Propane Refrigeration Loop leak Testing

[10815L]

Hi,
I'm going to leak test a propane refrigeration loop with Nitrogen and air in segments, vendor suggested this scheem while i'm suggesting to go with vaccume and break up with propane.
Please any one have experience in propane refrigeration system share their experience and advise for leak testing best technique.
Thanks
10815L

 

[TD2K]

We used to leak check everything after a turnaround by taping the flanges except for a small hole and then using snoop or a similar material that would bubble when gases flowed through it. Is this an operating system or one that you are leak checking for the first time?

The propane refrigerant systems I've worked on have all operated above atmospheric pressure (not much in the case of the first stage suction), have you just considered using an LEL detector and checking at the taped flanges.

 

[10815L]

TD2K,
Its 1st time commissioning and startup of this plant, propane refrigeration loop is big and vendor plus commissioning manager develop procedure for leak testing with air.
Thanks
10815L

 

[EmmanuelTop]

Pressurize to 80% of design pressure or maximum operating pressure, whichever the higher, and observe pressure decrease by time. There are no universal guidelines - it is up to the commissioning manager and the asset owner to set up the criteria for acceptable decrease of pressure. Make sure to account for ambient temperature changes.

I would use Nitrogen instead of Air.


Dejan IVANOVIC
Process Engineer, MSChE

 

[elridyeng]

As you said it is the 1st time so i think you may need to practice a pneumatic test and / hydro test and here are the information may help you :

After a pipe system is installed in the field, it is usually pressure tested to ensure that there are no leaks. Once a system is in operation, it is difficult, if not impossible, to repair leaks.
There are generally two types of pressure tests applied to a piping system:
1- Hydrostatic Testing
2- Pneumatic Testing
1- Hydrostatic Testing :
It is important to provide high point vents and low point drains in all piping systems to be hydrotested. The high point vents are to permit the venting of air, which if trapped during the hydrotest may result in fluctuating pressure levels during the test period.
The drains are to allow the piping to be emptied of the test medium prior to filling with the operating fluid. (Low point drains are always a good idea though since they facilitate cleaning and maintenance.)
A hydrotest is to be held at a test pressure not less than 1.5 times the design pressure. The system should be able to hold the test pressure for at least 10 minutes, after which the pressure may be reduced to the design pressure while the system is examined for
leaks. A test gauge should be sensitive enough to measure any loss of pressure due to leaks, especially if portions of the system are not visible for inspection.
The test medium for a hydrotest is usually ‪#‎clean_water‬, unless another fluid is specified by the Owner. Care must be taken to select a medium that minimizes corrosion.
2-Pneumatic Testing
The test medium must be nonflammable and nontoxic. It is most often ‪#‎compressed_air‬, but may also be ‪#‎nitrogen‬, especially for fuel gases or oxygen service. Note that compressed air often contains both oil and water, so care must be exercised in specifying an appropriate test medium.
A preliminary pneumatic test is often applied, holding the test pressure at 25 psig to locate leaks prior to testing at the test pressure. The test pressure for pneumatic tests is to be at least 1.2 but not more than #1.5 times the design pressure. The pneumatic test must be held at least 10 minutes, after which time it must be reduced to the lower of the design pressure or 100 psig (700 kPa Gage) until an inspection for leaks is conducted. If a high degree of sensitivity is required, other tests are available such as mass spectrometer or halide tests.

 

[10815L]

Thanks guys,
Further i need some more information on vacuum pulling before propane and lube oil charging,as my leak test stage has been completed and now vacuum pulling started, vendor need to pull vacuum up to 2 mmhg and i don't good vacuum pump.
As you all are aware of refrigeration process is positive process and vacuum pulling is just make surety of no water content remained in the system.
Is there any standard for maximum vacuum pulling if yes then please share here.
thanks
10815l

 

[georgeverghese]

Assuming current minimum ambient temp is 20degC at this location, sat vap press for liquid water is 0.02bar abs = 15mmHg, so if the system pressure has dropped below say 10mmHg, then there is no more liquid water in the loop. You may most likely have to purge with N2 before you start this vacuum pump. After gettting down to 10mmHg or so for a few hours, purge out remaining N2 + water vapor in the loop with propane gas.

If ambient temp is something other than 20degC, you can get the sat vap pressure for water from steam tables, and adjust accordingly.

 

[EmmanuelTop]

Apart from what George suggested, you can use dry Nitrogen purge for removing residual water. For cryogenic plants (even more stringent with regards to allowable moisture content than Propane units) we were targeting -10 to -20 degC water dew point. For Propane refrigeration you are probably good with any negative dew point (in Celsius).

Dew point is good indicator of actual moisture content. Vacuum can be a bit tricky - especially if one assumes that all water is gone once when vacuum is established in the system. There should be a good procedure adjusted to your system.

Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

Which is why we should keep this vac pump running for a few hours at 10mmHg abs to ensure that any residual liquid water gets to vaporise.

In air separations plants (which operate at -160 to -200degC), we used a portable Shaw dewpoint meter to check that purge gas exiting the columns was at -100degC water dewpoint at least.

 

[10815L]

Hi,
Here is my 24 hr vacuum parameters and i would like to know from your experience that is it ok or yet to go further down. Regarding the N2 continuous purge vendor is not accepting that idea.
Time Pressure Drop in PSIA Pressure in Bar Temp 0C
0745 0.34 -0.910 24.7
0900 0.50 -0.905 27.8
1200 0.84 -0.907 37.0
1500 1.01 -0.903 40.2
1800 0.84 -0.901 35.8
Thanks
10815L

 

[georgeverghese]

Based on the last temperature reading of 35degC, sat vap pressure = -0.96barg, and the vacuum guage reading is only -0.90barg. If indeed all the water has been removed, then there is some air leaking into this system , or the vacuum guage accuracy should be checked.

What does " pressure drop" mean ?

 

[10815L]

Hi George,
I understand pressure drop mean but, what about Ambient temperature effect on this pressure.
10815L

 

[georgeverghese]

As you can see from my last reply, ambient temp effect at 35degC is accounted for in the sat vap pressure of -0.96barg (approx 40mmHg abs). So you should try to get to below 35mmHg abs (-0.97barg) at 35degC at least.

 

[golpesar88]

Hi.
Because vendor is there, I think what he asked is in priority.
Always use nitroge, because Nitrogen penetration is higher and is safer. Find major leaks by nigrogen. Based on API test pressure is 90% of PSV set point.
You must categorize leak in 2 groups: external leaks from flanges. Internal leaks due to valve passing to flare or drain network.
Perform vacuum test to vaporize all water inside the cycle. Other wise hydrate fomation or water freezing expansion will cause siriues problem.
The pressure drop is high. It could have 2 reason. First is water trapped second is internal or external leak.
Compare results with N2 leak test. If you can find a similar trend then it is leak. Other wise it could be water. Specially if your plant situated at a humide place. And sorry for english mistakes.

 

[10815L]

Hi,
After hard working i'm able to bring down pressure to 0.0 psia but with pressure up and vacuuming with N2, here is my question if this was trapped water then by pressure up with N2 how easily removed but previously pump was not able to boil it up.
Can any one explain this philosophy?
Thanks
10815L

 

[aquafied]

The purpose of pulling a high vacuum on a refrigeration system is to verify that the system has no external leaks and to ensure that no moisture is trapped in the system. It is not uncommon to leave the vac test for 24 hours, a very slight loss of vacuum during this period can be attributed to ambient temperature rise, a substantial 10 to 20 microns would indicate moisture in the system and if system pressure is at atmosphere there is a leak in the system. Unfortunately I think you might be trying to measure the vacuum in the system with a compound gauge (not a vacuum gauge). Compound gauges are not designed to measure vacuums accurately.