View Full Version : Head PLUS pressure loss

06-18-2011, 03:25 PM
When figuring the size of a pump, the 'head' is a good starting point. Also, the performance chart, for a given pump needs to be considered. The thing often over looked is, the friction loss in pipe and fittings.

When installing pumps, you may consider over sizing your pipe for a few reasons...
-The next size pipe and fittings, does have less friction loss.
-The flow in our pipes start to slow, due to a bio-slime growing on the walls of the pipe.
-the pump will be operating at less pressure, generally, it will use less power and may last longer.

Most of us use smaller pumps in backyard systems. An average pump may have a 1/2, 3/4 or a one inch discharge.
The pvc fittings 'normally' available at the big box stores or hardware stores, in these sizes, are usually 'pressure' fittings. The 90's, are a sharp turn, which will have more friction loss, then using 2- 45 degree fittings.
You can see this for your self, by going to this link..
http://www.engineeringtoolbox.com/pvc-p ... d_801.html (http://www.engineeringtoolbox.com/pvc-pipes-equivalent-length-fittings-d_801.html)
The 'friction loss' is shown as added length of pipe.

Lets use 3/4 inch 90's and 45's as an example. According to the chart...
- a 90 adds 4.5 more feet of pipe to the run. (friction loss)
-2- 45's add, 2.2 more feet.

If you really want to get into figuring the total pressure loss....follow the directions on this page/link...
http://www.engineeringtoolbox.com/equiv ... d_804.html (http://www.engineeringtoolbox.com/equivalent-pipe-length-method-d_804.html)

Most backyard systems don't use too much pvc *, the cost of over sizing the pipe and using 2-45's instead of a 90 isn't that great, when considering the above mentioned benefits.(* I'm helping to build a commercial system, that is a lot of pvc. Even there, the pvc is being over sized.)

You can buy flex pvc or even $pecial order 'long radius' 90's in the smaller sizes...... http://flexpvc.com/cart/agora.cgi?cart_ ... rsSweep90s (http://flexpvc.com/cart/agora.cgi?cart_id=8844945.10866&product=PVC-Elbows-SpearsSweep90s)

The 1 inch lines with 2-45's are supply, coming from the pumps. The others are over flow drains.

06-20-2011, 12:11 AM
EXCELLENT. I never even knew about this. Thanks for sharing!

06-21-2011, 06:38 PM
I've been using the Crane's Manual for years. It gives you the methods to calculate all this stuff. Bit of a learning curve, but the bottom line is that there is a system curve and a pump curve. Where they cross is where you will operate. Design a safey factor of around 2 and add a throttle valve.