How to Review and Select Piping Specialty Items Safely

In my 20-plus years of piping engineering, I have seen many projects run smoothly on standard fittings only to grind to a sudden halt because of a single poorly specified specialty item. I remember a project in 2014 where a temporary cone strainer collapsed inside a compressor suction line during commissioning. The debris shredded the first-stage impellers, costing the client three million dollars in repairs and months of delay. That disaster could have been avoided with a simple, rigorous engineering review of the strainer’s structural collapse pressure.

Piping specialty items (often called SP items) are the unsung heroes and potential single points of failure in industrial plants. Because they do not fit neatly into standard ASME B16.5 or B16.9 dimensions, they require a dedicated review process. We must evaluate their pressure-containing capability, material compatibility, and structural interaction with the surrounding piping system.

Why Do Piping Specialty Items Require Custom Engineering?

When we design a piping system using standard elbows, tees, and flanges, we rely on the pressure-temperature ratings established by standards like ASME B16.5. However, for piping specialty items such as basket strainers, expansion joints, and sight glasses, no such universal standard exists. These are classified as “unlisted components” under ASME B31.3.

According to ASME B31.3 Paragraph 304.7.2, the pressure design of unlisted components must be verified using one of five strict methods:

• Extensive successful service experience under comparable operating conditions.
• Experimental stress analysis in accordance with ASME Section VIII, Division 2, Annex 5-F.
• Analytical calculations (such as Finite Element Analysis – FEA) per ASME Section VIII, Division 2, Part 5.
• Proof testing in accordance with ASME B16.9, MSS SP-97, or ASME Section VIII, Division 1, UG-101.
• Detailed calculations using recognized engineering standards.

Strainer Open Area Ratio (OAR) Calculation

One of the most common reviews I perform is for basket and Y-strainers. If the open area of the strainer basket is too small, the pressure drop across the strainer will be excessive, leading to cavitation, flow restriction, or element collapse. The Open Area Ratio (OAR) is calculated as follows:

Let us walk through a real-world design example. Suppose we have an 8-inch Schedule 40 pipe basket strainer.

• • Nominal pipe size = 8 inches
• • Inside diameter of 8-inch Schedule 40 pipe = 7.981 inches
• • Internal cross-sectional area of the pipe (A_pipe) = (3.1416 * 7.981^2) / 4 = 50.02 square inches
• • Strainer basket dimensions: Diameter = 9.5 inches, Length = 18 inches
• • Basket surface area (cylinder + flat bottom): A_basket = (3.1416 * 9.5 * 18) + ((3.1416 * 9.5^2) / 4) = 537.21 + 70.88 = 608.09 square inches
• • Perforated plate open area = 50% (0.50)
• • Mesh screen open area = 38% (0.38)

Now, we calculate the total open area of the basket (A_open):

Finally, we calculate the Open Area Ratio (OAR):

In my experience, for a basket strainer in liquid service, we look for an OAR of at least 4:1. A ratio of 2.31:1 is too low and will cause rapid blinding and high clean pressure drops. I would reject this design and request a larger strainer body (e.g., a 10-inch or 12-inch body with 8-inch nozzles) to achieve an OAR of 4:1 or higher.

Equivalent Pressure on Specialty Flanged Joints

Piping specialty items are often heavy and introduce external bending moments and axial forces onto adjacent flanges. To ensure these joints do not leak, we must calculate the equivalent pressure (P_eq) using the Kellogg method:

Where:

• • P_design = Internal design pressure (psi)
• • M = External bending moment (in-lb)
• • F = External axial force (lb)
• • G = Gasket reaction diameter (inches)

If the calculated P_eq exceeds the maximum allowable pressure rating of the flange at the design temperature per ASME B16.5, the joint is unsafe, and we must modify the piping support scheme to reduce the external loads.

To streamline the engineering review process, I have compiled a reference table of common piping specialty items, their primary design codes, and the key parameters that you must verify during the design phase.

How to Verify Piping Specialty Items on Site

Before any piping system is pressurized or handed over to operations, a physical walkdown of all piping specialty items is mandatory. In my experience, construction crews often treat these items like standard fittings, which leads to critical installation errors. Use the checklist below during your next field walkdown.

Field Case Study: Real-World Application

My direct recommendation is to always verify that piping guides are installed exactly per EJMA guidelines and that bellows spring rates are explicitly modeled in your stress analysis software. Never treat an expansion joint as a simple piece of pipe.