Valve Piping Classes and Specification Sheets: A Complete Engineering Guide

What Is a Piping Class (Pipe Spec)?

A piping class (also called a pipe specification, piping spec, or pipe class) is an engineering document created by the process or piping engineer that defines all the materials, dimensions, ratings, and testing requirements applicable to a specific group of pipe, fittings, flanges, and valves that operate within a defined service condition. For example, 'Pipe Class A1A' might apply to all carbon steel piping in general hydrocarbon service at ASME Class 150, while 'Pipe Class C3B' might apply to stainless steel piping in corrosive chemical service at ASME Class 300. Every pipe and valve on the plant P&ID is tagged with a pipe class, ensuring that only compatible, correctly-rated components are purchased and installed.

Structure of a Piping Class Document

A piping class document typically contains the following sections:

1. 1Header: Pipe class identifier, service description (e.g. 'General hydrocarbons'), design pressure and temperature range, pipe material and schedule, corrosion allowance, and applicable codes (ASME B31.3, B31.1, etc.)
2. 2Pipe and Fitting Schedule: Lists pipe material (e.g. ASTM A106 Gr. B), wall schedule (e.g. SCH 40), fitting standard (ASME B16.9 butt-weld, B16.11 socket weld, B16.3 threaded), and branch connection table
3. 3Flange Specification: Flange material (ASTM A105 for carbon steel), flange face type (RF, FF, RTJ), gasket type (spiral wound, ring type joint), bolting material (A193 B7 studs, A194 2H nuts)
4. 4Valve Specification Section: Lists each valve type (gate, globe, check, ball, butterfly) with body material, trim material, end connection (flanged, BW, SW, threaded), pressure class (ASME Class 150, 300, 600, etc.), leakage class, applicable standard, and any special requirements (fire-safe, fugitive emission, low-temperature)
5. 5Inspection and Test Requirements: PMI (Positive Material Identification), hydrostatic shell test, seat leakage test, radiography (RT), heat treatment requirements (PWHT), impact testing at low temperature
6. 6Applicable Standards and Company Specifications: Cross-reference to applicable API, ASME, ISO, and company internal specifications

ASME Pressure Classes and Their Meaning

Valve Datasheet / Specification Sheet Components

A valve specification sheet (valve datasheet) is completed for every valve type and size in a pipe class. It contains all technical requirements that the valve must meet. Typical datasheet fields include:

• Tag number and service description (e.g. 'HV-101 — Crude oil isolation')
• Valve type (gate, globe, ball, butterfly, check, plug, needle)
• Size (NPS or DN) and quantity
• Pressure class (ASME Class 150 through 2500) and end connections (flanged RF/RTJ, BW, SW, NPT)
• Body material (ASTM A216 WCB, A351 CF8M, A995 Gr 4A duplex etc.)
• Trim material (seat and disc/ball/wedge material — 13Cr, Stellite 6, PTFE, RPTFE, SS 316)
• Stem material and packing material
• Operator type (handwheel, gear operator, actuator — specify pneumatic or electric, fail mode)
• Leakage class (per ANSI FCI 70-2 or API 598)
• Test requirements (shell test pressure, seat test pressure, duration, test medium)
• Special requirements: fire-safe per API 607, low-temperature per BS 6364, fugitive emissions per ISO 15848-1, NACE MR0175
• Applicable design standard (API 600, API 6D, API 608, API 609, BS 5351, etc.)
• Inspection and certification requirements (third-party inspection, material traceability, heat number marking)
• Tag marking, painting, and preservation specification reference

Common Pipe Class Designator Systems

Different companies use different pipe class designator systems, but the most common structure encodes the ASME pressure class and pipe material in the identifier. A typical format is: [Number][Letter][Letter] where the number indicates the pressure class (1=Class 150, 3=Class 300, 6=Class 600, 9=Class 900, etc.), the first letter indicates pipe material (A=carbon steel, B=low-alloy steel, C or S=stainless steel, D=duplex), and the second letter is a sequential identifier. For example: 1A1 = Class 150 carbon steel; 6S1 = Class 600 SS 316L; 15D1 = Class 1500 duplex stainless. Note that different owner companies (Shell DEP, ExxonMobil GP, Saudi Aramco SAES, Reliance RES) have proprietary pipe class systems — always request the applicable company engineering standard before designing a valve package.

How to Correctly Specify a Valve from a Pipe Class

1. 1Identify the pipe class from the P&ID for the valve location — e.g. '6A2' pipe class.
2. 2Look up the valve section in the pipe class document for the valve type (isolation gate, NPS 4).
3. 3Read the valve specification: e.g. 'Gate valve, NPS 4, Class 600, ASTM A216 WCB body, SS 316 trim (13Cr seats, SS 316 stem), RF flanged, butt-weld or flanged per pipe class, handwheel operator, fire-safe API 607, API 600, hydro test 225 bar shell / 150 bar seat.'
4. 4Transfer all parameters to the valve purchase specification or inquiry document.
5. 5Obtain a manufacturer data sheet (MDS) or product submittal from the valve vendor and check each parameter against the pipe class requirement.
6. 6Review third-party inspection requirements: if the pipe class or project spec requires TPI at the manufacturer works, arrange this with your nominated inspection agency.
7. 7After delivery, verify PMI (Positive Material Identification) on body and trim as required by the pipe class inspection plan.

Small Bore vs Large Bore Valves in Piping Classes

Most piping classes define separate specifications for small-bore valves (NPS 1/2 to NPS 1-1/2) and large-bore valves (NPS 2 and above). Small-bore valves typically use socket-weld or screwed (threaded NPT/BSP) end connections and may be Class 800 or Class 1500 forged body construction rather than flanged. Large-bore valves from NPS 2 upwards use flanged (RF or RTJ) or butt-welded ends and ASME class-rated bodies. Some piping classes specify Class 800 socket-weld gate valves for all small-bore isolation, using integral flanged bodies only from NPS 2 upwards — always check the piping class for the size breakpoints.