True or False (T or F) A duplex wastewater pump station has two pumps. In South

Question

True or False (T or F) A duplex wastewater pump station has two pumps. In South Carolina, these pumps typically are arranged so they alternate lead and lag status. This means a wet well has at least three pump operation switches: lead pump on, lag pump on, and pumps off. By analogy, a simplex pump station has one pump and will have two pump operation switches: pump on and pump off. A rigid pipe can support the soil overburden and surface dead and live loads without deforming or bending. On the other hand, a flexible pipe deforms, e., changes shape, and /or bends longitudinally to transmit overburden and surface and dead loads to the surrounding soil. For South Carolina applications, the most common rigid pipes are concrete and corrugated metal. The most common flexible pipes are plastic (PVC and HDPE (high density polyethylene). When two or more pumps are connected in parallel at a pump station, the discharge is increased but the pressure head remains the same as with a single pump. Adding a second pump in parallel doubles the discharge from a pump station. Water flows downhill, meaning it flows from a region with higher EGL (region A) to a region of lower EGL (region B). even though region B may be at higher elevation with respect to mean sea level (z term in the modified Bemoulli equation) Cavitation in a pump occurs when the absolute pressure is less than the vapor pressure of water. The water vaporizes and forms small bubbles that collapse when they reach a region of higher pressure in the pump. This results in vibrations that can damage the pump. When two or more pumps are connected in series in a pipeline, the pressure head is increased but the discharge remains the same as with a single pump. The net positive suction head (NPSH) is the total absolute head above the vapor pressure of the fluid at the eye of the impeller I. There are two primary equations for head (energy) loss to friction in pipeflow, the Darcy-Weisbach and Hazen-Williams equations. They can be written in generic form with the respective coefficients and exponents as follow 8f L .73L The Hazen-Williams coefficient is in US Customary (English) units. Determine the flow, Q, in gpm through a 3200-ft of 6-inch, horizontal wrought iron pipe if the pressure drop is 12 psi. Solve this problem using both head loss equations. Did you get the same answer with both equations? If not, what do you think may bo the reason? 2. Not al engineers become designers; some work for regulatory agencies and review permit applications involving proposed designs. In this capacity, the reviewer often must verify the calculations by the design engineer. For this problem you are the reviewer and have been given the following solution for a piping system involving 4 parallel pipes. It is suspected the flow for one of the parallel pipes was calculated incorrectly and therefore the estimate of the total system flow is wrong. (a) State the two governing constraints, or properties, for this type of pipe flow problem. Ans: 1 b) For which pipe was the flow computed incorrectly? (c) What is the correct flow for that pipe? (d) What is the correct flow for the total system? Ans: Pipe Number Ans: Corect Flow Ans: System Flow- The following table gives data for the parallel pipe system. Flow rates are rounded to the nearest integer value. Pipe Hazen-WilliamsQ No 591 506

Explanation / Answer

1) True. the duplex waste water pump station has three opeartion swithches.

2) True. Rigid pipes show signs of distress before being vertically deflected 2%. A flexible pipe is the one that will deflect at least 2% without stuctural distress. After applying overburden pressure it will deflect more.

3)True. pumps connected in parallel , Q=Q1+Q2. But H1=H2

4)False. EGL should be more.

5)True.

6) False. twopumps connected in series, Q=Q1+Q2, and H=H1+H2

7)False. NPSH is below the vapor pressure of the fluid.

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