Suction and Discharge Recirculation

Cause and Effect:
Recirculation occurs at reduced flows and is the reversal of a portion of the flow back through the impeller. Recirculation at the inlet of the impeller is known as suction recirculation. Recirculation at the outlet of the impeller is discharge recirculation. Suction and discharge recirculation can be very damaging to pump operation and could be avoided for the continuous of pumps of significant energy level or pressure rise per stage.

Diagnosis from Pump Operation
Suction recirculation will produce the previously mentioned loud cracking noise and around the suction of the pump. Recirculation is of greater intensity than the noise from low NPSH cavitations and is a random knocking sound. Discharge recirculation will produce the same characteristic sound as suction recirculation except that the highest is in the discharge volute or diffuser.

Diagnosis from Visual Examination
Suction and discharge recirculation product evaporation damage to the pressure side of the impeller vanes. Viewed from the suction of the impeller, the pressure side would be the invisible, or underside, of the vane. This is unlike cavitations damage from inadequate NPSH that occurs on the low pressure surface of the inlet vanes. Damage to the pressure side of the vane from discharge recirculation is shown in the drawing. Similarly the casing tongue or diffuser vanes may show cavitations damage on the impeller side from operation in discharge recirculation.

Instrumentation
The presence of suction or discharge recirculation can be determined by monitoring the pressure pulsation in the suction and in the discharge areas of the casing. Piezoelectric transducer installed as close to the impeller as possible in the suction and in the discharge of the pump can be used to detect pressure pulsation. The data may be analysed with a spectrum analyser couple to an XY plotter to produce a record of the pressure pulsation versus the frequency for selected flows.

Corrective Procedure:
Every impeller design has specific recirculation characteristics. This characteristics are inherent in the design and cannot be changed without modifying the design. An analysis of the symptoms associated with recirculation should consider the following as possible corrective procedures:

• Increase the output capacity of the pump
• Install a by pass between the discharge and the suction of the pump
• Substitute an improve material for the impeller that is more resistant to cavitation damage
• Modify the impeller design

Centrifugal Pump Performance

Volume Flow Rate

Abbreviated to “flow rate” and known traditionally as “pump capacity” Q, this is the volume of liquid per unit time delivered by the pump. To US customary system unit, Q is expressed in US gallon per minute or USgpm, for which the abbreviation of gpm is used (1 US gallon = 231 in3). For very large pumps, the unit ft3/sec are used. The consistent SI unit m3/s are implied when an SI value of Q is, unless the numerically convenient liters per second (l/s) are specifically called out.


Datum for Pump Head

The total head has components of pressure, velocity, and elevation z. because pump head H more precisely ΔH is the difference of total heads evaluated at the discharge flange d and the suction flange s respectively, the elevation of the datum from which Z is measured cancel out. However, for purpose of identification, computing NPSH, and so on, the standard datum as shown is used.

The standard datum for horizontal shaft pumps is a horizontal plane through the centreline of the shaft. For vertical shaft pumps, the datum is a horizontal plane through the entrance eye of the first stage impeller. Because pump head is the difference between the discharge and suction heads, it is not necessary that the standard datum be used, and any convenient datum may be selected for computing the pump head.

Power

In USCS, the pump output is customary given as liquid horsepower (hp) or as water horsepower if water is the liquid pumped.

lhp = QH (sp.gr.)/ 3960

where Q is the gallon per minute, H in feet, and sp.gr. is specific grafity. If Q is in cubic feet per second, the equation becomes,

lhp = QH (sp.gr.)/8.82

In SI, the power P in watts (W) is given by

P = 9797 QH (sp.gr.)

Where Q in cubic meters per second and H is in meters. Where Q is in liters per second and H is in meters.

P = 9.797 QH (sp.gr.)

Efficiency

The pump efficiency, ή is the liquid horsepower divided by the power input to the pump shaft. The latter usually is called the brake horsepower (bhp). The efficiency may be expressed as a decimal or multiplied by 100 and expressed as percent. In this subsection, the efficiency will always be the decimal value unless otherwise noted. Some pump driver units are so constructed that the actual power input to the pump is difficult or impossible to obtain. Typically of these is the “canned” pump for volatile or dangerous liquids. In such case, only an overall efficiency can be obtained. If the driver is an electric motor, this is called the wire to liquid efficiency or, when water is the liquid pumped, the wire to water efficiency.

Principle of Oil Seals

Oil Seals
One of the most frequently used types of seal is the Rotary Shaft Seal. This is generally used for sealing lubricating oil or grease in rotary shaft applications. In exceptional cases, it is also used to seal other fluids, gases and powdered or granular solids. For trouble-free operation and optimum service life of a seal, shafts must have a satisfactory surface finish, within recommended limits and have no machine lay. Both correct design and material choice are critical if bearings and gears are to be sealed to prevent the leakage of lubricating oils and greases and the ingress of penetrating dust and dirt.

Sealing
A good lubricating oil forms a strong tenacious film on gears, bearings and shafts and is not easily removed from the pressure bearing surfaces of these. However, where the shaft extends away from the equipment, this oil film must be retained. In Oil Seals, the pressure or radial load exerted by the sealing lip must be sufficient to retain the oil film, whilst not so high that excessive friction losses or wear can occur. Good Oil Seal design is therefore a balance between optimum running properties of the material, lip design and integral garter spring.

Working principle
During rotation of the shaft, a hydrodynamic film of lubricant is produced beneath the sealing lip, the thickness of which depends on shaft speed, oil temperature, oil viscosity and the pressure or radial load exerted by the sealing lip on the shaft. Due to capillary forces and the surface topography of the shaft, the fluid being sealed forms a meniscus under the sealing lip and is prevented from leaking. The fluid, the seal material, the film thickness, the sealing lip geometry and the surface topography of the shaft are governing factors in the realization of these capillary forces. A used seal having a shiny wear flat with hardening and radial cracking is indicative that it had operated on a shaft which was too smooth and /or that the radial load exerted by the lip was too high. A used seal having a wide wear flat is indicative that it had operated on a shaft which was too rough, especially if there was no hardening or radial cracking and could also be associated with incorrect sealing lip geometry. inside outside.



Construction of the oil seal
The most commonly used type is type R. This type has a carbon steel insert and has rubber outside diameter. The rubber gives a good sealing capability, even when the housing is not fully in tolerance. The sealing lip with spring provides interference on the shaft for effective sealing. The outside diameter, with inner metal reinforcement case, allows press-fitting in the housing, with sufficient interference on the rubber to provide static sealing. The sealing element is produced from a high performance Nitrile rubber.

This in combination with a high quality galvanized steel garter spring gives the ERIKS Oil Seal an optimum life. In order to prevent leakages due to a hydrodynamic pumping effect is it necessary that the sealing lip contact area on the sleeve or shaft is without any traces of machine lay.

Metal components
Depending on the application, ERIKS Oil Seals are supplied with various types of metal.
The reinforcing case Carbon steel as standard but stainless steel or brass on demand.

TYPE GR
This type is fully covered with rubber on the inside of the reinforcing case. ERIKS GR Viton® Oil Seals are of this type and are fitted with a stainless steel garter spring. This type can also be supplied in Nitrile rubber on demand.

The garter spring
Galvanized steel as standard. Stainless steel, bronze or an elastomer can be supplied on demand.

Air Operated Diaphragm Pumps (AODPS)

In general diaphragm pumps of all types are sealless pump, have no dynamic seals or packing, are self priming, and have an infinitely variable flow rate and pressure rate within the pressure and capacity ranges of the pump. Air-operated diaphragm pumps (AODPS) can also run dry indefinitely, and the discharge can be throttled to zero flow indefinitely.

The most common types of AODPs are the double diaphragm pumps (duplex pumps). These contain two diaphragm chambers and two desired flexible diaphragms. The diaphragm are connected to each other through a connecting rod and are clamped at the outer edges of the diaphragm. The shaft connected diaphragms move in the same linear direction simultaneously. Compressed air directed to the back side of the left diaphragm moves both right diaphragm. After complexion of a stroke, an air distribution valve directs compressed air from the supply to the back side of the right diaphragm and exhaust air to the atmosphere from the left chamber. This continuous reciprocating motion, along wit properly operating internal check valve, create an alternating intake and discharge of pumped liquid into and out of each chamber that result in a nearly continuous action from the combined chambers.

A discharge pump air motor contains an air distribution valve that shifts position at the end of each stroke of the pump. The air distribution valve alternatively directs supply air pressure to one chamber and exhaust the other. Air motor often use a two stage valve to control the reciprocating motion of the pump.

Priming on Centrifugal Pumps

A centrifugal pump is primed when the passageways of the pump are filled with the liquid to be pumped. The liquid replaces the air, gas, or vapour in the passageways. This may be done manually or automatically.

When pump is first put into service, the passageways are filled with air. If the suction supply is above atmospheric pressure, this air will be trapped in the pump and compressed somewhat when the suction valve is opened. Priming is accompanied by venting the entrapped air out of the pump through a valve provided for this purpose.

At the rate capacity, a positive displacement pump will develop the necessary pressure to exhaust air from the chambers and from the suction piping. Centrifugal pumps can also pump air at their rated capacity, but only at pressure equivalent to the rated head of the pump. Because the specific weight of the air is approximately 1/800 that of water, a centrifugal pump can produce only 1/800 of its rate liquid pressure. For every 1 ft (1 m) water has to be raised to prime a pump, the pump must produce a discharge head of air of approximately 800 ft (m). it is therefore apparent that the head required for a conventional centrifugal pump to be self priming and to lift a large column of liquid (and in some cases to greater than the rating of the pump. Centrifugal pumps that operate with a suction lift can be primed by providing;

1. A foot valve in the suction line.
2. A single chamber priming tank in the suction line or a two chamber priming tank in the suction and discharge line.
3. A priming inductor at the inlet of the suction line or
4. Some form of vacuum producing device.

Foot Valves:

A foot valve is a form of check valve installed at the bottom, or foot of a suction line. When the pump stops and the ports of the foot valve close, the liquid cannot drain back to the suction well if the valve seat tightly. Foot valves were very commonly used in early installation of centrifugal pumps. Except for certain applications their use is now much less common.

Priming Chamber:

Single chamber primer is a tank with a bottom outlet that is level with the pump suction nozzle and directly connected to it. An inlet at the top of the tank connects with the suction line. The size of the tank must be such that the volume contained between the top of the outlet and the bottom of the inlet is approximately three times the volume of the suction pipe. When the pump is shut down, the liquid in the suction line may leak out, but the liquid in the tank below the suction inlet cannot run back to the supply. When the pump is started, it will pump this entrapped liquid out of the priming chamber, creating a vacuum in the tank. The atmosphere pressure on the supply will force the liquid up the suction line into the priming chamber.