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Is steam turbine expansion joint needed?

Is steam turbine expansion joint needed?

Is steam turbine expansion joint needed?

(OP)
Hello. I operate a water tube steam boiler that produces 35000 lbs per hour of super heated steam at a temp of 750 degrees at 475 PSI. We run a 3 MW steam turbine. We had a 15 year old inlet expansion joint, just prior to the turbine fail. We replaced with new, lighter duty model, with no support rods. It was rated to our specs, and it failed within a month, even though it was rated for 3000 cycles. Our managment has replaced the expansion joint with a solid pipe. All operators protested, to no avail. Is this unsafe?
Piping is 6 inch diameter, with a globe valve inline just prior to turbine. The elbow pipe going into the turbine is the  only support on the bottom. On top, our plant engineer put a couple of spring mounted steel straps to allow for movement. There is a 10 foot drop from the top of the line to the turbine inlet. Please tell me if this is normal proceedure, or  a major accident waiting to happen. thanks

RE: Is steam turbine expansion joint needed?

It would be very unusual to have an expansion joint in a 475 psi steam supply pipe.

We have about 10 steam turbines of the type you are describing.  All of them have expansion joints in the exhaust line.   None of them have any sort of flexible joint in the steam supply line.  A flex joint in not needed as long as the piping has adequate flexibility to keep the pipe strain at an acceptable low level.  I am not a piping design expert so hopefully someone else can help you understand your piping layout better.  I would suggest sketching out your piping arrangement and getting a piping expert to analyze it for thermally induced pipe strain. If the strain is predicted to be high, it may be a simple matter to change the supports to reduce the pipe strain.  As a general rule, the piping should be rigid close to the turbine and all thermal growth should be forced to grow away from the turbine.  

By the way, the support rods on most expansion joints are there to limit the deflection of the joint during assembly or transport.  Once it is installed, the rods are supposed to be backed off to allow clearance for movement and pinned in place.  If your support rods were locked tight, perhaps you didn't really have an expansion joint after all.  

RE: Is steam turbine expansion joint needed?

If adequate flexibility is provided without an expansion joint, you generally have a far superior system.  Since there were spring mounted supports added, I would tend to believe (I hope I hope) that somebody did a stress analysis on the new configuration.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Is steam turbine expansion joint needed?

There are two kinds of "bars" found on expansion joints.  As biginch said some are referred to as "shipping bars" and are meant to hold the joint at its design length and shape (and to protect it against "over extension") while being shipped.  It is likely that in the installation you describe these "shipping bars" were meant to be removed.  But we should all be cautious and follow the joint manufacturer's instructions on installing the joint that they ship us.  With a "tied joint" (or on a universal joint) the tie bars are part of the functional design and the survival of the joint depends upon them being in place during operation.

General Electric has a great publication regarding all of this (and more) and it is GEK-27060, "Design Recommendations for Steam Piping Systems Connected to Large and Medium Size Steam Turbine-Generators".  This publication includes a comprehensive discussion on when, how and where expansion joints should be used on turbine-generator applications.  The publication also describes how the piping loadings (on the turbine) should be controled. Maybe if you leave an email address someone out there will email the document to you.

RE: Is steam turbine expansion joint needed?

Well, on second thought Eng-Tips does not allow you to leave an email address.  I forgot.  Please excuse me.

Regards, John

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