Wherever we go, whether an industry, a commercial building or even a residence centrifugal pump is going to be the most common equipment all over. We can’t imagine any facility without a pump. It is there, for simple water supply application to even a complex structure like HVAC and even process heating and cooling.
Like every other electrical moving equipment pump is also meant to follow a definite path during its performance. It will not be exaggerated if we say it is the most systematic electrical moving equipment. If it is designed to follow a specific path then why it is not running at its best efficiency? Studies shows, in an air-conditioned commercial building about 20% electricity is consumed by pumps. The important thing here is that most of these pumps are working with pump efficiencies not more than 55%, whereas these pumps are designed to run with 70% or more efficiency. It means the consumption of pump is nearly 15% higher than what it should be. Studies shows that most of the people/facility operators are even not aware about the technical reasons behind this. If we ask people about the reasons of these lower efficiency operation we will get a very common answer “these are very old pumps”. Is it really so? According to manual on Operation And Maintenance Of Water Supply Systems prepared by Central Public Health and Environmental Engineering Organization, Ministry of Urban Development New Delhi the normal life of a centrifugal pump is 15 to 20 years. If we are talking about commercial buildings most of the applications are clear water pumping. It means there is hardly any wear and tear to the impeller or casing due to fluid characteristics. The major reason of these inefficiencies is less knowledge about the pump’s characteristics and sizing.
Figure 1 shows a typical characteristic curve of a centrifugal pump.
The simple interpretation of this curve is,
1. As flow increases head reduces and vice versa.
2. Pump can deliver a flow/head higher or lower than mentioned over its nameplate (thus nameplate parameters are not rated parameters. It is duty point to achieve best efficiency).
3. Unless and until there is wear and tear of pump parts, speed change or change in system, pump is meant to follow these characteristics.
If operating conditions matches the duty point conditions then pump will run at its best efficiency. Now let’s discuss why the pumps do not work at their best efficiency?
People might understood the pump curve/characteristic and select a good pump but they mostly forget about the system. What is system? System is the flow network consisting of pipe, bends, joints and valves. System also has its own characteristics. Pump manufacturers do not provide system characteristics. It is to be taken into account by the pump user. Figure 2 shows a system curve.
It is having static head and friction head. Static head is the effective straight height or it can even be the pressure drop across process/utility (like chiller, plate heat exchanger) against which liquid is to be pumped. Friction head is the frictional loss occurring while the liquid is moving through the piping network.
What flow and head the pump will deliver is decided by the system. The pump just has to overcome the system resistance (or say head) to deliver the required flow. System curve will help user to identify respective system head for his/ her required flow. For example, if user want a flow 80 m3/hr, he/she has to maintain a head of 32 m (see figure 2). Thus, the point of intersection of pump curve and system curve will decide the operating point of pump. It means if you have a pump of 80 m3/hr flow and 29-meter head but if your system head is more or less than this then the pump is not going to deliver 80 m3/hr at all. Let’s consider following case.
User need a flow of 80 m3/hr. As per system (see figure 2) head required for this flow is 32 m. If user has selected a pump of duty point 80 m3/hr and 29 m head; then the pump will be operated as shown in Figure 3.
Blue point indicates the Duty Point of pump and green point indicates the Operating Point. User selected a pump of desired flow of 80 m3/hr. However, he neglected the system requirements and wrongly selected the head. Due to which the instead of working on duty point, now pump is operating with some different flow and head which is not giving the best efficiency.
Thus, the system head is going to decide how the pump is going to operate, no matter what specifications we have selected for pump. Therefore, before selecting a pump, we should know what the system head is. How many of us are aware about our system head? Some people says my pump is designed for 100 m3/hr. But it is delivering only 60 m3/hr it means it is 60% efficient only and this may be due aging or bad selection. But it’s not the case. It is only because we ignore the system head.
We have seen that in order to select a pump we should know the system curve or at least system requirement. Then how to draw a system curve? If we have the piping design software and exact piping details (like diameter, length, height, number of bends, valves etc), then it is easy to design a system curve. Exact piping details can only be available if the project is under design or construction phase or the system is too simple like shown in the figure 4.
For a simple system as shows in figure-4, it is very easy to physically measure the static head, pipe diameter and pipe length also.
But if the system is a complex one, e.g. an old HVAC system, then it is very difficult to physically measure the pipe length. If you wish to replace the old pump in such system with a new pump, then it is very difficult to identify the system head requirement. Estimation may end up into wrong pump selection and loosing on the pumping efficiency.
With some experiment and mathematics, it is possible to calculate the system head for a particular flow requirement. The experiment and mathematical calculations and explained further.
Calculating System Requirements for an Existing Complex System
Let’s consider that the existing pump in a complex HVAC system is not working at its best efficiency and therefore, it is intended to replace this pump with a new efficient pump. The flow requirement is 500 m3/hr. In order to select an efficient pump, it is important to know the system head for 500 m3/hr flow. The system head can be evaluated by performing following experiment.
Step 1: Install new calibrated pressure gauges on the suction and discharge side of pump. Step 2: Install an ultrasonic water flow meter at the discharge of pump to measure the velocity and flow.
Step 3: Start the pumps with discharge valve 100% open. Measure the suction head, discharge head and discharge water velocity.
Step 4: Calculate total head for Step 3.
Step 5: Throttle the discharge valve for 50%. Measure the suction head, discharge head and discharge water velocity.
Step 6: Calculate total head for Step 5.
HT = HS + HF (equitation 1)
Where, HT : Total head
HS : Suction Head
HF : Frictional Head
Frictional head can be calculated using following formula HF = (F) x (L/D) x (V2/2g) (equitation 2)
Where, F : Friction factor related to the roughness inside the pipe
L : Length of the pipe
D : Diameter of the pipe
V : Average liquid velocity in the pipe
g : Gravitation acceleration constant
In equation 2 most of the parameters are constant except velocity. Thus equation 2 can be rewritten as,
HF = K x V2 (equitation 3) Where, K is constant.
So, the equation 1 can be rewritten as HT = HS + K x V2 (equitation 4)
Condition 1: Discharge Valve 100% open
As per the experiment conditions
HT1 = HS + K x V12 (equitation 5)
In equation 5, HT1 and V1 are known factors.
Condition 2: Discharge Valve 50% open
As per the experiment conditions
HT2 = HS + (1/0.5) x K x V22 (equitation 6)
In equation 6, HT2, and V2 are known factors. Because of 50% throttling the discharge diameter of pipe will be half of earlier diameter.
If we subtract equation 5 from equation 6 then we will get “K”.
By using value of “K” in equation 5, “HS” can be calculated.
Once “HS” and “K” are known, we can calculate system total head for any flow required or even can plot a system curve (using equation 4). Using values of “HS”, “K” and the required flow of 500 m3/hr total head requirement of the system can be calculated and then the pump can be selected. As this head, pump for 500 m3/ hr will match with the system requirement and will operate at its best efficiency.
This method gives better results than estimations and helps to select a pump which would operate at its best efficiency
