Tandem seal for pump and compressor

[DoraeS]

Hi,

If I have a tandem seal for pump and a tandem seal for gas compressor, what I understand is that the primary seal will be drawn from pump or compressor discharge to the stuffing box, and the secondary seal could be water for pump (API seal plan 52) and nitrogen for gas compressor. I would like to know if the above is correct? And if so, my question is, does the secondary seal pressure must be higher than the pump or compressor discharge pressure in order to prevent the primary seal from 'leaking'?
Also, does any one know which website can I find the relevant information?
Thanks a lot!

 

[flexibox]

A Tandem seal or Dual Unpressurized seal uses API Plan 52 on a pump. The primary seal is normally fitted with API Plan 11 but this depends on pump type and design. API plan 52 uses a Buffer fluid that must be compatible with the product being pumped and the fluid is not pressurized.

Tandem seals in a Gas Compressor is diffierent to a Pump. This equipment does not use API Plan 52. You must treat these as two different applications. The compressor would normally have a Gas Panel that delivers clean N2 to the secondary seal and dry filtered Process gas to the primary seals. This will vary depending on application.

trust this helps.

 

[DoraeS]

Dear flexibox,

Thanks for your explanation. I must admit that I am not familiar with the seal plans for both pumps and compressor!
Regarding your seal plan 52 on pump, what should be the buffer fluid pressure? If this pressure is lower than the primary seal (which is taken from pump discharge and hence discharge pressure?), how will the buffer fluid be able to prevent the primary seal from leaking?

Regarding the compressor seal, is the nitrogen pressure be higher than the compressor discharge pressure (which in this case, discharge fluid also will be the primary seal)?

Please provide more information and explanation
Thank you very much for your help.

 

[bk19702]

Plan 52 is unpressurized, therefore whatever leaks past the primary seal will leak into your buffer fluid. Whether the leakagae is mostly vapor or liquid depends on what you are pumping. Plan 52 reservoirs are typically vented to a flare system, so the primary seal vapor leakage has a place to go. If you want to contain the primary seal leakage completely, you pressurize the reservoir to a rating higher than what you are sealing against. This is no longer a Plan 52 at this point and subsequently becomes a Plan 53. In this arrangment, the barrier fluid (fluid in the reservoir) lubricates both primary and secondary seals. These particular flush plans address wet, contacting mechanical seals in pumps. Wet / dry tandems (contacting and non-contacting) would utilize different flush plans. Same goes for non-contacting pressurized seals in pumps.

Gas compressor tandem seals are different animals as mentioned, but there are similarities. You must keep in mind of course, that when discussing gas compressor seals of this type you are talking about a non-contacting seal design. The process gas will be applied to the primary seal; typically a lower pressure nitrogen sweep is applied the outer seal and leakage from both are sent to a flare. There are variations, but this would be the most common. A pressurized seal can also be used here, but you need adequate pressure differential over your sealed pressure (suction pressure typically in a beam type centrifugal compressor) for this to work.