Vertical and submersible turbine pumps for water and wastewater applications are generally specified by a Consulting Engineer. We basically match a pump(s) from our line to meet the required specifications.
Not many years ago, turbines were thought of primarily as deep well pumps and their use in public works and private development projects were limited to this application. There is still a viable market in deep well applications, and submersibles are rapidly moving into it.
Today, we are seeing vertical turbines pumping treated water out of clear wells to pressurize city water mains. These are often called High Service Pumps, and it was once the job of horizontal split-case pumps exclusively. Turbines are also in use as transfer pumps at various stages in the treatment process. You will also see them installed as filter backwash pumps in lime softening plants.
The reasons for this expanded use of vertical turbines are many. First is the very low maintenance they require. As opposed to a split-case pump, there is only one packing or mechanical seal to service. Use of turbines out of a clear well eliminates the suction lift. The use of a rigid motor to shaft coupling eliminates flex coupling maintenance and prevents misalignment with its’ attendant bearing wear.
In addition to the reduced maintenance and down time, when the inevitable time arrives for the major overhaul, a vertical turbine can be re-bowled at a fraction of the cost of replacing the rotating element of a horizontal split-case pump.
The slope of a vertical turbine curve is usually much steeper than that of a horizontal split-case performance curve. This simply means that when we fall off to the right hand side of the curve, in an area of low efficiency and high flow, the VT head is much lower than the split-case with a resultant drop in required horsepower. This can be important, as many specifications require that the motor be sized to be non-overloading at all points on the curve. It is not unusual for a horizontal pump to require a larger motor than a turbine of identical capacity to meet this non-overloading requirement. This frequently means a larger cable size to the motor, larger circuit breaker and a larger magnetic starter, all adding cost.
All of these factors combined have created a large and growing market for vertical turbines.
The simplest municipal application for vertical turbines is in wastewater treatment plants. These have become the pump of choice for disposal of chlorinated effluent. Effluent is the fluid by-product of the treatment process, and is a clear liquid. Chlorination removes the residual bacteria and renders the effluent safe for returning to the ground, usually through some sort of irrigation system, but some communities are using this successfully to create wetlands that help recharge the groundwater supply. Turbine pumps are not meant to handle solids or sludge so other types of pumps are used in the treatment scheme.
Water supply, treatment and transmission facilities use vertical turbines for a number of different functions. Deep wells supply most of our untreated water. Either, lineshaft or submersible turbine pumps are used to bring the water up out of the ground.
If the well, or wellfield, is located any distance from the treatment plant, the raw water is usually pumped into a large tank with an aeration device. It is also lightly chlorinated. When the plant requires additional water, a canned turbine pump, or pumps, will send it through a pipeline into the plant.
When lakes, rivers or reservoirs are used as a raw water source, vertical turbines are used as intake pumps in this service.
In the plant, vertical turbines may be used as transfer pumps to transfer the partially treated water from one treatment phase to another. Turbines are also used to pump backwash water through the filter.
Finally, the treated water is transferred to a large tank called a clearwell. High service pumps then pump out of the clearwell into the distribution system. These pumps are usually on VFD’s so that the out flow can be responsive to a varying system demand.
In some localities, pipe friction causes pressure losses at points that are distant from the clearwell. Booster pump stations, usually canned turbine pumps, are used to restore system pressure.
A technology, known as Reverse Osmosis or RO, has gained widespread use throughout the country. Simply stated, RO treatment consists of pumping water through a semi-permeable membrane at high pressure. We can think of these membranes as super-filters. Only pure water actually passes through the membrane; contaminated water is rejected and passed off to waste.
RO is capable of extracting pure water from sources untreatable by conventional methods. Salt water, brackish water, high total dissolved solids (tds) are the most common in our area. This treatment method has created a new and lucrative market for vertical turbine pumps. First, the heads tend to be much higher than those used in conventional treatment. Multi-stage turbines are a better fit for these service conditions than most horizontals.
The RO pumps are usually of stainless steel, as are the suction cans. The corrosive nature of the raw water dictates this. The motor horsepower tends to be high so that the pump can move product at the required heads.
There are several potential pitfalls when discussing alloyed pumps with engineers and municipalities. It takes some know-how to select and recommend a stainless steel pump.
The first consideration is that alloy pumps do not conform completely to the published performance curve. There can be as much as a 5% drop in flow, head and efficiency. This is due to the difference in machinability and shrink rate of the castings. It’s surprising that many engineers are not aware of this and specify right out of a standard catalog. In some cases, the difference in efficiency is great enough to require a higher horsepower than usual.
This is not as difficult as it sounds at first. Just about all alloy pumps are tested at the factory before shipping and the test files can give us most of the information we need on any given pump selection.
While on the subject of horsepower, it is also necessary to check the bowl shaft size and its ability to handle the required HP. 316 stainless steel shafts will handle only 75% of the HP that the standard 416 stainless steel shaft does. It may be necessary to oversize the shaft or go to a more exotic 17-4ph alloy for the shaft stock.
We will also run into some stainless steel fitted cast iron pumps for use in storm water control. This is usually a 316 stainless steel impeller in a cast iron bowl. These also require a re-rate, though not as radical as in all 316 stainless steel construction.