SYSTEM CURVE 18-5
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 size pump.
To create a system curve we plot the desired capacities against the required head over the total anticipated operating range or window 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:
This is the vertical distance measured from the centerline 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 (BEP) if the pump was selected correctly.
If the vessel we are pumping to is pressurized, this pressure converted to head units, will have to be added to the static head. To convert pressure to head units use one of the following formulas:
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 any type of 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 charts you will find in the charts section of this CD 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” states 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:
At 200 gpm the piping resistance calculated from published charts (you can find these in the charts section) is seventy-five feet (75 Ft.). What will it be at 300 gpm?300 / 200 = (1.5)2 = 2.25 x 75 feet = 168.75 x 90% of the change = 151.88 feet of resistance head
In other words, when we went from 200 to 300 gallons per minute the piping resistance increased from 75 feet to 151.88 feet.
The loss through the fittings and hardware was calculated at 25 feet. What will the new loss be?
300 / 200 = (1.5)2 = 2.25 x 25 feet = 56.25 x 100% of the change = 56.25 new feet of head
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.
The manufacturer generated these curves at a specific rpm. 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”:
Surprisingly there are only a few system curve shapes that you will encounter.
Figure “D” describes the first one.
This is also a very common application in many process systems, or aboard a ship that is frequently changing speeds (answering bells).
Filling a tank from the top and varying the amount of liquid being pumped is the normal routine in most process plants. The curve will look like this first one if the majority of the head is either static or pressure head.
The second system curve is the ideal one. Figure “E” describes it:
Most tank circulating pumps have a single point rather than a system curve. A steady state, power-generating boiler is another example.
A steady state process pump operates at a single point also.
Filling tank cars is a typical application.
System curve “G” is a another curve. It is a combination of static, pressure and system heads.
If the capacity is below 20 gallons per minute (4,5 m3/hr) you really should be using a positive displacement pump in this application or a really robust centrifugal pump.
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 number 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:
- A centrifugal pump will pump where the pump curve intersects the system curve. This may bear no relationship to the best efficiency point (BEP), or your desire for the pump to perform a specific task.
- The further off the best efficiency point (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 you 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 are experiencing any difficulties.
- If you connect the pumps in series the heads will add together so the capacities must be the same or one of the pumps will cavitate. You could also have a problem operating too far to the right of the best efficiency point with a possible motor “burn out”.
- When you vary the speed of a centrifugal pump the affect is almost the same as changing the diameter of the impeller. This means that the variable speed motor will work best on a system curve that is exponential (Figure “F”). 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. You can estimate that a 2% to a 5% slip is normal in these pumps with the amount of slip directly related to the price of the motor.
- You should also keep in mind that if the motor is running at its best efficiency point that does not mean that the pump is running at its best efficiency point (BEP).
Do not trust piping diagrams to make your calculations. The actual system always differs from that shown on the diagram 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 diagram.
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 nobody 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. These gages will show you how the head is varying with changes in flow. The trouble with these recording devices is they tell you what the present pump is doing. They do not tell you what pump should be in the system.
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 a large discount to get the spare parts business. If you are purchasing pumps at too big a discount 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 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.
If you find that your pump curve dos not match your system curve, please keep in mind that you will have to change valve positioning, or modify the piping system. A VFD (variable frequency drive) changes only the pump cure