S&T Gasket Replacement Before Nitrogen Purge 1

[Brandon181]

I have some questions/concerns regarding drying of a shell and tube exchanger after hydrostatic testing. My work is mostly in the sulfur/amine processes. If it is pertinent, which I do not think it is, the specific exchanger type I am looking at is a BEU. However, I would prefer general answers that could apply to all types if possible. Please excuse me if the following questions are ignorant, but as a recently graudated engineer I find they did not cover this stuff in school

1. My first question is whether the exchanger needs to be fully disassembled to ensure proper drying. I have heard of vendors that "rock" the exchanger to remove most the moisture, and then, if a nitrogen "purge" or "blanket" is not performed, they include drains to further remove moisture during shipment and storage. However, this does not seem to be adequate drying measures and could lead to corrosion.

2. Also, if a nitrogen "purge" or "blanket" IS to be performed, does the exchanger need to be disassembled in any way? I am not a 100% familiar with the procedure for performing a nitrogen purge on a shell and tube exchanger. My understanding is that the nozzles will be "blinded" off and a predetermined amount of nitrogen would be pumped into the unit and then the unit would be sealed until startup. If this is the case, it seems like disassembling strictly for the purpose of a nitrogen "purge" is unnecessary.

3. Finally, the most important question, is it common practice to replace ALL gaskets before performing the nitrogen "blanket" (I know if "testing" gaskets are used during the hydrotest that they will need to be replaced; My question is geared more towards when service gaskets are used for the hydrotest). It seems that it would be hard to ensure proper drying without replacing the gaskets after hydrotesting. I know that the new gasket seating might not be perfect and could potentially leak upon startup, but it seems this would be better than the alternative which is possible permanent water damage while the unit waits for startup for 6+ months. Any thoughts/opinions would be appreciated.

 

[muld0020]

Special drying requirements are usually provided in the project specification. Nitrogen purge or Nitrogen blanketing are usually extras. It depends on your materials and storage timeframe. Your exchanger vendor will be able to provide procedures and details to a specific exchanger.

1. The exchanger does not need to be fully dissassembled to be fully drained. A nitrogen purge can be done to assist in getting the air out and is usually suffecient for shorter term storage.

The easiest way to detmine if you are dry is to pull a vacuum after it is drained and watch the pump. There are varying ways/degrees to do this sometimes with hot air etc...

2. It is better that the exchanger remain sealed after testing for the a nitrogen blanket.

3. No it is not common to replace the gaskets. The test pressure is usually much higher than the nitorgen blanket pressure (10-15psi). If you are concenred about corrosion, you can request rust inhibitors.

 

[innovation2]

Through a couple of desicant packets in the equipment if you are really concerned with moisture. Also, specifying that the vessel be dry to a low dew point maybe required. Replacement of the service gaskets is not required as the amount of water trapped can be considered minimal.

I really do not see many tank specification that call for a nitrogen blanket unless it is carbon steel and they wish to prevent rust on the inside.

Remember it is going to be difficult to see if you have a leak with the nitrogen if you decide to replace all gasketing after hydro.

 

[Brandon181]

Thank you for the responses.

innovation2: You are correct in assuming this is a carbon steel shell. And it's my understanding that periodic testing of the nitrogen pressure would need to be made whether the gaskets are replaced or not, so leaking of nitrogen would be detected. Although I see the point that gasket leaks could cause more problems than those I mentioned at startup. I am not familiar with desicant packets and will research into those. And while most of the people I speak to agree that the water trapped by the gaskets is minimal, I have heard a small minority disagree.

muld0020: Our specifications do call for a nitrogen purge. And time before startup is 6+ moths. The problem is that we include a pressure vessel spec in our inquiries. Under that spec we have an item that states the gaskets included in the scope of the supply shall not be utilized for performing the test. According to spec, it seems pretty clear to me that the gaskets must be replaced. However, the vendor has protested this, saying that this is feasable for a regular vessel but not for a shell and tube. Due to the spec and previous units built, the gaskets will be replaced. I just wondered what other minds in the industry felt about this matter.

Any other opinions as to whether service gaskets should be replaced after testing would be appreciated. Without being able to do any longterm testing myself, I would like a pool of experienced people to draw opinions from. Thanks!!

 

[SnTMan]

Wow, an opportunity for one of my favorite rants

I HATE, HATE, HATE to get a PV spec as part of a S&T spec package. Too much confusion, too many clarifications required. Usually done by people who don't know enough about either product to have any idea what applies and what doesn't let alone why. Throw a bunch paper at the fabrication, let him figure it out. These are the people I have to get the clarifications from. A real "value subtracted" deal. Much better to explicity state the one or two things that are typically of interest. I'll do them happily.

Brandon181, no offense intended.

Regards,

Mike

 

[TJOrlowski]

Brandon181,
Concur with SnTMan: what is the reason that your pressure vessel spec requires that the test gaskets shall not be suppiied with the finished product? Does that reason make sense, and is it even applicable for shell & tube heat exchangers? Mandating that all gaskets be replaced after hydro, prior to nitrogen charge is your option, but adds unnecessary cost to your heat exchanger, while adding little quantifiable value in my opinion.

In the case of your BEU exchanger, mandating the installation of all fresh gaskets after hydro will require the fabricator to completely disassemble the unit. This cost could be significant in large diameter or long tube length U-tube units, where special care is required when handing the bundle. If you had, say, a type AES or AET shell & tube unit, now you're really adding time and money to the project.

-TJ Orlowski

 

[rmw]

Once you prove that it doesn't leak via a successful hydrotest, how do you then prove that it doesn't leak subsequent to the R/R of the gaskets? Don't introduce this variable into your project.

To a point made above, don't pull a vacuum on the Hx unless it is designed to withstand vacuum pressure (external pressure) although if you design it to do so, I like the vacuum method of drying (assisted by some gentle heating during the vacuum pull). Plus, if you pull the vacuum, once dry, then introducing the N2 insures that you get a good 'purge' and 'blanket'.

rmw

 

[Brandon181]

SnTMan: No offense taken. I do not write the specs or choose which specs get included into the inquires. This is above my pay-grade. And I understand your feelings that the shell and tube spec could be expanded to include those requirements found from the PV spec.

TJOrlowski: It is my understanding that the reason the pressure vessel spec requires that the test gaskets be replaced is due to moisture problems as well as not wanting to use gaskets that were subjected to the high test pressures. I also understand that they do not often deal with body gaskets as in a S&T. However, if moisture can be a problem for other gaskets, why not these? I understand your concerns with higher costs, especially if the replacements are unnecessary. I am trying to determine whether or not they are truly unnecessary.

 

[Duwe6]

". . . not wanting to use gaskets that were subjected to the high test pressures."

The test pressure was only 1.3 x Design Pressure, so the gasket was not taken anywhere near its upper limit. And this gasket set did not leak at Hydro Pressure. Why throw away a proven-to-be-good set of gaskets and put another set in? As soon as the 'fresh set' is installed, you have to test it for leaks prior to putting it into service.

Pulling every newly arrived S&T Hx apart is traditional, but you might want to rethink this. I see it as a waste of time, and a set of gaskets [that don't leak]. But this is ASSuming that the test gaskets meet the process spec's, and that the shell was properly cleaned-out prior to assembly for hydro.

 

[SnTMan]

Brandon181. Thank you for your forebearance, as it happened I had just gotten off a frustrating conference call with a client who is insisting on following a specification off a cliff

S & T exchangers are hydro'd, drained and dried all the time with the service gaskets installed. N2 blanket is very effective at preventing corrosion indefinetly as long as it is maintained properly. Use of dessicant only as noted by innovation2 is also common, although for shoter storage periods.

If API 660 applies it requires that any joint broken after hydro be fitted with new gaskets and re-hydro'd. Given a requirement to replace gaskets after hydro, or some other requirement to open the exchanger, this becomes a never ending cycle.

There are occasions where it is necessary to open an exchanger after hydro. This can be addressed, if the AI agrees, by a low pressure air test after reassembly. Not the same as a hydro, but it is something.

If there is a concern about damage to the service gaskets from a hydro I'd say the gasket selection is suspect in the first place.

Glad it's Friday

Regards,

Mike