Below we provide the answers to several frequently asked questions regarding hydraulic analysis in Ductile Iron Pipe design.
Q: The published Hazen-Williams flow coefficient “C” for cement-mortar-lined Ductile Iron Pipe is 140. What roughness value (e) does this correspond to on the Moody diagram?
A: The Moody diagram is employed to find the coefficient of friction (f) when using the Darcy-Weisbach formula to calculate energy loss (head loss) resulting from the flow of fluids. While the Darcy-Weisbach formula is widely used in academic circles, the Hazen-Williams formula is almost universally accepted by waterworks engineers in this country because it simplifies problem solution using calculators, tables, nomographs, and graphical charts.
Literature has been published regarding this issue. A.M. Friend, in his paper “Flow of Water in Pipelines,” which was presented at a national ASCE meeting, associated a roughness value (e) of 0.0004-feet to a “C” value of 140 for water flowing at 5 ft/sec in a 24-inch diameter pipe. Others have published roughness values of 0.000005-feet, 0.0001-feet, and “smooth pipe law applies” for centrifugally spun cement-mortar linings. Rather than using laboratory surface measurements or short-term flow test results, the roughness can be back-calculated using the Hazen-Williams “C” coefficient of 140 which has been established by many long-term flow tests conducted by DIPRA. However, when (e) is correlated to a Hazen-Williams “C” value, it varies with both the pipe diameter and flow velocity. Generally, (e) increases with increasing pipe diameter when holding velocity constant and decreases with increasing velocity when holding pipe diameter constant.
Back-calculating (e), using a velocity of 2.5 ft/sec and a Hazen-Williams “C” value of 140, yields values that range from approximately 0.0002-feet for 4-inch diameter pipe to 0.0006-feet for 64-inch diameter pipe. The relationship between (e) and pipe diameter is not quite linear and will vary with velocity.
Q: When did cement-mortar linings for iron pipe become prevalent, and should I be concerned about tuberculation of older iron pipe in my system that is not cement-mortar lined?
A: The first cement-lined Gray Iron pipe was installed in the water distribution system of Charleston, South Carolina in 1922. Since then, many improvements have been made in the production of cement-lined iron pipe. In 1929, ASA Sectional Committee A21 on Cast Iron Pipe issued a tentative standard for cement-mortar linings. This standard was published by AWWA as a tentative standard in 1932. After various revisions and refinements, it was officially adopted by ASA in 1939 under the designation of A21.4 (AWWA C104) “Specifications for Cement-Mortar Lining for Cast Iron Pipe and Fittings.” By the early 1950s cement-mortar linings were normally supplied with Gray Iron pipe. Cement-mortar-lined pipes are centrifugally lined at the factory to assure that the best possible quality control is maintained and that a uniform thickness of mortar is distributed throughout the entire length of the pipe.
Most of the Gray Iron pipe sold for waterworks service before the 1950s was provided with a hot-dip bituminous lining and coating. In those systems where the water was relatively hard and slightly alkaline, bituminous linings were generally satisfactory. Where soft-acid waters were encountered, however, problems occurred – such as the water being red or rusty and/or a gradual reduction in the flow rate through the pipe.
Today, such tuberculated Gray Iron pipe mains are readily-cleaned and cement-mortar lined in place. Water mains can be cleaned in place in a water distribution system by flushing or by using hydraulic-jets, fluid-propelled devices (pigging), and metal scrapers. AWWA C602 Standard for “Cement-Mortar Lining of Water Pipelines In Place – 4 in. and Larger” covers cement-mortar lining of pipelines insitu.
Q: What is the maximum recommended flow velocity for cement-mortar-lined Ductile Iron Pipe?
A: Normally, the maximum recommended flow velocity for cement-mortar-lined Ductile Iron Pipe is dependent on abrasion. Parameters involved in the abrasion phenomenon include flow velocity; the amount of solid particles; the size, shape, and hardness of the particles; the type of flow (turbulent or laminar); surface roughness and hardness of the lining; and the number of fittings per mile. Although the relative influence of these factors can be reasonably appreciated, there is no known equation able to predict abrasion resistance of different pipe materials in various situations. Inevitably, abrasion will occur at locations of changes in direction before it will occur along the length of a pipe barrel.
The abrasive characteristics of potable water are slight since this type of water contains limited amounts of solids and normally has velocities ranging from 2 to 10 fps. Cement-mortar-lined pipes in drinking water service for more than 77 years show no evidence of internal abrasion. In the absence of long term laboratory testing, the available literature lists satisfactory performance for cement/cement-mortar linings for potable water with velocities of 20 to 40 fps. However, one has to realize that all installations do not perform the same. Different installations will have different configurations, bend angles, flow characteristics, amount and shape of solids content in the water, etc. Using a velocity of 20 fps and applying a safety factor of 2, remembering that the kinetic energy of a particle is a function of the square of the velocity, will result in a velocity of 14 fps. This should normally be a good conservative maximum design velocity for continuous service for most applications. Please contact DIPRA member companies when velocities greater than 14 fps are anticipated.
Cement-mortar linings’ resistance to abrasion is more important in drainage and sewage pipelines where solid particles are present. In these applications, the size, shape, and hardness of the particles will greatly influence the abrasion rate. Again, cement-mortar lined pipe continues to perform satisfactorily in this type of service.
Q: If I specify ANSI/AWWA C151/A21.51 pipe, will it come standard with a cement-mortar lining or do I also have to specify that it be lined?
A: No, you do not need to specify that the pipe be lined although that was the case prior to ANSI/AWWA C151/A21.51-96. In the 1996 revision, Section 4.3.2 reads “Cement-mortar linings. The lining for use under normal conditions shall be a cement-mortar lining in accordance with the latest revision of ANSI/AWWA C104/A21.4, unless otherwise specified.” Also, the foreword of the standard has been revised under Section IIIA, “Purchaser Options and Alternatives,” to list the elimination of cement-mortar linings as an item that should be covered in the purchaser’s specification. Therefore, unless otherwise specified, pipe ordered in accordance with ANSI/AWWA C151/A21.51 will be supplied with a cement-mortar lining. However, if cement-mortar linings are desired by the purchaser, it is still good practice to specify that cement-mortar linings are to be supplied in accordance with the latest revision of ANSI/AWWA C104/A21.4, “Cement-Mortar Lining for Ductile-Iron Pipe and Fittings for Water.”
Q: What is the friction loss in C110 vs C153 fittings?
A: There is very limited data on the friction loss of C110 vs C153 fittings. Data that is available indicates that they are approximately the same. In general, C110 fittings have a larger bend radius than C153 fittings; however, C153 fittings have a larger inside diameter than C110 fittings. It is believed that for engineering calculations, these two differences can be considered off-setting and their friction loss considered the same. Because fittings normally result in a small percentage of the friction loss in a piping system, any error that may exist from this assumption should not affect the design or operation of the system.
Q: What Hazen-Williams flow coefficient “C” should be used for design of cement-mortar-lined Ductile Iron Pipe?
A: DIPRA and its predecessor, CIPRA, have long advocated a Hazen-Williams “C” value of 140 for use with cement-lined Cast and Ductile Iron Pipe. This recommendation of a “C” value for design purposes is sound. Over the years, DIPRA has conducted flow tests on cement-lined Cast and Ductile Iron Pipes in operational pipelines — new pipelines as well as those that have been in service for extended periods of time. These field tests have shown a “C” value of 140 to be realistic, and one that is maintained over time — even when transporting aggressive water. For additional information on this and related topics, request the following DIPRA brochures: “Cement-Mortar Linings for Ductile Iron Pipe” and “Hydraulic Analysis of Ductile Iron Pipe.”
