Feature
Understand the system curve for proper pump selection.
Centrifugal pump manufacturers aspire to recommend the perfect pump for your application. To do this, you need to provide manufacturers with an accurate system curve that describes the capacity and head needed for your various operating conditions. This will enable a manufacturer to plot pump curves on top of the system curve and hopefully select a pump that fills your needs.
Without this system curve, there is little chance that the right pump will be selected.
Centrifugal pumps contain rotating impellers or rotating vanes mounted on a shaft and use centrifugal force to deliver water in a steady stream to point-of-use (POU) devices.
To create a system curve, plot the desired capacities against the required head or pressure over the total anticipated operating range of the pump. The head is measured in feet and the capacity measured in gallons per minute (gpm).
However, creating a system curve can sometimes be confusing because there are three different types of pump pressure.
Static head
is the vertical distance measured from the center line of the pump to the height of the piping discharge inside the tank. Figure 1 shows the piping discharge is below the maximum elevation of the piping system. The maximum elevation is not used in calculations because the siphoning action carries 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 must develop enough head to fill the pipe, then the siphoning action takes over. The pump operating point should move back toward the best efficiency point (BEP) if the pump was correctly selected.
In dynamic or system head, liquid flowing through piping and fittings is subjected to 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 pump and piping manufacturers.
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 and heat exchangers, making the published numbers somewhat inaccurate.
A general rule of thumb is that friction loss in clean piping will vary, with approximately 90 percent of the square of the change in flow in the piping and 100 percent of the square with the change of flow in the fittings and accessories. Calculate the change in flow by dividing the new flow by the old flow, then square the number.
For example, in the original application system, loss was a combination of the loss through the piping and loss through the fittings for a total of 100 feet at 200 gpm. When the flow is increased to 300 gpm, the system head changed to 208.13 feet. This change has to be added to the static and pressure heads to calculate the total head required for the new pump.
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 err in your calculations. The suction head will be negative if water is lifted from below ground or pumped from a vacuum. It will be positive if water is pumped 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 2. 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 BEP as possible. The BEP will fall somewhere between 80 percent and 85 percent of the shut-off head (maximum head).
The manufacturer generated these curves at a specific number of revolutions per minute (rpm). The curves have to be adjusted to match the actual pump speed. Put a tachometer on the running motor and record the rpm difference between the pump and the speed shown on the pump manufacturer's published curve.
Positive displacement pumps
have a different shaped curve. In one system, the head remains a constant as the capacity varies. This is a typical application for a boiler feed pump that supplies a constant pressure boiler with a varying steam demand.
In the second system, the entire head is system head, so it will vary with the capacity. Look for this type of curve in the following applications:
a circulating hot or cold water heating/ cooling system
pumping to a non-pressurized tank, a long distance from the source with little to no elevation involved.
Selecting the Proper Pump
Once the pump manufacturer has a clear idea of the shape of your system curve, and the head and capacity numbers needed, the proper centrifugal pump can be selected. The shape of the curve will be largely determined by the specific speed number of the impeller.
Other items that can be selected include the impeller diameter, impeller width and pump rpm. The manufacturer also has the option of series or parallel operation and the possibility of using a multi-stage pump to satisfy your needs.
Most pumps are incorrectly chosen because of the desire to offer the customer the lowest possible price. A robust pump is still your best protection against premature seal and bearing failure when the pump operates off of its BEF.
Keep the following in mind as you select your pump:
A centrifugal pump will pump where the pump curve intersects the system curve. This may bear no relationship to the BEP or to your desire for the pump to perform a specific task.
The further off the BEP you go, the more robust the pump you will need. This is especially true if you have replaced the packing with a mechanical seal and no longer have the packing to act as a support bearing when the shaft deflects. Shaft deflection is always a major problem at start-up.
When you connect pumps in parallel, add the capacities together. The capacity of a pump is determined by the impeller width and rpm. The head of a centrifugal pump is determined by the impeller diameter and rpm. If the heads are different, the stronger pump will throttle the weaker one, so the impeller diameters and rpms must be the same if you connect pumps in parallel. Check the rpms on these pumps if you experience any difficulties.
If you connect the pumps in series, the heads will combine, so the capacities must be the same or one of them will probably cavitate. You could also experience pump "burnout" if you operate too far to the right of the BEP.
When you vary the speed of a centrifugal pump, the BEP comes down at an angle. The effect is almost the same as changing the diameter of the impeller, which means the variable speed motor will work best on a system curve that is exponential. Unfortunately, most process and boiler feed pump system curves are not exponential.
Pump curves are based on a speed of 1750, 3500, 1450, or 2900 rpm. Electric induction motors seldom run at these speeds because of "slip." A 2 to 5 percent slip is normal in these pumps, with the slip directly related to the price of the motor.
Pump selection is not always easy. Do not depend upon the knowledge of a pump supplier to select the correct pump for you. In many cases, the company is prepared to sell the pump at cost to get the spare parts business.
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 oversized.
By: Bill McNally
Bill McNally operates the McNally Institute, Clearwater, FL, a resource for the pump and mechanical seal industry.