HYDRANT PUMPS AND CONTROL .1 Pumps

Correct pump selection is vital in fuel hydrant design. Positive displacement pumps must not be used in this application.

Three types of centrifugal pumps may be considered;

6. Single Stage Self-Priming Pump. Generally this type of pump has a steep head/flow characteristic giving a wide difference between stall (no flow) and duty pressures. However, if the ‘pressure difference’ margins required can be met by proprietary pumps of this type they are recommended as being simple and reliable.

This type has a low efficiency and is slow to prime and is best suited to applications where there is a positive head on the pumps (i.e. with above-ground vertical

tankage).

7. Non-self-priming Single or Multi-Stage Pumps. Such pumps should be used only if there is absolutely no possibility of the loss of prime or the entry of air into the pump during operation. Where vertical, above-ground tanks are employed which are fitted with low level shut off valves and alarms, this type of pump may be suitable.

8. Multi-Stage Self-Priming Pumps. This pump has a much higher efficiency than the single stage pump and better self-priming capabilities. It is however more complicated and has additional seals, which increase the possibility of leakage. Its improved self-priming is not usually of significant importance as a separate priming arrangement is advised in any system where a positive head to the pumps cannot be guaranteed.

Experience has shown that simple, rugged, direct coupled, electric motor driven single stage centrifugal pumps are the most suitable.

Many self-priming pumps are slow to prime against a full discharge line and it is recommended that an automatic air separator be installed to enable the pump to vent to atmosphere during the priming period. The air separator should be installed on a vessel of adequate volume and can be conveniently fitted to the water separator on the discharge side of the pump.

In all but the smallest of systems, at least two or more pumps will be required. Thus, for reasons of pump control in multi-pump installations it is essential that the pumps are capable of running in parallel. The head/flow performance curves should have a gradual and continuously falling

characteristic with between 10 - 12% drop in head between stall and duty points. It is important that the pumps are suitably matched such that the head/flow curves are similar and that duty head and stall pressures are within a maximum tolerance of + 1.25% between the curves of individual pumps.

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Diagram XX shows a typical Head/Flow curve for a multiple pump operation.

In the operation of a fuel hydrant system the majority of the head

(pressure) loss occurs downstream of the hydrant pit valves and therefore such losses are paralleled for a number of fuellings, i.e. the flow

requirements of the fuel hydrant system vary over a wide range while there is little variation in head loss in the system.

Hydrant pumps in most modern systems have a stall pressure of approximately 10 bar. This level, together with a necessary safety allowance for surge pressure, is compatible with AP1 5L standard wall pipe, ANSI Class 150 fittings and some essential components such as hydrant pit valves.

With a stall pressure of 10 bar, the pump duty points should therefore be at a pressure of approximately 9 bar (i.e. 10 - 12% lower than stall). This pressure is necessary to ensure that the required 'into aircraft pressure' of approximately 3.5 bar is available making due allowance for system losses incurred by filtration, valves, pipelines and the hydrant dispenser.

For small fuel hydrant systems, catering for small uplifts at low flow-rates, pumps are normally chosen with a flow-rate of 136 m3/hour at 9 bars.

However, for busier airports, pumps with a flow-rate of 272 m3/hour are generally more suitable.

The appropriate pump duty is determined during detail design.

It is essential when operating multi-pump systems that the individual pump performance characteristics are the same (or matched). When ordering pumps for hydrant application, the duty head and stall pressures should be specified with a maximum allowable tolerance of ± 1.25% between the curves of the individual pumps.

Pump bodies should be of cast steel construction. The use of high-tensile iron may be accepted if the pumps are designed to withstand surge pressures of 16 kg/cm2 (225 lb/in2). Pump Glands should be of the mechanical seal type requiring no exterior lubrication. Pump and motor should be direct-coupled.

To facilitate servicing, pumps should have gate valves fitted on both suction and discharge piping. A non-return valve should also be fitted in each pump discharge and it is preferable to site the gate valve

downstream of this sot hat this unit can also be serviced. Pressure and vacuum gauges should also be fitted on the discharge and suction piping and if there is any possibility of ground settlement then flexible

connections are recommended.

In the operation of a hydrant system the majority of the head (pressure) loss occurs downstream of the hydrant pit and therefore such losses are paralleled for a number of fuellings. I.e. The flow requirements of the hydrant system vary over a wide range whilst there is little variation in head loss in the system. This feature is very relevant particularly in hydrant systems where only a single pump is required. Instances of electrical overloading, with consequent motor failures, have occurred when the pump has operated further down its performance curve than originally intended. Motor selection should cover the end of the curve +10% power requirement. In addition, it is recommended that thermal relays are installed which cut off the power supply should the end of curve condition

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be maintained for an extended period. If operations dictate extended

"Duty Point" running, then a further pump is required on the hydrant system.

This section refers to pumps for conventional hydrant pumping systems.

Consideration has been given at certain locations to the use of variable speed pumps for hydrant application, designed to maintain a constant delivery pressure at varying flow-rates, but after close examination it is felt that at the present time that such systems are too complicated and do not produce any economic advantage over conventional systems.

6.6.2 Control

In a fuel hydrant system the instantaneous flow-rate demand is constantly changing and the peak demand requires the output of several pumps operating in parallel. It is essential that the hydrant pumps are controlled automatically to meet the demand on the apron.

The basis of virtually all automatic fuel hydrant pump control systems is to maintain the fuel hydrant at pressure under no-flow conditions by the use of a line pressure switch in the circuit of the lead pump and non-return valves in the individual pump discharge lines, or, in the hydrant main. The former system is preferred.

When an aircraft tank valve is opened and a fuelling is initiated, pressure in the hydrant falls causing a pressure switch to start the first pump. As flow demand increases (when additional aircraft require fuel) further pumps are started (and stopped) by means of signals from flow sensing elements in the hydrant line.

If flow demand does not exceed the capacity of the lead pump, the pump is switched off when the pressure in the line rises to the upper pressure setting of the switch. Lower and upper pressure switch settings, are normally around 6 and 8 bar, respectively. It is also normal practice to install a time delay in the pump switching circuit (usually 20 to 30 seconds) to prevent frequent stopping and starting whilst enabling the pump to pressurise the line up to the stall pressure of the pump regardless of the upper pressure switch cut-out point. The cut-in switch setting at which the pump operates to re-pressurise the hydrant line can be critical with long, large volume, hydrant systems and the fall in pressure required to operate the switch should be a minimum, to avoid pump surging on start-up.

To cater for low flow-rate off-take from the fuel hydrant when the hydrant line pressure is approaching pump stall pressure and the pressure switch has operated to shutdown the pump, it is necessary to incorporate a low flow switch, wired in parallel with the pressure switch, to maintain the first pump in operation. It is recommended that this flow switch be set at approximately 135 litres/min (30 UK gal/min) such that when flow drops below this pre-set figure the switch contacts will open and the pump will stop after a predetermined time delay. Figure XX shows the control circuitry for a lead pump, or for a single electrically driven pump.

The location of the low-flow switch may vary depending on the accuracy and reliability of the components available but it can be in the lead pump discharge pipe-work, in the main line, or in by-pass around the main line non-return valve. If placed in the lead pump discharge pipe-work

consideration should be given to the installation of further low flow

switches in the discharge lines of other pumps so that increased flexibility in pump sequencing can be achieved. In general, this method of low-flow

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control is preferred as low-flow switches in main hydrant lines subject to very high flow-rates frequently give rise to switching problems. Similarly, problems have been experienced with switches in non-return valve by-pass lines due to the non-repeatability of non-return valve action at low flows.

When the hydrant flow demand approaches the capacity of the first pump, the second pump in the sequence is started by means of a flow sensing element, such as an orifice plate, venturi, Dall Tube or meter, in the main line, or alternatively flow sensing elements in the discharge pipe-work of each separate pump. Third, fourth and subsequent pumps are started and stopped by the same means.

Many types of flow element are currently in use or under evaluation and the more common systems are as follows;

Orifice Plate

The most common differential pressure flow-measuring element used as a basis for the switching of pumps. It is suitable with up to four pumps, but with high flow-rate hydrant systems it is impracticable due to its poor pressure recovery characteristics, which result in high-pressure losses through the orifice plate. Plates designed for the operation of more than four pumps at an acceptable pressure differential also give inaccurate switching (in and out) of the second pump.

Venturi or Dall Tubes

The venturi tube is a natural development from the orifice plate because of its better pressure loss recovery features. However, due to its complicated shape it is difficult and expensive to manufacture and has been

superseded by the Dall Tube. This is a modified venturi, which also has a better pressure loss recovery than an equivalent venturi. The Dall Tube may be used satisfactorily with up to six pumps.

Turbine Flowmeters

This system has been used successfully at a number of major airports, but a major disadvantage is the high-pressure loss and lack of sensitivity over a wide range of flows when using a single meter in the main line. The alternative of the installation of turbine meters in each pump delivery to cover the flow range, adds considerably to the cost of a control system.

Motor Current Supply

Systems are in operation in which the actuation of the pump starter for the next pump in sequence is controlled using the current supply to the pump motor of the preceding pump. Switches are operated from settings on the individual pumps ammeters and the total current supply ammeter. This system is complicated and is not generally recommended.

Insertion Meters

A system based on the use of small turbine meters which are inserted into the main line, on into the pipe-work of each individual pump, through a 40 mm (1 ½") gate valve. The latter system is preferable and provides

maximum flexibility of operation since the insertion meter can also be used as the low-flow switch, i.e. the pulse signal from the insertion meter in the pump discharge is used as a low-flow signal when the pump is the lead pump in sequence and thereafter the pulse is summated and the other pumps operated using the total pulse signal. This system is also of low

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cost and because the insertion meter can be removed whilst the system is still in operation, is easily maintained.

Motorised Gate Valve

This system utilises a standard gate valve and actuator and operates by maintaining a constant pressure drop across the valve (measured by a pressure differential switch) at all flow-rates by adjusting the valve position.

Pumps are started and stopped using switching mechanisms related to the valve spindle position.

It is also possible to utilise the gate valve as the low flow switch and since it is often, for emergency shut down purposes, necessary to install an actuated gate valve in the system, the cost becomes attractive. This system however, may not be fail-safe. A major disadvantage of this system is that it cannot be calibrated accurately prior to installation.

The flow measuring devices listed above, whilst differing in the flow-sensing element, operate in basically the same manner. The entire pump sequence of operations can be summarised as follows;

9. The first pump in the sequence is started by the fall in line pressure, which operates a pressure switch. The pressure switch can either have both low and high switching points, or simply use a high point for both switching on and off the pump motor. The single point switch is more common with recent systems. Providing there is sufficient flow to operate the low flow switch, more than 135 litre/min (30 UK gal/min), the lead pump will continue to run.

10. When the flow-rate of the lead pump reaches its duty point, the second hydrant pump is started by a signal from the flow-sensing element. The two pumps then operate together and the flow demand is shared equally. Should the demand equal the combined duty point of the two pumps then the third pump is also started.

Similarly, as flow demand falls the pumps are switched out in reverse order. It is normal to adjust the cut-out switching flow-rate to 230 litre/min below the cut-in point to avoid ‘hunting’ of the pumps are flow-rates close to the switching point flow-rates.

It is also necessary to install time delays on both the switching points to ensure that a fluctuating flow-rate does not continually operate the switches and overload the relays and contacts.

Figure XX shows the Control Circuit for a Multi-Pump system that utilises a pressure differential sensing element.

The various control circuits referred to above can be located in a dedicated console in the main depot or be part of a Programmable Logic Control (PLC) system incorporating other operational control functions.

The automatic control of diesel driven hydrant pump sets where these may be used at minor airfields or where the electrical supply is unreliable, is possible, although it is necessary to manually start the first pump set and leave the engine at idling speed. Figure XX shows the control circuit for a single pump system detailing the pressure and flow switches which act in the engine speed governor. A twin pump system would be basically similar except that the second pump can be started and stopped automatically using a second, higher flow-rate, flow switch.

Revision 1.0d - 6th January 1998