Valves & Actuators

Cycle Isolation testing utilizes acoustic monitoring instruments to help customers monitor valve performance. Photo courtesy: ValvTechnologies

By Russell Ray, Chief Editor

A single power plant uses hundreds of valves to control almost every aspect of its operation.

Valves, in conjunction with a controlling actuator, are used for pollution control, feed water, cooling water, chemical treatment, bottom ash and steam turbine control systems.

They work in harsh environments and are exposed to a variety of chemicals, abrasive materials and high temperatures. They are critical in optimizing efficiency, and they are often the final control element in the operation of a power plant.

What’s more, additional demands are being placed on valves and actuators as power plants are forced to be more flexible to accommodate the growth of intermittent sources of renewable power and mandates to curb carbon emissions. As a result, valves and actuators must operate at higher pressures, temperatures and frequency.

Although the basic technology for most valves and actuators has remained unchanged, innovative applications and design modifications are being developed to withstand these demanding environments. In addition, these improvements can reduce costs by supporting the control valve’s ability to throttle accurately, thereby providing better performance for high-pressure steam bypass, turbine bypass and other critical power plant operations.

Actuators regulate mass and energy flows by adjusting valves, flaps and cocks.

The actuator and valve create a single unit – the control valve. Actuators perform different motion sequences, including linear, pivoting and rotating motions, and they are powered by pneumatic, hydraulic or electrical energy.

Actuators receive a control signal from automation systems. The signal is converted into a motion so that the control element of the actuating element assumes a corresponding position. With control valves, this is a stroke motion. With flaps, ball cocks or rotary plug valves, this is a pivoting motion.

The Rotork CVA offers an accurate and responsive method of automating control valves without the complexity and cost of a pneumatic supply. Photo courtesy: Rotork

VALVE-ACTUATOR TYPES

There are three common types of actuators: Electric, pneumatic, and hydraulic.

Pneumatic valve actuators are powered with air or gas. The air pressure acts as a piston to create linear force to close and open the valve. Power plants have traditionally used pneumatic actuators to drive the many control valves throughout their facilities.

However, major improvements in electric control-valve actuator technology are helping power producers lower costs and boost efficiency. Valve actuators powered by an electric motor can withstand the demands of continuous movement. In addition, they work effectively in harsh environments, and provide superior performance in a wide range of applications. The benefits include better efficiency, less maintenance and enhanced performance of the control valves. What’s more, electric actuators do not require recalibration over time. Once calibrated, the electric control valve actuator can operate for months, even years, without adjustment.

Hydraulic actuators, which use pressurized hydraulic fluid to open and close valves, are increasingly popular because of their ability to achieve high torque. Hydraulic actuators are designed to carry out linear movement of all kinds. When a large amount of force is required to operate a valve, hydraulic actuators are normally used. The most common type of hydraulic actuator uses pistons that slide up and down within a cylinder containing hydraulic oil and a spring.

Young & Franklin offers electromechanically actuated (EMA) gas control valves designed specifically for the challenging operating conditions of industrial gas turbines.

Industrial gas turbines require precise control of the combustion process to drive efficiency, reduce emissions, and maximize availability. According to Young & Franklin, the company’s EMA valves offer substantial advantages over their hydraulically actuated counterparts.

Young & Franklin 3010 Series Choked flow valves are electromechanically actuated (EMA), single seat precision fuel control valves. These sonic flow valves are available in a range of sizes suitable for industrial or power turbines of any size.

The Y&F 3010 EMA gas control valve (GCV) is a modern, high precision control valve with excellent speed and valve position accuracy at low openings. This GCV electronically re-zeros its closed position reference every time the power is cycled and the valve position transmitter is greater than 300:1 position turndown.

The valve body is coupled to an actuator assembly that contains a fail-safe spring to quickly close the valve, halting fuel flow in the event of a power failure or turbine trip condition. When the valve and EMA are coupled to the Y&F series 1100 Digital Motor Controller, the complete system provides precise fuel flow delivery with reliable operation.

VALVE LEAKAGE

Leaking isolation valves are found everywhere in the steam generation industry and equally widespread is the detriment to P&L statements worldwide. How can a simple worn, damaged or improperly specified isolation valve have such far reaching effects?

Like all thermal engines, steam plants are powered by energy differences and the greater this difference, the greater the fuel efficiency. Valves maintain the separation by isolating the high energy processes from the low energy processes. When valves leak, they are acting in direct opposition to the forces that drive the plant by allowing energy to leave the high energy processes and enter the low.

Another key characteristic of the steam cycle is that production (or kilowatt-hours) is governed by the steam rate or mass flow through the cycle processes. Steam or energy that is bled out of the processes via leaking valves is not being put to beneficial use and thus may be proportionately reducing the amount of electricity or revenue being produced.

While plants are designed with ancillary equipment to compensate for some of the effects of valve leakage, this adds substantial costs to operations and the capability is limited. Recent client experience includes mitigating cycle water losses on a new generating plant in which cumulative valve leakage rates exceeded the make-up water system capacity. This forced the plant to curtail operations to allow the make-up system to catch up demonstrating how cycle isolation can directly impact plant reliability and availability.

The cornerstone to capturing these benefits is diagnostics. A systematic approach to accurately measuring valve leakage eliminates uncertainties that manifest as unnecessary added costs. Improvements to valve leakage diagnostic programs quickly result in plant performance improvements as well as sustained reductions to valve O&M costs.

ADVANCEMENTS IN TECHNOLOGY

Power plants are complex in that there are many different sub systems required to deliver electricity. These plants were an early adopter of distributed control systems to monitor and control the facilities. Due to the arduous nature of the environment, certain practices were adopted to allow for reliability and maintenance. Motor operated valves in particular are key to plant performance. Until 20 years ago motor operated valves tended to have motor control centers remote from the valve. This did not allow for the benefit of technology advances in electric actuators.

The early 1990s saw a trend towards smart actuators with integral data logging capabilities. These actuators could also be networked to provide the control system to receive data that had occurred in the actuator. Power plant designers started to take advantage of this in the past 10 to 15 years.

Today, there has been a major change in the availability of better information from the motor operated valves. Instead of being alerted after the fact, the electric actuators are now monitoring the systems and providing data ahead of potential failures in the equipment.

As an example, early actuators had torque switches which tripped after the valve had an internal failure which caused it to require more force than originally designed for. The more recent smart actuators have an internal data logger inside which has had the ability to monitor torque output.

The most recent electric actuator has these two features plus more. The newest feature is to have a monitoring set point above the baseline torque and below the over torque setting to alert the plant operator that there is an impending issue that needs to be addressed.

Miscellaneous trip alarms are also included to monitor things like starts per hour to insure the internal contactors are not being overused, perhaps due to an actuator that is hunting because of a faulty process signal. There are also maintenance interval settings that can be adjusted by the plant operator.

These newer actuators also have expanded screens at the unit to allow for better operator local diagnostics. These are just a few of the upgrades available today.

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