CONTENTS
4.6 Pump - Generic... Page 4,6 – 1
4.6.1 Introduction to Pumping Principles... Page 4,6 – 1 4.6.2 Background & History...Page 4,6 – 14 4.6.3 Hazard Assessment... Page 4,6 – 15 4.6.4 Operating Requirements... Page 4,6 – 21 4.6.5 Maintenance Requirements...Page 4,6 – 21 4.6.6 Single or Two Stage Centrifugal Pump Main Components... Page 4,6 – 23 4.6.7 Integration Aspects...Page 4,6 – 26
APPENDIX 1 - Net Positive Suction Head (NPSH)... Page 4,6 – 28 APPENDIX 2 - Pump Control & Management Systems... Page 4,6 – 29 This report section describes Dynamic and Positive Displacement pumping principles and the application of these principles to generic pump types. The information applies to common centrifugal pump types and is referred to from other sections, to avoid duplication.
4.6.1 INTRODUCTION TO PUMPING PRINCIPLES
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Pumps are used to transfer, or increase the pressure of, liquids.
Dynamic pumps work by sequential acceleration and deceleration of the working Positive Displacement pumps work by transferring measured quantities of fluid from pump suction to delivery, against a pressure differential.
Centrifugal pumps are the most commonly used type of Dynamic pumps, using Single stage pumps are simple robust pieces of equipment and run extended periods without problems. Mechanical seals remain the most vulnerable Multi-stage pumps are more complex and require higher standards of design and build for extended operation.
4.6.1.1 Working Principles of Dynamic and Positive Displacement Pumps There are two fundamentally different principles used to pump liquids.
Positive Displacement Pumps :- devices which move packages of liquid through the pump, transferring discontinuous packages from a lower pressure source to a higher pressure receiver. Dynamic Pumps :- devices which add energy (normally in the form of velocity) to the pumped fluid and which subsequently allow this energy to translate to pressure / liquid flow in a continuous process.
Positive Displacement pumps are discontinuous flow machines, they induce a fixed volume of fluid into a pocket, chamber or cylinder for pumping, by increasing the pocket volume mechanically. The pocket is then closed to suction & opened to discharge. The pocket volume is then reduced mechanically, forcing the fluid to be discharged from the pump at the pressure of the receiving system, normally at higher pressure. Only one stage of this mechanical displacement process is normally required, though to increase capacity, balance flows and compression forces there can be a series of pumping stages acting in parallel, but out of phase. There is never an open flow passage from delivery to suction (except for leakage through the clearances between moving parts).
Typical machines are :-
Piston pumps Screw pumps Gear pumps Lobe Pumps Diaphragm pumps
Progressive cavity pumps.
Piston, Diaphragm, or Plunger pumps use one or more reciprocating pistons in cylinders to pump fluid from a low pressure to a higher pressure. Self-acting suction and delivery valves permit forward flow and prevent back flow through the pump. The moving piston is sealed to the cylinder by specialised gland packing systems. The reciprocating motion is normally driven by a crankshaft, but can be operated directly by a pneumatic or hydraulic cylinder. A single reciprocating cylinder inevitably introduces pulsation, some capacity regulation systems make this worse by physically stopping the piston at a part stroke position. Triplex pumps have 3 cylinders with crankshaft driven pistons phased at 120 degree intervals. This minimises pulsation. Otherwise "pulsation dampers" can be fitted. These contain an elastomer bladder pre-charged with compressed gas, and absorb pulsations by further compressing the gas. Pulsation dampers are notorious for creeping deterioration in performance.
Screw and progressive cavity pumps use one or more helical screw shafts, these rotate continuously, drawing fluid up as the screw turns. To prevent back-flow, matched sets of screws seal each other, or a single screw runs in a soft moulded rubber stator. The clearances between screws and between screw and stator are critical, any wear here causes permanent loss of performance. Most screw pumps require the use of an inherently lubricating fluid to minimise wear. Certain special designs, which are more complex, can handle non-lubricating fluids and fine solids in suspension.
Gear pumps transport liquid in packages using simple meshing gear sets in close fitting casings to seal against back flow whilst the gear teeth transfer normally relatively viscous fluids, typically lubricating and heavy oils, around the gear periphery.
Lobe pumps use meshing pairs of lobed rotors, these are similar to gears but only have two or three lobes or "teeth" per rotor. To prevent lobe-to-lobe contact, they have an integral timing gear set. Lobe pumps can pump viscous non-lubricating fluids.
Peristaltic Pumps are an unusual and non-typical Positive Displacement pump design. They work by squeezing a soft rubber hose to create a seal, then moving the squeeze forward, mimicking the peristaltic pumping action of the human gut. In process pumps, the hose is curled into a pump casing and the squeeze is driven by cams or rollers mounted on a central rotor. These pumps are used for low-flow pumping of shear-sensitive fluids.
Dynamic Pumps are continuous flow machines, they use rotating impellers or propellers to sequentially accelerate the liquid (increasing its velocity) then decelerate it (trading kinetic energy for increased pressure). This may require a number of stages (maximum about 10), often within the same casing. Dynamic pumps always have an open flow route through the machine.
Typical machines are :-
Centrifugal Pumps Axial Flow Pumps
Hybrid (Mixed Flow) designs also exist. Centrifugal Pumps are by far the most commonly used dynamic pumps.
Centrifugal pumps work by spinning an impeller submerged in a bath of fluid. Vanes on the impeller impart rotary motion to the fluid, centrifugal effects push the liquid towards the outer edge of the impeller. This displaced liquid leaves the impeller and is collected by the shaped volute, which decelerates the fluid, converting kinetic energy to potential energy. This potential energy is expressed as increased pressure. The liquid being displaced from the impeller creates a partial vacuum at the centre of the impeller, permitting more liquid to enter from the pump suction. The impeller is a close fit in the casing to reduce the amount of back-flow which will by- pass the impeller. Centrifugal pumps may include several stages in series, each comprising one impeller, stage casing and volute.
Axial pumps are simply liquid propellers in pipes. Only a single stage is used as the downstream turbulence would greatly reduce the benefit of further stages.
Dynamic pumps have relatively few moving or contacting parts, low vibration levels and thus high intrinsic reliability. Hence they are preferred over other pump types where they can be used effectively. Pump selection needs clear statement of the intended duty and all temporary or transient duties to ensure capability over the complete operating range. Issues may arise if the pump is used for operation on higher density fluids when pressure containment capability can be exceeded when the developed head is translated into fluid pressure – typical situations arise when a hydrocarbon pump is used to pump water which can be 50% denser.
The materials of construction must be able to take the intended mechanical, thermal, and pressure loads and therefore are generally metallic; in addition those parts in contact with the process must be chemically compatible. Non-metallic materials are often used in seals.