Applications for electric vs. pneumatic valve actuators 1

[RobsVette]

Hey guys, a project that I am working on has me questioning some of the traditional beliefs I had regarding the use of electric vs pneumatic valve actuators. Can you guys give me your insight on the following issue.

Our company has been awarded the construction of an energy plant at a large university and I have noticed that the engineer has specificed all control valves (modulating and 2 position) to be pneumatic actuators. The valves with actuators in question are on the chilled and condenser water systems.

My beliefs on when you would use pneumatics vs electric where based on the following assumptions;

PNEUMATICS
- typically used for modulating applications such as compressor recycles, tank/steam drum level control where precise control is required and the valve will be opening and closing often. These are conditions that throughout the life of the valve would stress an electric actuator and in order to move the valve fast enough, electric actuators would take a decent amount of power to perform so air is used to drive a piston, rather than use an electric actuator.

- typically used for 2 position applications where there are concerns of safety dealing with a power outage (such as on a natual gas safety valve, or steam turbine safety valves) where you can have an air accumulator tank and know that there is always are to operate your valve or used where very high closing torques are needed due to closing a valve against a high differential pressure. I can't think of any good examples of the top of my head.

ELECTRIC
- Would typically be used for 2 position valves that are not going to close against very high differential pressures. The higher the DP the larger and more complex the motor/gearset gets (from probalby needing higher torque, the more power you consume, etc...) Also a factor is that the valves are not going to be moving very often, so there is little risk of damaging the motors from overheating.

- Electric actuators can also be used in modulating service, but would not be ideal where very precise control is needed, due to not wanting to have to have an overly large motor size and the constant abuse on the motor. Here I am thinking of small bellimo electric actuators that are often attached to valves. These tend to have very long stroke times.

Can you guys please confirm if my assumptions are correct and if not please correct me where I am wrong. There seems to be a general concensus that at utility plants most actuators are pneumatic, but I think that most actuators at utility plants are meetings the requirements stated above for pneumatic actuators. Where as the electric actuators that are seen in the HVAC world on isolation valves would meet the requirements listed above for electric actuators.

Thanks in advance for any help.

 

[mruboat]

RobsVette, you mentioned your “traditional beliefs” as you began this post. It’s interesting how far we’ve come with actuator technology, yet these traditional beliefs are still very common. At the risk of trying to teach granny to suck eggs, I thought some review of past trends might add perspective for your energy plant project.

The traditional use of pneumatic actuators on power boilers and supporting systems was heavily driven by Babcock & Wilcox which owned the Bailey Meter Co. for some portion of the last century. Bailey pneumatic controls and actuators were predominantly used in the post-war build up of the coal fired power industry in the U.S. and other developed countries. That made sense since the actuator technology matched the controls, and high torque, electric actuators for truly modulating valves and dampers didn’t exist in the 1950’s – 1960’s.

As modern control systems were developed, I/P transducers allowed for the use of electronic controls while reusing original pneumatics. Modern DCS’ could provide powerful control capabilities. Almost in parallel with controls developments, electric actuators capable of 100% duty cycle, and high torque were being developed.

The Clean Air Act of 1990 was a key juncture in the EU industry in America. Boiler owners were required to use the best available technology to upgrade boilers to lower NOx emissions, etc. Large investments were made in DCS retrofits on existing boilers to optimize combustion to reduce emissions. Improving boiler control and efficiency (fuels savings) helped justify the costs. Controls companies such as Foxboro, Honeywell, etc recognized that the old Bailey’s inconsistent performance would limit the capabilities of their new controls. Thus began the trend in the U.S. of installing precision electric actuators to replace pneumatics as part of large controls upgrade projects. Actuators from Harold Beck & Sons and Jordan Controls (mentioned earlier in this string) were heavily used for this purpose. The Electric Power Research Institute (EPRI) performed a commercial scale study on one of these projects: It’s described in a Power Engineer article on Beck’s website:

http://www.haroldbeck.com/pdf/SISR.pdf

Fast forward to 2012. The face of the U.S. power industry is changing as new capital is directed towards gas fired generating units that utilize far fewer actuators than a coal fired boilers. But, China is building new coal fired units at a break-neck pace. They’re using nearly all electric actuators, focusing on low first cost rather than control performance and lifetime cost. It’s not uncommon to see a five year old plant trying their second or third brand of electric in an attempt to keep things running. This just shows that the new trends aren’t necessarily improvements on tradition.

I hope this provides a little insight into how pneumatics became ingrained in the U.S. power industry. They’re still often used to replace existing pneumatics due to the cost of replacing airlines with AC. But, serious consideration should be given to high quality, high resolution electric actuators for important control applications.

Ed

 

[rmw]

Interesting. B&W was using electric actuators on their oil gun retracts years and years before 1990. And while B&W did own Bailey Meter Co and not only used pneumatic actuators, they use their pneumatic controls for every other control function on the boiler before the advent of electronic controls in the late '70's, early 80's. Their competitors, principally CE and FW in the mid to early last half of the century used a plethora of other brands of pneumatic controls and actuators. There were not as many actuators on the market as there were pneumatic controls so it wasn't unusual to see Bailey or Hagen actuators used with many different pneumatic control systems.

Back in the day when I was a pup, I put several Hagen Control systems in power plants and had the operators who were old WWII vets mention that they remembered the 'dog bone' controllers from warships they had served on. I put both Bailey and Hagen into service, but only heard the remark about Hagen but did remember installing Bailey Controls that looked an awful lot like the boiler controls on WWII vintage ships I had served on.

B&W often owned a major player in particular segments of the industry e.g. Diamond Power soot blowers. CE and FW were left with either using their competitors soot blowers on their boilers or using Copes Vulcan. It kept CV in business for years. Rare was a B&W boiler with CV soot blowers, but they did exist.

rmw