Valve Selection Guide: From Function to Class to Material

Select the valve by function first, then by pressure class and temperature, then by material. The order matters, because function eliminates more options than any other input.

A valve is a device with a job — stop flow, control flow, or prevent reverse flow — and the job decides the type before the operating conditions get a vote.

This guide walks the three cuts, gives the numbers our tool uses, and states where its coverage stops.

Cut 1 — What is the valve being asked to do?

It is being asked for one of three functions — isolation, throttling or non-return — and the function decides the type before the operating conditions get a vote.

Three functions cover the isolation and control duties this site handles:

Five valve types — ball, gate, globe, check and butterfly — account for roughly 85 to 90% of the industrial valve market, and that concentration is why this site covers exactly those five. The remaining types are not inferior; they serve duties that are specific enough that a screening tool should hand them over rather than approximate them.

Two functions are deliberately excluded, and the exclusions are decisions rather than gaps:

• Control valves. Control valve selection depends on Cv, on the actuator and on the control loop. We do not ask for Cv, because asking for it implies we can size on it. Control valves deserve their own treatment, not a side branch of an isolation valve tool.
• Safety and relief valves. Relief valve sizing is a code-governed calculation with a legal dimension. It is outside what a selection tool should touch, and we say so.

Cut 2 — Which class and temperature apply?

The operating temperature decides both: it sets what the pressure class can hold through the ASME B16.34 pressure-temperature table, and it sets whether the seat material is still usable — and the valve is limited by whichever of the two is lower.

The pressure-temperature rating is built in

Rising temperature reduces what a given class can hold, and the table that describes this is ASME B16.34.

Our tool applies it automatically, so a Class 150 valve at 538°C (1,000°F) is evaluated against what Class 150 actually holds at that temperature — which is about 1.3 bar (19 psi). That number is not an error and not a defect in a particular valve. It is the behaviour of the standard, and one of the most common misunderstandings in this category is attributing a class-collapse problem to the manufacturer rather than to the class.

The seat has its own gate

This is the part most selection guidance skips. The body and the seat have separate temperature limits, and the valve is limited by whichever is lower.

A valve built with a stainless body and a soft seat can pass the body rating and fail the seat rating, and the failure will not be visible on a pressure-temperature table.

Those figures carry a caveat we publish rather than bury: formulations of the same seat material family vary by roughly 10 to 20°C between manufacturers. We publish the conservative figure and treat the manufacturer's data sheet for the specific product as the governing document. The seat question is treated in full in Soft-Seated vs Metal-Seated Valves: Where the Temperature Line Is.

Material groups by temperature

Cut 3 — Which material does the medium force?

The medium leads to the material, and the material selection follows the failure physics of a valve rather than of a pump. The same acid behaves differently in a valve than in a pump, because a valve's exposure is through the body and the seat while a pump's is through the wetted parts and the seal. We do not move material ratings between the two, and this site does not inherit conclusions from our pump site.

The medium library here covers the same 37 media as the pump site plus three valve-specific ones — steam, sour oil and gas, and slurry — for 40 entries in total. Three groupings do most of the work:

• Wet and sour service. Sour oil and gas brings NACE MR0175 / ISO 15156 material requirements into the specification, and those requirements apply to the valve body, the trim and the bolting.
• Oxygen service. No oil and no copper. That is a material exclusion rather than a cleanliness preference.
• Slurry. The sealing surface faces abrasion rather than corrosion as the dominant failure mode, which changes the type choice before the material choice.

What diameter ranges do the valve types cover?

From DN300 upward the butterfly valve is preferred at Class 300 and below; ball valves move to trunnion-mounted construction at DN600; and DN1200, a class above 2500 and a pressure above 420 bar all sit past the range this site recommends automatically.

Four rules carry most of the diameter decision:

Between those lines the diameter bands overlap, and the choice falls back to the functions in Cut 1 — isolation or throttling — and then to the seat and material question.

The four rules are a boundary, not a matrix with a value in every cell. A diameter table that guessed the cells in between would look more complete and answer less.

What an answer from this site contains

It contains: a recommended valve type within the coverage window, the temperature limits that apply to the body and the seat separately, the material constraints the medium imposes, and the specification lines a supplier needs to quote.

It does not contain: a Cv value, an actuator selection, a relief valve sizing calculation, a compliance or certification determination, or a supplier recommendation. Each of those is a different discipline, and the coverage window is 40 media and five valve types — not the whole of flow control.

What people assume, and what the guide says

Common questions about valve selection

Why does function come before the medium?

Because a globe valve in the wrong medium can be corrected by material selection, while a gate valve in a throttling duty cannot be corrected at all — the damage is mechanical.

Does the tool cover control valves?

No. Control valves, safety valves, pressure-reducing valves, needle and instrument valves, diaphragm and pinch valves, plumbing valves and nuclear-grade valves are outside the five types covered here.

How do I handle a duty above 425°C?

The material group moves to chromium-molybdenum steels from that point, and above 538°C the duty goes to a human.

What if my medium is not in the library?

The enquiry is routed rather than approximated. The library holds 40 entries, and anything outside it goes to a human.

Is the output a purchase specification?

It is an RFQ specification sheet. It carries no price and no supplier recommendation, and the final selection belongs to the manufacturer's engineering team.

What this article does not cover

Note that this guide does not calculate Cv, size a relief valve, select an actuator or make a compliance determination, and it does not fill the gaps in the diameter table with estimates. It narrows a duty to a valve type and a material family, and states the boundaries of the coverage window.

Where the next constraint sits

A valve type and a material family are the end of the screening route, and the step that most often overturns them is the seat — a separate gate from the body rating, and the one that fails first above 230 °C, where every soft seat in the data set is past its limit. Soft-Seated vs Metal-Seated Valves: Where the Temperature Line Is carries those figures.

Where two of the five types overlap on function, what separates them is flow resistance and the damage mechanism behind it rather than preference: a gate valve sits at 0.1–0.3 of resistance coefficient and a globe valve at 4.0–10.0, and that gap is why both exist. Ball vs Gate vs Globe Valves: The Three-Way Comparison sets the three side by side.

The medium-by-medium version of the material cut — the properties that drive the choice and the materials that carry it — lives in the Media Guides.

Sources