|Rotork IQ electric actuator being installed at a power plant. Today’s modern generation of intelligent, electric actuators offers power plants a host of benefits, including the potential of significant energy savings. Photo courtesy of Rotork|
By Russell Ray, Managing Editor
Valves and actuators are critical in almost every aspect of power plant operations. They are used in a wide range of applications, including pollution control, feed water, cooling water, chemical treatment, bottom ash and steam turbine control systems.
They are exposed to a variety of chemicals, abrasive materials and very high temperatures. They are critical in optimizing efficiency, and they are often the final control element in the operation of a power plant.
Although the basic technology for most valves and actuators has remained unchanged, innovative applications and design modifications for problem solving have led to notable improvements in actuator technology. 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.
To better understand the options available to power producers, Power Engineering reached out to the following companies for more information about valves and actuators.
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.
|Cycle Isolation testing utilizes acoustic monitoring instruments to help customers monitor valve performance. Photo courtesy of ValvTechnologies|
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? The following summarizes the adverse impact on four critical plant performance measures: efficiency, production, reliability and maintenance costs.
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.
Young & Franklin
Advances in the design and manufacturability of electrically powered actuators have allowed for the replacement of hydraulically actuated control valves and guide vanes on heavy duty class combustion turbines. Asset owners and operators benefit from reduced life cycle cost, improved component reliability, and in many cases interrelated components can be eliminated, thereby improving system reliability.
The hydraulic valve product is mature, but to ensure their reliability frequent maintenance must be performed on a 24,000 hour cycle minimum. Lack of maintenance causes unpredictable control performance such as start-fails, forced trips, trips during controlled shutdowns and loss of flame during transitions. Hydraulic oil leaks of worn seals and vulnerable fittings create slip and fire hazards. On-site work includes messy filter changes, replacement of lube-oil varnish plagued servos and safety trip relays, unverified adjustment of valve stem packings, and re-calibration. Off-site work is typically a time-zero overhaul of the valve and actuator at 48,000 hour intervals.
Y&F set out to develop the EMA (electric motor actuator) product line with asset owners and operators in mind. Specifications were based on real world duty cycles, operating conditions and owner/operator inputs while meeting or exceeding original equipment performance metrics. Development testing was based on the most stringent requirement of each category. All products were designed to fit into the same envelope or smaller than existing hydraulic products and are suitable for many OEM turbine makes. Retrofit projects can be completed within typical outage duration with minimal on-site modifications required.
The Y&F EMA product line is designed for a 96,000 hour time-zero overhaul cycle.
Owner/operators benefit from improved reliability and realize maintenance budget gains by eliminating the recurring costs associated with lube oil powered hydraulic actuators.
The Ball valve has proven to be a highly efficient valve product providing good isolation with minimal pressure drop and the ease of quarter-turn operation. The simplicity of the designs makes it possible to offer a variety of seating materials, end connections, materials-of-construction, body styles, thru-flow in Full or Reduced Port designs, and is very adaptable from manual operation to electric/pneumatic/hydraulic automation.
There are as many different types of ball valves as there are applications for ball valves.
Two of the basic designs are commonly referred to as, the Floating Ball Design and the Trunnion Ball Design. Each design has various pros and cons, but typically the larger (6″ and larger) higher operating pressure/temperature valves will be of the Trunnion design and the smaller (1/4″ to 12″) floating design will typically be used in lower pressure/temperature conditions.
With the Trunnion design the Ball is fixed in-place by a bottom trunnion, and the stem-to-ball connection acting as the upper trunnion. With the trunnion design the Ball does not come into sealing contact with the Seats; the Seats are mechanically energized and move toward the ball to seal. Whereas, with the floating design, the ball is not fixed and is free to make contact with both seats; sealing is achieved by pre-loading the seats during assembly and test. Both of these designs offer a broad array of unique optional features to provide the best possible product to meet the end users applications and expectations.
Apollo Valves is headquartered in Matthews, N.C., with a bronze foundry and manufacturing facilities in Pageland, S.C., and a steel foundry and manufacturing facilities in Conway, S.C. Apollo produces valves for the Commercial and Industrial markets. Conbraco, Apollo’s parent company, offers a full line of bronze, steel and high alloy floating ball valves for the industrial market.
|The Valpres 766000 Series stainless steel valve. Photo courtesy of S&K Automation|
Bonomi North America has introduced a new line of ANSI Class 150 flanged industrial full-port ball valves, designed for easy automation with low operating torque and direct actuator mounting. The new split body valves accept any appropriately sized ISO standard actuator and are performance-matched to Bonomi’s Valbia electric and pneumatic actuators.
Designated the Valpres 766000 Series (stainless) and 766001 Series (carbon steel), the valves feature an ISO 5211 mounting pad and square stem, raised-face flanges and Fire Safe certification to API 607, 6th edition. In addition, they are also certified to API 6FA. They are designed to ANSI 16.5, ANSI 16.10 and ANSI 16.34. They meet NACE MR 0175, NACE MR 0103 and are TUV T.A. Lusft approved.
Valpres 766000/766001 Series valves are currently available in five sizes: 1″, 1½”, 2″, 3″ and 4″. They offer long service life with minimal maintenance in applications such as power generation, pulp and paper, chemical/petrochemical processing, water treatment and skid mounted pumping systems.
Standard features include virgin PTFE seats, stainless steel ball and blowout proof stem, and a locking handle that can be secured in either the open or closed position. The stainless steel body is cast from ASTM A351-CF8M. The carbon steel version is made from ASTM A216-WCB. Optional high-temperature seats are also available.
|Local control stations like this one are simple to install and operate. It gives the operator the ability to switch the actuator off or from “Remote” to “Local” mode. Photo courtesy of Promation|
Local control stations are self-contained signal generating units which are attached to an actuator, or in the case of an actuator in an inaccessible location, mounted on the wall close to the actuator it controls. It gives the ability to switch the actuator off or from REMOTE to LOCAL mode. An example of REMOTE mode is when a central control room controls the actuator. In case of emergency or during maintenance work, the actuator can be placed in LOCAL mode and then opened, closed, or jog to the appropriate safe condition. At the conclusion of the outage or maintenance work, the actuator is switched back to REMOTE mode and is returned to its normal role in the overall control scheme.
Local control stations are designed for either 2 position, floating, or proportional control, in many supply voltage options, and have a wide selection of switches and displays such as indicator lights for open/closed position, end of travel indicator lights, and LCD displays to show the current position of the valve. A light or display showing full closure of a valve is incredibly useful because if the valve/actuator is mounted high enough or in dim light, the actuator indicator may not be visible from ground level.
Having a local control station mounted to the actuator makes for a very quick and simple installation in the case of a valve retrofit, and allows for quick commissioning in a new installation as the installers will not need to fabricate a switch box for local control.
Operators really appreciate the convenience and safety offered by a well placed remote-mounted local control station which saves them from climbing a tall ladder to close or change the position of a valve located in an inconvenient or dangerous location.
Over time, there may not be another auxiliary system more maligned than that of a back-up liquid fuel system on a dual fuel gas turbine. The hardware shortcomings responsible for the inability to start on liquid fuel or transfer from gas to liquid fuel, check valve failures, flow divider failures, fuel system evacuation, exhaust temperature spreads and related turbine trips have all been addressed by designs developed by JASC Controls. Water cooled check valves, which prevent carbonization , metal to metal sealed water injection check valves rated for service at 600 degrees “F”, multiple use crush gasket technology which replace o-rings with a 1000 degree “F” rating at 2000 psi and a zero emissions equipment design which allows a turbine owner to test the readiness of the liquid fuel system without firing on oil are examples of the JASC’s unique technology.
These system specific products have two common design goals. First, they provide reliable operation for at least 24,000 hours or until a scheduled turbine service interval is reached. This provides the optimal timing for returning the valves to the JASC facility for refurbishment after three or four years of use.
Second, the valves must maintain an ANSI Class 6 sealing capability from installation until removal for refurbishment. Elimination of reverse flow results in separation between the turbine combustion process and auxiliary systems such as liquid fuel, purge air and water injection. With no reverse flow of combustion gasses into these systems, back-up fuel system availability and reliability typically exceeds 98 percent.
Finally, JASC’s solutions are configured to be interchangeable with the turbines existing hardware. This characteristic both minimizes the cost and reduces the time associated with performing fuel system upgrades on a turbine of any age.
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