After selecting material and valves (discussed previously), the next step is to ensure proper piping layout and design. This process will be iterative, even requiring a return to material selection in some cases, as the physical design can uncover issues not foreseen when looking simply at a schedule or process diagram. The following are a number of important considerations in designing process piping systems:
Piping Layout & Design Considerations for Installation
- Plan a degree of freedom. When fitting skids, tanks, pumps, and other equipment together in the field, it’s inconvenient to find centerlines off by an inch. Plan a pipe route that does not rely on unrealistically precise placement of large equipment with no plan for what happens if the concrete or other mounting surface isn’t perfect.
- When designing and planning pipe routes that may be difficult or impractical to install, consider the contractor who must build the pipe system.
- Plan clean routes. Not only do mechanical contractors need to connect the pipe system, they also need to provide adequate support. A straight and organized piping system is easier, faster, and cheaper to build and support. Straight runs are cheaper than elbows. Joints are time consuming no matter what the material.
- Measure twice, cut once. Appearance should matter to all involved.
- Place valves where they can be easily reached to avoid situations in which operations staff are inconvenienced by pipe location or other equipment.
- Once assembled, can the system be disassembled or removed for servicing? For example, if a valve were threaded between two parallel pipes, and then long runs of pipe on either side were installed, all the pipe would have to be taken apart just to unthread that valve. Unions or flanges should be used whenever possible.
- Consider how piping will drain when opened for servicing.
- Build manageable pipe sections. The cost of two extra flanges or a union may be well worth it considering the difficulty of assembly of complicated systems. Also, consider maintenance requirements for that pipe; removable sections facilitate any required changes and save time and money.
- Consider what happens if a valve fails, a tank overflows, or a syphon starts.
- Consider the suction requirements of pumps and design suction lines appropriately. All pumps are susceptible to cavitation, which has important consequences.
- Pipe friction matters. Need a drain to keep up with the inflow? Consider the head required to drive the required flow.
- Keep a certain distance between a pump discharge and check valve. A pump discharge may be much smaller than the pipe it is connected to. This leads to high velocity, which can be fatal for a check valve. For a typical centrifugal pump, it is best to use an expander to go up to the right pipe size and then install a check valve. This protects the check valve internals from high velocity flow. If the check valve is installed too close to the discharge, the internals may be damaged or missing entirely after they’ve been in service.
- If there are plans for future equipment additions or piping system expansion, consider ending headers with a flange instead of a cap.
- Pipes may need to be insulated to prevent heat transfer or formation of condensation. However, remember that insulation does not equate to freeze proofing. What happens when a process is left inactive during freezing temperatures? Heat tracing outdoor lines can be an important safety measure.
There are numerous types of pipe joints — selecting the best one for each application requires careful consideration. The following are some tips to help avoid costly mistakes on three of the most commonly used joint types:
- Threaded joints, such as NPT, are widely used and can be a great long-term reliable solution if applied correctly. Since they do not use elastomers to seal, they are more likely to develop leaks for a variety of reasons, from improper cutting of threads; lack of or incompatible sealant; or deformation of threads due to stress, expansion, and contraction or even material failure from overtightening, as is common when threading metals into plastics. Consider the media being utilized. When working with NaOH or NaOCl, it is common practice to never use a threaded fitting. Many chemicals have a tendency to weep through threads and form scale that never stops growing.
- Flanges are typically very reliable and easy to seal with the appropriate gasket yet, if used improperly, can cause numerous problems. Flanges require the pipe and bolt holes to align properly. When possible, at least one rotating flange ring on a joint should be used to allow a degree of adjustability to the joint for installation. This is especially true if a valve or other device is to be bolted in between the flanges. Most pumps will have a fixed flange ring cast as part of the volute or housing, so it is very helpful to have a rotating flange ring, such as a slip-on or van-stone style, to allow for rotational adjustments.
- Grooved pipe fittings can be very useful, but like any other joint, they need to be properly installed. Grooved couplings to pull pipes together or into alignment should always be avoided. Like all joints, when the coupling or bolts are removed, the pipe should remain in alignment on its own.
All piping must be held in place, whether on the floor or hanging from the trusses. Planning piping support is equally as important as piping routing. The following are some items to consider for planning supports:
- Structural steel (or other support material) should support the piping, not the other way around.
- Pipe hangers may hold the static weight of the flooded piping, but fluids flowing and stopping inside the pipe will impose dynamic loads and can cause it to move laterally if not properly restrained. This puts stress on the pipe and joints which in extreme cases can cause failures.
- Appropriate supports must be used. Never assume that a two-piece strap on a vertical pipe is supporting the weight of that pipe. It may keep the pipe against the wall and support the weight short-term, but it may also slide down over time, particularly where there are vibrations. Plan proper load-supporting clamps, especially in cases where non-metallic pipe straps are used for vertical support.
- In many cases, it is unrealistic to expect that equipment and piping will be held perfectly still. In these cases, having joint flexibility built into the piping (by using bellows, for example) can allow some movement without compromising rigid joints or welds.
- Proper pipe restraints are critical when using a pulsating pump, such as a piston or diaphragm type. Connecting the pump to piping by a hose and installing a pulsation damper will reduce surge and pipe movement but won’t stop it entirely.
- Consider thermal expansion impacts.
- When properly built, piping should not need the flange bolts to hold the right alignment. Avoid clamps and bolts to pull pipes into alignment, as this can cause cracks and failures.
- Drains need supports too. Since they are typically empty, drains may be lightweight and hold straight. This will not, however, be the case if a clog forms and the pipe fills with liquid.
- Avoid using flanges as a support point, even though they are convenient bolt holes with heavy-duty bolts. Mixing a mechanical support with a pressure rated joint could have serious consequences when maintenance on the support or tampering with the joint is required.
 Fluid mechanics and piping systems pressure drop calculations are core to piping sizing and design. Friction loss in piping systems is dependent on pipe diameter and material, and fluid density, viscosity, and velocity. This aspect of piping systems design is not addressed in this article.
Integrated Technologies, Inc. is an industry-leading engineering, design, and consulting solutions firm based in Burlington, VT. We offer project planning and development, full-service engineering and design, project and construction management, and services during construction to the surfacing finishing and industrial manufacturing industries.
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