Loads on suction nozzle conn. & combined resultants per NEMA fails

[MQM90]

Dear All,

We have already procured a steam turbine and its existing piping is being checked by piping specialist. The loads on suction nozzle connection and combined resultants (i.e. Limit 3) are not passed as per code NEMA SM 23.

I just want to get some sort of help or any engineering opinion to get the piping loadings and its impact reduced on suction nozzles according to the standard.

One idea is to make all support’s friction factor reduced by using Teflon or any other material that reduces the friction factor between the pipe and structure, because adapting above technique the turbine nozzle loadings as per NEMA SM 23 is within the limits.

I also want to confirm above engineering technique is viable in steam turbine piping or not recommended?

Appreciate your relevant help in above context.

Best regards,


Osama Nusrat Ali

 

[BigInch]

Be happy you have found some way to get the loads under allowables. Your only choice is to make judicious use of anchors and guides, sometimes with low friction teflon pads and/or spring supports as required to get under allowables.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[SNORGY]

Before you go much further...

Check to see if the turbine manufacturer gives you more than 1.0 x NEMA SM-23 allowables. About 20 years ago, I was involved in one of the first and most significant stress analysis problems involving hot rotating equipment in my career. The inlet to a new turbine was via piping from the pipe rack, the tie-in point for which was on the corner of an expansion loop. (It couldn't have been any worse...). I probably burned 200 CAESAR runs before I found something that got me to within 1.0 x NEMA.

Then, sadly, inspection of the fine print in the vendor documentation and layout drawings revealed a table in which the turbine manufacturer accepted 2.0 x NEMA SM-23 allowables.

Quoting my hero and idol...

"D'OH!"

Regards,

SNORGY.

 

[MQM90]

I got some answer to my points.

Thanks

But there is one confusion, that the method of using teflon pads between the weight support and the structure is a feasible solution to the problem or not ?

As per BigInch it seems to be feasible and as per SNORGY it is recommended to check the turbine manufacturer documentation.

So in the end, i want to know whether teflon pads can be used or not ?



Osama Nusrat Ali

 

[BigInch]

I obviously wouldn't have any problem with using teflon pads as long as steel/steel pads and adding flexibility was not producing a solution.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[SNORGY]

Teflon pads are probably OK. Your pipe will be insulated and on shoes anyway. Stainless steel and graphite are other options.

I read somewhere a while back (like 20 years ago?) that slide plate assemblies sometimes need a threshold activation contact pressure to be effective, so if your load is less than 500 psf across the contact surface, they provide little to no benefit compared to regular steel on steel. It might have been out of an old Fabreeka catalogue or somewhere.

Regards,

SNORGY.

 

[MJCronin]

"Your only choice is to make judicious use of anchors and guides, sometimes with low friction teflon pads and/or spring supports as required to get under allowables.

Nope, BigInch....wrong again...this is not the only choice for MQM90..

Another option is to more accurately model the equipment stiffnesses in the piping stress analysis.

Most programs default to a very high stiffness value.

If you can get the equipment vendor to give you more realistic numbers (he may have already developed an FEA model of his equipment for other purposes) you can use this to get more realistic numbers.

-MJC

 

[MQM90]

Dear All

I am being directed in right directions. Thankyou all of you

Now some point needs further clarification:

1. Regarding SNORGY reply, thanks he wrote in my favor i.e. using teflon pads seems no problem but the point regarding contact pressure to be effective or not? should be well taken care by the software (Bentley AutoPipe Plus). Because it runs AND simulates the actual operating conditions and finally applies imposed loading on all points (e.g.resting support with shoe configuration and placed on structure). The output therefore solves the reactions forces/ moments at all points.

And using teflon pads, reduces the reaction forces on suction nozzles i.e. to the extent that the reactions forces/moments / combination comes within the manufacturer allowable limits(NEMA SM 23).

This concludes SNORGY comment that teflon pads can be used as far the nozzles loading are within allowable.

2. Regarding BigInch reply, thanks he also wrote in my favor i.e. as steel/steel pads was not effecting reactions at all points. This comment again supports me using teflon pads on steam inlet lines. hah

3. Regarding MJCronin reply, thanks for joining me. I have asked my steam turbine manufacturer to provide me equipment stiffness. But MJCronin i was wondering, that in my piping software we normally use default stiffness values for the material. However, i will reconfirm the calculation using modified stiffness if provided by the manufacture.


Overall i am satisfied using Teflon Pads but anyother justifcation by all of you can really add a good taste to this project and my knowledge.



Osama Nusrat Ali

 

[BigInch]

MJC, if you like distorting your equipment, no problem for me.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[SNORGY]

MQM90:

Thanks...

Be cautious about how you interpreted my post. The intent was to inform you that *a long time ago*, I had read that unless you actually impose a load on the order of 500 psf on a teflon slide pad, it might not offer any advantage over steel-on-steel contact.

I'll look for where I read that...I could be wrong. That happens sometimes.

Regards,

SNORGY.

 

[SNORGY]

Looking into this a bit...

The value for loading of a teflon slid plate appears to be closer to 5000 psf, not 500 psf. Otherwise the message is the same: the coefficient of static friction rises sharply at lower contact pressure, to the point where marginal benefit is gained over steel on steel.

Regards,

SNORGY.

 

[ColonelSanders83]

To date I have designed piping for three seperate Nema SM-23 applications. One was to 1 times Nema allowable the other two were to 3 times Nema allowable.

My approch has been to allieviate the nozzle loads on the inlet side through the use of flexibility, and the main discharge by use of a pressure balanced bellows.

In all cases I could have made use of flexibility on the discharge but this was not a cost effective option in the long term time frame. Additional flexibility means long straigh runs and bends. This amounted to subtantial pressure drops on the discharge side.

After having been informed by the process engineer that 1 psi of pressure drop could eat up $100,000 of power we could generate instead, over the life of the plant a $15,000 bellows was chosen as the way to go.

I prefer flexibility as the way to go in all cases but alliviating the reaction on a 42" turbine dicharge with flexibility is no easy feat.

Just my two cents worth

A question properly stated is a problem half solved.

Always remember, free advice is worth exactly what you pay for it!

http://www.ap-dynamics.ab.ca/

 

[MQM90]

Thanks to all.

Finally i am going with the recommendation that all resting supports and guided supports to be modified with Teflon Pads.

In my case, allowble is 1 x NEMA Allowable and therefore, i have to go out of the way i.e. i cannot modify the existing piping routing and in this case only option left at my end is to apply Teflon Pads at all support locations.

hahhhh

Osama Nusrat Ali

 

[BigInch]

A reasonable decision IMO. Good work.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[racookpe1978]

In doing so, you are relying (for the life of the project) on the Teflon sliding resistance being the same - not getting rusty, jammed up with pigeon and rat poop, not pushed up against a slide edge and becoming immobile, not getting dirty or deteriorating.

Indoors (turbine building covered?) or outside? Check your paint spec's too.

Put a maintenance spec requiring a yearly inspection and verification that all parts are actually moving, and that wear points are clearly visible.

 

[BigInch]

and make the top plate bigger than the bottom plate by at least the amount it will move in any direction.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[MQM90]

Hi everyone,

What a nice exchange of technical informaation.

Thank you very much, I finally understood this issue.

See you all in other sessions.



Osama Nusrat Ali

 

[MQM90]

Guys I m back again

This time its very urgent matter.

I read in an article that coefficient of friction for teflon pads with steel varies from 0.04 to 0.2 and is only maintain at max. temp of 327 deg C which is low then 370 deg C (my operating temp) .

I have used 0.2 value of friction coefficient in my piping analysis.

Another problem with teflon pads is that its surface quality is an issue i.e. should be avoided from dust and greese environment.

Pls advice as i m stuck as i have already communicated to my site engineers to go for teflon pads at all piping supports.

Osama Nusrat Ali

 

[SNORGY]

If you have used m = 0.2 and your calculations predict no problems, I am inclined not to be too worried about it...unless m < 0.2 *does* predict problems.

Regards,

SNORGY.

 

[MQM90]

Dear SNORGY

The point is clear but in Pakistan we do have dusty environment and grease problems.

Alternatively there are two ideas:

1. To install steel rods under the T-shoe supports which means the support will roll on the structure upto certain limit i.e for example 5 mm as the maximum displacement at that some support locations are in the range of 5 mm i.e. the rolling will be stopped after 5 mm from either sides.

2. Graphite pads instead of teflon pads as graphite pads on steel gives sliding friction of coefficient = 0.15 - 0.2
And the temperature limit is also high i.e. > 350 deg.C

What is your opinion



Osama Nusrat Ali