Actuator type for shut off valve 5

[PAN]

In my company, the actuator for shut off valve always be cylinder type. If the vendors propose rack & pinion type, our engineers always reject them.

Please advise me. I would like to find the supporting reason.

 

[1969grad]

It is a mystery to me too. Seems to me that rack and pinion operators would be an economical application for small size actuators.

It is all in the sizing so perhaps the engineers in your company have been burned in the past.

 

[zdas04]

PAN,
What would be driving the rack if it isn't a pneumatic or hydraulic cylinder? What kind of valve technology are you talking about? For quarter-turn valves, the "cylinder-type" actuators all have a rack and pinion attached to the piston and to the valve stem.

David

 

[Sircrashalot]

David,

I think Pan is saying that they always insist on scotch yoke type actuators over rack and pinion.

I have often seen this in offshore specifications. The torque profile of the scotch yoke better fits most ball valve applications (high start and end torques, lower run) as opposed to many rack and pinnion units (torque decreases with stroke distance - i.e. high break to open and break to close, but end torques are lower). Also, most rack and pinion units are made of Aluminum materials, which make them lighter, but may present corrosion problems in certain envrionments. Personally as a designer and supplier, I prefer the flexibility to chose the best unit for a particular aplication.

I'd fight for a statement in the specification "Scotch yoke actuators are prefered, however alternate supply of rack and pinion actuators will be considered on a case by case basis." A 3lb 1/2" ball valve does not need a 25lb scotch yoke actuator...

Andy

 

[jsummerfield]

Sir Crash a lot has it right. Some plagiarized shutdown actuator sizing criteria follows:

Scotch yoke actuators are especially preferred for larger sizes. Consider rack and pinion actuators in non-hydrocarbon services for sizes 3” and below. Require spring return actuators for applications require where a failure position.

Generally, size each actuator to stroke the valve at a differential pressure equal to the maximum pressure for the ASME flange rating.

Include an air regulator set at the minimum air system pressure and size the actuator based upon this pressure.
Obtain the un-seating, running, and re-seating valve torque data, in both the opening and closing directions for actuator sizing. Select actuators capable of producing 1.5 times the required torque at each of these values.

John

 

[BSQUAREDBUCKEYE]

Gentlemen,

A rack & pinion actuator's torque decays through its travel? Why? The piston area doesn't change. The air pressure doesn't change. The perpendicular distance from the force to the point of rotation doesn't change. Are you thinking of a spring return actuator's torque output? The same would hold true for the scotch-yoke with a spring.

A double acting rack&pinion actuator has a straight line for a torque profile doesn't it?

A scotch-yoke torque profile is like a valley. The published torque output at either end and 50% less in the middle. Isn't that why you need to use a 1.5 multiplier when sizing?

Larger rack&pinions do get expensive in larger sizes and aluminum or stainless are the dominate materials and would have problems off shore.

 

[Sircrashalot]

Sorry - you are right that for Double acting Rack & Pinion, torque does not decay. I was thinking of spring return.

You use a 1.5 (or 1.25 or 1.3 or 2...) multiplier to get a safety factor to account for increases in operating torque over time, variances in the process, etc. You apply this at ALL points of the stroke when sizing. Sometimes you select your actuator based on the running torque, sometimes BTO, sometimes other points - it depends on the application.

Some valves have nice flat torque profiles that suit the rack and pinion - others have the valley and do well with a scotch yoke.

As for corrosion issues offshore, I have often supplied $400 actuators with $500 paint systems (and $200 butterfly valves with $2000 paint systems too, but that is another story...)

Andy

 

[BSQUAREDBUCKEYE]

Andy,

Perhaps I have this wrong and I don't know if I can explain it in words. My confusion is about the safety factors.

Valve manufacturers provide "valve torque requirements" in their literature. We then apply a safety factor to that number as you say depending on the process fluid. Gaseous oxygen is "sticky" and requires a 50% increase in the stated torque.

Relative to scotch-yoke actuators, it is my understanding that the two ends of the torque profile are the actuator's stated output for sizing and is 50% greater than the lowest or middle point of the torque profile.

Unlike a butterfly valve for obvious reasons, a ball valve has pretty much a flat line for it's torque requirment. That is the same torque required in the middle as well as the ends.

So when you are sizing a scotch-yoke actuator which has it's stated torque at the ends, you need to increase the requirement by 1.5 to get the middle of the stoke great enough to over come the valve toque requirement. Isn't that correct? In other words you need that 1.5 safety factor just to get the actuator large enough to handle the valve torque requirement. Not to provide additional safety. To size a scotch-yoke for the above oxygen valve you would need to double (2.0X) the valve torque requirement to select the proper scotch-yoke actuator. Is that correct?

 

[Sircrashalot]

Torque profiles for trunnion mounted valves are definately not flat (and floating too I think, but I don't actuate these much). The amount of torque required to cause a valve to move off its seat is related to the differential pressure across the valve. The running torque is more a function of the size of the ball and the seat material (as well as the fluid as you mention). An 8" 2500# valve with soft seats in PEEK will have roughly the same RUNNING torque as an 8" 1500# valve, but will have a significantly higher break to open (BTO) because of the higher differential pressure causing a higher friction force on the ball. Metal seated valves, of course will have flatter profiles than soft seated, but they will still dip a bit for the running torque.

When I am quoting a project, my valve vendor always gives me BTO, BTC, ETO, ETC, Run, and max stem shear torque (I know, I'm spoiled). Because of this I never have to quess what these other points are, and can size the actuator very close to the limits.

If you only know the BTO value (often what would be published in a catalog) it would be conservative to say the curve is flat and size based on this. Actuator will definately have more than enough torque - only potential issue is with your max stem shear - in some cases you could be bumping up against or exceeding this value. This only becomes an issue if the valve siezes or becomes jammed - during normal operation actuator will never hurt the valve. Still it is something you should be concerned with if you normally size like this.

Also your actuators are oversized and your competition could offer smaller, cheaper actuators. Often how I win work...

Andy

 

[BSQUAREDBUCKEYE]

Sircrashalot,

I think you are confusing the valve torque requirement charts which are given for valve size versus differential pressure, with valve torque profile which is the valve torque at different percentages of opening.

I would rather have my actuators oversized than undersized.

B2

 

[Sircrashalot]

I agree - oversized is better than undersized - up to a point.

Just finished up a project where requirement was for actuators to be 2x or more through the entire stroke range. This made selecting actuators VERY difficult as selecting actuator to provide 2x run torque could yield an actuator that provided 3x or 4x BTO. Sounds fine until you check the MAST and it's only 2.5x BTO...

Still in all of these valves, the running torque (torque required to keep the valve moving from about 10% open to 90% open) is less than the torque required to break the valve open or closed. How much less depends on the valve as I mentioned before. I wish there was as way to post diagrams and pictures, as I could post a couple examples of torque curves (torque required vs position opening) that I've been dealing with.

Andy

 

[GGOSCO]

There is another type of actuator that has not been mentioned here and is technically superior in both a double acting and spring return model.

A quality vane type actuator such as a Kinetrol, offers true rotary motion, without the hysterisis typically associated with rack and pinnion and scotch yoke. Not to mention, the Kinetrol offers the industries best warranty. I have personally seen Kinetrol actuators in high speed packaging applications, with over 30,000,000 cycles. Yes, 30,000,000

Rack & Pinnion and Scotch Yoke actuators transfer a linear force to rotary, while a vane actuator does not. The rack and pinnion has the parasitic losses of the geartrain, while the scotch yoke has a linkage, acting like a cam.

My experience has shown that several companies are working on the theory, 'If it ain't broke, don't fix it'. Unfortunately, as we all know, if new technologies are not explored, we don't grow and prosper.

I may be a little biased, as I am the Kinetrol rep. in Canada, but for a control application there is no doubt in my mind, it is the best choice. We have replaced hundreds of rack and pinnion, diaphragm and scotch yoke actuators and improved the perormance of the control package immensely, saving customers money and decreasing maintenance and downtime.

For critical service, fail safe operation, there is also an advantage to the vane actuators, in that the spring decay is minimal. Other actuators, use a compression spring, which decays rapidly, while a vane actuator uses a clock type spring. The other advantage to this is it is not subject to the same stresses as a compression spring and as a result, has a much longer life.

I would suggest visiting the Kinetrol website at

http://www.kinetrol.com

and taking a look at this style of actuator. It may be a little more expensive up front, but the ROI is much higher than any other type of actuator on the market.

 

[jsummerfield]

I do not know Kinetrol but it sounds like the Shafer vane type actuator. A problem with the vane actuator is that it lacks a spring driven failure position thus not fail safe.

John

 

[JimCasey]

Actuator type for shutoff valve is the title of this thread.

First consideration for an on-off valve is to get enough force to cause the flow to stop. That is dependent on the valve. There are more variables than anybody wants to read through, but those touched on earlier in the thread are valve type, size, seat material, probably shutoff pressure, process fluid lubricity, type of packing.... But it must be known so that the actuator can be specified.

The force required may change in service due to corrosion, product buildup, deterioration of the seats, "set" friction, and/or other variables. This is why there is a safety factor.

The actuator characteristics should be matched to the valve's torque characteristic. Butterfly valves have high breakout torque and a high dynamic torque at a near-open position. This corresponds nicely to the torque available from a scotch-yoke actuator. Ball vales have a breakout torque and the run torque drops off to 60-80% of the breakout torque, mostly because the pressure imbalance on the ball goes away after the ball opens. Rack and pinion actuators cope well with this.
But, the thread was not limited to rotary valves.
Gate valves are frequently if not predominantly used for isolation. R&P and scotch yoke actuators are not useful for rising stem valves. Gate valves need GOBS of force to force the discs into the wedge seating members used for shutoff (Even if the disc is the parallel-seat double-disc type) and after the valve cools down and grabs the disc they need as much or more force to yank the disc out of its seated position. after the end 1/16", the gate valve then only has packing friction and the weight of the components so it is relatively easy to move. To handle a gate valve you need a big electric (e.g.: Limitorque) with hammer-blow feature, or a huge oversized pneumatic cylinder, or a hydraulic cylinder.

GGOSCO brought up some interesting points about vane actuators. Every valve and, by extension, every actuator has its best application. However, and I hope this will be taken in the most respectful and constructive way: In My Humble Opinion GGOSCO went over the line:
"Promoting, selling, recruiting and student posting
are not allowed in the forums.
Click here to find out why."
This notice is posted right under the Message box. It's hard to miss. I work for a valve company, too, but I respect that this forum is intended to be nondenominational.

 

[rmw]

Dittos JimCasey.

rmw

 

[leecoop]

JimCasey,

Yes, gate valves are more usual for isolation. PAN asked about shutoff valves, and electric actuators have a hard time with failure modes. If you propose a gate valve, a linear pneumatic or hydraulic spring return is preferred.

Thanks
Cooper

 

[Ashereng]

There are many reasons why a site will only use a specific type of valves for isolation service. My current site is changing over to ball valves (from gate valves). I would think, price, is a goodly portion of the reason.

Once the valve is selected, like many have said above the actuator is then selected to match.

If a site manager has had a bad experience with, say rack and pinion, then that is a good reason for not using them again on that site. Not logical, but a good reason. And often happens.



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[JimCasey]

Ashereng: Ball valves are more expensive than gate valves, but pay for themselves in product saving, less leakage either through the seats or through the packing box, and easier maintenance. Lower cost of ownership with ball valves.

A typical ball valve will do a quarter million cycles. Try that with a gate valve and there will be a fountain where the packing used to be.
Servicing a gate valve means building up the seating surfaces with weld metal, and grinding them back to spec. Servicing a ball valve means replacing the seats and gaskets. Cheap-fast-No talent required.

Gate valves have no wetted elastomers and can serve at higher temperatures than Ball valves. Ball valve temperatures are restricted by the limitations of the seals. Softgoods in ball valves are usually TFE-containing blends. But then again there are specialty ball valves lines like Mogas that have no polymers inside and can operate at temps in excess of 1000F.

Gate valve actuators experience the same seal wear as the valve because of the long stroke required. A rack and pinion on a ball valve should last well past a million cycles with no attention as long as the air supply is clean. Wet air means rusty air, and rust particles are abrasive.

 

[Ashereng]

Full Port Trunnion Ball vs. Slab Gate. I think the balls are still cheaper.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[leecoop]

When comparing actuated ball valves to piston actuated gate valves, the price difference depends on size. I believe very large actuated ball valves are more expensive than the same size gate valve actuated with a piston. Metal-seated ball valves are very expensive. Through conduit gate valves are expensive too.