Continued from part 1…..Click here to read the part-1……….Centrifugal Pumps (Fig. 4):
A centrifugal pump is one of the simplest pieces of equipment in any process plant. The figure shows how this type of pump operates:
Liquid is forced into an impeller either by atmospheric pressure, or in case of a jet pump by artificial pressure.
The vanes of impeller pass kinetic energy to the liquid, thereby causing the liquid to rotate. The liquid leaves the impeller at high velocity.
The impeller is surrounded by a volute casing or in case of a turbine pump a stationary diffuser ring. The volute or stationary diffuser ring converts the kinetic energy into pressure energy.
A centrifugal pump has two main components. First, a rotating component comprised of an impeller and a shaft . And secondly, a stationary component comprised of a casing, casing cover, and bearings.
An impeller is a circular metallic disc with a built-in passage for the flow of fluid. Impellers are generally made of bronze, polycarbonate, cast iron or stainless steel, but other materials are also used.
The number of impellers determines the number of stages of the pump. A single stage pump has one impeller and is best suited for low head (= pressure)
Impellers can be classified on the basis of (which will determine their use):
- Major direction of flow from the rotation axis
- Suction type: single suction and double suction
- Shape or mechanical construction: Closed impellers have vanes enclosed by shrouds; Open and semi-open impellers; Vortex pump impellers. Fig. 4 shows an open type impeller and a closed type impeller
The shaft transfers the torque from the motor to the impeller during the startup and operation of the pump.
Casings have two functions
- The main function of casing is to enclose the impeller at suction and delivery ends and thereby form a pressure vessel.
- A second function of casing is to provide a supporting and bearing medium for the shaft and impeller.
There are two types of casings
- Volute casing (Fig. 5-A) has impellers that are fitted inside the casings. One of the main purposes is to help balance the hydraulic pressure on the shaft of the pump.
- Circular casing has stationary diffusion vanes surrounding the impeller periphery that convert speed into pressure energy. These casings are mostly used for multi-stage pumps. The casings can be designed as solid casing (one fabricated piece) or split casing (two or more parts together)
Assessment of pumps:
The work performed by a pump is a function of the total head and of the weight of the liquid pumped in a given time period. Pump shaft power (Ps) is the actual horsepower delivered to the pump shaft, and can be calculated as follows:
Pump shaft power Ps = Hydraulic power hp / Pump efficiency ηpump
or Pump efficiency ηpump = Hydraulic power / Pump shaft power
Pump output, water horsepower or hydraulic horsepower (hp) is the liquid horsepower delivered by the pump, and can be calculated as follows:
Hydraulic power hp = Q (m3/s) x (hd – hs in m) x ρ (kg/m3) x g (m/s2) / 1000
Q = flow rate
hd = discharge head
hs = suction head
ρ = density of the fluid
g = acceleration due to gravity
In practice, it is more difficult to assess pump performance. Some important reasons are:
Absence of pump specification data: Pump specification data (see Worksheet 1 in section 6) are required to assess the pump performance. Most companies do not keep original equipment manufacturer (OEM) documents that provide these data. In these cases, the percentage pump loading for a pump flow or head cannot be estimated satisfactorily.
Difficulty in flow measurement: It is difficult to measure the actual flow. The methods are used to estimate the flow. In most cases the flow rate is calculated based on type of fluid, head and pipe size etc, but the calculated figure may not be accurate. Another method is to divide the tank volume by the time it takes for the pump to fill the tank. This method can, however, only be applied if one pump is in operation and if the discharge valve of the tank is closed. The most sophisticated, accurate and least time consuming way to measure the pump flow is by measurement with an ultrasonic flow meter.
Improper calibration of pressure gauges and measuring instruments: Proper calibration of all pressure gauges at suction and discharge lines and other power measuring instruments is important to obtain accurate measurements. But calibration has not always been carried out. Sometimes correction factors are used when gauges and instruments are not properly calibrated. Both will lead to incorrect performance assessment of pumps.
Energy efficiency opportunities:
This section includes the factors affecting pump performance and areas of energy conservation. The main areas for energy conservation include:
- Selecting the right pump
- Controlling the flow rate by speed variation
- Pumps in parallel to meet varying demand
- Eliminating flow control valve
- Eliminating by-pass control
- Start/stop control of pump
- Impeller trimming
- Selecting the Right Pump:
Fig. 5-B shows a typical vendor-supplied pump performance curves for a centrifugal pump where clear water is the pumping liquid.
In selecting the pump, suppliers try to match the system curve supplied by the user with a pump curve that satisfies these needs as closely as possible.
The operating point is where the system curve and pump performance curve intersect (as explained in the introduction)
The Best Efficiency Point (BEP) is the pumping capacity at maximum impeller diameter, in other words, at which the efficiency of the pump is highest. All points to the right or left of the BEP have a lower efficiency.
The BEP is affected when the selected pump is oversized. The reason is that the flow of oversized pumps must be controlled with different methods, such as a throttle valve or a by-pass line. These provide additional resistance by increasing the friction. As a result the system curve shifts to the left and intersects the pump curve at another point. The BEP is now also lower. In other words, the pump efficiency is reduced because the output flow is reduced but power consumption is not.
Inefficiencies of oversized pumps can be overcome by, for example, the installation of VSDs, two-speed drives, lower rpm, smaller impeller or trimmed impeller
- Controlling Flow: speed variation-
A centrifugal pump’s rotating impeller generates head. The impeller’s peripheral velocity is directly related to shaft rotational speed. Therefore varying the rotational speed has a direct effect on the performance of the pump.
The pump performance parameters (flow rate, head, power) will change with varying rotating speeds. To safely control a pump at different speeds it is therefore important to understand the relationships between the two. The equations that explain these relationships are known as the “Affinity Laws”:
- Flow rate (Q) is proportional to the rotating speed (N)
- Head (H) is proportional to the square of the rotating speed
- Power (P) is proportional to the cube of the rotating speed
As can be seen from the above laws, doubling the rotating speed of the centrifugal pump will increase the power consumption by 8 times. Conversely a small reduction in speed will result in a very large reduction in power consumption. This forms the basis for energy conservation in centrifugal pumps with varying flow requirements.
- Controlling the pump speed is the most efficient way to control the flow, because when the pump’s speed is reduced, the power consumption is also reduced.
- The most commonly used method to reduce pump speed is Variable Speed Drive (VSD).
- VSDs allow pump speed adjustments over a continuous range, avoiding the need to jump from speed to speed as with multiple-speed pumps. VSDs control pump speeds use two types of systems:
- Mechanical VSDs include hydraulic clutches, fluid couplings, and adjustable belts and pulleys.
- Electrical VSDs include eddy current clutches, wound-rotor motor controllers, and variable frequency drives (VFDs). VFDs are the most popular and adjust the electrical frequency of the power supplied to a motor to change the motor’s rotational speed.
- The major advantages of VSD application in addition to energy saving are:
- Improved process control because VSDs can correct small variations in flow more quickly.
- Improved system reliability because wear of pumps, bearings and seals is reduced.
- Reduction of capital & maintenance cost because control valves, by-pass lines, and conventional starters are no longer needed.
- Soft starter capability: VSDs allow the motor the motor to have a lower startup current.
- Parallel Pumps for Varying Demand:
Operating two or more pumps in parallel and turning some off when the demand is lower, can result in significant energy savings. Pumps providing different flow rates can be used.
Parallel pumps are an option when the static head is more than fifty percent of the total head.
The Fig. 5 shows
- the pump curve for a single pump, two pumps operating in parallel and three pumps operating in parallel.
- that the system curve normally does not change by running pumps in parallel.
- that flow rate is lower than the sum of the flow rates of the different pumps.
- Eliminating Flow Control Valve:
- Another method to control the flow by closing or opening the discharge valve (this is also known as “throttling” the valves).
- While this method reduces the flow, disadvantages are
- It does not reduce the power consumed, as the total head (static head) increases. The Fig. 6 shows how the system curve moves upwards and to the left when a discharge valve is half closed.
- increases vibration and corrosion and thereby increases maintenance costs of pumps and potentially reduces their lifetimes
- VSDs are a better solution from an energy efficiency perspective.
- Eliminating By-pass Control:
The flow can also be reduced by installing a by-pass control system, in which the discharge of the pump is divided into two flows going into two separate pipelines. One of the pipelines delivers the fluid to the delivery point, while the second pipeline returns the fluid to the source. In other words, part of the fluid is pumped around for no reason, and thus is an energy wastage. This option should therefore be avoided.
- Start / Stop Control of Pump:
A simple and reasonable energy efficient way to reduce the flow rate is by starting and stopping the pump, provided that this does not happen to frequently. An example where this option can be applied, is when a pump is used to fill a storage tank from which the fluid flows to the process at a steady rate. In this system, controllers are installed at the minimum and maximum level inside the tank to start and stop the pump. Some companies use this method also to avoid lower the maximum demand (i.e. by pumping at non-peak hours).
- Impeller Trimming:
- Changing the impeller diameter gives a proportional change in the impeller’s peripheral velocity
- Changing the impeller diameter is an energy efficient way to control the pump flow rate. However, for this option, the following should be considered:
- This option cannot be used where varying flow patterns exist.
- The impeller should not be trimmed more than 25% of the original impeller size, otherwise it leads to vibration due to cavitation and therefore decrease the pump efficiency.
- The balance of the pump has to been maintained, i.e. the impeller trimming should be the same on all sides.
- Changing the impeller itself is a better option than trimming the impeller, but is also more expensive and sometimes the smaller impeller is too small.
- Figure 6 illustrates the effect of impeller diameter reduction on centrifugal pump performance.
The figure 6 illustrates the effect of impeller diameter reduction on centrifugal pump performance.
- With the original impeller diameter the flow is higher
- With the trimmer impeller the flow is lower
|Parameter||Change control valve||Trim impeller||VFD|
|Impeller diameter||430 mm||375 mm||430 mm|
|Pump head||71.7 m||42 m||34.5 m|
|Rate of flow||80 m3/hr||80 m3/hr||80 m3/hr|
|Power consumed||23.1 kW||14 kW||11.6 kW|
The above table compares three options to improve energy efficiency in pumps:
- changing the control valve, trim the impeller and variable frequency drive.
- The VFD clearly reduces power most, but a disadvantage is the high costs of VFDs.
- Changing the control valves should at all times be avoided because it reduces the flow but not the power consumption and may increase pump maintenance costs.