6 replies to this topic

[rsk]

Generally S & T HX specification, contains thermal rating in KW, but increase in LMTD due to change in steam pressure on shell side or decrease in feed water inlet temp tube side results in increased thermal rating. how thermal rating of a HX is arrived at. Also several name plate details, do not restrict inlet & outlet temp of feed water on tubeside. Please elaborate what are the minium details to be given for a HX to be completely specified.

[Art Montemayor]

RSK:

I think you are asking, "how is the thermal rating of a HX arrived at?". Is that correct? If so, the answer is that it is arrived at depending on the type of service it performs. Sensible heat transfer is differnent from latent heat transfer.

You also ask us to "elaborate on what are the minium details to be given for a HX to be completely specified". The easiest and simplest way to "elaborate" is to have you fill in a typical, detailed Specification Sheet for a Heat Excahanger. Do a SEARCH routine on our Forums and you will find an Excel Workbook calleld "Art's Specification Sheets" where you will find various specification sheets for a heat exchanger.

[djack77494]

A S&T heat exchanger is a mechanical peice of equipment, and it is necessary to only show mechanical parameters on the nameplate. There are code requirements that you must satisfy, such as showing the design pressure and temperature (and the MDMT) for both sides of the exchanger. I'm not real familiar with TEMA codes, and so don't know if they would impose additional requirements. You could choose to show other mechanical details such as surface area, number, diameter, and length of tubes, TEMA type, TEMA class, pitch, number of tube passes, etc. I am ambivalent about the need to have this information on the tag, and usually it is not done. You should show the materials of construction. You may note that the above is all unchanging mechanical information.

When you start to show duty, LMTD, "U" factors, fouling factors, etc. you are venturing into a whole different area. All of these are PROCESS (i.e. not mechanical) information. What is true today may not be true tomorrow if you change feeds, rates, or process conditions. This information has no strong connection to the equipment, and I oppose including it on a stamp. Nonetheless, I have seen many case where process data, especially duty, is included on nameplates. To me, it is meaningless, likely erroneous, and only confuses the issue.
HTH,
Doug

[mishra.anand72@gmail.com]

May be name plate must provide details about type of heat exchanger, design pressure and design temperaure for both side that is tube side and shell side and heat duty and name of OEM.

[djack77494]

"May be name plate must provide details about type of heat exchanger, design pressure and design temperaure for both side that is tube side and shell side and heat duty and name of OEM."

While I admit to being a bit less than 100% certain, I am fairly sure that the duty, which is a characteristic of the interaction between the process and the equipment, and the exchanger (TEMA?) type need NOT be shown on the nameplate. I have no idea about a requirement that the OEM's name be shown, though I know that I have seen it on exchangers many times. Exchangers are considered pressure vessels by the ASME and subject to normal pressure vessel stamping requirements.

[sq3169]

Hi guys

I am still a junior in the industry. I might be wrong and will be glad to be corrected...

To me, I think the thermal rating should be based on the possible worse case scenario. This is what we have done for some S&T exchangers on FPSO. We picked the did a check on the heat load studies for various design cases and gave another 10% of surface area in additional to that required based on calculations.

As for the name plate, both the shell and tube mechanically are designed according to ASME Sect VIII. I reckon the name plate should be similar to that for pressure vessel and perhaps the TEMA class type of the heat exchanger.

I have a question on the design temperature of S&T heat exchanger. On API 14C (Recommended Practice for Analysis, Design, Installation, and Testing of Basic Surface Safety Systems for Offshore Production Platforms), Appendix A-10 recommends the safety devices for S&T HX.

A brief description of the S&T HX that we have designed

Shell
Medium: Cooling water
Design: 15 Barg/100 deg C
Op temp: 37 deg C (IN) 55 deg C (OUT)

Tube
Medium: HC Gas
Design: 23 Barg/177 deg C
Op temp: 110 deg C (IN) 50 deg C (OUT)

The pressure safety device requirement is fulfilled. There is pressure sensor on the inlet and outlet of the HC gas to detect gas leakage into the shell side. The same recommendation doesnt require temperature safety device because both sections of the S&T are normally rated for the same maximum temperature of the heat medium. However, the cooling water interface on the ship is rated to 150# (15 Barg/100deg C). Should I interpret API 14C that if the design temp for both section is different, temp safety device should be in place?

Thanks

SQ3169

[djack77494]

To me, I think the thermal rating should be based on the possible worse case scenario. This is what we have done for some S&T exchangers on FPSO. We picked the did a check on the heat load studies for various design cases and gave another 10% of surface area in additional to that required based on calculations.

sq, you are describing how you did your DESIGN. I maintain that there is no such thing as a "worst case scenario" in this context. Work on a revamp where your process conditions completely change - flowrates, temperatures, and maybe even service. What changes? Duty, U factor, and anything related to the process. What does not change is the surface area, number and size of tubes, and other mechanical data. Even in your case, working on an FPSO, the duty, heat transfer coefficient, fouling and virtually ALL of your process conditions will significantly change over the life of the field. Values you would stamp onto the metal plate represent (theoretically) an instant in time. Actually, even that will almost surely not be fully realized.

On a revamp or when you go to an equipment "boneyard" and try to purchase a discarded old heat exchanger for a totally unanticipated service, any process data is completely meaningless. It is a characteristic of the PROCESS and not of the EQUIPMENT. You might even do minor changes like reworking the baffling, thereby changing the number of passes in the exchanger. It would still have the same basic mechanical characteristics, though its process characteristics would little ressemble the original design.

The same recommendation doesnt require temperature safety device because both sections of the S&T are normally rated for the same maximum temperature of the heat medium. However, the cooling water interface on the ship is rated to 150# (15 Barg/100deg C). Should I interpret API 14C that if the design temp for both section is different, temp safety device should be in place?

I am unaware of the need for any "temperature safety device" anywhere for any piece of equipment (though I don't doubt that such reqauirements probably exist somewhere). The ubiquitous PSV is strictly a pressure relieving device, and the common codes tend to be written around limiting pressures inside pressure vessels. I would be interested in hearing of codes that go beyond this common framework.