1.3.3 Pump selection
Pump selection will require the following:
the total head or pressure against which it must operate,
the desired flow rate,
the suction lift, and
characteristics of the fluid.
The total head, suction lift, and flow rate are dependent upon the piping system and the pump's characteristics. The piping system and the pump interact to determine the operating point of the pump — flow rate and pressure. The pump cannot independently control these parameters. As the flow rate is increased, the work to move each unit of liquid or tot dynamic head the pump must produce increases as shown in curve A in Fig. 1-4.
Figure 1-4. Pressure-flow rate (for a typical pipeline system and a typical centrifugal pump characteristics)
Pump manufacturers usually test a pump in the t place of manufacture and produce the test curve or set of curves. During a bench test, the pump is operated at constant speed, and the values flow Q and head H are determined for the various operating points. To allot a comparison between the various pump types, these measurements are carried out using only water as liquid. With these operating points a Q/H curve is then drawn connecting the points on the graph. Once the flow rate Q is defined and the delivery head H is calculated the operating point of the system can be determined.
When a pump is installed into a real system, that system can be expressed as a curvi I in the same format and on the same axes as the pump curve. The system curve is derive i by hydraulics calculation. When plotted on the same chart, there is normally only one point where the lines cross. This should be the actual operating point. If either the pump or system curves are changed, the operating point moves accordingly. If the lines cross at a shallow angle, or even worse, at two separate points, the operating point may be uncertain. The practical result is unstable operation, which must be solved by physically altering pump or system, thus changing the relevant curve shape.
The required operating point is obtained by adapting the pump to the specified operating conditions.
Figure 1-5. Viscosity characteristics for a typical non-Newtonian heavy crude oil
This can be done by the following actions:
throttling the flow
correcting the diameter of the impeller
adjusting the speed of the drive
Of importance is the fluid characteristics when selecting a pump. The type of fluid to be transported affects pipeline design and facility requirements and configuration, including pumping systems. The properties of fluid to be transported have significant impact on pipeline system design. For liquid lines, the liquid properties which affect pipeline design are:
viscosity,
density, and
specific heat.
These properties are influenced by the pressure and temperature. Temperature considerably affects the above properties in liquid pipelines and, specifically, crude oil or heavy crude oil pipelines. Viscosity is also affected by the liquids shear rate. Liquids that have a constant shear rate with respect to shear stress at any given temperature are termed non-Newtonian fluids, and the viscosity is a function of temperature only. It increases with decreasing temperature. Non-Newtonian fluids have viscosities which are not only a function of temperature but also of shear rate (Fig. 1-5) [10]. Non-Newtonian fluids are classified as tixotropic and rehopatric fluids, which are time-dependent. Others are pseudo-plastic-, yield-pseudoplastic dilatants-, and Bingham fluids and time-independent, Fig. 1-6.
Figure 1-6. Flaw curves illustrating Shear Characteristics for various products
As can be seen from Fig. 1-6, when transporting non-Newtonian products through the pipeline, a definite minimum stress must be applied to the product (to reach it yield point) before any flow of the product take place.
When the product being transported is always a low viscosity liquid such as water, gasoline, diesel oil, or very light crude oil, centrifugal pumps are cost-effective, reliable and efficient. However, as the liquid viscosity increases, the frictional losses within a centrifugal pump quickly reduce pumping efficiency dramatically (Fig. 1-7). For this reason, rotary, positive displacement pumps are often used when products such as heavy crude oil, bunker fuels (no. 6 fuel oil), low sulfur fuels, asphalt, Orimulsiona (a manufactured boiler fuel emulsion of 30% water and 70% bitumen), and similar products are to be transported.
Pumping capacity and power requirement must therefore consider the fluid condition over time because pumping operation will be influenced by any changes in liquid viscosity and density that may occur. Such change can affect
net positive suction head,
priming aflexibility,
Fluid condition and corrosion,
useful life,
maintenance,
quantity pumped,
pumping head,
power source, and transportation economics
|
Figure 1-7. Rotary versus centrifugal efficiency (light versus heavy products, 100 US GPM, 75psig discharge) |
Chapter 2. DESIGN AND OPERATION OF CENTRIFUGAL PUMPS_