Tag Archives: reservoir

API 682, mechanical seal

Cooling Water on Seal Pots

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Before retirement, I was regularly asked “Does that seal pot really need cooling water?”  The answer then, as now, usually was “Yes”.  Here’s a quick back-of-the envelope evaluation of a seal pot operating without cooling water and a simple guideline to when cooling water is needed.

The convective heat transfer coefficient to air is in the range of 2 to 5 Btu/hr sq ft°F depending on whether the wind is blowing or not.   This is very low heat transfer coefficient.  In comparison, water cooling coefficients are on the order of 50 to 300 Btu/hr sq ft °F.

If the surface temperature of the seal pot gets too hot then there can be a safety issue.  Many End Users do not want the pot to be at temperatures much higher than 150 °F or so.  This evaluation is based on a maximum seal pot (that is, buffer/barrier fluid) temperature of 150 °F.  Will the seal operate satisfactorily at a temperature hotter than 150 °F?  Almost certainly – the pot will just be hot.  Is the buffer/barrier fluid suitable for temperatures hotter than 150 °F?  That depends on the fluid but probably so.

Remember that the surface temperature of the pot will be hotter than that of the surrounding air.  If the pump and seal are to be operating in the summer then a summertime air temperature should be considered.  Also, for most places, sometimes the wind is not blowing.  Therefore, the base case is

Heat transferred = area x temperature difference x heat transfer coefficient; that is
Heat transferred from pot = pot area ft2 x (150 – 100) °F x 2 Btu/hr sq ft

The external area of a typical seal pot is (very roughly)

  • 2 gal –> 3 to 4 sq ft
  • 3 gal –> 4 to 5
  • 4 gal –> 6 to 7
  • 5 gal –> 7 to 8

So the base case heat transfer from an air cooled pot is roughly

  • 2 gal –> 300 to 400 Btu/hr
  • 3 gal –> 400  to 500
  • 4 gal –> 600 to 700
  • 5 gal –> 700 to 800

This is not very much heat transfer.  On a 50 °F day, with the wind blowing, those heat dissipation values could be multiplied by 5 but that is still not much heat transfer.  Of course, whatever heat is generated will be transferred by virtue of the seal pot temperature becoming as hot as necessary.

In addition to heat generation by the seal faces, there is heat absorbed from a hot pump into the buffer/barrier fluid; this is called heat soak.  For reference, according to API 682, the heat soak from a 200 °F pump to a 150 °F buffer/barrier is 1200 Btu/hr for a 2″ seal size pump.  Therefore, an air cooled pot should not be recommended even for a warm pump having small seals.

For tandem (unpressurized, Arrangement 2) seals, the usual requirement is that the seal pot has to dissipate the heat generated by the outer seal plus heat soak, if any.  As a point of reference, the outer seal of a tandem arrangement generates roughly 100 to 300 Btu/hr per inch of seal size at 3600 rpm (depending spring load, face width, materials, etc.).  Therefore, it turns out that the limits for an air cooled pot with Plan 52 on a hot summer day, no wind, 150 °F maximum allowable pot temperature  would be something like

  • 2 gallon pot –> 1.25″ seals and smaller
  • 3 gal –> 1.625″ seals and smaller
  • 4 gal –> 2.125″ seals and smaller
  • 5 gal –> 2.5″ seals and smaller

For double seals (Arrangement 3) using Plan 53, the usual requirement is that the seal pot must dissipate the heat from both seals plus heat soak, if any.  Therefore, an air cooled pot will almost never be adequate for Plan 53 since the minimum pot pressure must be at least 25 psig and heat loads would be more than double that of the tandem seals (Arrangement 2).  That is, heat loads for Plan 53 would be about 400 to 1200 Btu/hr per inch of seal size.

All the above calculations are very rough and very conservative but still representative.  In fact, the rough calculations readily demonstrate that most dual seal applications need water cooling and show why this is so.  But the rough estimate also shows that there is a place for air cooling and can provide some guidelines for defining those limits.

Based on the above analysis and estimates, here are some guidelines for application of cooling water:

  • Pump temperature above ambient — use water cooling
  • Plan 52, when pump temperature is ambient or cooler – use air cooling
    • 1800 rpm — pot size should be 1 gallon per inch of seal size (2 gallon minimum)
    • 3600 rpm — pot size should be 2 gallon plus 1 gallon per inch of seal size
  • Plan 53 — use water cooling

And a reminder:  Seal pots will always run hotter with oil (even the low viscosity synthetics) buffer/barrier fluids than with glycol/water solutions.

For those who really want to further investigate this question using more sophisticated calculations:  Good Luck!  There is not much information available.  A conservative approach is to order the seal pots with cooling coils but do not connect the cooling water to the coils unless proven necessary by actual operation.

API, API 682, mechanical seal

Hydrostatic Pressure Test

Apparently, the rules and guidelines for conducting a hydrostatic test on pressure vessels – including pumps – are about to be changed.  Perhaps those rules have already been changed and I’ve simply missed hearing about them.  I blame that on retirement.

The hydrostatic test is the way in which pressure vessels are tested for leaks.  The test involves filling the vessel with a liquid, usually water, and pressurizing it to the specified test pressure.  In my career, the rule for determining the hydrostatic test pressure has been to simply multiply the Maximum Allowable Working Pressure (MAWP) by 1.5.  That is, suppose a certain vessel has an MAWP of 600 psig; it would be hydrostatically tested at 900 psig.  Of course, there are other details such as the duration of the test, variations based on operating temperature, etc.  But basically, the hydrostatic test pressure has been 1.5 x MAWP for many years – certainly since the 1950s.  However, it has not always been that way.

Here’s an interesting little anecdote that I was told many years ago.  I may not have it quite right and, for all I know, it may not even be true, but here goes.  In the 1970s, I was told that very long ago, the practice was to test pumps at 2 x MAWP.  Perhaps this was because pumps are manufactured from castings.  Anyway, there was a pump standards meeting (perhaps this was even an early form of an API standards meeting) and a requirement was written to hydrostatically test pumps at 1.5 x MAWP.  After the meeting, the chief engineer for a major pump manufacturer was buying drinks for everyone at the bar.  Surprised by his generosity, a fellow committee member remarked that the 2x hydrostatic test must have been difficult.  “Not at all”, said the chief engineer, “In fact, I just increased all my pressure ratings by 33%!”  That is, pumps previously rated for 600 psig MAWP but hydrostatically tested at 1200 psig could still be hydrostatically tested at 1200 psig but then rated for 800 psig MAWP!

I’ve been told that the multiplication factor for determining the hydrostatic test pressure will be changed from 1.5 to 1.3 according to the ASME Pressure Vessel Code Section VIII.  Apparently this will be applied to both pumps and piping.  The same multiplier will probably be used for API 682 reservoirs such as are used with Piping Plan 52 and 53.  The pertinent 4th Edition clauses for reservoirs now read:

8.3.6.2.8 The reservoir is part of the pump piping system. Unless otherwise specified or required by local code, the reservoir shall be designed, fabricated, and inspected in accordance with ISO 15649 or ASME B31.3 using piping components.

8.3.6.2.9 If the reservoir is built entirely of piping components, ISO 15649 or ASME B31.3 can be applied and provides adequate design for the reservoir just as it does for the pump suction and discharge piping. It is the user’s responsibility to ensure that local codes do not require that reservoirs be built in accordance with a pressure vessel code such as EN 13445 or ASME VIII, Division 1.

With this wording, the default is a pipe based reservoir but an ASME certified reservoir is an option.  I hope we will be able to continue this approach in 5th Edition but I’m uncertain about higher pressure reservoirs.  From my past experience, old notes and browsing, here’s what I’ve learned.

ASME Certified Pressure Vessels

ASME certified pressure vessel fabricators undergo a rigorous program to ensure compliance with the rules and regulations of the ASME Boiler and Pressure Vessel code.  For unfired pressure vessels, these requirements are given in ASME VIII, Division 1 (EN 13445).  There are two variations of certification:  “U” stamp and “UM” stamp.

“U”stamped pressure vessels are required to have a 3rd party ASME inspector review and approve the calculations as well as inspect certain stages during manufacture of the reservoir.  This inspector also witnesses the ASME hydrostatic test which, apparently, is now 1.3 x MAWP.  This means that U stamped vessels cannot be manufactured in advance; that is, the U stamped vessel is customized for a particular service, has a serial number and therefore cannot be a stocked item. The fabricator undergoes an ASME inspection every three years.

In contrast to the U stamped vessels, there is a “UM” stamped vessel which is also subject to the same ASME specifications.  However, the “UM” stamped vessel is limited to a maximum of 1.5 cubic feet for a 600 psig rating and 600 psig is the maximum permitted pressure for the UM stamp.  ASME 3rd party inspection is not required; therefore “UM” vessels can be manufactured in advance, they do not have a serial number and they can be stocked as an inventory item.  The UM fabricator is inspected annually by ASME.

Pipe Based Reservoirs

Pipe based reservoirs for API 682 sealing systems must be built entirely of piping components and ASME B31.3, “Process Piping”, (ISO 15649) is the governing standard.  As far as I can tell, ASME B31.3 now requires hydrostatic testing at 1.3 x MAWP.

Mechanical Seals

It should be noted that the mechanical seal is not considered to be part of the pump pressure vessel and therefore does not fall under the pressure vessel rules.  Seal manufacturers have several pressure ratings for their products.  API 682 recognizes a static pressure rating, a dynamic pressure rating and a hydrostatic pressure test rating (see the SealFAQs version of these definitions).  Each seal OEM seems to use a different and proprietary method for determining these pressure limits.