long period standby pump

<...do to increase the reliability and avialability of this standby centrifugal pump?...>

Simply swap the duty at intervals so that the other pump runs.

However, it is not good practice to make the run hours of the duty pump and the standby pump the same: there is every chance that they will both wear out at about the same time, making replacement of the worn pump fraught with risk that the facility will shut down prematurely due to a similar failure in the second pump.

Standby pumps shall be available for un-interrupted operation in case running pump breaks down. Some times it may so happen that when tried to start stand by pump, found it it has problem. When stand by pump is not operated for long time, some times problems may arise due to corrosion, electrical or instrumentation related. So in my opinion it is advisable to swap the pump once in a week. For bigger pumps (run by HT motor), the change over period can be adopted as 15 days or a month depending on size.

"there are 2 or 3 pumps in parallel.. one of them online and the others are in standby mode sometime for long periods" If there are 3 pumps in parallel..only one of them online then select one of the standby pump for hibernation. Blank its suction, discharge and bypass line flanges, drain the liquid, dry all internals and electrically isolate it. Revive it only when you really require. It will prolong pump life by reducing damages caused by corrosion and aging of seal parts with contact of liquid.

Excellent comments so far. Just to add:

One of the adverse effects of leaving the standby pump idle for long periods is something called 'false-brinelling' of the bearings. This is caused by vibration from the duty pump causing the bearing balls or rollers in the standby pump to impact continuously on the same spot on the races, and it can result in rapid bearing failure if the standby pump is started after a long period of being idle. This alone is a good enough reason to recommend, as previous posters have, that the standby pump should be operated (intermittently).

If any additional pumps are hibernated, as pritam suggests, our old O&M manuals (for chemical process pumps) would have recommended that the pumps be turned by hand (if possible) a few revs every month or so. I'm not sure if this is really necessary, or if it is a relic of the days of gland-packing when these manuals were compiled. But it's worth checking your pump O&M manuals, or the pump manufacturers recommendations if not in the manuals. You might find this useful as well:

http://effective-maintenance.com/Documents/Dutystby.pdf