Task 6a Power Point - National Training Center

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HYDRAULIC DESIGN

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FACTORS AFFECTING DESIGN

o
SEASON OF USE

o
QUANITITY OF WATER NEEDED

o
WATER SOURCE PRODUCTION LIMITATIONS

o
ROUTE

o
GEOLOGIC LIMITATIONS

o
FISCAL LIMITATIONS

o
DEPTH OF BURY

o
MANUAL VS. AUTOMATIC

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STATIC HEAD

Static pressure is the pressure that is exerted by a liquid or gas
,


such as water or air. Specifically, it is the pressure measured when

the
liquid or gas is still, or at rest.

Pressure head is a term used in fluid mechanics to represent the internal

energy
of a fluid due to the pressure exerted on its container. It may also

be
called static pressure head or simply static head

(
but not static head pressure).

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Total Dynamic Head (TDH) is the total equivalent height that a fluid

is
to be pumped, taking into account friction losses in the pipe.


Pressure head is a term used in fluid mechanics to represent the internal

energy
of a fluid due to the pressure exerted on its container.

If the water

Is moving it
may also
be
called
dynamic
pressure head or
simply

Dynamic head

(
but not
dynamic
head pressure).

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FRICTION LOSS

o
Friction loss refers to that portion of pressure lost by fluids while

moving
through a pipe, hose, or other limited space.

o
The amount of friction loss (pressure loss) is due to four conditions:


1.
The velocity (speed) of the flow.

2.
Diameter of the pipe.

3.
Length of the pipe.

4.
Roughness of the pipe.

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Let's take a look at a pump curve, the common way of showing a centrifugal pump's performance.




Let's take a look at a pump curve, the common way of showing a
centrifugal pump's performance.


The size of the pump, 1
-
1/2 x 3
-

6 is shown in the upper part of the pump
curve illustration. Note that the size number 1
-
1/2 x 3
-

6 indicates that the
pump has a 1
-
1/2 inch discharge port, a 3 inch suction port, and a maximum
nominal impeller size of 6 inches. This type of nomenclature is common, with
some companies putting the 3 in the first position instead of the 1
-
1/2. In
either case, standard procedure is that the suction port is the larger of the
first two numbers shown and the largest of the three numbers is the nominal
maximum impeller size.


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Also in the upper right hand corner notice that the curve indicates
performance at the speed of 3450 RPM (a common electric
motorspeed

in 60
hz

countries). All the information given in the curve
is valid only for 3450 RPM. Generally speaking, curves which indicate
RPM to be between 3400 and 3600 RPM are used for all two pole
(3600 RPM nominal speed) motors applications.


look at a pump curve, the common way of showing a centrifugal pump's performance.




The pump's flow range is shown along the bottom of the performance
curve. Note that the pump, when operating at one speed, 3450 RPM,
can provide various flows. The amount of flow varies with the amount
of head generated. As a general rule with centrifugal pumps, an
increase in flow causes a decrease in head.

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performance.




The left side of the performance curve indicates the amount of head
a pump
is capable of generating.

Notice that there are several curves which slope generally downward as they move from left to right on
the curve. These curves show that actual performance of the pump at various impeller diameters. For this
pump the maximum impeller diameter is shown as 6 inches and minimum is 3 inches. Impellers are
trimmed in a machine shop to match the impeller to the head and flow needed in the application.

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below.



The point on the curve where the flow and head match the application's requirement is known as the duty point.
A centrifugal pump always operates at the point on it's performance curve where its head matches the resistance
in the pipeline. For example, if the pump shown above was fitted with a 6 inch impeller and encountered 100 feet
of resistance in the pipeline, then it would operate at a flow of approximately 240 gallons per minute and 100 feet
of head. It is important to understand that a centrifugal pump is not limited to a single flow at a given speed. Its
flow depends on the amount of resistance it encounters in the pipeline. To control the flow of a centrifugal pump
it is normally necessary to restrict the discharge pipeline, usually with a valve, and thus set the flow at the desired
rate. Note: Generally speaking, do not restrict a pump's flow by putting a valve on the suction line. This can cause
damage to the pump!

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AIR/GAS PROBLEMS

Air or gas gets into a pipeline in several ways.
These include:

1.
When a pipeline is drained, air enters the line through
hydrants or
any opening
.

2.
There are various forms of gasses in well waters. These
gases can
come out of solution during pipeline operation.
Some
wells have
more serious gas problems than others
.

3.
If the water level in a well or other source falls below the
pump intake
, air is drawn into the pipeline by the pump
.

4.
In gravity systems, air can be drawn into the pipeline when
water surface
falls below the pipeline entrance. In some
live
streams there
can also be air bubbles entrapped in the
water
.

5.
When you have a gravity line and the velocities in down hill
sections exceed the rest of the pipeline velocities.

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RELEASING AIR FROM PIPELINE

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AIR IN LOW HEAD GRAVITY PIPELINES

o
Air locks are a frequent problem in very low flow, low pressure

pipelines. An example of this type of system is a spring fed

installation. In this case the velocity of water is very low. Air

bubbles do not get pushed out, even if the summit in the line is only

one pipe diameter above the rest of the line.

o
The solution for air lock problems can be either of the following:


Install an open air vent at all summits in the line.


Install
the pipe so there are no summits in the line. Carefully

lay out the pipe so it is on either a constantly increasing or

decreasing grade.

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NRCS recommendation for very low pressure pipelines, experience


indicates that minimum pipe diameter should be:

1.
1
-
1/4 inch nominal diameter for grades over 1.0 percent.

2.
1
-
1/2 inch nominal diameter for grades from 0.5 to 1.0 percent.

3.
2 inch nominal diameter for grades from 0.2 to 0.5 percent
.

4.
For grades less than 0.2 percent, gravity flow systems are
not
recommended
.

Mike Montgomery recommendation:


1.
Try and standardize your pipeline pipe size.

2.
If you have grades less than 0.2 percent control the grade.

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AIR CONTROL IN HIGH HEAD, LONG PIPELINES

There are two ways to resolve air problems in high pressure pipelines:


Minimize the number of summits in the line by meandering the

pipeline along the contour to avoid high points. There is a

point where the extra cost of additional pipeline length makes

this a non
-
cost effective approach.


Install air valves at summits to control the entry
and exhausting

of air.

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There are three types of functions that air valves perform:


1. When
a pipeline is emptied, air must enter the line some
place.
If provisions
are not made for entry of air, a vacuum can be
created in
the pipeline. This can lead to collapse of the pipe or at
least breaking
of the water column, which creates gas or water
vapor pockets
in the pipeline. Although it is unlikely that the small

diameter pipe in
stockwater

lines will collapse due to vacuum, it

is a bad design practice to allow significant vacuum to develop in

the pipeline. It is therefore important to have a vacuum relief

mechanism at significant high points in the line.

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2. When
an empty pipe is filled with water, air in the line must be

released in large volumes. This can be done by leaving the hydrants

open. But what if the hydrants are closed? Air pressure will build

up in the pipeline. When a hydrant or float valve is opened, high

pressure air will escape and then, when water hits the end of the

line,
waterhammer

will probably occur.

For adequate system protection, there must be a mechanism to

automatically release large volumes of air from the pipeline during

filling. For best results, the mechanism should be located at all

significant summits in the line.

There are three types of functions that air valves perform:

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3. During
operation of the pipeline, air bubbles and other gasses come

out of solution and buildup as gas bubbles at summits in the line.

There are usually also remnants of the large volumes of air present

immediately after filling. If the summit is high enough, this air

will never push on through the line. Gases may eventually buildup

to the point where the flow rate is seriously reduced or flow may

even stop. It is not possible to predict how serious a problem this

may be when designing a pipeline.

There are three types of functions that air valves perform:

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1.
A line that has worked for years will
sometimes slow
down or stop. The
usual culprit is air in the line
.

2.
Adequate air handling
equipment should
always be designed into a
system at the time of
initial installation.

3.
In high pressure, moderate flow systems, there are frequently many


small
undulations in the ground surface and a few large humps. Trial


and
error on typical long
stock lines
in Montana has led to the


conclusion
that we can usually get away with not installing air vents


or
valves on summits that are less than ten feet high. So in most


cases
, it is recommended that air handling equipment be installed on


all
summits of ten feet or more, at the end of the pipeline and at the


first
high point of any kind past the pump
. (as a minimum every 2000

ft.
mjm
)


AIR VENT LOCATION:

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EXAMPLE 1, LOW HEAD GRAVITY SYSTEM

Figure 9.1 illustrates the profile for a very low head system. The

pipeline originates at a spring box and terminates at a stock tank. An

overflow is built into the stock tank. There is not float valve at the

tank and the entire spring flow goes to the tank. A gate
-
type valve

could be installed at the spring box to throttle the flow or shut it

off when water is not wanted. A valve at the tank allows drainage of

the pipeline during non
-
use. The pipeline is buried below the frost

line.

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Questions for exercise 1:

1.


What is the static pressure at
Station

10+00
, 15+00, 25+00, 30+00, and tank

station
?

2.

What diameter pipe should be used?

3.

Calculate the pressure rating of the pipeline

pipe for this project.

4.
If
the spring is flowing 5 gpm, and the water
at the tank if flowing 5 gpm, what is the
dynamic head at the tank
? This question
could be called a trick question.

5.
If
the spring will flow 10 gpm, and the water
tank has a flow restrictor of 5 gpm what is
the dynamic head at the tank?

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