Sizing Pipes and Pumps Concurrently

The cost of a pumped piping system clearly depends on both the cost of the pipes, and the pump. It is tempting to select pumps and pipes in stages, but this is unlikely to produce the lowest cost solution. Selecting a smaller pump may not appear to be viable, but changing the pipe sizes may make it viable. Like sizing pipes alone, overall costs can often be decreased by increasing the cost of one or a few components. Attempting to find the lowest cost option manually quickly becomes impractical even with a small number of variables.

Optimal Pumping System Operating Point (OPSOP)

A pump's BEP is considered the ideal place to run a pump as the efficiency is highest, and thus cost is lowest. Similarly, any piping system will have a lowest cost design.

Whether a pump is running at BEP, however, depends on the piping system, and the lowest cost piping system depends on the pumps it contains. What may be the lowest cost piping system for a given pump, may not me the lowest possible cost option. It is possible that a cheaper pump could be selected with the correct piping adjustments while still maintaining all Design Requirements.

The point at which no adjustments to pump or system will result in lower cost is known as the Optimal Pumping System Operating Point (OPSOP). One way to determine this point is to exhaustively size the system for different potential sizes of pumps. This is inefficient, but can be used to construct a visual representation of the OPSOP as shown below.

A graph showing the Optimal Pumping System Operating Point.

Figure 1: The OPSOP of a given system. Note that every point is an ideal sizing solution for the given pump size

If the sizing process is approached in stages, rather than concurrently, it is easy to get a result that says, for example, a 60 foot head rise pump is required. The best case system for the 60 foot head rise pump has a minimum cost of about $36,500. However, allowing the pump to change as well can result in a system more than 7% cheaper.

Process Outline

Find the Approximate Pump Size

Model the pump with the Pump Sizing option, fixing the flow to the desired amount. AFT Fathom will determine the required head rise.

Include an estimate of Nominal Efficiency.
Optionally, include Nominal NPSHR.

Add the pump to an Engineering Library.

Attach a Cost Library to the Engineering Library if one does not already exist.
Create a Power vs. Cost Scale Table for the pump with estimated cost values.

Select the appropriate options in the Sizing Window.

The cost for the pump will only be included in the Objective if it is specified to be included in Size/Cost Assignments.
If there are several pumps that will ultimately be of the same type, they should be placed into a Maximum Cost Group.

Run ANS.

Investigate the resulting pump size.
Update the Nominal Efficiency and NPSHR based on the result. Also update the cost information if necessary. If the pump accounts for a large portion of the overall cost, accurate values here are more important.
Repeat the process as necessary, until reasonable estimates for cost at the resulting operating point are obtained.

Compare Candidate Pumps

Locate accurate cost information and pump curves for the candidates.

Incorporate these values into Engineering and Cost Libraries.

Compare the sizing results for each candidate pump.

Estimating Pump Costs

Without knowing specifically what type or size of pump is going to be included in the system, the pump behavior must be approximated. The behavior is approximated with the Pump Sizing option, indicating a design flow. We also need to approximate cost. Because energy cost is often the largest component of a pump's overall cost, and this cost is directly related to the pump's power, it makes sense to correlate other pump costs to power as well.

A graph showing total initial costs for a series of pumps.

Figure 2 - Total Initial Costs for a series of pumps

Keep in mind that such a comparison is only valid for the selected range of pumps. Different designs of pumps, or pumps from different manufacturers may follow different relationships. One manufacturer may offer cheaper low-power pumps, while their competitor offers cheaper high-power pumps. This cost information is represented in ANS as a Power vs Cost Scale Table. This requires the creation of a Cost Library including the pump as a Custom Component.

Because we do not know what specific pump is going to be used, we also do not know exactly how much it will cost to run or maintain it.

As part of our sizing process, we must assume that the pump runs at some Nominal Efficiency. AFT Fathom will calculate the fluid power required at any given flow and head rise. However, a pump or motor is not perfectly efficient and will therefore require more power than the fluid power. This affects the pump selection, and therefore cost, so it is important to include an estimate on efficiency.

Note: It may seem that including a nominal efficiency is not important, as selecting a certain pump size will minimize the initial pump cost regardless of this value. However, this does not consider the cost of power over a long period, which is often a driving factor in sizing. Also, it is important to get the relative cost of the pump correct as this can affect the best sizing. The nominal efficiency will require more power and hence a more expensive pump - as this is included in the Objective, it is important to account for.

If maintenance costs are expected to be significant, it is important to include an estimate of these costs. Again, this could be accounted for with a Power vs Cost Scale Table.

The Approximate Sizing Solution

When the initial model is defined, the sizing configuration complete, and the pumps appropriately defined with approximate cost information, the automated sizing can take place.

ANS will size the pipes of the system to minimize cost while obeying Design Requirements. It will now include a power-based cost for the pump. Without cost information on the pump, ANS would tend to select small pipe sizes to reduce cost without any negative consequence. However, smaller pipe sizes have higher frictional loss, which requires greater head rise at the pump, and hence higher cost. By including pump cost information, the balance between pump costs and piping costs can be found.

However, this is still an estimate because we are not modeling a real pump. Fixing the flow and nominal efficiency are good design tools but it is important to represent a real pump curve not only to account for real effects, but also to ensure that the desired pump can actually be acquired.

If the pump costs are a significant portion of the overall cost, the estimates made for cost and nominal efficiency should be refined and the system re-sized before proceeding.

Sizing with a Real Pump

Now that there is a good idea of the size and cost of pump we are looking for, we can select a real pump we may want to use.

It will rarely be clear which pump is the best choice. First, the exact operating point determined from the approximate solution is likely unavailable in a commercially available pump. Second, different pumps have different costs - even if a pump with the exactly correct specifications is available, it may be much more expensive than a pump that is slightly mismatched. This cost difference could mean the cheaper pump is better overall, despite not appearing to be the ideal choice.

The best course is to select several candidate pumps, and size the system independently with each candidate pump. It is important that accurate pump curves and cost information be included for each candidate, as this no longer meant to be an estimation. After testing each candidate pump, the system with the lowest overall cost can be selected.

If the sizing results differ significantly in this stage this could indicate that the pump is poorly matched, that there was an error in the process, or that the system is dominated by pump costs and therefore very sensitive to the estimations.