Design & Simulation / Fluid Handling

Produce Proper Pipe Specifications

Include adequate detail and appropriate background information.

By Dirk Willard, Contributing Editor

Break a specification into two parts — engineering and tables.

The pressure to get the ammonia refrigeration pipe specifications done quickly was palpable. However, I’ve learned that, even on rush jobs, being correct is more important than being fast. Getting one complaint about late delivery beats facing never-ending gripes if the work is filled with errors.

I’ve written many pipe specifications. So, let me share some learnings.

First, it’s important to realize the specifications must consider regulations and the desires of the operating company and engineering firm. Generally, give the maintenance staff’s needs — improved reliability and reduced inventory — high priority. After all, they must maintain what you’ve specified.

Second, don’t count on regulations for clear guidance. To prepare my ammonia pipe specifications, I had to sift through several standards for material-specific instructions for valves. Generally, organizations such as the American National Standards Institute, the American Society of Mechanical Engineers (ASME) and the International Institute of Ammonic Refrigeration don’t provide comprehensive details you’ll need for a pipe specification. The Chlorine Institute does a better job of defining valves suitable for chlorine service.

Next, let’s dig into the weeds a little. Take, for example, hoses, a part of everyday plant life. Yet, organizations don’t publish specifications. The State of California provides a comprehensive guide I found useful; it requires a 500-psig test annually. Most companies won’t bother with the risk of poor documentation and just toss the hose.

In the ammonia pipe specification, I accepted nylon as the inner and outer material for hoses smaller than 1½ inches but said larger hose required braided stainless steel for the outer wall. This made sense to me because, for a given wall thickness, larger diameter pipe and hose must defend a bigger area than a smaller hose. You won’t find considerations such as these in regulations or standards.

Another issue is the use of lap flanges. These consist of two parts, a flange and a stub that’s welded onto the pipe. I worry a sloppy workman can compromise connection integrity. While these flanges may be useful in some low pressure applications, they tend to leak when subjected to thermal expansion or vibration. You certainly don’t want them on a 450-psig chlorine railcar. Design engineers should avoid them.

Now, let’s consider the structure of a pipe specification. Break it into two parts — engineering and tables. The engineering section supports the structure of the entire document while the tables section is meant for handing to constructors for use by their fabricators.

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For engineering, provide the following sections: 1) scope; 2) delivery, storage and handling; 3) codes and standards; 4) related documents (process and instrumentation drawings, etc.); and 5) special topics. In addition, if any changes exist from past work at the plant, include a separate engineering design report.

The delivery, storage, and handling section merely should send the reader to a particular specification for ensuring items are delivered to the fabricators intact and in order. Don’t just count on having a specification for this; review it and add to this section if it’s insufficient.

Be as specific as possible with codes and standards. Quote sections and even cite crucial passages. Confirm that each passage applies.

Special topics include ones like corrosion allowances, use of alternative materials (e.g., stainless steel instead of carbon steel), welding practices, incompatible chemicals or procedures, acceptable ASME pressure class and unique inspection requirements.

The table is the heart of the pipe specification. Columns should cover in order: item, notes, size, pressure rating, type of end connection and description. The description should reference an ASTM standard. Items include valves, gaskets, pipe, fittings, tubing, flanges, hose and fasteners (bolts, studs and nuts). If you discourage the use of some items like valves, explicitly say why.

Add copious numbered notes to the bottom of the table. Fabricators must know the reason for the choices in the table. You want to convince them to make the right choices.

Painting and insulation are crucial but keep in mind that ASME B31.5, Paragraph 538.2 requires pressure testing of bare uninsulated pipe. After testing, B31.5, Paragraph 520.1.6 mandates application of a “weather-resistant” coating to all “parts.” However, this doesn’t mean painting flange bolts; don’t do that. Sherwin-Williams recommends greasing the bolts to provide a protective coating.

Don’t think of pipe specifications as mundane boiler plate. It’s your chance to inspire safe construction practices.


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DIRK WILLARD is a Chemical Processing contributing editor. You can email him at dwillard@putman.net.

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