PumpCalcs: Be the Pump Designer


Written by:
Robert X. Perez
Published:
December 17, 2011

I have seen, and I am sure you have too, countless examples of installed pumps that are poor hydraulic fits. These selection mistakes are called off-design pump selections, bad actors, misapplied pumps, etc. We are all familiar with these pumps because they live in our repair shops and require inordinate amounts of time and care.

After much anguish and frustration, someone always asks the inevitable question: How did this pump ever make it into our plant? There are many reasons why these dubious selections make it into the field, including:

1. The pump supplier did not have a good hydraulic fit to offer, so he offered what he had available.

2. The user badly missed the actual flow requirements, so the pump manufacturer cannot be blamed.

3. More costly options were not considered, such as multistage or lower speed designs, as a means to keep initial capital costs down.

 

For example: Due to a low NPSHa specification, a pump supplier chose to bid a 3600-rpm pump that must operate well below its best efficiency point in order to keep the NSPHr low - instead of considering an 1800-rpm option. This selection "trick" places the pump between a rock and a hard place - dooming the pump to operate at low flow conditions forever! At lower flows, the pump will become unstable. At higher flows, the pump will cavitate.

Remember that there are usually at least four parties involved in the pump selection process: the process engineer, the project engineer, the rotating equipment engineer, and the pump supplier. Each party has a different point of view and different objectives.

In no particular order, I will briefly outline how each team member views pump selection:

The process engineer always wants to ensure the pump meets his or her process requirements, so he or she will typically add a safety margin to the flow and NPSHa specifications. These safety margins usually move the actual operating point away from the pump's best efficiency point and increase the pump's required suction specific speed. There is also some concern about energy efficiency, but this is usually secondary to ensuring the process requirements are satisfied.

The pump supplier wants to sell you the lowest cost pump possible so that he or she is the lowest bidder. If cost was no object, he would want to sell you the most expensive pump he has. But this is not the case. Due to extreme bidding competition, the pump supplier's goal is to bid the cheapest pump that meets your technical requirements and the manufacturer's selection guidelines.

The project engineer wants the least expensive pump that meets his design requirements. This means his or her objectives closely match those of the pump suppliers. However, by soliciting competitive bids from several pump suppliers, the project engineer guarantees he or she will obtain the lowest cost pump for the provided technical specifications

The rotating equipment engineer wants his or her pump to be as reliable as possible. He does not want to spend his nights and weekends fixing or worrying about this pump. Experience has taught him or her to:

1. Always select a healthy NPSH margin to protect the pump during process upsets.

2. Always select pumps with a suction specific speed (Nss) less than 11000 - better yet would be an Nss<9000.

3. Always select the slowest viable pump speed to ensure long-term mechanical reliability, i.e. operation at 1800-rpm is always better than operation at 3600-rpm. However, this is always a difficult sell. Slower always means more expensive, since slower pumps are larger, hence requiring more metal to construct, leading to the great chasm of pump selection. The project people like 3600-rpm pumps because of their lower cost, and the rotating equipment engineers like 1800-rpm pumps because of their improved reliability.

4. Always try and operate pumps above 70 percent to 80 percent of their best efficiency point (BEP).

All these requirements come with a price - one that the other selection team members may not think is justified. This is what makes the pump selection process fun!

Where does this put the rotating equipment engineer? He is placed into a debate of one against three - with the rotating equipment engineer having the least control over the final outcome. He has no money. He only has decision rights if empowered by his or her organization. This should not be considered a power struggle, but a collaborative selection process with the long-term benefits of the organization in mind.

I personally consider a pump selection a win-win if it represents the lowest cost of ownership over the project's life. What does this mean? It means if you add up the cost of the pump, installation costs, energy costs over its lifetime, repair costs, and cost of unreliability for all pumps considered, the pump selected should be the least expensive to own. If it is not, then the team failed to do its job.

But there is a catch in this reasoning. You only get to consider the pumps that are quoted. So you ask: How can I consider pumps that are not quoted? You cannot, but you can influence the types of pumps that are quoted - and I will show you how.

Pumps & Systems' new website PumpCalcs.com offers users an Expert Calculator that takes typical pump specifications and provides key hydraulic parameters that are useful in the pump selection process. By simply inputting flow, pressures, rpm, specific gravity, number of stages, number of suction eyes, NPSHa, and desired NPSH margin, the Expert Calculator will return the specific speed (Ns) and suction specific speed (Nss) for an ideal pump (See Figure 1).

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