6 Dec 05 19:00
NORZUL: Reliability and availability are two different animals, evin if they use some of the same information.
Your pump question is frequently posed. Let me pose it in a different form as in the case of a twin light engine airplane. Many people say a twin is more reliable than a single engine because there are two engines. In fact the additional engine decreases reliability because it adds more parts and two the twin engine airplane is supposed to fly on two engines. No pilot in his right mind would try to fly a twin on one engine to save fuel or other inane reason. There are times that twins have an engine failure and the pilot has to fly on one engine, but it takes skill and some practice.
The NUMBERS of availability are lower than reliability in your case, but Reliability and Availability are two different concepts. Your availability definition is a little restricted comapred to what we use. Our equation is:
Sum of operating and standby hours divided by the sum of operating and standby hours plus sccheduled and unscheduled maintenance hours plus administrative and logistics delay hours.
(operating + standby hours)/(operating + standy + maintenance + admin hours)
To continue your pump question, you have to look at the system and the reliability and what failure means to you. Let us say that each pump has a reliability of 90%. If one pump is needed and there is a spare then you have 100% redundency, but 90% reliability, since only one is needed. Assuming the second pump comes on when needed.
For the three pumps the reliability of the system is 90% times 90% or 81% since both pumps are required. reliable. The third pump increases the reliability from 81% to 90%, but it really gets tricky because if you have a pump failure and the standby pump comes online then you should immediately replace the broken pump to retore the system relaibilty. If both pumps fail togethethe standby pump cannot supply the needed flow.
If you do not mind send me your phone number and we can talk.