Circulation

Circulation is the last classification that exists upon a corner of our imaginary Golden Triangle. It can be defined as the flow of water through the swimming pool, all of it's equipment and piping. This flow of water is generated by the swimming pool pump's high speed impeller. The impeller is designed as a type of propeller not unlike that on a power boat. Water is thereby simultaneously pulled from and pushed back into the pool.

As we move through this discussion of swimming pool circulation it may be well to remember how important circulation is to the other two categories of our Golden Triangle, namely, filtration and sanitation. Strong circulation equals good filtration, because a greater volume of water is being pushed through the filter and therefore more dirt is removed from the pool per unit of time.

Strong circulation also equals good sanitation. Manual vacuuming, as well as the results produced by suction side and pressure side automatic cleaners, are enhanced because of the stronger flow of water. The increased general turbulence also helps keep organic debris in suspension, giving chlorine or other disinfectants an additional opportunity to kill.

There are two distinct modes to this circulatory system. The fluid dynamics of the system are such that the section of the swimming pool piping system that is just prior to the pump's impeller is under a vacuum and so is called the "suction side". That section of the swimming pool piping system that is just after the pump's impeller is under pressure and is called not surprisingly the "pressure side".

Swimming pools utilizing a pump will have therefore, a suction and a pressure side to their configuration, with the front half of the pump before the impeller, remaining on the suction side and that section of the pump that is after the impeller, remaining on the pressure side. The pump's impeller is the dividing line. This is a handy way to organize our thinking about the pool as a whole. Because of the fact of there being a pressure and a suction side, any problems that may be similar structurally, will however, manifest themselves differently, depending upon which side of the pool's system of circulation they exist.

As a simple example: If a hole exists somewhere in the pipe, on the suction side of the pool, then what will be noticed during the time the pump is on, is a stream of air bubbles that are being pulled from that hole. The developing bubble stream will move towards the pump from that suction side hole and continue along into and through the impeller to the pressure side of the pump. The bubbles will then collect and coalesce inside the filter. During this time the bubble stream may be observed by the pool owner or operator passing just under a clear pump pot lid.

Once the amount of air inside the filter reaches a certain threshold, air will begin to be expelled from the filter, back into the pool, by way of again, a stream of bubbles. The air bubbles will enter the pool by way of the inlets and float up to the surface where they again integrate with the atmosphere. As long as the pool's pump is running this process continues.

However, once the pool's pump is turned off by the pool's operator, or automatically, by the time clock, another sequence of negative events may occur. The air accumulating inside the filter has been compressing, because it's on the pressure side now, (air compresses, water does not). The action of turning the pump off will suddenly decompress that air, it will then expand to it's natural volume within the filter and drive the water that was inside the filter out.

The result is that the water flow is reversed suddenly, and flows at a high speed, backwards out of the filter back into the pump. The now backwards spinning impeller sometimes even unscrews! The water flow continues backwards through the pumps strainer basket, removing debris from that basket, and continues back to the skimmer where the backwards flowing water lifts the diverter valve and skimmer basket up off of their resting places and sometimes even blows the skimmer lid off! If the skimmer basket and diverter valve are not placed back into their respective positions, then any debris that enters the skimmer housing will travel back to and fill up the small pump strainer basket, the next time the timer starts the pool pump.

But that is not the worst of it. It may be that the suction line that runs back to the pump as well as the pump itself may now be devoid of water. With air in the line now, what may happen is that the pump may not be able to prime itself ( prime = sucking air for a bit before actually pulling water into the volute). The pump needs to have a water flow to carry away excess heat generated by the bearing supporting the impeller, as well as the heat generated by the impeller itself spinning in an enclosed space. Have you ever noticed how a blender will heat up food if it is left on too long? The same principal operates here, the high spin rate of the impeller will (by friction with the air) heat up the pumps interior (volute) and will cause the pump and any attached piping to warp.

Conversely, if the hole we are talking about, exists after the pump's impeller and is therefore on the pressure side of the pool, then the following negative process will be observed. The hole which may be in the pressure side piping, fittings, pump, or filter, now emits a drip or stream of water, when the pump is on. If the hole is very small and just slowly dripping, then it may be fixed at your convenience, but if the hole is larger, then immediate repair job is in order.

Most of the time, any water leak is minor, and the pool may be down just a fraction of an inch per day or if the hole is larger perhaps several inches in a twenty four hour period. I have seen swimming pools that have been half emptied out before the time clock shut the pump motor off! If the amount of water that is lost is not more than the capability of the automatic filler to compensate, then weeks or months may pass before the pool owner or operator becomes aware of this leak. They may finally notice wet areas around the equipment, a high city water usage or notice that the stablizer (cyanuric acid) levels are extremely low.

During the period of time when the pumps motor is turned off, air will leak back into the water system due to the hole being on the pressure side, gradually emptying the pool's equipment of water again, (but now for a different reason). When the pump is turned on it may just churn the air. If the hole is in the filter, then resulting mass of air will compress, perhaps causing the same difficulties as before, blowing the divertor valve, skimmer basket, and skimmer lids off their mountings.

I do not want to describe the leak discovery method or repair process that this type of problem requires. This type of problem will be discussed in detail, in later posts covering repairs. My intention here is to just have you understand how your pools circulation is organized. That it can be divided into a suction and pressure side, and that each side has its own particular type of fluid dynamic. In the next post the circulation discussion will continue.