Surge Pressure

Surge pressures in a pipeline occur when flow is suddenly terminated. In a fuel hydrant system, surge will occur if an aircraft suddenly shuts down its inlet valves or if a fuel hydrant pit valve is suddenly shut. The resultant surge pressure will depend on the closure time of the valve being shut and the deceleration in flow-rate within the pipeline.

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Aircraft are protected from surge pressures by equipment on the fuel hydrant dispenser but for satisfactory operation of this equipment it too must terminate fuel hydrant flow as quickly or quicker than the valves on the aircraft. Thus the fuel hydrant system pipe-work is itself always potentially subject to surge pressures as a result of sudden flow terminations.

In the design of a fuel hydrant system it is not normally necessary to consider the surge pressures associated with a rapid termination of the total Design Year peak instantaneous flow-rate. It is possible for all tank valves on one aircraft to close simultaneously and it is possible for flow to two aircraft to be terminated at the same instant by simultaneous closure of two fuel hydrant pit valves. Further instantaneous shutdowns are considered to be statistically improbable. However, the use of common Fixed Ground Power (FGP) supply to several aircraft on a Pier could potentially result in more than two aircraft losing power and, subject to battery back-ups on the aircraft, fuelling valves close at the same time.

Similarly, a loss of power to the fuel depot pump platform (or operation of the fuel hydrant emergency stop system) could result in several pumps stopping at the same time and surge pressures being generated.

Within the oil industry, the adopted practice is for fuel hydrant systems to be designed to cater for an instantaneous shutdown (or instantaneous reduction in flow) of approximately 7,500 litres/min without surge

pressures being generated which cause the maximum pipeline pressure (surge + operating pressure) to exceed 15.9 bar. In practice this allows a surge increment of approx. 6.9 bar over a normal system operating

pressure of 9.0 bar.

Flow deceleration of 7,500 litres/min covers the termination of maximum flow-rate of one Boeing 747 or two smaller aircraft. At small airports, it may occasionally be acceptable to design for a lower flow deceleration to reflect the aircraft anticipated.

At medium and large airports where a surge design flow deceleration of 7,500 is used, calculations have shown that single feeder pipelines should be 460 mm (18") diameter and 360 mm (14") for ring main systems. This is, of course, a generalisation, as each fuel hydrant system should be separately analysed for surge as part of the process of pipeline sizing.

Within Air BP, fuel hydrant systems are analysed for surge pressure using a computer programme.

Finger lines from fuel hydrant mains to individual fuel hydrant pit locations are usually small in diameter, commonly 150 mm or 200 mm; they are sized for pressure loss considerations only. Whenever possible, it is recommended that finger lines are avoided but where their use is essential their lengths should be kept to a minimum for surge pressure

considerations. This is one of the reasons why Air BP favours installing fuel hydrant pit valves on riser pipes welded to the top of the main fuel hydrant pipeline.

API 5L quality pipe and ANSI Class 150 fittings or equivalent are the standard for typical fuel hydrant operating pressures.

The surge pressure increment superimposed onto the running pressure conditions under conditions of "instantaneous" valve closure is calculated from the formula: -

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Where;

p = Surge increment in kg/cm2 ( lb/in2) W = Density of liquid in kg/m3 (lb/ft3) a = Shock wave velocity in m/sec (ft/sec) V = Reduction in fluid velocity in m/sec (ft/sec) g = Gravitational acceleration of 9.8 m/sec2

(32.2 ft/sec2) From this,

Where;

p = The difference between the normal fuel hydrant test pressure of 16 kg/cm2 and the approximate duty pressure point of the fuel hydrant system, normally 8.5 kg/cm2 (125 lb/in2) i.e. p = 7.5 kg/cm2 (100 lb/in2).

For Jet A-1:

W is approximately 800kg/m³ (50ib/ft³).

a in steel pipe is approximately 1200 m/sec (3900 ft/sec).

Putting these values into the equations above we get V = 0.765 m/s ( V = 2.38 ft/sec)

Thus the maximum permissible change in liquid velocity in the fuel hydrant line is approximately 0.75 m/s (2.5 ft/sec) and this equates to 7,500

litres/min (1,650 UK gal/min) with 450 mm (18 inch) pipe.

In Ring-Main systems where the feed to the apron is by means of a closed loop, a similar calculation shows that 350 mm (14 in) pipe is adequate for the looped portion.

The maximum design instantaneous shutdown flow rates are shown below:

P i p e S i z e F l

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It follows that if the Instantaneous Design Year flow-rate and maximum pressure loss considerations (1 kg/cm2 or 15 lb/in2) dictate the use of pipe larger than 450 mm (18 in) diameter for spur lines and 350 mm (14 in) for fuel hydrant loops, then no other surge alleviating equipment is required on the system itself in order to protect it from excessive surge pressures induced as a result of downstream valve closure.

When the Design Year flow-rate dictates that for pressure loss reasons spur fuel hydrant lines of less than 450 mm (18 in) diameter or fuel hydrant loops of less than 350 mm (14 in) diameter are adequate, either a lower safe instantaneous change in flow-rate must be accepted (recommended minimum 4,500 litre/min; 1,000 UK gal/min) or separate means of surge pressure control must be adopted.

Finger lines from the fuel hydrant mains to individual pit locations may be smaller in diameter, usually 200 mm (8 in) or 150 mm (6 in), sized in accordance with pressure loss considerations only. Whenever possible it is recommended that finger lines should be avoided, but where their use is essential their lengths should be kept to a minimum. Direct mounting of hydrant pits onto risers welded to the top of the main line is preferred with the fuel hydrant main routed accordingly. Such an arrangement lends itself to nose-in aircraft parking, particularly when pit installation precedes the laying of apron concrete i.e. on new fuel hydrant systems.

Where shock alleviating equipment is necessary within the system to reduce surge pressures caused indirectly by the closing aircraft tank valves, it is recommended practice to locate this equipment as close to the source of the shock pressure as possible. It is both impracticable and uneconomic to install shock alleviators at each fuel hydrant pit, and it is therefore general practice for such alleviators as may be required to be installed on fuel hydrant dispensers. (review VEHC 10 ??). It should be noted that fixed shock alleviators could impact on a fuel hydrant leak detection system.

6.4.5.1 Flanges and Fittings

For a Class 150 flange, the maximum non-shock working pressure (for temperature range -30° C to +38° C) is 1900 kPa. For a transient surge pressure, the allowable limit according to some codes could be higher than 1900 kPa. The installed equipment is probably the limiting factor e.g. fuel hydrant pit valves.

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6.4.5.2 Aircraft

The maximum surge pressure generated on the aircraft is a function of the aircraft valves, valve locations, fuel line lengths, fuel line diameter etc.

The dispenser vehicle and fuel hydrant characteristics have no bearing on the aircraft-generated surge pressure. AEG considers only the equipment up to the aircraft wing. Any surge pressure present in the dispenser/fuel hydrant is prevented from reaching the aircraft by means of pressure control valves. Experience has shown that an aviation hose-end pressure controller will allow no more than approximately 500 kPa to develop

downstream i.e. on the aircraft. If the aircraft has already developed a surge pressure in excess of 500 kPa the hose-end controller will neither increase the aircraft pressure nor relieve it. It is essential to prevent any surge pressure generated in the fuel hydrant or by the fuel hydrant

dispenser vehicle from damaging the aircraft being fuelled since fractured aircraft fuel lines could result.

6.4.5.3 Dispenser

The fuel hydrant dispenser vehicle is normally fitted with a shock alleviator to limit the vehicle surge pressure to 1600 kPa until the intake coupler closes (review ??).

6.4.5.4 Fuel Hydrant

The main concern is that the fuel hydrant is not over-pressurised.

Although aircraft valves are said to close in approximately 10 seconds, because they are pilot operated the effective closure time will be less. The pressure wave velocity in a fuel hydrant is approximately 1050 m/s so the periodic time of a major system several kilometres long is longer than the aircraft valve closure i.e. the valve closure must be considered to be

"instantaneous".

The allowance made for surge pressure must be judged by the designer to balance costs vs. benefits, in recognition that the worst possible case is statistically highly improbable;

• Probability of a section of fuel hydrant being isolated.

• Probability of peak fuelling in a concentrated area.

• Probability of FGPU (fixed ground power units) breakdown in the peak fuelling area.

The above probabilities combined would suggest that the chance of the worst case occurring in any one hour is several millions to one! Also it would be operationally possible to limit the fuel hydrant maximum flow when a section of fuel hydrant is taken out of service for maintenance.

6.4.5.5 FGP Units

Back-up power is certainly desirable but has no bearing on the surge problem unless it effectively creates an uninterruptible electrical supply to the aircraft, which is unlikely.

6.4.5.6 Depot Filters

When considering the effects of surge pressures, the depot filters and other equipment must be included. There is an industry tendency to rate filters at 150 psig working pressure, 225 psig test pressure. These are often incorrectly referred to as "Class 150 lb" however; these pressures are significantly below the correct full pressure rating of a true Class 150

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design. Consideration is being given to a higher rating for future filter purchases, or surge pressure protection.

6.4.5.7 Causes of Surge Pressure

The highest surge pressures are likely to be generated by the following:

• Failure of airport FGPU system causing the valves of several adjacent aircraft to closer simultaneously.

• Closure of an apron motorised fuel hydrant isolation valve either by an Operator or an ESD shutdown.

• Closure of the fuel depot motorised isolation valve either by an Operator or an ESD shutdown.

• Simultaneous shutdown of 2 or more dispensers.

• Stoppage of the fuel depot hydrant pumps whilst at high flow-rate.