The pump to select?

The pump to select? 

How to determine the pump to select? The correct pump.

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A centrifugal pump performance

curve is simply a tool which

enables anyone to literally see

how a pump will perform in terms of

HEAD and FLOW.

Every pump will be capable of

developing a specific PRESSURE

(PSI or BAR measurement translated into feet or meters head)

at a specific FLOW

(normally represented in gallons per minute or liters per minute)

A note on establishing flow.

If you do not know the flow you require

it is relatively simple to determine if you

just take it one step at a time and add

all the outlets together.

For industrial applications, for washing, heating or cooling

the equipment will have a flow and pressure

on the design plate.

Flow in a system can be established by

understanding the requirements.

In our example we will use a house which

has two bathrooms, a dishwasher, a washing

machine, an electric geyser in the roof and

a guest toilet with hand basin. Also a sink in

the kitchen and an outside shower for the

pool area.

So we have the following outlets :-

Bathroom hand basin taps 4

Bath taps 4

Shower taps 6

Toilet cistern 3

Guest basin tap 2

Sink in kitchen 2

Washing machine 1

Dishwasher 1

TOTAL 23

If we assume each tap will be required to

deliver 2 gallons per minute (GPM) at 44 PSI

or 3 bar, then we simply need to determine

how many outlets we would expect to

operate at any one time. It is unlikely that

all toilets, all showers, all baths etc will

be on at the same time. A good method is to

take a third of the total outlets as your flow

need for any single moment in time.

In our example that represents 23 divided by 3 or 7.6 make it 8 outlets.

This means a flow requirement of 8 X 2 = 16 GPM or 60 liters per minute.

A note on the concept of

PUMP HEAD.

Think about the highest point of your

body, the top of your HEAD.

It is the same idea with pump language,

Head refers to the measurement in

feet or meters from the center line of

the pump (the pump shaft center line)

to the highest point to which the unit

is expected to deliver fluid)

The above definition of head is

limited to what is known as the

Static head. in other words, this is

the measurement of the vertical

height which should never change, it is static.

Lets say you are needing

to deliver water from a tank at

the bottom of your garden, to the

geyser at the top of your roof. The

garden has a steep bank from the

house to the lower area where the

tank is located. The house is a double

story. If one measures the height from

the base of the tank to the exact spot

where the geyser rests in the house roof,

we get a measurement of 50 feet (15,5 meters)

. This is static head, it does not change.

However when it comes to fluid and

determining the total head the pump will

feel in order to deliver your required flow

to that vertical height, there are some extra

variables which will effect the head.

These variables are where the calculation of

total dynamic head becomes a little more

involved. We are not going to get into

a huge technical discussion with formula

and major math here, all we need you to

recognise is that there are dynamic forces at

work which affect the performance of

the pump and which one needs to apply to

the system curve. Doing this right will enable

you to determine the right pump for your needs

The total dynamic head is a combination of

static head, friction losses in the pipe system

called friction head loss, and losses caused by

the equipment to which one is delivering the

fluid. These losses change in measurement

depending on the volume of fluid which is

being pumped at any one time. As such the

losses ar dynamic, they change in relation to flow rate.

In our example, we have the tank at the bottom of the yard

and the geyser in the roof, we call this the static head.

Then we have the friction head which will result

from the desired quantity of water needed flowing

through a specific pipe size, over the total distance

from the pump to the geyser. Added to that we will

also need to add the required working pressure for the geyser.

So lets see how we can make the concept a lot

more simple, lets look at a picture.

In FIG 1 we see the static head of the pipe system.

That measurement of 50 feet will not change and

it is the vertical height measurement only.

The actual pipe length given bends and horizontal

distances covered in the system is 70 feet and we

intend to install a 1” inside diameter pipe.

The pipe inside diameter is important because

it is the measurement which is presented to

the water flow path. One needs to have the total

pipe length, pipe inside diameter and the pipe

material before one can apply the formula which

establishes friction loss in a pipe at any given flow.

FIG 2

To calculate the friction loss presented by a pipe system

to the pump TDH (Total Dynamic Head) one normally

would apply a formula like the Hazen-Williams equation as

shown below, BEFORE YOU BLOW A GASKET

AND GIVE UP, get our free simple friction loss calculator

here. (FRICTION LOSS CALCULATOR)

OPERATING PRESSURE

There is one more measurement that will

be required to determine the total dynamic head.

It is the actual back pressure of the geyser.

This will be on the plate of the equipment

which will be used. There will be two

measurements, working pressure and burst

pressure. The working pressure is the

pressure the equipment requires in order

to operate at design efficiency. In most

cases a home geyser requires between

30 and 60 PSI. (2to 4 Bar, 200 to 400 kPa)

Working pressure Working pressure will

need to be provided to the equipment and

therefore needs to be provided for by the

pump unit. The actual losses in the geyser

will be only around 3 or 4 PSI when new,

however we need to generate working pressure

in this instance.

So Total Dynamic head (which has many other

additions for more detailed pipe systems which

are not discussed here in detail) can be established

by keeping these basics in mind.

Get a handy calculator which will make this

far easier to establish. It is free, so have this

sent directly to you now.

HANDY HEAD CALCULATOR

BRINGING IT ALL TOGEATHER

When we know our total dynamic head and

our flow rate we require, the rest is simple.

We look up the head on the Y axis and follow

that line to the X axis flow value. Where these

two values intersect, we have our duty point.

We now search for a pump curve which will

allow this point to position on the “sweet spot”.

When we get that right, we have selected the correct

pump for our needs and we are able to purchase with

confidence.

If you used our example above you will have the following.

Flow required 16 GPM (60.48 liters per minute)

Head Static 50 feet

Friction 11.6 feet

Geyser 130 feet

(working pressure assumed to be 58 PSI)

BY adding all these we have a TDH 191.60 feet (58.95 meters)

We now are able to specify a pump that is

capable of delivering 16 gallons per minute

at 191.6 feet. I would add in about 10 percent

on the head for safety.(pipe bends and other

losses) so we look for a 63 meter head or 210 feet capablility in our pump.

We are to use the primary metric pump curves so

our measurements are flow in liters per minute on

the lower x axis and GPM on the upper, and

head in meters on the Y axis left with feet on the right.

FIG 3

We have taken a pump manufacturers catalogue,

looked through the various models available,

searching for a pump curve that has both the

right flow rate and head characteristics. Knowing

what we are looking for because we have worked

it all out makes the exercise relatively simple.

We have now selected the PLURIJET PUMP

RANGE (A PEDROLLO PRODUCT) so lets

see how the curve works. (SEE FIG 4)

FIG 4

Here we have shown the correct pump selection with

the geyser working pressure included in the design.

We also show the most common mistake made in

pump selection, not including the working pressure

of the geyser. As can be seen, if we do not include

the geyser working pressure, we have a major problem.

We hope this all assists you in understanding the basics

of a fluid system design. Should you wish to have far

more information literature and instruction is available

at the link below.

MORE DETAILED INFORMATION

Further technical information link click here

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this aspect for you, we do charge a small design

fee of US$10 per basic design and pump selection,

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