modes that could occur and the implications of such events. It should be demonstrated that wherever possible measures have been taken to prevent, control or mitigate the consequences of such events by the appropriate selection of materials of construction, fabrication methods, instrumentation and control or others.

Furnaces and boilers are items of equipment that are often found as part of process plant and are used for a variety of purposes such as waste heat recovery, steam generation, destruction of off-gases etc.

The design may involve the interaction of many different variables including water/steam circulation systems, fuel characteristics (liquid, gaseous or solid fuels), ignition control systems, heat input and heat transfer systems.

The design of the furnace or boiler enclosure should be able to withstand the thermal conditions associated with the system and specialist designs are often required. Many codes and standards exist for boiler design.

The elimination of hazards in burner design is a fundamental design requirement. Explosions can occur during start up if ignition design is not carefully considered. Leaks of fuel can cause explosive atmospheres when ignition is attempted. For these reasons consideration should be given to inerting /ventilation systems prior to ignition sequences to ensure explosive atmospheres are not present.

Isolation systems should be adequately designed to ensure leakage of fuel does not occur. Double block and bleed valves on fuel lines can be considered. Reliance should never be placed upon single valves for isolation. Careful consideration of the configuration of the pipework should also be considered to ensure that the flow of fuel into the system after the flame has failed or valves have been closed is minimised.

Purging facilities are essential to ensure that the firing space is free from a flammable atmosphere prior to start-up ignition.

A safety report should demonstrate that any furnace/boiler system is designed and maintained to the relevant codes and standards and that consideration has been given to the major hazards associated with the start-up, shutdown and operation of the equipment in terms of the fire and explosion potential of such systems. It should be demonstrated that the risks of an explosion occurring have been minimised by the design of the burner control management system and the layout and design of the fuel supply systems.

Process machines are particularly important items of equipment in process plants and in relation to pressure systems since they are required to provide the motive force necessary to transfer process fluids (liquids, solids and gases) from one area of operation to another. A machine system is any reciprocating or rotating device that is used to transfer or to produce a change in properties within a process plant. Examples may include items such as pumps, fans, compressors, turbines, centrifuges, agitators etc.

This type of equipment is a potential source of loss of containment. In addition due to the rotating/vibrating nature of such equipment pressure and flow fluctuations may be caused and these can affect the operation of other systems.

The basic requirements to define the application for pumps, fans and compressors are usually the suction and delivery pressures, the flow rate required and the pressure loss in transmission. Special requirements for certain industrial sectors may also impose restrictions on the materials of construction to be used or the type of device that can be considered. Many designs have become standardised based on experience and numerous standards (API standards, ASME standards, ANSI standards) have become available. These standards often specify design, construction and testing details such as material selection, shop inspection and tests, drawings, clearances, construction procedures etc

The choice of material of construction is dictated by consideration of corrosion, erosion, personnel safety and containment and contamination.

Many pumps are of the centrifugal type, although positive displacement types (such as reciprocating and screw types) are also used. Pumps are available throughout a vast range of sizes and capacities and are also available in a wide range of materials including various metals and plastics. Sealing of pumps is a very important consideration and is discussed later. The primary advantage of a centrifugal pump is its simplicity. Pumps are particularly vulnerable to mal-operation and poor installation practices. Proper installation and high quality maintenance is essential for safe operation.

Problems associated with centrifugal pumps can include bearing and seal failure. Cavitation (the collapse of vapour bubbles in a flowing liquid leading to vibration, noise and erosion) and dead head running (attempting to run a pump without an outlet for the fluid, for example against a closed valve) can also result in damage to the pumping equipment. Misalignment between pump and motor is also a common cause of catastrophic failure.

Seal-less or `canned pumps' are often used where any leakage is considered unacceptable. In a canned pump the impeller of the pump and the rotor of the motor are mounted on an integral shaft which is encased so that the process fluid can circulate in the space which is normally the air gap of the motor.

Key parameters for pump selection are the liquid to be handled, the total dynamic head, the suction and discharge heads, temperature, viscosity, vapour pressure, specific gravity, liquid corrosion characteristics, the presence of solids which may cause erosion etc.

Both positive displacement and centrifugal compressors are used in the process industry. They are complex machines and their reliability is crucial. It is very important that they are maintained to high operational standards. Centrifugal compressors are by far the most common although compression is generally lower than that given by reciprocating machines. They are used in both process gas and refrigeration duties. On centrifugal compressors some of the principal malfunctions include rotor or shaft failure, bearing failure, vibration and surge. Reciprocating compressors are utilised for higher compression requirements. They may be either single or multi-stage units. Air compressors for dry air require special consideration and specific codes and standards exist.

The main applications for fans are for high flow, low pressure applications such as supplying air for drying, conveying material suspended in a gas stream, removing fumes, or in condensing towers. These units can be either centrifugal or axial flow type. They are simple machines but proper installation and maintenance is required to ensure high reliability and safe operation.

One of the main causes of failure of rotating equipment is vibration. This often causes seal damage or fatigue failure and subsequent leakage and can result in a major accident. Numerous factors can result in vibration occurring including cavitation, impeller imbalance, loose bearings and pulses in the pipe. ASME standards recommend that pumps should be periodically monitored to detect vibration that should normally fall within prescribed limits as determined by the manufacturer. This should be initially confirmed on installation and then periodically checked. If measured levels exceed prescribed values then preventative maintenance is required and should be performed. By collection and analysis of vibration signatures of rotating equipment it is possible to identify which components of the system are responsible for particular frequencies of the vibration signal. It is then possible to identify the component that is deteriorating and responsible for the vibration that is occurring.

Seals are very important and often critical components in large rotating machinery and in systems which are flanged/jointed such as heat exchangers or pipework systems. Failure of a sealing arrangement can lead to loss of containment and a potential for a major accident. Numerous different types of sealing arrangement exist for rotating equipment. There are many factors that govern the selection of seals for a particular application including the product being handled, the environment which the seal is installed in, the arrangement of the seal, the equipment the seal is to be installed in, secondary packing requirements, seal face combinations, seal gland plate arrangements, and main seal body etc. The materials used for seals should always be compatible with the process fluids being handled.

There are three principal methods of sealing the point at which a rotating shaft enters a pump, compressor, pressure vessel or similar equipment:

Stuffing boxes and glands with packing are commonly used. Some product leakage is normal both lubricating and cooling the packing material. The chief advantages of this type of sealing arrangement are the simplicity and the ease of adjustment or replacement. The disadvantages are the necessity of frequent attention and the inherent lack of integrity of such a system.

Mechanical seals are the next most commonly employed arrangement. They are used in applications where a leak tight seal of almost any fluid is required. Mechanical seals find their best application where fluids should be contained under substantial pressure. They can range from the simplest single seal arrangement to complicated sophisticated double seals with monitoring of the interspace. Some mechanical seals are assemblies of great complexity and consist of components manufactured to very high tolerances. They are often fitted as complete cartridge type units. Some sealing arrangements require constant lubrication often from the process fluid itself whilst others require external lubrication arrangements.

Plant equipment may be monitored during commissioning and throughout its operational life. This monitoring may be carried out on the basis of performance or condition or both. Performance monitoring is not discussed in detail in this Technical Measures Document. However the predominant techniques and parameters employed are flow, pressure, temperature, power etc. The alternative to performance monitoring is condition monitoring of which there are a number of techniques. The aim of such techniques is to identify deterioration and pre-empt imminent failures and so secure reliable/available plant, particularly for production and safety critical items. Some of these techniques are identified below:

All machine systems should be assessed according to the hazard presented if the machine or any associated protective system should fail.

Machine systems that have been assessed to present unacceptable consequences if the machine or protective system should fail may be classified as a `Critical Machine System' and given specific attention during operation including additional maintenance and monitoring.

Assessments should be based on:

Structures are required to provide support for plant and should be able to withstand all foreseeable loadings and operational extremes throughout the life of the plant. Failure of any structural component could lead to initiation of a major accident. For full guidance on Design Codes - Buildings / Structures see relevant Technical Measures Document. Structural design should take into account natural events such as wind loadings, snow loadings and seismic activity and also plant excursions

Maps showing the wind speeds to be used in the design of structures at locations in the UK are given in British Standards Code of Practice BS CP 3: 1972: Basic Data for the Design of Buildings, Chapter V Loading: Part 2 Wind Loads. Typical values are around 50 m/s (112 miles per hour). The code of practice also gives methods estimating the dynamic wind pressure on buildings and structures of various shapes.

Protection against lightning strikes on process plant located outside buildings is required since lightning is a potential ignition source especially for fires involving storage tanks. Lightning protection should be provided and guidance is available in BS 6651 : 1992 Code of Practice for Protection of Structures against Lightning.

See also Technical Measures Document - Earthing.

For the following substances general published codes exist giving full design details for storage and handling.

The design of systems for chlorine requires special consideration since chlorine is highly toxic and, if wet, also very corrosive.

Chlorine is usually stored under pressure at atmospheric temperature, but may also be stored fully refrigerated (-34°C) at atmospheric pressure.

A number of publications are dedicated to the handling of chlorine and specific guidance is given in:

Also see:

The Euro Chlor organisation is an affiliate of the European Chemical Industry Council (CEFIC) and represents European chlorine producers at 85 plants in 19 countries. Euro Chlor produces a number of publications. Further details can be obtained via the website http://www.eurochlor.org.

Anhydrous ammonia, boiling point -33°C, is normally stored as a liquid either under pressure or at atmospheric pressure in refrigerated facilities.

A number of publications are dedicated to the handling of ammonia and specific guidance is given in:

HS(G)30 Storage of anhydrous ammonia under pressure in the UK : spherical and cylindrical vessels, HSE, 1986 (Not in current HSE list).

Gives advice for the appropriate materials of construction for ammonia storage vessels.

CIA Refrigerated Ammonia Storage Code

CIA Code of Practice for the storage of anhydrous ammonia under pressure in the UK: Spherical and cylindrical vessels. (The CIA has withdrawn this document).

CIA Guidance for the large scale storage of fully refrigerated anhydrous ammonia in the UK.

CIA Guidance on transfer connections for the safe handling of anhydrous ammonia in the UK.

Propane and Butane are referred to as liquefied petroleum gas (LPG) in accordance with BS 4250: Specification for commercial butane and propane. Fully refrigerated storage is required at atmospheric pressure and at the boiling points of the substances concerned. LPG can also be stored under pressure in horizontal cylindrical or spherical pressure vessels.

HS(G)34 Storage of LPG at fixed installations, HSE, 1987.

HS(G)15 Storage of liquefied petroleum gas at factories, HSE.

CS5 Storage of LPG at fixed installations, HSE.

LPGA COP 1 Bulk LPG storage at fixed installations. Part 1 : Design, installation and operation of vessels located above ground, 2000.

LPGA COP 1 Bulk LPG storage at fixed installations. Part 2: Small bulk propane installations for domestic and similar purposes, 2000.

LPGA COP 1 Bulk LPG storage at fixed installations. Part 3 : Periodic inspection and testing, 2000.

LPGA COP 1 Bulk LPG storage at fixed installations. Part 4 : Buried/mounded LPG storage vessels, 2000.

LPGA COP 15 Valves and fittings for LPG service, Part 1 Safety valves, 2000.

LPGA COP 17 Purging LPG vessels and systems, 2000.

EEMUA 147. Recommendations for the design and construction of refrigerated liquefied gas storage tanks.

Liquefied petroleum gas. IP Model code of safe practice: Part 9.

A number of standards and codes exist for the storage of petroleum products and flammable liquids generally. A range of different main types of storage tanks and vessels for liquids and liquefied gases can be considered:

The relevant standards and codes are:

It is critically important that following the detailed design of a plant that the construction phase is carried out according to the original specification and that no additional hazards are introduced to the plant during the construction phase. Poor construction can result in the integrity of the whole system being compromised resulting in an increased risk of a major accident.

Building and construction are covered by a series of different building regulation including the following:

Construction (General Provisions) Regulations, 1961;

Construction (Lifting Operations) Regulations, 1961;

Construction (Health and Welfare) Regulations, 1966;

Construction (Working Places) Regulations, 1966.

In addition the Construction (Design and Management) Regulations (CDM) clarify the responsibilities of the various parties in a construction project. Also available is the Approved Code of Practice for the CDM Regs: Managing Construction for Health and Safety. Construction (Design and Management) Regulations 1994, ref L54, HSE Books 1995, ISBN 0 7176 0792 5.

It is important to demonstrate that the correct materials of construction have been used and that appropriate construction techniques have been employed so as not to introduce construction faults and flaws into the plant. Evidence in the form of documentation which shows that checks were carried out during the construction phase are important to prove that the construction phase of the project has been adequately supervised.

Documentation should show that the equipment supplied and installed is of the correct material of construction (and has received the correct heat treatment if appropriate), is the correct item/part/unit number and is as specified in the design schedule.

Documentation should also show that the workmanship is of the quality specified and that inspection and acceptance tests were carried out as required under the contract.

Commissioning of equipment should be carried out and records kept of the commissioning exercises.

Evidence of the following should be available:

The following documentation should be available:

The management of the commissioning and verification stages should be identified under the Safety Management System. The system should focus on ensuring that the design intent is met, and that deviations are properly assessed and controlled. Systems should be in place to ensure that corrective action is taken on the identification of discrepancies between installed equipment and the design intent and to control any deviations from normal operation.

Evidence of a number of pre-commissioning and commissioning checks should be presented to verify that the equipment as installed has been tested and is suitable for operation and meets the design intent. These may include:

The following codes of practice may be useful reading for the assessor when considering the process design of plant and equipment. Codes and guidance associated with the design of specific items of equipment (as discussed in previous sections) are given below. Not all the codes or guidance documents identified below are currently available and many have been superseded. However equipment designed to these original standards may still be in operation.

Other Standards and Codes of Practice relating to Pressure Vessel Design

In the UK pressure systems are covered by the Pressure Systems Safety Regulations 2000 (PSSR regs).

News

Design Codes