Gate Valve pains 2

[Boynt]

I have read several posts that comment on gate valves being hard to operate or difficult to maintain. As a actuator supplier, I am curious as to what the issues are when automating a gate valve, specifically a rising stem valve.
As we are always looking to improve, can you comment on the problems you see with gate valves? Any industry (wet or dry), size, stoke or actuation methods.
Here's your chance to vent!
Boynt

 

[zdas04]

In upstream oil & gas, mostly we don't actuate rising-stem gate valves. I've seen actuators in steam plants that just rotate the valve until it is shut (never seen an actuator set to open a big rising stem gate valve, not sure why not), but not in the field. Maybe because of the amount of saleable field gas it would take (access to electricity in the field is not great).

I don't know why no one has replaced the bonnet bushing with a sleeve and made a linear actuator, mechanically it would be pretty easy, hydraulically it would be a real challenge to most processes that I know of.

At the end of the day, in my industry gate valves suffer from being too hard to operate and not providing the high-quality seal you can get from a ball valve. I sure wouldn't spend a lot of R&D money developing a working actuator for a valve that won't seal.

David

 

[gerhardl]

Hi Boynt, as always we are probably all talking out from our own experiences, as you are inviting us to do. However, as you are starting the discussion, it would be interesting to know what type of actuating you are most interested in (electrical, hydraulic or pneumatic), and for what sizes and pressure classes, and also which industry type you are concentrating on.

From my own experience the short answer is yes, there are challenges actuating a gate valve, but theese can also normally be overcome doing the job properly. especially by selecting sufficient torque or force, and by doing the mechanical connections and mounting properly.

Very often connections are done as firm and stiff connections between actuator and valve, where links or some sort of flexible connections would hinder forces from one or the other end acting with full leverage on bosses, connections and sealings on the other.

Rising stem gate valves are in my experience best actuated by electrical actuators. They would control the movement and you would avoid sudden force changes when loosening from closed position. Extra gearing to keep actuator size down is possible (as slow opening as allowed recommended).

The challengc is fast opening or closing.

For knife-gate rising stem valves the construction is readily adapted for pneumatic or hydraulic linear actuators, in addition to electrical.

This is also done by standard gatevalves, but more seldom, as few producers have standards for mechanical alterations for this, and modifications on your own would make the factory warrant for the valve invalid.

By some gatevalves the actuation can be solved by an electrical actuator placed on brackets above the valve with stem extension down to the valve, replacing the manual operation, without any rebuilding of the valve.

Most operational problems by rising stem actuatad gatevalves comes from uncorrect operation of the actuator, for instance lack of regular test operations, greasing or inproper application of the valve itself (other type of valve is better for the process application), or false installment or adjustment.

Please feel fre to ask direct questions if you have any special problems!

 

[Boynt]

Thank you for your feedback. I represent an electric actuator company, but am interested in issues from all types. I find your comment that the "challenge is fast opening or closing" most interesting. Is it full stroke (full open to close) that is most needed, or fast response to a comand to open or close? Or is it both?
We've had inquiries from coal handling, waste water, cement and several in between. I'm trying to understand why the current technologies aren't working well in the fist place.

As for sealing - is it the valves that can't seal, or the actuators can't provide the proper sealing force?

 

[rmw]

Gate valve actuators typically don't have a very high duty cycle rating. Being the type of valve that they are, they open when the plant starts and close when it shuts down with maybe the occasional stroke when some piece of equipment it is associated with is taken off line.

I suspect that with modern operation with lots of plant cycling, they are asked to cycle more often than the original designers anticipated. But that's just a guess.

Also, since not a lot of new plants are being built in our country, I'd suspect (verifiable with a little research that I will leave to you) that most of the old name reliable gate valve actuator companies have gone by the way.

Some of those things were beasts. They were made when the business model was the more the plant cost to make, the more money the regulated power company made. They didn't spare much iron in those days.

I have some (electric) actuator background and about the last thing I think I would want to set out to do in these days is to develop an actuator for gate valves. But... that's just me. Good luck to you.

rmw

 

[tr1ntx]

"We've had inquiries from coal handling, waste water, cement and several in between. I'm trying to understand why the current technologies aren't working well in the fist place."

Rising stem gate valves are going to be multi-turn & high torque, with the torque increasing substantially as the valve builds up sludge and/or scale. To get a tight seal, the ending torque (seating torque) required can be a large multiple of that required for the rest of the stroke. Sometimes it has to be brought on and off the seat several times to make seal, or the operator needs to feel it set firmly, not a "spongy" stop.

For these reasons, and others I'm sure, plus the things that zdas-4, gerhardl, and rmw mentioned, other types of valves tend to be used where actuation is needed. Advances in polymer seal technology have allowed high temp service for other types, for instance.

 

[tr1ntx]

One other thing I thought of regarding the "multi-turn" aspect of the operation, the number of turns will vary with valve size and manufacturer. There can be metric or US threads, or it can be fine or coarse threads depending on the pressure class. Or they could make a non-standard thread. So you can't count revolutions, it would need stop signals which would require calibration for each installation. And custom installation hardware due to stem differences.

 

[reverman]

My experience is primarily with wedge gate valves. A future opportunity I see in the market is to come up with a cheaper method of dealing with thermal stem growth.

Most power plants being built are running hotter and hotter steam for better efficency. This causes linear stem growth which can lock up the motor. The current solution to this is a thermal spring compensation pack in the motor however they are a very expensive option. I think they can add up to 20K to the motor cost on a really large motor

The spring packs are also used on extremely high speed motors to prevent the wedge from getting stuck in seat.

If somebody were to develop a cheaper solution I think you could sell it to power plants as a added safety option with a lot of success. especially plants which are running steam +1000°F

 

[Boynt]

Thanks for all of the feedback. We have found opportunities to replace existing actuators on gate and knife valves when the stem is a linear rising stem (not a rising rotary stem) Reasons for the retrofits varied, so I wanted to get a better understanding of why the industry has issues with this actuation. Most were hydraulic replacements.

 

[sjrfc2]

I remember wedge gate valves have some unique problems. The solid wedges can get jammed in place while they're closed when the plant comes up to temp. This can be from piping flexure that causes binding on the wedge as the valve body flexes. I hadn't heard of the stem growth but it sounds like a similar principle.

Flexible wedge gates with non-drainable bonnets can have the potential to have water trapped in the bonnet while the valve is shut. If the plant comes up to temperature and/or if the by-pass is open, the trapped fluid can expand and overpressurize the bonnet or make opening difficult.

I think I read some info about the MOV gates and binding (not operating in emergency shutdown installations) on the NRC website. I don't remember if they were solid wedges or flex wedges with heated fluid while closed but it's worth reviewing their info if you have time.