Page 197 - Shale Shakers Drilling Fluid Systems

180 SHALE SHAKERS AND DRILLING FLUID SYSTEMS

diameter of the impeller at a particular rotation flow when it reaches the height of the water. (The
speed controls the head produced by the pump last several feet of movement of weighted drilling
and not the flow rate. fluid may be much slower than the water if the
As an example of another common misconcep- horsepower of the pump is very low—but the fluid
tion, consider the following scenario: A centrifu- will eventually reach the same height.)
gal pump is connected to a 200-foot length of cas- When liquid leaves the impeller of a centrifu-
ing vertically secured next to a derrick. The top gal pump, and thereafter the pump casing, its ve-
end is open and the lower end is connected to a locity is roughly the same as that of the impeller
centrifugal pump. A tank full of water is connected vane tips. In addition to providing a containing
to the pump suction. When the pump is turned on, chamber, the casing acts as a guide, directing the
water stands 120 feet above the water level in the fluid to the discharge. At the discharge of a cen-
suction tank. Water is drained from the system trifugal pump, the kinetic energy (velocity energy)
and replaced with a drilling fluid that is twice as creates a hydrostatic pressure, or head. Centrifu-
dense as water (16.7 ppg). gal pumps produce a constant head depending on
The misconception is that the heavy-weight the tip velocity of the impeller,
drilling fluid will not rise as high as the level of Centrifugal pumps are generally described by
the water. In reality, the dense fluid will rise to the the size of their suction and discharge pipe flange,
same height as the water. Head is defined as the For example, a 3 x 4 pump has a 3-inch diameter
height of the fluid column. Constant head means discharge pipe flange and a 4-inch diameter suction
that all fluids, regardless of density, will rise to the pipe flange. Most pumps used in oil fields can be
same height. Additionally, the horsepower of the outfitted with different impeller diameters. The head
pump does not have an impact on the height of generated by a particular diameter impeller spinning
the heavy-weight drilling fluid. For example, if a at a particular rotary speed, will be the same no
motor is capable of lifting water to a height of 120 matter what size housing contains the impeller,
feet, it has sufficient horsepower to lift the heavier Figure 11-1 shows a head curve for several dif-
weight drilling fluid. When a drilling fluid is flow- ferent pump sizes with the same impeller diameter,
ing, the required horsepower increases with fluid Each curve shows the head decreasing as the flow
density. However, in this case the fluid ceases to rate increases. This is caused by the internal friction

FIGURE 11-1