Process hydraulics

Authors: Thomas Considine, Sean Kelton, Michael Gleeson

Introduction

The transportation and storage of fluids is essential to a chemical process plant. Piping, valves, pumps and compressors comprise the major components of fluid handling equipment. The goal of process hydraulics in a design setting is to overcome frictional losses in piping and equipment, provide correct operating conditions, and overall assist in the controls of the plant. All three objectives must be design in concert, and before the final controls system is designed. (Towler, 1207).

Hydraulic systems & Pressure drop

Overall pressure drops created by pumps and compressors must also include those created by the connecting pipes. These components must be designed in concert, to account for changes in elevation and friction losses in the pipe.

Total Pressure Drop

Pressure drops throughout the flow of a fluid can be summed to find the overall pressure drop of a defined system. For example: If a fluid A, initially at zero gauge pressure, is pumped to a pressure of 300 kPa, then flows through 10 meters of pipe resulting in a loss of 50 kPa, the final gauge pressure at the end of the pipe is 250 kPa. This type of analysis is useful when designing pressure systems over many components.

Pressure Drop in Pipes

When designing pumps and compressors, the loss of pressure due to piping is not negligible, and must be appropriately accounted for (Turton, 537). The pressure change in pressure $P$ across a pipe is calculated as follows:

$P=(4*c*L/d)*(rho*v^{2}/2)$

where $c,L,d,rho$ and $v$ are specific coefficient (typically 0.005 for turbulent flows), the length of piping, the diameter of piping, the density of the fluid, and the velocity of the fluid.

An added term accounting for the pressure difference due to height is also necessary if there is a change in elevation.

Additionally, the first term in the equation can be altered to include an additional factor:

$(n+4*c*L/d)$

which accounts for piping bends, restrictions, and other variables.

Heuristics

Both the process hydraulics and the economics of a system is affected by pipe sizing (Peters, 500). Heuristics, or "Rules-of-thumbs" have been developed to assist in optimizing pipe selection. While more detailed optimization techniques are available and commonly used, the rules of thumb provide a good starting point for pipe selection.

Suggested pipe velocities, in ft/s, for gases, liquids, and super-heated steam are approximately 60-100, 6, and 150, respectively (Towler Presentation, 9). Additionally, for liquid flow, the following equation provides a rule-of-thumb for optimal pipe diameter, in inches:

$D=sqrt{Flow/10}$

Pumps & Compressors

TJ: Check out (Seider, 132) (the dark grey book) for a pumps/compressor rule of thumb