Ball Harp
The Ball Harp test is the recommended test method for supporting fluids used in construction of diaphragm walls according to EN 1538. The Ball Harp consists of several glass and steel balls, each with different diameters. These balls are immersed in the supporting fluid at the same time. With a given density of the supporting fluid, each of these balls is assigned a different yield point. Balls whose yield points are smaller than the yield point of the suspension float. Balls with a yield point greater than the supporting fluid are immersed in it.
The balls of the Ball Harp are numbered 1 to 10 in the order of their increasing yield point. The yield point of the supporting fluid is thus between the yield point of the largest numbered ball that is still floating and the yield point of the smallest numbered ball that is immersed in the fluid.
The yield points of the standard ball set are specified for densities between 1.02 and 1.32 g/cm³ (851.23 and 909.65 lb/gal). Heavy ball sets are available to determine the yield point of higher density suspensions.
The Ball Harp is not only used in diaphragm wall construction. The Ball Harp also helps to adjust the composition of drilling fluids that block invasion and prevent formation damage during drilling operations.
Drilling fluid does not only reduces the friction between the drill pipe and the annulus by forming a lubricating film. It also supports the near wellbore region. If the formations pore size is small, it can be stabilized by generating a relatively thin outer filter cake. In the case of a very porose formation, the drilling fluid must penetrate the near wellbore region and formation damage can be avoided with the help of stagnation. Thus, the drilling mud must be mixed and prepared in such a way that it hardens within the pores of the formation. Only then sufficient supporting pressure can be applied in the annulus. The static yield point is the decisive benchmark. It is measured with the Ball Harp.
On the other hand, the yield point of the drilling mud must not be too high, otherwise the mud pump can no longer be operated. Since drilling fluids tend to be thinner than the support fluids in diaphragm wall construction, a special light ball set has been developed in recent years, with which low yield points can also be determined.
