During the movie the following dialog takes place between the Captain and Luke. Luke just finished time in “the box” after an unsuccessful escape attempt. In addition he was given a set of leg irons “to slow him down.”
Captain: You gonna get used to wearing them chains after a while, Luke. Don't you never stop listening to them clinking, 'cause they gonna remind you what I been saying for your own good.
Luke: I wish you'd stop being so good to me, Cap'n.
Captain: Don't you ever talk that way to me. (pause, then hitting him) NEVER! NEVER! (Luke rolls down hill; to other prisoners) What we've got here is failure to communicate. Some men you just can't reach. So you get what we had here last week, which is the way he wants it. Well, he gets it. I don't like it any more than you men.
(Carroll & Rosenberg, 1967)
As a result “What we’ve got here is failure to communicate” has become a common phrase in the American lexicon.
Many of the problems we experience are caused by the failure to communicate. In piping systems, the failure to communicate causes pumps to be over-sized leading to increased operational, maintenance, and capitol cost and reductions in system reliability and overall output. One of the most common communication problems is the failure to accurately state the process requirements when selecting a pump. It works like this:
The owner of the system design provides a capacity requirement based on future system needs knowing full well that the system will be operating at a lower capacity for an extended period of time until the market need catches up with capacity. So instead of specifying the expected 500 gpm for process design flow, 1,000 gpm is given so as to plan for the future.
The engineer designing the system takes the capacity provided by the process group and adds a 20% design margins of flow (to allow for future capacity increases). In addition a design margin for head is added (to account for system uncertainties during the design process) when specifying the equipment. As a result the pump design point is 1,200 gpm and 200 ft of head.
The individual selecting the pump chooses a pump with the design point left of the pumps Best Efficiency Point (BEP) to allow the pump to better accommodate future system capacity increases.
“What we've got here is failure to communicate.”Each group in the process added their design margin, just to be on the safe side. Yet they fail to document or communicate their design margins to the entire group. This is the way things are done today because that’s the way things have always been done. We as engineers, have a tendency to add design margin just to make sure thing work. There is nothing wrong with that except that problems arise when we don’t consider the consequences of compounding design margins on the system operating costs, maintenance costs, capitol costs, and plant reliability.
That is where PUMP-FLO comes in handy; it’s a great way to communicate. Anytime during the process the user can enter basic system operating information and the program calculates the static head and dynamic head. Combined with the pump curve you can see the interaction between the pump, process, and control. For example the following pump/system curve was generated with the PUMP-FLO program using a pump curve from the Crane Deming Pumps catalog.
Notice the design point for the selected pump was 1,200 gpm with 200 ft of head based on the calculations performed by the engineer. The individual selecting the pump chose one with the design point left of the pump’s best efficiency point of 1,550 gpm. As we can see the pump is the most efficient at a flow rate that will never be achieved.
The blue lines represent the system curve. The upper blue line was used in calculating the system static head for the pump selection calculations; this represents the maximum static head expected even though the possibility of the system operating under this condition is less than 1% of the time. The lower system curve shows the typical valve for static head. The pump was sized for the maximum static head that occurs infrequently and at a flow rate that is not expected to be needed for 10 years.
What is this costing us? Cost is a primary driver for most operating system, and one again PUMP-FLO is able to help you communicate how much the system costs to operate. For example the projected pump operation during the first 5 years is 500 gpm resulting in a 72 psi pressure drop across the control valve. When the system is running as described for a year it consumes 395,000 kWh and with a power cost of $0.10/kWh costs $39,500/yr to operate.
If the differential pressure across the control valve was reduced to a more reasonable pressure drop of 28 psid, by reducing the impeller diameter on the pump to the minimum allowed by the manufacturer, the pump will consume only 214600 kWh per year with an operating cost of $21,460/year.
The Captain was wrong, we don’t have to get used to the leg irons of system inefficiency if we just communicate our process and total cost to everyone involved.
By the way if you haven’t seen Cool Hand Luke I would suggest renting in on Netflix® or Amazon.com® and watch an excellent story with an outstanding cast. And please let me know what you think about the article or about Cool Hand Luke.
References:
Carroll, G. (Producer), & Rosenberg, S. (Director). (1967). Cool Hand Luke [Motion picture]. United States: Warner Bros.-Seven Arts
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