Mechanical pumps maintenance

Fig. 7.16 A suggested log book layout for routine mechanical pump maintenance in the laboratory.

This report briefly describes a group of reactor coolant pump (RCP) seal failures that occurred at Arkansas Nuclear Unit 1, Calvert Cliffs Unit 1, and Indian Point Unit 3. Both mechanical and maintenance-induced RCP failure are discussed. For each event, the following information is provided the... 

To maximize turbine efficiency (by minimizing turbine back-pressure). This is achieved by condensing steam and creating an adequate vacuum. The level of vacuum created by the reduction in steam-to-water volume is typically on the order of 26 to 29 inches of mercury and is in large part a function of the cooling water inlet temperature. A contribution to the maintenance of the vacuum is obtained through the mechanical pumps and air ejectors, which form part of the condenser system. 

Application ranges of the various kinds of devices for maintenance of subatmospheric pressures in process equipment are shown in Table 7.3. The use of mechanical pumps—compressors in reverse— for such purposes is mentioned earlier in this chapter. Pressures also can be reduced by the action of flowing fluids. For instance, water jets at 40psig will sustain pressures of 0.5-2.0psia. For intermediate pressure ranges, down to O.lTorr or so, steam jet ejectors are widely favored. They have no moving parts, are quiet, easily installed, simple, and moderately economical to operate, and readily adaptable to handling corrosive vapor mixtures. A specification form is in Appendix B. 

Among the duties of oils in standard mechanical pumps are those of protection. Since scroll pumps do not have oil, they are dependent upon traps and baffles for protection against gaseous and particulate contamination. Although backstreaming of pump oils is not a concern from a scroll pump, the need for traps still remains. Ironically, the demands of maintenance on traps for scroll pumps is greater than for regular pumps due to the total lack of protection the pump has for itself. 

Maintenance. Probably the most common problem with mechanical pumps in labs is inattention to the pump oil. More pumps have died because of LSETCOI (Let Someone Else Take Care Of It) than as a result of any heinous intentional act of destruction. The reason is simple Changing pump oil is not fun or glamorous. The proper maintenance of a mechanical pump, however, is as necessary as cleaning glassware, keeping a proper lab notebook, and backing up a computer hard disk. 

In this section on mechanical pumps, constant attention is placed on the protection and/or maintenance of pump oils. This is because of the incredible demands placed on these oils. Ideally, all mechanical pump oils should ... 

Mechanical seals have fully established themselves for all types of centrifugal pumps in a variety of services. Though more costly than packing, the mechanical seal reduces pump maintenance costs. 

Choose the pump to use. B ased on the actual pump-down time, either the compound mechanical pump or the combination mechanical booster pump can be used. The final choice of the pump should take other factors into consideration—first cost, operating cost, maintenance cost, reliability, and probable future pressure requirements in the system. Where future lower pressure requirements are not expected, the compound mechanical pump would be a good choice. However, if lower operating pressures are anticipated in the future, the combination mechanical booster pump would probably be a better choice. 

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... 

The maintenance cost of a jet mixing system is also usually lower than that of a mechanically agitated system—the jet mixing system is very simple, and the pump is remote from the tank. Pump maintenance will not require the tank to be drained and purged. 

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... 

Two types of vacuum producing devices are used jet ejectors and mechanical pumps. Jet ejectors usually have lower initial costs, lower maintenance costs, but higher operating costs than equivalent mechanical systems. Mechanical pumps are more efficient users of energy. Jet ejectors require no moving parts and therefore are simplest of all vacuum producers. 

The plant layout of a LSPR plant is shown in Figures XXV-19 and XXV-20. The conventional fuel handling system is not available for a long life core. However, maintenance handling machines and maintenance spaces are accommodated and the pullout space necessary for mechanical pump impellers and purifying units is provided. When the reactor vessel lifetime expires, the current assumption is to comply with the need to exchange the reactor vessel with a new one. 

Troubleshooting pumping difficulties is a job that operating engineers face every week. Count the number of pumps on your unit the total will likely exceed 100. A major part of a unit s mechanical budget is allocated to pump maintenance. Many pumps sent to the shop are just disassembled, reassembled, and returned to the unit. The shop machinists have found nothing wrong with them. Also, many pump mechanical failures are caused by operator errors. 

Other factors that favor the choice of the steam ejector are the presence of process materials that can form soflds or require high alloy materials of constmction. Factors that favor the vacuum pump are credits for pollution abatement and high cost steam. The mechanical systems require more maintenance and some form of backup vacuum system, but these can be designed with adequate reflabiUty. 

The most common maintenance problem with centrifugal pumps is with the seals. Mechanical seal problems account for most of the pump repairs in a chemical plant, with bearing failures a distant second. The absence of an external motor (on canned pumps) and a seal is appeahng to those experienced with mechanical seal pumps. 

As we ve di,seussed, system design is responsible for much of cavitation. Yet, the maintenanee mechanic is responsible for stopping and preventing eavitation. And certainly, it s the maintenance mechanic who has to deal with the results of cavitation, the constant changing of bearings, meehanical seals, damaged impellers, wear rings and other pump parts. 

At times you ll find that the information is the same, but the presentation of the curves is different. Almo-St all pump companies publish what are called the family of curves . The pump family curves are probably the most useful for the maintenance engineer and mechanic, the design engineer and purchasing agent. The family curves present the entire performance picture of a pump. 

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