19 replies to this topic

[iplan]

We have two equal sized parallel operating control valves on the steam letdown station sharing the load equally. Once there was a problem of the one of the control valve closure due to a mechanical failure. The other control valve eventually managed to handly the flow requirements.

The failed control valve was replaced and the lone control valve which took load was isolated and exmained. A mechanical damage was noticed in this valve. My endeavour is to ascertain the failure of this valve.

Though broadly it can be said that due to the sudden load shifting, the control valve has suffered, how to calculate the sudden force it was subjected to at that moment (provided the flow values are available).

I am thinking of calculating this by

1. From flow reduction, calculate the momentum endured by the valve stem by mass x velocity and compare with control valve data sheet ( though i doubt the value will be there or not).

2. From the flow reduction and downstream pressure, calculate the delta P offered by the valve at that point of time. compare this with the max delta P the valve is designed.

This excercise is aimed to redesign the valve to eliminate such future problems..

Requesting for your insights please.

Thanks in advance
iplan

[Qalander (Chem)]

It's somewhat resembling to a 'water hammer like' scenario although not exactly.

There is a sudden change in flow rate associated with great velocity change indeed,I believe;

Thus shocks are generated along with highly vibrant conditions even for shortest period of time contribute to such damages.

[iplan]

Hi

I wrote,
"The failed control valve was replaced and the lone control valve which took load was isolated and exmained. A mechanical damage was noticed in this valve. My endeavour is to ascertain the failure of this valve."


Please read the last sentence in the above paragraph as " ascertain the cause of failure".

thanks
iplan

[Qalander (Chem)]

Sorry,

I assumed discussion was on and that's the reason I entered.

My apology.

[JoeWong]

iplan,
When you said mechanical failure of first control valve, what kind of failure ?

The failed control valve was replaced and the lone control valve which took load was isolated and exmained. A mechanical damage was noticed in this valve. My endeavour is to ascertain the failure of this valve.

Some confusion here. Mechanical damage on the duty control valve (failed and being replace) or the spare control valve ?

There are several issues in this kind of system :

i) flashing and may be cavitation
ii) water hammer
iii) thermal shock

[iplan]

I should restate the problem, which I hope will be much clearer:

Two, parallel, equal-percentage, control valves A & B were in service. Control valve A failed first. The DCS showed some opening of the valve but, in the field the control valve stem was found to be detached.

Upon valve A failure, Valve B assumed full load. Valve A was, meanwhile, repaired and placed back into service. Then, Valve B was examined. Valve B was also found mechanically damaged. The reason for Valve A failure was seal ring damage. What is of interest to us is: when valve B took full load, what exactly caused the valve B to experience mechanical damage?

I apologize for not adding a sketch.

thanks again
iplan

[mishra.anand72@gmail.com]

Where r u?? may be valves were due to service. Said part life was due to expire. Control valve actuator has gone wrong. Control valve was hunting a lot. blah blah blah. I fail to assume process problems. Wn one control valve can take full load, why u hv been given 2 c/v??

[Qalander (Chem)]

Dear iplan Hello/Good Morning And thanks,

For the explanation that was already grasped.

Dear mishra Hello/Good Morning, Its always good to see you contributing in forums,
but I anticipated your positive cooperative response 'regarding use of better written' English language indeed!

[mishra.anand72@gmail.com]

Hai Qlander

Good day.

Thank u for ur appreciation.

u no japanese classify ppl as operator and operator users; engineer and engineer users. What difference does it make?? I don't no. What is the problem in being engineer users?? v all r engineer users?? don't v??

Don't worry about ur problems?? I assure u my problems are bigger than u.

[djack77494]

mishra,
If you have any interest in making postive contributions to these forums, please heed the advice you've been given and learn to utilize proper English. If nothing else, it will prepare you for useful engineering reporting, a valuable asset in engineering. If you cannot be clear, accurate, and precise, your value as an engineer is greatly diminished. The safety of personnel and property could well depend on your ability to accurately convey a design or procedure to others, and gibberish like the above convey nothing. For gaming, try one of the many websites that provide that service; this site is for engineering.

[ankur2061]

Doug,

I think now there is a serious case for banning trash on the forum. Of late, certain members have been doing a great disservice to the forum and the engineering community by posting trash on the forum.

If I recall, eng-tips has been very strict about the quality of posting on the forum. I would request the forum administrators to look into the matter seriously and completely ban trashy postings and the posters from the forum.

Such postings really diminish the stature of the forum.

Regards,
Ankur.

[Art Montemayor]

Ankur:

I can assure you that your and Doug's concern has not gone undetected and is shared. I personally have taken a step back in recent weeks and tried to be be patient and willing to wait for positive changes. This optimistic and "benevolent" attempt has obviously not produced fruit and I have already taken one step to remedy this distasteful and sad situation. I am sure that you and Doug share my sadness in seeing a fellow professional comport himself/herself in a distasteful and insulting manner. We all suffer when our profession is invaded and mistreated by such comportment and there comes a time when exposure to such tactics cannot be tolerated any longer. Human endurance and tolerance has a limit.

Our Forums have a reputation for being a resource and help to other professionals and future professionals while demonstrating mutual respect. Tolerating abusive and inconsiderate treatment of our members as well as language lacking in communications and value to our members is not adding value to our contributions and it goes against the spirit under which these Forums were formed, and I am sure our administration will help us in resolving this issue.

[Qalander (Chem)]

Dear All,

Although I tried my level best(knowing all my weaknesses) to take the situation towards positive and constructive,sustainable discussion.

But I feel to have failed disappointingly indeed!

[davebartran]

shifted flow is not the likely cause alone.

my own experience was the valve sizing condition had increased as the plant was modified. The valve discharge velocity (superheated steam let-down) eventually exceeded 100 m/s. Produced extreme noise and severe vibration levels in the valve. The valve trim battered itself to failure. Similar to you discription.

the replacement valve would have required re-piping the entire steam letdown piping and there was no room for the increased pipe so the decision was increased maintenance.

We eventually used inconel valve internals, but it only increased valve life by a few months, before it required replacement.

[JoeWong]

iplan,

I don't see a straight forward answer to your question without series of troubleshooting. Problem may come from start-up / operation i.e. valve intended for full flow, valve limitation, quality of the valve after being long time not in service, etc, may be started from design stage i.e. correct material used (infact you still have not clearly stated the mechanical problem of valve B that you mentioned, which part experience problem, correct trim used, etc.

Try to assess from valve selection, material used, correct trim, etc in design and specification stage. Next assess if selected valve meeting the specification and properly certified. Next interview operator how this valve is put in operation, how they operate this valve, compare system performance when it is in operation, how they aware the valve got problem, noise, vibration level, etc to ease your troubleshoot, next get vendor input on this matters...

I am sorry for not being helpful in this thread but that's all i can offer...

[Qalander (Chem)]

We suffered similar damages scenario at my old employer's one 600# Boiler Let -down stations; On two parellal Control Valve(s)

and frequent damages had almost become a routine and
sometimes,we had no other option except

to repair these at refinery's metal workshop with shorter service life span ;since original one's Import lead cycle very long.

[iplan]

Many thanks, I found that the Valve maximum flow specified in the data sheet is less than the actual flow that can occur ( and also ocurred) through this valve during this upset. As Davebartran says, it may be a case of velocity exceeding the limits.

Also, as JoeWong says , the trouble could have started with a poor selection. or a maloperation in line up. But we are looking for any process issue that could be the reason so that it can be addressed.

Again, once the higher flow ocurred through the valve, the velocity may be the major source of the valve damage. Nevertheless, since they are parallel control valves, on failure of one, there was a time delay before which the other one took load ( as always in control valves on auto- the response is gradual). What happens at valve upstream during that time?

Once again, many thanks for your time and inputs,
iplan

[Qalander (Chem)]

Incidentally my very first reply, 25th March response to your query said this all please see above, even before davebartran's post.

Still since you got some degree of satisfaction that's enough achievement for me indeed!

[iplan]

Qalander,

Your point is taken and agreed. Cannot thank you enough. Just one more thing. The velocity effect will be the same under the same flow irrespective of circumstances under which the flow ocurred through the valve. But I feel there is a difference between a gradual decrease in flow and a sudden decrease. In my case, the sudden closure of the control valve subjected the control valve that closed to a momentum of approximately 5000 lb-ft/sec ( flow around 200000 lbs/hr in 8"line-900#steam @ 800F). Had there been a gradual decrease in flow, the momentum effect would be much less.

Again, I am not sure of the above concept and I want to hear from distinguished forum members on this.

thanks again,
iplan

[Qalander (Chem)]

Thanks;Somehow its my gut feeling that if service is for very drastic pressure drop as we had i.e (something from 600# to 150#).

1)Then there is very little one can do with gradual change(s) except to prolong slightly the cycle life.

2)As regards having single valve in service instead of two designed for parallel service (presumable distributed load shared design);We can not stop the damages to occur may be able to marginally decrease as mentioned in first point above.

There are much more learned and skillful colleagues here to guide further indeed.