Sequoia’s LISST-Infinite and LISST-Hydro products generate data on sediment concentrations in water entering turbines. However, there has been no guidance for plan operators on what levels are dangerous, or at what level the operator should shut down turbines. At a conference of the IAHR in Grenoble, France, 4-8 July 2016, Sequoia president Yogi Agrawal proposed a simple idea. According to this idea, a cost of abrasion is calculated at any instant. This quantity is the cost of repair of a turbine multiplied by the ratio of current sand transport rate (tons/hr) divided by the total sand transport after which repair is necessary. The shutdown is recommended when this cost exceeds revenue generation. The same idea was proposed simultaneously by Prof. Robert Boes of Switzerland.

The abstract for the paper presented at the conference is below.  The full text of the paper as a PDF can be found here.

Abstract:

It is known that sediments cause abrasive damage. It is recognized that, generally, large grains do more damage than
smaller ones. Grain hardness and water velocity plays a role, and the type of turbine plays a role, e.g. Peltons suffer
more severe damage than Francis. A few formulations relating rate of abrasion and these 3 parameters have been
proposed; verification and broad acceptance remains in progress.
Given that grain size is important, monitoring technologies must measure size-specific concentration of sediments
running through turbines at any time. So far, only one technology has emerged that can do this with accuracy. This
technology is laser diffraction, embodied in the LISST-Hydro and LISST-Infinite instruments (Sequoia Scientific, Inc.).
Installations in Latin America and in the Himalayas have been operating for a few years. The technology has been
proven, though in one case in the Nepal Himalayas, radically extreme conditions produced damage to instrumentation
itself (clogged sample pump). In all cases, rapid rise and slow decline of sediment concentrations have been found.
In one case, the turbine operator provided numbers on turbine repair cost as related to volume of sediment passage
between maintenance stoppages. As our first key new contribution, we propose an objective strategy to set alarms that
guide turbine shut-down only when rate of loss of revenue is exceeded by rate of cost of repair.
Finally, we shall introduce a new high-frequency acoustic backscatter instrument, LISST-ABS for monitoring sediments
coming into turbines. Though not as accurate or size-sorting as laser diffraction, it is none-the-less far superior to optical
turbidity type sensors used in some countries. It’s response (output per unit mass concentration) is relatively flat over
30-400 microns showing only 30% variation (and as square-root of diameter for larger sizes), in contrast with a loss
of sensitivity to large grains by 1400% by turbidity sensors over the same size range (and as one over diameter). The
LISST-ABS is more economical, and can serve the needs of small power plants where cost is an issue.

The full text of the paper as a PDF can be found here.