A combined cycle power plant customer was having issues with severe erosion at the elbow of their reheat interstage attemperation loop due to increased cycling at the plant. This caused the plant to perform extensive repairs costing millions of dollars. The attemperators are located in heat recovery steam generators (HRSG) between the primary and secondary superheaters on the reheat (RH) lines, requiring accurate temperature control across a wide range of operations from startup to full-load of steam turbine.
The IMI CCI team worked closely with the customer to define the true operating cases based on distributed control system (DCS) data from all four units, including: startup, shut down, normal operation, and transition between units. These actual process conditions allowed the team of Valve Doctors® to evaluate the cooling water spray atomisation based on the kinetic energy, degree of superheat, and the pipe velocity. The team concluded the limiting factor to be the insufficient distance to the downstream bend for the installed probe style desuperheaters.
Based on the request from the customer’s corporate office, the IMI CCI team and the customer’s nominated consultant collaborated to provide a detailed report of the root cause analysis and recommended solution. The solution included changing probe style desuperheaters to ring style desuperheaters and increasing the downstream distances to ensure sufficient water droplet evaporation. The following features of the DAM-B ring style desuperheaters enable superior steam temperature control:
– A flow profiling liner is welded to the inside of the DAM body to increase steam velocity, achieve better secondary atomisation.
– The nozzles receive water from a common spray water pipe encircling the steam pipe, supplying water evenly across the nozzles.
– Nozzle design prevents flashing inside the nozzle and maintains a certain water atomisation pressure at any flow condition to achieve superior primary atomisation.
The 100DSV spraywater valve, using DRAG® disk stack technology, was supplied separate from the attemperator, so that the internal controlling elements are not subject to the thermal stresses of an integrated water valve-probe design. The final solution was delivered to the customer in eight weeks to meet the plants outage/installation requirements. A month later, the customer extended the outage and requested an additional two units to be upgraded with the IMI CCI solution.