Typically, standby pumps are installed with various configurations and capacity to provide operational flexibility and increase system availability. The switchover of duty to standby pumps is required for numerous reasons. From optimizing operations through making both duty and standby pumps available when required, to allowing functional operational checks and periodic predictive maintenance, as well as ensuring any upstream and downstream piping conditions are checked, switching over from duty to standby pumps not only optimizes reliability but also plays a role in cost efficiency as well.
By Mohammad Rahman, Mechanical Integrity & Reliability Engineer, Zeon Chemicals L.P.
It is important for operators to understand that both duty and standby pumps are available when needed, meaning that if a duty pump fails, the standby pump can take over. This minimizes any downtime in the pumping system. Additionally, it is essential for both variations of equipment to refrain from accumulating the same number of running hours, allowing for replacement equipment to be scheduled for installation at a suitable interval. This approach allows for various tasks, such as function checks, periodic and preventative maintenance tasks, like bearing lubrication, collection of vibrational data, and oil sampling to be performed by the standby pump. The replacement pump will readily take on the system load when required. Lastly, regular checks are conducted on the upstream and downstream piping and valve conditions.
This is to prevent the accumulation of process fluid, which can lead to line plugging, corrosion, or valve seizure over time. If the standby pump remains inactive for an extended period, the idle bearing may experience false brinelling, which is caused by low-amplitude vibrations from surrounding machinery. This can result in damage to the bearing and potentially lead to premature failure when the pump is put into operation, ultimately resulting in multiple failures where both pumps become unavailable simultaneously.
Following the Start-Stop Cycle
Typical duty-standby pump switchover follows a 90% – 10 % start-stop cycle. Several studies have been carried out to demonstrate good practice. Usually, a duty pump is run for 9 weeks, and then a standby pump is in operation for one week afterwards. This is to maintain the 90% – 10% cycle with this arrangement, the number of start-stop cycles can be kept minimum in a year. For example, a duty pump can start 6 times, and have 5 stops. A standby pump can can perform 5 starts and 5 stops.
When starting a pump, most of the strain on the pump comes from its seal. Whether it is a packing or mechanical type seal, the pump’s primary and secondary seal faces may start to rub against each other prior to being lubricated by the process fluid and form an effective sealing. That is why the start-stop cycle needs to be minimized to keep the wear level minimum on the pump seal. However, there are some challenges that come with the 90–10% start-stop switchover approach.
This approach solely emphasizes reducing the frequency of start-stop cycles to minimize wear-and-tear within a specific timeframe. However, it neglects the economic aspects associated with pump operation and maintenance.
The start-stop cycle approach does not provide a specific switchover time; instead, it suggests keeping the start-stop cycle to a minimum percentage of the total time without specifying the actual duration.
Lastly, this approach fails to consider the impact of other failure modes and its consequences. For example, certain processes may involve corrosive elements, some require extensive cleaning to prevent line plugging, and certain seals may be more expensive to replace. Additionally, health and safety challenges may arise that are not accounted for in this approach.
The Economical Aspect- Using an Analytical Approach
An analytical approach is developed to find an economical duty to standby pump switchover frequency that considers reliability and cost optimization in the operation and maintenance processes.
For example, if the duty pump fails, the standby pump starts and meets operational demands. If each pump fails randomly and the failure data follows the Weibull Distributions, this approach may be beneficial to operators. Multiple failures only occur when the standby pump fails, while the duty pump is simultaneously in a failed state.
The probability of multiple failure can be calculated as per SAE JA1012:
If the failure rate of the standby pump is represented by λ, the reliability of standby pump that it will operate at time t > 0 is calculated by:
Expanding this as:
By ignoring higher order terms:
Instantaneous unavailability of standby pump can be expressed through:
Average unavailability of the standby pump is expressed as:
Where T is the standby pump switchover frequency to ensure the pump is periodically restored in working condition.
The average multiple failure rate is shown here:
μ – demonstrates the failure rate of the duty pump.
Optimizing Duty Pump to Standby Pump- Part Two
In order to achieve the most efficient duty pump to standby pump switchover interval, a balance is needed between switching too frequently. By evaluating total cost per switchover cycle, cost of multiple failure and cost of the pump switchover, users can seamlessly determine switchover frequency.