Plumbing valve types and selection field guide

Pick the valve by what it has to do

A valve is a device that starts, stops, throttles, or protects flow in a piping system. That sounds simple until you stand in front of a supply house wall of valves and realize the first decision is not the brand or the size. It is the job. A valve that opens and closes a line is built differently from one that meters flow, and both are different from one that only lets flow go one way or one that opens to dump pressure in an emergency.

Three things pick the valve, in order. First the function: are you isolating, throttling, checking backflow, or relieving pressure. Then the media and conditions: potable water, hot water, drain, gas, steam, chemical, and the pressure and temperature it runs at. Then the size and the end connection that matches the pipe. Get the function wrong and nothing else saves you. The most common valve mistake on a job is not a cheap valve. It is the right valve doing the wrong job.

Which pipe you are running and how you join it are separate decisions with their own tradeoffs, covered in the piping materials guide and the joining methods guide. The valve has to land on top of both: its body material has to suit the media, and its end connection has to match how you are joining the pipe. Pick a valve in isolation from those two and you end up with a threaded bronze valve on a system you meant to press, or a sweat valve you cannot solder near finished work.

Isolation or throttling: the split that decides everything

The first fork in valve selection is isolation versus throttling, and it is the one that gets violated most. Isolation valves are built to be all the way open or all the way closed. Throttling valves are built to sit partway and regulate flow. They are not interchangeable, and using one as the other is how valves die early.

A gate or a ball valve is an isolation valve. The flow path is either open or shut. Crack one partway to throttle and the high-velocity flow scours across the partly exposed seat or disc, which erodes the sealing surface and, on a gate, sets up vibration and chatter that wears the wedge and the guides. Do it long enough and the valve will not seal when you finally do close it. A globe valve is the throttling valve. Its seat and disc are shaped to take that velocity head-on and to give you fine control across the travel, which is why it has a much higher pressure drop wide open than a gate of the same size.

The practical rule is blunt. If you need to set a flow rate, reach for a globe. If you need to shut a line so you can work on it, reach for a ball or a gate. A ball valve will take light throttling in a pinch, and there are V-port ball and butterfly designs made for control service, but a standard isolation valve held partway open as a habit is a valve you will be replacing.

The ball valve, the modern shutoff

A ball valve is a quarter-turn isolation valve with a drilled ball that lines up with the pipe when open and turns 90 degrees to block it when closed. It is the workhorse shutoff in modern plumbing for one reason: it is fast, it seals tight, and the lever tells you its state at a glance. Handle in line with the pipe means open. Handle across the pipe means closed. You can read a whole mechanical room from across the floor.

The two versions you choose between are full-port and standard, or reduced, port. A full-port ball has a bore the same size as the pipe inside diameter, so it adds almost no pressure drop and lets you drain or flush the line at full flow. A standard-port ball has a ball roughly one pipe size smaller drilled through it, which restricts flow and adds pressure drop, but costs less and packs smaller. For a main shutoff, a pumped line, or anything you want to drain, spend the money on full-port. For an ordinary branch isolation where a little restriction does not matter, standard port is fine.

Ball valves seal on a seat, commonly PTFE, that takes a set if the valve sits in one position for years. The classic field surprise is a ball valve that has not been moved since the building was built: the seat has cold-flowed around the ball, and the first turn takes real force or weeps for a minute before it seats. Exercise the important ones. And treat a ball as on or off, because parking it half open chews the seat the same way it chews any isolation valve.

The gate valve, the older isolation valve

A gate valve isolates by driving a wedge-shaped disc, the gate, down across the flow path with a multi-turn handwheel. Wide open, the gate lifts clear of the bore and the valve passes nearly full flow with very little pressure drop, which is its one real advantage over a ball. It is the older isolation valve, and you still find it everywhere on existing systems and on larger lines where a full-bore shutoff matters.

Two stem styles show up. A rising-stem valve, often an outside-screw-and-yoke or OS and Y design, moves the stem up as you open it, so the stem height shows you the position and the stem threads stay out of the water where they cannot corrode or pack with scale. A non-rising-stem valve keeps the stem in place and is more compact, which suits tight or buried spots, but the threads run inside the body in contact with the media. On potable and corrosive water, the rising stem ages better.

Do not throttle with a gate. It is the headline rule for this valve. Partly closed, the flow tears across the bottom edge of the gate and sets up vibration that wears the seat and the gate face, and the disc can hammer in the guides. The other gate failure is the one you find at the worst time: a gate left fully open for years seizes from corrosion or scale on the stem threads and will not budge when you finally need to close it, or it closes but will not seal because the seat is pitted. When a quick, reliable shutoff matters, a ball valve is the better choice.

What is a globe valve used for?

A globe valve is the throttling valve. It regulates flow rather than just shutting it. Inside, the flow makes an S-turn up through a seat and past a disc that you raise and lower with the handwheel, and that path is what lets it control flow smoothly across its travel and shut off cleanly at the end. The same path is why a globe has a much higher pressure drop wide open than a gate or a full-port ball of the same size. That drop is not a defect. It is the feature that makes throttling stable.

Reach for a globe anywhere you need to set and hold a flow rate: a bypass you balance, a fill line you want to ease open, a hose station, a point where you trim flow to a piece of equipment. The disc and seat take the velocity that would erode a gate, and many globe seats are renewable, so a globe in throttling service can be reseated instead of replaced. The angle valve is a globe built with the outlet at 90 degrees to the inlet, which saves a fitting and a little pressure drop at a corner.

For fine metering on small lines, the needle valve is the globe's close relative: a tapered needle into a small seat gives precise control of a small flow, which is why you see them on gauges, instruments, and sampling points rather than on a 2 in water main.

What is a check valve?

A check valve is a one-way valve that lets flow pass in one direction and closes to stop it from reversing. There is no handle. The flow itself opens it, and gravity, a spring, or back-pressure closes it. You put one wherever reverse flow would cause a problem: on a pump discharge so the system does not drive the pump backward, on a water heater feed, ahead of equipment, or where two sources could push back into each other. The arrow cast on the body has to point in the direction of flow, and a check installed backward is a valve that will not pass flow at all.

The styles trade off the same way every time. A swing check has a hinged disc that flaps open and falls closed, passes flow with little restriction, and is usually limited to horizontal runs or vertical flow-up. Its weakness is the slam: when flow reverses, the disc waits until the reverse wave reaches it and then bangs shut, which is a classic source of water hammer on a pump system. A spring-loaded check, the silent or wafer check, closes the instant flow stops, before the reverse wave builds, so it controls hammer and installs in any orientation as long as the arrow follows the flow. It costs more and passes a little less flow for its size. A ball check, where a free ball seats on back-flow, handles dirty water and is common on sumps and drainage.

Backflow prevention for cross-connection protection, the testable assemblies that keep contaminated water out of the potable supply, is a separate device class governed by its own standards, covered by topic in the cross-connection material. A plain check valve is not a code backflow preventer. Do not let one stand in for the other.

The butterfly valve, for large pipe

A butterfly valve is a quarter-turn valve with a disc that rotates inside the body on a shaft: parallel to the flow it is open, rotated 90 degrees it seats against a liner and closes. Its case is large pipe. On 4 in and up, a butterfly is far lighter, shorter face-to-face, and cheaper than a gate or a ball, which is why mechanical rooms run them on chilled water, condenser water, and large building risers. It isolates well, and a resilient-seated butterfly will take some throttling, unlike a gate.

Throttle it in the middle of its travel, not near closed. Held just off the seat, the high-velocity flow whips across the disc edge and the resilient liner and can chew the seat, so control service keeps the disc roughly in the 30 to 70 degree range. Push fine control near the seat and you damage the liner.

Lug versus wafer is the mounting decision and it matters for isolation. A wafer body sandwiches between two flanges with through-bolts and depends on both flanges to hold it, so it cannot be used at the end of a line or with one side open. A lug body has threaded lugs around the rim that bolt to each flange independently, so it can hold pressure with one side disconnected and serve as an end-of-line shutoff, which is exactly what you want when you need to isolate equipment and unbolt it. Lug bodies also distribute bolt load better and generally carry the higher pressure rating. If you will ever open one side for service, spec lug, not wafer.

Angle stops and supply stops at the fixture

Supply stops are the small isolation valves at the fixture: the chrome angle stops under a sink, behind a toilet, and at the ice maker and the dishwasher. Angle stop, supply stop, fixture stop, and shutoff all name the same thing at this level. The point of them is local isolation. You shut one fixture to change a faucet or a fill valve without killing water to the rest of the building.

The two kinds you install are quarter-turn and multi-turn. A quarter-turn stop is a small ball valve, snaps from open to closed in a 90 degree throw, and reads at a glance. A multi-turn stop is the old compression style with a washer-on-stem globe action that takes several turns and, after ten or fifteen years, sticks, drips at the stem, or will not reseat when an apprentice cranks on it during an emergency. Quarter-turn costs a couple dollars more and outlives the multi-turn by years. Most service plumbers swap multi-turn stops to quarter-turn on sight.

End connection at the stop is its own small decision. A compression inlet uses a nut and ferrule squeezed onto the copper or CPVC stub-out, a sweat inlet solders to a copper stub, and threaded and push-fit inlets cover the rest. Match the inlet to the stub coming out of the wall, and match the outlet, commonly 3/8 in compression, to the fixture supply riser.

The pressure-reducing valve

A pressure-reducing valve, a PRV, takes a high, variable upstream pressure and holds a lower, steady downstream pressure regardless of how the supply swings. It is a regulating valve, not a shutoff. A spring-loaded diaphragm senses downstream pressure and modulates a seat to hold the setpoint, commonly adjusted into the 45 to 60 psi range for a building, then tuned to the system.

Most plumbing codes require a PRV on the building water service where the static street pressure exceeds 80 psi, because pressure above that range stresses fixtures, fill valves, and supply tubing and drives leaks. Confirm the threshold and the requirement against the adopted code edition, because the number and the trigger vary by jurisdiction and amendment.

Putting a PRV on the service creates a closed system: the reducing valve does not let pressure relieve back toward the street. Now when the water heater fires and the water expands, that thermal expansion has nowhere to go, pressure spikes, and the relief valve on the heater weeps or fixtures hammer. The fix is thermal expansion control, a properly sized expansion tank, installed with the PRV. The expansion side and the relief side are covered by topic in the thermal expansion material. If you install a PRV and skip the expansion tank, you have built the problem in.

The pressure relief and T&P safety valve

A relief valve is a safety valve. It stays shut until pressure, or pressure and temperature together, reaches a set limit, then it opens automatically and dumps to protect the vessel and the people near it. On a water heater or a hot-water storage tank the device is the temperature-and-pressure relief valve, the T and P, which opens on excess pressure or excess temperature, whichever it hits first. This is the one valve in the building whose whole purpose is the worst-case day.

The setpoints are stamped on the valve and set by its listing, commonly around 150 psi and 210 degrees F for a residential water heater T and P, and you do not adjust them in the field. The valve has to be the right rating for the vessel, mounted in the listed port, and its discharge piped down to a safe termination, full size, with no valve, no cap, and no trap in the line. Plug or valve a relief discharge and you have disabled the one safety device that keeps a tank from becoming a hazard.

Relief valves stick. Mineral scale builds on the seat, and a T and P that has never been tested can be seized shut when it is needed. Many manufacturers call for periodic testing by lifting the test lever, and a relief valve that weeps after testing or shows scale is replaced, not nursed. Hedge the exact setpoints and the test interval to the valve's listing and the manufacturer's instructions.

The backwater valve on the drain side

A backwater valve is a check valve for the sanitary drain. It sits in the building drain or the lateral and lets sewage flow out toward the main while a flapper closes against flow coming back in, so when the municipal sewer surcharges or backs up, the building's low fixtures do not flood with it. It protects basements, below-grade fixtures, and anything downhill of the street main.

Plumbing codes commonly require a backwater valve where a fixture drains below the upstream manhole cover or the crown of the street sewer, the fixtures that flood first in a backup. The exact trigger, the location, and the required access are set by the adopted code and local amendment, so confirm them rather than assuming. The valve has to be installed with a cleanout and accessible cover, because a backwater valve that cannot be reached cannot be cleaned, and a flapper fouled with debris is a flapper that will not seal when the sewer surcharges.

Reference the specific product standard and the code section by the adopted edition. A backwater valve is only protection if it is serviced, so it belongs where someone can open it and clear it.

Thermostatic mixing valves

A thermostatic mixing valve blends hot and cold to hold a set outlet temperature, and adjusts the blend automatically as the inlet temperatures or the flow change. It does two jobs that a manual blend cannot. It lets a water heater store hot enough to control bacteria while delivering safe water at the fixture, and it holds temperature when someone elsewhere draws cold and would otherwise scald the user.

Two listings cover the common cases. A point-of-use device, listed to ASSE 1070, limits temperature at the fixture for scald protection and is set into a low delivery range, commonly around 105 to 110 degrees F for lavatories. A master or system mixing valve, listed to ASSE 1017, tempers the hot water leaving the heater for distribution. They are not interchangeable: a system valve at the heater does not replace point-of-use scald protection at a fixture, and the listing is what tells you which job a given valve is rated for.

Mixing valves rely on internal checks to stop hot from crossing into cold and back, and a TMV that drifts or will not hold temperature is often a fouled or failed internal check or a cartridge limed up by hard water. Verify the listing and the setpoint range against the application by topic; do not assume one valve covers both system tempering and point-of-use protection.

Diaphragm, pinch, and other specialty valves

A handful of specialty valves show up when the media is the problem. A diaphragm valve seals by pinching a flexible diaphragm down onto a weir, which keeps the operating parts out of the flow path entirely. That makes it the choice for slurries, abrasives, and clean or sterile service where you cannot have the media reaching a stem or a seat. A pinch valve goes further and squeezes a flexible sleeve shut, with nothing but the sleeve touching the media, which suits gritty and abrasive slurries that would destroy a metal seat.

These are not your everyday building-water valves, and you reach for them by the media, not by habit. When you do, size and rate them to the manufacturer's data for the service, because their pressure and temperature limits are tied to the diaphragm or sleeve material and run lower than a metal-seated valve of the same size.

Valve body materials

The valve body has to suit the media, the pressure, and the temperature, the same way the pipe does. Bronze and brass dominate small potable and hot-water valves: threaded and sweat ball and gate valves up through about 2 in to 3 in are mostly bronze or brass because the metal handles potable water, takes a tight seat, and joins to copper easily. Cast iron and ductile iron carry the large flanged and grooved valves on building risers and mechanical systems, where iron is strong and economical at size and the media is treated water, not aggressive chemistry. Ductile iron carries higher ratings than gray cast iron.

Stainless steel goes where corrosion or temperature would eat bronze or iron: aggressive water, chemical service, high-purity, and higher-temperature lines. PVC and CPVC plastic valves handle chemical, irrigation, and drainage service and cold or moderate-temperature water, with CPVC taking more heat than PVC, and both are limited by temperature in a way metal is not. Match plastic valves to the same temperature and pressure derating as the plastic pipe they sit on.

For potable water there is a hard overlay on all of it. Wetted components have to be lead-free, defined under the Safe Drinking Water Act and certified to NSF/ANSI 372 as a weighted average lead content of no more than 0.25 percent across the wetted surfaces. A valve sold for non-potable service can look identical and not meet that limit, so on potable systems you buy valves marked lead-free and listed for potable water, and you confirm it rather than assuming.

End connections: match the valve to the joining method

A valve's end connection has to match how you are joining the pipe, and this is where a valve ordered in isolation bites you. Threaded ends take NPT and thread onto steel, brass, or threaded adapters. Sweat or solder ends slip over copper tube and solder like a fitting, which means a torch near whatever is around the valve. Press ends crimp onto copper or stainless with a tool and no flame, which is why they win near finished work. Grooved ends clamp into a coupling on roll-grooved pipe and dominate large mechanical lines. Flanged ends bolt to a mating flange and carry the big iron and steel valves. Push-fit ends shove onto the pipe with an internal grab ring for fast repair and tight spots, and solvent-socket ends glue onto PVC and CPVC.

The joining methods themselves, how each one is made and where each fails, are covered in the joining methods guide. The selection point here is narrow: the valve end and the pipe joint are one decision. If the system is pressed copper, the valves are press, not sweat, so you are not lighting a torch on a press job. If it is grooved, the valves are grooved. Order valves with the wrong ends and you are buying adapters, adding joints, and adding leak paths the design never had.

One more field note. Sweat valves with soft seats can be cooked by the torch during soldering. When you solder a ball or a globe valve, open it partway and keep the heat moving, or you melt the seat you are trying to install. Press and threaded versions sidestep that risk, which is part of why they keep gaining ground.

What does WOG mean on a valve?

The numbers stamped on a valve body are its pressure rating, and reading them keeps you from putting a valve where the system will overpressure it. WOG, water-oil-gas, and CWP, cold working pressure, are the same thing: the maximum non-shock pressure the valve handles at ordinary ambient temperature, commonly the range of about minus 20 to 100 degrees F. A valve marked 600 WOG is good for 600 psi cold. WSP, working steam pressure, is the separate, lower rating for saturated steam, and on steam you read the WSP, never the WOG.

Flanged valves also carry an ANSI class, 125, 150, 200, 300 and up, which is a pressure-temperature rating, not a single psi. The allowable pressure for a class falls as temperature rises, because the metal weakens when it is hot. A class-150 valve holds more cold than it does at 300 degrees F, and the rating tables, tied to the body material, give the actual allowable at each temperature. Class 125 and 250 are the cast iron families; class 150 and 300 cover ductile iron, steel, and stainless.

Read the rating as a ceiling that depends on the service, and hedge it to the valve and the standard rather than carrying one number for everything. The marking system itself follows MSS practice, and the pressure-temperature basis follows the valve standard the body is built to. When the media is hot, when it is steam, or when the system can shock, confirm the rating for that condition against the manufacturer's data, not the cold WOG alone.

Full-port versus standard-port: when the bore matters

Full-port and standard-port describe the bore through the valve relative to the pipe. A full-port valve has an opening the same size as the pipe inside diameter, so flow passes straight through with almost no added pressure drop. A standard, or reduced, port valve necks down to roughly one pipe size smaller through the valve, which restricts flow and adds drop, in exchange for a smaller, cheaper valve.

Most of the time on a branch shutoff the difference does not matter, and standard port is the economical pick. Full port earns its cost where flow and drop actually count: a building or main shutoff, a pump suction or discharge where you cannot afford the restriction or want to avoid cavitation, a line you need to drain or flush at full bore, and any run where you are already fighting for pressure. On a long, loaded system, a string of reduced-port valves quietly adds up to real pressure loss.

The honest field test is whether you would notice the restriction. On a 1/2 in lav stop, no. On a 3 in pumped riser feeding the top of a building, the reduced port is a tax you pay every minute the pump runs. Spec full port where the bore is in the flow path that matters and standard port where it is not.

Manual and automated actuation

How the valve is operated is a selection axis of its own. Manual is the default: a lever on quarter-turn valves, a handwheel on multi-turn gates and globes. For most building isolation and throttling, manual is all you need, and a lever you can read across the room beats anything fancier for a shutoff.

Automated actuation comes in when the valve has to operate on a signal, on a schedule, or faster than a person can get to it. An electric or pneumatic actuator bolts to a quarter-turn or rising-stem valve to open and close it from a control system, which is how zone valves, building automation, and large mechanical isolation get driven. A solenoid valve is a small, fast, electrically operated valve for on-off control of a line, common on fill, irrigation, and equipment feeds. The automation and controls side is its own topic; the selection point is that the valve and the actuator are a matched pair, sized to the torque and the service together, not bolted up after the fact.

Where to put isolation valves

The isolation strategy is decided at design, and it is the difference between fixing one fixture and shutting down a building. The principle is simple: put a shutoff at every point where you would otherwise have to drain a larger section to do a repair. Money spent on isolation valves up front is money you do not spend on building-wide shutdowns for the life of the system.

The places that earn a valve: at each fixture, so a faucet or a flush valve is changed without touching the rest. At each branch, so a restroom group or a tenant space drops out on its own. At the base and top of risers, so a stack is isolated floor by floor instead of all at once. At every piece of equipment, so a heater, a pump, or a coil is pulled with the line still up everywhere else. And at the main, so the whole building can be shut and the rest of the system worked on. On large buildings the valves also let you sectionalize for testing and for finding a leak by closing in on it.

The mistake is the building with too few valves. Somebody saved a hundred dollars in valves at rough-in, and now changing a single lavatory faucet means draining a wing because the nearest shutoff is two floors down. The crew that has to live with the system pays for the valves the estimator cut, over and over.

The building main and the meter stop

Every building has a main shutoff, and knowing where it is, and that it works, is basic competence that fails at the worst moment. The building main is the valve just inside where the service enters, commonly a full-port ball or a gate, and it kills water to the whole building. Upstream of it is the meter stop on the meter setter and, out at the property line, the curb stop the utility operates with a key.

The trap is the seized main. A main shutoff installed twenty years ago and never touched corrodes or its seat takes a set, and the day a pipe bursts is the day you discover it will not close, or closes and still passes water. The professional move is to exercise the main and the key isolation valves on a schedule so they actually function when a flood is running across the floor. Find the curb stop too, and confirm the utility key fits, because when the building main fails the curb stop is the backstop.

On a service call, the first thing a competent plumber locates is the shutoff, before the repair, not during the emergency.

Why is my valve leaking or stuck?

Valves fail in a short list of ways, and reading the symptom tells you the cause. A drip from around the stem, under the handle, is almost always the packing or the stem seal, not the seat. On a gate or globe with a packing gland, a careful snug of the gland nut often stops it; on a cartridge or O-ring stem, you replace the seal. Do not crank the gland down hard to stop a weep, because you bind the stem and round off the next person who has to turn it.

A valve that will not seal when closed has a seat problem: debris caught on the seat, a seat that took a set from sitting in one position for years, or wire-drawing, the fine erosion channel cut across a seat by flow leaking past a not-quite-closed valve, often the legacy of someone throttling an isolation valve. A gate or main that will not move at all is seized, corroded or scaled on the stem threads from never being exercised. A check valve that lets the system run backward, or that bangs on every pump stop, is a failed or wrong check, and the slam is usually a swing check where a spring check belonged.

The thread running through all of it is exercise. Valves that move occasionally keep working; valves that sit untouched for a decade seize, set, and fail when you finally need them. Cycle the important isolation valves and test the relief valves on a schedule. The valve you never touch is the one that lets you down.

Isolation for critical systems that cannot go down

On systems that cannot tolerate a full shutdown, a data center cooling loop, a hospital line, a process feed, the valving is designed so a section can be isolated and serviced while the rest keeps running. That changes the valve count and the layout. You see valved bypasses around equipment so a pump or a heat exchanger drops out without breaking the loop, paired isolation valves so a component sits between two closed valves, and double-block-and-bleed arrangements where two valves close and a bleed between them proves the isolation is tight before anyone opens the line.

The selection logic is the same as the rest of this guide, applied harder: full-port for low drop on the critical loop, the right body for the media and pressure, and reliable quarter-turn or actuated isolation that closes when commanded. The design point is that you can take any one component out for maintenance without taking the service down, which means the isolation has to be planned into the piping, not added later. Confirm the redundancy and the isolation scheme against the project requirements and the reliability target for the facility.

What to document

A valve schedule that lives only in someone's head is a schedule that dies when they leave. The record that matters is which valve does what, where it is, and how it is rated, so the next person can isolate the right section without guessing and can replace a valve with one that actually fits.

Capture the valve type and function, the size and the end connection, the body material and whether it is lead-free for potable, the pressure rating, the location and what it isolates, and the setpoint on anything adjustable like a PRV, a relief valve, or a mixing valve. On a building of any size, a valve tag and a valve chart that maps each tagged valve to what it serves turns an emergency shutoff from a search into a single move.

Valve, function, and best use at a glance

The whole selection collapses to one question repeated: what does the valve have to do, and which type is built for that. This table is the short version to carry to the supply house.

Field checklist

Common mistakes

• Throttling with a gate or a ball valve, which erodes the seat until it will not shut off.
• Putting a gate where a quick, reliable shutoff is needed, then finding it seized when it matters.
• Skimping on isolation valves, so changing one fixture means draining a wing or the building.
• Installing a check valve backward, or using a swing check where the slam causes water hammer.
• Reading the WOG on a steam line, or ignoring that a flanged class derates as temperature rises.
• Picking the wrong body material or a non-potable valve for a media or potable system it does not suit.
• Ordering valves with the wrong ends for the pipe, adding adapters, joints, and leak paths.
• Installing a PRV and skipping the expansion tank, so the closed system spikes and the relief weeps.
• Capping or valving a relief discharge, which disables the one safety device on the vessel.
• Mistaking a plain check valve for a code backflow preventer on a cross-connection.

Standards and references

Valve construction and ratings follow a set of standards, and citing the right one for the valve is what keeps a submittal honest. The Manufacturers Standardization Society, MSS, writes the standard practices most plumbing valves are built to. MSS SP-80 covers bronze gate, globe, angle, and check valves in the common classes for threaded, solder, and flanged ends, and it sets the pressure-temperature ratings, materials, and testing for them. The MSS SP series also covers iron gate and check valves, ball valves, and butterfly valves under their own numbers, so confirm the specific SP number and edition for the valve you are submitting rather than carrying one citation for all of them. MSS SP-25 is the marking system you read on the body.

For larger and industrial valves, ASME B16.34 governs pressure-temperature ratings for flanged, threaded, and welded valves, and API standards cover heavy industrial gate, ball, and check valves by topic. Mixing valves are listed to ASSE standards, point-of-use scald protection to ASSE 1070 and system tempering to ASSE 1017, and backflow prevention assemblies and backwater valves each fall under their own ASSE and ASME product standards by topic. On potable water, NSF/ANSI 61 covers the health effects of materials in contact with drinking water and NSF/ANSI 372 covers the lead-free content limit.

The installation rules, where shutoffs are required and must be accessible, where a PRV is required above a pressure threshold, and where a backwater valve is required, live in the adopted plumbing code, the IPC or the UPC depending on jurisdiction, with local amendments. Standards and code editions change between cycles and ratings are tied to the specific valve and its listing, so verify the current edition and hedge the pressure-temperature numbers to the valve and the standard it is built to, not to a remembered figure.

Units, terms, and conversions

Valve specs read across a few naming systems, and the same valve shows up under different labels on a drawing, a catalog, and a body casting.

Pressure is psi in US sources and kPa or bar in metric ones, where 1 bar is about 14.5 psi and 100 kPa is 1 bar. Valve and pipe size is nominal pipe size, NPS, in inches, with DN the metric nominal in millimeters. WOG and CWP both mean the cold non-shock pressure rating, WSP is the steam rating, and the ANSI flange class is a pressure-temperature rating rather than a single psi. Port describes the bore: full-port matches the pipe bore, standard or reduced port necks down. Quarter-turn means a 90 degree throw, multi-turn means several handwheel turns.

Isolation valve

A valve built to run fully open or fully closed, such as a ball or gate

Throttling valve

A valve built to regulate flow at intermediate positions, such as a globe

Full-port / standard-port

Bore equal to pipe inside diameter, versus a bore one size smaller that adds pressure drop

WOG / CWP

Water-oil-gas or cold working pressure, the maximum non-shock pressure at ambient temperature

WSP

Working steam pressure, the separate lower rating for saturated steam

PRV

Pressure-reducing valve, holds a steady lower downstream pressure

T and P relief

Temperature-and-pressure relief valve, the safety valve on a water heater or hot-water vessel

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