Get Pipe Specifications Right in the Beginning!

Good pipe specifications can avoid costly and dangerous mistakes

By Dirk Willard, Contributing Editor

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My company couldn’t see all the fuss. We had been hired by Bechtel as a subcontractor for the ill-fated Goro project to refine nickel ore in New Caledonia. Bechtel wanted us to match its pipe specifications. To us, this was just another symptom of a bloated project. We used to joke that Bechtel had two engineers for toilets: one for the men’s room and one for the women’s room. Looking back, Bechtel’s approach made sense: get the pipe specifications right or live with a project fraught with headaches — never mind other minor issues, i.e., safety, reliability and profitability.

The first decision in establishing a pipe standard is choosing whether it will be based on function or condition. A function-based specification would be something such as a sewer pipe or vent duct. A condition-based specification would discriminate between sewer pipe for corrosive versus sanitary service. Usually, a condition-based specification is best.

Collecting information is the next step. Pipe specifications often are presented in tables kept in massive volumes. What’s missing is supporting information explaining the basis for decisions like the choice of type 316 stainless steel over type 304L, or selection of Inconel 601 instead of Inconel 600. This omission in company records almost justifies the reluctance I’ve witnessed to challenge pipe specifications.

Once information is collected, you should budget time for careful analysis by a consultant. The idea is to look for discrepancies between the pipe specifications and for areas of improvement. For example, Trevor Kletz, in his book “Process Plants: A Handbook for Inherently Safer Design,” (http://www.amazon.com/Process-Plants-Handbook-Inherently-Engineering/dp/1560326190) suggests eplacing fiber gaskets with spiral-wound gaskets because their leak rate is lower.

With the analysis complete, it’s time for a meeting. Bring together maintenance staff, project engineers, vendors and contractors to pencil in the details of the new pipe specifications. Leave detailed discussion of any contentious points to follow-up meetings. Rely on empirical data!

Sadly, plants seldom use one important source of empirical data — the corrosion coupon, which is a welded strip of metal designed to be inserted into a process. Instead, they allow analysis of the effects of corrosion on parent metal and welds. Coupons, which also can be used for gasket materials in some applications, provide real data well beyond those found in textbooks and from laboratory analysis, which seems artificial by comparison. The downside is exposure time; material evaluation takes months. But, believe me, it’s worth it.

Consider what happened on another project. We had to handle a feed stream of aluminum chloride added to a stream containing wet chlorine gas and vaporized titanium tetrachloride. A world-renowned valve material expert claimed that zirconium oxide would survive our process. Being prudent, I suggested coupons but was overruled because of time constraints. The expert was dead-wrong! What we saw was quickly dubbed “the jawbreaker effect” by our operators. Over about six weeks, the balls in our ball valves shrunk, flaking away from thermal shock. Coupons would have saved us more than $1 million a/year in downtime during the next few years and justified a ball made of a more expensive ceramic.

With all basic facts now in hand, it’s time to schedule follow-up meetings to review the draft pipe specifications. The best format for these specifications consists of a single page summary followed by details, exceptions and references. The details should include gaskets, valves, fasteners (e.g., bolts), special fittings, construction aspects such as weld inspection and paint preparation, etc. If possible, review the specifications with your mechanics. They may want to pass on their wisdom on construction details.

Exceptions may exist, and where they do, they must be clarified and documented. Sometimes exceptions arise because a vendor can’t change a valve specification and an acceptable alternative hasn’t been found yet. In one company, we ordered a PTFE-lined plug valve and modified it to fit an actuator.

Keep the summary table relatively simple. It should include pressure rating; application, i.e., for which chemicals; connections, e.g., threaded or socket-welded; and material of construction. It’s best to isolate the summary table so that it’s compact and usable by those in the field.

References should include the ANSI number and the old pipe specification number it replaced if there is one. Clearly state temperature limits; separately cite test pressures for ambient pressure tests. If you must note vendor information, also try to include acceptable alternatives. Keep your options open with vendors. Saving a little money now on a sole-source contract often isn’t worth the headache later of finding a replacement when a part fails to meet quality standards.

An often-overlooked item is the cross-comparison table, which matches the new specifications against the old ones. This table is crucial for working with old vendors and old inventories.

Pipe specifications are one of the keys to safe plant operation. By not keeping them current or allowing them to become confusing or hard to use, you may encourage deviation from good engineering practices. Make specifications simple and easy to follow.

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