Mr. J. SELTZ
Every aquacultural rearing calls for a more or less important water circulation, depending on it's intensification. Therefore for intensive rearing of fish in basins, an hourly or even every half hour renewal is necessary. The water outflow which supplies the oxygen and evacuates the toxic materials (nutriment wastes, animal excretion matter…) permits the survival and the development of the animals reared. Any water supply deficiencies will mean the rapid deterioration of the rearing conditions (especially drops in oxygen rates) leading sometimes to partial or total mortality of the fish, and this can happen quite rapidly (in some cases within one hour).
The above described shows the importance that the water supply has on aquaculture rearings. For marine aquaculture, the circulation of water is carried out in different ways depending on the rearing habits.
Therefore, structures located at sea, such as the shell fish tables or floating cages, use the flows caused by the natural current in the zone. The structures on land can have either a gravity supply by the tides or a supply by means of pumping.
The use of tides brings about an intermitting water circulation, which has a variable outflow depending on the tidal coefficients. This is why, save in exceptional cases, this procedure is only used in extensive or semi-intensive rearing.
When pumping is used a total control of the water renewal is obtained. But then a economic problem arises. The depreciation and running cost of a pumping station impels great fish loads and thus a rearing intensification.
A pumping structure consists of 4 elements which are:
- the pumping unit, consists of the pump itself and the motor that drives it,
- the pumping station, sheltering one or more pumping units, as well as diverse annex equipments, such as the control press, the transformer with an emergency generator set,
- the water intake
- the discharge or evacuation pipe.
The difference kinds of pumps used for aquaculture are:
- centrifugal pumps,
- propeller pumps,
- archimedean screw conveyors,
- current accelerators.
All these pumps are described in the annexes from 1 to 7 which also shows the different versions possibles (surface or immersed pump, with vertical or horizontal axes, etc, …)
The pressure losses are caused by the different obstacles that block the flow of the fluid pumped.
linear pressure losses caused by the water movement against the inside of the pipe and singular pressure losses caused by all the other local phenomena, such as, section or direction changes in the pipe (sluice gates, etc…)
In practice, the pressure losses are determined with the help of tables or graphs, such as those represented in annexes 8 to 10.
On these graphs the linear pressure losses are calculated from the COLEBROOK formula ; the common pressure losses are expressed by a length of piping which has an equivalent pressure loss.
When pumping; the pump should not only supply which is equivalent to that corresponding to the difference of the level between suction and discharge (named geometric head) but also the pressure which is necessary to overcome the pressure losses. J in the suction and discharge canalizations (respectfully J suc. and J dis.).
The total head of water is the addition of the geometric head and the pressure losses at suction and discharge.
With centrifugal pumps, it's the depression or suction caused by the turbine the raises the water.
If this depression should reach a complete vacuum stage, the water could not reach a height greater than the atmospheric pressure in any case (TORICELLI's experiment) which is 10,33 m at zero altitude.
In practice, this height is never reached because part of the available pressure is needed to:
- overcome the pressure losses in the suction pipes,
- gives the desired speed to the fluid,
- maintain pressure at the pump entrance at a minimum value, as the pressure steam corresponding to the temperature of the liquid to be pumped, must not be reached especially. If this should happen steam bubbles would appear in the liquid, which in entering the turbine at the highest pressure point, would clash violently together creating here specific, very high pressure, which could completely destroy the materials in these places.
This phenomenon is known cavitation
So as to avoid such as phenomenon, it is necessary for the pressure at the entrance of the pump to be kept well below the steam pressure of the liquid.
In practice; the suction possibilities of a centrifugal pump will be determined by the NPSH (net positive section head) which is the value of the pressure, measured at the entrance of the pump. This value must not drop below a minimum which would mean the apparition of the cavitation, named required NPSH.
The curve representing the required NPSH According to the outflow for the given pump is fournished by the engineer.
Therefore, the calculation of the available NPSH which can be expressed in a first approximation by the relation:
| available NPSH = 10 - (Ha + Ja) | Ha : Geometric head of suction |
| Ja : Pressure loss at suction |
and the verification that this value is well above the required NPSH fournished by the engineer is sufficient.
With curves, we can also find the solution to this problem.
with Ha : Geometric head of suction
Ja : Pressure loss in the suction pipe
I : cavitation point
To obtain satisfactory operating pump conditions, the operating point of the pump must be to the left of point I (intersection of the two curves, required and available NPSH) corresponding to the apparition of the cavitation. The distance which separates both curves shows the efficiency of the structure with regards to the cavitation.
Remark : the suction height for a centrifugal pump is in general limited at 7 m maximum.
There are three principal characteristic curves for a pump (apart from the required NPSH curve):
- theoutflow-height curve
- the brake power curve
- the efficiency curve
A graphical representation of these curves is given is Annex 11.
Brake Power
The power absorbed on the shaft of the pump is given by the following formula:
 | Pcv = Puissance cheval vapour Q = Outflow in l/s |
 | Hmt = Total head of water in m N = pump efficiency |
Remark: The power can be expressed in CV, Kw or HP. The different units are linked together by the followings relations:
Cheval vapour: 1 cv = 75 Kg m/s = 0.745 Kw
Horse power: 1HP = 550 ft 1bf/s = 0.745 Kw
with: KW = Kilowatt
kgm/s = Kilogramme meter per second
Ft.lbf/s = Foot pound force per second
Efficiency
All engineers fournish the efficiency curve. For each type of pump, there is a maximum around which it is suitable to use the pump.
For your guidance, the optimum efficiency for centrifugal pumps is generally between 0,70 and 0,80.
The determination of an operating point of a pump allows us know the outflow and the head of water produced by a given pump delivering into a given network system or pipe.
It is easily determined by drawing on the same graph the characteristic curve of the pipe (which represents for each outflow the addition of the geometric head and the pressure losses in the pipe) and the characteristics H - Q curve of the pump. At the intersection point S of both curves, the head of water for the pump will be equal to the addition of the total geometric head and the total pressure loss in the pipe. Consequently, point S is the operating point of the pump.
The choice of a pump type must be made while taking into account the hydraulic characteristics of the structure in view (outflow, head of water) and also the particular conditions of use (pumping at the edge of a lagoon or sea, type of land protection, etc…)
As there is nearly always a continuous need of water, it should be better to give privilege to the proportion of power consumed by outflow supplied.
Generally, we can give the use scope of the different types of pumps:
- big outflows - feeble height for raising :
(> 1001/s)
propeller pump (h < 15m)
Archimedean screw conductor (h < 10m)
- all outflows - height> 15m
centrifugal pump
- Creation of a water current - raising height of a few centimeters : current accelerators
The motor that drives the pump can be electric (in most cases) or thermic. when immersed units are used, they can only be electric.
We already know the power absorbed by a pump. It should be equal to the power available on the shaft of the motor, which is the power multiplied by the efficiency.
The pumping power necessary is as following:
with Q : Outflow in m3/h
H : Head of water in m
N : pump efficiency
K : motor efficiency
For your guidance, the output for a thermic motor in 0,75 and that for an electric one, 0,90.
The electric motors nearly always have a three-phase alternative current feed with 380 V. This current is supplied either by the network system (if in the proximity) or by a generator set which is placed near the pumping units.
Remarks: At it is necessary to have incessant water pumping in aquaculture, a pumping structure supplied by the network system will also have an emergency generator set which starts off immediately when there are power cuts.
It can be connected to the network system when the latter has a low tension food (220/380 v).
However in most cases, the station shall have a mean voltage supply between 5.5 and 15 KW. The implementation of a transformer for the supply to the motors and the different elements is then necessary.
The power of a transformer is given in KVA. Having determined the total P power of the station in CV, and considering an average cos ℓ equal to 0,85, we can calculate the power of the transformer:
A generator set can be used as the principal means for power (when there is no electric network system) or intervene in emergency cases when there are power cuts in the network system. Within the calculation for the electric power of the unit (carried out in KVA), it must be noted that electric motors use greater power when starting, then during the normal running times. This being so, the power of the generator get will be more than that for the pump (see annex 12).
The electric connections between the transformer or the generator set and the pumping station can be underground or overhead.
The feed cables must be of sample dimensions; for length of loss than 100 m, 3 to 4 per square of the section, is acceptable; for greater lenghts it would be better to reduce these quantities to 1,5 and 2A.
The voltage drops on a wire can be calculated in the following manner:
with L : Length of the cables in meters
I : Amperage intensity
Cos ℓ : power factor of the motor
S : Section of the lead main wire in mm2
V: Voltage
Generally, a motor can not be supplied when it is at a distance of more than 6 to 700 m from the transformer, as the voltage become too important.
In certain cases, it is necessary to implement pumps which are directly driven by a thermic motors (diesel or petrol)
Diesel motors are used for average size or great structure. For your guidance, the consumption for diesel motors vary from 0.15 to 0,25 1/h/CV.
For powers of scores to a few hundred CV, motors with rotations between 1000 and 1500 RPM are used
For the petrol pump, the heavy fuel consumptions limit their use to small pumps which are often used intermittently.
The pumping station regroups all the equipment described here above (pumps, motors, transformers, etc…) as well as control, regulation and protection equipment for the units and structure (contact switch, cut-out switch, pressure controller, clock, etc…).
For security and reliability reason, a pumping station is normally equipped with:
- two power supply sources (network system and the emergency generator set)
- two or more pumps which are implanted on parallal.
Generally, it has either two pumps each supplying the maximum outflow which is necessary for the rearing basins (which is Qm) and operating alternatively, or three units, two or which supplying half of the maximum outflow each. the third one being for emergencies. The latter solution also permits to reduce the outflow with a simple pump when the instant fish stock permits so.
An example is given in annex 13.
It is a building in which a pump is placed, the latter must obligatory be connected to a suction and evacuation pipe.
Certain rules must be respected for the implantation of the canalizations (see annex 14). The most important point being, to be sure that the suction canalization is protected against any entrance or accumulation of air which could bring about the drainage of the pump (vortex, high point, etc…)
These stations have a rectangular section lank or preferably a circular one embedded into the ground at a level which is below sea level; so that the tank can receive a gravity supply, the units only operating for the discharge.
There are two types of underground stations according to the pumping unit used:
With a turbine unit, the tank has only one section compartment (Annex 15)
With a propeller pump (annex 16), the tank two compartments:
- a suction room or lower part into which flows the intake water,
- an upper room or high part into which the pump raises the water to the evacuation pipe.
The propellor pumps are placed at the intersection of the two rooms which are hermetically scaled off by these pumps.
Annex 17 shows the implantation conditions for these units.
Remarks: All underground stations must have sluice-gates on the inflow pipe which permits to isolate the tank from the sea, so that the drainage of the pipe may be carried out, for the dismantling and upkeep works of the units.
There are many types of water intakes, generally used, to supply hydraulically, the sea water pumping unit.
These water includes are classed into two categories:
- the water intake by suction
- the water intake by gravity
It is the suction pipe for a surface pumping unit. This pipe runs partly on land and partly at sea, the latter must be more or less protected depending on the types of site (exposed or protected). The end of this pip is equipped with a screen which blocks the entrance of foreign bodies which could dammage the pump and a flap valve. These hinder the drainage of the canalization when the pumps stop operating (see annex 18 B).
At beaches (sand) the suction pipe can have a drainage network system (annex 18 C).
If the site is well protected (ex: lagoon), the water intake can be by means of a simple intake canal.
At sea, there is generally a canalization protected by environments (annex 18 A).
There can be alternative, by using a small canal which is hemmed in between two groynes protecting the end of the canalization which supplies the station. This solution which is more expensive has the advantage of having an accessible water intake, an easy upkeep, which is not the case for the canalization which is situated underneath a groyne.
The evacuation into the sea water intake rearing basins, using pumps, is done by means of discharge pipes, which ensure the connection between the pump and the supply structure for the rearing basins.
The fact that the pumping is carried out in marine zones causes a certain number of technical difficulties such as:
- corrosion,
- transport of sand,
- growth of marine organisms (mussel, Balanus) in the canalizations
Modern materials, such as stainless steel, plastic materials (vinyl polychlorure, polyethylene, polyester resin) permit to contend with corrosion, efficiently.
Sand is much harder to combat. Frequent cleaning of the pumping tanks and of the settings tanks is necessary.
Also, the problem of marine organisms which tend to invade the canalisations has not been solver. Great speeds (> 1,5 m/s) and wash out holes must be provided in the canalizations.
When a pump stops operating, the inertia hinders the water from stopping abruptly. This water continues to flow, causing depression wave which could create cavitations.
When this depression wave arrives at the extremity of the tank, it is transformed into a excess water pressure wave which has the same absolute value as the depression wave obtained earlier.
This abrupt variation of pressure is known as water hammering.
To combat these phenomena, devices such as, tanks or security valves are placed.
Data
Water supply for the rearing basins with 50 l/s from an underground structure which has a gravity water intake 100 m in length and an outflow towards the basins by means of an immersed centrifugal unit and a discharge pipe of 50 m in length (see drawing annex 19).
Water intake
The head of water is 2 m, which is 0,02 m per meter of pipe. The outflow corresponding to this pressure loss is obtained in a canalization having an inside diameter of 170 mm, according to the graph (annex 8). So as canalization having as inside diameter of 170 mm found on the market, shall be chosen.
Discharge pipe
If the water speed on the inside of the discharge is at 1,5 m/s, a diameter of 210 mm for an outflow of 50 l/s is required.
The corresponding pressure loss is 0,009 m per meter of pipe which is for 5 0 m ; 0,45 m.
If there were two bends in the discharge pipe, the pressure loss at the level of the bend would be equal to 1,5 m of pipeline, which would be 3 m for two bends. This correspondence to 3 × 0,009 m or 0,03 singular pressure loss.
Summary
Linear pressure loss caused by discharge: 0,45 m
singular pressure loss: 0; 03m
So the total head of water:
Hmt = H gem + J
= 5 + 0,45 + 0,03 ≠ 5,5 m
The immersed unit power would be:
= 5,5 CV (ou 4 KW)
- “Hydraulique Générale et Appliquée” by M.CARLIER - Ed.Eyrolles
- “In land Aquaculture Engineering” - ADCP - FAO
- “Aquaculture Engineering” - Ed.J.WILEY and Sons
- “Les pompes et leur application” par D. THIN - Ed. Eyrolles
- “Les pompes et les petites stations de pompage” - Technique rurale par la SOGREAM
- “Machines hydrauliques” by CARLIER - ENGREF
- “Pertes de charge dans les canalisations d'adduction d'eau en PVC rigide” Syndicat des fabricants de tubes et raccords PVC
- “Memento technique de l'eau” - Société DEGREMONT - Ed. Technique et Documentation
- “Inventaire des prises et rejets d'eau de mer sur le littoral breton entre Douarnenez et morlaix” par QUINTIN - Rapport IFREMER
| pump | pump power KW | Generator power KVA |
| Pompe | Consommation kW | Générateur kVA |
| L150 | 1.4 | 5.0 |
| S150 | 1.4 | 5.0 |
| L200 | 2.7 | 7.5 |
| L500 | 5.0 | 15 |
| C550 | 7.5 | 20 |
| L554 | 8.5 | 20 |
| L556 | 9.0 | 20 |
| L700 | 13.5 | 35 |
| L700R | 13.5 | 35 |
| L900 | 30 | 70 |
| L1100 | 65 | 150 |
1 kVA (kilo-volt-ampere) correspond a environ 0.8 kW
1 kVA ↔ 0, 8 kW
CONDITIONS GENERALES D'INSTALLATION D'UNE POME CENTRIFUGE
CONDITION GENERALE D'INSTALLATION
CONDITIONS DE MONTAGE
| 1o Cas: Pompe avec eau non charge | 2o Cos: Pompe avec eau contenant des groviers |
 | D indique is Ø du cóne d'espiration et non celui de to pompe |
| E : Pompe Ø 400 Tuyou Ø 400 cóne d'asp Ø 600 | |
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