- Email: [email protected]
Why is pump piping so difficult to design?
FEATURE •
W h y is p u m p p i p i n g so d i f f i c u l t to design? The main problem with pump piping design is that pumps are a very diverse group. The range of possibilities is very wide. There are many different styles of rotodynamic pump and over 40 different types of positive displacement pump. Pump sizes range from 5 W to about 300 MW. If we discount pump/turbines used for power generation schemes, which are usually built onsite from concrete, and only consider pumps manufactured in factories, the maximum size is reduced to about 47 MW. Obviously, notes Brian Nesbitt, that still leaves a wide variety of piping requirements.
he average API end-suction centrifugal pump can be taken as a reference point (Table 1). Pipework for these pumps generally works well enough. Some significant features are obvious. Sometimes the pipework is heavier than the flowing liquid and sometimes the flowing liquid is much heavier than the pipework. All pumps produce pressure pulsations. Rotodynamic pumps produce relatively low level pulsations, most of the time, compared to PD pumps. (A two-stage vertical centrifugal pump
T
Z
may produce +_5% suction pulsations at BEP but these increase to over 20% at minimum flow.) Low level pulsations can produce considerable axial thrust in large diameter pipework. The pressure pulsations from PD pumps can also produce considerable ~xial thrust when pulsation dampening is ineffective. The flowing liquid in large diameter pipework possesses an enormous amount of momentum and kinetic energy. The effects of momentum are felt when the liquid is forced to change direction at a bend, for example. Most pumped process applications do not operate at high liquid speeds. Surge pressures produced by waterhammer are usually easily contained by normal metal pipework. Surge pressures will be magnified if the liquid is required to reverse direction. The axial thrust produced by surge pressures can be very large. Special anchors may be necessary to restrain the pipework in the event of a waterhammer excursion.
Figure 1. The ideal pipe run
Z
kx
Figure 2. A pipe run w i t h flexibility
WORLD
PUMPS
October 2 0 0 0
There is one effect which most people do not address (Table 2 and Table 3) the potential energy stored in the liquid due to compression. Liquids are relatively incompressible. Although cold water is one of the least compressible liquids, pressurising water 690 bar(a) does increase its potential energy considerably. Compressing hot water to 319 bar(a) has a similar effect. The potential energy will become available whenever there is a reduction in pressure.
The impact of pressure surge Data sheets submitted to pump manufacturers, for selection and quotation purposes, concentrate on conditions at a "steady-state rated or normal" duty point. All pumps have at least two transient conditions: starting and stopping. If pump startup is very quick then surge effects may be present in the suction or discharge pipework. Startup surge in the suction pipework creates a negative pressure pulse. If the negative pressure pulse is large enough, the liquid column can separate and produce a vapour or gas void. If the vapour/gas passes into the pump, cavitation may damage the pump internals. If the vapour/gas passes through the pump then additional surge pressure problems will be experienced in the discharge pipework. If the vapour/gas remains in the suction pipe, the positive surge pressure will occur when the separation void collapses. Pressure reductions inside large diameter thin wall pipe can pose mechanical problems through elastic instablilt,c: tf the negative pressure pulse is large enough, the pipe will buckle. During start-up the discharge pipework may be subject to the positive surge pressure effects. During stopping the effects in the pipework are reversed. The surge effects created during starting and stopping can be minimised by controlling the rate of change of the flow. Normal flow control strategies may not be effective in the event of a
0262 1762/00/$ - see front matter © 2000 Elsevier Science Ltd. All r~ghts reserved
FEATURE •
Pump type Peristaltic laboratory pump Small industrial triplex plunger pump Small hygienic single-stage centrifugal pump Small API triplex plunger pump Large API septuplex plunger pump Average API end-suction centrifugal pump Average 2-stage vertical centrifugal pump Large duplex double-acting piston pump Large industrial end-suction centrifugal pump Large API end-suction centrifugal pump Large segmental multi-stage centrifugal pump Large multi-stage centrifugal boiler feed pump Large axial flow pump Large double-suction centrifugal pump Large multi-stage vertical centrifugal pump power cut or an emergency shutdown (ESD). There is little point implementing a structured ESD strategy which damages the pipework while trying to save the plant. Positive displacement pumps may be less affected by surge problems than rotodynamic pumps. PD pumps do not suffer from run-out - - the pump does not try to push much larger volumes of liquid through the pipework just because the differential pressure is low. The initial rate of liquid acceleration may be much lower. Many PD pumps are fitted with pulsation dampers to attenuate the pressure pulsations created by flow variations. The gas charged pulsation damper is the most popular style and is very effective at cushioning surge effects. They work during power cuts and ESDs too! The modem trend towards variable speed pumps has a very significant impact on the surge effects produced by pumps. The ability to control the acceleration and deceleration of the pump means surge effects can be eliminated during normal operation. Power cuts and ESDs, however, may still be a problem.
Onus on the system designer The system designer must evaluate information, as shown in Tables 2 and 3, on a pump by pump basis and decide
www.worldpumps.com
on the most appropriate approach for pipework and pump installation. Figure 1 shows the ideal 'hydraulic' pipe for any application; a straight length of pipe between two rigidly located pieces of equipment. The pipe is straight but not level - - every pipe should be self venting. The system designer must arrange the equipment to accommodate the straight pipe. The ideal pipe run cannot be vibrated by pressure pulsations or waterhammer pulses but both may apply forces to the equipment. The method of pipe support adopted is unimportant because this pipe run is n o t dynamic. The pipe will not move. This pipe run will not be acceptable to the pipe designer if there are any changes in medium or ambient temperature. The ideal hydraulic pipe is rigid and has no flexibility to accommodate expansion/contraction. Flexibility could be introduced by fitting an expansion joint or bellows. The choice would be dependent upon pressure/temperature ranges and the nature of the liquid. Figure 2 shows the next logical step in good pipe run design: a run with a single bend. This pipe run can overcome the flexibility problem experienced with the ideal pipe. If a bend is located so that both straight legs have a reasonable length then bending can occur without imposing high stresses in the pipe or large
bending moments at the equipment connection. Pressure pulsations and water-hammer pulses will apply forces on the bend but the scope for movement is restricted. Movement in the 'X' or 'Z' directions is restrained by tensile/compressive forces in the pipe. Flexibility is added without significantly increasing the chances of vibration problems. Pipe supports must allow the bend to 'spring' to accommodate expansion and contraction. Pipe runs as shown in Figure 2 are seen frequently around pumps. Unfortunately the pipe designer nearly always contrives to have a very
Figure 3. Pipework installation
WORLD PUMPS
October 2000 25
Pump type
Vel m/s
Peristaltic laboratory pump Small industrial triplex plunger pump Small hygienic single-stage centrifugal pump Small API triplex plunger pump Large API septuplex plunger pump Average API end-suction centrifugal pump Average 2-stage vertical centrifugal pump Large duplex double-acting piston pump Large industrial end-suction centrifugal pump Large API end-suction centrifugal pump Large segmental multi-stage centrifugal pump Large multi-stage centrifugal boiler feed pump Large axial flow pump Large double-suction centrifugal pump Large multi-stage vertical centrifugal pump
Pipe w t kg/m
c
Figure 4. A pipe run with two bends Z
d
g
Figure 5. Normal pipework
26
WORLD PUMPS
October 2000
Pulses pk - pk %
Axial KE Thrust kg J/m
W-Hammer Press bar
Axial Thrust kg
a.6
short straight run next to the pump. The effect is to increase the nozzle loading on the pump. When the short straight run occurs on the suction side there is an increased probability of flow turbulence upsetting the pump. Typical problems thereafter include vibration, short bearing life and in extreme cases, cavitation. Pseudo-cavitation is the most likely - where the dissolved gas emerges from solution to Z
Water w t kg/m
form bubbles which partially constrict the suction connection. The system designer has ultimate responsibility for avoiding these problems. Figure 3 shows the suction and discharge pipework for a high pressure vertical plunger pump. One wonders how much information the system designer supplied to the pipe designer. Notice the lower portions of the pipes are anchored to the baseplate. Did the pipe designer consider the thermal growth of the pump? Did the pipe designer consider the differential expansion/contraction between the pump and the pipes? Did the system designer tell the pipe designer the exotic high pressure liquid end was mounted on a cast iron box? Do we all remember the amount of elongation when cast iron fails? Did the system designer tell the pipe designer to mount the dampers as close to the pump as possible? They could have been much closer! In this particular case we know the dampers are working within specification limits. The rest of the pipework vibrated so badly the damper performance was checked. Figure 4 shows a simple pipe run with two bends. Sections 'ab' and 'cd' are restrained from axial movement in the 'X' direction because one end is anchored. The nodes 'b' and 'c' may move very slightly in 'X' depending upon the elasticity of the pipe material. Section 'bc' may be able to move in 'Z'; this depends upon the
type of pipe supports fitted to 'ab' and 'cd'. If the horizontal sections are simply supported or are on hangers then 'bc' will be able to vibrate. The amplitude will be dependent upon the mass of 'bc' and the bending stiffness of 'ab' and 'cd'. The horizontal sections could be fully restrained in 'Z' but this would remove flexibility for 'bc' to expand/contract. The simplest solution is probably to have simply supported horizontal sections but provide an anchor at the mid-point of 'bc'. The movement at 'b' and 'c' would require checking to see if other restraints were necessary.
Real life pJpework All the pipework considered so far is simple. All the pipe sections lie on a single 'X-Z' plane. In fact, very few installed pipe runs are like this. Because insufficient attention is given to equipment layout at the earliest design stages, the pipework shown in Figure 5 is the norm. This style of pipework is applied to all pumps irrespective of dynamic characteristics regarding pressure pulsations and surge. It is usually assumed that rotodynamic pumps do not have pressure pulsations. It seems very likely that the pipe designer is not aware of the specific pump type or the style of construction. The pipe designer is definitely not aware about normal pressure pulsation levels and damper requirements. Notice that the pipework is not self-venting, a major
www,worldpumps.com
Pumps • Hydro Turbines • Valves Kirloskar Brothers Limited (KBL) has over 75 years of specialisation in fluid handling and a sophisticated manufacturing and testing set-up all-under-one-roof. The company manufactures a wide range of pumps, hydro turbines and valves in a variety of materials which can meet diverse needs across the world. Over the years, Kirloskar Brothers has exported its products to over 60 countries around the world.
KIRLOSKAR
BROTHERS
LIMITED
o
Regd. & Corporate Office: Udyog Bhavan, Tilak Road, Pune 411 002, India. Tel.: 91-20-4444444 • Fax: 91-20-4444198/4442780/4440156 E-mail: [email protected] • Website: www.kbl.co.in • Group Website: www.kirloskars.com
THE ENQUIRY NUMBER FOR THIS ADVERTISEMENT IS 419
I.Z <( ./
no time ed f f performance curve in the main-frame he dimensions from volume IX, page 1097 r the viscosity conversion JrauJiq,Department ,~~areS of last year 0uat of data sheets e ~ s list in the archives...
SOFTWARE
~J~~'vll~w
Pump SelectionSoftware
THE ENQUIRY NUMBER FOR THIS ADVERTISEMENT IS 420
Nature has developed ingenious solutions ...
Asthe atmosphereprotectslife on earth, so our patentedHCV~-Technique allowsthe uninterruptedoperationof our high-pressure water systemsevenunderthe mostextremeconditions,
WOMA Apparatebau GmbH Werthauser Str 77-79 D-47226 Duisburg Tel +49 2065 304-0 Fax +49 2066 304-200 Internet http//www woma de E-mall info@woma de
Water as a tool for a clean environment
THE ENQUIRY NUMBER FOR THIS ADVERTISEMENT IS 421
FULL PLASTIC CORROS/ON .,roof ~ . , z ~ PUMPS SAVINO BARBERA s.n.c, pumps prevent corrosion and help media clean during processing. More than 30 years' experience in the design and manufacture of centrifugal pumps made in PP, PVC and PVDF materials has put SAVINO BARBERA in the position to offer a solution to the problems of pumping and handling aggressive fluids. The product range consist of more than 70 moders, which include vertical type AS/NT and horizontaF type OMA/NTS, sized of 0.5" to 6", open or closed impeller, a maximum capacity 150 m3/h and 60 m.w.c.
OMA/NTS
~bS~BAJ~ Via Torino, 12 - 10032 Brandizzo (TO)ITALY Tel. 0039.011.913.90.63 Fax 0039.011.913.73.13 e-mail [email protected]: www.savinobarbera.com THE ENQUIRY NUMBER FOR THIS ADVERTISEMENT IS 422
FEATURE •
Pump type
] Velocity I m/s
Peristaltic laboratory pump Small industrial triplex plunger pump Small hygienic single-stage centrifugal pump Small API triplex plunger pump Large API septuplex plunger pump Average API end-suction centrifugal pump Average 2-stage vertical centrifugal pump Large duplex double-acting piston pump Large industrial end-suction centrifugal pump Large API end-suction centrifugal pump Large segmental multi-stage centrifugal pump Large multbstage centrifugal boiler feed pump Large axial flow pump Large double-suction centrifugal pump Large multi-stage vertical centrifugal pump problem when handling hazardous liquids. Notice also that the equipment is not rigidly located but is on flexible mountings. This does not mean, necessarily, that the equipment is supported from something rigid, like concrete, and uses rubber inserts. Much equipment is mounted on structural steelwork which is effectively flexible. The ability of the equipment to move adds a new dimension to the piping design. All anchors are external to the equipment. All sections of this pipe run can be loaded by axial, bending and torsional forces provided by the mass of the pipe/liquid and the pulsation/surge pressure variations. The magnitude of the various mechanical forces are dependent upon the positioning and the degree of restraint provided by the supports. The hydraulic forces are dependent upon the levels of pressure pulsations and surge. The pipe run shown in Figure 5 does not include the essentials found in most pipes: flanges/connectors, isolating valves, control valves, and branches to associated systems. All of these items increase the complexity of pipe supporting and restraints. Pipe design is usually accomplished by proprietary computer software. Inadequate input data, which only describes part of the problem, will produce inaccurate results.
www.worldpumps.com
Pipe w t kg/m
1.2
~149
¢1
~794
Water wt kg/m
Pulses pk - pk %
O:O46
20
3,5
.
3.6
7~
2,5
378,
s
3.8 123
1~3,
155
7U 1~,
3
3,8 6,0
863
9i.
9.3
~3
3: 2 5,
2.8 4.2
Axial Thrust kg
.
..
8.4 :
3"
948t ' .
2A
1316
3.8
678
3e43
5
~3{}>
3.7
868
5991 '
1
978'
pitfalls
It seems the computer software usually used by pipe designers is for the design of static pipework. In reality pumps inject very complicated pressure signals into the suction and discl~arge pipe runs. Software analysis to see if the pipe runs will be excited, mechanically or acoustically, by the pump signals, are not conducted as a matter of routine. These checks normally only follow after commissioning when the piping dynamics are considered to be unacceptable, and the pump has been blamed first. Only when the pump is shown to be operating properly are other avenues explored. Modifying the dynamics of installed pipework can be very costly. This article has concentrated on the mechanics of pipework design, The mechanical design is very important but not as important as the hydraulic design. Process pipework connected to pumps is fitted to convey liquid. This prime function must be accomplished efficiently. The mechanical design must not interfere with the successful conveyance of the liquid. The proprietary software used to design pipework seems to concentrate on 'Code' compliance rather than ensuring that the prime objective is accomplished successfully. The system designer must arrange the equipment so that properly designed pipework will allow the pump to function correctly.
,2
46,
3391 ~,
Software
W-Hammer Press bar
°,
18:s - . 3 2S3: , , O:S
43
Axial KE Thrust kg J/m
A premium expertise
:
on
Today, industry places great reliance on computers and software, tf experienced engineers are to be replaced by software, then the software must capture and re-deploy the wisdom and experience of these engineers. The mechanical software used to ensure code compliance of pipe runs will only be completely successful when it incorporates the hydraulic rules-of-thumb. There is considerable difference in designing suction pipework compared to discharge pipework. Software which is not developed using experienced engineers can only solve textbook problems. The real pump world is dominated by problems which don't appear in textbooks.
Note: System designers should be aware that the European Pressure Equipment Directive, which comes fully into force after 29 th May 2002, requires "that due consideration is given to the potential damage from turbulence and formation of vortices", • BRIAN NESBITT IS AN INDEPENDENTCONSULTANT SPECIALISING IN POSITIVE DISPLACEMENT PUMPS. BRIAN HAS BEEN INVOWED WITH PUMPS AND PUMPING SYSTEMS SINCE 1974. A MEMBER OF THE BSI M C E / 6 COMMITTEES, BRIAN REPRESENTSTHE UK ON CEN AND ISO PD PUMP COMMI'n'EES AND IS CURRENTLY WORKING WITH THE API 6 7 4 TASKFORCE. BRIAN CAN BE CONTACTED AT BRIANNESBITT~)EMAIL.MSN.COM
WORLD P U M P S
October2000 : 2 9
W h y is p u m p p i p i n g so d i f f i c u l t to design? The main problem with pump piping design is that pumps are a very diverse group. The range of possibilities is very wide. There are many different styles of rotodynamic pump and over 40 different types of positive displacement pump. Pump sizes range from 5 W to about 300 MW. If we discount pump/turbines used for power generation schemes, which are usually built onsite from concrete, and only consider pumps manufactured in factories, the maximum size is reduced to about 47 MW. Obviously, notes Brian Nesbitt, that still leaves a wide variety of piping requirements.
he average API end-suction centrifugal pump can be taken as a reference point (Table 1). Pipework for these pumps generally works well enough. Some significant features are obvious. Sometimes the pipework is heavier than the flowing liquid and sometimes the flowing liquid is much heavier than the pipework. All pumps produce pressure pulsations. Rotodynamic pumps produce relatively low level pulsations, most of the time, compared to PD pumps. (A two-stage vertical centrifugal pump
T
Z
may produce +_5% suction pulsations at BEP but these increase to over 20% at minimum flow.) Low level pulsations can produce considerable axial thrust in large diameter pipework. The pressure pulsations from PD pumps can also produce considerable ~xial thrust when pulsation dampening is ineffective. The flowing liquid in large diameter pipework possesses an enormous amount of momentum and kinetic energy. The effects of momentum are felt when the liquid is forced to change direction at a bend, for example. Most pumped process applications do not operate at high liquid speeds. Surge pressures produced by waterhammer are usually easily contained by normal metal pipework. Surge pressures will be magnified if the liquid is required to reverse direction. The axial thrust produced by surge pressures can be very large. Special anchors may be necessary to restrain the pipework in the event of a waterhammer excursion.
Figure 1. The ideal pipe run
Z
kx
Figure 2. A pipe run w i t h flexibility
WORLD
PUMPS
October 2 0 0 0
There is one effect which most people do not address (Table 2 and Table 3) the potential energy stored in the liquid due to compression. Liquids are relatively incompressible. Although cold water is one of the least compressible liquids, pressurising water 690 bar(a) does increase its potential energy considerably. Compressing hot water to 319 bar(a) has a similar effect. The potential energy will become available whenever there is a reduction in pressure.
The impact of pressure surge Data sheets submitted to pump manufacturers, for selection and quotation purposes, concentrate on conditions at a "steady-state rated or normal" duty point. All pumps have at least two transient conditions: starting and stopping. If pump startup is very quick then surge effects may be present in the suction or discharge pipework. Startup surge in the suction pipework creates a negative pressure pulse. If the negative pressure pulse is large enough, the liquid column can separate and produce a vapour or gas void. If the vapour/gas passes into the pump, cavitation may damage the pump internals. If the vapour/gas passes through the pump then additional surge pressure problems will be experienced in the discharge pipework. If the vapour/gas remains in the suction pipe, the positive surge pressure will occur when the separation void collapses. Pressure reductions inside large diameter thin wall pipe can pose mechanical problems through elastic instablilt,c: tf the negative pressure pulse is large enough, the pipe will buckle. During start-up the discharge pipework may be subject to the positive surge pressure effects. During stopping the effects in the pipework are reversed. The surge effects created during starting and stopping can be minimised by controlling the rate of change of the flow. Normal flow control strategies may not be effective in the event of a
0262 1762/00/$ - see front matter © 2000 Elsevier Science Ltd. All r~ghts reserved
FEATURE •
Pump type Peristaltic laboratory pump Small industrial triplex plunger pump Small hygienic single-stage centrifugal pump Small API triplex plunger pump Large API septuplex plunger pump Average API end-suction centrifugal pump Average 2-stage vertical centrifugal pump Large duplex double-acting piston pump Large industrial end-suction centrifugal pump Large API end-suction centrifugal pump Large segmental multi-stage centrifugal pump Large multi-stage centrifugal boiler feed pump Large axial flow pump Large double-suction centrifugal pump Large multi-stage vertical centrifugal pump power cut or an emergency shutdown (ESD). There is little point implementing a structured ESD strategy which damages the pipework while trying to save the plant. Positive displacement pumps may be less affected by surge problems than rotodynamic pumps. PD pumps do not suffer from run-out - - the pump does not try to push much larger volumes of liquid through the pipework just because the differential pressure is low. The initial rate of liquid acceleration may be much lower. Many PD pumps are fitted with pulsation dampers to attenuate the pressure pulsations created by flow variations. The gas charged pulsation damper is the most popular style and is very effective at cushioning surge effects. They work during power cuts and ESDs too! The modem trend towards variable speed pumps has a very significant impact on the surge effects produced by pumps. The ability to control the acceleration and deceleration of the pump means surge effects can be eliminated during normal operation. Power cuts and ESDs, however, may still be a problem.
Onus on the system designer The system designer must evaluate information, as shown in Tables 2 and 3, on a pump by pump basis and decide
www.worldpumps.com
on the most appropriate approach for pipework and pump installation. Figure 1 shows the ideal 'hydraulic' pipe for any application; a straight length of pipe between two rigidly located pieces of equipment. The pipe is straight but not level - - every pipe should be self venting. The system designer must arrange the equipment to accommodate the straight pipe. The ideal pipe run cannot be vibrated by pressure pulsations or waterhammer pulses but both may apply forces to the equipment. The method of pipe support adopted is unimportant because this pipe run is n o t dynamic. The pipe will not move. This pipe run will not be acceptable to the pipe designer if there are any changes in medium or ambient temperature. The ideal hydraulic pipe is rigid and has no flexibility to accommodate expansion/contraction. Flexibility could be introduced by fitting an expansion joint or bellows. The choice would be dependent upon pressure/temperature ranges and the nature of the liquid. Figure 2 shows the next logical step in good pipe run design: a run with a single bend. This pipe run can overcome the flexibility problem experienced with the ideal pipe. If a bend is located so that both straight legs have a reasonable length then bending can occur without imposing high stresses in the pipe or large
bending moments at the equipment connection. Pressure pulsations and water-hammer pulses will apply forces on the bend but the scope for movement is restricted. Movement in the 'X' or 'Z' directions is restrained by tensile/compressive forces in the pipe. Flexibility is added without significantly increasing the chances of vibration problems. Pipe supports must allow the bend to 'spring' to accommodate expansion and contraction. Pipe runs as shown in Figure 2 are seen frequently around pumps. Unfortunately the pipe designer nearly always contrives to have a very
Figure 3. Pipework installation
WORLD PUMPS
October 2000 25
Pump type
Vel m/s
Peristaltic laboratory pump Small industrial triplex plunger pump Small hygienic single-stage centrifugal pump Small API triplex plunger pump Large API septuplex plunger pump Average API end-suction centrifugal pump Average 2-stage vertical centrifugal pump Large duplex double-acting piston pump Large industrial end-suction centrifugal pump Large API end-suction centrifugal pump Large segmental multi-stage centrifugal pump Large multi-stage centrifugal boiler feed pump Large axial flow pump Large double-suction centrifugal pump Large multi-stage vertical centrifugal pump
Pipe w t kg/m
c
Figure 4. A pipe run with two bends Z
d
g
Figure 5. Normal pipework
26
WORLD PUMPS
October 2000
Pulses pk - pk %
Axial KE Thrust kg J/m
W-Hammer Press bar
Axial Thrust kg
a.6
short straight run next to the pump. The effect is to increase the nozzle loading on the pump. When the short straight run occurs on the suction side there is an increased probability of flow turbulence upsetting the pump. Typical problems thereafter include vibration, short bearing life and in extreme cases, cavitation. Pseudo-cavitation is the most likely - where the dissolved gas emerges from solution to Z
Water w t kg/m
form bubbles which partially constrict the suction connection. The system designer has ultimate responsibility for avoiding these problems. Figure 3 shows the suction and discharge pipework for a high pressure vertical plunger pump. One wonders how much information the system designer supplied to the pipe designer. Notice the lower portions of the pipes are anchored to the baseplate. Did the pipe designer consider the thermal growth of the pump? Did the pipe designer consider the differential expansion/contraction between the pump and the pipes? Did the system designer tell the pipe designer the exotic high pressure liquid end was mounted on a cast iron box? Do we all remember the amount of elongation when cast iron fails? Did the system designer tell the pipe designer to mount the dampers as close to the pump as possible? They could have been much closer! In this particular case we know the dampers are working within specification limits. The rest of the pipework vibrated so badly the damper performance was checked. Figure 4 shows a simple pipe run with two bends. Sections 'ab' and 'cd' are restrained from axial movement in the 'X' direction because one end is anchored. The nodes 'b' and 'c' may move very slightly in 'X' depending upon the elasticity of the pipe material. Section 'bc' may be able to move in 'Z'; this depends upon the
type of pipe supports fitted to 'ab' and 'cd'. If the horizontal sections are simply supported or are on hangers then 'bc' will be able to vibrate. The amplitude will be dependent upon the mass of 'bc' and the bending stiffness of 'ab' and 'cd'. The horizontal sections could be fully restrained in 'Z' but this would remove flexibility for 'bc' to expand/contract. The simplest solution is probably to have simply supported horizontal sections but provide an anchor at the mid-point of 'bc'. The movement at 'b' and 'c' would require checking to see if other restraints were necessary.
Real life pJpework All the pipework considered so far is simple. All the pipe sections lie on a single 'X-Z' plane. In fact, very few installed pipe runs are like this. Because insufficient attention is given to equipment layout at the earliest design stages, the pipework shown in Figure 5 is the norm. This style of pipework is applied to all pumps irrespective of dynamic characteristics regarding pressure pulsations and surge. It is usually assumed that rotodynamic pumps do not have pressure pulsations. It seems very likely that the pipe designer is not aware of the specific pump type or the style of construction. The pipe designer is definitely not aware about normal pressure pulsation levels and damper requirements. Notice that the pipework is not self-venting, a major
www,worldpumps.com
Pumps • Hydro Turbines • Valves Kirloskar Brothers Limited (KBL) has over 75 years of specialisation in fluid handling and a sophisticated manufacturing and testing set-up all-under-one-roof. The company manufactures a wide range of pumps, hydro turbines and valves in a variety of materials which can meet diverse needs across the world. Over the years, Kirloskar Brothers has exported its products to over 60 countries around the world.
KIRLOSKAR
BROTHERS
LIMITED
o
Regd. & Corporate Office: Udyog Bhavan, Tilak Road, Pune 411 002, India. Tel.: 91-20-4444444 • Fax: 91-20-4444198/4442780/4440156 E-mail: [email protected] • Website: www.kbl.co.in • Group Website: www.kirloskars.com
THE ENQUIRY NUMBER FOR THIS ADVERTISEMENT IS 419
I.Z <( ./
no time ed f f performance curve in the main-frame he dimensions from volume IX, page 1097 r the viscosity conversion JrauJiq,Department ,~~areS of last year 0uat of data sheets e ~ s list in the archives...
SOFTWARE
~J~~'vll~w
Pump SelectionSoftware
THE ENQUIRY NUMBER FOR THIS ADVERTISEMENT IS 420
Nature has developed ingenious solutions ...
Asthe atmosphereprotectslife on earth, so our patentedHCV~-Technique allowsthe uninterruptedoperationof our high-pressure water systemsevenunderthe mostextremeconditions,
WOMA Apparatebau GmbH Werthauser Str 77-79 D-47226 Duisburg Tel +49 2065 304-0 Fax +49 2066 304-200 Internet http//www woma de E-mall info@woma de
Water as a tool for a clean environment
THE ENQUIRY NUMBER FOR THIS ADVERTISEMENT IS 421
FULL PLASTIC CORROS/ON .,roof ~ . , z ~ PUMPS SAVINO BARBERA s.n.c, pumps prevent corrosion and help media clean during processing. More than 30 years' experience in the design and manufacture of centrifugal pumps made in PP, PVC and PVDF materials has put SAVINO BARBERA in the position to offer a solution to the problems of pumping and handling aggressive fluids. The product range consist of more than 70 moders, which include vertical type AS/NT and horizontaF type OMA/NTS, sized of 0.5" to 6", open or closed impeller, a maximum capacity 150 m3/h and 60 m.w.c.
OMA/NTS
~bS~BAJ~ Via Torino, 12 - 10032 Brandizzo (TO)ITALY Tel. 0039.011.913.90.63 Fax 0039.011.913.73.13 e-mail [email protected]: www.savinobarbera.com THE ENQUIRY NUMBER FOR THIS ADVERTISEMENT IS 422
FEATURE •
Pump type
] Velocity I m/s
Peristaltic laboratory pump Small industrial triplex plunger pump Small hygienic single-stage centrifugal pump Small API triplex plunger pump Large API septuplex plunger pump Average API end-suction centrifugal pump Average 2-stage vertical centrifugal pump Large duplex double-acting piston pump Large industrial end-suction centrifugal pump Large API end-suction centrifugal pump Large segmental multi-stage centrifugal pump Large multbstage centrifugal boiler feed pump Large axial flow pump Large double-suction centrifugal pump Large multi-stage vertical centrifugal pump problem when handling hazardous liquids. Notice also that the equipment is not rigidly located but is on flexible mountings. This does not mean, necessarily, that the equipment is supported from something rigid, like concrete, and uses rubber inserts. Much equipment is mounted on structural steelwork which is effectively flexible. The ability of the equipment to move adds a new dimension to the piping design. All anchors are external to the equipment. All sections of this pipe run can be loaded by axial, bending and torsional forces provided by the mass of the pipe/liquid and the pulsation/surge pressure variations. The magnitude of the various mechanical forces are dependent upon the positioning and the degree of restraint provided by the supports. The hydraulic forces are dependent upon the levels of pressure pulsations and surge. The pipe run shown in Figure 5 does not include the essentials found in most pipes: flanges/connectors, isolating valves, control valves, and branches to associated systems. All of these items increase the complexity of pipe supporting and restraints. Pipe design is usually accomplished by proprietary computer software. Inadequate input data, which only describes part of the problem, will produce inaccurate results.
www.worldpumps.com
Pipe w t kg/m
1.2
~149
¢1
~794
Water wt kg/m
Pulses pk - pk %
O:O46
20
3,5
.
3.6
7~
2,5
378,
s
3.8 123
1~3,
155
7U 1~,
3
3,8 6,0
863
9i.
9.3
~3
3: 2 5,
2.8 4.2
Axial Thrust kg
.
..
8.4 :
3"
948t ' .
2A
1316
3.8
678
3e43
5
~3{}>
3.7
868
5991 '
1
978'
pitfalls
It seems the computer software usually used by pipe designers is for the design of static pipework. In reality pumps inject very complicated pressure signals into the suction and discl~arge pipe runs. Software analysis to see if the pipe runs will be excited, mechanically or acoustically, by the pump signals, are not conducted as a matter of routine. These checks normally only follow after commissioning when the piping dynamics are considered to be unacceptable, and the pump has been blamed first. Only when the pump is shown to be operating properly are other avenues explored. Modifying the dynamics of installed pipework can be very costly. This article has concentrated on the mechanics of pipework design, The mechanical design is very important but not as important as the hydraulic design. Process pipework connected to pumps is fitted to convey liquid. This prime function must be accomplished efficiently. The mechanical design must not interfere with the successful conveyance of the liquid. The proprietary software used to design pipework seems to concentrate on 'Code' compliance rather than ensuring that the prime objective is accomplished successfully. The system designer must arrange the equipment so that properly designed pipework will allow the pump to function correctly.
,2
46,
3391 ~,
Software
W-Hammer Press bar
°,
18:s - . 3 2S3: , , O:S
43
Axial KE Thrust kg J/m
A premium expertise
:
on
Today, industry places great reliance on computers and software, tf experienced engineers are to be replaced by software, then the software must capture and re-deploy the wisdom and experience of these engineers. The mechanical software used to ensure code compliance of pipe runs will only be completely successful when it incorporates the hydraulic rules-of-thumb. There is considerable difference in designing suction pipework compared to discharge pipework. Software which is not developed using experienced engineers can only solve textbook problems. The real pump world is dominated by problems which don't appear in textbooks.
Note: System designers should be aware that the European Pressure Equipment Directive, which comes fully into force after 29 th May 2002, requires "that due consideration is given to the potential damage from turbulence and formation of vortices", • BRIAN NESBITT IS AN INDEPENDENTCONSULTANT SPECIALISING IN POSITIVE DISPLACEMENT PUMPS. BRIAN HAS BEEN INVOWED WITH PUMPS AND PUMPING SYSTEMS SINCE 1974. A MEMBER OF THE BSI M C E / 6 COMMITTEES, BRIAN REPRESENTSTHE UK ON CEN AND ISO PD PUMP COMMI'n'EES AND IS CURRENTLY WORKING WITH THE API 6 7 4 TASKFORCE. BRIAN CAN BE CONTACTED AT BRIANNESBITT~)EMAIL.MSN.COM
WORLD P U M P S
October2000 : 2 9