Giranti per pompe acqua mare disponibili in diverse dimensioni, configurazioni e numero di pale.
To correctly identify an impeller for a sea water pump, it is necessary to check the part number, dimensions, number of blades, material and insert type.
The impeller is one of the most important components of a sea water pump. Its function is to draw water from outside the vessel and deliver it to the engine cooling circuit.
An incorrect impeller may fit inside the pump and rotate, but this does not necessarily mean that it will provide the required flow rate, priming performance or service life.
Part number, dimensions, number of blades, material and insert type must always be checked together.
Why the Engine Model Is Not Always Sufficient
The engine manufacturer and model provide an initial reference, but they do not always identify the impeller with certainty.
Apparently identical engines may use different pumps depending on:
- production year;
- engine version;
- application;
- marinisation system;
- pump manufacturer;
- modifications carried out over time.
The same basic engine may also be used for propulsion, generator sets or auxiliary applications, with different pumps and impellers.
For this reason, it is always advisable to check the part number of the installed pump.
Where to Find the Pump or Impeller Part Number
The pump part number may be:
- stamped on the pump body;
- shown on an identification plate;
- engraved on the cover;
- listed in the engine documentation;
- included in the spare parts manual;
- printed on the old impeller packaging.
The impeller part number may be printed on the hub, moulded into the elastomer or shown on the packaging.
Even a partially readable part number can help compare equivalent products from JMP, Sherwood, Jabsco, Johnson Pump and other manufacturers.
When the complete pump part number is available, identifying the impeller is generally more reliable than comparing dimensions alone.
Which Impeller Dimensions Should Be Measured?
When the part number is unavailable, the impeller must be measured accurately.
The main dimensions to check are:
Outside Diameter
This is the maximum diameter of the impeller, measured from one blade tip to the opposite blade tip.
The measurement should be taken without excessively compressing the elastomer.
Width
This is the axial depth of the impeller.
It should be measured across the impeller body, excluding any hub projection unless it is included in the manufacturer’s stated dimension.
Shaft Bore Diameter
This is the internal diameter of the hub where the pump shaft is fitted.
Even a small difference may prevent installation or create excessive play on the shaft.
Number of Blades
The blades must be counted individually.
The number of blades is a technical specification, not merely a visual detail.
Insert Type
It is necessary to check how the impeller is connected to the pump shaft.
Common insert and drive types include:
- keyed;
- pin drive;
- splined;
- double-flat;
- threaded;
- special-profile;
- impeller moulded directly onto the shaft.
Two impellers with identical outside dimensions but different inserts are not interchangeable.
Tipo di inserto
Occorre verificare come la girante viene collegata all’albero della pompa.
Gli inserti più comuni possono essere:
- a chiavetta;
- a perno;
- scanalati o millerighe;
- a doppio piano;
- filettati;
- con profilo speciale;
- girante stampata direttamente sull’albero.
wo impellers with identical outside dimensions but different inserts are not interchangeable.
How to Measure a Used Impeller Correctly
- A = impeller outside diameter
- B = impeller width
- C = bore diameter or insert seat
Main dimensions to check when identifying an impeller for a sea water pump..
.A used impeller may be deformed, swollen, worn or have permanently bent blades.
Measurements taken from an old impeller must therefore be treated with caution.
We recommend:
- using a vernier calliper;
- measuring the body at several points;
- avoiding compression of the elastomer;
- checking that the hub is not oval;
- checking whether parts of the blades are missing;
- comparing the dimensions with the manufacturer’s table;
- photographing both sides of the impeller.
When the impeller is badly damaged, it is better to identify the pump first and use its part number to determine the correct replacement.
Impeller Material
Flexible impellers may be manufactured from different materials selected according to the pumped liquid and operating conditions.
Depending on the manufacturer, compounds may be available for:
- fresh water;
- sea water;
- fuel and oil;
- food-grade liquids;
- particular temperatures;
- industrial applications.
The material should not be identified solely by the colour of the impeller.
Apparently identical impellers may be made from different compounds and may not be suitable for the same liquid or operating temperature.
For marine engine cooling, the material specified by the pump or impeller manufacturer must be used.
Impellers with the Same Dimensions but a Different Number of Blades
This is one of the most frequently misunderstood points.
Two impellers may have:
- the same outside diameter;
- the same width;
- the same bore diameter;
- the same insert type;
but a different number of blades.
This does not automatically mean that they are interchangeable.
The number, shape, thickness and flexibility of the blades form part of the hydraulic and mechanical design of the pump.
Changing the number of blades may affect:
- actual flow rate;
- flow uniformity;
- priming capability;
- generated pressure;
- suction performance;
- power absorption;
- blade flexing frequency;
- operating temperature;
- vibration;
- impeller service life.
Manufacturers therefore list the number of blades as an essential characteristic when distinguishing between impellers with very similar dimensions.
Does a Higher Number of Blades Always Mean More Flow?
Not necessarily.
As the number of blades increases, the space between each blade generally becomes smaller. The flow may become more uniform, but this does not automatically mean that the pump will deliver more water.
Flow rate also depends on:
- pump chamber profile;
- pump eccentricity;
- cam shape;
- hub diameter;
- blade height and thickness;
- material flexibility;
- rotational speed;
- system back pressure;
- cover sealing.
It is therefore incorrect to assume that an impeller with more blades is always better.
What May Happen When Using an Impeller with Fewer Blades?
An impeller with fewer blades may have larger spaces between the blades.
Depending on the pump design, this may cause:
- more pulsating flow;
- a different flow rate from the intended value;
- less consistent priming;
- greater deformation of each blade;
- increased stress on the elastomer;
- vibration or noise;
- premature wear.
What May Happen When Using an Impeller with More Blades?
An impeller with more blades generally has smaller chambers and a greater number of blade passages over the cam during each revolution.
This may result in:
- more uniform flow;
- a different flow rate from the intended value;
- more frequent blade flexing;
- increased friction;
- higher operating temperature;
- different power absorption;
- a different service life from the specified impeller.
The Impeller Fits inside the Pump: Does This Mean It Is Compatible?
No.
The fact that an impeller fits inside the pump body and can be installed on the shaft only proves partial dimensional compatibility.
It does not confirm that the following are correct:
- number of blades;
- compound;
- flexibility;
- geometry;
- flow rate;
- priming capability;
- service life.
An impeller must not be considered compatible simply because its diameter, width and insert match. The number of blades must also correspond to the specified model or be expressly approved by the manufacturer as an equivalent alternative.
The fact that the pump initially operates does not prove that the replacement is correct. A different impeller may appear to work, but provide a different flow rate or have a significantly shorter service life.
Why Similar Impellers May Have Different Part Numbers
.Differences may involve characteristics that are not immediately visible:
- number of blades;
- blade thickness;
- material;
- compound hardness;
- hub diameter;
- depth;
- drive type;
- insert length;
- gaskets included in the kit;
- specific application.
This confirms that identification cannot be based on a single dimension.
How to Count the Blades Correctly
o avoid mistakes:
- place the impeller on a flat surface;
- select one blade as the starting point;
- mark it with a removable marker;
- continue counting in the same direction;
- count each blade only once;
- repeat the count for confirmation.
If some blades are broken, inspect the blade roots still visible on the hub.
When the roots are also damaged, the original number of blades should not be estimated. It is better to identify the pump or locate the part number of the previous impeller.
What to Do When the Old Impeller Is Destroyed
A severely damaged impeller may not allow reliable measurement.
In this case, collect the following information:
- engine manufacturer and model;
- engine serial number;
- pump manufacturer and part number;
- photographs of the pump body;
- photograph of the pump cover;
- shaft diameter;
- drive type;
- internal chamber width;
- any remaining impeller fragments;
- part numbers shown on the previous packaging.
It is also important to locate any missing blade fragments in the cooling circuit.
Pieces of the impeller may become trapped in:
- hoses;
- heat exchanger;
- thermostat;
- fittings;
- cooling-water injection point in the exhaust.
Installing a new impeller does not solve an obstruction caused by fragments from the previous one.
Which Photographs Should Be Provided?
To assist with identification, we recommend sending:
- front view of the impeller;
- rear view;
- side view;
- close-up of the insert;
- photograph showing the calliper across the diameter;
- photograph showing the calliper across the width;
- stamped or moulded part number;
- photograph of the pump;
- photograph of the engine identification plate.
Photographs should be clear and taken in good lighting.
A ruler may be placed beside the impeller as a general reference, but precise measurements should be taken using a vernier calliper.
When Should the Impeller Be Replaced?
The impeller should be inspected periodically according to the recommendations of the engine or pump manufacturer.
Replacement is advisable when there are:
- cracked blades;
- cuts;
- permanent deformation;
- missing blades;
- separation between the elastomer and hub;
- hardened material;
- uneven wear;
- excessive play on the shaft;
- corrosion or damage to the insert.
Flexible impeller pumps should not be operated dry beyond the time strictly necessary for priming, as the absence of liquid may damage the impeller very quickly.
Checks to Carry Out during Impeller Replacement
When replacing the impeller, it is also advisable to inspect:
- condition of the cover;
- wear of the cam;
- condition of the gasket;
- scoring inside the pump body;
- shaft play;
- leakage from the drain hole;
- debris inside the cooling circuit;
- condition of the hub;
- correct initial lubrication.
The correct gasket thickness must be used because an unsuitable gasket may alter the axial clearance and contribute to premature impeller damage.
Common Mistakes to Avoid
Do not select an impeller based only on:
- colour;
- outside diameter;
- width;
- approximate number of blades;
- photograph found online;
- general engine model;
- visual similarity to the old impeller;
- the fact that it fits inside the pump;
- lower price.
Two very similar impellers may have different technical characteristics and applications.
How to Request an Impeller Check from Scandiesel
Unsure Which Impeller You Need?
Please send us:
- engine manufacturer and model;
- serial number;
- pump part number;
- old impeller part number;
- outside diameter and width;
- shaft bore diameter;
- number of blades;
- insert type;
- photographs of the impeller and pump.
We will check the available information to identify the impeller specified for the pump or a compatible replacement expressly approved for the same application.
View our range of JMP sea water pump impellers.