Pressure changes, pumps, compressors, and piping systems

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·         Pressures

• Unless otherwise stated, all feed, product and waste streams should be at 1 atm.  A combustible product stream from a condenser must be cooled below its flash point, which is normally given in the MSDS for that substance.
• The exit stream from a mixer will be near the minimum pressure of the entering streams.   Thus, if an entering stream has a significantly lower pressure than the other(s), this represents a loss of energy.  (A mixer is normally just a pipe Tee.)
• Do not use zero pressure drops for equipment.  You can obtain rough values for heat exchangers, valves and piping from Walas's heuristics (see Chapter 11 in Turton et al.). You can assume a small pressure drop if you do not have any information.  Use the Ergun equation to have a process simulator (HYSYS, UniSim, Aspen Plus, etc.) calculate the pressure drop in a plug-flow reactor.  The pressure drop in a fluidized-bed reactor is approximately equal to the weight of the particles supported per cross sectional area.  For a liquid feed to a distillation column, set the pressure at the bottom of the column to the feed pressure.  (This is because the nominal feed pressure is for the feed pipe at about the same height as the reboiler.  In rising to the feed tray, the feed pressure drops because of the static head.)
• Show pressure increases only for pumps and compressors and not for other equipment.  For example, it is a very serious error to show a pressure increase for a stream going through a heat exchanger or reactor.
• In distillation, absorption, stripping and extraction columns the pressure must decrease as one goes up the column.  For tray columns, this pressure increase corresponds approximately to the head of liquid on each tray.
• To reduce the pressure of a gas, consider the use of an expander rather than a valve.  This produces electricity that reduces energy costs for the plant (i.e., the plant’s electricity use is reduced and a credit is taken for this in cost estimation).

·         Pumps

• Each recycle loop requires a pump or compressor somewhere in the loop to compensate for pressure drops.
• Set only the inlet T and P and the outlet P.  Let the software calculate the outlet T assuming adiabatic conditions (Q=0).
• When hand calculating the power required by a pump, simply multiply the increase in head (pressure) times the volumetric flow rate, with suitable unit conversions.
• For a condenser located at ground level, estimate the energy requirement for the reflux pump by multiplying the height of the column X the mass flow of the reflux X the acceleration due to gravity.
• If possible, increase the pressure of a stream while it is a liquid rather than when it is a vapor.  For the same pressure increase, compressors are much more expensive and their operating costs are much higher than pumps.
• Gas in a stream being pumped can damage many types of pumps, and so should be avoided.
• To avoid cavitation, make certain the inlet pressure exceeds the required Net Positive Suction Head.
• Pump Handbook, 4th Edition (available free for members)
• Centrifugal pump operation and specification
• “Profiting from your Pumping System (Identify ways to minimize the lifecycle costs of your pumping system),” Chemical Engineering Progress (September 2003) 46-53.

·         Compressors

• Each recycle requires a pump or compressor somewhere in its loop to compensate for pressure drops.
• Set only the inlet T and P and the outlet P.  Let the software calculate the outlet T assuming adiabatic conditions (Q=0).
• To hand calculate the energy required by a compressor, see Section 10 in Perry's.
• If possible, increase the pressure of a stream while it is a liquid rather than when it is a vapor.  For the same pressure increase, compressors are much more expensive and their operating costs are much higher than pumps.
• When a large compression ratio is required, you may save money by doing the compression in stages with heat removal between stages.
• Liquid in a stream being compressed can damage many types of compressors, and so should be avoided.
• Compressor Handbook

·         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)

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·         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.

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

• Compressible Gas Pipe
• Pipe Segment
• Relief Valve

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.

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.