reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

Where diesel generator systems are installed in close Proximity to the sea, due regard is given to corrosion Problems arising from the salt-laden atmosphere.

•The fuel supply system must remain functional.

•The air intake must allow full-load running of the engines at all times.

•The atmospheric coolers must remain intact and functional.

•The exhaust must remain functional, i.e., unblocked, although not necessarily intact.

The starting and fuel supply systems are totally protected against damage from windborne cladding.

The air intake system allows the air flow required for full power running despite damage by the windborne cladding and possible blockage by windborne debris. It is capable of removing excessive windborne seaspray, rain and/or snow such that operation of the engine itself is protected. it is also capable of withstanding the strongest wind gust without significant loss of efficacy.

Atmospheric coolers can withstand the greatest wind gust without damage and are protected against significant damage caused by windborne cladding. They are also protected against loss of efficiency due to blockage by winclborne debris or snow.

The exhaust system and silencer are not required to survive the hazards intact but are designed always to provide a free passage for exhaust gases, bearing in mind possible damage caused by high wind or windborne cladding.

These design criteria ensure that the plant remains running without significant loss of efficiency.

Seismic requirements

Diesel generators are designed to operate after an earthquake in order to provide supplies to safely shut down nuclear reactors within the station.

The magnitude of seismic vibrations can vary from station to station and also from one floor level to another within a building. To enable the manufacturer to design equipment to withstand seismic vibrations in combination with normal operational forces, the necessary details to calculate acceleration forces are given in the appropriate specification for a station. Diesel generator systems are designed to withstand twice the response accelerations which are calculated from the acceleration forces in combination with horizontal and vertical floor response spectra for the particular building.

The ability of the equipment to withstand seismic vibration is established preferably by analytical methods, or by a combination of analysis and low level induced vibration tests. Where these methods cannot be used with confidence, items comprising the first production unit are subjected to shaker table tests. The equipment is energised during these tests and, where practical, it is at normal operating temperature. Monitoring equipment is used to evaluate the performance of the equipment before, during and after these tests. Full paticulars of seismic qualification procedure are contained in the CEGB Technical Specification and Schedules for Seismic Qualification of Electrical Plant (E/TSS/EX32000).

Avoidance of common mode failure

In order to avoid common mode failure, the CEGB always reserves the right to obtain some of the machines for a particular station from each of two dif-

ferent diesel engine manufacturers. This subject is giv en careful consideration during the tender stage, w it h particular respect to the tenderer,' information o n proven standard design and reliability and servic e ex- perience on similar duties.

Location of diesel generators

Diesel generators are usually located in engine room s in pairs, with careful attention to ensuring that failure of a component or system does not render both diesel generators inoperable (see Fig 9.7). In particular, where common systems are employed, standby plant and ficient pipework and valving is provided to ensure that alternative supply routes are available for each diesel generator. Individual services to each diesel generator are separated but, where it is necessary to run syste ms in close proximity, then adequate protection is provided to each system to prevent common failure.

Fire protection

The whole installation is designed to minimise both the risk and the effects of fire. Nevertheless, accidents can occur and the diesel generator installation, with the exception of the bulk fuel storage tanks (see belco.%), is therefore protected against fire by an automatic system of high velocity watersprays. It is a design requirement that the operation of the fire protection system does not adversely affect the operation of a diesel generator running at the time of operation of the watersprays. Careful attention to waterproofing of equipment is therefore essential. Whilst individual certification of equipment for this condition is not required, a site test on the completed installation will subject the running diesel generator to high velocity watersprays, when any waterproofing deficiencies are made good.

Any ancillary plant or electrical equipment likely to be affected by smoke, excessive heat or by the operation of the fire protection equipment is located outside the engine room in a separate plant room.

The fuel oil bulk storage tanks are equipped to inject low-expansion foam as a fire extinguishing medium.

5.2 Engine and auxiliaries

5.2.1 Engine types and characteristics

The engines specified by the CEGB are of the cold starting, compression ignition (diesel) type, pressure charged, operating on a four stroke cycle. The cylinder configuration is usually a 'vee' arrangement and, to obtain long life, the running speed does not usually exceed 750 r/min. In general, they comply with BS5514 and are suitable for use with BS2869 Class A fuel oils over the full range of engine operating conditions and ambient temperatures. A block diagram of a typical diesel generator system is shown in Fig 9.8.

5.2.2 Engine design and construction

, :;:me assembly

I IL eniline crankcase and frame are designed to be \!reineip robust and rigid constructions, with a prefor high grade castings rather than a fabricated Wlicre the latter is employed, careful

.,.! en:ion is paid io the manufacturer's construction, and inspection procedures, with particular quallty of welding (see Figs 9.7 and 9.9).

\thc engines spend long periods in the stationcondition, the manufacturer is required to give

n.irtieular attention to the tensile stresses induced in

iron components to reduce the risk of long term ,recp damage to the engine structure. The CEGB

. ..quires such tensile stresses not to exceed approxiiyly 25N of the material yield strength with the stationary. Allowance is made for the effects ■katerside corrosion, which reduces material

icthe crankshaft bearing housings can form part of hedplate or crankcase. Shell type, lined, high fatigue main bearings are used. They are split and ranged so that both halves may be removed without

disturbing the crankshaft. Any thrust bearings that require periodical adjustment are arranged to facilitate this operation.

The crankshaft is a one-piece alloy steel forging, fully machined incorporating oil ways and an integral driving flange at one end. The whole of the rotating and reciprocating masses is balanced.

All bearings, pistons, connecting rods, cam shafts, crankshaft, etc., are accurately machined to standard sizes. The aim is to ensure complete interchangeability of components throughout the engines of a particular size for a particular station and, whenever possible, on a national basis, thus reducing the cost of spares holdings.

Cylinder blocks are provided with renewable cylinder liners which are separately cooled to minimise thermal stresses induced by the rapid start requirements. Liners are fitted with seals to prevent the interleakage of oil, water or gases within the engine. Means of detecting failure of the liner lower seals are provided.

Each cylinder is provided with its own individual cast iron cylinder head, complete with starting air inlet valve, aspiration air inlet valves, exhaust valves, pres-

sure relief valve, indicator cock and centrally located fuel injector. The heads have a high resistance to thermal stresses and are separately cooled to meet the fast response requirements. Heads are totally enclosed by individual covers except for the injector, which is located outside to avoid contamination of the lubricating oil by the fuel oil.

To facilitate maintenance access, the crankcase is provided with inspection doors on both sides of the engine and is vented to atmosphere. Altogether, the whole engine is arranged to allow easy and rapid maintenance with a minimum of special tools.

To satisfy statutory and personnel safety requirements, pressure relief valves and flame traps are fitted.

The engine is cooled by a water cooling system, as detailed in Section 5.2.4 of this chapter.

Lubrication

The CEGB gives preference to engines operating on a wet sump system and incorporating a forced lubricating oil system to provide correct lubrication under all operating conditions to all surfaces requiring an oil supply. The exceptions are the generator and exciter bearings which are separately lubricated (see Section 5.3.1 of this chapter).

The lubricating oil capacity of the engine sump/ tank is sufficient for at least 24 h continuous operation of the engine at rated load without make-up.

Occasionally a manufacturer may put forward a dry sump design, but this must be demonstrated to be able to maintain a positive suction head on the lubricating oil pumps at all times.

The main lubricating oil pump is of the positive displacement type directly driven off the engine. The pump takes oil from the wet sump via upstream coarse strainers.

A standby electrically-driven pump, complete with automatic changeover controls and alarm initiating devices, is usually provided. The initiation of the electrically-driven standby pump is at a higher oil pressure than the 'low oil pressure' engine trip setting to permit continued engine running in the event of failure of the engine-driven pump.

Oil coolers, using water as a cooling medium, are provided and are designed to avoid contamination of the lubricating oil by water due to leakage across the separate oil/water circuits. Interconnecting pipework is arranged for easy removal of the cooler end boxes for cleaning the cooler tubes.

Thermostatic control is provided to regulate the system, safely maintaining the optimum oil temperature under all conditions of operation.

Because the engine is stationary for long periods, it is essential to maintain the oil at the optimum tem -- perature for starting. This is achieved by fitting thermo statically controlled oil heaters (see Section 5.3.4 of this chapter).

JACK. 1 COOL IN(, WATEH.

FIE1URN PIPE

PNEUMATICALLY OPERATED

AIR START VALVES

FIG. 9.9 General arrangement of a diesel generator unit

sioleaeue5 iesa!G

Duplicate full-flow fine filters, with replaceable elements and onload changeover facilities, are provided. Drip trays are fitted in this area and in all other positions of possible oil spillage.

A dirty lubrication oil collection system is provided in each diesel house to cover each pair of engines. The tank is sized to contain the sump oil of two engines. Drainage of the sump to the tank is by means of a portable pump and flexible hoses. The tank is arranged for discharge to a road tanker.

En addition, drainage points are provided at he lowest points of oil filters, heaters, coolers, etc. I

are arranged to keep oil spillage to a minimum and all pipework is self-draining. Magnetic drain plugs are fitted to trap any metallic particles in the lubricating oil.

A portable lubricating oil centrifuge is provided to service any of the diesel generators in a station and is sized to give a minimum of three changes per 24 h per engine. To facilitate this arrangement, using hoses, the necessary connections and valves are provided on each engine.

It will be noted from the above that any draining or centrifuging of the oil involves the use of hoses. This requires a deliberate action on the part of maintenance personnel connecting the two ends of a hose and avoids the accidental opening of a valve on a fixed pipework drainage system which could cause an inadvertent loss of oil endangering the engine and jeopardising its availability.

Compression fittings for pipe joints have given trouble in the past because of possible poor workmanship during assembly. They are reduced to a minimum nowadays and preferably not used at all in positions 1 m or less from the engine, to avoid the fire risk caused by oil spraying onto a hot engine.

Lubricating oil priming

When the engine is stationary, oil will drain from bearings and cylinder liners. This is avoided by supplying these items with oil during stationary periods, either on a continuous basis or intermittently, controlled by a timer; this operation is known as priming.

To safeguard the engine, duplicate (duty and standby) electrically-driven pumps are fitted for each engine, arranged so that in the event of pump failure the standby pump will take over the duty. An air-driven priming pump is also provided, which comes into operation if the electric pumps are unavailable.

The priming system is designed to avoid lubricating oil entering the cylinder combustion chamber or exhaust system but, at the same time, cylinder liners receive adequate lubricating oil to prevent pistons sliding on dry walls on start-up.

The priming pumps are capable of supplying sufficient oil to the bearings to enable the machine to be barred and also to allow the machine to be safely brought to rest should the main oil pumps fail when starting or running.

Fuel injection

Fuel injectors, fuel injection pumps (one per cyli n der) and fuel filtration equipment are usually of the manu. facturer's standard type, thoroughly proven in servic elsewhere under similar operating conditions. Adequatee redundancy is provided in the operating drive and metering control to ensure the engine is not made u n . serviceable by individual component failure.

Fuel injectors, incorporating filters, atomise the f ue l sufficient for complete combustion. The injectors are easily removable for maintenance purposes, for wh ic h a maintenance jig (including a test pump), are provid ed .

The fuel injector pumps are individually cam-op_ erated, of the spring-return plunger type and are driv en off a common camshaft. They have means of ensuring equal distribution and metering of the fuel to all cylinders under all conditions of loading. Each pump is capable of being manually primed.

The fuel system is self-venting and any fuel spill from the injectors is returned directly to the fuel oil bulk storage tank.

Fuel pipes from the pumps to the injectors are sheathed such that any leakage is piped to a tank with a level alarm.

Dual full-flow fine filters are provided upstream of the fuel injector pumps.

Flywheel

Dependent on the inertia of the total diesel engine/ generator assembly, a flywheel is sometimes provided to overcome the cyclic variation in torque of a diesel engine. It is manufactured from a material free from significant defects and is subjected to an overspeed test at 150% normal engine running speed.

The dynamic system, comprising engine and driven machinery, is so designed that critical speed vibration stresses do not exceed the recommended values given in the latest edition of Guidance Notes of Lloyd's Register of Shipping.

Barring gear

Motor operated barring is provided to turn over the engine at slow speed at regular intervals in conjunction with the priming equipment described above, during long periods of idleness and for maintenance purposes. Interlocks are fitted to prevent engine operation with the barring gear engaged. Provision is also made for manual operation. The electrically-driven gear is made inoperative when hand operation is in use and starting of the engine is prevented with hand barring gear engaged. Barring gear 'engaged' indication is provided at the control panels.

Facilities are available for 'inching' the diesel gen - erator rotors to enable them to be positioned angularly with precision for purposes of erection and maintenance (see also Section 5.3.4 of this chapter).

The drive motor of the electrically-powered barring gear is designed to turn the diesel generator from rest.

Emergency supply equipment

air for about 2.5 minutes of running which is considered sufficient to provide adequate lubrication prior to the first start attempt.

Air compressors and drives

Each compressor is of the air cooled type, mounted on a common bedp1ate with its electric motor and associated equipment to make a complete packaged unit for floor mounting (Fig 9.11). It is aligned in the manufacturer's works for easy erection on prepared foundations designed to the requirements of the manufacturer. Emphasis is again placed on a standard design and construction of proven reliability.

Equipment associated with each compressor comprises air suction filter, silencer, intercooler, relief valves, automatic start and stop features in the form of pressure switches, automatic unloading gear and automatic drain valves. Means are also provided for separating oil and moisture from the compressed air.

Each compressor is capable of charging the two largest receivers from atmospheric pressure to normal working pressure in the minimum time commensurate with the requirement that the compressor is kept running for a period of about 20 minutes under normal charging conditions. This period is chosen to enable the compressor to reach its normal working temperature and reduce wear and tear due to frequent stops and starts. Whilst it is generally left to the manufacturer to specify the above times, in no circumstances is the recharging time allowed to exceed one hour.

Air receivers

Each receiver is fitted with a relief valve, non-return valve, drain connection and valve, moisture trap, pres-

Chapte r 9

sure gauge, a pressure operated alarm switch (receiv er low pressure) and facilities for regular statutory inspections. The receivers and fittings are manufac. tured and tested in accordance with 3S1123 and BS5169.

The relief valves are piped to a safe discharge poi nt to avoid danger to personnel. For maintenance pu r. poses, the air supply pipework to the receiver inco r . porates an isolating valve.

To prevent corrosion, the internal surfaces of air receivers are either galvanised or coated with a flame resistantcoating approved by the CEGB.

Air pipework and valves

Pipelines are sized to allow the required air quantities to flow without prejudicial pressure drop and they in. corporate drainage slopes. Pipework and valves are designed to withstand a hydraulic pressure of twice the working pressure (Fig 9.12).

Manually operated isolating valves are provided in the diesel generator room to isolate the starting air system from the diesel engine whilst work is being carried out on the engine. The valves have padlocking facilities to enable them to be locked off in compliance with the CEGB Safety Rules. Similar isolating valves are provided for all other pneumatically operated auxiliaries associated with each diesel generator.

The manual isolating valves are also interlocked with the barring gear to prevent it from being engaged until the isolating valve is in the closed position.

Internal surfaces of pipework are coated, similar to the interior surfaces of air receivers. For small bore pipework coating is not possible, arid this is usually supplied in stainless steel to BS3605.

where vibration can become a problem, e.g., in cinity of compressors, flexible pipework is pro- \ kleLl to avoid fatigue fracture and noise transmission.

5.2.4 Cooling system

diesel generator is provided with a radiator system (I 9.13) with separate circuits for cooling of:

•Pie engine jacket water.

•The secondary water system, supplying coolers for engine lubricating oil, charge air system, generator coolers, etc.

!• ach radiator system is complete with its own water ;n.ike-up tank, radiator, electrically-driven fans, filters, niPework and valves. Pipework is arranged to conrleci separate sections of the radiator to the jacket and

'cLondary cooling circuits and valved to enable one

...] ftuit to be taken out of commission without affecting th,: others. There is no interconnection between cooling

circuits, with the exception of the common make-up supply.

To maintain the optimum engine working temperature under all conditions of operation, the engine jacket cooling water circuit is thermostatically controlled. This includes keeping the engines in a suitable pre-start condition, which may need the use of off-load heaters (see Section 5,3.4 of this chapter).

To prevent air locks, continuous or automatic venting from the high parts of the water system is provided.

The engine jacket cooling system is arranged to feed water direct from the engine to the radiator, with circulation maintained by an engine-driven water pump. It is also usual to provide a standby electrically-driven pump with automatic start facilities in the event of failure of the engine-driven pump.

The separate secondary water system, for the other coolers mentioned above, feeds water direct to a separate section of the radiator with circulation maintained by a separate engine-driven water pump and a

MAKE-UP

TANK

ROEPS

PUMP

Tr RADIATORS

CC>

4

VALVE

CAGES

standby electrically-driven pump, similar to the one already described.

A flow indicator is provided on all engine circulating water discharges.

Comprehensive precautions are taken against freezing of any part of the cooling system by including off-load heaters, antifreeze solution and trace heating. Also, where necessary, lagging is provided.

and out to prevent corrosion. They are of the closed (fully covered) design, vented to atmosphere and sized to ensure that no overflow occurs due to expansion of the coolant at any operating temperature. They are fitted with a calibrated level indicator visible from ground level. Tanks are erected on steelwork at a height which provides a gravity feed to the radiator systems. Personnel access platforms are fitted for cleaning.

Tanks are constructed generally in accordance with BS4I7 Part 2 from mild steel and galvanised inside

A radiator with fan assemblies is provided for each diesel generator unit (Fig 9.14). The assembly forms