Double mechanism

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. 

There are trade-offs for magnetic drive and canned pumps versus centrifugal pumps with double-mechanical seals. The former have no seals to leak, but need active interlocks to prevent high temperature for temperature sensitive materials. Similarly, diaphragm pumps, that have... 

The leak size can be reduced by using double mechanical seals or a mechanical seal and a throttle bush, the space between the two being vented to a safe place. Major leaks may still occur, however, due to collapse of the bearing or seal. LFG pumps should therefore be fitted with emergency isolation valves (see Section 7.2.1), particularly if the temperature is low or the inventory that can leak out is high. 

Figure 3-31A. Typical seal flush arrangement for double mechanical seals.

Figure 3-31C. Double mechanical seal, two rotary elements against common stationary. (By permission, Fischer, E. E., Chem Processing, Oct. 1983 [24].)...

Double Mechanical Seols With All Ports Accessible Without Removing the Drive... 

PROCESS INDUSTRY REQUIREMENT DOUBLE MECHANICAL SEAL... 

Figure 6-46C. Double mechanical seal used for special gas sealing requirements in Figure 6-46A, and substitutes for the single seal of Figure 6-46B. By permission, Roots Division, Dresser Industries, Inc.

Where it is necessary to prevent any leakage of fluid to the atmosphere, a double mechanical seal is used. The space between the two seals is flushed with a harmless fluid, compatible with the process fluid, and provides a buffer between the two seals. 

The standard pressurized dual mechanical seal (previously referred to as a double mechanical seal) shall be... 

The kinetics and mechanisms of the displacement deposition of Cu on a Zn substrate in alkaline media was studied by Massee and Piron (5). They determined that at the beginning of the deposition process, the rate is controlled by activation. The activation control mechanism changes to diffusion control when the copper covers enough of the Zn surface to facilitate further deposition of copper. This double mechanism can explain the kinetic behavior of the deposition process. 

FIG. 10-113 Internal bellows-type double mechanical seal. 

Figure 7.13. Types of seals for pump shafts, (a) Packed stuffing box the sealing liquid may be from the pump discharge or from an independent source, (b) Water cooled stuffing box. (c) Internal assembly mechanical seal the rotating and fixed surfaces are held together by the pressure of the pump liquid which also serves as lubricant a slight leakage occurs, (d) Double mechanical seal with independent sealing liquid for handling toxic or inflammable liquids.

A substantially new and critical feature in bottom-entering agitators is the sealing of the shaft double mechanical seals are used here. On modem designs, they are provided with additional standstill seals for emergencies. The standstill seal allows the mechanical seal to be replaced while the reactor is full and under pressure. In a number of polymerizations, particularly in vinyl-chloride polymerization, the rotating elements must be protected by additional water flushing (4) other-... 

Fig. 11. Double mechanical seal accessories Pressurized thermal convection system with liquid level switch and 1-gal ASME vessel.

Sample purification and concentration used double-mechanism (cation exchange and reversed phase) solid phase extraction cartridges. The extracts from 5 mL of urine were dried, redissolved with 100 /aL of running buffer, and injected, to achieve a detection limit of 100 ng/mL. For peak identification, not only the retention times were used, but also the on-line-recorded UV spectra of the peaks, which were compared to computer-stored models. 

Quite recently, Schafroth et al. (1994) determined the major urinary compounds of eight common benzodiazepines (flunitrazepam, diazepam, midazolam, clonazepam, bromazepam, temazepam, oxazepam, and lorazepam). The used MEKC with 75 mM SDS in a phosphate-borate buffer (pH 9.3). After enzymatic hydrolysis and extraction with commercial double-mechanism cartridges, the sensitivity was reportedly better than that obtained with the common immunoassay EMIT (enzyme-multiplied immunoassay technique). 

Figure 2 — Double mechanical seal consumes greater horsepower...

To determine the change in pump efficiency for a given seal size, material and construction, a coyistant difference in horsepower should be used, not a constant decrease in points of efficiency (rj). Fig. 4 shows pump horsepower at operating conditions vs. loss of efficiency when using double mechanical seals. (Fig. 4 is to be applied to pumps that have been tested at 1,750 or 3,550 rpm.)... 

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