• Friction factor, which is an empirical measure of the resistance to flow by a pipe or duct.

• Equivalent Length of a pipe segment, which characterizes bends and fittings as an equivalent length of straight pipe.

• Pressure drop due to friction for liquids and gases, both isothermal and adiabatic.

• Restriction orifices.

• Control valves.

• Two-phase flow.

• What is the required flow range, now and in the future? Plant systems often operate at a range of flows, due to production rate variances, start-up differences, multiple uses of the same system (such as production followed by flushing and cleaning), and future de-bottlenecking. Ensure that all of the components, such as control valves, account for these variations.

• How might the chemical composition and temperature change? Again, different production scenarios may result in wide variances in chemicals in use, and in temperatures. It may be prudent to create material and energy balances for different scenarios, and perform fluid flow calculations on several of these. Gaseous systems are especially sensitive to composition and temperature changes.

• Are the piping specifications fixed, or is there flexibility to optimize material selections? Construction materials may affect the pressure drop calculations due to differences in their surface roughness and dimensions. Specified wall thicknesses may be adjusted to save money (thinner walls reduce initial capital cost and potentially operating cost since the pressure drop will be lower at otherwise identical flow conditions).

• What are the status of flow diagrams, piping and instrumentation diagrams (P&IDs), general arrangement drawings, and elevation drawings? Each of these are needed for a thorough analysis. But preliminary results from fluid flow calculations may be needed to complete the drawing.

Design Documents

Block flow diagram (minimum requirement)

Process Flow Diagram (PFD) (highly recommended)

P&ID (desirable)

General Arrangement drawing with major equipment from the P&IDs shown, in scale, in plan and elevation

Piping specifications. Minimum requirements are materials of construction, pressure and temperature limits, and dimensional specifications (e.g., acceptable diameters, wall schedule vs. diameter, long or short elbow radius).

Material and Energy Balance

For each pipe in the analysis, the material composition, flow, and temperature are needed. Various scenarios might be needed.

Physical Properties of the Materials

For liquids, the density, coefficient of thermal expansion, and viscosity at flowing temperature(s) are required.

For gases, provide molecular weight, ratio of specific heats (Cp/Cv), and compressibility factor, Z. Z is needed at high pressures and/or low temperatures. Chapter 27 discusses the determination of Z.

1. List the pipes that are included in the analysis. They are usually the lines shown on the P&IDs.

2. Using the General Arrangement drawings, estimate the length of each line and the number of each type of fitting (elbow, tee, etc.). If the drawings are already completed in CAD, and piping is already drawn, then an actual take-off of the piping should be obtainable from the CAD system. For conceptual or preliminary work, a very quick take-off of each pipe is made by roughly measuring the distance from the origin to the destination of the pipe using x-y-z coordinates, then adding a contingency factor of about 25%. See sidebar “Quickly estimating pipe lengths.”

3. If the piping is not yet sized, use velocity to tentatively select pipe diameters.

4. Calculate pressure drop due to friction for selected scenarios. This step is used to determine the optimum size for the pipe during early design work, and to support sizing pumps and control valves during detailed design.

5. Size pumps, fans, and control valves. Iterate through all of the steps as information is developed.

6. Create system curves (see Chapter 5).

7. Update the P&IDs and General Arrangement drawings.

8. Conduct Process Hazards Analyses (PHAs), especially when flammable or toxic chemicals are being processed (see Chapter 20).

Quickly Estimating Pipe Lengths

Here’s a simple way to get started on a project. The objective is to create a list of pipe segments, with size and length for each, which can be used for preliminary calculations and cost estimation.

Example: Refer to the sketch in Figure 1-0.

Figure 1-0

Line 215 length = 2 m + 2.5 m = 4.5 m (ignore liquid level in R-200)

Line 216 length = 6 m + 2.5 m + 6 m + 10 m + 1.5 m for = 26 m

• Low velocity may indicate a larger pipe diameter than is necessary which raises cost.

• Low calculated velocity may result in the pipe running partially full in horizontal runs.

• Low velocity can lead to laminar flow conditions which may promote fouling and will definitely hinder heat transfer (if applicable).

• High velocity may be noisy.

• High velocity can cause damage to the pipe due to erosion.

• Certain components such as check valves and control valves are designed to operate with a specific flow range; manufacturers of the components recommend minimum lengths of pipe of specified diameter upstream and downstream of their device.