For several decades it has been recognised that deposition on the surfaces of steam turbine blades during operation can result in significant loss in thermal performance and, in some cases, a large reduction in the steam swallowing capacity. One principal cause of deposit fouling on HP turbines is copper, although other elements, for example silicon, can also be problematic. Copper is initially corroded from condenser and feedheater tubes by the water which then contaminates the inner surfaces of the boiler as the water is evaporated. The steam from the boiler becomes contaminated with copper oxides as a result of the copper fouling inside the boiler. The solubility of copper compounds in steam is a strong function of pressure. As the steam expands through the turbine and pressure reduces, the copper oxides deposit out onto the blade surfaces, roughening them and resulting in loss of performance .
A test facility is being developed by Durham University to allow copper deposition under real steam conditions to be investigated in a laboratory environment. The facility consists of a non-flow ‘box test’ type arrangement. The initial experimental arrangement consisted of a single reactor vessel. Superheated steam at typical boiler conditions was created in the reactor vessel and held at these conditions for several 10’s of hours. The reactor vessel also contains a copper sample and a sample of target blade material. During this first stage of the test, copper dissolves into the steam, contaminating it with copper metal and its oxides. In the second stage of the test the steam conditions are quickly reduced to lower pressure values that are representative of the latter stages of a typical HP turbine cylinder from a large fossil-fired unit. The reduced solubility of copper in steam at the lower pressure results in copper depositing out onto the sample of blade material.
Conditions are held constant again for 10’s of hours during this second stage of the test, to allow sufficient time for a reasonable amount of deposition to occur. The reactor vessel is then cooled and the sample of blade material removed for analysis.
Results from some initial testing using the single reactor vessel arrangement are described in this paper. The results demonstrate that it is possible to create a copper transport and deposition process under representative steam conditions using a test facility of this type.
It was found to be difficult to control, accurately, the single reactor vessel tests, particularly during the second phase when the steam conditions were reduced. A revised test set-up is proposed consisting of two reactor vessels, in order to improve the operability of the facility.
The ultimate aim of the work is to use this facility to investigate, systematically, deposition under different steam conditions and to produce a physically based model of the process. The facility will be validated by comparing test results with deposit samples taken from real turbines that experience copper fouling during operation.