The flow system as shown in the Figure, the centrifugal pump(3500 R.P.M.) is delivering water at 100 gal/min and 70 °F into a boiler operating at a pressure of 35 psig. The water levels in the feed tank and boiler are constant. Because corrosion is expected to be negligible, select the suitable material to be used for this purpose. Assume frictional pressure drop of 5 psi across the heat exchanger. The following data are available:
Note:
(a) For NPSH required:
Total Head (ft.)
15 18 20 25 30 40
Flow rate, (gal/min)
25 30 85 105 120 155
(b) Use the physical properties of water from textbooks.
Find:
1. The suction and discharge pipe sizes
2. The head the pump must deliver
3. The break horsepower
4. The electric-motor horsepower
5. Draw the required curves
6. If cavitation will occur in the pump
This sounds like a nice engineering project. Do you have any questions about it?
To answer the given question, we need to follow a step-by-step approach. Let's go through each part and find the answers:
1. The suction and discharge pipe sizes:
To determine the appropriate pipe sizes for the suction and discharge pipes, we need to consider the flow rate and the frictional pressure drop. Calculate the velocity of water through the pipes using the formula: Velocity = Flow rate / Cross-sectional area. Decide a suitable velocity value based on acceptable design criteria, typically between 3-10 ft/s, and calculate the cross-sectional area of pipes accordingly. Once the area is known, use standard pipe sizing charts or formulas to determine the appropriate pipe sizes.
2. The head the pump must deliver:
The head the pump must deliver can be calculated using the following formula:
Head = (Discharge pressure - Suction pressure) + Frictional pressure drop + Elevation pressure difference.
In this case, the discharge pressure is given as 35 psig, the suction pressure is atmospheric pressure, the frictional pressure drop is given as 5 psi, and the elevation pressure difference can be assumed to be negligible unless specified otherwise.
3. The break horsepower:
To calculate the break horsepower, use the following formula:
Break horsepower = (Flow rate * Head) / (3,960 * Pump efficiency).
Note: The pump efficiency value needs to be known or assumed based on the pump type and specific characteristics.
4. The electric-motor horsepower:
The electric-motor horsepower required is the same as the break horsepower since the motor needs to deliver the same power as the pump.
5. Draw the required curves:
To draw the required curves (such as pump performance curve or system curve), we need to understand the pump's characteristics, such as head-flow relationship and efficiency at different operating points. With the given data, we can plot a pump performance curve showing head vs. flow rate. Additionally, we can plot the system curve, representing the frictional pressure drop and elevation pressure difference as a function of flow rate.
6. Determining if cavitation will occur in the pump:
To determine if cavitation will occur in the pump, we need to compare the available Net Positive Suction Head (NPSH) with the required NPSH for the pump. From the given data, we have NPSH required values at different flow rates. To calculate the available NPSH, consider factors such as suction pressure, elevation, frictional pressure drop, and velocity head. If the available NPSH is greater than the required NPSH, cavitation should not occur. However, if the available NPSH is lower than the required NPSH, cavitation may occur, and steps should be taken to address it, such as adjusting pump speed or modifying the system design.
Remember, the specific calculations can be performed using the given data and equations relevant to fluid mechanics and pump performance.