Troubleshooting Poor Regeneration Performance in a New Softening Vessel
An egg-processor in the midwestern USA purchased a new four-vessel softener system from a highly reputable water treatment system OEM and filled the vessels with SSTC60 Shallow Shell™ resin. The vessels were installed in two separate timeframes—with two installed initially and the remaining two being installed at a later date. After the installation of the second set of vessels, one of two new vessels had shorter service runs than the two vessels that were installed initially. The shorter service runs were resulting in increased salt consumption, which was increasing overall operating costs. Because high-efficiency resins should enable longer service cycles and decrease the use of regenerant chemicals, this was especially problematic.
Purolite has a long-standing relationship with the OEM as well as the local distributor who provided the resins. We were asked to troubleshoot the system to determine the root cause of the short service runs as neither the equipment company nor the distributor was able to determine what could cause the issue through their routine system checks. They weren't sure if it was a design problem, a mechanical problem, a resin problem—or some other issue.
To begin the troubleshooting process, Purolite requested the end-user’s elution study graphs from their periodic elution studies. Drawings of the vessel internals were also requested from the equipment company.
Once received, we reviewed the information and then visited the customer’s facility to perform our own series of elution studies to confirm the data that was provided.
We also looked at all aspects of the softener operation to try to identify the root cause(s) of the short runs. Some of the processes monitored included:
- The length of time for the regeneration steps
- The flow rates of backwash, brine and rinse water
- The volumes of backwash, brine and rinse water
- The regenerant (brine) concentration averages
Our investigation showed that one of the vessels had a flow rate of 80 gpm, which would yield a bed expansion of ~35% (about 14")—which is less than the optimal expansion of 50% – 75% (20" to 30").
Additionally, a drawing of the upper internals showed that the opening of the upper drain standpipe allowed only 23" of free board space above the resin bed. This was problematic because increasing the backwash flow rate to achieve the optimal 50% to 75% backwash expansion (20" to 30") would pose a risk of resin loss during backwash as it could expand the resin bed above the level of the upper drain standpipe.
We asked the customer to confirm the resin bed depth (and therefore resin volume) in the vessel experiencing the short runs. The customer confirmed that the vessel at issue did lose resin during backwash cycles, and that the reduced resin volume correlated with the reduction in the length of the service cycle.
Investigating the vessel’s resin volume and backwash flow rate, it was discovered that a backwash flow restrictor that was installed on the first three vessels was not in the underperforming vessel.
The lack of backwash flow restrictor:
- Caused an unacceptably high backwash flow rate
- The high backwash flow lead to a resin bed expansion beyond the top of the upper drain standpipe
- The resin expansion pushed resin down the drain, reducing the resin volume
- The reduced resin volume resulted in the shorter service run time
After installing a backwash flow restrictor in the vessel and confirming the backwash flow rate, the vessel performed as it was expected to with superior operating efficiency and performance that enabled them to save on regenerant chemicals, waste water and have longer service runs.
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