body gas testing

[mahimetal]

We manufacture API 6A gate valves and Wellhead equipments. Here we use Nitrogen to perform high pressure static tests. It is mentioned (in API 6A) the gas could be nitrogen,methane,air or other mixtures of gases.
I'm wondering how would it be if I replace Nitrogen with Air?Would the oxygen content be a major safety issue???

 

[zdas04]

Short answer is "no". People have two concerns about testing with air: (1) creating an explosive mixture with production gases already present or that will be added later; and (2) increasing corrosion risk.

On the first one, there is no risk for valve testing at the manufacturer's facility--after the test you are not going to evacuate the valve or inert the atmosphere and seal it are you? The ultimate user is not going to take any particular steps to purge the valve prior to installing are they? The answer to both questions is "no" so how could testing with air increase explosion risk? It can't.

Same with corrosion. The valve is shipped and stored in an air environment and doing the test with air isn't going to create any new corrosion risks.

Depending on your air source (mostly people just use an air compressor without any special drying or filtration) you might add some compressor oil to the inside of the valve which is not a problem in most service, but in a valve going to food service could be a big deal. An extra coalescing filter can fix even that.

David

 

[mahimetal]

You are right, but my concern is the high pressure oxygen,I have read "high pressure oxygen can make explosion in contact with oil based lubricants" so consider that the partial pressure of oxygen in a 5000 psi compressed air chamber is somehow about 1000 psi.
Is it possible that the oxygen content and the lubricants combination lead to an explosion???

 

[zdas04]

First, you are not proposing testing with oxygen, you are talking about AIR which is around 20% oxygen. Second, what is your ignition source? If you had 5,000 psig upstream of a shut valve and zero psig downstream and you slammed open the block valve, you could compress the trapped air fast enough to develop very high heat of compression numbers, BUT the mass of air that is trapped in a single valve is too small to be a problem--by the time the pressure wave hit the backside of the valve and mixing began, the incoming air would quench the heat to nearly supply temperature.

If you ganged 10 30-inch valves for a test, you might have enough trapped air to ignite a fuel source, but then you have to determine the autoignition temperature of any oils or greases and again it becomes a tempest in a teapot.

Finally, slamming that valve open would be a really stupid thing to do. As the Engineer, you have an obligation to write the pressurization/depressurization procedures that must include a rate of pressure change. If you take 3-4 minutes to get to 5,000 psig, you'll never get a supply stream that can act as a piston (I call this a pseudo-piston in my purging class) and you won't have an ignition source at all. You want to be even more careful with your rate of depressurization. If you go too fast you'll develop a lot of JT cooling and can easily drop the valve to the brittle fracture region while you still have enough pressure to break something.

David