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TYPICAL SURFACE ROUGHNESSMaterial Nature of

Material     Roughness [mm]   

Roughness [inch]

 Steel pipe drawn, new 0.02 - 0.1 0.0008 -

0.004welded, new 0.05 - 0.1 0.002 - 0.004galvanized, new 0.15 0.006used, cleaned 0.15 - 0.2 0.006 - 0.008lightly corroded 0.1 - 0.4 0.004 - 0.016severely corroded 0.4 - 3 0.016 - 0.12light scaling 1 - 1.5 0.04 - 0.06heavy scaling 1.5 - 4 0.06 - 0.16bitumed coated 0.05 0.002

 cast - iron pipe new 0.25 - 1 0.01 - 0.04

corroded 1 - 2 0.04 - 0.08with scaling 1 - 4 0.04 - 0.16

 concrete pipe smooth finish 0.3 - 1 0.012 - 0.04

rough 1 - 3 0.04 - 0.12 Sheet steel smooth 0.07 0.0028 Glass, lead, copper, brass

0.0001 - 0.0015 4·10-6 - 60·10-6

Help for calculating pipe friction loss and the pipe economic size analysis applet

Applets are programs based on the java language that are designed to run on your computer using the Java Run Time environment.

This document will cover two topics, one a general discussion of this subject and how the equations were developped. The other some specific comments on how the applet functions.

General

The following is an excerpt of the afore mentioned book. This java applet does friction head loss calculations for any Newtonian fluid for which the viscosity is known in the turbulent flow regime only which is most cases. The applet provides data on pipe roughness the source of which can be obtained in a pdf file at the bottom of this page.

PIPE FRICTION HEAD DIFFERENCE FOR NEWTONIAN FLUIDS

The Friction Head is the friction due to the movement of fluid in a piping system and is proportional to flow rate, pipe diameter and viscosity. Tables of values for Friction Head are available in references 1 & 8.

The Friction Head, as defined here, is made up of the friction loss due to the fluid movement and the friction loss due to the effect of pipe fittings (for example, 90° elbows, 45° bends, tees, etc.):

the subscript FP refers to pipe friction loss and the subscript FF to fittings friction loss.

NEWTONIAN FLUIDS

Newtonian fluids are a large class of fluids, whose essential property VISCOSITY, was first defined by Newton (see Appendix A for a list of Newtonian and non-Newtonian fluids). Viscosity is the relationship between the velocity of a given layer of fluid and the force required to maintain that velocity. Newton theorized that for most pure fluids, there is a direct relationship between force required to move a layer and its velocity. Therefore, to move a layer at twice the velocity, required twice the force. His hypothesis could not be tested at the time, but later the French researcher, Poiseuille, demonstrated its validity. This resulted in a very practical definition for viscosity.

The Darcy-Weisbach formula expresses the resistance to movement of any fluid in a pipe:

where f is a non dimensional friction factor. Often, the tables give values for friction loss in terms of ft of fluid per 100 ft of pipe. When the appropriate units are used (Imperial system), the Darcy-Weisbach equation becomes:

[3-16]

and in the metric system

[3-16a]

The friction factor is proportional to the Reynolds number which is defined as:

[3-17]

[3-17a]

The Reynolds number is proportional to the kinematic viscosity, the average velocity, and the pipe inside diameter. It is a non dimensional number. The kinematic viscosity is the ratio of the absolute viscosity

to the fluid specific gravity SG.

Viscosity data of common liquids can also be found int the Goulds pump catalogue.

Laminar flow - RE < 2000

Distinct flow regimes can be observed as the Reynolds number is varied. In the range of 0 to 2000, the flow is uniform and is said to be laminar. The term laminar refers to successive layers of fluid immediately adjacent to one another, or laminated. Looking at a longitudinal section of the pipe, the velocity of individual fluid particles is zero close to the wall and increases to a maximum value at the center of the pipe with every particle moving parallel to its neighbor. If we inject dye into the stream, we would notice that the dye particles maintain their cohesion for long distances from the injection point.

Figure 3-16 Laminar and turbulent flow velocity profiles.

The friction loss is generated within the fluid itself. Figure 3-16 shows that each layer (in this case each ring) of fluid is moving progressively faster as we get closer to the center. The difference in velocity between each fluid layer causes the friction loss.

The friction factor f is given by:

[3-18]

For viscous fluids (i.e.: n > 50 SSU), the combination of velocity and viscosity usually produces a low Reynolds number and therefore laminar flow. Pumping viscous fluids at a faster rate may cause the fluid to become turbulent resulting in high friction losses. The tables for viscous fluid friction loss given in references 1 & 8 are based on the equation for laminar flow, equation [3-18]. This equation can be theoretically derived and is found in most fluid dynamic volumes (see reference 11). An interesting aspect of laminar flow is that pipe roughness is not a factor in determining friction loss.

Unstable flow - 2000 <RE <4000

The flow is pulsing and unstable and appears to possess characteristics of both laminar and turbulent flow.

Turbulent flow - RE > 4000

At Reynolds number larger than 4000, it is very difficult to predict the behavior of the fluid particles, as they are moving in many directions at once. If dye is injected into the stream, the dye particles are rapidly dispersed, demonstrating the complex nature of this type of flow. Reynolds, who originally did this experiment, used it to demonstrate the usefulness of a non-dimensional number (the Reynolds number) related to velocity and viscosity. Most industrial applications involve fluids in turbulent flow. The geometry of the wall (pipe roughness) becomes an important factor in predicting the friction loss.

Many empirical formulas for turbulent flow have been developed. Colebrook's equation is the one most widely accepted:

[3-19]>

where is the average height of protuberances (absolute roughness) of the pipe wall surface (for example, 0.00015 ft for smooth steel pipe). The term /D is called the pipe roughness parameter or the relative roughness. Since it is not possible to derive an explicit solution for f, L.F. Moody (see Figure 3-18) developed a graphical solution. The diagram shows the linear relationship of the friction factor (f) with the

Reynolds number (Re) for the laminar flow regime. For Reynolds numbers in the medium range (4,000 to 1,000,000, turbulent flow), the friction factor is dependent on the Reynolds number and the pipe roughness parameter, which is known as the transition zone. For high Reynolds numbers (1,000,000 and higher, fully turbulent), the friction factor is independent of the Reynolds number and is proportional only to the pipe roughness parameter. This is the zone of complete turbulence.

Some typical values for the absolute roughness :

PIPE MATERIAL Absolute roughnessSteel or wrought iron 0.00015 ftAsphalt-dipped cast iron 0.0004 ftGalvanized iron 0.0005 ft

Another equation developed by Swamee and Jain, gives an explicit result for f and agrees with the Colebrook equation within 1%, this is the equation used for this applet.

[3-20]

The pressure head loss per 100 feet of pipe is obtained using equation [3-16]. You can get the total head loss by multiplying by the length of the pipe and dividing by 100. Then to convert to pressure loss, use equation [3-21].

[3-21]

Specific

How to use the applet

Data for the system (see next figure) is entered in the area marked general data. When you select the type of pipe to be used, standard values for nom. diameter, and inside diamater are inserted into the pipe data table in columns 1 and 2. The pipe diameter values used in this applet are available here . Installation cost for the pipe is also inserted into the pipe data table in column 3, these are typical values only and you need to replace them with values applicable to your area.

The pipe data table is editable by double-clicking on any item in the 3rd column. Once this is done you can press the Calculate button and the results will appear at the bottom. The first line of the results give the diameter selected that is closest to a standard diameter based on the flow rate and target velocity. The second line provides information on what the power cost and installation cost would be if you had selected the next biggest diameter. These costs can then be compared to the costs for the smaller diameter providing the cost savings for one year which in turn determines how many years are required to pay back the pipe installation cost or the ROI period. The next largest diameter is then selected and the same calculation is done based on the smallest diameter.

The intent of this applet is to help make a reasonned decision on pipe size that is more involved than selecting a pipe diameter on the basis of a target velocity. If a system has a low static head, the cost of power for such a system over a year may well justify installing a larger pipe than would normally be called for based on matching a target velocity. To do this, the cost of the pipe, hangers, supports, flanges, etc., must be known per foot of pipe. The power consumption is calculated based on the fluid properties, the length of pipe and the flow rate. The pump efficiency must be known and if the pump has not been purchased then a reasonable estimate can be provided using this curve. The cost of a kiloWatt-hour must also be known as well as the motor efficiency

and then the annual savings that can be achieved by installing a bigger pipe can be calculated.

The power consumed is calculated with the standard formula:

The pipe roughness is selectable and based on the values in this table. You can also specify any pipe roughness by clicking on the specify

text of the pipe roughness selection box. This will make another textfield appear where the pipe roughness can be entered.

The pipe diameter used is the inside diameter. This diameter varies depending on the construction of the pipe. Various standards such as carbon steel schedule pipe are used and are selectable. These values are then displayed in a grid on the applet. The values in the grid can be changed at any time.

The annual operating cost of power is calculated based on the number of operating hours in a year, the motor efficiency and the cost per kW-h. This is done based on a pipe size that closely matches the target velocity. These calculations are done for the next biggest pipe sizes and are compared with the installation cost of these pipes.

The ROI (Return On Investment) period is the ratio of the pipe cost (includes purchase and installation ) difference between the initial selection and the next available diameter and the power cost difference of the the initial selection and the next available diameter. In the graphic above the ROI period

6.5 years = ($10,500 -$7500)/($1196-$1658).

A small period, for example less than 2 years means that it will take 2 years for the power savings to pay back the increased cost of the larger pipe size. Remember that it is difficult to change pipe size after

the fact, the cost of dismantling and production time loss is usuallly very high.

The friction calculation in this program uses the Swamee-Jain equation. There is no head loss calculation for fittings friction. Therefore the real friction will be higher which means that the power consumption will be slightly higher. However this is not expected to impact the ROI period since fittings friction loss is normally a small portion of the total head.

The static head of the system must be known and this is added to the calculated friction head. If the discharge or suction end of the system is pressurized then this should be included in the static head. If there are any other process equipment such as control valves, heat exchangers, etc., the sum of their total head loss can be entered in the equipment head loss textbox.

The pipe construction types provided are carbon steel schedule pipe and ID pipe in Imperial units. The poly ethylene, PVC-M and UPVC pipe size original data is metric and the sizes have been converted to Imperial units.

The applet offers you two choices of pipe sizes that are larger than the initial selection based on a target velocity and you can decide which of these is appropriate based on the ROI period.

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CENTRIFUGAL PUMP SYSTEMS TIPS This is a list of ideas or DOS AND DON'TS for pump systems. You may not of thought of some of these and they will help you design and trouble-shoot pump systems and select the proper pump. Also there is information here that is hard to find elsewhere. You can think of this list as GUIDELINES for the pump system designer.

You can also download these tips in pdf format.

1. Flow and pressure relationship of a pump

When the flow increases, the discharge pressure of the pump decreases, and when the flow decreases the discharge pressure increases (ref. tutorial2.htm).

2. Do not let a pump run at zero flow

Do not let a centrifugal pump operate for long periods of time at zero flow. In residential systems, the pressure switch shuts the pump down when the pressure is high which means there is low or no flow.

3. Use pressure gauges

Make sure your pump has a pressure gauge on the discharge side close to the outlet of the pump this will help you diagnose pump system problems. It is also useful to have a pressure gauge on the suction side, the difference in pressure is proportional to the total head. The pressure gauge reading will have to be corrected for elevation since the reference plane for total head calculation is the suction flange of the pump.

This applet will help you calculate total head from pressure

4. Do not let a pump run dry, use a check valve

Most centrifugal pumps cannot run dry, ensure that the pump is always full of liquid. In residential systems, to ensure that the pump stays full of the liquid use a check valve (also called a foot valve) at the water source end of the suction line. Certain types of centrifugal pumps do not require a check valve as they can generate suction at the pump inlet to lift the fluid into the pump, see http://www.watertanks.com/category/43/. These pumps are called jet pumps and are fabricated by many manufacturers Goulds being one of them.

Make use of check valves to isolate pumps installed in parallel.

5. Suction valves

Gate valves at the pump suction and discharge should be used as these offer no resistance to flow and can provide a tight shut-off. Butterfly valves are often used but they do provide some resistance

and their presence in the flow stream can potentially be a source of hang-ups which would be critical at the suction. They do close faster than gate valves but are not as leak proof.

6. Eccentric reducer

Always use an eccentric reducer at the pump suction when a pipe size transition is required. Put the flat on top when the fluid is coming from below or straight (see next Figure) and the flat on the bottom when the fluid is coming from the top. This will avoid an air pocket at the pump suction and allow air to be evacuated.

7. Use a multi-stage turbine pump for deep wells

For deep wells (200-300 feet) a submersible multi-stage pump is required. They come in different sizes (4" and 6") and fit inside your bore hole pipe. Pumps with different ratings are available, see http://www.webtrol.com/waterwell%20homepage.html

8. Flow control

If you need to control the flow, use a valve on the discharge side of the pump, never use a valve on the suction side for this purpose.

This is an excellent treatment of the types of control systems for a centrifugal pump. Thanks to Walter Driedger of Colt Engineering a

consulting engineering firm for the petro-chemical industry in Alberta, Canada.

9. Plan ahead for flow meters

For new systems that do not have a flow meter, install flanges that are designed for an orifice plate in a straight part of the pipe (see next Figure) and do not install the orifice plate. In the future, whoever trouble-shoots the pump will have a way to measure flow without the owner having to incur major downtime or expense. Note: orifice plates are not suitable for slurries.

10. Avoid pockets and high points

Avoid pockets or high point where air can accumulate in the discharge piping. An ideal pipe run is one where the piping gradually slopes up from the pump to the outlet. This will ensure that any air in the discharge side of the pump can be evacuated to the outlet.

11. Location of control valves

Position control valves closer to the pump discharge outlet than the system outlet. This will ensure positive pressure at the valve inlet and therefore reduce the risk of cavitation.

When the valve must be located at the outlet such as the feed to a tank, bring the end of the pipe to the bottom of the tank and put the valve close to that point to provide some pressure on the discharge

side of the valve making it easier to size the valve, extending it's life and reducing the possibility of cavitation.

12. Water hammer

Be aware of potential water hammer problems. This is particularly serious for large piping systems such as are installed in municipal water supply distribution systems. These systems are characterized by long gradually upward sloping and then downward sloping pipes. Solutions to this can involve special pressure/vacuum reducing valves at the high and low points or additional tanks which provide a buffer for pressure surges (see http://www.ventomat.com/default.asp).

see also the pump glossary

For pumps 500 gals/min or larger use semi-automatic manual valves at the discharge that are controlled to open gradually when starting the pump. This will avoid water hammer during the initial start and damage to the piping system.

13. The right pipe size

The right pipe size is a compromise between cost (bigger pipes are more expensive) and excessive friction loss (small pipes cause high friction loss and will affect the pump performance). Generally speaking, the discharge pipe size can be the same size as the pump discharge connection, you can see if this is reasonable by calculating the friction loss of the whole system. For the suction side, you can also use the same size pipe as the pump suction connection, often one size bigger is used (ref. tutorial3.htm). A typical velocity range used for sizing pipes on the discharge side of the pump is 9-12 ft/s and for the suction side 3-6 ft/s.

See this calculator for velocity and flow

A small pipe will initially cost less but the friction loss will be higher and the pump energy cost will be greater. If you know the cost of energy and the purchase and installation cost of the pipe you can select the pipe diameter based on a comparison of the pipe cost vs power

consumption, this applet on the economic analysis of pipe size will help you do this, and you can view this applet's help file here.

14. Pressure at high point of system

Calculate the level of pressure of the high point in your system. The pressure may be low enough for the fluid to vaporize and create a vapor pocket which will be detrimental to the performance of the system. The pressure at this point can be increased by installing a valve at some point past the high point and by closing this valve you can adjust the pressure at the high point. Of course, you will need to take that into account in the total head calculations of the pump.

See this video showing this phenomena in action .

15. Pump pressure rating and series operation

For series pump installations make sure that the pressure rating of the pumps is adequate. This is particularly critical in the case where the system could become plugged due to an obstruction. All the pumps will reach their shut-of head and the pressure produced will be cumulative. The same applies for the pressure rating of the pipes and flanges.

16. Inadequate pump suction submersion

There is a minimum height to be respected between the free surface of the pump suction tank and the pump suction. If this height is not maintained a vortex will form at the surface and cause air to be entrained in the pump reducing the pump capacity.

see the pump system glossary also see this experiment on video that shows vortex formation .

17. Pump selection

Select your pump based on total head (not discharge pressure) and flow rate. The flow rate will depend on your maximum requirement. Total head is the amount of energy that the pump needs to deliver to account for the elevation difference and friction loss in your system (ref. tutorial3.htm).

Pump selection starts with acquiring detail knowledge of the system. If you are just replacing an existing pump then of course there is no problem. If you are replacing an existing pump with problems or looking for a pump for a new application then you will need to know exactly how the systems is intended to work. You should have the P&ID diagram and understand the reasons for all the devices included in your system. You should make your own sketch of the system that includes all the information on the P&ID plus elevations (max., min., in, out, equipment), path of highest total head, fluid properties, max. and min. flow rates and anything pertinent to total head calculations.

The next figure is a typical example:

Typical example of flow schematic used for total head calculations.

The control method is important (on-off, control valve, re-circulating, variable speed) as it may affect your selection. Besides the system

sketch, here is a pump data sheet that you can use to record some of the data.

Depending on the industry or plant that you work in, you will be forced to either select a certain type of pump or manufacturer or both. Manufacturers are normally a very good source of information for final pump selection and you should always consult with them, do your own selection first and confirm it with the manufacturer. They can help you select the right type, model, and speed if you have all the operating conditions and if not they will rarely be able to help you. This form will help you gather all the information pertinent to operation and selection

of your pump .

Aside from the normal end suction pump, vertical turbine and submersible pumps, there is a wide variety of specialized pumps that you should consider for your application if you have unusual conditions.

In the selection process, you will be trying to match your flow rate with the B.E.P. of the pump. It is not always possible to match the flow rate with the B.E.P. (best efficiency point), if this is not possible, try to remain in the range of 80% to 110% of the B.E.P..

Desirable selection area for impeller size for centrifugal pumps.

Operating outside this range will lead to excessive vibration, recirculation and cavitation, see the next two figures. The first one from the Pump Handbook from McGraw-Hill which shows how the axial force increases with the distance in terms of percent flow from the B.E.P. and the second from Goulds essentially shows the same information but in terms of vibration.

Radial force vs. % flow of BEP

Vibration level vs. flow

Electronic pump curves have been created for many (over 50) manufacturers, see a list of them here. They have all been developed by Engineered Software located in Lacey Washington USA of which I am a representative. Their pump sizing software PUMP-FLO can help find the best pump for the application, it can select the closest one to the B.E.P. for you and do all kinds of searches based on NPSHR, efficiency, size, etc.

When you order your pump make sure that the motor is installed with spacer blocks so that the next largest motor frame can be installed.

See also item 30 on selecting high speed centrifugal pumps.

18. Air in pump reduces capacity

When air enters a pump it sometimes gets trapped in the volute, this reduces the capacity, creates vibration and noise. To remedy, shut the pump down and open the vent valve to remove the air. If the pump is excessively noisy do not automatically assume that the problem is cavitation, air in the pump creates vibration and noise. Cavitation produces a distinct noise similar to gravel in a cement mixer. If you have never heard the sound of cavitation here's a recording of it in WAV format, courtesy of my friend Normand Chabot, water hammer specialist here in Montreal.

see also the pump glossary

Also see articles on entrained air on this web page: pumpworld.htm

19. Effect of viscosity on pump performance

Viscosity is the main criteria which determines whether the application requires a centrifugal pump or a positive displacement pump. Centrifugal pumps can pump viscous fluids however the performance is adversely affected. If your fluid is over 400 cSt (centiStokes) in viscosity consider using a positive displacement pump. Use this

calculator to determine the correction for viscosity to the water performance curve of the pump.

 see also the pump glossary

Also see articles on viscosity on this web page: pumpworld.htm

20. Avoid running pump in reverse direction

Avoid running a pump in reverse direction, pump shafts have been broken this way especially if the pump is started while running backwards. The simplest solution is to install a check valve on the discharge line.

21. Minimum flow rate

Most centrifugal pumps should not be used at a flow rate less than 50% of the B.E.P. (best efficiency point) flow rate without a recirculation line. (What is the B.E.P.?) If your system requires a flow rate of 50% or less then use a recirculation line to increase the flow through the pump keeping the flow low in the system, or install a variable speed drive.

see also the pump glossary BEP

How is the minimum flow of a centrifugal pump established (answer from the Hydraulic Institute http://www.pumps.org/content_detail.aspx?id=2138)

The factors which determine minimum allowable rate of flow include the following:

* Temperature rise of the liquid -- This is usually established as 15°F and results in a very low limit. However, if a pump operates at shut off, it could overheat badly.

* Radial hydraulic thrust on impellers -- This is most serious with single volute pumps and, even at flow rates as high as 50% of BEP could cause reduced bearing life, excessive shaft deflection, seal failures, impeller rubbing and shaft breakage.

* Flow re-circulation in the pump impeller -- This can also occur below 50% of BEP causing noise, vibration, cavitation and mechanical damage.

* Total head characteristic curve - Some pump curves droop toward shut off, and some VTP curves show a dip in the curve. Operation in such regions should be avoided.

There is no standard which establishes precise limits for minimum flow in pumps, but "ANSI/HI 9.6.3-1997 Centrifugal and Vertical Pumps - Allowable Operating Region" discusses all of the factors involved and provides recommendations for the "Preferred Operating Region".

22. Three important points on the pump characteristic curve

The performance or characteristic curve of the pump provides information on the relationship between total head and flow rate. There are three important points on this curve.

1. The shut-off head, this is the maximum head that the pump can achieve and occurs at zero flow. The pump will be noisy and vibrate excessively at this point. The pump will consume the least amount of power at this point. See also the pump glossary.

2. The best efficiency point B.E.P. this is the point at which the pump is the most efficient and operates with the least vibration and noise. This is often the point for which pump’s are rated and which is indicated on the nameplate. The pump will consume the power corresponding to its B.E.P. rating at this point.

3. The maximum flow point, the pump may not operate past this point. The pump will be noisy and vibrate excessively at this point. The pump will consume the maximum amount of power at this point.

Sometimes the characteristic curve will include a power consumption curve. This curve is only valid for water, if the fluid has a different density than water you cannot use this curve. However you can use the total head vs. flow rate curve since this is independent of density.

Typical centrifugal pump characteristic curve.

If your fluid has a different viscosity than water you cannot use the characteristic curve without correction. Any fluid with a viscosity higher than 10 cSt will require a correction. Water at 60F has a viscosity of 1 cSt.

23. Normal, flat and drooping characteristic curves

There are three different characteristic curve profiles for radial flow pumps. Figure 4 shows the various vane profiles that exist and the relationship between them. This tip is related to the radial vane profile which is the profile of the typical centrifugal pump.

Pump vane profiles vs. specific speed.

There are three different curve profiles shown in the next figure:

1. Normal, head decreases rapidly as flow increases

2. Flat, head decreases very slowly as flow increases

3. Drooping, similar to the normal profile except at the low flow end where the head rises then drops as it gets to the shut-off head point.

Different types of radial pump characteristic curve profiles.

The drooping curve shape is to be avoided because it is possible for the pump to hunt between two operating points which both satisfy the head requirement of the system. This is known to happen when two pumps are in parallel, when the second pump is started it may fail to get to the operating point or hunt between two points that are at equal head. Thankfully not to many pumps have this characteristic, here are a few:

Drooping curve (Goulds).

Drooping curve (Sundyne).

A flat curve is sometimes desirable since a change in flow only causes a small change in head, for example as in a sprinkler system. As more sprinklers are turned on the head will tend to decrease but because the curve is flat the head will decrease only a small amount which means that the pressure at the sprinkler will drop only a small amount, thereby keeping the water velocity high at the sprinkler outlet. The National Fire Prevention Association (N.F.P.A.) code stipulates that the characterictic curve must be flat within a certain percentage. This code can be purchased at ANSI.

The normal curve can be more or less steep. A steep curve can be desirable from a control point of view since a small change in flow will result in a large pressure drop. The steepness of the curve depends on the number of vanes and the specific speed.

24. Suction piping

Many people are way to CONSERVATIVE about suction piping design. The usual advice you get is make the piping as straight, as big and short as possible.

I have seen a suction line 300 ft long, now that's not short.

I believe the important considerations are:

- by all means make the pipe as short and straight as possible, particularly if the fluid has suspended solids which may cause plugging or hangups.

- make sure there is sufficient pressure at the pump suction (this means check the NPSHA against the NPSHR);

- make sure that the stream flow lines are coming in nice and straight at the pump suction.

This generally means having 5 to 10D straight pipe ahead of the pump inlet.

Avoid the use of filters at the pump inlet if at all possible. Their maintenance will often be neglected and the pump will suffer from poor performance and perhaps cavitation.

Use a 90° or 45° elbow at the pump’s inlet pipe end. This will allow almost complete drainage of the tank and is especially useful in the case of fluids that can not be readily dumped to the sewers. It also provides additional submergence reducing the risk of vortex formation.

Also be careful of elbows that are too close to the pump suction, see the pump glossary.

25. The meaning of specific speed

If you are having trouble with a pump or want to check whether the new pump to be installed is appropriate, check the specific speed and the suction specific speed of the pump. The specific speed provides a number which can help identify the type of pump (for example radial or axial flow) that is best suited for your application. The specific speed of the pump type selected (see Figure 4) should be close to the specific speed calculated for your application. The suction specific speed will tell you if the suction of the pump is likely to cause problems in your

application. Calculator for specific speed.

see also the pump glossary.

26. Different types of centrifugal pumps

There are many different types of pumps available other than the standard end suction, submersible or vertical multi-stage pump. In this article, you will see a number of pumps that are specialized and may suit a particular need.

27. Unusual aspects of pump systems

This article discusses unusual aspects of pump systems : variation in pressure throughout the system and effect of fluid properties. To

calculate the pressure anywhere in a system use this applet.

28. Predict pump efficiency

Save time in the initial phase of the project and calculate power requirement prior to the final pump selection using a chart that

predicts the efficiency of standard end suction centrifugal pumps. Alternatively, compare the efficiency of the final pump selection with the industry average.

You will notice that efficiency increases with specific speed, this means that a pump with a higher speed (rpm) that meets your requirements will be smaller and more efficient and therefore cost less to operate, see item 30.

29. Predict pump N.P.S.H.R.

Predict the pump N.P.S.H.R. with this chart. If you have an old centrifugal pump and no data from the manufacturer this chart can help you predict the NPSH required and avoid cavitation. You will need to know the suction eye velocity which depends on the eye diameter.

The following two figures come from this web site: http://www.tpub.com/content/fc/14104/css/14104_128.htm

30. Consider higher speed centrifugal pumps

The standard motor induction speeds that are most often considered for pumps are 900, 1200, 1800 and 3600 rpm. The specific speed of a pump depends on its speed and the higher the specific speed the higher the efficiency (see the chart in item 28). By choosing a higher speed pump the pump will be smaller and maybe less expensive. Modern anti-friction bearings are quite capable of handling these higher speeds without compromising their useful life. Will there be increased wear within the casing? Probably not since for the same head the fluid particle speed within the casing will be the same because of the smaller impeller diameter turns at a higher rpm giving roughly similar linear velocities.

Therefore, if you want to conserve energy and reduce costs always check to see if a centrifugal pump running at a higher speed can meet

your requirement. However, you should check the suction specific speed number and make sure it is below 11000 to avoid problems with cavitation.

The following is a selection of two pumps at different speeds for an application requiring 55 l/s at 70 m, by using the higher speed pump, the power savings could be very significant over time.

Also see articles on the effect of pump speed on this web page: pumpworld.htm

31. Flywheel effect, pulley diameter

Many pumps are still driven by pulleys, and large pulleys become flywheels. Flywheel pulleys have a limiting safe rotational speed. The

flow can be increased by changing the pulley diameters, be careful not to exceed the safe operational speed of the pulleys.

32. 10 ways to select a happy pump

(from Robert Perez of www.pumpcalcs.com)

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US Grit UK Grit Ra Raref Ref. µm µ inch 120 3 125 180 2 8580 1.65 70 240 1.50 50 320 0.75 30180 0.62 25240 0.45 18 500 0.40 15320 0.25 10

Sl No. US Grit UK Grit Ra µm Ra µ inch1 120 3 1252 180 2 853 80 1.65 704 240 1.5 505 320 0.75 306 180 0.62 257 240 0.45 188 500 0.40 159 320 0.25 1010