Steam is another of the basic utility systems and as such an extensive distribution system is required to distribute steam throughout the Complex.
Steam is an important utility and a useful means of transporting stored energy around a site for use as a heating media in processes and offices. Steam generated in boiler plants can be distributed around site at a variety of pressures and temperature (commonly known as HP – High Pressure steam, MP – Medium Pressure steam, LP - Low Pressure steam etc.) depending on the required use, equipment design specification etc. A common feature however is that all steam (as a gas) is at high temperature relative to the surroundings through which the pipework distribution systems pass and that in order to minimise the loss of heat to the surroundings steam mains must be protected by insulation. Steam in distribution systems also travels at high speeds through the pipework.
If cooling occurs in steam mains, as is inevitable due to the long distances frequently involved, then an amount of hot liquid condensate (hot water) is produced as the gas changes back to a liquid. (This is the process where steam being emitted from a kettle condenses on nearby surfaces when rapidly cooled). Liquid condensate occupies less volume than the equivalent mass of steam (a gas) and its presence can cause problems in steam distribution systems. Conveying liquids at high speeds in gas pipelines is undesirable due to the considerable damage that can be caused by the impact of the liquid on fittings such as instruments, valves and bends.
Steam system pipework is protected by a number of different systems. Safety relief valves are installed along lengths of steam main in order to relieve overpressure of steam in the line and hence protect the steam main from catastrophic failure should the steam supply pressure rise. Safety valves for steam are generally designed and sized to relieve steam but not condensate. The presence of safety relief valves on steam systems relieving to atmosphere is clearly visible due to white plumes of steam that are present. Following a steam release to reduce the pressure the safety valves normally re-seat. Safety valves are designed to relieve the pressure allowing for a maximum rate of steam release. The capacity of a safety relief valve to relieve hot liquid condensate is less than that for steam due to the different characteristics of gases and liquids. Safety relief valves are not designed to vent condensate and under these circumstances can not be guaranteed to provide adequate pressure relief for the pipework system concerned.
Heat loss to the surroundings occurs as steam is distributed through long distances of pipework, through machinery and fittings etc. despite lagging and cladding that is designed to minimise the heat loss. As the temperature falls steam can be condensed to form hot liquid condensate. This must be removed from the steam mains in order to prevent flooding of the pipework and the carryover of liquid into instruments, machines and process areas. An extreme condition known as water hammer results in slugs of liquid being conveyed at high velocity through the steam main pipework carried by the steam. The liquid impacts on valves, bends and instrumentation and can cause sudden and catastrophic damage. Removal of condensate is achieved by the installation of steam traps which are designed to capture and then release the condensate from the steam main in a controlled manner. Detailed calculations must be carried out on steam main layouts to correctly size steam traps and safety valves for condensate removal and pressure relief.
Hot condensate is a valuable resource and its collection and return to the steam raising system minimises the quantity of fresh water and energy required for steam raising at the boilers. It is therefore preferable but not always possible to capture and return condensate by means of a condensate return system. Where it is not possible to capture the condensate this is generally released directly to ground. Whilst this does not give rise to any significant environmental concerns since it is only hot water that is being discharged care must be taken when approaching steam traps due to the potential for contact with hot water and scalding/burning which may occur. Steam traps discharging directly to atmosphere are often clearly visible due to a small quantity of accompanying steam which may also be released.
For steam flowing in pipelines an energy balance is achieved whereby fresh steam being fed into the pipeline system from the boilers replaces the steam being fed to the users. The system is effectively balanced with heat losses from the pipework system being replaced by heat input from the distribution system. Condensate should therefore not build up in the steam system. However in the event of the steam system being isolated or in a "dead leg" situation heat loss to the surroundings will still occur but there will be no heat input from fresh steam. As a result steam will cool and condense resulting in gradual condensate build-up unless it is removed.
The design of steam distribution systems requires detailed knowledge and understanding of the mechanisms by which overpressure may be generated and condensate formed and careful design in order to ensure that the steam traps and safety valves are correctly positioned. Precautions against excessive build up of condensate include careful pipe layout and the removal of condensate from dead-legs and low points in the system.
On-going maintenance of the steam supply system is also required in order to ensure that the system is fully functional. Isolation of either safety valves or steam traps impairs the integrity of the overall steam system and introduces the possibility of overpressure or condensate build-up in the steam system.
Steam is used in large quantities on the Complex for a variety of different purposes including heating. It is generated for the Complex in the main power station which is situated on the North Side of the Complex. Seven water tube boilers generate the steam which is then distributed throughout the Complex by an extensive steam pipework supply system. Steam is exported from the North Side of the Complex to the South Side of the Complex by pipework systems which run beneath Bo’ness road in two separate culverts carrying general electrical and mechanical services known as the West Gemec and East Gemec culverts. Having two supply routes available ensures that in the event of a problem with one of the distribution routes that steam supply can be maintained to the South Side of the Complex.
The steam is originally generated at a pressure of 1850 psi (125 barg) and then reduced in pressure in stages through a system of turbo-alternators, de-superheaters and pressure let down stations into a system of steam distribution pipework headers which give an MP steam supply system at 14 barg (200 psi).
A 450mm (18") MP steam main runs through the West Gemec tunnel and a 700mm (30") main through the East Gemec tunnel. All the pipework (carbon steel) is insulated /clad to minimise the effects of heat loss.
A manifold arrangement on the South Side enables the supply of steam to the chemical plants to be achieved from either of the supply systems. A number of valves on both the North Side and South Side in the vicinity of the two Gemec culverts are associated with the distribution pipework for the steam to allow isolation of various sections of the system as necessary and to ensure the correct routing of steam through the pipework system.
The portion of the pipework systems which run beneath the road essentially form the bottom portion of a "U" of horizontal length 30 metres and vertical drop approximately 6 metres. The pipework runs aboveground adjacent to the Bo’ness road. The culverts are low points compared to the surroundings and are prone to flooding. Pumps are required to keep the water levels in the culverts under control and to prevent the lagging on the steam mains from becoming wet and hence damaged and ineffective.