Design & Simulation / Fluid Handling / Ground

Challenge Pipe Specifications

Opting for alternative materials or designs may offer significant benefits.

By Dirk Willard, Contributing Editor

Ineos hired Organic Technologies (OT) of Coshocton, Ohio, to toll manufacture synthetic lubrication oil. I think part of the reason why Ineos’ engineers visited during commissioning was to see how the toller undercut the competition by a quarter. As a consultant hired by OT to help with the campaign, I was curious myself.

There are many ways to safely cut corners.

OT’s secret, or at least one of its secrets, was tubing: the company used 1-in.-dia. tubing for the process pipe. Strong, easy to assemble, and ideal for small processes, i.e., ones below 25 gpm, the tubing significantly speeded up construction; it only took days instead of weeks. No welding was required, which meant no qualification tests for welders. OT didn’t even need isometrics because the whole thing would be torn down after the campaign. This is but one example of safely cutting corners in pipe specifications.

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Let’s consider some others.

First, check if fiberglass pipe can handle the service. Such pipe doesn’t require welding, is easily cut and glued, and stands up to most chemicals. I reckon the total construction cost — i.e., materials, labor and profit — of a 2-in.-dia. fiberglass pipe is 46% of a schedule-40 stainless steel one. Some sources give less-favorable material cost comparisons, sometimes even showing stainless steel cheaper than fiberglass. However, they don’t consider construction and lifetime costs.

Part of this lifetime savings comes from reduced friction losses — and, thus, lower pumping costs. For new pipe, fiberglass can cut energy costs 25% versus steel. When replacing old steel pipe, the savings could reach up to 200%. This might allow a designer to use a smaller line size for fiberglass compared to steel, thereby making construction easier or reducing construction costs further. However, under no circumstances use fiberglass pipe less than 1½ in. in diameter because it’s difficult to prevent glue globs from restricting flow at small fittings.

Limit ordinary fiberglass pipe to operating temperatures no higher than 150°F, pressure below 200 psig and vacuum no more than 500 torr abs. Some high-temperature resins can handle services of 250°F or more.

Other plastic pipe such as PVC and CPVC can offer alluring savings compared to steel but at the expense of lower operating temperatures and pressure. Never use PVC if the maximum operating temperature can rise above 140°F. Instead, consider CPVC because it can operate up to 200°F at an allowable pressure of 50 psig for 2-in. schedule-40 pipe. Hybrid materials, such as CPVC wrapped with fiberglass, can provide higher temperature and pressure capabilities. (For more on using CPVC piping, see “Put CPVC Piping in Its Place.”)

You can cut costs in construction in other ways besides choosing plastic pipe. Why not avoid heat tracing and insulation by burying pipe? This could raise a red flag with environmental regulators — but there are ways to construct reliable double-pipe systems that avoid costs and spill problems.

Here’s another thought. For product isolation, use a double-isolation-and-bleed (DIB) valve with internal relief instead of two double-block-and-bleed (DBB) valves. A DIB valve is more compact; you’re buying two valves in one. It utilizes bidirectional seats that provide isolation, without contamination, from either direction.

DBB valves typically have unidirectional seats and, so, provide isolation only from one side, allowing contamination when the body cavity relieves to the low-pressure side. If a flammable, volatile liquid is present, this could pose a safety issue. If a heated liquid is present, this could result in seal rupture if the liquid fouls or thermal relief is insufficient. (For more about DIB and DBB valves, see: http://goo.gl/YYqcNs; and http://goo.gl/n9mN7X.)

Early in my career I ran into a dilemma over material standards. Millennium Inorganic Chemicals required a nickel/copper alloy for control valves used in pure oxygen service. It turns out that Union Carbide blew up an O2 furnace a century ago and the leading cause was rust rushing through a control valve at high velocity; Carbide’s solution was a nickel/copper alloy (Monel). The trouble is that it takes six months to get a 4-in.-dia nickel/copper-alloy control valve. There had to be a better solution — there was. I knew steel makers used pure O2 in their furnaces, so I started making calls; the steel companies had changed to type-316 stainless decades ago. Type-316 stainless steel valves are off-the-shelf items. The moral of this story is to always challenge pipe specifications and design practices. They could be obsolete.

And always remember that good pipe specifications are crucial both for economics and safety. (See: “Get Pipe Specifications Right in the Beginning.”)


dirk.jpgDIRK WILLARD is a Chemical Processing contributing editor. He recently won recognition for his Field Notes column from the ASBPE. Chemical Processing is proud to have him on board. You can e-mail him at dwillard@putman.net