ESD Valve actuator 3

[cam72]

Hi

I am not familiar with ESD valves and new to its design. What are the factors that determine the torque requirements of ESD valve actuator? In case of emergency how the torque is taken care of? Are there any arrangements(protection) in actuator which ensures that there is no shearing of actuator in case of excess torque?

Thanks.

 

[pennpiper]

An ESD valve is not some special mystery design or contraption. It is just a line spec valve with some kind of motor or pneumatic operator on it.

The torque for the valve should be the same if operated manually or by motor.

The main protection that should be considered for the operator is fire proofing to prevent damage and allow the valve to still function even when fully engulfed in flames.

 

[JLSeagull]

Torque numbers exist for breaking from open toward travel toward closed, from closed toward travel to open etc. At the lowest air supply pressure the actuator must have sufficient torque to exceed the valve torque with a reasonable safety factor such as 1.4. And if lacks a spring return it isn't an ESD valve actuator.

 

[itascot]

ESD (Emergency Shut Down)
Valves have torque safety factors that can vary from 1.3 up to 2, depending on project specifications, and can also vary from Break to Open, running & End to close positions.

Speed of emergency stroke operarion can also vary from 1 second total, to 1 second per inch of diameter, to 3 seconds per inch daimeter.

The Safety factor has an effect on (some) valve shaft diameter, or material, due to much higher requisites (due to increase in resistance torque) as requested torques exceed valve MAST.
Most of ESDV are low pressure pneumatic operated (unless you are in the desert on a gas pipeline, where line pressure= to actuator sizing pressure), by low pressure anything between 3 & 5 Barg is considered...a Air Filter Regulator should be included in instrumentation to ensure that supply pressure to actuator does not increase over sizing pressure...increase of pressure to a piston = increase to torque output of actuator.

The speed of operation can also cause problems, that are mostly solved by intervention on actuator & instrumentation;
QEV (Quick Exhaust Valve) are fitted on discharge to allow faster stroke.

This may cause problems to valve, when slamming shut (damaging seal/seat & possibly connections between valve & actuator...lots will argue that it's enough to set mechanical stoppers in actuator, however these are not designed to take this sort of shock...it would not be stopping only actuator stroke (fast) but would also be taking high loads caused by inertia (valve disk can be quite a heavy mass traveling at high speeds).

The actuator manufacturer needs to add a hydraulic damper to come into action during the last few degrees of closure (slowing the speed of stroke to a "rasonable" speed before it makes contact with travel stops, but still respecting stroke time requirements).

Hope this gives a general idea & can be of some assistance.
Ciao,

 

[hsbcn]

You might find this paper useful:

http://www.safetyusersgroup.com/documents/AR060001/EN/AR060001.pdf

Heather's focus: Alloy Valves Duplex Valves Super Duplex Valves Monel Valves Inconel Valves [link titanium-valves.com]Titanium Valves[/url] & 6 Moly Valves

 

[cam72]

Hi
Thanks everybody .. its good beginning for me with valuable info. itascot ur info was great, thats what I was looking for. G8..

 

[leecoop]

Dont forget...Its not just torque, it could be thrust if its a gate valve or globe valve. Typically, we use a spring as well.

 

[itascot]

Ieecoop, you are quite right...in case of linear valve/actuator, same goes as the above, but dont forget to consider a hammer-blow coupling between valve stem & actuator rod, this will aid in opening valve.
Ciao,

 

[JimCasey]

Also with ESD valves it is appropriate to have a system/device to perform Partial Stroke Testing (PST). This verifies that the valve will at least move away from its full-open position and gives implication that it will close on command. Partial stroke is used so as not to disrupt the process-the valve is typically moved from full-open to 85% of travel, and the PST positioner records the valve's signature, compares it to a recorded baseline, and reports whether or not the valve is trustworthy to shut down the plant in an emergency. PST is usually scheduled on some sort of regular interval.

Back up to Itascot's post: Valves become progressively more difficult to move if left in place. Even if sedimentation/plate-out of process products/corrosion damage don't happen, the seals microscopically cold-flow to conform to surface irregularities, and the break-torque increases with time. If an ESD works the one-time it is really needed, it pays for itself. Put all the actuator on it that the shaft/stem will take.

 

[terje61]

JimCasey: be carefull with doing partial stroke testing. Although it has now become a more common requirement, this is mostly for dubious reasons. In my opinion:
- PST does not increase safety levels.
- PST does not prove that an ESD valve will close.
- PST does not show that valve forces have increased.
- With PST you can not skip regular maintenance.

The only thing PST shows is that the valve is not stuck in the full open position. The reason for having an ESD valve is to shut-off a pipeline. These are two very different things.
It might be better to select a valve that doesn't have the tendency to get stuck in the open or any other position.

Regards,
Terje

 

[itascot]

Sorry Terje61, I dissagree with comments on PST.

- PST DOES increase safety levels (reasons are below and self explanatory).
- PST does show when valve forces have increased.
- With PST you cannot skip regular maintenance, but can avoid (or at least reduce)unforseen shutdown...Preventive maintenance.
The reason LSS (Long Stand Still) service valves need PST is to reduce the effect of sticktion hence giving a lower probability of failure due to this sticking.
Analitical PST have reduced shutdown costs just about anywhere automated valves are in use & have contributed to increased safety on plants due to giving information that otherwise, would have been impossible to determine "before total malfunction" and we all understand what that can mean on Emergency Shutdown situation....
Ciao,
itascot

 

[terje61]

Hi Itascot,

I will explain my arguments:

1)
If a valve is susceptible to sticking you do not require PST for safety, but for NORMAL operation. Hence you cannot use the PST for increasing your safety level.

2)
Valve forces depend on many items:
- temperature (changing viscosity of your grease)
- internal pressure (will increase pressure forces on your stem / will increase seal friction)
- flow conditions (flow dynamics will work against operational forces)
- air supply to the actuator
- valve sticking
- etc.

Conclusion, it is extremely difficult to determine the reason for an increase or decrease in valve force (if at all measured correctly).

3)
If you plant depends on your ESD valve, buy a valve that is not susceptible to sticking and do serious tests. If you want the valve to close -> test valve closure.
Note that most PST test units do not:
- measure the pressure sensing
- measure the systems logic
- measure the solenoid valves operation
- measure the valve closing speed
- measure valve closure

They only measure if a valve intents to close. Which should be extremely easy because at this point the actuator springs are fully compressed and at their strongest!!
It is much more difficult to perform the final closing when the actuator springs are fully elongated.

Therefore my conclusion: be carefull with partial stroke testing, it is not a cure for poor ESD-valves.

Regards,
Terje

 

[itascot]

PST are of little use if poor quality valves are used for ESD service. I firmly beòieve this is 100% correct.
How many specifications call for SIL III when dealing with ESD servie...The PST you are considering must not be the ones I am accustomed to seeing every time I read specs.
Find a reliable doc. in link, that may explain better all PTS (good ones) functions.

http://www.valve-world.net/actuation/ShowPage.aspx?pageID=564

Ciao,

 

[terje61]

Hello Itascot,

The supplier mentioned in the article you refer to has been running around our equipment for several days. This system only measures the outlet air from the actuator. We (the supplier and I) have found that this is the worst indicator of anything ever.

Example:
- 16" 900# ESD valve, closing time < 2 sec.
- We made a valve signature at 10.00h. and adjusted the PST device to close the valve approx. 20%.
- After lunch we wanted to perform the SAT and we found that the fast operating ESD valve closed for more than 50%. The valve signature did not match in any way to the initial signature.
- So we re-adjusted it, back to 20% and made new signatures.
- Then our inspector asked for several valve strokes and a new PST.
- This time the valve closed fully (!) before it started opening. Again a completely different valve signature.
- We stopped the SAT.
- Without any adjustments we did a re-run early the next morning. The valve closed for less than 10%.

Our conclusion was that the device is not very reliable. Probably due to temperature differences (warmer grease/hydraulic dampening) the PST will give completely different outputs. Evaluating the curves as mentioned in the article can be compared to astrology. The PST technician provided answers from the graphs that did not match the stroking we saw on the video later.
Note that we also found uncomparable results when the pipeline pressure changed and when the actuator supply pressure changed.

If you do PST, as a minimum use a device with pneumatic feedback AND stroke position feedback.

Sorry if I sound like a know-it-all, but I have become very disappointed by all promises that have been made by PST suppliers.

Regards,
Terje

 

[itascot]

Terje,

You may be right regarding this particular equipment manufacturer. I do know that some of them have problems with quick stroke when actuator has a large volume of air to exhaust, as they use QEV & this does not help response time from the PST device and can give irregular/unreliable data and working condition, however this can be solved by using a dedicated loop in control system (by-passes main control for PST and only for these purposes).
Other products are a little more refined & can give the indications you mentioned.

http://valveproducts.metso.com/neles/TechnicalBulletins/en/7ND9020EN.pdf

Dont worry about sounding like a know-it-all...Ive bee around valves & actuators (or gearboxes) for ove 1/4 century & still learn something new all-too-often (main reason I take part of this eng-tips community).

Ciao,