U tubes test pressure

[Eabrar]

It is a common practice that each U tubes used in exchangers are hydraulically tested individualy. This test is carried out after bending and heat treatment and at a pressure lower than the test pressure specified by the code and design pressure.

My question is that why this test is carried out in the first place? If the tube is likely to fail at pressure near to its design pressure then their is no use of this test.

Secondly what is the criteria of defining this test pressure? In some places I have read 70 bars but I want to confirm if their is some code requirment for this test pressure.

 

[metengr]

We buy heat exchangers, and I am unaware of any requirement for testing individual U-bend tubes until after installation, as part of the hydrostatic test for acceptance of the heat exchanger, per ASME Section VIII, Div 1 requirements.

The only reason I could think of for someone wanting to test each u-bend after forming/stress relief heat treatment is to look for defects that have formed during the bending process. We typically perform a Liquid penetrant inspection on a small percentage of tube bends to assure we have no problems. The NDT is our requirement for a QC check.

 

[unclesyd]

The main reason is that you don't want to install a bad tube/tubes. All the HE's built for or on-site had the tubes tested prior to installing into the tube sheet. Low pressure(<100 PSIG) tubes were tested at design pressure. Anything above the 100 PSIG was test at 100 PSIG prior to installing. All tubes were rolled and then seal welded and most rolls and welds were tested post fabrication at 100 PSIG. The final test pressure was based on design conditions in and out of the shell depending on design and operating conditions. We also preformed vacuum test on tube rolls and welds.

I don't know where the 70 bar came from as the test pressure of the shell and tubes is based on design pressure.

Come back with your design conditions and what code your design is based on and you will get a much better answer.

 

[rmw]

In high pressure heat exchanger tubing, such as Feedwater heater tubing, the tubes are tested individually at the tube mill where they are bent, just for the reason that Unclesyd stated. Once a tube bundle is assembled, and tested, by whatever means, it is then too late to discover that there is a bad tube (tubes) deep within the bundle.

The pressure, however, depends upon service and/or design conditions for the heater as a pressure vessel, which would dictate the metallurgies, and tube thicknesses, so a single value of 70 bar would be limited to a specific application.

I will consult some technical information I have in a different location, and if I find more information that will add to this thread, I will re-post.

rmw

 

[bmoorthy]

Some of the buyer's specification in fact specifies 70 Bar. You would find 70 Bars as the max test pressure given in the commonly used material spec ( A 179).

Also it is a common practice to test at higher of 70 Bar or at the Tube side Hydro test pressure.

This hydro test of U tube as a component is not mandatory as per Code. But it is a prudent way for buying tubes.

Testing at a lower pressure as compared to design pressure is not an objective requirement, especially if you are specifying hydro test after making of "U" but before making of Bundle.

The rationale is that, after rhe formation of tubes the tubes thin (Although thinning is taken care during design). If the "U" maker dose not follow good engineering practice, the actual thinning may be more than the one anticipated by designer. Remembering the tubes are very thin as such and further thinning say by 0.3 or 0.4mm in a 2mm thickness would be a very high percentage.

I personnally know of situations where in, during shop hydro test of exchanger some of the inner row tubes koncked off and the buyer rejected the idea of plugging (Buyer was using Shell spec). Hence the manufcturer had to cut all the tubes and and buy new set of tubes and tube sheet (It was a welded tube and tube sheet construction). Imagine the loss!!!

It is a good idea to spend a little bit more while purchasing U tubes and specify hydro test. Atleast this is good for the fabricator if ot for the User.

For the user you can look at this requirement more as Expediting requirrement than as a n inspection requirement although in the face of it it appears to take more time.

 

[rmw]

It is common practice on high pressure Hx's that the inner two (minimum, maybe more) rows are at least one gage thicker tubing than the rest of the bundle, to address the thinning in the tight radius of the inner row U bends.

rmw