Sulzer Technical Review 2 / 2019

Air supply for water treatment filter

April 28, 2019

Water treatment facilities rely 24/7 on filtration systems to remove waste products as part of the process of recycling the water before it returns to the river. These filters are periodically cleaned by backwashing them with air bubbles produced by large blowers. Following two unexpected blower failures, Sulzer was asked to investigate the cause and improve the reliability of the system.

Air supply for water treatment filter cover
Author Mathew Knight
Author: Mathew Knight, Bristol, United Kingdom.

In sewage water treatment, the system must be constantly available in order to guarantee the proper water quality.

In the United Kingdom, one of the regional water companies has an agreed framework contract with Sulzer for the repair of the blowers on its sites. The water company prefers to have a single source for blower repairs, rather than having to deal with numerous original equipment manufacturers (OEMs).

The blower pump unit before repair
Fig. 1 The blower unit before repair.

Filtration cleaning with air

The water treatment facility operates three blowers as part of a complex cleaning cycle for the granular media in its filtration system. Two blowers had failed in quick succession and been sent to Sulzer’s service center (Fig. 1). Sulzer completed the repairs (Fig. 2) and returned the assets to be installed by the site’s own engineers.

The maintenance team at the water treatment facility had only just reinstalled the two repaired blower units when, shortly after they were commissioned, they failed again. This left the cleaning process with only one blower, the bare minimum for operation, but there was also considerable concern for the reliability of the remaining unit.

The blower pump unit after repair
Fig. 2 The blower unit after repair.
We can repair blowers from any manufacturer. We have a very good working relationship with this customer and the blower repairs are just one aspect of the service we provide. The fact that we complete all of our repairs in-house gives the customer complete confidence in the quality of the repairs that we deliver. It also means we can give progress updates whenever they are required and return the repaired equipment with minimal delays. Mark Teale, Bristol Service Center Manager, Bristol, United Kingdom

The heat was on

The units were removed once again and returned to Sulzer’s service center. Once stripped down, the engineers found several damaged parts in the blower units. The heat build-up was so intense that the plastic cabinet extraction fan had melted. Additionally, the blower rotors had expanded and become wedged.  One of the rotors had even friction-welded itself to the housing. The fact that both blowers exhibited the same failure indicated an obstruction, either on the inlet side or in the discharge line.

Investigating the cause

Sulzer offered to carry out an investigation at the water treatment facility to find the root cause of the problem. The engineers inspected the inlet filters along with the discharge pipework using a borescope, but no obstructions were found. Attention then turned to the pressure relief valves (PRVs), which were situated inside the acoustic cabinets and should have lifted in the event of the blowers operating against an obstruction or excessive load. Both PRVs were found to be stuck, however while this accounted for the failure of the blowers, these valves were not the cause of the obstruction.

The unload valve that was the root cause of the failure
Fig. 3 The unload valve that was the root cause of the failure.

Understanding the application

The blowers operate on a 24-hour cycle, producing scour air that backwashes the filter media. During the cleaning cycle, the blowers are not required to operate all of the time. They are regulated by an unload valve, which vents excess air into the atmosphere. This avoids stop/starting the blowers during the cleaning cycle.

All three blowers are regulated by this valve (Fig. 3) and closer inspection found that the diaphragm chamber of the valve was full of water, preventing it from operating. This, combined with the PRV failures, had led to the blowers becoming distressed very quickly; only the full function of the remaining PRV had prevented the last blower meeting the same fate.

The unloading valve was stripped and cleaned, but Sulzer’s recommendation was to replace all of the PRVs as well as the unloading valve to be sure the blower circuit would continue to function reliably. With over 15 years of service, these valves had performed well, but without a regular maintenance schedule, their operating condition had remained unchecked.

Sulzer’s engineers refurbished the rotors and casing to OEM dimensions
Fig. 4 Sulzer’s engineers refurbished the rotors and casing to OEM dimensions.

Delivering the solution

While the on-site investigation was completed, the engineering team at the Bristol Service Center removed the damaged rotors. Using OEM drawings, they machined the rotors and the casing back to original dimensions, ensuring the correct clearances were provided (Fig. 4). The blowers were reassembled and returned to site, where they were reinstalled. The Sulzer team handed over a field report to the customer with detailed findings and recommended implementing a maintenance routine for the PRVs and all valves, to ensure reliable operation in the future.

Damien Weston, Customer Support Engineer at the Bristol service center, concludes: “Without the on-site investigation, we would have just repaired the blowers and they would have continued to fail. The combination of our technical expertise and application knowledge has been invaluable in resolving this issue and ensuring long-term reliability for our customer.”

Sulzer Technical Review