Open and Shut Case
Small and plentiful in most designs, valves do not often get the same attention as the big ticket items, even though each valve is a crucial point in the total system. With that in mind, this month's panel of experts discuss the latest developments in valves and actuators, touching on topics like electronic vs.
Small and plentiful in most designs, valves do not often get the same attention as the big ticket items, even though each valve is a crucial point in the total system. With that in mind, this month's panel of experts discuss the latest developments in valves and actuators, touching on topics like electronic vs. pneumatic; matching up actuators and valves; and the expectations of consulting engineers.
CSE: Starting off, what are the latest valve innovations?
RATTENBURY: The types of valves have not changed significantly. Probably the only noteworthy innovation is the now-common ball valve, which has virtually replaced the globe valve and gate valve in most on/off applications.
That being said, the advancements in technology of chemical processes, water purification and high-purity systems—particularly in the microelectronics manufacturing industry—have driven new options in valve materials and interior surface finishes to address corrosion resistance and purity.
EGBERS: I would add these recent valve innovations: direct-coupling, which allows for easy attachment and interchangeability of actuators; caged-trim throttling plugs, which allow for reduced noise and susceptibility to wear from vibration; optimized stem packings, for selecting the optimal packing material for the media being controlled, reducing the chance for leakage; and finally, high-rangeability throttling plugs, which allow for more precise control.
FELKER: We consider the characterized control valve with the DIN-Standard equal percentage curve, which Belimo developed, to be the only new type of valve designed in the last 20 years. Other than that, most changes in existing globes or ball valves are merely ways to manufacture them cheaper.
CSE: How about actuators?
EGBERS: Generally, there has been a development of application-specific end devices, such as the introduction of high-position resolution actuators. Other developments include capacitive return, direct-coupling and auto-calibration of the control signal—which allows the input signal to be matched with an individual valve's stroke length. Additionally, electronic position feedback is now available.
As far as damper actuators , we've observed the development of self-centering shaft adapters; wide-operating temperature ranges; outdoor applications on rooftop units; auto-calibration of control signal; and plenum-rated devices.
FELKER: We've seen the development of BACnet- and LonWorks-compatible actuators. That's a major step.
CSE: Are electronic actuators dominating the market, or do pneumatic and other more traditional actuators still have a place?
GEERS: Electronic actuators are becoming more of a factor, but not dominant. Pneumatic actuators are still very effective—and widely used for automating quarter-turn valves.
RATTENBURY: I agree. Pneumatic actuators are actually more reliable, plus they are lower in cost and can provide more power (torque) with motive compressed air for larger valve sizes.
EGBERS: But electronic actuators dominate new installations. In addition, the price point for electronic actuation is approaching that of pneumatics.
FELKER: Actually, we feel that electric actuators have dominated the total market for 50 years. On the commercial market today, 90% of actuation is electronic, including fire and smoke dampers. Virtually 100% of direct-digital-control, variable-air-volume actuators are now direct-coupled with no linkages.
In some applications, the powerhouse 30-in. isolation valve-type applications go with pneumatic—but power, not accuracy, is the issue for two-position valves. Where accuracy is the prime goal, electronic is used.
CSE: Is the growing popularity of electronic actuators tied to better building automation strategies, or is it more a result of improving control of process handling?
RATTENBURY: Although I agree that computers are taking over the "thinking" processes of building and process automation, I believe the actual mechanical control tends to remain mechanical.
EGBERS: But the continued integration of building-control, fire-alarm, lighting-control and security systems necessitates electronic control systems. The natural extension of the electronic control system is to directly drive an electronic actuator, rather than driving an electronic-to-pneumatic transducer. Additionally, there is no need to procure, install and maintain the pneumatic compressor and air distribution lines.
FELKER: As I mentioned before, another big factor is price. In the VAV box, it is less expensive to use an electric actuator with DDC, as a transducer to pneumatic would require two trades, and the transducer costs as much as an actuator.
CSE: In general, is it better just to match valves and actuators from the same manufacturer, or is it simply a matter of doing one's homework?
GEERS: Speaking as a supplier of both valves and actuators, I believe that there are benefits when one manufacturer supplies the complete package. If the application criteria warrant the use of one manufacturer's product, then the customer will be able to take advantage of a number of things: In particular, the service and technical support of the package, where the customer has the ability to contact one company.
RATTENBURY: Yes, a single point of responsibility is the most advisable, even if the valve or actuator manufacturer relies on an OEM purchase to provide a complete package.
EGBERS: I would also point out that only the manufacturer truly understands all the component tolerance implications between the valve and actuator. It is in this way that valve parameters such as flow control characteristics, close-off pressure and life cycles can be optimized.
CSE: Are there still problems with engineers undersizing or oversizing valves?
GEERS: Proper sizing remains the most important aspect of automated valve packages. Factors to consider include: pressure, temperature; type and viscosity of the fluid; and phase changes of the material. All these factors can affect the operating torque of the valve. Economics, however, does play a key role in many decisions about sizing.
FELKER: It's been our experience that very few consulting engineers choose valve sizes. But when they do, oversizing is chronic. In fact, we've found that the ASHRAE sizing method is generally ignored. For example, valves that should be line-sized for water temperature control—such as in the case of perimeter resets—are often sized to take 5 psi.
Overall though, it seems that few of us have realized that it does not matter what size valve is used. What is important is that less energy and pumping will be required if a line-size valve is employed.
EGBERS: But if valves are oversized, flow control and, ultimately, product life may be compromised.
CSE: In terms of expectations, are specifying engineers often unrealistic, calling for requirements such as bubble-tight shut off?
GEERS: I don't believe that engineers' expectations—such as bubble-tight shut off—are unrealistic. The engineer is designing the system and specifying the products in order to provide the highest quality, most economical system for the owner. I do believe that the manufacturer needs to provide as much information as possible regarding product features and benefits.
EGBERS: Control valve specifications should be chosen to match system requirements, but the specifier should not be overzealous—specifying high numbers for the sake of showing high numbers. The specifier should understand the true system requirements—with respect to close-off, temperature, pressure and leakage ratings—and select valve products accordingly.
FELKER: When it is required—for example when a control valve may be called upon to serve as an isolation valve—Class-IV leakage should be specified. Technically, "bubble-tight" is slang, and few engineers know or use the American Society of Mechanical Engineers' formal definitions.
RATTENBURY: There is a tendency among professionals to be highly conservative in order to protect themselves, but there is also a legal precedent whereby someone who does not apply the latest techniques, equipment or procedures can be found liable for damages or injuries that could have been avoided. Such legal precedent may seem unreasonable or unfair to practicing professional engineers. Consequently, they will tend to look for the most sophisticated, robust, high-performance equipment for a given application. If there is ever a failure of the equipment or system, the engineer will argue that they applied the latest technology and are therefore not at fault. In such circumstances, the owner is burdened with the added cost of expensive equipment. This is tantamount to the professional engineer transferring his professional liability premiums to the owner.
However, this is not the reasonable or realistic approach. Engineers are expected to use judgment while applying their knowledge or skills. In the example of "bubble-tight" valves, the professional must research the actual need for such a valve property, and whether the failure to apply such a property can be reasonably foreseen as the proximate cause of damage.
CSE: What other common mistakes are made in specifying this equipment? And what are some common-sense recommendations?
FELKER: I see budget constraints and the lack of respect for mechanical engineers as the biggest problems. More time to design, not just cookie cut, is required to engineer a system.
GEERS: A lack of application information is the most common cause of incorrect sizing and selection. Thus, access to technical information to assist in the selection process is very important, and up-to-date product information and technical service from manufacturers can generally be obtained by visiting the web site.
EGBERS: Four valve pressure ratings are commonly confused:
Static pressure refers to the pressure within the piping system from the non-moving media.
Maximum differential pressure for modulating service is the maximum differential pressure across the valve that, when the valve is modulating, will still allow for quiet, smooth control.
Design pressure drop is the pressure drop used to size and select a valve.
Close-off pressure refers to the maximum differential pressure that a valve can withstand without leakage while in the full closed position.
Also, care should be taken to understand the published ratings. In the case of electronic damper actuators, some manufacturers have timing and torque ratings that vary based on temperature or undervoltage conditions. These can lead to torque outputs far less than published values. This has the potential to greatly affect the building control system. Look for a manufacturer whose published torque and timing ratings are valid throughout the entire temperature and voltage ranges required.
Dave Egbers , Sr. Product Mgr., Building Automation Div. Siemens Building Tech., Buffalo Grove, Ill.
Bill Geers , Sr. Product Manager, System Control Nibco, Elkhart, Ind.
Larry Felker , Operations Manager Belimo, Danbury, Conn.
John Rattenbury , P.E. Owner, UV Ameridrain, Hull, Mass.
Temperature-Actuated Valves are Gaining Ground
In the centrifugal booster pumps used to pressurize water in multi-story buildings, preventing water from overheating is a serious matter. Water that's too hot can cause a pump's rotating parts to expand—resulting in seizure; damage to packing or seals; and flashing of the liquid with consequent vapor binding. If left unchecked, overheated water can also pose a serious safety hazard to anyone in a building who turns on a spigot.
The overheated-water problem begins with the pump itself: to provide enough water at sufficient pressure, pumps are sized to meet maximum demand. As long as demand is high, water temperature remains within an acceptable range. However, when demand is low, the heat generated by the excess power will overheat the water.
In conventional systems, a thermostatic switch senses the temperature of the water and sends a signal to a solenoid valve, which then opens to allow the overheated water to drain. But thermostat failure, often brought about by electrical problems, can cause the solenoid valve to fail—either entirely open or closed. If the valve fails in the open position, water will drain continuously from the system. If it fails closed, the valve will be unable to offer any protection from overheated water. The result in both situations is high operation and maintenance costs.
A simpler solution
Temperature-actuated valves are becoming more widely used in centrifugal pumps for an important reason: their overall simplicity. Unlike conventional electrical systems, completely mechanical temperature-actuated valves combine temperature sensing and bleeding in a single unit. Each valve contains a thermal actuator that continually senses pump water temperature. When it rises above the desired limit—usually 105°F—the actuator opens the valve. A small sample of water then flows past the sensor, and the valve will continue to modulate open until the overtemperature is eliminated. When makeup water temperature returns to the safe range, the valve modulates closed. Because the sensor is typically located directly on the pump volute, water is eliminated at the precise temperature desired.
By Nick Tallos, Vice President, Engineering, Therm-Omega-Tech, Inc., Warminster, Pa.