Page 38 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Installation, Maintenance, and Repair of Horizontal Pumps 23
Shutting Down the Pump

The discharge valve on a centrifugal pump should be partially closed
before the driver is stopped in order to prevent reverse flow. Usually,
there is a check valve in the discharge line to prevent such reverse flow.

Diagnosing Pump and Seal Problems in the Field

Severe operating conditions in most refineries and chemical plants sub-
ject process pumps to high temperatures, abrasion, corrosion and prema-
ture bearing and mechanical seal failures.
Damage to the pump can occur not only inside the mechanical sur-
faces, but on the outside as well. Surrounding atmospheric conditions can
also shorten the life of any pump, especially in corrosive environments.
The life expectancy of pumps and mechanical seals in this type of envi-
ronment is very dependent on proper maintenance procedures.
Many mechanical seal failures have been the result of wear or deterio-
ration of pump bearings or internal pump components. Troubleshooting
pump and mechanical seal difficulties should begin at the pump while it
is installed and running. Maintenance and operating personnel need to
determine first if a process deficiency might be causing pump or mechan-
ical seal problems. The investigation should involve a thorough study of
pump hydraulics to determine if the pump is performing per design. Ac-
curate suction and discharge pressure readings need to be taken. The
pump should also be checked for excessive vibration, shaft deflection,
noisy bearings, and excessive temperature. If pump hydraulics appear to
be normal, but the pump is noisy and vibrating, it’s quite possible that the
pump could be misaligned, or the coupling could be faulty, or possibly
the pump and/or motor bearings are defective. By using a vibration ana-
lyzer and monitoring the frequency of the vibration, one can determine
the probable source, and the problem can be eliminated. If the pump
bearings have been subjected to severe vibration, the pump will have to
be removed to the shop for repairs, and if the mechanical seal is leaking it
will also need replacing.
A more thorough coverage of this subject can be found in Volume 2 of
this series, Machinery Failure Analysis and Troubleshooting.

Pump Preventive Maintenance

Earlier we had attempted to define the components of machinery main-
tenance strategy. We believe that preventive maintenance activities
around process pumps have to be shared by vigilant operators and main-
tenance personnel. Table 1-1 is presented as a guide for this task.