Test Pressure Higher Than Design Pressure, How?

Simply, the design pressure is the maximum pressure the vessel is designed to withstand under normal operation for the extent of its expected lifetime. The test pressure is always larger than this, and will reveal defects in the vessel periodically, which may be significant for obtaining indemnity insurance cover on the vessel for normal operation.

Apart from measuring the thickness of their components by ultrasound, among other tests, heritage steam locomotive boilers are subjected to test pressures either 133% or 150% of their design pressure using water as a test fluid as part of their annual insurance examinations. The insurance company's Engineer-Surveyor will assess the condition of the boiler and state requirements and conditions upon the operation of the boiler that are mandatory for continuing insurance cover on the boiler.

Dear sir,

Thanks for your answer.Still I have confusion , for example

we design a vessel for 50Kg/cm2 as design pressure, but we test it more than 75Kg/cm2 means how is it withstand. Normally designed it for to work up to 50Kg/cm2 if it is so, it exceeds the value during testing how the vessel withstand the exceed pressure & not cause damage to the vessel.Please explain.

Thanks

The longevity of the components are likely less with the higher pressure.

How long is the expected life of your machine?

Check the life expectancy of a bearing at different loads. A bearing will last longer with lighter loads.

When I worked on doubled wall pressure vessels (cookers for the rendering industry) the inner wall was always thicker because it would be worn away by the cooking process.

Is your pressure vessel subject to wear or corrosion?

When a vessel is pressurized it stretches. The test pressure is based on the designed working pressure + the safety factor. During a test, the vessel is stretched to the working pressure plus the specified safety margin. The vessel should recover its size and shape after de-pressurization to within a certain percentage. If it does not, the material is nearing its end of life expectancy and must be destroyed.

Good design will take into consideration such things as metal (or other material) fatigue due to age, wear, crystallization, oxidation, environment, etc. The final designed wall thickness must have a extra safety factor added in. If the test pressure is 150%, then the wall thickness should be designed at; about 180 to 200% of the working pressure depending on the application and planned life expectancy. At 200%, the wall could have a rust pit up to 25% of original manufactured thickness before it fails.

Not in all cases the design pressure must equal to 150, 180, or 200% of operating pressure. A lot of design handbooks said it must be 120%, and also this statement is not correct at all cases.

The design pressure mainly depends on operating pressure, and also depend on the type of process, fluids toxicity, fluid flammability, hazards, environment, safety precautions, ..... etc.

For example, for operating pressure 100 kg/cm², can we say that its design pressure must be 200 or 180 kg/cm²? It will be logic if we used a design pressure of 108 kglcm², and we have to use a safety means like safety valve to assure that pressure will not reach the design pressure. In this case the design pressure shall be about 1.08 times the operating pressure. But if the fluid is toxic and there is a problem of any leakage, we can use a design pressure of 1.2 or 1.3 times of operating pressure.

But for small operating pressures like 2 kg/cm², we can use a design pressure 4 or 5 kg/cm², i.e. the design pressure is 2 times (200%) or more of the operating pressure.

Also determining the design pressure depends on type of fluid and process, I mean we can't equalize the design pressure for air or water at a pressure vessel which have the same operating pressure of the same vessel when handle a flammable or toxic fluid.

Agree

Because the design equations have a built in 3 to 1 or 4 to 1 safety factor. If the steel used and welding performed is per the standars, your 50 Kg/cm vessel can hold 200 Kg/cm brfore the steel of weld goes plastic and yields.

Definitions (chem vessels)

Design Pressure - The maximum design pressure used to determine the metal thickness.

Maximum allowable working pressure (MAWP) – Maximum pressure that the pressure vessel is capable of safely holding at a design maximum temperature.

Operating pressure- The normal maximum operating pressure of the pressure vessel. The normal operating pressure must always be below the MAWP. As a rule of thumb the normal operating pressure should be no greater than 25% of the MAWP.

Test pressure – The maximum test pressure shall be 130% of the MAWP at the design temperature. Room temperature tests can be used if the 130% of the MAWP is multiplied by the ratio of (allowable stress of the vessel material at room temperature/allowable stress of the vessel material at the operating temperature).

Metal thickness – The thickness of the pressure vessel wall (not including a corrosion allowance) to be used in the pressure calculations.

Design safety factor on metal tensile strength – A vessel shall be designed with a safety factor of 4 on the metal tensile strength. All vessels shall be designed to operate with wall stresses below the yield point of the metal used for fabrication.

regards

sameer

Good post.

Design safety factor on metal tensile strength – A vessel shall be designed with a safety factor of 4 on the metal tensile strength.

I think this gets to the source of the original poster's confusion. A safety factor of 4 allows for a test pressure well above the design pressure.

To the original poster: You can imagine that if test pressure were only equal to design pressure, then we would have many pressure vessel failures as a result of nicks and scratches, metal fatigue, higher temperatures, faulty gauges, shock loadings from abrupt valve closings, etc. So you must test to something higher than the design pressure (and this is true for other things as well: structures, etc.), and if you want to tank to survive the test routinely, then you must provide a large safety margin.

Think of it like this.

If you are a mountain climber and you weigh 150#. Do you want to use a rope that has been tested and breaks at 150# or do you want a rope that has been tested and will hold 600#?

It boils down to margin of safety. Remember, failures will occur. In light of this you want to test your device (rope, boiler, beam, whatever) to a load which is higher than the load which it will see in service will be.

Also, typically hydraulic components are designed to operate at 3000 psi and are typically tested at 4500 psi or even 6000 psi. The logic and reason being that if the component will perform at the higher loads then it will operate satisfactorly at the lower (rated and intended) load.

(Homework?)

WTS

Agree with above. Wish to add that often pressure testing actually involves 2 tests of differing pressures and duration which may or may not be performed concurrently.

On piping systems we will pressure to 100% and percentage plus for a short duration. This equates to a burst or failure test. In tandem to this test the pressure may or may not be reduced to 100% and minus percentage to fulfill a pressure decay test requirement. A lower (usually) pressure over a longer duration.

cr3

Yes I also agree with the above, if the yield of the material is 240MPa ( varies with materials) then you design in a safty factor and you would normally design to a 60Mpa working load.

Regards JD.

sounds like you are getting working and design pressure confused... in my experience, a pressure vessel is usually designed to at least 2x the expected normal working pressure and tested to at lest 1.5x expected working pressure.

Often, as others have said, the design pressure is 3 or 4 + times the working (normal operating) pressure. So, the design pressure is your actual working pressure times the factor of safety you deem reasonable. Testing should always be done at 1.5x working pressure if not higher. That is for design and workmanship verification.

Design pressure should equal or exceed test pressure. You are confusing design pressure and operating pressure. Test pressures must always exceed the operating pressure but should not exceed the design pressure.

Sorry, No. Please refer to post 11.

No confusion there, since the reason is very simple.

At design stage, a lot of safety factors must be taken into consideration, so for the test pressure which is higher than the design pressure, we know that the stresses arises into the member shall not be greater than the permitted values, and will not reach the yield stress.

Therefore we have to calculate the stresses arises at all different loading conditions which the hydrotest is one of them, and compared with the permitted figures of stresses.

Test pressure in accordance with ASME code, Section VIII, Division 1 is:

P_h = 1.3 * P * (S _{at ambient temp.} / S _{at design temp.}) ,

where,

P = Design Pressure

S _{at ambient temp} = Allowable tensile strength of material at ambient temp. (test temp.)

S _{at design temp.} = Allowable tensile strength of material at design temp.

So, the test pressure must be higher than design pressure, and design pressure must be higher than operating pressure.

i would like to put it in other words... test pressure has a caping that the primary stresses should not exceed 0.9* yield stresses at test conditions. so you are still in elastic limits.

2. Its a test of integrity if due to some defect/ flaws the vessel were to fail catestrophycally, its always better it fails during testing and not during operation.

3. you get leakage test done for all welds

4. stresses redistribution and peak stress relieving by local yielding at certain location.

I would say its an accelerated integrity test that is: higher pressure shorter duration....

There are two reasons (in my mind) to perform a Hydrostatic test on a pressure vessel or any pressure holding component.

1- Make sure there is no leak

2- More importantly is to initially pressurize the vessel in a control environment, you need to increase the pressure (stress) on the vessel gradually in steps and hold the pressure for a while in every step until you reach the test pressure, this is because there is localized area in the vessel has very high local stresses and you don't want to suddenly increase these stresses and have a failure accordingly.

In a nutshell you have to perform a hydrostatic test to prevent the local failure in certain areas due to high local stresses

The test pressure for vessels has to be at least 1.3 times the design pressure times the stress ratio, and less than 90% of the yield stress.

This means the test pressure is higher than the design pressure, the reason is that you perform the test pressure at the room temperature not at the design temperature, 1.3xP is to compensate the design temperature at the operating conditions.

.There are two reasons for pressure tests; one as you stated, is to discover any leaks - that is why the tests have to last at full pressure for a certain length of time, The other is to prove that there will not be a catastrophe failure during operation!

...and that's why it is done with incompressible fluids, such as water.