Centrifugal pump failures

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. 

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. 

So in this chapter of failure analysis and corrective methods, we decided to consider some problems, symptoms, and remedies particular to PD pumps. We re using two tables. The first table lists the few symptoms that send a PD pump into the shop. These symptoms are mated to another column of possible causes listed in numerical order. The numerical causes are on the second tabic starting with the. source of the problem in the left column and the probable cause/suggested remedy in the right column. As you go through the list, you ll see again that PD pumps and centrifugal pumps have a lot in common. Enjoy. 

Data from an existing collection system were analyzed for failure modes and distribution. The results of Pareto analyses indicate the principal causes of failure. A few values of mean times to maintenance action (MTBM) are given for ethylene plant pumps (85 electric driven centrifugal pumps over a 19-month period), and ethylbenzene-styrene monomer plant equipment from 10 months data 4 gas compressors, 3 screw conveyors, 121 pumps, and 235 other items... 

More pumps fail as a direct result of improper installation than any other single factor. The predominant reasons for these failures include starvation, caused by inadequate or inconsistent suction conditions distortion, caused by pipe strain or improper foundation and turbulent that results from piping or entrained gas problems. Centrifugal pump installation must follow Hydraulic Institute Standards, which provide specific guidelines to prevent these installation and performance problems. This chapter will address the fundamental requirements for proper installation. 

In a continuous operation to bleach a fat-derived acid, the latter was stirred and treated at 80°C with a slow stream of 50 wt% hydrogen peroxide. Un-noticed failure of the agitator led to peroxide build-up, layer formation and eventual formation of an explosive mixture. When this passed into a centrifugal pump, it detonated. 

On a long HAZOP, the team may find that certain findings are repeating themselves. For example, it may be that all centrifugal pumps of a certain type have an unusually high rate of seal failure. In such cases, the team should develop generic findings and recommendations. 

Centrifugal pumps are subject to mechanical damage when they lose suction pressure or when the discharge flow is blocked. Furnace tubes may overheat and fail when liquid flow through the tubes is greatly reduced. Low-flow trips protect process equipment against these failures. This trip may be used to shut down a pump or block off fuel to a furnace. To test an orifice-type low-flow trip, proceed as follows ... 

Centrifugal pumps are especially sensitive to (1) variations in liquid condition (i.e., viscosity, specific gravity and temperature) (2) suction variations, such as pressure and availabiUty of a continuous volume of fluid and (3) variations in demand. Table 21.1 lists common failure modes for centrifugal pumps and their causes. 

Table 21.1 Common failure modes of centrifugal pumps...

Cavitation in a centrifugal pump, which has a significant, negative effect on performance, is the most common failure mode. Cavitation not only degrades a pump s performance, but also greatly accelerates the wear rate of its internal components. 

Positive-displacement pumps are more tolerant to variations in system demands and pressures than centrifugal pumps. However, they are still subject to a variety of common failure modes caused direcdy or indirectly by the process. 

Rotary-type, positive-displacement pumps share many common failure modes with centrifugal pumps. Both types of pumps ate subject to process-induced failures caused by demands that exceed the pump s capabilities. Process-induced failures are also caused by operating methods that either result in radical changes in their operating envelope or instability in the process system. 

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