SUBJECT: Understanding the pump system curve 5-12
Every pump manufacturer would like to recommend the perfect pump for your application. To do this he would like you to provide him with an accurate system curve that would describe the capacity and head needed for your various operating conditions. Once he has your system curve, he can plot his pump curves on top of the system curve and hopefully select something that will come close to your needs. Without this system curve, neither one of you has much of a chance of coming up with the right pump.
To create a system curve we plot the desired capacities against the required head over the total anticipated operating range of the pump. The head will be measured in feet or meters and the capacity will be measured in gallons per minute or cubic meters per hour.
Some of the confusion begins when we realize that there are three different kinds of head:
STATIC HEAD This is the vertical distance measured from the center line of the pump to the height of the piping discharge inside the tank. Look at figure "A" and note that the piping discharge is below the maximum elevation of the piping system. We do not use the maximum elevation in our calculations because the siphoning action will carry the fluid over this point once the piping is full of liquid. This is the same action that lets you siphon gasoline out of an automobile to a storage can.
The pump will have to develop enough head to fill the pipe and then the siphoning action will take over. The pump operating point should move back towards the best efficiency point (B.E.P.) if the pump was selected correctly.
DYNAMIC OR SYSTEM HEAD As the liquid flows through the piping and fittings, it is subject to the friction caused by the piping inside finish, restricted passages in the fittings and hardware that has been installed in the system. The resulting "pressure drop" is described as a "loss of head" in the system, and can be calculated from graphs and charts provided by the pump and piping manufacturers. These charts are not included with this paper, you can find them in the Hydraulic Institute Manuals. This "head" loss is related to the condition of the system and makes the calculations difficult when you realize that older systems may have "product build up" on the piping walls, filters, strainers, valves, elbows, heat exchangers, etc., making the published numbers some what inaccurate.
A general "rule of thumb" says that the friction loss in clean piping will vary approximately with 90% of the square of the change in flow in the piping, and 100% of the square with the change of flow in the fittings and accessories. You calculate the change in flow by dividing the new flow by the old flow and then square the number. As an example:
In the original application system, loss was a combination of the loss through the piping and the loss through the fittings for a total of 100 feet at 200 gallons per minute. When we increased the flow to 300 gallons per minute our system head changed to a total of 208.13 feet. This change would have to be added to the static and pressure heads to calculate the total head required for the new pump.
Please note that the pump is pumping the difference between the suction head and the discharge head, so if you fail to consider that the suction head will be either added to or subtracted from the discharge head, you will make an error in your calculations. The suction head will be negative if you are lifting liquid from below ground or if you are pumping from a vacuum. It will be positive if you are pumping from a tank located above ground. If the suction head is pressurized, this pressure must be converted to head and subtracted from the total head required by the pump.
A centrifugal pump will create a head/capacity curve that will generally resemble one of the curves described in figure "B" The shape of the curve is determined by the Specific Speed number of the impeller.
Centrifugal pumps always pump somewhere on their curve, but should be selected to pump as close to the best efficiency point (B.E.P.) as possible. The B.E.P. will fall some where between 80% and 85% of the shut off head (maximum head).
The manufacturer generated these curves at a specific R.P.M.. Unless you are using synchronous motors (you probably are using induction motors on your pumps) you will have to adjust the curves to match your actual pump speed. Put a tachometer on the running motor and record the rpm. difference between your pump and the speed shown on the pump manufacturer's published curve. You can use the pump affinity laws to approximate the change.
POSITIVE DISPLACEMENT PUMPS have a different shaped curve. They look something like Figure "C".
In this system, the head remains a constant as the capacity varies. This is a typical application for:
The second system is the ideal one, Figure "E" describes it:
In this system the entire head is system head so it will vary with the capacity. Look for this type of curve in the following applications:
System curve "G" is a common one. It is a combination of static, pressure and system heads.
Once the pump manufacturer has a clear idea as to the shape of your system curve, and the head and capacity numbers needed he can then select the proper centrifugal pump. The shape of his curve will be pretty much determined by the specific speed number of the impeller.
In addition to specific speed he can select impeller diameter, impeller width, pump rpm., and he also has the option of series or parallel operation along with the possibility of using a multi-stage pump to satisfy your needs.
The sad fact is that most pumps are selected poorly because of the desire to offer the customer the lowest possible price. A robust pump, with a low L3/D4, is still your best protection against seal and bearing premature failure when the pump is operating off of its best efficiency point. Keep the following in mind as you select your pump:
Since you will be using pumps that were supplied at the lowest cost, you can do the following to resist some of the shaft displacement:
Do not trust the system prints to make your calculations. The actual system always differs from that shown on the print, because people tap into the lines, using the pumped fluid for a variety of purposes and after having done so forget to change or "mark up" the original system print. You are going to have to "walk down" the system and note the pipe length, the number of fittings, etc., to make an accurate system head calculation. Do not be surprised to find that the discharge of your pump is hooked up to the discharge of another pump further down the line. In other words, the pumps are connected in parallel and no body knows it. Pressure recorders (not gauges) installed at the pump suction and discharge is another technique you can use to get a better picture of the system or dynamic head. They will show you how the head is varying with changes in flow.
Pump selection is simple, but not easy. Do not depend upon the knowledge of the local pump salesman to select the correct pump for you. In many cases he is prepared to sell his pump at cost, to get the spare parts business. If you're purchasing pumps at too big a discount&emdash;something is wrong, there is no free lunch. Keep in mind that if several people are involved in the selection process each of them will commonly add a safety factor to the calculated pump size. These factors added together can cause you to purchase a pump that is very much over sized.
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