Using Industrial Hydraulics |
Applications of Computer-Aided Manufacturing
__3.17 Non-electric submersible pumps
Electric submersible pumps are potentially very dangerous.
Many small submersible pumps are used with water. Water and electricity is of course a dangerous combination. The motor manufacturer takes extreme precautions to prevent the water and electricity mixing. Large submersible pumps are used for handling hazardous liquids, such as crude oil. It’s flammable but is not a pure liquid. Crude oil is a mixture of liquids, and possibly gases such as carbon dioxide and hydrogen sulphide. Hydrogen sulphide is potentially poisonous as well as flammable.
Removing electricity from the pump installation therefore removes a very serious hazard!
Fig. 22 Cross-sectional arrangement of a heavy duty submersible pump
Fig. 23 Installation of submersible pump in sump
Sometimes electric power is not available, on building sites For example; and so other sources of power must be used, such as engines, compressed air from mobile compressors or hydraulically from a separate power pack, or one which is built into a vehicle. This type of pump is also suitable for intermittent use in construction work such as road making, rock blasting and excavating or as a fire precaution when carrying out stubble burning or site clearing.
Compressed air-driven rotodynamic submersible pumps are of similar construction to those which are electrically driven. They are equipped both with and without special heavy-duty components for pumping large solids. The high speed compressed air motor, lubricated by oil mist, can compete on performance with electrically driven units, see Fig. 24. Also remember, the air-operated double-diaphragm pump can be treated as a submersible pump. The air exhaust should be piped to above the maximum liquid level to prevent 'flooding' and potential corrosion of the air valve.
Jet pumps can be submerged, these pumps can handle large solids providing large quantities of liquid are moved, and large quantities of driving fluid are provided.
__3.18 Submersible pumps for deep well applications
Submersible deep well pumps in smaller sizes have the same range of applications as deep well pumps with ejectors. The difference here being that the pump is placed directly in the borehole connected by means of a delivery hose and electric cable.
Since the smallest borehole diameter is 4", the diameter of the pump must be somewhat less, see Fig. 25. Impellers are between centrifugal and mixed-flow designs with as many as 100 stages. The pump is a multi-stage centrifugal pump driven by a special type of long thin electric motor located beneath the pump and cooled by the water in the borehole. For small pumps, suitable for domestic applications, the pump length L varies according to the depth of the borehole between 500 and 2500 mm.
Submersible deep well pumps of larger sizes have outside diameters of up to approximately 500 mm and lengths of up to 5 meters. Their hydraulic data varies considerably, the maximum values being: flow 5000 m^3/h and differential head 1000 m. Motor outputs of several megawatts are available when using high voltage motors.
These pumps are used for municipal drinking water supply, reduction of ground water levels and mine drainage. Deep well pumps find favor in oil field applications, on and offshore. Units are used for secondary oil recovery prior to water injection. Typical applications are to be found in restricted wells or shafts, Fig. 26. They can also be used for pressure boosting in drinking water supply networks, where the pump may be integrated in a section of pipe.
The electric motors are always squirrel-cage liquid-filled induction motors, which naturally makes very great demands upon the motor's electrical insulation. The liquid filling is either clean water, water-oil emulsion or oil depending on the requirements of each specific case. The liquid filling is separated from the pumped liquid by a mechanical seal and a diaphragm to compensate for variations in volume, due to temperature changes.
The pump and motor rotors run in liquid-lubricated plain bearings. The axial thrust bearing is the most heavily loaded, which is why some manufacturers employ opposed impeller construction to reduce unbalanced axial forces. Special models are available for sand contaminated or corrosive water.
Fig. 24 Compressed-air driven submersible pump
Pump section Largest outside diameter less than 100 mm Water intake with strainer Electric motor
Fig. 25 Small submersible deep well pump
Fig. 26 Deep well pumps placed in wells illustrations 1 and 2, and placed in pipe section for pressure boosting illustrations 3 and 4
Fig. 27 Engine driven self-priming pump
Hale Products Europe
Fig. 28 A typical small portable self-priming pump; Varisco Srl
__3.19 Portable self-priming pumps
Most pumps must be vented and primed prior to start-up. For pumps which have flooded suctions this usually involves opening small valves on top of the discharge pipework, so that all the air trapped in the suction pipework and in the pump will be forced out and replaced by liquid. Some pump designs require vent valves on the casing to fill the pump completely. When the pump suction is not flooded the vent valve in the discharge pipework is coupled to an air ejector. The air is withdrawn by a partial vacuum and the liquid fills the void.
Self-priming pumps are also designed to overcome these problems. The casing of a self-priming pump is constructed to retain sufficient liquid so that the impeller is always flooded. The casing needs priming only once provided the liquid does not evaporate or leak away through poor seals. The pump casing is shaped so that any gas or air bubbles become completely surrounded by liquid and are pumped from the suction to the discharge. Since there is only a limited amount of liquid in the pump casing during evacuation, the gas and liquid must be separated, whereupon the liquid is returned to the suction side to recirculate. The internal design and shape of the pump casing to achieve this effect varies considerably depending on the manufacturer. When choosing a pump it’s important to take into account the wearing effect which sand, For example, may have upon the pump's evacuating capabilities. When considering the suction system design, the volume of gas in the suction piping should be minimized.
Engine-driven self-priming pumps are usually single-stage centrifugal pumps as shown in Fig. 27. In order to be self-priming the pump must be filled with liquid prior to starting.
The evacuation times quoted usually refer to the time taken to obtain the full pressure at the pump discharge for a specific suction lift, using a hose having the same diameter as the pump's suction connection and whose length exceeds the suction lift by 2 meters. The pump performance can be adapted to actual requirements by varying the engine speed. Fig. 28 shows a typical small portable self-priming pump.
Engine-driven pumps can handle solid particles of up to 10 to 15 mm. The suction hose is reinforced because of the vacuum caused by pumping. The normal suction strainer can usually be replaced with a much shallower design in order to facilitate pumping down to within a few millimeters off a floor. The pump should be equipped with a protective safety frame and runners, skids or a wheeled trailer to enable it to stand on soft ground.
Priming of engine driven units can be assisted by using the engine inlet manifold vacuum or a separate air pump.
Engine-driven single cylinder reciprocating pumps are also used as self-priming pumps especially for construction sites and road work. Engine-driven pumps are available with suction sizes up to 150 mm which can handle flows of 400 m^3/h at heads up to 30m. Engines sizes up to 30 kW are not unusual.
Single cylinder mechanical diaphragm pumps are also available, see Fig. 100. If compressed air is available, then the air-operated double-diaphragm pump can be considered.
These pumps won’t handle such large solids but are useful for 'clean' applications. Care should be taken to supervise these pumps. If the liquid supply 'dries-up' the pump will race and consume a large quantity of air while doing zero work.
Engine-driven units can be very noisy. Current designs pay special attention to noise reduction, levels of below 80 dBA at 1 m are desirable. To achieve this level of noise it’s usually necessary to fit silencers to both the inlet and exhaust sides. The engines are normally splash-lubricated which means that the pump must be placed on a relatively flat surface the inclination may seldom exceed 20 ~ . Engine-driven compressors can also be extremely noisy.
__3.20 Horizontal two-stage axially-split centrifugal pumps (includes ISO 13709, API 610 designs)
These pumps are similar in many respects to the double-suction and multi-stage pumps described later. The use of two-stages provides extra differential head. The advantage of the split casing is usually found in maintenance. Suction and discharge connections can be located in the bottom half, the top half being easily removed without disturbing piping. Process connections can be located in the top half with the obvious extra maintenance work. A radially-split design would remove this complication; see later. A radially-split design is recommended by ISO 13709 and API 610 for 'hazardous liquid' applications.
Impellers are usually mounted back-to-back. Two-stage pumps of this design are usually larger than the end-suction two-stage pumps and require the impellers to be placed between bearings. Impellers usually have a front wear ring. A common back wear ring, a center bush, reduces leakage between the stages.
The center bush also acts as a bearing and provides some shaft stiffness. Engineering plastics can really improve the bearing effectiveness. A double-suction first stage can be used when NPSHa is low but this increases the cost significantly. The increased cost should be less than that of providing a separate booster pump and driver plus associated electrics and controls.
A reorganization of the plant layout may improve NPSHa and obviate a double-suction. Casings may be diffuser or volute.
Diffuser pumps can be more efficient if the duty point is calculated accurately.
Depending upon the size of the pump, bearings may be rolling contact or sliding. Back-to-back angular contact ball bearings have become popular as a combined radial and thrust bearing.
A very compact bearing housing is possible. Oil-ring lubrication is simple and effective. Larger pumps, with sliding bearings, usually have a tilting pad thrust bearing and a lube oil system.
Some users prefer oil mist for rolling contact bearings and build a central supply system.
Shaft sealing is accomplished by a wide range of mechanical seals, although soft packing may be possible. Cast iron casings probably have restricted sealing space as cast iron is not recommended for handling many hazardous liquids. Connecting small bore vent and drain piping is only possible via threaded holes in many casings.
These two-stage pumps are manufactured in a wide range of materials, see Tbl. 1. Pumps in super stainless steels may be available to order.
Tbl. 1 Two-stage axially-split pumps --materials of manufacture Two-stage pumps can usually handle flows up to about 2000 m^3/h and differential heads of up to 600 m.
__3.21 Horizontal two-stage radially-split centrifugal pumps (includes ISO 13709, API 610 designs)
These pumps are similar in many respects to the double-suction pumps described in Part. 3.8 and multi-stage pumps described later in Part. 3.25. A radially-split design is recommended by ISO 13709 and API 610 for 'hazardous liquid' applications. The use of two-stages provides extra differential head. The advantage of the radially-split casing is found in application and maintenance. These pumps can be centerline-mounted to remove thermal growth problems experienced in high temperature applications.
The use of circular gaskets, for the main pressure containment closure(s), removes problems associated with large flat gaskets and bolting. Elastomer 'O'-rings can be used when temperatures permit. Hybrid fiber and metal gaskets can be used for higher temperatures. Metal 'O'-rings can be used for the most arduous applications. Suction and discharge connections can be placed at any orientation, virtually, to suit the plant layout.
Some connection layouts are more popular than others; top-top is the manufacturers default arrangement; this is the shortest delivery time because there are no pattern modifications. If the plant layout is considered, practically, for piping venting and priming, then good arrangements can be accommodated in the appropriate time scale. Maintenance of the pump does not require the casing connections to be broken.
Two-stage pumps of this design are usually larger than the end-suction two-stage pumps and require the impellers to be placed between bearings. Impellers are usually mounted back-to-back to minimize axial thrust and have front and back wear rings. A double-suction first stage can be used when NPSHa is low but this increases the cost significantly. The increased cost should be less than that of providing a separate booster pump and driver plus associated electrics and controls.
A reorganization of the plant layout may improve NPSHa and obviate a double-suction. Suction pipe venting should be checked. Casings may be diffuser or volute; larger pumps may have double volutes. Diffuser pumps can be more efficient if the duty point is calculated accurately.
Depending upon the size of the pump, bearings may be rolling contact or sliding. Back-to-back angular contact ball bearings have become popular as a combined radial and thrust bearing.
A very compact bearing housing is possible. Oil-ring lubrication is simple and effective. Larger pumps, with sliding bearings, usually have a tilting pad thrust bearing and a lube oil system.
Some users prefer oil mist for rolling contact bearings and build a central supply system.
Shaft sealing is accomplished by a wide range of mechanical seals, usually to EN ISO 21049/API 682. These two-stage pumps are manufactured in a wide range of materials. Lowest cost is carbon steel with cast iron stationary internal parts and cast iron wear rings. Materials progress through all carbon steel, 11-13Cr, austenitic stainless steel and super stainless steels. Some manufacturers produce pumps in 19Cr 13Ni 3Mo low carbon as standard. Pumps can be built with any range of compatible materials, to order, if sufficient time is allowed in the contract timetable.
Fig. 29 Large 4-stage segmental pump with balance drum, sleeve bearings and tilting pad thrust bearing
Two-stage pumps can usually handle flows up to about 2000 m^3/h and differential heads of up to 800 m with pressure ratings of 100 bar(g). Operating temperatures up to 425 degr. are typical.
__3.22 Multi-stage segmental centrifugal pumps dH > 300 m
These pumps are a larger version of those in Part. 3.12; horizontal pumps only with flows up to 3000 m^3/h at heads up to 4500 m are available requiring drivers up to 10 MW. Special designs for boiler feed pumps allow operation over 200 ~ at 500 barg. Larger pumps are available in a wider range of materials; cast iron, steel, bronze, 18-10-2 austenitic stainless steel and duplex stainless steel. Some manufacturers have opted for fabricated suction and discharge heads, using thick boiler plate machined to the required profiles. Interstage connections are possible. Hydraulic axial thrust reduction can be implemented by disc, drum or double-drum (a balance drum with two diameters). Pumps can be fitted with rolling contact or sliding bearings. Tilting pad thrust bearings are available. The previous comments regarding hazards liquids still apply. Higher head segmental pumps are popular as boiler feed pumps and seawater pumps for desalination and reverse osmosis. Also useful in mining, irrigation and gas scrubbing processes. Fig. 29 shows a large 4-stage segmental pump. Examples of high pressure pumps are shown in Figs. 30, 31 and 32.
__3.23 Horizontal multi-stage axially-split centrifugal pumps (includes ISO 13709, API 610 designs)
Large multi-stage axially-split pumps have a very wide range of applications, notably mine drainage, boiler feed and process industries. A sectional view of this pump type, during routine overhaul and general view, are shown in Figs. 33 to 1.35 respectively. They are also used extensively in the refining, petrochemical industries and in oil field production. The design and construction being determined by the specific functions and components.
--First stage impeller
The theory of rotodynamic pumps shows that the kinetic energy of the liquid produced in the impeller must be converted to pressure energy by efficiently reducing the velocity of the liquid. Two methods are used; vaned diffusers and vaneless diffusers called volutes. The two main groupings of axially-split pumps are diffuser pumps, meaning vaned diffusers, and volute pumps. Modern trends are leaning towards volute pumps as these are cheaper, in theory, to produce.
Fig. 30 A high pressure segmental pump with balance disc and roller bearings; KSB A G
Fig. 31 A typical installation of a hot high pressure segmental pump; KSB A G
Fig. 32 A double-suction arrangement for a high pressure segmental pump; KSB A G
Fig. 33 Sectional view of a multi-stage axially-split case pump, API 610 code BB3 Weir Pumps Ltd Fig. 34 Multi-stage axially-split case pump during routine overhaul Weir Pumps Ltd
Fig. 35 Multi-stage axially-split case pump, API 610 code BB3; Weir Pumps Ltd
Fig. 36 An example of cap-nuts on a horizontal joint; Weir Pumps Ltd
There are also two choices of first stage impeller; single entry and double entry. Double entry impellers are used in low NPSHa applications to try to obviate the need for separate booster pumps.
For moderate pressure increases it’s possible to absorb the axial thrust directly in the thrust bearings. However, it’s generally necessary to balance out the thrust. The most common method of balancing thrust is by opposing the direction of the impellers and balance discs or by means of balance drums. Opposing impellers leads to some complications with the casing castings due to the transfer passages and can lead to poor casting integrity and weld repairs. Experts argue endlessly about the merits of balance disc and drums; personal experience with the specific application may be the best guide.
Depending upon the pump size, ball bearings are probably the standard with angular contacts, back-to-back, for the thrust.
Sliding bearings with tilting pad thrust bearings may be an option or become the standard as pump speed increases. At higher speeds the life of rolling contact bearings is reduced whereas sliding bearings can have an almost unlimited life.
Lube system options will cover all requirements.
Small multi-stage pumps, like borehole pumps, can have many stages, 50 or 100. Larger multi-stage pumps are limited by shaft deflection and critical speeds and the number of stages rarely exceeds 15. It’s often required to run them at higher speeds than those produced directly by electric motors, this means using steam or gas turbines or geared-up electric motors. The speed is generally 5000 to 8000 rpm. Some recent development trends are towards high speed single stage pumps but this is not universal.
Pumps are available in many material combinations with casings in carbon steel, 13% Cr steel, 18-10-2 stainless steel and duplex stainless steel. Pumps at 2900 rpm can handle 2400 m^3/h at heads over 1400 m. Running at higher speeds will produce more flow at higher heads. Standard casing designs are available for pressure ratings up to 280 barg. The maximum rating to date is thought to be 430 barg. Connections are usually in the bottom half so that all maintenance can be accomplished without disturbing the process pipework. Other connection positions are sometimes optional. Stuffing boxes and seal cavities tend to be big enough to cope with most requirements. Some manufacturers can incorporate intermediate connections on the pump so that liquid can be added or extracted at an intermediate stage.
Pumps are available to API 610 requirements. API excludes the use of axially-split pumps when:
--Liquid temperature of 205 degr or higher
--Flammable or toxic liquid with SG 0.7
--Flammable or toxic liquid over 69 barg
The main weakness in the axially-split design is the joint between the casing halves. Some designs employ gaskets and others metal-to-metal joints. As the pump pressure increases the joint bolting becomes more heavily loaded and tends to stretch reducing the interface pressure at the joint faces. The number and size of bolts or studs which can be fitted around the joint is finite. Capnuts can be used to eliminate the space required for spanners but the highly loaded stud/bolt cross section still sustains the tensile stress, see Fig. 36. This weakness in design is cured by the pump design in the next Section.
__3.24 Horizontal multi-stage radially-split centrifugal pumps (includes ISO 13709, API 610 designs)
As developments within the process industries have increased so the range of hazardous liquids has increased. The limitation of the axially-split multi-stage pumps was addressed to remove fears of potential leaks of hazardous liquids and the radially-split pump design evolved.
Consider the multi-stage segmental pump shown in Fig. 29. It has already been stated that this pump design has many potential leak paths. However if the stack of segments is housed with a pressure vessel any leakage would be contained; this is the basic design principle of radially-split pumps is illustrated in Fig. 37. Fig. 38 shows a horizontally-split casing assembly encapsulated in a radially-split pressure containment. The process pipe connections can be flanged or prepared for direct pipe welding as shown.
The small clearance around the outside of the segments is usually at discharge pressure so that the segments don’t have to be designed to cope with high tensile stresses caused by internal pressure. Unlike axially-split pumps, radially-split pumps all have vaned diffusers and usually balance compensation by disc or drum. Some very special pumps have opposed impellers. The segmental stack, together with seals and bearing assemblies, is built as a complete assembly, called the cartridge, which is inserted into the pressure vessel from the non-drive end. This allows all work on the pump to be carried out without disturbing the motor or the pressure vessel and process pipework.
Fig. 37 Cross-section of a multi-stage radially-split pump
Fig. 38 A horizontally-split casing assembly encapsulated in a radially-split pressure containment, Flowserve Corporation
Three types of construction of radially-split pumps are built as standard, the variation being in the pressure vessel which is normally called a barrel. Hence the popular name for this type of pump, the barrel pump; not to be confused with very small barrel emptying pumps described in Part. 6.2. Small and low pressure pumps can have the barrel fabricated from standard pipe or rolled plate; this type of construction has been called segmental-pump-in-a-pipe. Larger pumps have cast or forged barrels. Forged barrels may have the process connections welded to the barrel requiring additional quality control.
The barrel may be foot or center-line mounted depending upon the application and inter-stage connections may be possible.
Double suction first stage impellers are an option as is compliance with AP! 610.
Radially-split multi-stage pumps are used for the most arduous high pressure applications. Temperatures over 400 degr. have been accommodated. Many material combinations are available to cope with temperature, pressure and corrosion. Flows over 400 m^3/h at 3000 rpm are possible; operating speeds up to 6500 rpm are not uncommon. Special pumps are available for direct drive by gas turbines. Total head rise of over 3000 m is not unusual. Pressure ratings up ANSi 2500LB, 430 barg, are available. One of the largest barrel pumps built was a boiler feed pump in a power station, rated at 55 MW. Process connections can be to any standard or special. Boiler feed pumps tend not to be flanged but butt welded directly to the pipework; as the casing is not disturbed during maintenance this is not a problem. Stuffing boxes and seal cavities can be made to suit any sealing requirements.
__3.25 Large vertical multi-stage centrifugal pumps (includes ISO 13709, API 610 designs)
Vertical multi-stage pumps tend to be wet-pit pumps. Some versions are available where the pump is suspended within its own tank and effectively becomes a dry-pit pump. These vertical pumps are used for high flows in applications such as cooling water where one or two pumps feed a ring main which supplies a whole site. Construction details vary considerably including centrifugal and mixed-flow impeller types. Pumps are produced individually to order based on a range of standard designs.
Countless material combinations are available at every conceivable level of quality.
Pumps can be built with flanged vertical motors mounted directly on top of the pump. Small pumps can be coupled with rigid couplings utilizing thrust bearings built into the motor.
Larger pumps have self-contained bearing assemblies, rolling or sliding, and are driven through flexible couplings. Some pumps are built with a right-angle gear box mounted on top; these pumps must have self-contained bearings. The pumps are driven via flexible couplings by motors or engines. This style of pump is popular in offshore installations. Flows over 70000 m^3/h are possible, differential heads vary from 50m to over 400m.
Vibration can be a problem on vertical pumps. It would be reasonable to purchase large pumps from manufacturers who have facilities to test pumps in the fully assembled condition.
Balancing requirements on motors and rotating elements may need tightening to reduce problems.
__3.26 Single-stage centrifugal pumps with integral gearing
The process industries have shown considerable interest in high speed pumps. Lower speed multi-stage pumps can be replaced with smaller, more compact packages. Pumps consist of a specially designed radial vane centrifugal impeller and casing which incorporates one or two step-up gears driven by a standard 2-pole motor. The maximum, practical, gear ratio for a pair of gears is 6:1 so that 60 Hz pumps could run up to 120000 rpm and 50 Hz to 105000 rpm. A very wide speed range is available at both frequencies. Only vertical pumps are available for process applications; horizontal single-stage pumps proved difficult to operate successfully. Horizontal models are only available for very small duties such as domestic or commercial water boosting. When seal flushing is fitted pumps can run dry for short periods.
Some operational problems have occurred with some installations. The high pump speed can cause mechanical seal, soft packing, and bearing problems. Due to the speed, rolling contact bearings may be replaced by sliding bearings with attendant lube systems. High speed pumps can be relatively intolerant of rapid process transients and particularly distressed by NPSHa inadequacies and solids. Liquid viscosity can be a problem. The variation between manufacturers in the pump's resilience to cope with operating problems has lead users to consolidate with a specific manufacturer for specific applications.
Although the possible speed range is great, most pumps operate in the range from 15000 to 30000 rpm. Differential heads of 1500 m are common and powers up to 500 kW are possible.
Radial vane impellers can have a smaller stable operating range than the standard backward swept vane impellers. A variable flow by-pass line may be necessary for some applications.
Speed-up gear boxes tend to give more problems than speed reducing gear boxes. Contemporary developments in electronics and electric motors may make this mechanical technology redundant. Variable frequency inverters are used routinely to vary the speed of standard squirrel cage induction motors.
Cheaper electronic inverters can be used to power switched reluctance motors. The rotor of a switched reluctance motor can be made in one piece from steel; no windings or rotor bars required. This motor can run at very high speeds without problems of integrity caused by centrifugal forces on assembled components. A single-stage pump could operate at the highest speeds and be limited by impeller construction and materials.
Fig. 39 Horizontal multiple high speed pump
__3.27 Multi-stage centrifugal pumps with integral gearing
High speed multi-pump packages are an extension of the design philosophy of the high speed single-stage pumps. A speed-up gearbox is driven by an electric motor. The gearbox has two or three output shafts, each of which drives a single stage centrifugal pump. The pumps can be connected in series or parallel to produce higher flow or higher head. An option is available to have a slower speed first stage pump which is specially designed for low NPSHr. Inducers are available for some impellers. The multi-pump packages are restricted to horizontal pumps and an example is shown in Fig. 39.
This type of packaging results in a compact powerful unit. Mechanical seals must be used as there is no access to the stuffing box for adjustment. Seal cavities are large enough for double and tandem seal configurations with flush and buffer liquids.
The individual pump casings must be removed in order to inspect or maintain the seals; the process pipework and the inter-stage pipework must be removed to accomplish this. A special liquid injection feature allows the use of standard mechanical seals.
The individual pumps run at speeds from 12000 to 25000 rpm.
Bearings may be a combination of rolling and sliding or all sliding. Lube systems can be tailored to customers requirements including API 614 compliance. Flows up to 225 m^3/h are possible at differentials up to 4500 m. High suction pressure capabilities are available up to 150 barg with casing pressure ratings up to 310 barg. Absorbed power can be to 1150 kW. Most pump users tend to standardize on regularly used wear parts such as mechanical seals and coupling diaphragms. The choice of manufacturers for high speed seals may be very restricted. The comments at the end of Part. 3.26 regarding by-passes and gearboxes also apply here.
High speed pump packages have replaced multi-stage centrifugal pumps and reciprocating pumps in some applications.
High speed multi-pump packages are smaller but not as efficient. High speed multi-pump packages cannot reproduce the reciprocating pump characteristic but only cover the duty point.
__3.28 Centrifugal pumps for pulp
For handling paper pulp, depending on the concentration of fiber in suspension, it’s necessary to use specially designed pumps as follows:
~ Up to 0.5%
--Standard pumps 0.5 to approximately 2%
--Pulp pumps or standard pumps fitted with special semi-open impellers
--Approximately 2 to 5% m
Approximately 5 to 6-7%
--Pulp pumps with specially shaped inlet blading
* Above 6-7%
--Positive displacement dense pulp pumps
For unscreened pulp of concentrations above 3% the pump flow passage should be at least 40 mm. The impellers for pulp pumps are semi-open with back vanes for simultaneous balancing of axial thrust and cleaning behind the impeller. Fig. 40 shows such a pump, with a replaceable wear plate in the pump casing and axial adjustment of the rotor. The choice of material is normally stainless steel and for raw product sometimes grey cast iron. Due to the presence of entrained air in pulp suspensions it’s necessary to design the blades of the impeller intake so as to prevent air locks. The concentration of air also causes rough running with increased levels of vibration and mechanical stress.
Compared with a water pump, the mechanical construction of a pulp pump requires an extra safety factor of approximately 2 in order to achieve satisfactory reliability. For pumping twigs and waste from the macerater, non-clogging free-flow pumps are best. Horizontal end-suction pumps with mixed-flow impellers are capable of 6000 m^3/h at 35 m up to 110 ~ with pressure ratings of 7.5 barg.
Fig. 40 Pulp pump with semi-open impellers with back vanes
Fig. 41 Cross-section through a heavy duty hard metal solids handling pump
Fig. 42 Typical heavy duty hard metal solids handling pump GIW Industries Inc Fig. 43 Solids handling pump with replaceable rubber lining Fig. 44 Typical solids handling pump with a replaceable elastomer lining GIW Industries Inc
__3.29 Centrifugal pumps for handling solids < 10ram
For very abrasive mixtures with particle sizes of up to approx 10mm, the components in contact with the pumped fluid are rubberized or rubber coated. Typical solids handled would be some sands, iron and copper ore and mine tailings. Pumps which are rubber coated can have the coating repaired or replaced if the damage is not severe. Rubber lined pumps are built so that the lining of the casing can be removed and completely replaced.
Smaller pumps, 2" to 4" suctions, may limit solid sizes to 6 mm or less. Stuffing box requirements and drive arrangements are similar to those shown in Part. 3.30. Small pumps are similar in hydraulic performance to all metal pumps. Larger pumps can handle more flow; a 16" suction will pass 5000 m^3/h at up to 48 m.
__3.30 Centrifugal pumps for handling solids > 10 mm
Solids handling pumps are used for pumping suspensions and liquid/solid mixtures of various solid particle sizes up 70% solids by weight. Two types of construction for rotodynamic pumps are available; all metal and rubber coated or rubber lined. The particle sizes and the abrasiveness of the solids determine the choice of construction. Abrasiveness can be measured by the Miller Number according to ASTM G75-82, Test method for slurry abrasivity by Miller Number.
All metallic materials are used in the case of larger particles and higher liquid temperatures, see also Section 7, Section 7.5.
Some manufacturers restrict the size of small particles which can be mixed with the large. With particles over 50 mm it’s unlikely that 70% concentrations can be achieved. Pumps are available in various materials from plain cast iron to chrome irons with 2.5% to 27% Cr. Despite the use of extremely wear-resistant materials the operational life is relatively short of the order of a few months and depending greatly upon the speed of flow, rpm, particle size and nature. Figs. 41 to 1.44 show examples of typical heavy duty hard metal solids handling pumps and those with replaceable rubber and elastomer linings.
For delivery heads above 50 to 60 m it’s normal to employ several pumps in series. Pump impellers are of the closed or semi-open type with suitably thick shrouds and vanes. The number of vanes are relatively few to reduce the chances of impact. Some models have adjustable casings to correct for wear.
Shafts, bearings and bearing housings are dimensioned for heavy-duty loading due to the out-of-balance forces and high SG. Some manufacturers limit pump speed as SG increases.
Solids pumps are usually V-belt driven, speeds from 2600 to 400 rpm, so as to be able to adjust the flow without machining the impeller. Throttling, to correct pump performance, is particularly difficult and prone to constant adjustment; variable speed is preferred.
Due to the abrasive nature of the liquid mixture particular attention has to be given to the design of the shaft seals. Design solutions such as stuffing box flush with clean liquid; hydrodynamic seals as in Section 8, Section 8.3; centrifugal seals with back vanes and as glandless pumps with overhung impellers for vertical models, as in Fig. 41, are available. This pump type is particularly suitable for frothing fluids, where air can rise and not block the impeller intake. A certain degree of self-regulation is obtained by the air mixing at low level.
The maximum solid size handled continuously is a function of the pump size and impeller design; open or closed. Tbl. 2 can be used as a guide for sizing pipework.
Maximum particle size mm
Suction nozzle; Discharge nozzle; Closed impeller; Open impeller
Tbl. 2 Solid sizes for various pump sizes
A 2" suction pump can handle up to 30 m^3/h at heads of 60 m; a 20" suction pump can handle 2250 m^3/h at heads up to 38 m.
Pumps are available which can handle solids larger than 120mm. Obviously to pump large solids in quantity, rather than an occasional piece as with a contractor's self-priming site pump, a great deal of liquid is required and a large pump.
Pumps up to 36" or 900 mm suction are available. Efficiency tends to be low, because the kinetic energy given to the solids cannot be converted into increased pressure, although one pump was advertised at 90% on water between 500 and 900 rpm. It’s essential to consult manufacturers very early in the system design and feasibility studies and be prepared to supply plenty of NPSHa, up to 10 m.
__3.31 Non-clogging pumps
Non-clogging pumps are mainly used as sewage pumps for pumping untreated sewage in sewage treatment plants. This Section deals with normal models for dry installation and with only the pump section immersed in the liquid. Part. 3.32 covers submersible models where both pump and drive motor can be operated below the surface of the liquid.
Sewage handling pumps are completely dominated by rotodynamic pump designs. The following types are used: Centrifugal pumps with through-flow impellers, special enclosed impellers with large almost straight passages
--Free-flow pumps, recessed open impeller
Pumps having semi-axial impellers and axial pumps (propeller pumps) for larger flow rates and lower differential heads The free area, the through-flow for non-clogging pumps, is normally 60 to 100mm for the smaller pump sizes and 125 to 150 mm for the larger pumps. The obviously desirable feature that particles as large as the connection diameter should be able to pass through the pump is not usually possible because of hydraulic problems and pump efficiency. Any solids should, in theory, be surrounded by liquid. See Tbl. 2 for guidance. For small flow requirements, in order to obtain sufficient liquid flow, oversize pumps must be used.
All sewage handling pumps comprise single suction impellers.
This is to avoid the necessity of locating the pump shaft in the intake. Pump materials chosen are usually grey cast iron or SG iron for municipal sewage and grey cast iron or stainless steel for industrial effluent.
Pumps with channel impellers have always been the most well developed and constitute a special class of their own on account of the number of different types and the numbers of pumps in use. In practice most pumps have impellers with single or double channels, although larger pumps sometimes have three channels, Fig. 45. As implied by the description 'throughflow' impeller, the pumped liquid passes through the pump impeller and out through the delivery connection after leaving the pump intake.
Pumps with through-flow impellers are characterized by their high degree of efficiency over a large part of the H-Q curve. The overall efficiency at the nominal duty point for medium size pumps is about 60%. The wear resistance of the impeller is moderately good when used in combined systems and good in separate systems.
The shape and design of free-flow recessed impeller pumps differs from through-flow pumps in that the location of the impeller and the utilization of the pump casing is different, Fig. 46.
The pump impeller is symmetrical with open vanes. The profile is low and its pulled back location, recessed in the stuffing box wall, leaves the pump casing completely or partially free. The free-flow pump can be described as a centrifugal pump having very large sealing clearance. The liquid and contaminants flow freely under the pump impeller and out through the delivery connection. The absence of sealing clearance results in a reduction in efficiency compared with through flow pumps. The overall efficiency at the nominal duty point for small and medium size pumps is 40 to 42%. A special version of the semi-axial design pump has an impeller like a corkscrew. This style of pump has been called a screw-centrifugal pump. The flow passages are large and the leading and trailing edges are sharp to cut fibrous material.
These are low head machines but water efficiency can be quite high, up to 84%. Gentle handling !!!
Fig. 45 Various types of channel impeller
Fig. 46 Free-flow recessed impeller pump
Fig. 47 Waste pump with "compression screw" for feeding the centrifugal pump unit
Fig. 48 Oblique disc pump (Gorator pump) for simultaneous disintegration and pumping
Pumps, similar in design to sewage handling pumps, are used for transporting larger delicate solids, such as whole fish, fruit and root vegetables by means of water. The channel areas and shapes are designed to cause the least amount of damage to the product being pumped. See Section 16, Section 16.10 for an example of the pumping of live fish and food.
In direct contrast there are other types of non-clogging or chokeless pumps where deliberate attempts are made to finely disintegrate the material being transported. Apart from sewage handling pumps with a integral fine disintegration device or macerater there are pumps specially developed for the handling of sludge and waste within the food processing industry, Fig. 47 and Fig. 48.
Fig. 49 Submersible sewage handling pump suitable for both dry and submersed installation
Fig. 50 Single cartridge mechanical seal for submersible pumps
Fig. 51 Non-metallic pump with surrounding reinforcement casing
__3.32 Submersible non-clogging pumps
Nowadays the submersible pump is predominantly more important then the conventional dry pump type. Submersible pumps are usually capable of functioning in dry locations and so can be used in low locations where there is risk of flooding, For example in the case of an electricity power cut.
Submersible pump construction is recognized by the fact that the impeller is located directly onto the combined motor and pump shaft, Fig. 49. The drive motor, a squirrel-cage induction motor, is completely sealed by axial and radial O-rings and the pumped liquid is sealed from the drive motor by double mechanical seals which run in an oil-bath. The type of bearings, size of shaft and method of sealing are deciding factors in reliability and operational safety of the whole system. Deflection of the shaft due to radial loads should not exceed 0.05 mm at the shaft seal. The shaft seal components should be assembled in a seal cartridge, which enables pre-assembly and pressure testing of the shaft seal prior to fitting a replacement, Fig. 50. From the point of view of pump service and repair, it’s advantageous to be able to fit separate motor units of varying sizes to a number of different pump casings. Such a range system makes it possible to maintain a complete community with a small stock of motor parts. Some pump systems also require facilities for converting on site from through-flow to free-flow impeller.
__3.33 Mixed-flow pumps
Mixed-flow pumps are used for large flows and low differential heads. The differential head can be increased by using multiple impellers a multi-stage mixed-flow pump. These pumps are nearly always installed with the impeller immersed in the liquid, flooded, and the motor located above, so called extended shaft pumps for immersed installation. For relatively clean fluids the central support tube also functions as a delivery pipe while for contaminated liquids the delivery pipe is separate, similar to Fig. 17 in Part. 3.13.
Typical of immersed installation pumps, at least for the larger sizes, is that the delivery connections and mounting plates are located according to each particular installation, if the total height of the pump exceeds the available height required for assembling and disassembling then the support and the delivery pipe as well as the shaft may be divided into segments. For long shafts it’s necessary to fit intermediate bearings to prevent vibration and to increase critical speeds. There is a clear trend towards replacing extended shaft pumps with submersible pumps in the smaller sizes.
The hydraulic design and number of stages is varied according to the duty requirements. Typical applications include wells and boreholes, condenser circulating water, condenser extraction and drainage pumps. Pump sizes start at 20m^3/h and approximately 30 m differential per stage. Pumps are available in all popular pumping materials.
__3.34 Axial flow pumps
Many of the comments in Part. 3.33 can be applied to axial flow pumps. Most pumps are vertical, the vast majority are single stage. Some horizontal versions are available which look like pipe elbows. Axial flow pumps can be equipped with adjustable blades which can be set, either when the pump is at rest, pre-set, or during operation, variable, by means of mechanical or hydraulic actuators. The differential head produced by axial flow pumps is much less than mixed-flow or centrifugal pumps; typically 15 m per stage. Because of the simplicity of the construction they are easy to build in large sizes. Axial flow pumps are generally used for flows greater than 400/h; flows of 30000 m3/h are not uncommon.
A special version of multi-stage axial propeller pump has become popular in the last few years. A heavy duty version of the pump has found some applications in the multi-phase flow field.
The cost of oil well exploitation can be significantly reduced if the well products can be piped or pumped to a central gathering station or pre-treatment plant. The cost of individual wellhead installations can be prohibitive if gases and liquids are separated and then pumped/compressed.
A pump which can handle crude oil, light hydrocarbon liquid fractions, sour water and natural gases is extremely useful. The pump looks like an extremely rugged version of an axial compressor. The rotor drum is of relatively large diameter and the blade height is unusually short. The rotor stages have few blades but each blade extends for 60 or 70 degrees around the circumference. The casing stages have more nozzles and resemble the high pressure nozzles from a steam turbine. Horizontally-split casings are used for ten or more stages. Good experience has been accumulated with flows of 500 m3/h and differential pressures up to 14 bar. Gas volume fractions are generally in the region of 80 to 90% at suction but pumps have continued to operate with 100% gas for 15 minutes. These multi-stage axial flow pumps operate at speeds up to 4000 rpm.
If higher differential pressures are required, or the mixture contains solids, then twin-geared screw pumps, in Part. 5.4, should be considered.
__3.35 Non-metallic rotodynamic pumps
Non-metallic pumps are most suitable when used for pumping acids, alkalis and corrosive salt solutions. A general review of physical properties and corrosive resistance of various non-metallic materials is given in Section 7, Sections 7.2.12 and 7.3.
Larger sizes of non-metallic pumps must, because of the physical properties of the material, be equipped with an outer reinforcement casing to carry the nozzle loads and to absorb forces due to the fluid pressure, Fig. 51. This outer casing is not necessary for smaller pumps or if the particular non-metallic material has suitably good physical properties or is fiber reinforced internally. Since the liquids handled are of a dangerous nature, great care must be given to the type of shaft seals. The construction generally being similar to that of the ISO 2858, ANSi B73 and AP1610 standard pumps. The pump shaft is usually 18-10-2 stainless steel minimum and can be completely isolated from the liquid by a non-metallic shaft sleeve under the mechanical seal.
Pumps are available for 400 m3/h at differential heads up to 80m; pressure ratings of 7 bar and temperatures up to 100 degr. are possible.
__3.36 Hygienic-quality rotodynamic pumps
Within the food processing, chemical and pharmaceutical industries, special centrifugal pumps are used, Fig. 52. Their shape and construction is distinctive and determined by the hygienic and sterile requirements.
The materials used are usually stainless steels with O-ring gaskets etc. of approved hygienic quality. Both exterior and interior surfaces must be smooth and polished to approved hygienic standards, it should also be possible to disassemble the pump quickly for cleaning and washing. When cleaning without disassembling, Cleaning In Place or CIP, is specified, it must be possible to carry out this process quickly and effectively. This means that all components and clearances must be designed for this purpose, especially with respect to temperature. To simplify external cleaning, the electric motor is often encased in a polished stainless steel casing.
For reasons of hygiene many foodstuffs pumps may not have oil lubricated shaft bearings. Food processing centrifugal pumps are normally available up to approximately 30 kW. Centrifugal pumps cannot be used for viscous and shear sensitive products such as yoghurt, cheese, liver paste, etc. In such cases lobe rotor pumps are generally used, see Part. 5.8.
CEN Standard EN 1672 covers the requirements for hygiene for machinery used in the preparation of food for human or animal consumption. The Standard describes methods of construction and the types of acceptable fittings, couplings and flanged joints.
Single-stage end-suction pumps can handle over 400 m3/h at heads up to 100m. Close coupled end-suction pumps can be constructed like a segmental pump, shown in Sections 1.3.12 and 1.3.21, with up to three stages. These pumps can handle 50 m3/h at heads up to 180m. ISO is producing new standards for hygienic machinery requirements. ISO 14159 deals with machinery design. The American Standard T-02-09, which defines the requirements for 3-A Sanitary Standards originally for the dairy industry, is popular in many countries.
Traditional hygienic pumps have been used in food manufacturing processes. European standards use the term Agrifoodstuffs to include food products for animals as well as food for human consumption. The biotechnology industry has become an important user of hygienic pumps. EN 12690 deals with the risks to personnel and the environment due to the emission of micro-organisms from assembled working pumps or inadequately cleaned pumps which are disassembled. A biohazard is defined as danger or harm associated with a biological agent able to cause infection, allergy or toxicity in animals, humans, plants or the environment.
Fig. 52 Hygienic-quality centrifugal pump
Fig. 53 Synchronous magnetic coupling for "sealless" torque transfer