Diffusion Pump Maintenance

Figure 7.9. Some types of centrifugal pumps, (a) Single-stage, single suction volute pump, (b) Flow path in a volute pump, (c) Double suction for minimizing axial thrust, (d) Horizontally split casing for ease of maintenance, (e) Diffuser pump vanes V are fixed, impellers P rotate, (f) A related type, Ihe turbine pump.

The connection of the pumping system to the manifold should be by way of a large-size trap (Fig. 3-10), which is cooled with liquid nitrogen in order to remove contaminants. In operation, the mechanical pump is started first, the pressure in the system is reduced below 100 /X, and the diffusion pump is turned on. Once a low pressure has been attained, the pumps are not ordinarily shut off unless this becomes necessary for maintenance or repair. If the system to which the pumps are connected is to be raised to atmospheric pressure regularly, a bypass system should be supplied so that the system may be evacuated... 

Efficient high-vacuum pumps generally do not operate well near atmospheric pressure. Thus the vacuum system must have a mechanical vacuum pump to evacuate the system to a pressure where the high-vacuum pumps are effective. Mechanical pumps require routine maintenance, such as ballasting and replacing the pump oil. The diffusion pump is the least expensive and most reliable high-vacuum pump. Turbomolecular pumps and cryopumps are also used on mass analyzers. The high-vacuum pumps also require... 

A pumping system is available, capable of evacuating the retort to a pressure of 0 05 fi (mercury equivalent). This consists of an oil diffusion pump, backed by a mechanical pump. In order to reduce the maintenance rate of the main pumping system, a small portable vacuum pump is used to achieve a pressure of a few millimetres, before the main pump is started. The furnace... 

In all mass spectrometers a vacuum of 10 Pa or better is maintained to avoid further ion formation from residual gas species or collisions of analyte ions with these species. Nowadays, turbo-molecular pumps are preferred over diffusion pumps as their maintenance is easier and oil back-flow does not occur. 

Under normal circumstances, there is no need for the operator to be concerned about routine maintenance of the mass analyzer. With modem turbomolecular pumping systems, it is highly unlikely there will be any pump- or sample-related contamination problems associated with the quadrupole, magnetic sector, or TOP mass analyzer. This certainly was not the case with some of the early instruments that used oil-based diffusion pumps, because many researchers found that the quadrupole and prefllters were contaminated by oil vapors from the pumps. Today, it is fairly common for turbomolecular-based mass analyzers to require no maintenance of the quadrupole rods over the lifetime of the instrument, other than an inspection carried out by a service engineer on an annual basis. However, in extreme cases, particularly with older instruments, removal and cleaning of the quadrupole assembly might be required to get acceptable peak resolution and abundance sensitivity performance. 

Active Transport. Maintenance of the appropriate concentrations of K" and Na" in the intra- and extracellular fluids involves active transport, ie, a process requiring energy (53). Sodium ion in the extracellular fluid (0.136—0.145 AfNa" ) diffuses passively and continuously into the intracellular fluid (<0.01 M Na" ) and must be removed. This sodium ion is pumped from the intracellular to the extracellular fluid, while K" is pumped from the extracellular (ca 0.004 M K" ) to the intracellular fluid (ca 0.14 M K" ) (53—55). The energy for these processes is provided by hydrolysis of adenosine triphosphate (ATP) and requires the enzyme Na" -K" ATPase, a membrane-bound enzyme which is widely distributed in the body. In some cells, eg, brain and kidney, 60—70 wt % of the ATP is used to maintain the required Na" -K" distribution. 

In one case report a 37-year-old male maintenance fitter was accidentally exposed to a large cloud of hot acetic acid while disconnecting a pressurized pump. The patient suffered first-degree burns on the hands and face and developed progressive dyspnea. At 3 months there were persistent extensive crackles in the basal area of the lungs, widespread bronchial inflammatory changes, and diffuse moderate interstitial pneumonitis that promptly improved after treatment with corticosteroids and bronchodilators. 

The most important requirement for utilisation of this kind of ionic diffusion as a means to information transfer is the maintenance of the non-equilibrium ionic concentration gradient. This is a relatively unstable state - it requires energy to counteract the natural entropy-increasing flow back to equilibrium. This is best illustrated by the pump storage model. Ions are actively pumped through... 

Jet ejectors require very little attention and maintenance and are especially valuable with corrosive gases that would damage mechanical vacuum pumps. For difficult problems the nozzles and diffusers can be made of corrosion-resistant metal, graphite, or other inert material. Ejectors, particularly when multistage, use large quantities of steam and water. They arc rarely used to produce absolute pressures below 1 mm Hg, Steam jets are no longer as popular as they once were, because of the dramatic increase in the cost of steam. In many instances where corrosion is not a serious consideration, they have been replaced by mechanical vacuum pumps, which use much less energy for the same service. 

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