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You Get What You Pay For…… or do you?

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chart of Static Head Changes

You Get What You Pay For…… or do you?

By: Empowering Pumps & Equipment Staff
One of the most costly mistakes that almost every company makes with their pumps, is actually buying the wrong pump. Sadly, this is a situation that occurs with much more frequency than anyone would care to admit. On top of it all, the ramifications are huge..!! Reliability plummets and maintenance costs escalate dramatically….!!
 
Two of the most common reasons behind buying the wrong pump are;
            1.  Providing the supplier with incomplete information.
            2.  Buying the cheapest pump.
 
Providing Incomplete Information to Suppliers
 
Frequently, when a pump is being selected, it is known that it will have to operate at more than one single condition. Sadly, this information is not always transmitted to the supplier, and the Engineer will decide for which of these conditions the pump will be sized. It is not uncommon for that decision to be made based on what is considered to be the Worst Condition. The thought process being that, “if it can handle the worst condition, it should be able to handle all the others”. Sadly, such is not the case. In fact, when the pump is selected for the Worst Condition, then that immediately becomes the Best Condition, simply by virtue of the fact that it is the duty for which the pump has been selected.
 
The resultant problems show up in two ways; when the Static Head in a system undergoes a change, and also when Friction Losses change.
 
Static Head Changes
 
A classic example of such a situation is in a batch transfer system, where the Total Head is constantly adjusting as a result of the change in tank levels throughout the batch process. Consequently, when the supplier is given only one set of operating conditions for this application, he is being supplied with inaccurate and misleading information. 
 
Let’s assume that the operating conditions supplied will be the Startup conditions where the level of liquid in the supply tank will be at it’s highest, while the level in the discharge tank could be zero. This will translate into a low value of Static Head as is depicted on Figure 1. The pump will also be selected to operate close to the Best Efficiency Point (BEP) which also happens to be the most reliable operating condition of that pump.
 
chart of Static Head Changes

Figure 1 – Static Head Changes

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
However, as the level of liquid in the supply tank drops and the level in the discharge tank increases, the Static Head will steadily increase. By the time there is no longer any liquid in the supply tank, the level in the discharge tank will be at it’s maximum. At this point the Static Head will also be at it’s maximum and the Pump Performance Curve will be as shown.  At this point, the pump should be shut down.
 
As the pump operation moves steadily from Startup to Shutdown, there will be a corresponding change in pump capacity. However, as the system approaches the Shutdown point, the pump performance will become unstable, thus resulting in low reliability and high maintenance costs.
 
However, had the complete system information been provided to the Supplier, an alternative selection with a steeper performance curve could have been made which would place the BEP midway between the Startup and Shutdown condition. This provides a more stable operation within a smaller range of flow rates.
chart of More Reliable Pump Selection

Figure 2 – More Reliable Pump Selection

 
 
A similar situation can occur in a boiler feed system, where the feed pump takes it’s suction from a deaerator under vacuum and supplies a boiler under pressure. In this system, the Differential Pressure is not a function of the flow rate and will have similar consequences as the Static Head change in the previous example. Any change in pressure in either the deaerator or the boiler, will cause the system curve to fluctuate as identified above.
 
Friction Loss Changes
 
A closed loop system is one in which the entire system is pressurized by the pump. To achieve this, the pumpage is fully contained within a series of pipes and pressurized process equipment all the way from the pump discharge, through the system, and back to the pump inlet. In such a layout, the Static Head in the system is effectively zero, and the pumping conditions are usually controlled by adjusting the friction losses.
 
A change in Friction Loss can be caused by a variety of conditions such as manual operation or automated controls opening and closing a different valving system. This will result in the System Curve adopting a different slope that will pivot about it’s point of origin at zero capacity.
 
chart of friction loss changes

Figure 3 – Friction Loss Changes

 
The same effect can also be realized when the bore of the pipe in the discharge side of the pump reduces in size owing to some kind of buildup such as scaling, etc. Such a buildup may also occur inside process equipment such as filters or heat exchangers. These buildups automatically reduce the bore of the pipe and therefore increase the Friction Losses in that pipe. The result of these changes will take years to become evident.
 
When we are selecting a pump for a particular service, it is important to be aware of all the ramifications of that service before deciding on which pump to use. Frequently, the basic operating data is insufficient for an optimum selection. Knowledge of any extreme or upset conditions must be made available to the supplier in order that the correct pump can be purchased. This will minimize maintenance costs and maximize pump and system reliability.
 
Buying the Cheapest Pump
 
The policy of always buying the cheapest product or service is one that few of us practice in our private lives, yet it continues to be the single biggest mistake made by end users of pumping equipment? Why?
 
I believe the problem is ignorance. Many people think that if all the pumps meet the specification (assuming there is one!), then the cheapest one is the best buy. Unfortunately, that is simply not true and the problem often resides with the specification itself.
 
Most pump specifications are either inappropriate or incomplete. This is a more serious situation and happens more frequently than we would care to acknowledge. It does not always refer to any vindictive misrepresentation or withholding of data, but rather to the limitations of knowledge of the field conditions and the understanding of how these conditions might impact the pump performance and reliability.
 
All pump reliability problems arise from either internal or external stresses. The internal stresses mostly occur as a result of upset hydraulic conditions, which are rarely discussed in any specification, regardless of how integral they are to the system in which the pump must operate. The external stresses come from inappropriate installation or operation, and they too tend to be ignored in the specification.
 
The problem is that the pump selected must be able to withstand these (sometimes) unknown stresses. The cheaper pump rarely does.
 
Negotiating the Price
 
Traditional pricing negotiation frequently shows up in the guise of “Your price is too high!” which instantly takes the emphasis away from the need to buy the best value, and not the lowest price. Many salespeople have not yet realized that this statement is simply a set piece in the informal script of the negotiation process, which has become part of the pump purchasing scenario over the years. Any purchasing agent worth their salt will use that phrase at some time during negotiations, regardless of the specific numbers in front of them.
 
Unfortunately, this problem is aggravated by the fact that many of those who purchase pumps don’t know how to evaluate one pump bid against another on anything other than a subjective basis. Consequently, the low bid policy continues to rule.
 
The sad part is that, over the years, this strategy has resulted in the elimination of the availability of some good quality products in certain markets. This, in turn, leaves those industries with inappropriate and usually inefficient equipment with which to transfer and process the liquids needed in their operation.
 
The long-term consequence of this scenario usually finds these same end users negotiating low bids on the prices of the over abundance of spare parts they need to keep their pumps operational. Many of which are now supplied by third party organizations that, in turn, rarely accept responsibility for any inappropriate changes in hydraulic operation of the pumps for which they provide the parts.
 
The Consequences
 
The trouble is that, when we buy the cheapest pump, it almost always is less efficient, breaks down more easily and frequently, and often doesn’t even do the job that was expected.
 
This results in more power draw, which increases the cost of running the pump. More frequent breakdowns increase the cost of spares and expended manpower as well as a reduction in reliability.
 
The Correction
 
Value Based Purchasing helps us buy the pumps that are the best available in the market, and the ones most suited to the operation for which they are being purchased.
 
This involves a detailed evaluation and comparison of pump quotes, and will always require some degree of subjective evaluation about the accuracy of the data presented. In this area, some previous experience with pumps and potential suppliers will be invaluable. However, the overall consideration must continue to be the best long-term value for the money.
 
To do that, we must consider specific aspects of the equipment being purchased, including:
 
                        1.         Hydraulic suitability to the service.
                        2.         Efficiency of operation.
                        3.         Mechanical suitability to the service.
 
The better companies already include some type of evaluation for the first two factors in their considerations. The only comment that can be made would be in relation to the hydraulic suitability of the pump for the service. This can only be established if all the extremes of operation are considered, and not just the ‘normal’ conditions.
 
With respect to efficiency of operation, an evaluation of power cost is fairly standard, and is based on the particular cost of usable power in that plant. To ensure a real appreciation of the value of an efficient pump, it is strongly recommended that the actual power consumption cost for each pump under consideration be calculated. Do not be tempted into only calculating the difference in efficiency quoted, as it tends to give a false impression.
 
It is also recommended that life cycle cost considerations for various system setups be considered as well as the various mechanical options within the pump itself. In view of the ready availability of computer based design programs and pump selection programs this is a far simpler exercise than it used to be in the olden days when slide rules were accurate. One major benefit that has been identified in recent years is the ability to change the size of the pipes in the system design and show the resulting comparison of the system curves. With a larger pipe diameter, the flow velocities and friction losses are reduced, the Head required from the pump is also lowered, together with the power draw needed to drive that pump.
 
Consequently, as we don’t buy the “cheapest” of anything else, let’s start working with a value based approach to pump purchasing, and stop buying the wrong pumps. By purchasing the right pumps we will be able to increase reliability and reduce maintenance costs.
 
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  can be reached at 1-800-465-6260 or through his web site atwww.practicalpumping.com

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