Effect of hydrotest on component stress 3

[CWicker]

What is the effect of hydrotesting a steel vessel to pressures ranging from 1,2 to 1,35 times the design pressure in terms of approaching or exceeding the yield strength? Can limited yielding be expected? If a vessel has been hydrotested to 1,35x design pressure at commissioning and then periodically thereafter to 1,2x design pressure, will there be any adverse effects on total stresses, or will the periodic hydrotests have no effect at all?

 

[metengr]

CWicker;
The initial or commissioning hydrotest at 1.5X MAWP is conducted below the yield strength of the component material. I have seen a number of technical articles that indicate the benefit of an initial 1.5X MAWP hydrostatic test on components that contain inherent stress risers.

Apparently, the component material at areas of stress concentration could locally exceed the yield strength resulting in a strengthening affect (locally) and crack blunting if minor fabrication flaws are present. This of course provides some benefit for future service.

After the initial hydrotest, periodic hydrotesting at or below 1.5X MAWP, but above MAWP, serves no real purpose other than to check for gross material defects. I for one, do not advocate pressure tests above normal operating pressure on in-service components. This is especially true for boilers or high temperature pressure vessels that could be adversely affected by introducing water on warm or hot boiler components.

There are state-of-the-art nondestructive testing methods today that can provide a condition assessment of a component without the need for subjecting it to hydrotest pressures above normal operating pressure.

 

[athomas236]

If a boiler has undergone major repair work, I would always test to the original hydro test pressure.

athomas236

 

[unclesyd]

I agree with metengr that testing to original test pressure on equipment is not essential in most cases nor desired in every case. This is not saying that the rules and regulations of a jurisdictional authority should be purposely violated.

This is especially true with working boilers. We have always tried limited the test pressure to a maximum of 1.1 x OP for hydrotesting of inservice boilers. Several times we have gagged the low pressure relief valve and tested to the set pressure of this valve leaving the high pressure valve in service.

For other vessels the testing pressure is evaluated in each case and with concurrence of everyone a test pressure is chosen and incorporated into a testing procedure. Very seldom was the original test pressure used.

It had been my experience that in many cases not enough engineering/designing input is used in the pressure testing procedure or a good procedure is overruled by operations. In my notebook I have a list of probably 50-60 reasons for not doing a hydrotest, what went wrong, etc. I took the approach that the only valid excuse for not achieving the desired test conditions was that the equipment came apart before the test conditions were achieved, never lost one. I didn’t always win but 99% of the time I did.

It's a mechanical requirement, but the main purpose is the Safety of Personnel.

 

[athomas236]

unclesyd

What are the 50-6o reasons then.

athomas236

 

[metengr]

The main reason that I do not believe hydrotesting provides a benefit for in-service components is that it had been done once during commissioning and you have proven the design.

If you are using a hydrotest to find gross defects or to burst weaken components in service, then these are the wrong reasons for hydrotesting. I have worked closely with the National Board Inspection Code to down play the hydrotesting (pressure above normal operating pressure) of in-service components as a means of showing remaining useful service life. Going above normal operating pressure will harm safety valves, unless special precautions are considered. Second, the cost involved to exceed normal operating pressure is not worth the effort. For our boilers, we would have to rent a pumper truck to provide the boost in pressure.

Third - on aged boilers we do have waterwall tube circuits that could be overstressed because of existing thermal/mechanical fatigue damage or corroison fatigue damage that is not an issue at normal operating pressure. Fourth and very important is that you could be introducing water into areas of the boiler circuit (SH or RH) that may not be able to handle the added weight of water because of aged hanger systems.

If all you are looking for is to check for defective tube welds after a major boiler overhaul, performing an operating pressure squeeze will achieve the purpose. Above and beyond this pressure will serve no benefit.

Rigorous boiler inspections, supplemented with NDT, tube sampling and pressure tests at or below working pressure after overhauls will provide a safe boiler for continued service.

 

[Sharik]

I agree with metengr. We actually take it a step further. For boiler inspections or if repairs to watertubes, superheater, reheater or economizer tubes are completed, we only pressure test to just below the superheater outlet set pressure. These tests are completed under controlled conditions with respect to water temperature and the rates of increasing and decreasing the pressure.

The only time we would do a full 1.5X pressure test would be if we had completed any weld repairs to the pressure retaining parts of the drums or headers (excluding welding the header handhole covers).

 

[LSThill]

Team Member

What do the FRANCH do?

 

[GenB]

They test, test and test again

 

[LSThill]

TEAM MEMBER "PLEASE HELP"

RE: Metal and Metallurgy engineering what was the area of specialist in research:

Mr. Jean Marie Vialle
Professeur de Sideuegie
Ecloe National Superuraie des Mines
De Sanit Etienne
Cours Fauriel
12, Saint Etienne, France
Telephone: 77-33-20-23


Please,

Leonard@thill.biz

http://www.thill.biz

 

[athomas236]

how can we harm safety valves we take them off

 

[unclesyd]

athomas236

If you don’t know the rules you don’t have to go by them;
This is kinda silly on this old stuff;
Under was authority;
We didn’t do this last time;
Can't find the pump;
No hose for the pump;
Hose too short;
Can’t find the right gasket;
Wrong flange on hose;
What do you mean hard pipe it;
Pump don't work;
Pump will not hold pressure;
Pump will not get to test pressure;
The check valves will not hold;
Can’t find the repair kit;
Stores is out of everything;
The stores clerks have gone home;
Can't get the air out;
Vent valve frozen;
How are we going vent;
The pump is in use;
The pump is loaned out;
Can't find the proper blanks;
Why do I have to change out the blanks;
Don't you think its too cold;
How are we going to blank a 18" line;
The whole vessel is wet;
I didn’t know we were looking for leaks;
It's gonna rain before we are through;
Its too dark to see a leak;
Can't find the gauges;
Can't find the correct range gauge;
This gage is calibrated and still want work.
Calibration sticker on gauge is out of date;
It will be too late if we have the gage calibrated;
The instrument shop doesn't like to work overtime;
The gauge isn't at 0;
What! use a hand pump;
Where is the air;
Why do we have to use DI water;
Who will sign-off on the test;
Operations is on my case;
Operation is going to startup at shift break;
The foreman is going to pull blanks in 10 min;
Why can't I use a Fire Water connection;
Who’s going to pull the blanks after shift break, I’m going home;
Why do I have to hold it for x minutes;
Why pull a vacuum and then pressure test;
The block valve leaks through;
Why can’t I pressure test using the instrument valve;
Why change out the gaskets;
What is the point of this as we have never had a leak;
These are Class 2500 flanges why are you testing at 3500 PSIG;
Why put a standpipe on the tank;
Can’t we open the manway;
Show me that on SPI’s;
What’s this going to prove;
Why start over if we have a leaking flange;
I’ll have to leave the water in overnight;
Why can’t I use the process material;
Why can’t I use air, we used it other areas;
Why can’t I use N2 it want explode;

 

[athomas236]

rubbish rules not a single technical reason, u must be american

athomas23

 

[metengr]

athomas;
My question to you is why perform a hydrotest after major work - sounds like an expensive method to check for leaks?

 

[deanc]

athomas236

I agree with the others about hydrotesting. Bad practice to use 1 1/2 for in service items. One example is anything with threaded stay bolts-undue stress at the root of the threads.

I have seen more damage caused by the 1 1/2 hydro then the item that was repaired.

By the way-being American has nothing to do with it. How about common sense and experience.

 

[davefitz]

Please note the following:

a)The latest version of some ASME codes has increased the allowable stress for low temp ( ie below 800F) steel components, and likewise had to lower the hydro test to 1.3 times design pressure. DO NOT HYDROTEST those components at 1.5 times design pressure- these components ( if later retrofit onto an older system) will define the max hydrotest pressure of the system

b) Newer boilers and power piping systems are using "advanced" alloys such as P91, P92, P122 etc- these components have very low fracture toughenss at room temperature, and there are major QC issues related to ensuring correct room temp properties vis a vis PWHT procedures. Bottom line: do not hydrotest these components until you are absolutley sure (a) adequate room temp toughness exists and (b) the metal temp of these parts during hydrotest is above 70 F

 

[rmw]

Davefitz,

Regarding your comment "a" above, is this for fired vessels as well as unfired???

Yours is the post I have been waiting for, as I only recently became aware of the code changes, but the conversations I was involved in were strictly related to unfired vessels.

You get a star.

athomas236,

My reaction to your comment would be that codes and local laws trump common sense and experience, often in a detrimental way.

Davefitz,

What about where a local code hasn't caught up with the newer ASME codes, and still might require a 1.5X hydro??

rmw

 

[davefitz]

If the local code requires a 1.5 hydro, then you will have to overspecify the design pressure of any new component added to an old system to ensure it is not overstressed during hydrotest. But I think you could instead forward a page from the newer ASME design code to thelocal buildign code inspector and obtain a waiver fot he 1.5 rule based on the new code allowable stresses and permitted hydrotest pressures.

I do not believe the 1.3 hydrotest applies to section I ( boilers) , but it does apply to Section VIII and perhaps B31 parts.

 

[metengr]

rmw/davefitz;
For any in-service boiler, the National Board Inspection Code governs. The NBIC provides options for re-rating components using revised allowable stress values (see RC-3024, RD 3000) applicable to boilers, pressure vessels and other pressure retaining items. RD-3000 lists the conditions for taking advantage of materials with higher allowable stress values.

Typically, for replacement of major boiler components the hydrotesting should be performed at the shop NOT in the field (where the entire boiler would be subjected to an unnecessary hydrotest - for the reasons you stated above. The NBIC provides alternative methods for using NDT, in lieu of hydrostatic testing, regarding installation of altered components.

 

[davefitz]

meteng:
The situation of concern is where the ASME has changed the basis for assigning allowable stresses in some codes ( sect VIII for example). In order to compete with the less conservative european codes, the allowable stresses of low temp carbon steels was increased by making use of a higher fraction of the yield stress than previously assumed. By assigning the allowable stress as a higher fraction of min yield stress, one is left with the result that the component would be stressed above yield stress if one were to use a 1.5 hydro test pressure. For these components using teh new ASME allowablwe stresses, one CANNOT safely use the 1.5 hydrotest pressure.