HYDRAULIC DESIGN

FACTORS AFFECTING DESIGN SEASON OF USE QUANITITY OF WATER NEEDED WATER SOURCE PRODUCTION LIMITATIONS ROUTE GEOLOGIC LIMITATIONS FISCAL LIMITATIONS DEPTH OF BURY MANUAL VS. AUTOMATIC

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).

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). Total Dynamic Head (TDH) is the total equivalent height that a fluid is to be pumped, taking into account friction losses in the pipe.

FRICTION LOSS Friction loss refers to that portion of pressure lost by fluids while moving through a pipe, hose, or other limited space. The amount of friction loss (pressure loss) is due to four conditions: The velocity (speed) of the flow. Diameter of the pipe. Length of the pipe. Roughness of the pipe.

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.

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

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.

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!

AIR/GAS PROBLEMS Air or gas gets into a pipeline in several ways. These include: When a pipeline is drained, air enters the line through hydrants or any opening. 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. If the water level in a well or other source falls below the pump intake, air is drawn into the pipeline by the pump. 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. When you have a gravity line and the velocities in down hill sections exceed the rest of the pipeline velocities.

RELEASING AIR FROM PIPELINE

AIR IN LOW HEAD GRAVITY PIPELINES 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. 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.

NRCS recommendation for very low pressure pipelines, experience indicates that minimum pipe diameter should be: 1-1/4 inch nominal diameter for grades over 1.0 percent. 1-1/2 inch nominal diameter for grades from 0.5 to 1.0 percent. 2 inch nominal diameter for grades from 0.2 to 0.5 percent. For grades less than 0.2 percent, gravity flow systems are not recommended. Mike Montgomery recommendation: Try and standardize your pipeline pipe size. If you have grades less than 0.2 percent control the grade.

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.

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.

There are three types of functions that air valves perform: 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: 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.

AIR VENT LOCATION: A line that has worked for years will sometimes slow down or stop. The usual culprit is air in the line. Adequate air handling equipment should always be designed into a system at the time of initial installation. 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)

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.

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. 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. 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?