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.

 

[zdas04]

Your thoughts are pretty close to mine, but I have always used pneumatic preferentially over electric for virtually all applications. I've used very small electric on/off valves (solenoid) to send gas pressure to pneumatic valves. I've also used a few electric motor valves in specific applications where there just wasn't a reliable source of gas pressure.

Last week I went to a meeting with a guy from Fisher and that preference may be changing. They started marketing a Fisher easy-Drive[sup]TM[/sup] electric Actuator that basically puts a DC brushless motor driving a set of gears on their D3 and D4 globe valves (traditionally both have been pneumatic). I'm considering this valve in a high dP dump-valve application instead of setting an air compressor. I've never seen an electric valve that I would use for a dump valve before. (no, I am not in any way affiliated with Fisher or Emerson, I just saw some technology that excited me).

I have an application where a 1-inch valve needs to open for 2-3 seconds every 45 seconds--using a motor valve required a 250 W solar panel and I still ran out of power occasionally. The specs on this valve say that the application should be able to run off a 24 W panel (assuming 6 hours of sun a day, if I round 24 W up to 150 W I'm still way ahead).

The easy-Drive[sup]TM[/sup] may change my selection in specific applications, but pneumatic valves will still be my first choice for the lion's share.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[RobsVette]

Hi Dave,

Thanks for the clarification. A dump valve is a good example of a valve that could be 2 position that would still have pneumatic actuator.

The reason for my question, was we are seeing an application (chilled and condenser water system isolation valves) where I think a pneumatic actuator will be more troublesome over the long run. There are no unique requirements that merit the use of a pneumatic actuator, so I was going to question why this was selected, but wanted to make sure I wasnt missing something first.

My personal feeling is that if you can apply a simple 120V electric actuator to a valve and it will work, why overcomplicate the actuation by involving air and electric signals. Also some of these valves are at a cooling tower, so now heat trace and insulation become involved on the air lines.

Just to recap my point from above, in the modern day HVAC industry, if you have one of the 2 specific requirements above for using a pneumatic actuator, than you should select one. However if not, it is easier and more trouble free over the long haul to stay with a standard electric motor. Most of these valves are strictly for convenience. So in that case a simple actuator works fine, whereas when you are performing a control or safety function a pneumatic actuator is the best selection because a normal electric actuator can not handle the stress of the situation or will not provide accurate enough control.

When moving to utililty plants, probably 90% of the valves meet the criteria for using a pneumatic actuator.

I am going to inquire with our local Fisher rep about the Easy-Drive. That actually sounds very interesting. Thanks for the heads up.

Rob

 

[btrueblood]

I can understand why Dave, who's in the biz of pumping/moving highly flammable fluids, would use an intrinsically safe technology like air to provide motive power for valve actuators. I've never heard of air causing sparks.

But chilled water? You are taking line power to drive a compressor, piping it thru leaky pneumatic tubes/fittings, and then (typically) putting a leaky positioner stage at the valve/actuator, and losing power at every stage due to efficiencies, then dumping most of the work you've paid for in compression away when the regulators and valve expand the air into a cylinder. Just never made much economic sense to me. Electric actuators with speeds of 5 or 10 seconds are a dime a dozen these days, so that should not be a concern, and accuracy for proportional control valves on a chilled water system would dictate speeds down in the 30-90 seconds range, so even more options are available. I'm not buying the stress arguement either - well made actuators of either variety will survive hundreds of thousands of cycles, and the same cycles that wear electric motor bearings/gears cause equivalent wear on pneumatic linkages and seals, etc. And I'm laughing out loud when I hear about accuracy of control from pneumatic actuators and positioners - when you have any real world valve, with its assoicated seal friction/stiction, and similar friction/stiction in the actuator seal, the stupid things ALWAYS overshoot, because the piston pressure has to rise to a point to "break loose" from static friction. Maybe that's not critical, and if you are talking about bang-bang actuation certainly not, but for control valve service pneumatics are plain worthless in comparison to a properly sized electric actuator.

Oh and fail safe? Spring return and battery-backup actuators are available for electric actuators too. We find that most people who specify spring-return valves really don't know what they are doing - the best fail position for most control valves in a chilled water system is "fail in place", if you think through the scenarios.

 

[RobsVette]

btueblood,

I feel much better about the situation after reading your response. Sometimes I find that if I can't figure out someone else was thinking, neither can they.

Suffice to say, in the last couple of HVAC projects we have done, everything has been electric actuators, constant flow and variable flow applications included. We have had no problems whatsoever, with any of these actuators.

Thats for the insight guys

 

[moltenmetal]

Do you have air? Is the service Class 1 Div 1 or Div 2 (or class 2)? Do you care about having your valve rapidly and reliably go to a deterministic failure position? Then pneumatic is probably still the right solution.

General purpose area classification? Don't have air? Don't care about failure position, or want valves to all "fail in last position"? Are spring-fail air actuators impractical for your preferred type of valve? Electric actuators are then worth considering.

Compressed air definitely costs money to compress and dry. Wasting it to produce a minor amount of cooling using a vortex cooler is questionable. Using it to run an air diaphragm pump might also be a suboptimization. But to actuate valves in a plant setting, air is still king.

 

[btrueblood]

Modifying this statement, " We find that most people who specify spring-return valves [in HVAC chilled water systems]really don't know what they are doing ".

Molten, chilled water HVAC is about as far from Class1 Div1 as is possible to get. Thermal response of a heat exchanger dominates in HVAC, so response times are on the order of minutes, no reason for fast acting valves.

This statement: "Do you care about having your valve rapidly and reliably go to a deterministic failure position? Then pneumatic is probably still the right solution. " is a bit misleading. If you care about failure conditions, then you want the valves to move when the power to the pumps fail. Not after the air pressure in your actuator/instrument lines finally bleed down. Spring return electric actuators can respond at speeds equal to pneumatics (go figure, they both use the same energy storage mechanism, right?), and begin do so instantaneously with loss of power, no delay for the air pressure to bleed off. Battery powered electric fail-safes can be programmed to drive at high speeds as well.

Finally, most of the reason air is used (in any situation) is because it's already there, i.e. the original controls are all pneumatic and the cost of running lines and installing compressors and driers is already sunk. For a new plant, I'd lay 1:1 odds that industrial quality electric actuators can do everything pneumatics can do, and probably cheaper or at least at equal cost; but it would depend on whether other processes demand instrument quality air, or if hazards exist that preclude any significant electric voltage from being used. If air was really such great technology, then we'd put air compressors on power line poles, and plumb air lines into our homes...

 

[moltenmetal]

Electric actuators with spring-driven failure position are typically much more expensive than their pneumatic counterparts. Battery back-up systems are even more expensive still, and their realistic availability on demand isn't stellar either. The only way the non-facetious portion of your third paragraph is accurate in my opinion is if you take into account the total cost of ownership over many, many years in an environment where the energy to compress and dry air is very expensive and the air system is poorly maintained.

In a pneumatic system, for an on/off valve you go to the failure position if you lose electric signal to your solenoid valve, or if instrument air bleeds off. If you use a 4/3 way solenoid, you can assist the valve to the failure position if air remains available.

I'm not saying electric actuators don't have a place- they definitely do, as do solenoid valves, wax actuators, self-operated controllers etc. They do have weaknesses that need to be honestly acknowledged though, in the same way that air actuators do.

 

[btrueblood]

I think you need to look beyond Fischer or Rotork for electric actuators if cost is an issue...but that's just my opinion. We shall have to agree to disagree.

 

[rmw]

Each has their place and each has its strengths and weaknesses. Many of those have already been brought out. The electric utility industry long ago accepted electric actuators and uses a lot of them for critical valves and dampers of all sizes.

On the other hand, the petrochemical and refining industries seem to prefer pneumatic and predominantly Fisher. I have heard it said that one can walk from Baton Rouge to Houston on the tops of green valves.

Electrics are slow. Even the fast ones are slow compared to pneumatics. Sometimes slow is good.

Electics (except the spring and battery versions mentined) fail in place upon loss of power while a pneumatic valve is going somewhere if it loses motive air, open or closed. Pick the one that does what you want done there.

Spring return electrics devote a lot of the drive train torque to merely overcoming the spring compression force and the net remaining for driving the valve or damper is not terribly high compared to the amount of power it takes.

Pneumatic valves suffer from "stiction" or effects of packing tightness/contaminants on the valve stem. Yes, a positioner will overcome that, but there is always a delay while the positioner figures out that the valve didn't go where it told the valve to go as a response to the change in the instrument signal and starts to apply more motive air until the valve "jumps" and overshoots where the positioner really wanted it to go.

An electric actuator is going to start, move the driven device or trip out trying. Sometimes they will break gears trying to move a device. I have seen electrics bend boiler steel trying to move a damper.

Either type can have manual operators added, but not in 100% of cases.

Electrics come in more than one class. There is positioning - meaning it generally drives from a preset position to a second preset position, say closed to open and once there generally stays.

There is a mid range type that is good for 500 - 600 starts/hr. Not good for tight control.

Then there is full modulating - meaning having the ability to follow a constantly changing control signal up to 3600 starts/Hr. That is one motor start per second. When you get up in that class, the players that can meet that requirement are very few. Rotork bought the old Jordan Controls and the other one is Harold W. Beck & Sons. (David, I think that Fisher Easy Drive looks like the old Jordan MV-1000 series which I know that Fisher did use at one time.)

Electrics come driven by AC, DC brush and brushless, stepper motors, and AC synchronous motors. Take your pick and choose the one that gives you what you need. Electrics are capable of VERY tight position accuracy and repeatability.

I'd use them in a heart beat if they gave me what I needed. My last several picks have been pneumatic because they gave me what I needed and some of those picks were to replace electrics that were too slow.

Insrument air ain't free as previoudly stated, and:

Remember one thing about pneumatics. "Clean dry Instrument Air is an oxymoron."

rmw

 

[gerhardl]

The actuator discussion above is interesting, but perhaps a bit US-dominated? Seen from a European/Worldwide viewpoint I believe electric actuation is increasing in use. El- actuator solutions with integrated advanced control systems have been vastly improved the last ten years. Apart from that you will find almost 'religious' beliefs for both el. actuator and pneumatics, with engineers singularily promoting the one or the other.

If we stick to actuators for water-handling valves (in any form) my (typical?) European experience could be noted in some main points:

1. Both electrical and pneumatic actuators have a wide range in quality. If you select from the best and take care of selecting after real use, with performance tests and maintenance after given instruction, both types will perform excellent, no difference in dependability and accuracy. (Note: require operation within the actuators given performance data)

2. Cost aspects, lifecost and buying cost: not easy to generalize, but could be fairly similar in comparison (experienced several cases for smaller or larger plants), but could also be different, both ways. BUT: See note on integrated control systems under point 4.

3. Typical for selecting pneumatics: smaller valves, many on/off, many valves together, near eachother, and/or pneumatic system with sufficient capacity already in place.

4. Electrical actuators could also be used here, but high-quality electrical actuators must be selected. Because of price (at present) for smaller high-quality electrical actuators, either pneumatics are selected. If electrical are selected inferior quality el. actuators could be selected of price reasons, and this could lead to problems.

Note: some electrical actuators have advanced in-built control cabinets, saving more than the cost of separate control cabinets and extra cabling.If advanced control is necessary this could point to an el. actuator solution.

5. Typical where I would have preferred electric actuators: larger valves (12 inches and up), normal or larger pressures, both on/off and slower (damped) regulation. Also for singel or fewer actuators remotely placed. Battery-back up and electronical (threadless) steering and control are commercial available and dependabel if done correctly. (Experienced for valves and actuators un-axessible during winter, without problems).

6. Personally I am a bit sceptical to spring-return electrical actuators, if failsafe valve position is needed. I would rather have used pneumatics or oil-hydraulic with fluid (pressurized) reservoirs for back up, and failsafe solenoids for release if actuator failsafe position. Alternatively el.actuator with UPS - (Uninterrupted power system - battery with charging system)

One example of recent European advanced product development could for instance be worldwide AUMA (Werner Riester), advanced on integrated control systems, or perhaps Bernard or others.

 

[vpl]

RobsVette

Let me throw my two cents in. First, so you know, my background is nuclear (different from some of the others) so I have a bit of different perspective on things.

In the nuclear field, we'd look at failure modes -- electric valves will fail "as-is" or in the last position they were in when power is lost. Pneumatic valves fail either open or closed, depending on how the solenoids are set up. Both have their advantages and in nuclear plants there are a mixture of electrically operated and pneumatically operated valves.

For that reason, I don't recommend going 100% all electric or all pneumatic.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[Boynt]

I'm a bit late to this discussion, but I’d like to comment on how the market has grown well beyond the conventional MOV electric actuators. It’s time to kill the myths.

“Electrics are slow. Even the fast ones are slow compared to pneumatics.”

Electric actuators are no longer slow. Technologies that convert the rotary motion of a motor directly into a linear force through the use of integrated roller screws rather than gear tranes provide speeds up to 40 inches/sec.(configurable) You used to need a hydraulic to do that. It’s why turbine governor control is moving to this electric technology.

“Then there is full modulating - meaning having the ability to follow a constantly changing control signal up to 3600 starts/Hr. That is one motor start per second.”
The use of servo motors puts this start/stop modulating duty cycle to bed. Servo motors are meant to continuously change position. They have response times in milliseconds, and positioning accuracies to 1/1000th of an inch. Tht is why they are used in robotics or weld guns.

Design life – MOV’s (the good ones) boast a life cycle of 10,000 “cycles” to 1,800,000 “starts” (Modulating).

Servo valve actuators have a travel life of over 10,000 million inches.

“Is the service Class 1 Div 1 or Div ?”

I think most electric technologies have this covered. Hazardous, explosion proof, outdoors, -40 to 65C. In fact the electric actuators have an advantage outdoors in the northern climates. No frozen air lines.

“An electric actuator is going to start, move the driven device or trip out trying. Sometimes they will break gears trying to move a device”
Servo motor actuators can be current limited to prevent this, and can easily send a feedback signal telling the operator that something is wrong. This same capability allows them to change from an analog position mode to a seating mode, providing a firm valve seat.

I will admit that this technology is new to the process market outside of gas or steam turbine control, but it has been used for years in automotive, packaging and military application where the environment can be extreme.

It isn't the right fit for all applications, (nothing is) but it does compare nicely with other technologies.

Check out Exlar as an example.
Barb

 

[zdas04]

Good post Boyant. You really should post more, you seem to have a lot to say.

I proposed one of those linear actuators to a client in a place where traditionally we would have used a pneumatic globe valve. His instrumentation guy said it was too new and we should use a MOV actuator on a 2-inch 1/4 turn ball valve because the technology was "proven". My response was "proven to not work in a separator dump application" and explained why (valve linearity being the big one). We're going with the new valve (this one has actually only been on the market since January, 2012 so it really is new), but it was a close thing.

I frequently despair that automation guys will ever open their eyes.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[rmw]

Barb,

Good post, nice link. Some pretty impressive sctuators.

However - no disrespect meant, but these were pretty much toys compared to the electric actuators I use. I deal with stuff measured in ft-lbs and in the thousands and tens of thousands.

Notwithstanding that, I do have some applications where these will fit nicely and will keep this site in mind. I'll give you a star for it.

rmw

 

[Boynt]

RMW

I concur that high torque rotary is not the top application for servo gear motors, but for linear force apps electric servo technology can handle 0 to 80 thousand pounds and up. That’s a big “toy”. Scotch yoke hardware can be used with two linear actuators to get the high torque values you mention, however the sweet spot is really modulating applications rather than brute force on/off.

Barb

 

[JasonRobinson]

Hey everyone,

A little late to the conversation but I just wrote a small blog post on this topic. It consists of basic and rather general knowledge of pneumatic and electric valve automation pros and cons but it may help, who knows.

Blog Post

 

[zdas04]

Jason,
Not a bad blog, I can't say that your arguments fully supported your conclusion, but that is pretty common.

I really like the cartoon at the beginning. What would I have to do to get permission to use it in a training course?

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[JasonRobinson]

Permission granted, a company name drop would be nice even if it is in tiny print at the bottom of the page lol. I'm glad you like the cartoon my wife drew it up for me. In my defense you can't say that I didn't tell you the information was veg and general. When writing blog posts on the company website, my posts are picked through with a fine toothed comb, and the main things my marketing team is looking for is understand-ability. So that it is reader friendly for the potential first time actuator buyers. I also have to try not to sound bias due to the fact that the company represents both pneumatic and electric actuators. So you can probably imagine writing to those guidelines is rather difficult to make a resounding argument. If you would like I have more detailed documents involving technical specifications and forecasted annual average kWh rates.

If you would like to provide your email address I would be more than happy to send you further research, and the full size carton image.

 

[zdas04]

Jason,
If you follow the link in my signature, my e-mail address is toward the bottom of the page that the link takes you to. I'll cite the company name at the top of the blog post, if you want a different citation let me know.

My problem with your conclusion is if I have a valve that changes position twice a year, it would take several hundred years for the cost savings per actuation to recover the difference in the cost of acquisition. On the other hand, I have an application where I actuate a 1-inch valve 2-3 times/minute, comparing solar powered electricity to setting a gas-fired air compressor I get a payout of the extra capital (of the pneumatic valve, air compressor, and driver vs. the electric valve, solar panel, and batteries) in a few days.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.