Double seals

This eliminates the vapor space but sealing the edge can be a problem. Double seals can help and sometimes a fixed roof is also added above the floating roof to help capture any leaks from the seal. However in this case, the space between the fixed and floating roof now breathes and an inert gas purge of this space would typically be used. The inert gas would be vented to atmosphere after treatment. 

Double seals are standard with toxic and lethal products, but maintenance problems and seal design contribute to poor reliabihty. The double face-to-face seal may be a better solution. 

Do not use a double seal in dirty seiwice—the inside seal will hang up. 

First, one pump running in a parallel system tends to suffer from cavitation because operation to the right of the BF.P indicates that the NPSHr of the pump rises drastically. To survive this condition, you should use dual mechanical seals on these pumps. Dual or double mechanical seals can withstand cavitation better than a single seal. There is a discussion on this in the mechanical seal chapter of this book. Many engineers perceive that parallel pumps are problematic becau.se they appear to suffer a lot of premature seal failure. Parallel pumps deserve double seals even if it s only a cold water system. 

The solution is Parallel pumps should have dual or double seals installed to withstand cavitation when one pump is running solo. 

Double seals are also known as dual seals. They arc used ... 

The general tendency with double seals is to think that one seal in a dual arrangement could fail and the other would kick-in as a spare or back up. This is Vi y cars have a spare tire, why a passenger jet has a pilot and a co-pilot and v y —" bodies have two kidneys and two eyes. One could become incapacitated, damaged or fail and the other would assume the functions. This is the case with other do ble seals but not this one in the applications where it is most specified. As soon as one fails the other will follow. 

This type of double seal (with back to back faces) is pressurized above the pressure inside the seal chamber (Figure 13-20). It is recommended for toxic, explosive, costly, dangerous, and volatile liquids. It is important to maintain the seal pressure above the pumped pressure inside the seal chamber. 

This facc-to-facc double seal is quite versatile (Figure 13-21). It is reeommended in a wide range of applieations depending on the piping arrangement of its support system. 

With its support system pressurized above the seal chamber pressure, this double seal functions well with toxic and dangerous liquids (like the back-to-back dual seal). If the support system is pressurized at / the seal chamber pressure, this seal can handle higher pressures where a single seal would fail. If the support system should be non pressurized... 

Double seals require some type of support system. The reason is simple. With two seals mounted onto the same shaft, one seal is the principal or primary seal and the other becomes the secondary or back up seal already installed. If the primary seal is performing its function and sealing the pumped liquid, the. secondary. seal would be running dry, overheat, burn and self-destruct. Then when the crucial moment comes, we won t have a second seal to assume the ftinctions, which was the original reason to consider a dual seal. 

There are three distinct support systems for double seals. They are often referred to as barrier tanks . The term barrier tank was initially applied to the thermal convection tank, although the term today refers to any of the three support systems. Each support system has different attributes. 

Drop in Pressure Double seal with pressurized barrier fluid... 

Contact with air Quench and Drain or Double Seal... 

Non-Lubricating Liquids No lubrication or heat dissipation. Remove the heat with a double seal and a barrier rank with forced eonveetion How. 

Very High Pressures Use balanced o-ring cartridge seals up to about 500 psi. Above 500 psi use the tandem double seal with the barrier fluid pressurized at / the seal chamber pressure. Remember as pressure goes up the o-rings will extrude and metal parts will distort. Use a torsion balanced seal. 

Do not use a double seal in dirty serviee—the inside seal will hang up. The API standard is a good guide to the use of balaneed and unba-laneed seals. Applieation of a balaneed seal at too low a pressure may eneourage faee lift-off. 

Figure 3-31D. Tandem double seal. (By permission, Fischer, E. E., Chem. Processing, Oct. 1983 [24].)...

The plate heat exchanger consists of a frame in which closely spaced metal plates are clamped between a head and follower. The plates have corner ports and are sealed by gaskets around the ports and along the plate edges. A double seal forms pockets open to atmosphere to prevent mixing of product and service liquids in the rare event of leakage past a gasket. 

On double seal installations, be sure the sealing liquid lines are connected, the pressure control valves are properly adjusted, and the sealing liquid system is operating before starting the equipment. 

Face lubricated-type seals must be connected from the source of lubrication to the tap openings in the seal gland before startup. This is another predetermined environmental control feature that is mandatory for proper seal function. Where double seals are to be operated, it is necessary that the lubrication feed lines be connected to the proper ports for both circulatory or dead-end systems before equipment startup. This is very important because all types of double seals depend on the controlled pressure and flow of the sealing fluid to function properly. Even before the shaft is rotated, the sealing liquid pressure must exceed the product pressure opposing the seal. Be sure a vapor trap does not prevent the lubricant from reaching the seal face promptly. 

Remove all clothing and personal effects segregating them as either durable or nondurable items. Although it may be possible to decontaminate durable items, it may be safer and more efficient to destroy nondurable items rather than attempt to decontaminate them. Items that will be retained for further processing should be double sealed in impermeable... 

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