INDUSTRIAL PUMPS
To keep pumps operating successfully for long periods of time, several elements are required. These elements include careful pump design selection; proper installation; careful operation; the ability to observe changes in performance over time; and, in the event of a failure, the capacity to thoroughly investigate the cause of the failure and take measures to prevent the problem from recurring. Pumps that are properly sized and dynamically balanced; that sit on stable foundations with good shaft alignment and with proper lubrication; that operators start, run, and stop carefully; and that maintenance personnel observe for the appearance of unhealthy trends, usually never experience a catastrophic failure
The family of pumps comprises a large number of types based on application and capabilities. The two major groups of pumps are dynamic and positive displacement.
Dynamic Pumps (Centrifugal Pump)
Centrifugal pumps are classified into three general categories:
Radial flow – a centrifugal pump in which the pressure is developed wholly by centrifugal force.
Mixed flow – a centrifugal pump in which the pressure is developed partly by centrifugal force and partly by the lift of the vanes of the impeller on the liquid.
Axial flow – a centrifugal pump in which the pressure is developed by the propelling or lifting action of the vanes of the impeller on the liquid
Positive Displacement Pump
A positive displacement pump has an expanding cavity on the suction side of the pump and a decreasing cavity on the discharge side. Liquid is allowed to flow into the pump as the cavity on the suction side expands and the liquid is forced out of the discharge as the cavity collapses. This principle applies to all types of positive displacement pumps whether the pump is a rotary lobe, gear within a gear, piston, diaphragm, screw, progressing cavity, etc.
A positive displacement pump, unlike a centrifugal pump, will produce the same flow at a given rpm no matter what the discharge pressure is. A positive displacement pump cannot be operated against a closed valve on the discharge side of the pump, i.e., it does not have a shut-off head like a centrifugal pump does. If a positive displacement pump is allowed to operate against a closed discharge valve, it will continue to produce flow which will increase the pressure in the discharge line until either the line bursts or the pump is severely damaged or both.
Types of Positive Displacement Pumps
Single Rotor - Vane, Piston, Flexible Member, Single Screw
Multiple Rotor -Gea,r Lobe, Circumferential Piston, Multiple Screw
Component Description
Centrifugal Pumps
Impeller and volute The two main components of a centrifugal pump are the impeller and the volute.
The impeller produces liquid velocity and the volute forces the liquid to discharge from the pump converting velocity to pressure. This is accomplished by offsetting the impeller in the volute and by maintaining a close clearance between the impeller and the volute at the cut-water. A centrifugal pump impeller slings the liquid out of the volute.
Positive Displacement Pumps
Single Rotor:
Vane The vane(s) may be blades, buckets, rollers, or slippers that cooperate with a dam to draw fluid into and out of the pump chamber.
Piston Fluid is drawn in and out of the pump chamber by a piston(s) reciprocating within a cylinder(s) and operating port valves.
Flexible member Pumping and sealing depends on the elasticity of a flexible member(s) that may be a tube, vane, or a liner.
Single screw Fluid is carried between rotor screw threads as they mesh with internal threads on the stator.
Multiple Rotor:
Gear Fluid is carried between gear teeth and is expelled by the meshing of the gears that cooperate to provide continuous sealing between the pump inlet and outlet.
Lobe Fluid is carried between rotor lobes that cooperate to provide continuous sealing between the pump inlet and outlet.
Circumferential piston Fluid is carried in spaces between piston surfaces not requiring contacts between rotor surfaces.
Multiple screw Fluid is carried between rotor screw threads as they mesh.
Relief Valves:
NOTE: It is essential to have a relief valve on the discharge side of a positive displacement pump.
Internal relief valve Pump manufacturers normally have an option to supply an internal relief valve. These relief valves will temporarily relieve the pressure on the discharge side of a pump operating against a closed valve. They are normally not full ported, i.e., cannot bypass all the flow produced by the pump. These internal relief valves should be used for pump protection against a temporary closing of a valve.
External relief valve An external relief valve installed in the discharge line with a return line back to the supply tank is highly recommended to provide complete protection against an unexpected over pressure situation.
Basic Measures to Improve Pump Efficiency
-Shut down unnecessary pumps.
-Restore internal clearances if performance has changed.
-Trim or change impellers if head is larger than necessary.
-Control by throttle instead of running wide-open or bypassing flow.
-Replace oversized pumps.
-Use multiple pumps instead of one large one.
-Use a small booster pump.
-Change the speed of a pump for the most efficient match of horsepower requirements with output.
Pump Efficiency Actions Large Horsepower Pumps (25 HP or greater)
-Here are some actions to take to improve pump efficiency. (Listed in order of decreasing potential for efficiency.)
-Excessive pump maintenance – this is often associated with one of the following:
Oversized pumps that are heavily throttled.Pumps in cavitation. Badly worn pumps.Pumps that are misapplied for the present operation.
-Any pump system with large flow or pressure variations. When normal flows or pressures are less than 75% of their maximum, energy is probably being wasted from excessive throttling, large bypass flows, or operation of unneeded pumps.
-Bypassed flow, either from a control system or deadhead protection orifices, is wasted energy.
-Throttled control valves. The pressure drop across a control valve represents wasted energy that is proportional to the pressure drop and flow.
-Fixed throttle operation. Pumps throttled at a constant head and flow indicate excessive capacity.
-Noisy pumps or valves. A noisy pump generally indicates cavitation from heavy throttling or excessive flow. Noisy control valves or bypass valves usually mean a higher pressure drop with a corresponding high energy loss.
-A multiple pump system. Energy is commonly lost from bypassing excess capacity, running unneeded pumps, maintaining excess pressure, or having a large flow increment between pumps.
-Changes from design conditions. Changes in plant operating conditions (expansions, shutdowns, etc.) can cause pumps that were previously well applied to operate at reduced efficiency.
-A flow-flow, high-pressure user. Such users may require operation of the entire system at high pressure.
-Pumps with known overcapacity. Overcapacity wastes energy because more flow is pumped at a higher pressure than required.
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موضوع: INDUSTRIAL PUMPS 
