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Pressure changes, pumps, compressors, and piping systems

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·         Pressures

·         Pumps

·         Compressors

·         Piping system design steps

The following is recommended for students to estimate the pressure change between two units.  It is assumed that you have had a course in fluid mechanics.

  1. Decide about how long the piping needs to be, the fittings and valves required, and the change in elevation between the two ends.  Note that except for adjacent units, piping is often elevated to permit people and even vehicles to pass underneath it.  Large condensers for distillation-towers are usually located on the ground for ease of construction and maintenance.  Following are some useful references for piping systems:

·         Section 10 in Perry's Chemical Engineers’ Handbook, 8th edition (available free for AIChE members)

·         Facility Piping Systems Handbook (2nd Edition)

·         Piping Handbook (7th Edition)

·         Valve Selection Handbook

·         Piping Materials Guide

·         Flow Measurement in Engineering Handbook: Ref 681.2 M649f2

·         Encyclopedia of Fluid Mechanics: Ref 620.106 E5

  1. Determine the vapor-phase fraction, mass flow rate, volumetric flow rate, density r and viscosity of the stream.  In HYSYS/UniSim, double-click on the stream.  Note that the “Liq Vol Flow @Std Cond” on the Worksheet Conditions page is NOT the volumetric flow rate.  Use the “Act. Volume Flow” on the Properties page.  The methods described below are most accurate for a stream that is 100% liquid, Newtonian, and in the turbulent flow regime.  Special methods are required for highly non-Newtonian flow, compressible flow with a pressure drop large compared to the total pressure, and multiphase flow.
  2. Estimate the economic pipe diameter.  Use these results to select an internal diameter for the type of piping to be used (see, for example, Perry's Chemical Engineers’ Handbook, 8th edition; available free for AIChE members), for the following calculations.
  3. Calculate the Reynolds number.  Is the flow turbulent or laminar?  (Usual industrial practice is turbulent flow.)
  4. Calculate the pressure drop (head loss) due to friction:

a.    Determine the pressure drop for straight pipe by using one or more of the following:

·         On-line calculator for incompressible flow. 

·         Nomograph from Perry’s 5th edition (right click and Save Target As to download to your computer).  Multiply by length.

·         In section 6 in Perry's Chemical Engineers’ Handbook, 8th edition (available free for AIChE members).  Multiply by length.

·         Your fluid dynamics text.

·         The HYSYS/UniSim Pipe Sizing utility for calculating pressure drop per unit length.  The stream must have already been entered into HYSYS/UniSim.  Then go to Tools/ Utilities and select Pipe Sizing.  Enter the economic pipe diameter from 3 above. 

b.    Determine the pressure drop for valves and fittings using one or both of the methods below:

·         Equivalent pipe length method.  Use either Table 3.3.11 in Marks’ Standard Handbook for Mechanical Engineers, 11th Edition or Section 6 in in Perry's Chemical Engineers’ Handbook, 8th edition (available free for AIChE members) to get the equivalent pipe length for each valve and fitting.  Add these and multiply by the pressure drop per length of straight pipe from 4.a. above.

·         Velocity head loss method.  Use Section 6 in in Perry's Chemical Engineers’ Handbook, 8th edition (available free for AIChE members) to get the fraction of the velocity head (V2/2) lost by each fitting and valve.  Add these and multiply by V2/2, where V is the velocity of the fluid flow (volumetric flow divided by pipe cross-sectional area).

c.    Add the pressure drop for straight pipe to that for fittings and valves. 

d.    Add the pressure drop (or increase) due to the elevation change h between inlet and outlet, i.e. DPh = rgh, where g is the acceleration due to gravity and NOT the conversion factor between mass and force   Take care with units (see, for example, common conversion factors for units).   Note that there is NO elevation change for a loop, because the starting and ending points are the same!

e.    Check to make certain that the inlet pressure to the pump is sufficient to avoid cavitation as required for the pump’s Net Positive Suction Head.  If not, lower the elevation at the inlet and/or lower the temperature of the liquid.

Additional HYSYS tools

Either use F12 or Flowsheet, Add Operation.  Under Piping Equipment you will find:

UniSim Process Pipeline Manager

The UniSim Heat Exchanger suite includes the Process Pipeline Manager (PPL), which has nothing to do with heat transfer unless you want to account for heat transfer with the environment.  PPL includes the UniSim components and properties estimator database.  See Help or Documents for information on how to use it.  PPL does pipeline calculations between any two points for mixtures that are all vapor, all liquid, or mixed vapor-liquid (“quality” is vapor fraction).  You can include elevation changes and various fittings to calculate two of the following three given the other two: pressure drop, flow rate, pipe diameter.

 

Distillation

 

Last updated September 15, 2012.   Please submit all questions, comments and suggestions to W.R. Wilcox

 

Disclaimer: The material on these pages is intended for instructional purposes by Clarkson University students only.  Neither Clarkson University nor Professor Wilcox are responsible for problems caused by using this information.

 

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