“Call the engineer, the whole piping system is screwed up!”
This is the direction from the production foreman as they are tired of troubleshooting your design. Probably been working through the weekend and calling you choice names in the process. They are headed to your office now while you are knee deep in another design issue. You have no idea they are coming and its been 6 months since you have dealt with the pipe system in question.
So lets back up a bit. You designed this cooling system several months ago. Since then, several designs, issues, concepts, etc. have passed through your office. You never know when the previous designs are being implemented because you are constantly working on the next thing while also addressing other concerns. However, after installation of the aforementioned design, the crew starts the pump to cool a heat exchanger, but the equipment runs hot! It was a simple system and no problems were anticipated. Those installing it start to contemplate plugged pipe, reverse polarity, shaft binding and other possible issues. When troubleshooting in the field is exhausted, you get the knock at your door (or cubical)… “What is wrong with your design? Need it fixed yesterday!” You also hear in the background something like, “Stupid Engineers”!
So after discussing the issue and a enduring the fateful field trip (plus some flavorful cussing along the way), you start to clear the cobwebs out of your brain concerning the design that was previously completed and you now fumble through your notes to confirm you made no mistakes. At this point is was a good thing that you had learned from experience that you should make notes about each of your designs during and immediately after completion… Since most of us do not have photographic memories months later! Knowing your reasoning process at design time as well as the intended function helps tremendously.
*See free download below for the Centrifugal Pump Affinity Law Spreadsheet*
As you review the requirements, you notice that the vendor provided a flow requirement that matched other equipment. How did you miss it when you were only juggling 20-30 tasks at a time and people were rushing through your office everyday with multiple issues and concerns. Not to mention the endless supply of meetings, technical specs, purchasing reviews and other daily skirmishes!
Anyway, here you are with a design issue concerning fluid pumped by a centrifugal pump that you already purchased and installed. Your friendly production crew has you in the cross hairs while you think of ways to get this system going on the right track.
Couple of scenarios could result, either you need more or less flow while minding the head as well. The crew knows that if you need more head, a larger impeller is necessary. However, they usually don’t know what happens if you just stick a larger impeller in the casing, granted you have the room to expand. So if you can install a larger impeller, what does this do to flow, power and head? Same question if you need to trim the impeller. Well, I’m glad you asked!
Centrifugal pumps come with standard casing sizes with impellers that are typically available among a range of sizes. The impeller diameter can be trimmed to match the desired operating point within a pumps flow envelope. In the event that the pump impeller above was maxed out when purchased, the only option is to increase the pump size or align more pumps if standby pump(s) were already installed in the mix. However, where you can increase the impeller, it doesn’t necessarily mean you have the power necessary to produce the desired flow. The available power depends on what was specified on the initial order as well.
When you trim an impeller in small amounts or change the speed of the pump, the resulting flow, power and head varies with respect to the Centrifugal Pump Affinity Laws. These mathematical relationships are limited to centrifugal pump types. Other pump types should be consulted with the manufacturer.
See a short video below concerning the Pump Affinity Laws. *Apologies in advance if I put you to sleep 🙂
You can find the equations below in several different references, but these came from Cameron Hydraulics Data, 19th Ed. You can get a copy at www.flowserve.com for a nominal fee.
D = Impeller diameter in inches
H = Head in feet
Q = Capacity in gpm
S = Speed in RPM
BHP = Brake Horsepower
Subscript 1 is for the original design and 2 is for the new or resulting design condition.
So you can see that depending on your purchased pump casing size, the motor and if you have the ability to change speed, you may have some options without too much pain. If you bought a multi-speed pump or one that is driven by a VFD (Variable Frequency Drive), the speed can be changed to possibly accommodate the flow. However, this all depends on the flow requirement and the capabilities you have at hand.
Click the link below for a free download of the Affinity Law spreadsheet mentioned in the video. Use it to easily manipulate your options.
Hope this helps. Please comment if you have any questions.