How To Read A Pump

HOW TO READ A PUMP PERFORMANCE CURVE. (exit at any time by scrolling to end of this document) 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)

f = 0.2083 (100/c)1.852 q1.852 / dh4.8655 (1) where f = friction head loss in feet of water per 100 feet of pipe (fth20/100 ft pipe) c = Hazen-Williams roughness constant q = volume flow (gal/min) dh = inside hydraulic diameter (inches) 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

FOR PERSONAL ASSISTANCE If you would like one of our designers to complete this aspect for you, we do charge a small design fee of US$10 per basic design and pump selection, please contact us at this link (PUMP SYSTEM DESIGN SUPPORT)