Volumetric pumps also known as positive displacement pumps are composed by an expanding cavity with a suction valve and a discharge valve.
Generally speaking, the working principle is based on a liquid flowing into an expanding cavity when the suction valve is open; then, when the suction valve is closed and the discharge valve is open, the liquid flows out by forcing the cavity to collapse.
The volume is constant given each cycle of operation.
Positive Displacement Pumps, unlike a centrifugal pump, will produce the same flow at a given speed (RPM) no matter whatthe discharge pressure.
Volumetric pumps operate at constant flow.
The positive displacement pumps can be classified in two main classes:
Typical reciprocating pumps are
Plunger or piston pumps are composed by a cylinder with a reciprocating piston inside.
In the head of the cylinder are mounted the suction and discharge valves.
In the suction stroke the piston retracts and the suction valves open causing suction of fluid into the cylinder.
During the forward stroke the piston pushes the liquid out through the discharge valve.
In the simple case of a single axial cylinder, the fluid flow varies between maximum flow rate when the plunger moves through the middle positions, and zero flow when the piston is in the end positions.
Vibration and “water hammers” due to pressure fluctuation and intermittency can represent a serious problem.
In general these problems are compensated by using two or more cylinders working in opposite phase to each other.
DIAPHRAGM PUMPS AND ROTARY PISTON PUMPS
In diaphragm pumps the piston is used to flex a diaphragm in the pumping cylinder.
Diaphragm valves are used to pump hazardous and toxic fluids.
The difference between piston pumps as compared to rotary piston pumps is the actual mechanism used to transfer the fluid.
Rotary piston pumps typically have an internal rotating mechanism that moves the piston.
Lobe pumps are used in a variety of food processes.
They are quite diffused because they provide high sanitary qualities, high efficiency, reliability, corrosion resistance, and good clean-in-place and sterilize-in-place (CIP/SIP) characteristics.
These pumps are designed in several lobe configurations including single, bi-wing, tri-lobe and multi-lobe.
Rotary lobe pumps are non-contacting and have large pumping chambers, allowing them to handle solids without any damage.
They are also used to handle slurries, pastes, and a wide variety of other liquids.
If wetted, they present self-priming characteristics.
A gentle pumping action minimizes product degradation.
Lobe pumps can work with reversible flows and can operate dry for long periods of time.
Flow is relatively independent of changes in process pressure, so that output is constant and continuous without pressure fluctuation.
Rotary lobe pumps range from industrial designs to sanitary designs.
Different sanitary designs depend on the service and specific sanitary requirements
Lobe Pumps: main features.
The performance of a centrifugal pump can be shown graphically introducing a characteristic curve.
In a typical characteristic curve the total dynamic head, efficiency, brake horsepower, and net positive suction head are shown and plotted over the capacity range of the pump.
Figure below shows that the head curve for a radial flow pump is relatively flat and that the head decreases gradually as the flow increases.
The brake horsepower increases gradually over the flow range with the maximum normally at the point of maximum flow.
The affinity laws express the mathematical relationship between the several variables involved in pump performance.
They apply to all types of centrifugal and axial flow pumps.
They can be assumed as follows:
With impeller diameter D held constant: (img 1)
Q = Capacity
H = Total Head, Feet
BHP = Brake Horsepower
N = Pump Speed, RPM
With speed N held constant: (img 2)
When the performance (Q1, H1, & BHP1) is known at some particular speed (N1) or diameter (D1), the formulas can be used to estimate the performance (Q2, H2, & BHP2) at some other speed (N2) or diameter (D2). The efficiency remains nearly constant for speed changes and for small changes in impeller diameter.
For a specified impeller diameter and speed, a centrifugal pump has a fixed and predictable performance curve.
The point where the pump operates on its curve is dependent upon the characteristics of the system in which it is operating, commonly called the System Head Curve, or the relationship between flow and hydraulic losses in a system.
This representation is in a graphic form and, since friction losses vary as a square of the flow rate, the system curve is parabolic in shape.
By plotting the system head curve and pump curve together, it can be determined:
NO STATIC HEAD – ALL FRICTION
If the levels in the suction and discharge are the same, there is no static head and, therefore, the system curve starts at zero flow and zero head and its shape is determined solely from pipeline losses (bends, valves, section restriction, filters, etc…).
The point of operation is at the intersection of the system head curve and the pump curve, while the flow rate may be modulated by using a throttling valve.
CASE 1: POSITIVE STATIC HEAD
In this case positive static head is present and it must be considered together with friction losses through the system including all bends and valves. The shape of the system curve is again parabolic.
This static head does not affect the shape of the system curve, but it does indicate the required head of the system curve at zero flow rate.
The operating point is the intersection of the system curve and pump curve. The flow rate can be modulated by throttling the discharge valve.
NEGATIVE (GRAVITY) HEAD
In the illustration below, a certain flow rate will occur by gravity head alone. But to obtain higher flows, a pump is required to overcome the pipe friction losses in excess of “H” – the head of the suction above the level of the discharge. In other words, the system curve is plotted exactly as for any other case involving a static head and friction head, except the static head is now negative. The system curve begins at a negative value and shows the limited flow rate obtained by gravity alone. More capacity requires extra work.
MOSTLY LIFT- LITTLE FRICTION HEAD
The system head curve in the illustration below starts at the static head “H” and zero flow. Since the friction losses are relatively small (possibly due to the large diameter pipe), the system curve is “flat”. In this case. the pump is required to overcome the comparatively large static head before it will deliver any flow at all.
The requirements for the selection of pumps can be summarized infour main points:
To define the process requirements, necessary information to choose a pump in verifying the needs of production.These include:
More in detail, process requirements must be defined not only on the basis of present needs, but also on future ones.
For example by calculating how many liters per minute of product are required to empty a tank in a certain time, or those necessary to supply the production lines.
The scope of operation is related to production, while the maximum should be calculated taking into account the possible expansion of production rate or particular operations which need to be carried out quickly.
The pressure operating conditions are equally important.
One must take into account the design pressure of the pump and piping, the product sensitivity to pressure, and energy requirements.The pipes used in the food industry are generally assembled with welded joints or clamps for quick disassembly, cleaning and connection to the pumps.The maximum pressure in the pipes used in the food varies from 5 to 300 atm in relation to the type of connection, temperature of the product, installation and maintenance procedure.
The characteristics of the product strongly influence the choice ofpump, and they can be summarized in:
Moreover, if we known the name of the product, we often known all the remaining features, while in other cases it is necessary to conduct laboratory tests.
The temperature is a very important parameter, since the rheological properties change with it.Also, some pumps can operate only under certain conditions of temperature, since the thermal expansion of parts could result in damage to the pump due to the tolerances required in moving parts.
The viscosity of the product provides an important indicator of resistance to mass transport. The size of any solids in the product are needed to understand whether there may be danger of damaging the pump or the product itself, while its chemical activity is important for choosing the material for parts in contact with the product.
The transport of goods or highly acidic solutions may require the use of special steel with low carbon content, such as 316L, or other materials that prevent corrosion.
Sensitivity to shear rfers to the possibility that the rheology of the product, when subjected to severe stress because of the interaction with the impeller, can vary significantly.
For vapor pressure we mean the pressure of an area above the liquid in equilibrium conditions.If the pressure in the pumping system is below the vapor pressure of the liquid, the so-called flashing occurs, or the sudden vaporization of the liquid.
The gas bubbles that form can also collapse when the pressure of the system increases, giving rise to the phenomenon of cavitation that can occur usually downstream of the impeller.
The operational requirements to consider when choosing a pump for aseptic operations can be summarized in the following points:
21. Liquid Mixing