How Much Energy does Your Piping Burn?
In earlier articles we have discussed the procedure of piping size selection for a given process flow requirement, and the way this affect the pump power consumption. In this issue, we will supplement this with two computer programs for a quick and simple basic evaluation of the piping effect on pump energy and associated cost. As always, the best illustration of a concept is a specific example. So let’s say you need to pump 8000 gpm of water 5 miles away. Let’s also assume a flat terrain, with no appreciable elevation differences along the way, i.e. most of the hydraulic losses come from the pipe friction in this case. A non-stop 24/7 operation is also assumed, although that can be also accounted for, as shown below.
To follow along, you need to sit down with a cup of coffee and access to a computer, as well as some patience for the “homework puzzle assignment” which is coming at the end of the example. Log into www.pump-magazine.com/pump_magazine/pump_magazine.htm – and get ready to use two programs listed there:
* Pump Performance Estimator and (Pump-Cal)
The first program will estimate the friction losses associated with pumping the given amount of flow thru a selected piping diameter and length. A straight run of pipe is assumed, with no additional losses for bends, turns, and other restrictions along the way. More elaborate programs exist to handle complex piping, but for our purpose a simplified straight piping run will suffice. Clean (or somewhat clean) water is assumed to flow thru a reasonably new pipe. For old, rusty, or partially clogged piping, additional adjustments can be made, such as, for example, applying a smaller pipe diameter, to simulate partial blockage.
Below is the snap shot of the first iteration of the Pipe-Cal run:
Fig. 1 Pipe-Cal initial run
The 5 miles of pipe (26,250 feet) resulted in 836.3 feet of hydraulic losses, which needs to be handled by the pump. Pump efficiency was assumed 80%, and the resultant required pump power of 2,112 hp, which, at $0.10 per Kw-Hr would translate at $1,380,028 energy cost per year.
Once we know the pump head, we can run the Pump-Cal program, to get a better value of the pump efficiency (many of our readers have already used this program in the past, but the value of head at that time was enter without the benefit of a Pipe-Cal program):
Fig. 2 Pump-Cal efficiency adjustment run
Notice that the actual pump efficiency has been adjusted to 83.9%, reducing the power to 2013 hp by re-running the Pipe-Cal:
Fig. 3 Pipe-Cal run with a pump corrected efficiency by the Pump-Cal
Let’s see what happens when we try to increase the pipe size by just one step up (20 inch):
Fig. 4 Pipe-Cal run with increased pipe size (20 inch instead of 18 inch)
The head losses dropped almost by half – and so did the wasted energy! Also, a smaller (less expensive) pump can now do the job.
Piping size is a strong player in energy savings, keep that in mind. Use these programs for a quick estimate of your piping effect. By entering actual pipe length, diameter, flow, and percentage of approximate operating time thru the year, you can get a good feel for the impact of the changes, even within the simplified assumption used.
Home work assignment: by using Pipe-Cal program, evaluate how much additional energy is wasted if the pipe in the above example gets clogged up at half the initial area? The answers will be posted in the future articles, and the detailed solution will be presented at the next scheduled Pump School training during the upcoming PumpTec Conference in September: www.pumpconference.com
I look forward to see you there! Until then – Keep-On-Pumping!
Dr. Lev Nelik, P.E.
Editor, Pump Magazine On-Line