Flujo

Sizing flow valves is a science with many rules of thumb that few people agree on. In this
article I'll try to define a more standard procedure for sizing a valve as well as helping to
select the appropriate type of valve. **Please note that the correlation within this article are
for turbulent flow
STEP #1: Define the system
The system is pumping water from one tank to another through apiping system with a
total pressure drop of 150 psi. The fluid is water at 70 0F. Design (maximum) flowrate of
150 gpm, operating flowrate of 110 gpm, and a minimum flowrate of 25 gpm. The pipe
diameter is 3 inches. At 70 0F, water has a specific gravity of 1.0.
Key Variables: Total pressure drop, design flow, operating flow, minimum flow, pipe
diameter, specific gravity
STEP #2: Define amaximum allowable pressure drop for the valve
When defining the allowable pressure drop across the valve, you should first investigate the
pump. What is its maximum available head? Remember that the system pressure drop is
limited by the pump. Essentially the Net Positive Suction Head Available (NPSHA) minus
the Net Positive Suction Head Required (NPSHR) is the maximum available pressure drop
forthe valve to use and this must not be exceeded or another pump will be needed. It's
important to remember the trade off, larger pressure drops increase the pumping cost
(operating) and smaller pressure drops increase the valve cost because a larger valve is
required (capital cost). The usual rule of thumb is that a valve should be designed to use
10-15% of the total pressure drop or 10 psi,whichever is greater. For our system, 10% of
the total pressure drop is 15 psi which is what we'll use as our allowable pressure drop
when the valve is wide open (the pump is our system is easily capable of the additional
pressure drop).
STEP #3: Calculate the valve characteristic

For our system,

At this point, some people would be tempted to go to the valve charts or characteristiccurves and select a valve. Don't make this mistake, instead, proceed to Step #4!
STEP #4: Preliminary valve selection
Don't make the mistake of trying to match a valve with your calculated Cv value. The Cv
value should be used as a guide in the valve selection, not a hard and fast rule. Some other
considerations are:

a. Never use a valve that is less than half the pipe size
b. Avoid using thelower 10% and upper 20% of the valve stroke. The valve is much
easier to control in the 10-80% stroke range.
Before a valve can be selected, you have to decide what type of valve will be used (See the
list of valve types later in this article). For our case, we'll assume we're using an equal
percentage, globe valve (equal percentage will be explained later). The valve chart for this
type ofvalve is shown below. This is a typical chart that will be supplied by the
manufacturer (as a matter of fact, it was!)

For our case, it appears the 2 inch valve will work well for our Cv value at about 80-85% of
the stroke range. Notice that we're not trying to squeeze our Cv into the 1 1/2 valve which
would need to be at 100% stroke to handle our maximum flow. If this valve were used,
twoconsequences would be experienced: the pressure drop would be a little higher than 15
psi at our design (max) flow and the valve would be difficult to control at maximum flow.
Also, there would be no room for error with this valve, but the valve we've chosen will
allow for flow surges beyond the 150 gpm range with severe headaches!
So we've selected a valve...but are we ready to order? Not yet,there are still some
characteristics to consider.
STEP #5: Check the Cv and stroke percentage at the minimum flow
If the stroke percentage falls below 10% at our minimum flow, a smaller valve may have
to be used in some cases. Judgements plays role in many cases. For example, is your
system more likely to operate closer to the maximum flowrates more often than the
minimum flowrates? Or is...