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A pump is a machine driven by some external power for transmitting energy to fluids.This power may be provided by the operators own effort (i.e) as in hand pumps) or by coupling the pump to a suitable engine or motor.

Although there are many different types of pump, one in particular has come to be regarded as the best for brigade work , this is the centrifugal pump .

A centrifugal pump has no valves, pistons or plungers and does not work by displacement. It makes use instead of centrifugal force i.e (the tendancy of a revolving body to fly out from the centre of ratation.)

The construction of the centrifugal pump is relatively simple, it consists of two main parts, the impeller and the casing, each of which has a separate function, that of the inpeller being to impart a high velocity to the water, while the casing transforms this velocity energy into pressure energy.

Priming systems

Priming is only necessary with centrifugal pumps.Positive displacement pumps i.e force ,lift , bucket and plunger and rotary pumps are themselves capable of exhausting air from the suction, and so have no need for a supplementry primer, They are for this reason frequently described as self-priming pumps.

There are three ways of priming a centrifugal pump,

(a) Gravity

(b) by pressure fed supply from a hydrant

(c)by use of a primer

The priming devices suitable for use with centrifugal pumps are

(1) Reciprocating #

(2) Exhaust ejector

(3) Rotary (a) water ring

(b) sliding vane

(4) Water seal

Of these the rotary sliding vane and the water seal primers are not widely used.

(1) Reciprocating primers

This primer consists of a small piston which is driven from the main pump drive shaft by a friction clutch or a wheel type friction drive, either of which are brought in or out of engagement by means of a lever. The priming pump inlet is connected to the suction side of the main pump by means of a pipe in which is situated the priming valve (a) this is also known as an air or atmospheric valve and is normally kept closed by means of a spring, thus isolating the priming pump from the main pump.

When the pump is to be primed the priming valve is opened and the primer is thus connedted with the pump casing and suction.

The drive is then engaged and the piston on its downstroke causes a reduction in pressure in the cylinder until the piston uncovers the inlet port then air from the pump and suction flows in . On its upstroke the piston causes the air to be forced through the outlet valveand discharged through the waste pipe. The priming pump is then disengaged and the valve closed.

On modern pumps arrangements are generally made for the priming valve to be opened automaticlally when the priming pump drive is engaged .

(2) Exhaust gas ejector primers

This type of primer is operated by the exhaust gases from the engine.

When the pump is to be primed, the priming valve is opened and the exhaust valve closed . The effect of the closed valve is to divert the exhaust gases from their normal passage down the exhaust pipe to the silencer and conduct them to a nozzle .The gas discharges from this nozzle into a throat (piped)and, on the principle of the jet pump draws the air from the suction and pump casing through the valve.

When all the air has been exhausted water will follow and steam will be seen discharging from the pipe.The valve should then be closed and valve opened ,when the water and exhaust gases will follow their normal courses into the pump and silencer respectively.

Valves have been treated in the above description as if they are opened independently.In fact the two valves are linked in such a way that operatinga single lever closes the one , and opens the other and vice versa.

(3) Rotary primers

A water ring primer may be automatic in action, being driven directly from the main pump shaft, ormayh be brought into engagement as required be means of a friction wheel which engages with a driving wheel on the main shaft.These primers depend on a water ring to form a seal .

An impeller with a hollow centre, rotates in an elliptical housing , Inside the hollow centre is a stationary boss which is a projection from the housing end cover. This boss has two suction and two delivery ports in its perphery which communicate with the primer suction and delivery connections. When the impeller rotates, the liquid in the housing is compelled by centrifugal force to move outwards, forming a ring of water within the contour of the housing thus creating a hollow elliptical vortex.This liquid ring rotates in the housing with the impeller and as it rotates from the minor diameters of the ellipse towards the major diameters it moves radially outward between the impeller vanes. After it passes the major diameter and rotates towards the minor diameter it moves radically inwards.As the liquid moves radially outwards between the vanes air is drawn into the impeller through the suction ports.As the liquid moves inwards this air is forced into the discharge ports in the central boss. Since the impeller is located centrally in the elliptical housing there are two pumping actions in each revolution.

A pump is a machine driven by some external power for transmitting energy to fluids.This power may be provided by the operators own effort (i.e) as in hand pumps) or by coupling the pump to a suitable engine or motor.

Although there are many different types of pump, one in particular has come to be regarded as the best for brigade work , this is the centrifugal pump .

A centrifugal pump has no valves, pistons or plungers and does not work by displacement. It makes use instead of centrifugal force i.e (the tendancy of a revolving body to fly out from the centre of ratation.)

The construction of the centrifugal pump is relatively simple, it consists of two main parts, the impeller and the casing, each of which has a separate function, that of the inpeller being to impart a high velocity to the water, while the casing transforms this velocity energy into pressure energy.

Priming systems

Priming is only necessary with centrifugal pumps.Positive displacement pumps i.e force ,lift , bucket and plunger and rotary pumps are themselves capable of exhausting air from the suction, and so have no need for a supplementry primer, They are for this reason frequently described as self-priming pumps.

There are three ways of priming a centrifugal pump,

(a) Gravity

(b) by pressure fed supply from a hydrant

(c)by use of a primer

The priming devices suitable for use with centrifugal pumps are

(1) Reciprocating #

(2) Exhaust ejector

(3) Rotary (a) water ring

(b) sliding vane

(4) Water seal

Of these the rotary sliding vane and the water seal primers are not widely used.

(1) Reciprocating primers

This primer consists of a small piston which is driven from the main pump drive shaft by a friction clutch or a wheel type friction drive, either of which are brought in or out of engagement by means of a lever. The priming pump inlet is connected to the suction side of the main pump by means of a pipe in which is situated the priming valve (a) this is also known as an air or atmospheric valve and is normally kept closed by means of a spring, thus isolating the priming pump from the main pump.

When the pump is to be primed the priming valve is opened and the primer is thus connedted with the pump casing and suction.

The drive is then engaged and the piston on its downstroke causes a reduction in pressure in the cylinder until the piston uncovers the inlet port then air from the pump and suction flows in . On its upstroke the piston causes the air to be forced through the outlet valveand discharged through the waste pipe. The priming pump is then disengaged and the valve closed.

On modern pumps arrangements are generally made for the priming valve to be opened automaticlally when the priming pump drive is engaged .

(2) Exhaust gas ejector primers

This type of primer is operated by the exhaust gases from the engine.

When the pump is to be primed, the priming valve is opened and the exhaust valve closed . The effect of the closed valve is to divert the exhaust gases from their normal passage down the exhaust pipe to the silencer and conduct them to a nozzle .The gas discharges from this nozzle into a throat (piped)and, on the principle of the jet pump draws the air from the suction and pump casing through the valve.

When all the air has been exhausted water will follow and steam will be seen discharging from the pipe.The valve should then be closed and valve opened ,when the water and exhaust gases will follow their normal courses into the pump and silencer respectively.

Valves have been treated in the above description as if they are opened independently.In fact the two valves are linked in such a way that operatinga single lever closes the one , and opens the other and vice versa.

(3) Rotary primers

A water ring primer may be automatic in action, being driven directly from the main pump shaft, ormayh be brought into engagement as required be means of a friction wheel which engages with a driving wheel on the main shaft.These primers depend on a water ring to form a seal .

An impeller with a hollow centre, rotates in an elliptical housing , Inside the hollow centre is a stationary boss which is a projection from the housing end cover. This boss has two suction and two delivery ports in its perphery which communicate with the primer suction and delivery connections. When the impeller rotates, the liquid in the housing is compelled by centrifugal force to move outwards, forming a ring of water within the contour of the housing thus creating a hollow elliptical vortex.This liquid ring rotates in the housing with the impeller and as it rotates from the minor diameters of the ellipse towards the major diameters it moves radially outward between the impeller vanes. After it passes the major diameter and rotates towards the minor diameter it moves radically inwards.As the liquid moves radially outwards between the vanes air is drawn into the impeller through the suction ports.As the liquid moves inwards this air is forced into the discharge ports in the central boss. Since the impeller is located centrally in the elliptical housing there are two pumping actions in each revolution.