Hysteresis & Dead Band Tests on (electric actuated) ON/OFF valves (!) 3

[NGiLuzzu]

Dear All,
as long as I know, the tests in object are typically applicable to control valves, especially when (smart) positioners are present.

Maybe You'll want to refer to IEC 60534 standard, Part 1 (July 2005) and Part 4 (June 2006) for definitions and/or to Shell DEP 32.36.01.17-Gen specification (just to make an example) for more information.

Notwithstanding our objections, one Customer insisted to have this kind of tests done on on-off valves, operated by means of electric motors and gear reducers (!).
Is this just a persevering error, or are there any actual reasons to investigate what happens to the valve obturator far from the fully closed position?

Has anybody ever faced a similar situation?


THANKS to All in advance, 'NGL

 

[ione]

Just my opinion.

Does your customer know the definition of hysteresis?
If the answer is yes, he should know it does not make sense to consider this parameter for an on/off valve. Otherwise he knows the definition but does not understand it.

You could show him a diagram which illustrates the meaning of hysteresis, applied to a proportional control valve, that is in a case where it is meaningful to talk about hysteresis.
Then explain him that it takes a complete cycle for the valve to establish the max difference (as measured along a horizontal line) between the characteristic valve curves with increasing and decreasing input.
For an ON/OFF the valve has just two positions of interest (fully open or fully closed). What to do with an hysteresis value that, no doubt, occurs for an intermediate position of the stem?

 

[bcd]

Exactly as ione mentions. Unless the actuator has the capability to accept a variable electrical signal range (for example 4 - 20 mA), how do you position the actuator mid-stroke to do the tests?

Some electric actuators have this capability. If so, then the test can be done. Also, consider that some on/off valves have a dual purpose and can be used for throttling during upset plant conditions.

More than likely, someone copying a specification to write the new one didn't know what they were doing. If the specified actuator is not capable of placing the valve mid-stroke, then the test cannot be accomplished per the IEC or ISA standards for control valves.

 

[MikeHalloran]

A geared valve may take a long time to get from the ON position to the OFF position, and vice versa.

If the customer is doing a transient analysis, and/or is planning to modulate the valves, then the hysteresis and dead band would be of interest.

Agreed, recycled boilerplate is a more likely cause.


Mike Halloran
Pembroke Pines, FL, USA

 

[itascot]

anegri,
Did the mentioned electric actuators come equiped with fieldbus communicator & smart logic ? as you probably know, actuator manufacturers market all the electronic goodies for all kinds of services...was you customer asking you to do a valve performance test OR what he really wanted to see, was the actuators capabilities ? The gear reducers mantioned were connected to electric motors OR were they actually manually operated gearboxes ?
As far as valve obturator behaviour far from closed position, this can vary depending on all the play you may have in actuator/valve stem connection , valve stem to obturator connection...these can cause constant (even is small) changes in obturator (easier to write Disc) position do these sound like possible reasons to test ? to some people may-be.
Ciao,
Ian

 

[NGiLuzzu]

itascot,
there was no communication equipment on the actuator: we performed the test just stopping the motor at the required values of stroke percentage by means of pushbuttons and checked the stem position against an angular graduated scale we added of our own (I know that the valve obturator position was to be measured instead, and I also know that a more accurate isntrument, like a potentiometer or an encoder, could be used... but we didn't feel like to give a too sharp answer to an obtuse question) ;-)

In the specific case, as ione and bcd said above, the End-Users do not probably understand the meaning of hysteresis (even if we tried to explain with graphs and references to the definitions from international standards), but still mantain this requirement in their specification for on/off valves (probably due to a typical copy/paste error)... Moreover, the Engineering Company find easier to get us perform meaningless tests than to make them apply reason.

That's just my reconstruction of facts... but I think everybody can easily acknowledge it, once it is summarized as per the attached file:

http://files.engineering.com/getfil...5b-9077591001ca&file=History_of_a_Project.JPG

In more general terms, my question was just a curiosity about the possible actual applicability of hysteresis and/or dead band testing to on/off valves...(?).


THANKS to All, 'NGL

 

[terje61]

Hello Anegri,

I cannot see any actual application for hysteresis and/or dead band testing to on/off valves.

What I do see is the growing interest in making valve/actuator footprints. Which is actually also some kind of hysteresis + dead band test. With the footprint we try to establish if the valve is working as good as when it was delivered (no sticking / additional friction etc). And then we say that we do not need to do maintenance which saves us a lot of money. (I hope it is clear that I have some doubts regarding this method.)

Regards,
Terje

 

[NGiLuzzu]

Hi terje61,
I was aware of a similar kind of testing performed - for example - by SCORE Europe Ltd. (

http://www.score-group.com/)

for a BP project in Norway.
As far as I can remember (and as I could understand), the scope was to state the initial performance of actuated valves when they were brand new (at the factory acceptance testing stage) mainly in terms of torque requirements, in order to establish a "starting point" for their monitoring system; partial stroke testing was also included in the procedure.
They called it "Benchmark Performance Test", but I believe it may have more or less the same meaning of your "footprint"... would you agree?


Thanks and Regards, 'NGL

 

[terje61]

The "Benchmark Performance Test" looks the same as what I meant with a "Valve footprint". When using pneumatic operated valves it is almost standard available in smart positioners. It more or less consists of a hysteresis test / dead band test / response time test.

As mentioned, it is used on several plants to delay the moment of preventive maintenance. Personally I believe you would be walking on thin ice, but good salespeople can do miracles. A Shell plant manager explained me that they are saving a lot of money because of preventive maintenance that is not done. I hope that I will not be there when it would have been better to do the maintenance job. The good part is that you are at least testing your valve.

Partial stroke testing is in my opinion even more tricky. By only moving the valve partially or having the intention of moving it, some people think that they can predict if a valve is still in good shape.

But the idea is the same as what I was trying to explain regarding your question. Your hysteresis / dead band requirement may have come from the monitoring people.

Regards,
Terje

 

[itascot]

I recall the term "valve signature" applied when related to Valveguard system (I dealt with this product for phneumatic actuators) fuctions are more-or-less described in above replies, but aimed specifically at predictive maintenance (change in signature, meaning change in working conditions, meaning close monitoring till next opportunity to access valve/actuator & then carry our preventive manitenance, avoiding unsheduled shut-down & related costs).
Anegri, unfortunatelly, I have been in your position...at the end of the day, the customer is the one that pays for what he wants, maybe you need to be involved with your sales department when quotations are issued & assist them in avoiding this kind of situation (state clearly at bidding stages what tests are considered applicable to your product & deviate from non applicable requests at bidding stages).
Ciao,

 

[NGiLuzzu]

terje61, itascot,
THANKS for Your intresting contributions!

Just to reply on the last issue: we did raise the concern since the bid stage, also saying that the test was meaningless during meetings overseas... and the Engineering Company answer was more or less: "Yes, we understand... but just propose a test procedure for our Customer anyway!"
That's how it goes... ;-)

Best Regards, 'NGL