Hydrotest Question 1

[JABoomer]

Hi All,

Can across a situation yesterday and I am still unclear on the details.

Had a 150# piping system with a design pressure of 1770 kPA. Now that would make the minimum hydrotest pressure 2655 kPa. The Client actually had the hydrotest pressure set at 2948 kPa which I was fine with. However they later requested the hydrotest pressure be increased to the ANSI maximum allowable non-shock pressure for a 150# flange rating of 3105 kPa (450 psi), as per the following table:

http://www.engineeringtoolbox.com/ansi-flanges-pressure-temperature-d_342.html

Their reasons for the change is they like to make the hydrotest the maximum allowable in case the design pressure is change down the road, that way they don't have to re-test the pipe if the design pressure is increased.

I see several problems with this: The inspector will always want to see the test pressure during the hydrotest indicating equal to or above the stated hydrotest pressure, which means there is a danger of exceeding the maximum allowable non-shock pressure during the test. Also, if you were testing a vessel, or even a large vertical run of pipe, the additional hystrostatic pressure from the water used during the hydrotest could further exceed the maximum allowable pressure.

I think you are allowed to exceed your hydrotest pressure by 15%?

But can you exceed the maximum allowable non-shock pressure AT ALL? And if you do do you have to perform stress calculations to ensure the pipe has not been over-stressed?

Can anyone help me out on this one. Is it okay to have your hydrotest at the maximum allowable non-shock pressure as indicated by ANSI?

Thanks Much,

JABoomer

 

[XL83NL]

Had a 150# piping system with a design pressure of 1770 kPA. Now that would make the minimum hydrotest pressure 2655 kPa.

1) The hydro test pressure is determined by your design code. Not knowing what code you're using, and assuming its B31.3, the test pressure is dependent on the design pressure (times 1.5 times the stress ratio (ambient / design pressure)). However, most specs are flange limited, therefore allowing the 'easy' rules to test a system @ 1.5 times the flange ambient pressure rating for the whole system.
2) Not knowing your MoC, it hard to tell if the test pressure should be 2655 kPa(g).

However they later requested the hydrotest pressure be increased to the ANSI maximum allowable non-shock pressure for a 150# flange rating of 3105 kPa (450 psi),

Okay, so you're using 1.2 materials. From B16.5:2013, I get 1.5 x 19.6 bar(g) = 2970 kPa(g), which should be rounded to 30 bar(g) (3000 kPa(g)) as per B16.5 para 2.6.

Their reasons for the change is they like to make the hydrotest the maximum allowable in case the design pressure is change down the road, that way they don't have to re-test the pipe if the design pressure is increased.

Sounds like they want to have a flange limited spec.

I think you are allowed to exceed your hydrotest pressure by 15%?

Based on?

In the end, the system's test pressure should not exceed 90% of the yield of any component in the system (again, assuming it's B31.3 piping).

 

[Duwe6]

What you are describing is what the petrochem folks call a "Full Flange Hydro" 1.5 x room-temp rating of flange. Very good for replacement spools - you do not have to research the original design conditions. And if the piping needs to be rerated upwards, your hydro is already done.

I prefer them - with an RT/UT rate of 5%, poor [or bad] quality welds can slip through. Any weld that passes Full Flange is just fine for use. I'd rather get my bad news today, in the fab shop, rather than next year while attemption to make product and $$ in a plant.

 

[LittleInch]

JA Boomer,

The root of your issue, and that of your client is IMO tied up in your first sentence - "Had a 150# piping system with a design pressure of 1770 kPA" No, you have a piping system with a design pressure of 17.7 barg which happens to have #150 flanges welded on it. That is the crucial difference. Your piping system has a design pressure for a reason - I don't know what that is, though my guess is that this is the de-rated pressure at a higher temperature in which case you are allowed in B 31.3 to test at a higher pressure to take account of that.

However, and this is where I don't agree with XL83NL, many systems have non flange rated design pressures and the piping and other fittings are designed to that design pressure. Therefore to blindly go to the maximum flange rating without actually doing any checks is dangerous and not good practice. If you look at a line list in any decent sized plant, you will find a large variety of design pressures for piping systems with identical flanges.

To be honest at #150, you are unlikely to run into serious trouble unless your design pressure is below 15bar, but for higher rated systems you could easily have a design pressure of 25bar and pipe sized accordingly, but #300 flanges with a max CWP of 50 bar. Now that would / could be lethal.

B 31.3 states a min of 1.5 x Design pressure (highest point) and no more than yield stress at the maximum (345.4.2.(c ), so yes, you need to perform checks to ensure that the yield stress on the piping is not exceeded. Any connected equipment / fitting with a lower design pressure limits the design pressure and pressure test.

Your clients desire to test to a higher pressure is understandable, but needs to be done properly otherwise you will eventually overstress something - the worst case being that you don't quite reach UTS, but you have seriously work hardened some piece of pipework or fitting which could then fail catastrophically. As said above, the higher the flange rating, the higher the difference might be between the design pressure of a bit of pipe and the max working pressure of the flange that just happens to be welded to it.

Also I don't know where you get the "maximum allowable non shock pressure". what the figure you quote is 1.5 times the highest working pressure as a test pressure. This is not the same as the maximum allowable working pressure which is what your phrase implies.



My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[JABoomer]

Okay, maybe we can take this back a step. The hydrotest pressure indicated in the following link:

http://www.engineeringtoolbox.com/ansi-flanges-pressure-temperature-d_342.html

Are they no the maximum allowable hydrotest pressures for each ANSI pressure rating class?

 

[XL83NL]

Therefore to blindly go to the maximum flange rating without actually doing any checks is dangerous and not good practice.
If you look at a line list in any decent sized plant, you will find a large variety of design pressures for piping systems with identical flanges.

Our plants also have such line lists, however, if a spec is flange limited, i.e. you verified it, then there's nothing wrong with going to the max. flange rating.
Though my experience is limited, I guess yours and mines are different then. The specs Ive seen, most of em are flange limited.

 

[LittleInch]

JA boomer - Yes they are the max hydrotest pressure (290 x 1.5 = 435 then rounded up to the nearest 25 psig = 450). It was just that the words "Max allowable non shock pressure" implies the max pressure in operation which is not correct.

Also maybe I missed it before, but testing vessels is different to testing pipe systems. Vessels are custom designed and cannot just be raised in MWP just because it has a certain class of flange on it.

XL83NL - Yes may piping specs and lines are limited by the flange rating, but my point was that simply because a pipe has a flange welded to it, the design pressure of the pipe and other things attached to it cannot be assumed to the same max pressure that the flange can withstand.

At #150, the full rating is fairly common, but at #300 and above, designing the pipe and all the fittings to the max flange rating when you don't need it for the process is just a waste of metal in the vague chance that one day in the future maybe you would want to operate that piece of pipe at a higher pressure.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[XL83NL]

but my point was that simply because a pipe has a flange welded to it, the design pressure of the pipe and other things attached to it cannot be assumed to the same max pressure that the flange can withstand.

Youre absolutely correct. However, the intend of my previous comments was solely to mention "that when ... then .. if ...".

At #150, the full rating is fairly common, but at #300 and above, designing the pipe and all the fittings to the max flange rating when you don't need it for the process is just a waste of metal in the vague chance that one day in the future maybe you would want to operate that piece of pipe at a higher pressure.

Right again, spot on side note!

 

[moltenmetal]

Flange limited design for PIPING has its proponents, and they have reasons that aren't totally idiotic for recommending it. At 300# class, unless it's exotic alloy or very large piping, flange limited design for piping doesn't have much of a cost associated with it- until it's extended to an entire line including all its valves and instruments. That can create all manner of trouble for no good reason.