Relationship Between Velocity and Performance/Costing of the Piping System

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Is there any study on relationships between velocity (v= 0.6 to 2m/s) and the performance/costing of the piping system including pumps? For example: if the client is insisting only on 0.6m velocity in MS pipe and economical dia works out with higher velocity.

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8 Answers

  1. Dear Pramod,

     

    To do a detailed estimation, this estimation must be done using the Total Owner Cost that include all the items that could generate any cost durint the project life (cost of the system, operation, maintenance, others).

     

    In this case you must evaluate the cost of any pipe (diameter), pump, electrical component, accessories and others as its operation, maintenance cost, and any other cost. All this must be projected over the time and financial analysis and then compared with the disponible aternatives to chose the best one.

     

    The water velocity is based on the minumum velocity to dont let any solid precipitation and the maximum velocity to rech the best cost-effective performance.

     

    Remember that the drop pressure in a ratio with the square velocity, and it is rated with the cost of the enerrgy to pum the liquid.

     

    You could not use the same velocity in a 15 m pipe to feed 1 l/s steam boiler generator that works at 10 Bar than the used in 14 km pipe to transport 1000 l/s with 1 m H2O in level difference.

     

    Regards,

     

    Orlando D. Gutiérrez Coronado

  2. In case of adoption of a pipe of size other than the required size ,calculated  based on economical sizeof rising main which takes into consideration  ,capitalised cost of pipe  and it’s related energy requirement ,the Capital cost of pipe will be high, if a higher size is adopted. Besides this, settlement of solids in the liquid pumped, will be more resulting in increased maintenance cost ,if velocity is less than required self cleaning velocity.

  3. The relationship between pipe size and pumping cost involves many variables including: future demands, network configuration, energy costs, pump efficiency, power factors and time of day discounts. The free water system hydraulic model TdhNet (tdhnet.com) can be used to model and provide information on all these factors and present pumping cost results in tabular format and charts. 

  4. The condition of the inner wall of the pipe, angle and numbers of joints, most importantly the distance of each dynamics and elements not only influence but are factors that could impeach on the performance of any pipeline transportation system as the pressure and sound transmitted  are key to the accompanying sensor mechanisms (UMEADI, 2008).  

  5. All arguments mentioned are valid, but keep in mind that the pipe diameter is not the only factor affecting the head loss. Smoothness of the inside of the pipe can vary much, extremes between like concrete and stainless steel, have a bigger influence at higher velocities. Also you need to take into account the number and type of curves, valves, reduction sections and T's. A wide curve creates less loss, than a knee, a saddle type valve more than a butterfly valve, etc. And these losses are more than linear at higher flow velocity.

    As a general rule, for clean water v should not exceed 3 m/s.

  6. Velocity v = Volumetric flow rate Q ÷ pipe cross-sectional area A. For the pressure, calculate your total head loss & required delivery pressure. Once you have these two you can check the pump curve to size the pump. Normal practice is to chose the nearest larger size pipe diameter to reduce head loss. This is especially applicable if v = 0.6m/s is the maximum velocity your client will accept. If it's the minimum velocity then choose the next smaller pipe size, but ensure your head loss is acceptable

  7. What I'd suggest would be to use the length of the pipe and determine the pressure loss at both pipe diameters, remember to adjust the friction factor for the velocity and diameter change. Then take the differential pressure between the two pipe diameters and back calculate the BHP to flow the fluid using the delta P as the TDH. Assume a pump efficiency of 70% (or whatever the actual pump is) and a 94% efficient motor to convert the BHP to KWH. Then multiply the KWH by the electric cost per KWH. That will give you the differential cost to operate the line.

    BHP = GPM * TDH /( 3960 * EffPump * EffMotor) (English Units)

    KWH = BHP * 0.746

    The additional cost to pump the liquid with a smaller diameter pipe will tell you if it is more economical. Naturally, you'll have to factor in the increased cost of larger pipe and any increased installation costs but this method will give you a good idea of which is more economical.

    If you think of this as what's the cost to pump the fluid at the differential pressure between the two pressure losses is the amount that you're going to save using the larger diameter pipe.

    Keep in mind that recommended pipe size is a generality and may not fit your application.

    P.S. An easy way to calculate velocity of water (English units): v = 0.408 * GPM / Dia^2

    v in Ft/Sec, Q in Gal/Min, Dia in inches

    2 Comments

    2. Historically, the specification on water velocity has been primarily to prevent settling of solids in gravity flow situations.  The recommended minimum velocity for sewers is around 2' second which is about 0.6 m/ second.  Lower velocities don't make sense unless one is looking at overall demand.

      You might want to consider the "shower flush syndrome" when one is scalded or frozen with a blast of off temperature water when the system demand hogs all the hot or cold water, as when you are in the shower and the dish washer kicks in and demands hot water, or alternatively when the toilet is flushed when you are showering.  

      It's a question of head losses and demands.  Frankly, I'd rather use larger pipe sizes to reduce the fluctuations due to the instataneous demands.

       

  8. It will depends a lot of the application. You need more data to make a decision about water velocity! 

    1 Comment