Pumps, the good, the bad and the ugly……
This issue’s column is focused on centrifugal pumps. They are the most widely used water mover in our industry and in most cases the most misunderstood. In theory, a 2hp pump is supposed to draw 2 HP to operate properly regardless of who manufactures it. However, not all pumps are equal. Many pump manufacturers “play” with the HP number by not adding in the efficiency of the motor to obtain wire-to-water HP.
There are several factors that should be addressed when determining which size and style of the pump should be used for which application. The important things to consider are the required flow rate, the required head pressure, and efficiency. Tucked in between those three are the horsepower, service factor and construction.
There are several approaches to determining how much flow you need. Some facilities prefer to provide redundancy and select lower overall flow rates and multiple pumps. Other facilities select a flow that can allow them to use one pump to do all the work. The decision to have a redundant pump in the system is always a safer plan so that when a single pump fails because they all fail eventually, the system will still run at a lower flow rate which should help mitigate any losses.
The next factor is the physical position of the pump in the system. The first question/factor to answer is to determine if the pump will be physically below the water level it’s pumping from or above. If it is above the water level, then the suction lift must be considered as a factor to add to the calculations. Head pressure is the total pressure that the pump will work against once it leaves the outlet of the pump plus friction loss head to overcome. Suction lift is the vacuum pressure or suction head that the pump will need to provide in order to flow the water into the pump. This might seem confusing, and in some cases annoying, but this information is key to ensuring a successful pump installation. In our experience, suction lift is often ignored which can cause premature motor failure due to cavitation and in some cases, nitrogen saturation.
All pumps have a curve that is based on the flows it can produce with a given impeller diameter and pump speed, performing against a given suction lift, head pressure sometimes called the system curve which is unique to each system. All of these can change based on the application if the pump is engineered for the project. Off the shelf pumps generally use a full impeller, full speed (3450 rpm) and may or may not have an induction rated motor which is required with using the pump with a variable frequency drive (VFD). Engineered pumps have a specific impeller diameter and speed based on providing the most efficient solution.
When reviewing a pump curve, the best thing to do is try to choose the pump that has your needs in the middle of the curve or BEP, (Best Efficiency Point), which is the most efficient area of the curve. It is rare to see manufacturers of off-the-shelf pumps include their efficiency curves in their technical data, putting the buyer at an unseen disadvantage.
Here is why, most off the shelf centrifugal pumps perform at high pressure, medium flow. In other words, their curve doesn’t start until the head pressure is around 40 feet and ends in the 80-100 foot range. Have you ever seen a pump installed with a valve on the outlet and the valve is semi-closed? The reason you see that is because the pump needs artificial pressure in order to work properly. By doing this, the user is paying for the extra electricity required to run the pump artificially high as well as creating heat and unnecessary wear and tear on the plumbing and valves. As a rule-of-thumb, we say pressure usually costs double what flow costs. The affinity laws are more precise in this regard.
Pumping pressure requirements are usually based on the filtration components that follow the pump as well as the geometry, (friction loss), of the plumbing. If the requirement of the pump is to replace an existing pump, the best thing to do would be to install an oil-filled pressure gauge on the outlet and an oil filled compound gauge (measures pressure and vacuum) on the inlet. This is the easiest way to get the pressure and vacuum information on an existing system. It’s also recommended to install these on all pumps, but that’s another article altogether. If the pump is for a new application, then it is best to spend the time to estimate the plumbing run and filtration design. That information is then used to determine the theoretical head pressure.
This is the crux of the matter when it all gets boiled down. Pump efficiency is probably the most ignored factor in pump selection. Every pump purchase is a virtual contract to buy power. The less efficient a pump is, the more power it will take to do the job. Over the years, we have encountered quite a bit of misinformation on this topic. Most of it stems from our equipment being sourced from other industries and mostly in residential industries where liberties are taken in order to allow the consumer to feel like they are saving money and electricity. The more efficient the pump, the cooler it runs, the longer it lasts and ultimately the less it costs to run.
Let’s consider pumps that are 5hp and lower for a moment. Spoiler alert, anything over 5hp is usually considered industrial and plays in a different realm of marketing and sometimes bracketed (volume) power rates. For the smaller pumps, the majority are repurposed from the pool industry. The pool industry has a marketing strategy for uprating their pumps. This is a practice where a smaller horsepower motor is used with a higher service factor to make it seem that it is a bigger, more efficient pump motor. It is important to take note of the service factor on the motor when choosing a pump. When the pump is running with a service factor higher than 1, the efficiency is reduced, the pump will produce more heat and if there is any variability in the supply voltage, the motor can fail prematurely.
One of the biggest factors in buying a pump tends to be the purchase price. Marketing materials focus on it and so do consumers. If the pump has a lower capital cost but the higher operational cost and shorter life, it might not save money for the facility over the long run. More efficient, purpose-driven pumps are definitely more expensive to purchase. However, the savings is realized with lower electrical costs and longer life. We know of cases where pumps paid for themselves in electrical costs saved in record time.
For the larger horsepower pumps, the marketing strategy is more straightforward. It is rarer to see high service factors since the decision makers for these pumps are more likely to consider the efficiency and return on investment.
Parts and pieces…..
The volute, housing for the impeller, is an important piece of the pump. The majority of the smaller horsepower pumps are made with a plastic volute. These tend to be similar amongst the manufacturers with small differences in the inlet and outlet geometry. The mechanical seal is a critical consideration when choosing a pump. Many of the aquaculture duty pumps have changed the seal to a stainless steel/plastic configuration. Some manufacturers take it further and used a metal freestyle seal which is best for most applications. Off the shelf pool, pumps use a brass seal which can cause water quality issues for some more sensitive species. It is best to avoid using pumps with brass in aquaculture if possible.
Other larger pumps may use reinforced plastic or cast iron volutes which both have their uses. The reinforced plastic is inert and is generally more efficient and more expensive. The cast iron comes with a variety of options, including epoxy linings, bare and in some cases FRP lining. The lined or armored pumps are a better choice if a cast iron volute is the choice. The coatings help keep the water from interacting with the iron as well as provide a smoother surface for the water which increases efficiency.
It is also important to know what back-up parts are good to have on the shelf in the case of a breakdown. Many of the pool pumps and reworked aquaculture duty pumps are supplied with a private label motor that can’t be repaired. This can make sourcing a replacement difficult in an emergency. We recommend that each facility have at least one back up the motor, set of seals and o-rings and impeller on the shelf regardless of the brand of pump. Alternatively, a complete pump as a backup will work as well.