Specification Explainer · Engineering Perspective · Published: October 9, 2026
Table of Contents
Key Takeaways
- WOG rates a valve’s maximum non-shock working pressure for water, oil and gas at ambient temperature, while WSP rates the same valve for saturated steam service at elevated temperature [1].
- On any given valve the WOG figure is always higher than the WSP figure. Industry references note that a bronze gate valve marked 200 WOG may carry a WSP rating of only 125–150 psi [3].
- CWP (cold working pressure) is functionally the same concept as WOG, and manufacturers commonly use the two terms interchangeably; the ambient band is generally understood as −20 °F to 100 °F [5].
- A Class number is a designation, not a pressure: Class 150 carbon steel is good for roughly 285 psi at ambient temperature and Class 300 for about 740 psi, both read from material-specific tables [4].
- Ratings fall as temperature climbs. A valve good for 1,000 psi at room temperature may hold only a fraction of that in hot service [2].
- Specify by reading the pressure-temperature table at the actual operating temperature, never by treating the nameplate figure as one universal number.
The Marking on the Body Is a Contract
Every cast or forged valve body carries stamped markings, and the pressure rating is the one buyers most often misread. The stamp is not decoration and not marketing shorthand. It is the manufacturer’s statement of the maximum pressure the pressure boundary may hold, under defined media and at a defined temperature. The two most common letter codes on general-purpose valves, WOG and WSP, encode exactly those two definitions, and the difference between them is almost entirely about temperature.
Misreading the code has real consequences. A valve selected for a hot line using its cold-service figure operates above its true rating every hour it runs, with leakage, seat deformation and eventually pressure-boundary risk as the typical progression. The good news is that the rating system is simple once the acronyms are separated.
WOG: The Cold-Service Baseline
WOG stands for water, oil and gas. It is the maximum non-shock working pressure the valve may hold at ambient temperature for those media, and it is the most common rating stamped on general-purpose brass, bronze and stainless valves [1]. The “non-shock” qualifier matters: the rating presumes steady pressure, not water hammer or hydraulic spikes, which is why surge analysis on pumping systems is a separate exercise from rating selection.
Two scope notes prevent the most frequent misuse. First, although “gas” appears in the acronym, fuel-gas service — natural gas, propane and similar media — generally requires separate certification to dedicated gas standards, and a standard WOG-marked valve should not be assumed listed for fuel-gas distribution [1]. Compressed air and inert gases, by contrast, are ordinarily within the scope of the WOG figure. Second, WOG describes the media range and the pressure; it does not by itself certify a seat material, a packing design, or a temperature limit beyond the ambient reference.
WSP: The Hot-Service Counterpart
WSP stands for working steam pressure — the rated pressure for saturated steam service at elevated temperature [1]. Standardization guidance draws the distinction cleanly: WSP indicates the pressure at which the valve can continue to operate within its maximum temperature range, whereas WOG indicates only the pressure at ambient temperature [3]. Steam service implies both elevated temperature and a stored-energy hazard far greater than cold water, so the WSP figure is always lower than the WOG figure on the same valve [3].
The gap is not academic. A bronze gate valve with a 200 WOG rating may carry a WSP rating of only 125–150 psi, and a steam line whose relief valve is set above that figure has no business being throttled by that valve [3]. When a system runs hot — boiler feed lines, steam-adjacent piping, heat-recovery loops — the WSP figure, not the WOG figure, is the number that governs.
CWP and the Alphabet Soup
Walk a valve warehouse and you will see CWP stamped nearly as often as WOG. Cold working pressure is the maximum working pressure at ambient conditions, commonly understood as the band from −20 °F to 100 °F, and manufacturers use CWP and WOG largely interchangeably for the same rating [5]. Related legacy markings — working water pressure and similar variants — appear on older bronze and iron lines and convey the same idea: an ambient-temperature rating for defined media [5].
The practical takeaway is to treat WOG and CWP as two spellings of the cold-service baseline, then ask the only question that actually changes the answer: how hot does this valve get?
Why Heat Steals the Rating
The reason a single number cannot serve both cold and hot service is materials. Tensile strength of the body alloy, creep behavior, gasket and seal temperature limits, and differential thermal expansion between trim components all degrade as temperature rises. A pressure-temperature rating table is the engineering summary of that degradation: the pressure a valve may hold is a function of its material and the temperature of the pressure-retaining parts, and it declines continuously from the ambient figure [2]. The decline can be dramatic — a valve rated 1,000 psi at room temperature may be good for only a fraction of that in hot service [2] — which is why the nameplate value is properly understood as the starting point of a curve, not the whole curve.
For water treatment equipment the exposure is usually cold, which is why softener and filtration valves are selected on WOG-style figures. But boiler-feed pretreatment rooms, condensate-adjacent piping, and any line downstream of a heat exchanger can hold fluid well above ambient, and those positions deserve a rating checked at temperature.
Class and PN: Two Rating Languages
Alongside the letter codes, two standardized rating systems dominate engineered piping. In American practice, valves are designated by Class — Class 150, Class 300, and so on — where the Class number is a designation rather than a pressure. The actual allowable pressure comes from material-specific pressure-temperature tables: a Class 150 carbon-steel valve carries roughly 285 psi at ambient temperature, and a Class 300 valve about 740 psi, with both figures falling as temperature rises [4]. European practice uses PN designations on the same logic, with the number referencing nominal pressure at a reference temperature and the exact allowable value again read from the applicable material tables [4]. Either way the rule is identical: match the valve’s rating system to the mating flange system, then verify the actual allowable pressure at the operating temperature rather than at the nameplate reference [4].
Reading a Rating Correctly
| Marking | What it tells you | Watch-outs |
|---|---|---|
| WOG (e.g., a three-digit figure stamped on the body) | Maximum non-shock working pressure for water, oil, and gas at ambient temperature [1] | Ambient reference only; fuel-gas service needs separate certification [1] |
| WSP | Rated pressure for saturated steam at elevated temperature [1] | Always lower than WOG on the same valve; verify against relief-valve settings [3] |
| CWP | Cold working pressure, generally −20 °F to 100 °F [5] | Used interchangeably with WOG; same ambient-reference caution applies [5] |
| Class 150 / Class 300 | Designation whose actual psi comes from material pressure-temperature tables [4] | Not 150 or 300 psi — Class 150 carbon steel ≈ 285 psi at ambient [4] |
| PN designations | European nominal-pressure classes, allowable value read from material tables at temperature [4] | Match valve PN to flange PN; re-check the table at the hottest operating condition |
A Selection Routine That Holds Up
A five-step routine keeps rating selection honest. Identify the highest temperature the valve will see, including upset conditions rather than averages. Look up the allowable pressure at that temperature from the manufacturer’s pressure-temperature data for the actual body material. Compare that figure against the system’s maximum operating pressure including relief-valve set points, not just the normal operating pressure. Where a line can run both cold and hot, size to the hot case. Finally, keep the rating documentation with the project file, because an auditor or inspector will ask for the temperature basis, not just the stamped number.
Transparency helps here. Shanghai ChiMay publishes pressure and flow figures alongside its control valve documentation rather than gating them behind inquiries — we publish the numbers other suppliers make you ask for [6]. For a buyer comparing valves across suppliers, the fastest screening question is simply whether the pressure-temperature basis is published at all. Suppliers who publish it are usually the ones who test to it.
The stamped code on a valve body is short because the engineering behind it is not. Read WOG as the cold baseline, WSP as the hot ceiling, and the tables as the truth in between — and the rating system stops being an acronym puzzle and starts being what it was designed to be: a contract you can hold.
References
- Apex Flow Solutions — “What Does WOG Mean on a Valve? Pressure Ratings Explained” — Definitions of WOG (ambient-temperature water/oil/gas rating) and WSP (saturated-steam rating), with fuel-gas certification caveats. https://apexflowsolutions.com/pages/what-does-wog-mean-valve-ratings
- Apex Flow Solutions — “Valve Pressure & Temperature Ratings Reference Chart” — How ratings derate with temperature; notes a 1,000 psi room-temperature valve may hold only a fraction in hot service. https://apexflowsolutions.com/pages/valve-pressure-temperature-ratings-chart
- GOZ Valve — “What does W.O.G mean in Valve Field” — Explains the WSP versus CWP/WOG distinction; WOG always exceeds WSP on the same valve; bronze 200 WOG vs 125–150 WSP example. https://www.gozvalve.com/en/news/889873546724069407.html
- ValveMan — “Ball Valve Guide: How They Work, Types, Ratings, and Selection” — Class numbers are designations, not psi; Class 150 carbon steel ≈ 285 psi and Class 300 ≈ 740 psi at ambient per ASME B16.34 tables. https://valveman.com/blogs/the-valve-authority/ball-valve-guide
- Jan Hen Valve — “CWP on Valves: Meaning and Pressure Rating Explained” — CWP definition over −20 °F to 100 °F; notes manufacturers use CWP and WOG interchangeably. https://janhenvalve.com/cwp-on-valves-meaning-and-pressure-rating-explained
- Shanghai ChiMay — control valve product overview — Product-site overview of Shanghai ChiMay softener and softening-filtration control valves with published hydraulic and pressure data. https://www.chimaytech.com/
Specification values cited from Shanghai ChiMay product documentation reflect the company’s published materials as of the publication date; confirm current specifications against the latest datasheet before purchase.
About the author: Written by the Shanghai ChiMay Technical Editorial Team — the specification and quality engineers behind Shanghai ChiMay’s softener control valve line, covering pressure-boundary design, materials selection, and datasheet transparency.
