This
Month's Tech Tips
Here's a few rules-of-thumb for you keep around because sooner
or later you'll need to size a pump (or you'll need to
know a system's Pump Head for valve sizing) or you'll
be asked to specify or install a Bypass Valve based
on the system's differential pressure.
First off, lets keep in mind that these are rules-of-thumb,
and any significant factors involved in your project should
be considered. (That's our legal disclaimer similar
to recommending that you consult your doctor before starting
a diet...)
CALCULATING PUMP HEAD
First, measure in feet the pumps longest run.
Now add 50% to this total. Then multiply by .04.
The resulting number is your pump head!
df = longest run distance in feet
df + (df x .50) x .04 = Pump Head
VALVE SIZING FOR DIFFERENTIAL
PRESSURE BYPASS VALVES
When sizing control valves, it is common to use a 4
to 5 pound pressure drop, to maintain good control without
adding greatly to the system's pumps work load.
But what about the valve that is installed between the
supply and return better known as the DP Valve, or differential
control valve? This valve remains closed, most,
if not all the time. It comes into play only with
an increase in pressure between the supply and return.
This pressure increase is caused by many of the control
valves in the system being closed or throttled down all
at the same time (some newer buildings have Variable
Frequency Drives on the pumps to reduce the pumps rpm's
when this occurs). This is when the DP Valve starts
to open to relieve the pressure on the system and "rat
race" the water to the return side and back to the boiler.
How do you size this valve? Line sizing, or using
a valve equal to the pipe size at the pumps will be a
costly error as large proportional valves can be expensive
and quite simply a waste of money as this valve will seldomly
if ever be utilized to full capacity. Better
to simply size the valve like any other control valve
in the system but instead of using the 4 or 5 pound drop,
use a pressure drop of 20 psi.
This will save money on the initial cost of the valve,
yet it will be adequately sized to handle any over pressure
situations that will normally be temporary in nature. If
it is not a temporary situation, you have a more serious
problem on your hands!
