Hidrostal Centrifugal Screw Pump

A Unique Solution

Ever heard of an immersible centrifugal screw pump? Say that five times fast. Odds are you probably haven’t heard of this style pump, as they are definitely unique. I know I had not until I took over my current plant. I took over as manager right at the start of a major upgrade and expansion and I was too busy learning everything else to ask questions. I just stared at these weird fiber glass stalls and goofy pumps with a metal tube stuck at the bottom of the impellor sitting in my newly constructed influent building and nodded. Well, little did I know, these odd pumps would end up being one of my favorite pieces of equipment within the whole expansion. Why you may ask? They worked from day one, haven’t caused me any headaches, and can pump high solids influent with ease. You may ask why I have high solids in my influent. I’ll get to that later.

Every plant has there own unique needs and specifications and sometimes unique situations call for unique solutions. One of the problems that was solved in the recent upgrade at my facility was the fact the wet well and old screw pumps couldn’t keep up with the main interceptor flow very well. Several rain events caused the main interceptor to back up, obviously creating a bad day for all involved. To remedy this situation, the engineers specified a much wider and deeper wet well that almost acts like a small retention basin. These pumps hold the level per our setpoint and call on lag pumps accordingly based on flow, all while trying to maintain our level setpoint. Anything above 6 MGD is then diverted to our actual EQ tank. The pumps and logic have handled several rain events with ease now. The plant formerly used to have three traditional screw pumps that were replaced with five Hidrostal centrifugal screw pumps. These pumps are also unique as they sit in fiberglass stalls and are capable of being partially or fully submerged. They have a long metal tube at the bottom of the volute that allows them to completely suck down the fiberglass stalls and wet well several times a day. This is clever choice of pump and solves the high solids issue I was talking about earlier, as the wet well directly receives the pressate water from my solids handling Schwing screw presses (we’ve got quite a few screw related processes in this plant. Archimedes would be proud). This would sometimes cause a build up of solids bound with polymer in my wet well. These pumps allow us to clean this wet well out and not build up massive amounts of solids. We noticed that at the beginning of startup the solids would build up and then when it did the daily pump down cycle it would cause us to get huge slug load in our influent. We were able to reprogram the logic to allow us to pump down the wet well up to 3 days per day, evenly spreading the load out throughout 24 hours. This was better on the entire treatment process, particularly the BNR system that can be finicky if you slam it with high levels of P04.

Below is a video of the pump down cycle on these pumps. As you can see, these pumps pump down to the floor of the fiberglass stall. This does a great job of cleaning out the wet well. In addition, the pumps automatically rotate each pump down cycle.

One of the things I focus on when selecting equipment is the ease of use. These pumps have had zero issues since startup. Which I really like, because I had a lot of other equipment that did have a lot of issues after we completed the upgrade at this facility. We can change the pump down cycles, wet well level and numerous other settings very intuitively all at the HMI. The interface is very easy to use and everyone at the plant has had no problem changing the pump cycle, rotation, or wet well levels if needed. There is a lot to be said of an easy interface. No one wants to be bogged down trying to figure out how to change a setting when things need to be changed quickly.

Main Panel HMI

Another factor I always look at when selecting equipment is maintenance. So far we have completed the annual oil change and the pump was no worse for wear. We contracted the manufacturer rep to come assist, as the operators had never worked on this style of pump, and honestly, it wasn’t much different than most of our other submersible pumps. Fairly straight forward and easy. They were easily pulled from the fiberglass stall, making it a quick and efficient process. As of right now we haven’t had to replace anything else on the pump.

Overall these pumps have been the workhorse of our plant (obviously since they are the influent pumps of the plant), and were able to replace three old style screw pumps with ease. They’ve been in operation for about two years now and have had no major problems. I like that. I like something that is sold to work for a specific application and does what it was sold to do. You’d be surprised how many things get sold in the wastewater industry that do not work as designed. From the easy to use interface, unique design and function, these pumps really solved a specific problem for our plant (and one somewhat unforeseen one) and have worked well from day one.

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