1.5x on Valves and Fitings 1

[tlh222]

Hydrostatic pressure testing on piping systems. Should all valves, fittings, etc on the system be designed to withstand the 1.5x requirement? Seems like this would add a lot of cost to a job considering the system will most likely never see the 1.5x pressure during operation.

Assume a system has been constructed with such fittings not rated for the 1.5x pressure. How is this handled during the pressure test? Or is it such that the system was not designed properly?

Thanks.

 

[LittleInch]

The system should never normally see any more than 1.0 x design pressure....(of the lowest rated piece of equipment).

The 1.5 is a recognized safety level to test a set of components such that at their normal operating pressure they have an adequate amount of safety margin.

What costs a lot of money is when something breaks and then damages people, the environment or other items.

If the system contains items not rated for the test pressure, then it either needs to be removed or the test pressure and hence the design and operating pressure reduced accordingly.

Sometimes you get a situation where the majority of the system is designed to one pressure but contains, for some reason, a lower rated component. You can test the rest of the system to higher pressure, but the maximum operating pressure then needs to be reduced to lower than the lowest rated component. If that component is then changed out for higher rated one, you don't then have to re-test the system to its now higher MAOP.

This is fairly rare so normally you test to 1.5 x the lowest rated component DP for piping systems or whatever your particular design codes states or recommends.

I think your last sentence is correct....

Some more details would be good to see what it is you're talking about.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[moltenmetal]

I think the OP is confused about what "rated for" means.

A component- valve, fitting, flange or pipe etc.- with a calculated or code-rated maximum allowable working pressure of 1000 psig can be hydrostatically tested to 1500 psig without damage. That's because the safe allowable stresses (SAS) used in the design of the component are a fraction of the ultimate tensile stress and the yield stress of the material. SAS is typically less than 1/4 of the UTS or 2/3 of the yield, whichever one limits- unless something else gets in the way like embrittlement or creep and lowers the SAS values even further.

The hydrotest may bring components near yield, but the test conditions must be limited to keep components below 90% of yield also.

Given that the hydrotest brings components nowhere near their UTS, it isn't really a very challenging test- but bringing components near their yield is actually about as stringent a test as you can do without changing the shape of what you're testing. Fortunately, the nature of a properly executed hydrotest is that very little energy is stored during the test, and that energy can't really do too much work on the surroundings if it were to be released suddenly. That is of course no longer true when you're testing giant vessels, and tests are often done improperly with trapped pockets of stored gas that make them much less safe. Caution is always required during pressure testing.

The fact that SAS is generally less than 1/4 of UTS leads some people to (falsely) believe that there is a 4x safety factor on piping components. There is not! Nor does the 1.5x hydrotest give you confidence that piping can be operated higher than the MAWP for extended periods. The SAS values are selected to be conservative, but that's for good reasons. Pressurized systems store energy and when energy is released suddenly, people can be injured or killed.

Note also that there are other meaningful ratings for valves rather than just their body MAWP. Some valves which may be safely hydrotested to 1.5x MAWP will be damaged or will leak through copiously if tested while closed. Hydrotests of systems containing valves need to be done with the valves open, or partially open, and the ends plugged. Leakage at stem packings during a test may not be considered a failure of the test, because a valve packing isn't a piping joint. Valve packings must not leak during an assembled system leakage test which is done at maximum operating pressure, which of course must be quite a bit lower than the MAWP much less the hydrotest pressure.

 

[jte]

Just to clarify things a bit...

Some folks believe that vessels and piping have a design margin of 4. There is a bit of logic to this number, but it is outdated, and has been for roughly 16 years. Also, the value of 4 never applied to B31.3 piping. For vessels built from sometime in the mid 1950's to 1998 to Section VIII and after 1968 to Section VIII Div. 1, the allowable basic stress in tension for most materials was limited by the lowest of 1/4 UTS, 2/3 yield, creep, or fatigue. Now, show me a vessel and I'll show you a few areas in which the basic allowable stress is routinely exceeded. Simply put, keep in mind the 1.5*S and 3.0*S (or more recently Sps) criteria. So when at MAWP, I can have parts of the vessel at stresses above yield and approaching ultimate. Since 1999, the design margin against UTS has been 3.5 for VIII-1.

For B31.3, the design margin against UTS has never been 4. Try 3. For VIII Div. 2 the value would be either 3 or since 2007, 2.4 at ambient temperatures only.

Moving on to the OP's question which relates not to vessels or pipe, but to fittings and valves:

For B16.5 components, refer to B16.5 2013 paragraph 2.3 for reference to 1.5* ambient temperature pressure rating.
For B16.9 components, refer to B16.9 2012 paragraph 10 for discussion of suitability for system hydrotests.​

In ideal circumstances, I'll agree with LittleInch that the system should never see more than 1.0 times the design pressure. However, in less than ideal circumstances with B31.3 piping I have no problem leaning on B31.3 paragraph 302.2.4 which, with certain restrictions, allows for operation not exceeding 33% of the rated pressure.

 

[LittleInch]

I did say "never normally see...."

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[tlh222]

Thanks to all for responding. I was more concerned with the effects on the valves, etc. Not necessarily the body, but more the packing, etc. I think Molten and the rest answered my question. Thanks again.

 

[davefitz]

As I recall, B16.34 allows a max valve overpressure 1.1* design pressure, based on limits of the nominal P,T rating for the class of valve selected. I seem to recall that that limit is usually based on bolting and bonnet limitations more than the limit of the valve body and flange.

ASME sect I requires a 1.5x MAWP hydrotest following initial fabrication, but there are alternate rules to confirm adequate safety using other NDT measures + leak testing at design pressure to avoid repeated application of the 1.5 hydrotest; some level of damage is incurred after each 1.5 hydrotest, it if that can be avoided, then one avoids it. In particular , aged, high temperature components may have degraded room temp ductility and a 1.5 hydrotest at 60 F may lead to a weld crack proceeding to significant damage .

Section VIII requires a 1.3 hydrotest ( or lower pneumatic test) , and to subject a sect VIII vessel to a 1.5 hydrotest is not appropriate. The sect VIII relief valves are to prevent more than a 1.1 * design pressure overpressure when lifting, assuming they were sized appropriately and the process casualty conditions have not evolved over time( as may occur if an upstream Pressure reducing valve was replaced ).

Having said that, one would much rather have damage detected at room temp with water than at operating temp while the process is containing a dangerous fluid or gas.

"Nobody expects the Spanish Inquisition!"

 

[jte]

and to subject a sect VIII vessel to a 1.5 hydrotest is not appropriate

I think that's a bit over the top... My current company standard, similar to those I've seen in many others, requires a MAP based hydrotest (UG-99(c)) rather than the code minimum in UG-99(b) for VIII Div. 1 vessels. I'd have to run some numbers, but I'm pretty sure we sometimes exceed 1.5*MAWP (adjusted for temp) in performing a 1.3*MAP test.

Nobody expects the Spanish Inquisition!

I prefer comfy chairs myself.

 

[davefitz]

jte,
I would have to defer to the code and to stress analysts, but if a sect VIII vessel was designed for the max expected hydrotest pressure of 1.3 * allowable and then it is actually tested at 1.5 * MAWP, the damage that occurs at stress raisers ( near nozzles, penetrations, reinforcing pads) is in excess of the designer's assumption.

"Nobody expects the Spanish Inquisition!"

 

[TGS4]

The hydrostatic test pressure specified in the Code (ASME Section VIII, Division 1) is a Minimum test pressure. There is no maximum-specified test pressure. There is a limit on visible deformation, but that is subjective to the AI.

It is doubtful that there would be anything resembling "damage", in the context that you mean, at a test pressure less than 2.5-3*MAP. If you think otherwise, then the cold-forming operations and weld-residual stresses likewise cause "damage".

 

[moltenmetal]

There is a maximum test pressure, isn't there? Isn't there a "< x % of yield" criterion that has to be satisfied? The ratio of stresses for temperature is adjusted downward sometimes to keep parts out of yield if I recall correctly, but that's in B31.3 not in Section VIII- correct me if I'm wrong.

 

[TGS4]

moltenmetal, B31.3 in 345.2.1(a) limits the hydrostatic leak test to be the pressure that would generate nominal hoop yield stress away from discontinuities.

VIII-1 has no limit.

VIII-2 has a limit of 95% of yield of general membrane (hoop) at the hydrostatic test condition - 4.1.6.2(a).

In all cases there is no limit on any stresses other than nominal hoop.

 

[jte]

To follow up on the comment above by TGS4, by the time hoop stress is approaching yield, local stresses and certainly peak stresses have long since exceeded yield and most likely linearly calculated ultimate strengths. So I'll explicitly draw attention to TGS4's last sentence.

 

[davefitz]

Right you are. sect VIII-1 par UG-99 requires a 1.3*MAWP * LSR as the minimum test pressure , with no upper limit, but the inspector may fail the test if he detects distortion. LSR= lowest stress ratio, based on allowable stress ratios at design temp vs test temp., considering all materials used in construction of the PV.

"Nobody expects the Spanish Inquisition!"