Tandem and Double Seals
Since almost all seals utilize the process liquid or gas to lubricate the seal faces, they are designed to leak. Process liquids and gases containing hazardous vapors, dangerous toxic chemicals or flammable petroleum must not be allowed to leak into the atmosphere or onto the ground. In these applications a second "containment" seal is placed after the primary seal along the pump shaft. The space in between these two seals is filled with a neutral or compatible liquid or gas (generally nitrogen) called a buffer seal (unpressurized) or barrier seal (pressurized).
In a tandem seal, the seal will leak into the buffer fluid contained in the unpressurized cavity commonly known as thermosiphon pot. If the cavity registers a dramatic increase in pressure or fluid level, the operator will know that the primary seal has failed. This can be achieved by using pressure/level switches or transmitters. If the cavity is drained of liquid, then the secondary seal has failed. In both instances, maintenance will need to be performed. This arrangement is commonly used when sealing fluids that would create a hazard or change state when contacting open air. These are detailed in API 682 Piping Plan 52
In a double seal, the barrier liquid in the cavity between the two seals is pressurized. Thus if the primary seal fails, the neutral liquid will leak into the pump stream instead of the dangerous pumped fluid escaping into the atmosphere. This application is usually used in gas, unstable, highly toxic, abrasive, corrosive, and viscous fluids. These are detailed in API Piping Plan standards #53a, 53b, 53c; or 54. Plan 74 may also be considered a double seal piping plan, although it is used exclusively when describing a dry gas barrier seal support system. The barrier fluid used in a Plan 74 system is simply a gas, not a liquid. Typically, nitrogen is used as its inert nature makes it advantageous due to mixing with the process stream being sealed.
Tandem and double seal nomenclature historically characterized seals based on orientation, i.e., tandem seals mounted face-to-back, double seals mounted back to back or face-to-face. The distinction between pressurized and unpressurized support systems for tandem and double seals has lent itself to a more descriptive notation of dual pressurized and dual unpressurized mechanical seal. This distinction must be made as traditional 'tandem seals' can also utilize a pressurized barrier fluid.
Read more about this topic: End Face Mechanical Seal
Famous quotes containing the words tandem and, tandem, double and/or seals:
“We are bare. We are stripped to the bone
and we swim in tandem and go up and up
the river, the identical river called Mine
and we enter together. No one’s alone.”
—Anne Sexton (1928–1974)
“The Cad is the entire epitome, the complete blossom and fruit in one, of what we are told is an age of culture. Behold him in the vélodrome as he yells insanely after his kind as they tear along on their tandem machines in a match, and then ask yourself candidly, O my reader, if any age before this in all the centuries of earth ever produced any creature so utterly low and loathsome, so physically, mentally, individually, and collectively hideous?”
—Ouida [Marie Louise De La Ramée] (1839–1908)
“The Cossack eats Poland,
Like stolen fruit;
Her last noble is ruined,
Her last poet mute:
Straight, into double band
The victors divide;
Half for freedom strike and stand;—
The astonished Muse finds thousands at her side.”
—Ralph Waldo Emerson (1803–1882)
“Social and scientific progress are assured, sir, once our great system of postpossession payments is in operation, not the installment plan, no sir, but a system of small postpossession payments that clinch the investment. No possible rational human wish unfulfilled. A man with a salary of fifty dollars a week can start payments on a Rolls-Royce, the Waldorf-Astoria, or a troupe of trained seals if he so desires.”
—John Dos Passos (1896–1970)