Need More Megawatts? Try Your Valves

Running a power plant at peak performance cannot be a reality until all of its systems operate successfully over the long term. This includes the system’s control valves, which are commonly the most underrated and ignored elements in plant performance. This omission is due to a combination of factors: their treatment as only “necessary evils” in control, poor recognition of their contribution to plant productivity, and a lack of systematic methods in quantifying their effect on efficiency. However, control valves, especially those in severe service, can play a big role in improving a plant. Since they are the final control elements in any process, this places them in a position to affect plant efficiency directly. Selection of severe service valves incorporating the right technology designed for the specific application offers one of the quickest and most effective means of improving plant performance. The benefit of this is clear: every additional MW in capacity that is recovered is like installing a new 1 MW unit.

CCI has helped powerplants around the world achieve up to 5% improvement in performance by modernizing their critical service control valves

Most of the problems in severe service applications are caused by valves that have been misapplied; they simply cannot perform beyond their limits to meet the conditions originally specified. These performance limits can often be attributed to the design of the installed valves. By identifying this as the root cause of many problems, a first step is taken to ensure the appropriate technology is selected for the specific application.

At the Hegang power plant in China, the existing boiler feedpump recirculation valves was leaking badly, and the plug was jamming often. It started with a small amount of flashing steam and water mixture which cut the seating surface like a rough wire and in a short while, the shutoff function of the valve was lost. More importantly, excessive leakage reduced the efficiency of the plant both through direct energy loss and because the boiler could not be fed the full rated flow.

Utilizing an angle design, the Hegang replacement valve provided a solution to combat the erosion: using the advanced design of the multi-stage, multi-path DRAG® disk stack, the fluid velocities were controlled by forcing the fluid through a series of right angle turns.

In general, poor valve performance in severe service applications like Hegang is due to lack of velocity control. The consequence of this presented itself again at Dominion Generation’s Surry power station, where the installed valves were experiencing vibration, noise and poor control. The existing valve had no velocity control. Hesitant to try a manufacturer with unproven multi-stage, multi-path technology, Dominion relied on DRAG’s experience to help eliminate the problem. The six CCI feedwater regulator valves are designed to achieve the desired level of control in the system, which will improve Surry power station’s ability to increase output.

The problems of fluid velocity are not unique: high velocities produce cavitation, erosion and abrasion in most applications, which can quickly destroy a valve. Such side effects were recently felt at CFE in Mexico , where the older installed valves were experiencing erosion in a spraywater application. Often in a plant, steam temperature control can be the cause of problems. The importance of controlling steam temperatures within narrow limits is evident by the fact that a change of 35-40 F (19-22 C) corresponds to a change of about 1% in heat rate at pressures above 1800 psi (124 Bar).

For this reason, CFE replaced the older valves with two CCI DRAG® valves. Specifically engineered for superheater spraywater applications, these valves allow for the fine temperature control at a lower control signal, an advantage in improving plant efficiency.

Improvement in valve performance throughout a plant can progress overall performance by 2% to 5%. Although there are different approaches to achieving performance improvement in a power plant, experience to date makes a compelling case for the economic focus on control valves in order to achieve performance gains. The fact of the matter is, the cost of solving these valve problems is much smaller compared to the losses they cause. This awareness is prompting power plant constructors and operators to accept the knowledge that the best available technology in control valves is necessary for good long-term performance. With the help of DRAG® technology, peak performance in severe service valve applications is assured, and can be the essential ingredient in maintaining an optimum running power station.

Published in SOLUTIONS Fall 2001