Pumps maintenance

Figure 1-7 presents the causes of losses for the largest chemical accidents. By far the largest cause of loss in a chemical plant is due to mechanical failure. Failures of this type are usually due to a problem with maintenance. Pumps, valves, and control equipment will fail if not properly maintained. The second largest cause is operator error. For example, valves are not opened or closed in the proper sequence or reactants are not charged to a reactor in the correct order. Process upsets caused by, for example, power or cooling water failures account for 11 % of the losses. 

Pressure maintenance (jockey) pumps should be provided to maintain a predetermined pressure on the system and make-up normal leakage in the distribution system. Normally, the pressure maintenance pump will maintain 10-15 psi (69-103 kPa) above the starting pressure for the automatic starting of the main fire water pump. (See 7.4.4.3.9.)... 

When the fire water system will be routinely used for purposes other than firefighting (e.g., washdown), the pressure maintenance pump, typically 150-300 gpm (570-1,150 Ipm), should have sufficient capacity for such use or a separate service pump should be provided. The service pump need not meet the requirements for fire water pumps. 

With such good intentions, what could go wrong After completion of maintenance, Pump B was put into service. After 45 min of operating time, major problems appeared. Flammable gas detectors alarmed, warning that a large amount of process fluid was escaping containment in the Pnmp B area. The cloud ignited. 

H. P. Bloch, "Consider a Low-Maintenance Pump," Chem. Eng., July 1988. 

Fig. 8-4. 6000-gpm top-maintenance pump for circulating solutions through a 50-M v reactor, being built by Reliance Electric Company. 

From a maintenance standpoint, a top-maintenance pump appears to be advantageous. Direct-maintenance practices can be used to bolt and unbolt the main flange. The pump casing is a permanent part of the piping system. A top-maintenance pump being developed for the HRE-3 is illustrated in Fig. 8-4. 

The failure mode of an equipment item describes the reason for the failure, and is often determined by analysing what causes historic failures in the particular item. This is another good reason for keeping records of the performance of equipment. For example, if it is recognised that a pump typically fails due to worn bearings after 8,000 hours in operation, a maintenance strategy may be adopted which replaces the bearings after 7,000 hours if that pump is a critical item. If a spare pump is available as a back-up, then the policy may be to allow the pump to run to failure, but keep a stock of spare parts to allow a quick repair. 

The maintenance of a constant pressure in a system during distillation under diminished pressure is of great practical importance if trustworthy boiling points are desired. Devices which maintain a constant pressure in a system that is higher than the minimum pressure that the pump will give are termed manostats. A simple manostat, due to M. S. Newman, is illustrated in Fig. II, 23, 4. 

Storage areas for maintenance, janitorial, and other service organizations must be provided. Safety items such as fire extinguishers, firehose cabinets, safety hoops on permanent ladders, guard rads, shielding for acid pumps, clearance for electric panel boards, etc, are needed. Manholes and cleanouts for sewer pipes within the facility as well as in the landscape and parking areas should be provided. 

Example 3. A centrifugal pump moving a corrosive Hquid is known to have a time-to-failure that is well approximated by a normal distribution with a mean of 1400 h and a standard deviation of 120 h. A particular pump has been in operation for 1080 h. In order to plan maintenance activities the chances of the pump surviving the next 48 h must be deterrnined. 

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. 

Figure 12 shows the plan and elevation views of a process unit piping (9). A dmm is supported off the piperack. Heat exchangers are located far enough back from the support columns so that they are accessible and their shell covers can be removed. Pumps are located underneath the piperack, but sufficient room is provided for maintenance equipment to access the motors and to remove the pump if necessary. The motor is always oriented away from the process equipment and located on that side of the piperack. Instmment valve drops are shown supported from the columns. The instmment trays themselves mn on the outside of the support columns. Flat turns are only made from the outside position of the piperack. Nozzle-to-nozzle pipe mns are made whenever possible. Larger lines are located on the outside of the piperack. Connections to nozzles above the rack are made from the top... 

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. 

Another option is the dry-pit design (Fig. 7c). This pump is installed in a dry pit and coimected to a weU via a pipe. Because the dry pit is usually dug out wider than the wet pit, Tenough room is available for pump maintenance, troubleshooting, and repair without pulling the pump to the surface for servicing. 

An axially spHt pump typically has a volute, and offers the advantage of easy maintenance because of quick upper-half casing and rotor removal that is similar to a single-stage version. The piping, attached to the no22les located at the lower half of the casing, remains undisturbed. 

The rate of contamination from the pump set is <10 molecule/(m -s) for molecular weights >44 (23). This is the maximum contamination rate for routine service for a weU-designed system that is used constantly and subject to automatic Hquid-nitrogen filling and routine maintenance. 

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