How to calculate the suction pressure of a pump

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How to calculate the suction pressure of a pump
Operating pumps move fluids in piping systems by creating low suction pressure at the inlet side and high discharge pressure at the outlet side. (water pump image by nutech21 from Fotolia.com)

Operating pumps move fluids in piping systems by creating a low suction pressure at the inlet side and a high discharge pressure at the outlet side. You can calculate suction pressure expressed in feet for a water distribution system in the "United States Customary System Units" using suction pressure definition. Total suction pressure (hs) in feet equals static pressure (hss) in feet plus surface pressure (hps) in feet minus the vapour pressure (hvps) in feet minus the friction pressure in the piping, valves and fittings (hsf) in feet. Values for "hvps" depend on whether the region above the fluid's surface in the tank on the suction side of the pump is open to the atmosphere, pressurised, or is a vacuum.

Skill level:
Easy

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Things you need

  • Calculator
  • Hydraulic Institute Engineering Data Book
  • Vapour pressure chart

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Instructions

  1. 1

    Obtain the proper information to make the calculation. Since you are using an equation, you need to know all the required parameters. Because you can only add or subtract like terms, you must perform all calculation in either "feet of water gauge" or "feet of water absolute." Absolute means that you have added atmospheric pressure (head) to the gauge reading. You, therefore, need to know "hss," "hps," hvps," and "hsf" in feet.

  2. 2

    Calculate the suction surface pressure (hps). If the question provides the value for "hps" in feet, you will use them directly in your calculation of total suction pressure. If the question provides "hps" in another unit other than feet, you can convert the value to feet using an appropriate formulas: (a) the suction surface pressure in feet of liquid equals the suction surface pressure in inches of mercury times 1.133, divide by specific gravity, (b) suction surface pressure in feet of liquid equals the suction surface pressure pounds per square inch times 2.31, divide by specific gravity, and (c) suction surface pressure in feet of liquid equals the suction surface pressure in millimetres of mercury, divide by 22.4, divide by specific gravity. Specific gravity of the liquid changes with temperature, type of fluid, and fluid concentration. For fresh water, specific gravity is 1.0.

  3. 3

    Calculate the static suction pressure (hs). That is the vertical distance in feet between the suction centre line and the suction liquid level in the suction side of the system. "hs" is positive if the liquid level is above centre line and "hs" is negative if the liquid level is below the pump centre line. The centre line is a reference horizontal passing through the pump and the pipe adjoining to the pump.

  4. 4

    Calculate the vapour pressure head (hvps). That is barometric pressure of the suction vessel converted to feet. If the question does not provide the vapour pressure you need for your calculation, you can obtain it from a vapour chart. For example, vapour pressure of water at 68 degree Fahrenheit equals 0.27 pound per square inch. You can convert it to head by multiplying pressure in pounds per square inch times 2.31, divide by the specific gravity with the aid of a calculator to obtain the suction vapour pressure in feet.

  5. 5

    Calculate the suction friction pressure (hfs). Suction friction head, hfs, equals the sum of all the friction losses in the suction line. You will use the k-value equation to calculate "hfs," which states that the pressure drop (h) in feet is equal to the total resistance coefficient (k) of the fitting times velocity head (velocity squared, divide by twice acceleration due to gravity) in feet (ft). If you know total k and velocity (v), calculate "hfs" directly with the aid of a calculator. If you have to compute total k. Make a chart for all fitting types. Use an Excel work sheet to make a chart showing all fittings in your suction line in column one, k value in column two, quantity of fitting type in column three and total k value for each fitting type in column four. For each fitting, obtain its k value from the Hydraulic Institute Engineering Data Book, then multiply it by the quantity of that fitting type in the suction line to obtain a subtotal k. Sum all the subtotals to obtain the total k. Multiply the total k by the velocity head with the aid of calculator to obtain the suction friction pressure in feet. Acceleration due to gravity (g) is a constant and its value is 32.17 feet per second squared.

  6. 6

    Perform the calculation. Calculate suction static pressure plus surface pressure minus the vapour pressure minus the suction friction pressure with the aid of a calculator to obtain total suction pressure in feet of water absolute.

Tips and warnings

  • Process designers achieve pump performance and reliability by maintaining suction and discharge pressures within systems operating specifications.
  • Your calculated suction pressure for a given pump must be greater than suction pressure required for the pump, which engineers established by actual test and vary from pump design to another.
  • Atmospheric pressure in feet equals 14.7 pound per square inch times 2.31 divide by specific gravity
  • To simplify determination of "K," use an Excel spreadsheet to calculate subtotal "K" for each fitting type and add all subtotals to obtain total "K" for all fittings. This is particularly helpful for a complex systems consisting of several of fittings.
  • The result you obtain is suction pressure head; it is not system pressure head. If you want to predict the system pressure head, you will have to subtract total suction pressure head from total discharge pressure head with the aid of a calculator.
  • The k-value method assumes velocity of water (V) in feet per second to be constant throughout the piping system, which can be determined from volumetric flow rate in gallon per minute. This assumption may not be valid if you are using another method, in which case you will have to determine velocity in each section of the system.

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