July 1, 2014
Over the last few months, I’ve been plagued with a series of warning lights on the dashboard of my car, telling me that my engine is overheating. Repeated tests have identified that the engine is not overheating, but the warning lights persist.
After a few attempts, my independent mechanic admitted defeat. (A man of some integrity!) I then took the car to the dealership where I had purchased it. After all, they’re the experts on this particular brand and model, so they must be able to fix it.
Sadly, such was not the case.
After a few attempts, it became obvious that they were relying heavily on the computers in the engine to tell them the problem and the solution. Unfortunately, the computers didn’t cooperate. The problem persisted and they too were stumped. (No, they didn’t acknowledge it, but that’s another story.)
As a result of some intensive inquiries, I finally found an experienced independent mechanic who understood the background and history of this particular model and was able to add some old fashioned trouble-shooting to the computer readouts and arrive at the solution.
Today’s Pump Problems
When I consider the extent of the computer based systems and equipment that we utilize in plant maintenance today, I can’t help but wonder if our growing dependence on some of them is replacing the old fashioned trouble-shooting knowledge and procedures that are disappearing with the retirement of long term employees in the industry.
With my work with plant engineers and technicians, I frequently come across a lack of understanding of how a centrifugal pump really functions within the operating system. This quickly leads to pumps that are being measured, monitored and analyzed, but where the ongoing, repetitive and expensive failures persist.
For example, a pump is designed and produced to supply a whole range of head-capacity conditions as identified on it’s performance curve. The pump will operate on that curve if it is driven at the particular speed for which the curve is drawn. However, the specific conditions at which the pump will run, will be determined by the system in which it operates.
In other words, for all practical purposes, the system controls the pump, and will operate that pump at whatever conditions it sees fit, regardless of the Head and Capacity for which it was bought (and designed) to achieve.
First we need to understand how a pump works!
For this we need to understand the Characteristic Pump Performance Curve. To the uninitiated, this diagram may seem like a mass of lines set up simply to confuse the reader. In fact, it is only a picture of how the pump works.
However, we do need to be able to make some sense out of that picture.
To fully appreciate the relationship between Head and Capacity, consider a Centrifugal Pump discharging into a straight vertical pipe.
The liquid will eventually reach a level beyond which it is unable to move. This can be considered as the maximum Head the pump can develop. Although the pump will continue to run it will be unable to push the liquid any higher in the pipe.
Under these conditions, liquid is being churned around in the pump casing, but there is no flow passing through the pump, therefore the Capacity at this maximum Head is zero.
Expressing pressure as “Head” in this way makes the pump curve applicable to every liquid regardless of it’s Density.
If we cut holes in the discharge pipe at progressively lower levels, the Head is effectively reduced, and the pump will steadily develop an increasing Capacity.
By graphically depicting these results, a characteristic pump performance curve is drawn. You will note that this curve is not completed down to Zero Head, as a centrifugal pump does not operate reliably beyond a certain Capacity, at which point, the curve is usually discontinued.
This curve identifies the Capacity (Q) that this pump can develop, and the Total Head (H) it can add to a system, when it is run at a particular speed with a specified impeller diameter.
Consequently, the only practical way you can change what the pump is capable of doing is by physically adjusting how fast the pump is rotating, or by changing the impeller diameter.
That’s what it’s designed to do… so why doesn’t it do it?
If the Centrifugal Pump is controlled by the System, we should now understand some aspects of a Pumping System. For this we look at the System Curve.
The System Curve is created by the combination of factors that resist the flow of liquid from one end of a System to the other. The common factors in all systems are Gravity and Friction.
To overcome Gravity in a typical system, the liquid must be raised through the vertical distance represented by the change in elevation between the originating source to the final destination.
Referred to as the Total Static Head, this distance is measured in one of two ways, depending on the layout of the system.
It can be measured between the free surface of the liquid in the suction source and the free surface of the liquid in the discharge tank. If the pump discharge line enters the Discharge Tank from above, the Static Head would be measured between the free surface of the liquid in the suction source and the highest point of the discharge line. In either case, the Total Static Head is not a variable of the flow rate, and a graph comparing the two would show up as a straight line. Friction is the resistance to flow in the system and must be considered for three separate areas individually.
The Friction Losses in piping is usually calculated from the Friction Loss Tables available from a variety of sources such as the Standards of the Hydraulic Institute or The Practical Pumping Handbook. These sources also identify the losses through the more common pipe fittings and valve types.
Losses in Filters, Heat Exchangers, etc., can be obtained from the original equipment manufacturer, or from measuring the inlet and outlet pressures on site. As the flow increases, so too does the Friction Loss, but at a far higher rate.
The only other conditions we need to take into account are the pressures at the Suction Source and in the Discharge Tank. If they are both open to atmospheric pressure, they balance each other out and we can ignore them. If however, they are closed vessels under different pressures, the difference in pressure has to be added to the Total
Head required from the pump.
A combination of that Differential Pressure, the Total Static Head, the Friction Loss, and the Velocity Head is referred to as the Total Head. When plotted against the
Capacity, the resultant curve is known as the System Curve.
Therefore, when a specific Flow Rate is selected for a system, the System Curve will identify the Total Head that must be overcome.
The Flow Rate through a system can only be supplied by a pump, and is therefore the Capacity required from the pump.
When the pump is properly selected, its Characteristic Performance Curve will intersect the System Curve at the point at which the pump will operate.
An increase in Static Head can be caused by reducing the liquid level in the tank, and will move the System Curve straight up on the graph, thus reducing the Capacity required.
An increase in Friction Loss can be caused by a variety of conditions such as automated controls opening and closing a different valving system. This will result in the System Curve adopting a steeper slope and again reducing the Capacity required.
Therefore, if we don’t physically changing the pump speed or impeller diameter, then any change in Head or Capacity noted at the pump means that the system has been changed either purposely, or accidentally.
Remember, the System controls the Pump. So don’t be surprised if your Pump starts to overheat, or otherwise doesn’t do what it’s supposed to do. Bring in some old-fashioned trouble shooting expertise. Check out what’s changed in the System and why.
Ross Mackay is an internationally renowned expert in pumping reliability. He specializes in helping companies increase their pump asset reliability and reduce operating and maintenance costs through pump training programs. He is the author of “The Practical Pumping Handbook”, and “The Mackay Self-Directed Pump Reliability Training System”. He can be reached at 1-800-465-62601-800-465-6260.